Chapter 2 Flashcards
How do you know what’s alive?
Growth, reproduction, metabolism and response to stimuli. [Also, made up of living cells.] However not everything that looks alive is! (a cut mushroom or flower could be dead already). And some living things may not exhibit the above characteristics e.g. women who are not (or no longer) able to reproduce.
How do you know what’s alive, Descartes edition.
As you may recall from the topic introduction, Descartes (a French philosopher, 1596–1650) defined life from a philosophical perspective, ‘I think, therefore I am’ (originally in Latin, ‘Cogito ergo sum’), which relates to the idea of ‘consciousness’. There is an argument that computers with artificial intelligence could be deemed a form of life. These machines can move and respond to stimuli, they can even expand in size so apparently grow, but they are not made up of cells – so as a biologist I would argue that they are not alive.
How do I know what’s alive?
My attempts! How do I know I am alive? I eat, breathe, am aware of myself, I move. How many life forms on Earth? Hundreds of thousands of different species. All life forms have four characteristics in common: what do you think they are? Generate energy, have at least one cell, excrete, have a nervous system. All life forms are made up of something. What is it? Matter.
What is biodiversity? How many life forms are out there?
Biodiversity refers to the diversity of living organisms. There are over 1.5 million officially registered life forms or types of organism, known in biological terms as a species, of animals, plants and fungi living on Earth (excl. bacteria; viruses are different kettle of fish!). Estimates as to the actual number of species cover a vast range upwards of 3 million. In 2011, scientists estimated that there were 8.7 million, but 2016 studies suggest this number could exceed 1 trillion.
What is an organism?
An organism can be simply defined as an individual living thing.
What is a kingdom?
A kingdom is a group of species which all have certain fundamental characteristics in common.
How many species predicted by mathematical modelling?
animals: 7.77 million (12% described and named)
fungi: 0.61 million (7% described and named)
plants: 0.30 million (70% described and named).
What is taxonomy?
The ranking or organisation of living things is known as taxonomy.
What is a taxon and what is the plural?
Taxon is a group or a rank within a taxonomic scheme.
What is a domain? Who introduced the 3 domain system, what is it and what characteristics is it based on?
The top taxon is called a domain. Carl Woese introduced the 3 domain system in 1990 which became the most generally accepted one. The three-domain system is based on genetic differences between life forms found in each domain and the domains are called Archaea, Bacteria and Eukarya.
Archaea
Archaea are classified as prokaryotic based on their physical structure (i.e. they do not have internal cellular compartments). However, they do have similarities with eukaryotic cells on a genetic level. Based on these characteristics, Archaea are classified in their own domain.
Bacteria
The Bacteria domain is comprised of prokaryotic cells. Prokaryotic cells do not have separate internal compartments.
Eukarya
The Eukarya domain is composed of eukaryotic cells. Eukaryotic cells have their internal contents packaged up within compartments in the cell.
Prokarya v Eukarya
Both have cell membrane and genetic mat. Pro doesn’t have nucleus or internal compartments.
Humans: tree
eukaryote animalia chordate mammalia primate hominids homo sapiens
Tree!
Domain Kingdom Phylum Class Order Family Genus Species
What are the domains?
All of ‘life’ is classified into three domains: Archaea, Bacteria and Eukarya. Humans are part of the Eukarya domain.
What is a kingdom? How many are there?
The second highest taxonomic rank for living organisms is kingdom. It is generally accepted that there are six kingdoms. Organisms are placed into kingdoms according to their ability to make food and their cellular structure.
The Archaea domain consists of one kingdom, Archaea also called Archaebacteria. The Bacteria domain consists of one kingdom, Bacteria. The Eukarya domain consists of four kingdoms: Animalia, Plantae, Fungi and Protista.
Humans are part of the Animalia kingdom.
Phylum.
The third taxonomic rank for living organisms is phylum. The animal kingdom consists of approximately 35 phyla and plants have about 12 phyla. It is not possible to give an approximation for the number of phyla for the other four kingdoms. Humans are classified in the ‘chordata’ phylum. All chordates have a backbone at some point in their life.
Class.
The fourth taxonomic rank for living organisms is class. Mammalia is a good example of an animal class; humans are mammals and belong to this class.
Order.
The fifth taxonomic rank for living organisms is order. Humans are part of the ‘primate’ order, which means ape-like animals, in the animal kingdom.
Family.
The sixth taxonomic rank for living organisms is family. There are specific rules for determining whether a species is a member of a specific family. Humans are classified as ‘hominids’ or ‘human-like animals’.
Genus.
The seventh and penultimate taxonomic rank for living organisms is genus. In binomial nomenclature the genus name is the first part of the species name. The binomial name for humans is ‘Homo sapiens’; ‘Homo’ is the genus name, meaning ‘humankind’. It is important to note that the binomial name is written in italics and the first name has an initial capital letter.
Species.
The final taxonomic rank and the basic unit of biological classification is species. All species have a Latin or ‘binomial’ name which is written in italics. The first part of the name should have a capital letter at the start of the word and the second part of the name is in lower case. Homo sapiens are ‘modern humans’, although ‘sapiens’ means ‘capable of discerning’ or ‘wise’, so one could say that Homo sapiens means ‘wise humankind’.
Linnaeus.
In 1735, Carolus Linnaeus, a Swedish botanist (‘plant biologist’) and zoologist (‘animal biologist’), classified over 10 000 plants and animals and described them in his publication Systema naturae. Linnaeus realised that there had to be an organised and systematic method of naming the animals and plants described, so he developed the naming system for living things which we still use today. The reason that the naming system is vitally important is so that the same name can be used for the same type of living organism wherever it may be in the world, regardless of the local language or common names used.
Binomial system.
The system of naming species is known as the binomial (two-name) system and the language used is mainly Latin. The two parts of the binomial system are the genus and the species. The convention for the binomial name is that it is always written in italics with an initial capital letter for the genus and an initial lowercase letter for the species.
Bacteria.
Prokaryotic - Single Cell. Bacteria are microscopic organisms, generally single cell (unicellular), and are believed to be among the first life forms to inhabit Earth. There are fewer than 10 000 species of bacteria classified but estimates of the actual number of bacteria species range from 10 to 7 to 10 to 9. A gram of soil contains 40 million bacterial cells/ There are approximately ten times as many bacterial cells on the surface of a human body as human cells inside the human body.
Five groups: spherical, rod, spiral, comma and corkscrew.
Bacteria are also classified by a staining technique known as ‘Gram staining’. Gram staining uses a chemical called crystal violet which is taken up by Gram-positive bacteria (bc thicker cell wall) making them appear dark blue or purple in colour; Gram-negative bacteria do not take up crystal violet stain and appear red in colour.
Escherichia coli: rod-shaped and Gram-negative. Some strains cause disease, such as respiratory illnesses, sickness and diarrhoea, but most strains are harmless.
Archaea.
Neither Prok nor Euk. Archaea (singular: archaeon) is the most recently recognised kingdom. The classification of Archaea as prokaryotic or eukaryotic is the subject of intense debate because Archaea have features that are very similar to bacteria. Archaea are single cells and can be rod-shaped, spherical and many other weird and wonderful shapes. They tend to live in the most inhospitable parts of planet Earth which explains their relatively recent discovery. Archaea can be autotrophic or heterotrophic and seem to thrive in extreme environments. They are found in deep sea rift vents where temperatures exceed 100 °C and hot springs. They are also found in environments lacking in oxygen, such as mud in marshes and in ocean floors, in very salty water and even in petroleum deposits underground. Some Archaea produce the smelly gas, hydrogen sulfide, which can be characteristic of sulfur-rich environments such as Yellowstone National Park in the United States of America.
Protista.
Eukaryotic - single/multi-cell. Protista (singular: protist) are a large and very diverse group of organisms and include around 200 000 species such as slime moulds and Euglena. The designation of Protista as a separate kingdom is a matter of debate because of the diverse nature of the species within the kingdom. Protista can be defined as eukaryotic, single-cell organisms that are not plants, animals or fungi. However, they can display animal-, plant- or fungus-like characteristics. Some Protista are autotrophic, creating their food from the energy in sunlight, and others are heterotrophic, ingesting their food stuff. You can see why Protista are such a difficult group to classify!
Plantae.
Eukaryotic - multi-cell - autotrophic (photosynthetic)
Plants have existed on Earth for millions of years and plants and can be found pretty much everywhere; there are thought to be more than 300 000 plant species.
All green plants, along with algae, some bacteria and some archaea are autotrophs, i.e. capable of synthesising their own food. Plants do this by using sunlight energy in a process called photosynthesis. Plants convert energy from sunlight into food and oxygen. The oxygen is released into the atmosphere and the food is stored within the plant.
Green plants are essential for the survival of animals because animals cannot get energy directly from the sun. Herbivore animals get their energy by eating plants, whilst carnivores eat other animals, which have eaten plants. Animals also need oxygen, produced by plants, in the atmosphere to survive.
Most, but not all, plants are green due to the presence of chlorophyll, the essential pigment for photosynthesis. There are some plant species which are not green and do not make their own food, i.e. they are heterotrophic. The heterotrophic plants can be further divided according to their type of nutrition. Saprophytes are organisms that get their nourishment from dead or decaying organic matter, many fungi are also saprophytic. Parasitic plants live off other living plants to get their nourishment. Carnivorous plants (also called insectivorous) get their nutrition from insects, there are about 450 species of insectivorous plants and the pitcher plant shown in the figure is an example.
Fungi.
Eukaryotic - multicellular - heterotrophic - absorptive nutrition. Fungi used to be part of the plant kingdom and they do have some features in common; many for example live in the soil and they don’t move, but a fundamental difference is that fungi are heterotrophic, unlike most green plants which are autotrophic. Fungi are found everywhere on Earth, from the poles to the Equator and perform the essential role of decomposition, i.e. recycling of nutrients from dead and organic matter by breaking them down. You may well have noticed fungal growth on some of your food products, such as that on the tomatoes shown in the figure below, perhaps after storing your products for a little too long in the bottom of your fridge.
Some species of fungus such as the single-celled yeast are used by humans in bread making and beer brewing, and mushrooms and truffles (which are multicellular) are consumed by humans.
Fungi are also used in medicine; in 1928, Alexander Fleming noted a green mould Penicillium notatum growing in a culture dish of Staphylococcus bacteria. He noticed a clear margin around the mould where no bacteria grew and isolated the substance from the mould that appeared to inhibit the bacterial growth. The substance turned out to be penicillin, an incredible medical discovery and the first antibiotic which revolutionised medical practice.
Animalia.
Eukaryotic - multicellular - heterotrophic - ingestive nutrition. Animalia is a very diverse group of living things. There are approximately 35 phyla in the kingdom of Animalia - scientists cannot agree on exactly how many phyla there should be. Many phyla can be identified in the fossil record from 542 million years ago.
One of the phyla is chordata (from Latin), which includes all animals with a spinal cord or backbone. The non-chordate phyla are animals without a spinal cord, and these represent all of the other animal phyla.
There are a significant number of classes, orders, families and species, making the animal kingdom the most diverse kingdom in terms of the total number of species (more than one million recognised species so far).
Cell characteristics.
Cells are the smallest units of living things that show the attributes of life: reproduction, growth, metabolism and sensitivity.
Which cell type was thought to be the first life form on Earth?
Prokaryotic cells, specifically bacteria.
What size are cells?
Cells are found in an enormous variety of sizes and shapes; they can range from the smallest bacteria with a length of 0.1–10 μm (1 μm is 1 × 10−6 m) to plant cells with typical diameters of 50–100 μm. Human cells typically have a diameter of 10–50 μm.
Function determines shape.
The function of nerve cells is to transmit information around a multicellular organism so the cells have evolved to be long to allow this transmission of information, rather like electric wires transporting an electric current around a house.
Epithelial cells are a group of cells that line the cavities in multicellular organisms, and this one would be found lining the gut. The finger-like projections (known as microvilli) act to increase the surface area of the cell to allow more area for absorption of nutrients into the body from the gut.
The sperm cell has a long tail which allows it to swim.
Cell membrane. Cytosol.
The balloon, or outer boundary of the cell, is called the cell membrane and it is an extremely thin, yet very complex, structure made of phospholipids.
Its main function is to keep the cell contents separate from the external environment and at a fairly constant concentration. The concentration of a liquid is defined as the number of particles within a particular volume.
The cytosol (viscous internal environment) needs to be maintained at a particular concentration to allow metabolism (the chemical processes) within the cell to occur. The membrane does this by restricting the movement of molecules from the inside of the cell to the outside, and vice versa, so effectively acting as a barrier.
Protein solution.
What are organic molecules?
Organic molecules are naturally found in all living organisms and systems and are composed of carbon atoms in long chains or rings with other atoms such as oxygen, hydrogen and nitrogen attached.
Components of the phospholipid molecule.
The hydrophobic or ‘water-hating’ end of a phospholipid molecule is made up of fatty acids. In Figure 2.2, these are represented by the black squiggly lines in the yellow background. These hydrophobic ‘lipid’ portions make up the ‘tail’ of the phospholipid. The phospholipid molecules associate together because of the non-polar properties of the lipids, which means that they group together away from the polar water molecules (which you met in Topic 1) in the aqueous environment.
The hydrophilic or ‘water-loving’ end of the phospholipid molecule is made up of phosphate groups, represented by blue circles in Figure 2.2. The phosphate groups associate with the aqueous environment on the outside of the cell membrane as they are polar in nature and hydrophilic. The phosphate groups, or ‘heads’ of the phospholipid, end up in the aqueous environment on the outside of the group.
This ‘bilayer’ (or double-layer with lipids on the inside, phosphate groups on the outside) that acts as a protective barrier to the external environment.
Selective permeability.
The phospholipid barrier is not completely impenetrable as there are ‘pores’ in the membrane which allow transport of substances in and out of the cell. Although most substances cannot cross the cell membrane, it is not a total barrier to the movement of substances. Rather than being completely permeable (allowing any substance to pass through), the cell membrane is selectively permeable which means that it allows only certain substances to transfer through.
Diffusion. Concentration gradient.
Molecules of some substances (e.g. oxygen, carbon dioxide and sodium chloride) move freely from one side of the cell membrane to the other by a process known as diffusion. Diffusion is simply the process by which molecules tend to move from areas where they are plentiful to areas where they are scarce.
The movement of a chemical substance from an area where it is at high concentration to an area where it is at low concentration is a process known as diffusion. The difference in concentration is known as a concentration gradient, and the particles move down the concentration gradient, from high to low. The higher the temperature, the faster the particles move and the faster diffusion takes place.
Osmosis
Water molecules move across membranes by the process of osmosis.
Organelles. What purpose does partitioning serve?
Both prokaryotic and eukaryotic cells have cell membranes, but in eukaryotic cells the contents are packaged up into a number of different compartments known as organelles.
The partitioning enables some cell functions to be restricted to particular parts of the cell. Prokaryotic cells have no such internal organisation – all their contents are mixed together in the internal environment.
What is the difference between Brownian motion and diffusion?
Both Brownian motion and diffusion are processes involving the movement of molecules. Brownian motion is the random movement of large particles whereas diffusion is the movement of molecules down a concentration gradient from an area of high concentration to an area of low concentration.
Passive diffusion across a membrane.
The diffusion of a chemical substance can also occur across a permeable barrier, i.e. one that the substance can pass through freely. The lipid bilayer of a cell membrane is permeable to some small molecules, including gases like oxygen. These small molecules can freely diffuse across the cell membrane whenever there is a concentration gradient. The molecules will move randomly in both directions across the membrane, but the net (overall) movement will be from the high concentration side of the membrane to the low concentration side, down the concentration gradient. This type of movement across a cell membrane is sometimes called passive diffusion because it doesn’t require the cell to expend any energy. Diffusion ceases when the concentration of the molecule is the same each side of the membrane.
Direct v special channel diffusion.
Oxygen molecules can diffuse directly across the lipid bilayer of a cell membrane, but other molecules can only diffuse through special channels in the cell membrane formed by membrane proteins.
Osmosis.
Osmosis is a process that occurs when two different solutions are separated by a selectively permeable membrane, i.e. one that lets some substances through but not others. Water molecules can move freely across cell membranes, but solutes dissolved in the water (e.g. sugar molecules or sodium chloride) may not be able to cross the membrane. Osmosis is the movement of water through a partially permeable membrane down the water concentration gradient from a dilute to a concentrated solution.
Dilute v Concentrated.
A dilute solution (where there is a relatively high concentration of water and a low concentration of solutes). A concentrated solution (where there is a relatively low concentration of water but a high concentration of solutes).
Solution volume change in osmosis.
The volume increases on the left side which started out with the more concentrated solution (more solute molecules and fewer water molecules) and decreases on the right side which started out with the less concentrated solution (fewer solute molecules and more water molecules). This is because water molecules move down their concentration gradient by osmosis from the right side to the left side.
Dehydration.
The salt in the blood becomes ever more concentrated so more and more water is drawn out of the cells. This is also the reason why animals must drink fresh water and cannot drink salty seawater. Dehydration has uncomfortable symptoms like dizziness, fatigue and confusion. Severe dehydration is fatal if not treated.
Application of osmosis for food preservation.
Concentrated solutions of salt or sugar can also be used to preserve foods like meat, vegetables and fruit. When foods are placed in a highly concentrated solution of salt or sugar, water is drawn out of the food by osmosis. Similarly, water is also drawn out of any bacteria contaminating the food, killing them very effectively.
Virtual microscope. What pics does it take.
This virtual microscope is a type of light microscope, where light is shone through the cells so that their features can be seen and imaged. Those images are known as micrographs. There are many other types of microscopes, including those that use electrons instead of light (transmission electron micrograph, TEM); and fluorescence microscopes.
Plant v animal cell.
Both the animal and plant cells have cytosol, mitochondria, a nucleus, nuclear membrane and cell membrane.
Plant cells have chloroplasts, a cell wall and a large vacuole, whereas animal cells do not. The plant cell has a regular shape defined by the cell wall; the animal cell has an irregular shape defined by the cell membrane.
The plant cell is much larger than the animal cell. The scale in the plant cell drawing is 7 μm, whereas it is 1 μm in the animal cell.
Animal cell.
Animal cells are eukaryotic cells surrounded by a cell membrane (plasma membrane). With the exception of red blood cells (erythrocytes), all animal cells have a large organelle called the nucleus, which is surrounded by a double membrane called the nuclear envelope. The nucleus is generally the most prominent feature of a eukaryotic cell. The nucleus contains the hereditary material deoxyribonucleic acid, usually known as DNA, which is packaged into chromosomes. The cytoplasm is the gel-like substance within the cell membrane in which all intracellular organelles sit. The organelles are membrane-bound structures and include mitochondria, endoplasmic reticulum (smooth and rough) and lysosomes. The endoplasmic reticulum nearest the nucleus is studded with ribosomes and is described as rough; while further out from the nucleus is the smooth endoplasmic reticulum, which lacks associated ribosomes. Free ribosomes can also be found in the cytoplasm. The structure and content of a cell depends on its function.
Cell membrane.
The cell membrane (plasma membrane) is the outer membrane of a cell and is composed of a single phospholipid bilayer with proteins embedded in it, although no membrane proteins are shown in this schematic image of a cell. The cell membrane separates the contents of the cell from the outside environment. Particles can be incorporated into the cell by endocytosis, a process by which the membrane invaginates to enclose the substance, then pinches off to form a vesicle that moves inside the cell. In the opposite process, exocytosis, cytoplasmic vesicles fuse with the cell membrane to expel their contents from the cell
Lysosomes
Lysosomes are cellular organelles bound by a single membrane. They are known as the ‘dustbins’ of the cell as they contain digestive enzymes that breakdown unwanted cellular contents such as ‘worn out’ organelles or engulfed bacteria. They are not thought to be present in most plant cells, where the vacuole may perform a similar function
Mitochondria
Mitochondria are often known as the ‘batteries’ of the cell and are found in the majority of eukaryotic cells. They produce the majority of the cell’s stored chemical energy in the form of adenosine triphosphate (ATP). A mitochondrion (singular for mitochondria) has two membranes, the outer membrane and the inner membrane, with the intermembrane space lying between them. The inner membrane is ‘convoluted’ or wiggly in shape
Nucleus.
The nucleus is known as the ‘control centre’ of the cell, it coordinates many of the cell’s activities including protein synthesis, growth and cell division. The nucleus is the largest organelle in the cell and contains the hereditary material, deoxyribonucleic acid or DNA, packed into chromosomes. The nucleus contains a small nucleolus. The nucleus is enclosed by a double phospholipid membrane called the nuclear envelope. The outer membrane has holes known as ‘pores’ to allow transport of large molecules from the nucleus. Immediately outside the nuclear envelope is the rough endoplasmic reticulum which has ribosomes on its outer surface.
Ribosomes.
Ribosomes, which are found in all cells, are complex structures that are responsible for synthesising proteins. Ribosomes are most abundant in the cytoplasm, either free in the cytosol or attached to the rough endoplasmic reticulum (RER).
Rough ER (RER)
The rough endoplasmic reticulum (RER) is an organelle consisting of a network of single membrane-bound tubes that is continuous with the outer layer of the nuclear envelope and which is studded with ribosomes. The ribosomes of the RER are located on the outer surface only, which faces the cytosol. The RER is the site of proteins synthesis, including membrane and lysosomal proteins, and proteins that are to be exported from the cell. The endoplasmic reticulum acts as a ‘transport system’ for eukaryotic cells.
Smooth ER (SER)
The smooth endoplasmic reticulum (SER) is a single membrane-bound organelle. Unlike the rough endoplasmic reticulum (RER), it has no ribosomes attached. The SER is the site of many activities, including the synthesis of lipids. The endoplasmic reticulum acts as a ‘transport system’ for eukaryotic cells.
Plant cell.
Plant cells are eukaryotic cells with several distinctive features that distinguish them from animal cells. Plant cells are surrounded by a cell wall, which is a rigid barrier outside the cell membrane. Plant cells usually have a large central vacuole, a water-filled space enclosed by a membrane. As in animal cells, the chromosomes are enclosed in a nucleus bounded by a double phospholipid membrane called the nuclear envelope. The cytoplasm includes similar organelles to animal cells, including mitochondria and endoplasmic reticulum, but also some other types of organelles called plastids, where molecules are synthesised or stored. Chloroplasts, for example, are plastids that contain chlorophyll, the green-coloured pigment used in photosynthesis. Chloroplasts are found only in plant cells, not animal cells.
Plant cell membrane.
The cell membrane in plants is known as the plasmalemma. It is composed of a phospholipid bilayer with embedded proteins. As in animal cells, it separates the contents of the cell from its outside environment.
Plant cell wall.
Plants (and also fungi and bacteria) have a rigid cell wall structure outside of the cell membrane. Cell walls are composed predominantly of cellulose, which helps to provide shape, mechanical support to the plant and prevents the entry of large molecules that may be toxic to the cell. In comparison, animal cells do not have cell walls. Cell walls are composed predominantly of cellulose.
Chloroplasts.
Chloroplasts are the organelles in which photosynthesis takes place. They are found in the green parts of plants (leaves and stem). Chloroplasts are bounded by a double phospholipid membrane. The green pigment chlorophyll is found in chloroplasts and is essential for the process of photosynthesis. Photosynthesis is the conversion of water and carbon dioxide into glucose and oxygen, using light as an energy source.
Plant ER.
The endoplasmic reticulum (ER) in plants, as in animal cells, is an organelle consisting of a network of membrane-bound tubes and sheets that is continuous with the nuclear envelope. The rough endoplasmic reticulum (RER) is studded with ribosomes. The smooth endoplasmic reticulum (SER) has no ribosomes attached and is more tubular than rough ER, forming a separate network that usually extends throughout the cytoplasm. The endoplasmic reticulum acts as a ‘transport system’ for eukaryotic cells.
Plant mitochondria.
Mitochondria in plants, as in animal cells, are the cell organelles that produce the majority of the cell’s form of stored chemical energy, adenosine triphosphate (ATP). A mitochondrion has two membranes, the outer membrane and the inner membrane, with the intermembrane space lying between.
Plant nucleus.
In plant cells, as in animal cells, the nucleus is the large organelle that contains the hereditary material, DNA, packed into chromosomes. It coordinates many of the cell’s activities including protein synthesis, growth and cell division. The nucleus contains a small nucleolus. The nucleus is enclosed by a double phospholipid membrane called the nuclear envelope. The outer membrane has holes known as ‘pores’ to allow transport of large molecules from the nucleus. Immediately outside the nuclear envelope is the rough endoplasmic reticulum which has ribosomes on its outer surface.
Plant ribosome.
Ribosomes, which are found in all cells, are complex structures that are responsible for synthesising proteins. Ribosomes are most abundant in the cytoplasm, either free in the cytosol or attached to the rough endoplasmic reticulum (RER).
Vacuole.
A vacuole is a cell organelle that is very prominent in plant cells. A vacuole is enclosed by a membrane and filled with water, ions and small organic molecules. Most mature plant cells have a single large vacuole which has several roles. Firstly, it helps to maintain the internal hydrostatic pressure, or turgor, that maintains the cell shape and which plants rely on to remain upright and support important structures such as leaves. It is also involved in the storage of some chemicals and has a similar disposal role to that of the lysosome found in animal cells.
Hormone
Hormone – a signalling molecule within a multicellular organism; effectively hormones deliver messages between cells.
Arthropods
Arthropod phylum. Arthropods do not have a backbone, i.e. they are ‘invertebrates’. They have an exoskeleton, a segmented body with jointed legs and are symmetrical along their length. Arthropods include classes of: insects (butterflies, ants, etc.), arachnids (spiders, mites, scorpions, etc.), chilopods (centipedes) and diplopods (millipedes).
What are the three processes involved in growth?
Cell division, synthesis of new structures from raw materials and cell expansion.
What sort of growth pattern do vertebrates show?
Slow, fast, slow as adulthood reach.
What sort of growth pattern do arthropods show?
Unlike other invertebrate, arthropods do not increase slowly and gradually over time. They grow in steps and must shed their exoscheleton to do so. Mayflies shed 50 times and are the only ones to do so after getting their wings.
What are meristems and where are they found?
Meristems are actively dividing cells found in plants at the tips of roots and shoots.
What factors determine growth in plants and animals?
External factors include temperature, diet and light. Growth is determined internally by hormones.
Why don’t cells just grow by getting larger?
This would result in very large cells which would become unsustainable. The maintenance of the cells (e.g. regenerating tissue and regulating the internal chemistry through homeostasis) would be too energy intensive. As a result, the intracellular functions would become compromised as the energy required for homeostasis and repair became too great.
Secondary growth
Growth in trees takes place in both the stem and the roots by the division of meristematic cells located in the vascular part of the plant.
Cell division.
Cell divisions and growth cycles. Each cell produces two progeny cells when it divides. These grow and then divide in turn. This mechanism of cell reproduction to enable growth allows the replenishment of worn-out cells as well as the overall growth of a multicellular organism.
Mutation.
Both prokaryotic and eukaryotic cells reproduce by copying their chromosomes (which contain their DNA) and passing on an identical copy of their genetic material to each progeny or ‘daughter’ cell. Occasionally the reproduction of chromosomes is not completely accurate – when mistakes occur the process is known as mutation.
How do single-celled organisms reproduce?
Single-celled organisms do not grow as individual organisms, rather their growth is measured by an increase in the total number of organisms within the population.
Single cells must reproduce themselves and this is known as asexual reproduction because only one parent is needed to produce two offspring, whereas in sexual reproduction two parents are required to produce offspring.
How do prokaryotic cells reproduce?
Most prokaryotic cells divide by a process called binary fission.
What shape are e coli? How do they multiply?
Rod. The bacteria elongate, appearing to roughly double in length and then split into two equally long bacteria.
In what way are prokaryotic cells simpler than eukaryotic cells
Prokaryotic cells do not have internal cellular compartments.
How do prokaryotic cells multiply?
Prokaryotic cells have a single circular chromosome that is not contained within a nucleus. Replication (copying) of the chromosome starts at one point on the chromosome, known as the ‘origin of replication’ and moves in both directions around the circular chromosome. The end result is a new, copied chromosome and an original chromosome, both of which move to opposite ends of the cell as the cell elongates. The cell then splits down the middle creating two daughter cells, each containing an identical chromosome.
What is a cell cycle and how long does it take?
The division of eukaryotic cells occurs within the cell cycle which is a series of processes that take place in a cell. The cell cycle can take anything from a few hours to many weeks to complete. The rate at which a cell proceeds through the cycle depends on many factors, including the type of organism, the type of cell and its size, and the environment in which the cell is growing. For instance, cells in the roots of many plants take about 12 hours to divide, those in the gut of a mouse take about 17 hours and those in human skin take about 24 hours to divide.
Describe the cell cycle.
The cell cycle consists of four main phases:
G1 (gap 1) phase S (synthesis) phase G2 (gap 2) phase M (mitosis and cytokinesis) phase.
G1, S and G2 are collectively known as interphase.
In G1, the first phase of the cell cycle, the cell grows, increasing the number of organelles, and prepares for DNA synthesis. In S phase, the DNA is replicated. By the end of this phase, the cell has copied each chromosome. The cell prepares for division in the G2 phase, the last stage of interphase.
The final stage of the cell cycle, known as the M phase, is relatively short but highly dynamic and consists of two processes. In the first process, known as mitosis, the two sets of chromosomes condense and segregate equally into two separate nuclei. In the second process, known as cytokinesis, the cytoplasm of the cell divides to form two completely separate daughter cells. Cells can immediately re-enter the cycle and divide again. Or they can exit the cycle to a resting phase called G0, where they will remain, until stimulated to enter G1 again.
The G0, or resting, phase can be long or short dependent upon the cell type. For example some types of nerve cells have an extremely long, maybe even never-ending, resting phase. Other cells, such as skin cells, do not even enter the resting phase; they simply continue from division to G1, and consequently their cell cycle is much shorter.
What is mitosis?
Mitosis is the dynamic phase in the cell cycle. In this phase, the chromosomes condense into two sets, and the two sets of chromosomes segregate equally into two separate nuclei. Two completely separate daughter cells are then produced following cytokinesis. Before we look at mitosis in more detail, you firstly need to understand a bit more about chromosomes.
Mitosis is the division of the nucleus, which has to occur before the cell itself can divide.
Mitosis is a dynamic and continuous process, but for convenience, biologists identify stages or phases of the process.
Mitosis begins when the DNA molecules in the nucleus become tightly coiled or condensed, thereby making the chromosomes visible using a microscope. As each DNA molecule made a copy of itself during the replication stage of interphase (S phase), there are double the original number of DNA molecules. Somewhere along its length, each DNA molecule is joined to its copy. This point of attachment is called the centromere and each newly copied DNA molecule is called a chromatid.
How many human chromosomes? What type? Which are the cells that have a different number and how are these cells created?
The number of chromosomes in a cell is characteristic of each type of organism. For example, a human cell generally has 23 pairs of chromosomes, making a total set of 46 chromosomes. Half of the chromosomes come from the mother and half from the father. Twenty-two of the chromosome pairs look similar to each other – these are called the autosomes. The chromosomes in the 23rd pair can appear different from each other – these are the sex chromosomes, and they determine whether humans are male or female. The only human cells that do not have 23 pairs of chromosomes are the gametes (the sex cells), which contain one of each autosome and one sex chromosome, generated through the process of meiosis.
Describe chromosomes. What are they made of? When can you see this material?
Chromosomes within the nucleus of eukaryotic cells are made up of DNA molecules. During interphase, most DNA exists as very long, thin threads; this makes it very difficult to see the chromosomes even through a microscope. However, during mitosis the long thin threads of DNA bunch together and become tightly coiled, that is the chromosomes become condensed which makes them easier to see. NB: broad bean Vicia faba (magnification × 2000).
What is interphase?
This animal cell is in interphase; it is not yet dividing, but is actively growing and replicating the DNA in its chromosomes. The chromosomes are not visible at this stage.
What is S phase?
As a result of DNA replication during the preceding S-phase of the cell cycle, each chromosome is made up of two identical strands of DNA. These strands are known as chromatids. Each pair of chromatids is joined at the centromere.
For clarity, this cell has just two pairs of homologous chromosomes, two long and two short. One chromosome from each pair, shown in red, was inherited from the organism’s maternal parent; the other, shown in blue, from the paternal parent.
What is prophase?
Mitosis begins when the DNA molecules in the chromosomes become tightly coiled, or condensed. At this point they become visible under the microscope.This is prophase, the first stage of mitosis.
In the earliest phase of mitosis the nuclear membrane disappears so that the cell no longer has a nucleus. Each chromosome now consists of a pair of identical chromatids joined together at the centromere. Delicate threads anchored at one or other end of the cell become attached to the centromeres.
What is metaphase?
At the end of prophase, the nuclear membrane breaks down, and microtubules assemble at opposite poles of the cell to form a structure called the spindle. Spindle microtubules from opposite poles attach to the centromere of each chromosome.
The chromosomes line up at the centre or equator of the cell.
The threads attached to the centromeres exert tension on the chromosomes, eventually aligning them across the middle of the cell.
Anaphase.
Each centromere splits and the two chromatids are drawn to opposite poles of the cell by the shortening of the spindle microtubules.
In anaphase, which follows metaphase, the chromatids separate, so that each becomes a chromosome in its own right. One member of each former pair of chromatids is drawn to one end of the cell, while its partner is drawn to the other end.
Telophase.
The chromosomes have now reached the poles of the cell. This phase is called telophase.
The microtubules forming the spindle disassemble and a nuclear membrane reforms around each of the two groups of chromosomes. The chromosomes uncoil and decondense. Nuclear division, or mitosis, is now complete.
In telophase, once the chromosomes have reached one or other end of the cell, the threads that were attached to them disappear. There is now a set of chromosomes clustered at one end of the cell and an equivalent set of chromosomes clustered at the other end. The DNA molecules then start to become uncoiled. At the same time, a nuclear membrane forms around each chromosome cluster so that the cell temporarily contains two nuclei.
What forms a belt around the cell? What is the constriction called?
The final event in cell division is cytokinesis, the division of the cell cytoplasm to form two separate daughter cells.
In animal cells a contractile ring of microfilaments made from actin and myosin forms a belt around the cell.
As the ring contracts a constriction, or cleavage furrow, forms between the nuclei and eventually completely separates the two daughter cells.
What is a diploid cell?
Binary fission.
Prokaryotic cells divide (reproduce) asexually by the process of binary fission and this results in reproduction of the parent cell into identical progeny cells.
Advantages of sexual reproduction.
If all individuals in a population are identical they will respond in a similar manner to any external influence. The external influence could be, for example, lack of a food source, how attractive they are to a predator, or susceptibility to a particular catastrophic disease. In any of these three examples the entire population could be wiped out and the species could become extinct. In comparison, species that reproduce sexually will have individuals that differ from each other from a physiognomy and physiology pov, which means that the species could respond to external influences differently and have better chances of survival.
When did hominins appear?
5.8 m years ago.
Sickle cell.
Red blood cells (erythrocytes) are essential for transporting oxygen around the body to every living cell (Figure 4.2). All living cells need oxygen to allow their chemical processes to occur – without oxygen the cells die. You will find out more about this in Part 5 Metabolism.
Normal red blood cells are flexible and disc-like in shape as shown in Figure 4.2. However, in sickle cell anaemia the red blood cells become rigid and ‘crescent-shaped’ or ‘sickle-like’ (Figure 4.3), hence the name of the disease.
Sickle cell anaemia.
Sickle cell anaemia is a hereditary condition, which means it is passed down the generations from parents to offspring. It is mainly found in people of African, Caribbean, Middle Eastern, Eastern Mediterranean and Asian origin. Sickle cell anaemia can cause severe pain, tiredness and lethargy and other more serious symptoms that can be potentially fatal. However, despite these symptoms, sickle cell anaemia has also been shown to protect sufferers from malaria.
Malaria.
Malaria is a serious and sometimes fatal disease that is widespread in many tropical and subtropical countries. Malaria is caused by a parasite carried by the Anopheles mosquito. The mosquito carries the malarial parasite, called Plasmodium falciparum, in its saliva. The malarial parasites enter the host’s bloodstream when the host is bitten by an infected mosquito. The Plasmodium falciparum then migrate to the liver where they multiply, before returning back into the bloodstream to invade the red blood cells.
Video 4.1 shows Plasmodium falciparum infecting red blood cells. The video is very difficult to interpret, but look closely at the red blood cell right in the middle of the picture. Initially this orangey-red blood cell is vibrating and it appears to have some visible components within it. These ‘components’ are called trophozoites which is the parasite during the growing and feeding stage. Note how the parasites continue to multiply inside the red blood cell, to produce mature schizonts. These reproduce asexually to form daughter parasite cells that are known as merozoites.
In the video the infected cells burst to release large numbers of merozoites into the bloodstream, causing a fever that is characteristic of the malaria disease. These released parasites will go on to infect more red blood cells and the cycle of infection continues.
What is the offspring cell called and the cells which combine to form it? What are they in humans?
Sexual reproduction requires two individual organisms belonging to the same species. Each individual produces special types of cell, called gametes, which fuse together to form a zygote from which the offspring grows. In humans, the gametes are sperm or egg cells.
Mitosis ensures that each progeny cell contains chromosomes that are exact copies of those of the parent cell in every respect.
What is the second type of cell division? What is the point?
There is a second type of cell division, involving a process known as meiosis (pronounced ‘my-oh-sis’), which ensures that each daughter cell or gamete contains exactly half the chromosomes of the individual parent cell. So, as human cells normally contain 46 chromosomes, there should be 23 chromosomes in a human gamete produced by meiosis. Sexual reproduction requires a male and female gamete to fuse together to form a zygote, in which there are chromosomes from both gamete cells.
What two types of chromosomes are there?
Autosomes and sex chromosomes.
the number of chromosomes in a cell is characteristic of the species. With the exception of gametes, all human cells contain 46 chromosomes arranged into 23 pairs. Twenty-two of the chromosome pairs look similar to each other. These are called autosomes. The 23rd pair of chromosomes can appear different from each other; these are the sex chromosomes and they determine whether humans are male or female.
x v y
You can see an obvious difference in the pair of sex chromosomes in a male in comparison to a female – the second chromosome in a male is shorter than the first chromosome, whereas the sex chromosomes are the same length in females. These sex chromosomes are individually called X and Y, and it is the Y chromosome that is the shorter chromosome. Females have two X chromosomes, whereas males have an X and a Y chromosome. It is the presence of a Y chromosome that determines male development in humans.
Diploid v haploid.
Eukaryotes, including humans, are referred to as being diploid because their chromosomes appear as pairs. A gamete cell with half the diploid number of chromosomes, such as a sperm or egg cell gamete, is known as haploid. You may find that noting that ‘ha’ from ‘haploid’ is similar to ‘ha’ from ‘half’ will help you to remember the difference between haploid and diploid cells.
Describe haploids.
Some organisms (e.g. some fungi, algae and insects) spend a large part of their life cycle with all their cells in the haploid state. Other organisms (e.g. mammals) use haploid cells only for sexual reproduction. In such sexually-reproducing organisms, haploid cells constitute an extremely special group because only haploid cells directly contribute DNA to the next generation. These haploid cells or gametes are called ova (singular ‘ovum’, sometimes called an egg) in female animals and spermatozoa (or sperm, for short; singular ‘spermatozoon’) in male animals. The equivalent terms in plants are ‘ovules’ for ova and ‘pollen grains’ for sperm.
Why meiosis
Why is it important that each gamete cell (i.e. egg or sperm) contains half of the genetic material of the parental cells?
Gametes join together in the process of sexual reproduction to form a zygote. It is vitally important that gametes are haploid, with 23 chromosomes in each gamete, so that at fertilisation the resultant zygote is diploid (i.e. has 46 chromosomes). Organisms undergo meiosis in order to produce gametes that each contain half of the genetic material of their parents; this prevents the zygote having twice the number of normal chromosomes on fertilisation.
Adv of sexual reproduction.
Sexual reproduction allows genetic variation, which may give the offspring a competitive or survival advantage compared with its parents, as we have seen with the genetic trait of sickle cell anaemia. Individuals with the condition are more likely to be resistant to malaria and hence survive and produce offspring.
Virus alive?
Needs host cell machinery to reproduce so no!
Another test of aliveness.
Presence or absence of cells. Hence virus not alive.
What is metabolism?
Metabolism is one of the basic functions of life and metabolic processes either release energy or require an input of energy. Metabolism was defined at the start of this topic as all the chemical processes that occur within a living organism to maintain life.
Anabolism v Catabolism.
All chemical processes are linked to energy input or output. Some chemical processes require an input of energy to happen, other processes result in a release of energy and these distinguish the two main subdivisions of metabolism:
Anabolism (biosynthesis) is the building of new organic compounds. Anabolic reactions require an input of energy to occur and are involved in growth and reproduction.
Catabolism. Catabolic reactions serve to release energy from the breakdown of biological molecules. Energy is stored in the chemical bonds of organic and inorganic compounds. The breakdown of these compounds releases energy.
What are biochemical processes and what substances are involved in accelerating them?
The chemical processes undertaken by living things are biochemical processes, i.e. ‘bio’ from biology (the science of living things) and ‘chemistry’ (the science of the composition, properties and changes of matter). Biochemical reactions often use biological catalysts (catalysts are substances that accelerate chemical reactions without being changed by the reaction themselves) and enzymes are the biological catalysts that speed up reactions.
How many blood cells are destroyed every second and how long before your blood is completely replaced?
2.5 m and 4 months.
What is the process through which anabolism and catabolism take place? What is the chemical reaction? Illustrate on a graph.
Both anabolism and catabolism usually occur through a series of steps rather than just one reaction, known as a metabolic pathway. The figure below is an illustration of the catabolism of glucose. In reaction (a) glucose is broken down in one step to carbon dioxide and water and a certain amount of energy is released. In reaction (b) the same amount of energy is released from the breakdown of glucose, but the energy is released in a stepwise process via a series of intermediate compounds.
Why is it important that energy be released gradually?
It is important that energy is released in small steps within a cell as this way the energy can be stored in specific high energy compounds. If the energy was released all in one go it would destroy the cell.
What is energy. Types of energy.
Energy can be defined as the ability of an object to do work. There are many different forms of energy, and some are defined below:
chemical energy – energy stored in fuel which is released when chemical reactions take place
kinetic energy – energy which an object possesses by virtue of being in motion
electrical energy – energy transferred by an electric current
thermal (heat) energy – energy of an object due to its temperature
nuclear energy – energy stored in an atom’s nucleus
solar energy – energy from the Sun transferred through waves and light particles (photons)
sound energy – energy transferred via sound waves.
Cyclist going up hill. What types of energy?
Kinetic and potential. Max potential at top of hill.
ATP: what is it and how does it act as an energy transfer molecule?
https://academic-eb-com.libezproxy.open.ac.uk/levels/collegiate/article/adenosine-triphosphate/3722
What is photosynthesis?
Photosynthesis, the Sun’s energy is converted into chemical energy in a form of organic chemical compounds that can be stored in plants.
Photosynthesis is responsible for the current diversity of life on Earth – only a few known organisms do not ultimately depend on it (e.g. bacteria living in thermal vents).
Photosynthesis is an example of biosynthesis: in this case, a process that builds up a large molecule (i.e. the sugar glucose) from small molecules (carbon dioxide and water).
Photosynthesis is an anabolic reaction because a large molecule, glucose, is made rather than broken down.
Biosynthesis features.
An important feature of all biosynthetic processes is that they require energy.
Biosynthetic pathway reactions are catalysed by enzymes.
Glucose synthesis and breakdown.
The organic compound made by the process of photosynthesis is the common sugar, glucose. The chemical energy stored in glucose molecules can be used to power growth and other life processes.
The energy is released by the chemical breakdown of glucose in steps. The energy released from the breakdown of glucose is then transferred to ATP molecules.
Biology / chemistry / physics of photosynthesis.
The biology of photosynthesis includes the anatomy (structure) and physiology (functions or processes) of the parts of a plant (normally leaves) where photosynthesis occurs.
The chemistry of photosynthesis includes the chemical molecules (of which chlorophyll is one) and chemical processes that do the work of conversion of light energy into chemical energy.
The physics of photosynthesis includes the conversion of energy from one form to another.
Photosynthesis word reaction and chemical reaction.
Photosynthesis actually involves the reaction of water and carbon dioxide with light as an energy source. The products of the reaction are glucose and oxygen.
Reactants: carbon dioxide + water (+ solar energy) -> Products: glucose + oxygen
This is a ‘word equation’ for a chemical reaction. The reactants are the chemicals that combine to form the products at the end of the reaction. As all chemicals have formulas this can also be written as a chemical equation as follows:
co2 + h2o -> c6h12o6 + o2
What are the environmental variables which affect photosynthesis?
The environmental variables are: light, water, carbon dioxide and nutrients. Temperature is also a variable factor that affects photosynthesis.
Light: Photosynthesis requires the energy from sunlight for the process to occur. Plants grown at the Earth’s Poles, where there is either 24-hour light or no light, are less productive plants grown at the Equator with constant 12 hours of light and 12 hours of dark throughout the year. Tropical rainforests (equatorial) are very productive.
Water: Water is an essential reactant in photosynthesis. The Okavango Delta shows lush plant growth near the river, whereas further away in the desert there is no plant growth.
Carbon dioxide levels: Carbon dioxide (or CO2) is an essential reactant in photosynthesis. The concentration of carbon dioxide does not vary much over the plain, but tiny differences in carbon dioxide levels can be observed in dense parts of a rainforest canopy. Human-made emissions can also have an effect on carbon dioxide levels.
Nutrients: Nutrients are essential for plants to live and function metabolically. Lack of specific nutrients is likely to have adverse effects on chlorophyll and other chemical compounds essential in photosynthesis.
Temperature: Temperature will affect photosynthesis. If the temperature is too hot or too cold plant cells can die and photosynthesis will not then occur.
Phases
The process of photosynthesis is not quite as simple as the overall equation suggests. It actually consists of a number of individual chemical reactions or stages because photosynthesis is a metabolic pathway and this pathway is actually composed of two discrete phases:
Phase 1 – or the light reactions – here solar energy is used to manufacture or synthesise ATP.
Phase 2 – or the dark reactions – is not dependent on light.
Chloroplasts are found in the cytosol of the cell.
Chloroplasts are bound by an outer membrane and also have internal membranes. The internal membranes are folded back on themselves many times to form stacks called grana (singular: granum), which contain the chlorophyll pigments. Starch molecules, shown as black spheres in Figure 5.4, are made from glucose molecules produced from photosynthesis. Glucose cannot be safely stored in cells, but starch molecules can. Water and carbon dioxide are converted into carbohydrates by the chloroplast in the process of photosynthesis.
Each granum of a chloroplast houses the all-important light-absorbing chlorophyll molecules and it is here that ATP is generated using energy captured from the Sun. The stroma of the chloroplast is a fluid-filled space and is the site of the biosynthetic processes that manufacture sugar molecules. This type of compartmentation is a characteristic feature of cells and is important for the functioning of the chloroplast.
What is respiration?
The term ‘respiration’ means the intake and exhalation of air in animals and this can easily be confused with ‘cellular respiration’, which is the pathway of reactions that occurs in cells. In both types of respiration oxygen is taken in and utilised.
In cellular respiration, oxygen is used to break down glucose in a metabolic pathway to create ATP. In fact cellular respiration is really the reverse of photosynthesis. Whereas photosynthesis is an anabolic reaction resulting in the production of glucose, cellular respiration is a catabolic reaction resulting in the breakdown of glucose.
Cellular respiration results in the production of ATP, the energy transfer molecule which is used as a fuel for all energy-requiring cellular chemical processes.
Respiration equation.
Glucose + oxygen -> water + carbon dioxide (+ATP)
Light energy (sunlight) - chemical energy (carbohydrate) - chemical energy (ATP) - movement, biosynthesis, heat
The breakdown of glucose, using oxygen, takes place in many small, enzyme-catalysed steps. The reason for the numerous steps involved is to ensure that all the energy released from the reaction is not released in one ‘big bang’. If this was the case the cell would be unable to cope and probably would explode! It is better for the cell to have its energy in small packets, which can happen in a multi-step process, and the packets of energy are transferred to ATP molecules.
In fact, there are four distinct stages in the breakdown of glucose to release energy, each of which comprises many separate chemical reactions but we will not go into the biochemical detail of these reactions here. The important thing to remember is that both cellular respiration and photosynthesis occur in steps. There are other similarities and differences which you will determine in the next activity.
Mitochondria are associated with energy production. Mitochondria are often known as the ‘batteries’ of the cell and are found in the majority of eukaryotic cells. They produce the majority of the cell’s stored chemical energy in the form of adenosine triphosphate (ATP).
What is a stimulus? What’s an example of a stimulus for a human being?
A stimulus (plural: stimuli) is something that elicits a response. One of the respondents to the question in the introductory video of ‘How do you know that you are alive?’ said that he would ‘pinch himself’ and this is a great example of a stimulus. The response, if elicited, would be determined by the severity of the pinch. Other stimuli could include changes in the environment of an organism (e.g. temperature, amount of sunlight, water, etc.). Organisms respond to stimuli to keep themselves in favourable conditions.
Why do living things need to be able to respond to stimuli?
Heterotrophs need to be able to detect where their food will be coming from and autotrophs need to be able to detect where the reactants for making their food are in the environment. This section will start with single-celled organisms and their responses to stimuli, and end with multicellular animals and their responses to stimuli.
What are chemotaxis and what are the types?
Chemotaxis (from chemo meaning chemical and taxis meaning movement) is the movement of organisms due to chemical stimulation. Chemicals can be either chemoattractants or chemorepellents: the chemoattractants result in the organism moving towards the chemical whereas the chemorepellants result in the organism moving away.
What are flagella and what function do they have on an E Coli?
The primary role of flagella is for locomotion, but they can also function as sensory organelles, being sensitive to chemicals and temperatures outside the cell.
What are amoebas and what are their protrusions called?
Amoebae are single-celled eukaryotic organisms that move by flowing cytoplasm into protruding areas of the cell membrane. These protruding areas are called pseudopods (or false feet).
What is a ligand? What is the process of transferring the message from outside to the inside of the cell?
So, how do amoebae and bacteria know to move towards a chemoattractant? Both organisms must first recognise the chemical and secondly respond to the stimulus. Recognition of the chemical occurs on the outside of the cell membrane and cell surface receptors are biological complexes found in the cell membrane that allow the recognition of a specific extracellular chemical.
The recognition by cell surface receptors of one particular type of chemical compound is a key concept in understanding chemotaxis (i.e. movement towards or away from the chemical).
The chemical compound is known as a ligand and it is usually a small compound which binds to one type of cell surface receptor only and induces a change in shape of the receptor. This results in the message being transferred into the cell. This transference of the message from the outside of the cell to the inside is known as signal transduction.
An illustration of the membrane receptor action and the process of signal transduction.
The top of the illustration, with a pale lavender background, represents the outside of the cell (extracellular) in which there are four pale orange spheres, one of which is labelled ‘hormone or environmental stimulus’.
One of these orange spheres is embedded in a red ovoid labelled ‘receptor’ that is situated in the blue thick line, which represents the cell membrane.
A black arrow goes down from the receptor to a pale blue sphere. A second black arrow goes down from this ovoid to a second pale blue ovoid and a third black arrow goes down from this second ovoid to a third pale blue ovoid. These three ovoids are labelled ‘relay molecules’.
A fourth black arrow goes down from the third ovoid to a larger pale orange ovoid, with four lines below it to represent ‘activation of cellular responses’.
Below the cell membrane, in which the receptor is situated, the pale blue background represents the cytosol.
The actions are labelled on the left of the illustration and each one is indicated by a left square bracket starting with ‘signal’ at the top (where the orange spheres are in the extracellular environment), followed by ‘receptor’ (in the cell membrane), then ‘transduction’ and finally ‘response’ at the bottom within the cytosol or intracellular environment.
What is the plant’s response to a stimulus called? What do green plants grow towards?
In plants the response to a stimulus is known as tropism. Plants cannot move in the same way that animals can, but they do still respond to external stimuli by growing either towards or away from the external stimuli.
Green plants need to grow towards sunlight and water.
Describe various tropism and reasons why they occur.
Plants grow towards the light – this is known as phototropism. Plants also grow up, defying gravity, the force that attracts a body towards the centre of the Earth. This phenomenon is known as gravitropism. Plant stems are positively phototropic, i.e. they grow towards the light, whereas plant roots are negatively phototropic, i.e. they grow away from the light. Stems are negatively gravitropic while roots are positively gravitropic.
The reason for these tropisms is quite clear. Roots growing down and away from light are more likely to find the soil, water and minerals they need, whereas stems that grow towards the light will be able to expose their leaves so that photosynthesis can occur.
What are plant hormones or chemical messengers called?
ike bacteria and amoebae, plants need to be able to sense a stimulus and then react to the stimulus. Plants have hormones or chemical messengers which are called auxins, from the Greek, auxein, meaning ‘to grow’. Auxins are produced in the tips of the shoots and roots and can diffuse to other parts of the shoots and roots. Auxins change how long plant cells become. For example, in a shoot the shaded side contains more auxin which means that the cells in the shaded side grows longer, causing the plant to bend towards the light.
Cress experiment.
The cress seedlings grew towards the light source, a good example of positive phototropism. The seedlings changed their direction of growth towards the light source very shortly after the change in the direction of the light source. Also, the cress seedlings did not grow at the same rate; one ‘giant’ cress seed can be seen from day 3 amongst the other smaller seedlings.
What is the nervous system? Describe its complexity across species.
Most animals employ the nervous system to respond to external stimuli, and the complexity of the nervous system varies with the complexity of the multicellular animal. Basically, the more complex the body system, the more complex the nervous system must be. The nervous system is one of the body’s control systems and, in the most evolved species, such as humans, it has a ‘control centre’, which is the brain. Some of the simpler multicellular animals, such as sponges, do not have a nervous system.
It is the job of the nervous system to take information about the external environment, compute the information and elicit a suitable response. Think for a moment about how you know that you have stubbed your toe or can feel if water in a bath is too hot or too cold. In both of these examples, the message needs to be transmitted either from your toe or fingers to your brain where the information is computed or understood and a suitable action initiated.
What are neurons?
Cells of the nervous system include neurons. Neurons are essentially like all other cells in terms of their cellular contents; however, they are specialised to transmit information in the form of an electrical signal.
What do all eukaryotic cells have?
All cells have a cell membrane, nucleus, mitochondria and cytosol.
The cell’s function determines its structure. Exemplify.
Neurons are an excellent example of how a cell’s structure directly relates to its function. Many neurons are very long and can be more than 1.5 metres in length in humans. One nerve called the ‘recurrent laryngeal nerve’ runs from the heart to the larynx (voice box) and is more than 4 metres long in giraffes.
The micrograph of a Drosophila (fruit fly) neuron shows the ‘cell body’ as the round section of the cell. Note the long appendages that emerge from the cell body – it is these appendages that can make the cells so long.
Describe the movement of an electrical impulse through a neuron. What are the gaps between neurons called? What means does the nervous system use for relaying information?
You will note that in the journey of the electrical impulse it jumps over a small gap from one cell to another. These ‘gaps’ between neurons in the nervous system are called synapses. The interesting thing about the synapse is that it does not conduct electricity, therefore the message needs to change its method of transport. It does this by means of chemical transportation. So, the nervous system uses both electrical and chemical means to relay information.
