Chapter 1 Flashcards
Cell biology
Rules of cell theory
- Living organisms are composed of cells- cells are the building blocks of organisms
- Cells are the smallest units of life- a cell is the basic unit capable of carrying out all the functions of a living organism
- Cells come from pre-existing cells- cells don’t show spontaneous generation
What are 3 exceptions to cell theory?
- Striated muscle
- Giant algae: Acetabularia
- Aseptate fungal hyphae
Striated muscle
- composed of repeated units called sarcomeres
- these show a characteristic striped (striated) patter under a microscope
Atypical because it’s multinucleated and larger than a typical cell
- challenges idea that a cell has one nucleus, as muscle cell is multinucleated
- average muscle fibre cell is 30mm long, much larger than a typical cell
Giant algae (Acetabularia)
- a genus of a unicellular green algae of gigantic size (0.5-10cm in length)
Atypical because it’s a single-celled organisms
- challenges notion that cells must be simple in structure and small in size
Aseptate fungal hyphae
- long threads w/ many nuclei
- they have no dividing cell walls (septa)
- results in a shared cytoplasm and multiple nuclei
Atypical because fungal hyphae are multinucleated, very large and possess a continuous, shared cytoplasm
- it challenges idea that a cell is a single unit
Nanometres, micrometres and millimetre conversions
1000 nm = 1 μm
1000 μm = 1 mm
Formula for magnification
Magnification = size of drawing/actual size
Unicellular
- single-celled organism
- whole body is made of only one cell
- one cell needs to carry out all life processes
eg. bacteria, archaea, protozoa, unicellular algae etc.
Functions of life
- Metabolism
- Reproduction
- Homeostasis
- Growth
- Response
- Excretion
- Nutrition
MR H GREN
Metabolism
regular set of life-supporting chemical reactions that take place within the cells of living organisms
Reproduction
he production of offspring, either sexually or asexually, to pass on genetic information to the next generation
Homeostasis
the maintenance of a constant internal environment by regulating internal cell conditions
Growth
the maintenance of a constant internal environment by regulating internal cell conditions
Response
- to a stimulus
- a reaction by the living organism to changes in the external environment
Excretion
the removal of waste products of metabolism and other unimportant materials from an organism
Nutrition
The intake of nutrients, which may take different forms in different organisms
- nutrition in plants involves making organic molecules (during photosynthesis)
- nutrition in animals and fungi involves the absorption of organic matter
Why aren’t viruses regarded as living organisms?
- single living cell is capable of carrying out all life functions
- a virus is a non-living example because it can’t carry out all the processes of life
- a virus has a protein coat and, like living organisms, has genetic material (DNA or RNA)
- but, viruses don’t metabolise or reproduce – this function is carried out by the infected host cell
- as they exhibit no properties of life outside the host cell and do not have a cellular structure, viruses are not regarded as living entities
Two examples of unicellular organisms
- Paramecium
- a genus of unicellular protozoa
- eukaryote
- usually less than 0.25mm in size
- widespread in aquatic environments
- they are heterotrophs
- move in all directions using cilia (cover the whole body and beat rhythmically to propel cell in a given direction) - Chlamydomonas
- a genus of unicellular green algae
- eukaryote
- range from 10-30μm
- have a cell wall, a chloroplast, an ‘eye’ that detects light and two flagella
- use flagella to swim
- they’re autotrophs: can manufacture their own food using their large chloroplast to photosynthesise
Paramecium and its functions of life
- Metabolism:
- most metabolic reactions are catalysed by enzymes and take place in the cytoplasm - Reproduction:
- can carry out both sexual and asexual reproduction
- asexual reproduction is more common
- cell divides into two daughter cells through binary fission (asexual reproduction) - Homeostasis:
- a constant internal environment is maintained by collecting excess water in the contractile vacuoles and then expelling it through the plasma membrane
- this is osmoregulation and helps Paramecium to maintain water balance - Growth:
- as it consumes food, Paramecium enlarges
- once it reaches a certain size it will divide into two daughter cells. - Response:
- wave action of the beating cilia helps to propel Paramecium in response to changes in the environment, e.g. towards warmer water and away from cool temperatures. - Excretion:
- digested nutrients from the food vacuoles pass into the cytoplasm, and the vacuole shrinks
- When the vacuole, w/ its fully digested contents, reaches Paramecium’s anal pore, it ruptures, expelling its waste contents to the environment - Nutrition:
- Paramecium is a heterotroph- engulfs food particles in vacuoles where digestion takes place
- soluble products are absorbed into the cytoplasm of the cell
- it feeds on microorganisms, such as bacteria, algae and yeasts
Chlamydomonas and functions of life
- Metabolism:
- most metabolic reactions are catalysed by enzymes and take place in the cytoplasm - Reproduction:
- it can carry out both sexual and asexual reproduction
- when Chlamydomonas reaches a certain size, each cell reproduces, either by binary fission or sexual reproduction - Homeostasis:
- a constant internal environment is maintained by collecting excess water in the contractile vacuoles and then expelling it through the plasma membrane
- this is osmoregulation and helps Paramecium to maintain water balance - Growth:
- production of organic molecules during photosynthesis and absorption of minerals causes organism to increase in size
- once it reaches a certain size it will divide into two daughter cells - Response:
- Chlamydomonas senses light changes in its environment using its eye spot and then uses its flagella to move towards a brighter region to increase rate of photosynthesis - Excretion:
- it uses the whole surface of its plasma membrane to excrete its waste products - Nutrition:
- Chlamydomonas is an autotroph
- uses its large chloroplast to carry out photosynthesis to produce its own food
Heterotroph
an organism that feeds by taking in organic substances (usually other living things)
Autotroph
an organism that can produce its own food from inorganic sources
Limitation on cell size
- a cell can’t grow indefinitely due to its SA:V ratio
- to survive a cell needs to import molecules and expel waste products through its plasma membrane
- if a cell’s SA is too small compared to its volume, not enough of the necessary molecules can get in, and not enough waste can get out
- by dividing into two smaller cells, a larger SA:V ratio is restored
- Hence, SA:V ratio limits overall size of a cell
Evolution of a cell
- Life on this planet probably started out as small unicellular organisms
- Over the course of evolution, some of these cells clumped together and over long periods of time began to work together, evolving into simple multicellular organisms
- Organisms grew larger because they were no longer limited by the size of one cell
- Cells in such an organism were able to specialise through differentiation
- Multicellular organisms displayed emergent properties
- whole organism can do more than what individual cells are capable of, due to the interaction between different parts
Genome
the complete set of genes, chromosomes or genetic material present in a cell or organism
- human genome consists of 21, 000 genes
- all these genes are present in each cell of your body, but not all genes are active in all types of cells in the body
Cellular differentiation
when an unspecialised cell changes and carries out a specific function in the body
- cells differentiate to form different cell types due to the expression of different genes
Differentiation
a process in which unspecialised cells develop into cells w/ a more distinct structure and function
- involves the expression of some genes and not others in a cell’s genome
Cell differentiation after fertilisation
- at an early stage, the fertilised egg starts to divide.
- up to this stage, cells in an embryo are pluripotent embryonic stem cells- they can develop into any type of body cell
- although each cell has the same genome, only certain genes are expressed in certain cells and not in others
- this gives rise to the synthesis of certain proteins, which can trigger specialised development of that specific cell and its descendants
- so, groups of cells differentiate along different paths to form different specialised tissues of embryo
- once a cell starts to differentiate, it’s irreversible
NB/ process of differentiation involves expression of some genes and not others in a cell’s genome
Stem cell
an undifferentiated cell of a multicellular organism that can form more cells of the same type indefinitely
- from which certain other kinds of cell arise by differentiation
- are unspecialised cells that can give rise to a wide range of body cells by differentiating along different pathways
- they retain capacity to divide indefinitely and have potential to differentiate into specialised cell types when given right stimulus
- but, not all stem cells can give rise to all body cells
Totipotent stem cells
- the 8 cells of the morula- first cells formed following fertilisation
- can differentiate into any type of cell including placental cells
- can give rise to a complete organism
Pluripotent stem cells
- embryonic stem cells of the blastocyst
- can differentiate into all body cells, but can’t give rise to a whole organism
Multipotent stem cells
- umbilical cord stem cells
- can differentiate into a few closely related types of body cell
Unipotent stem cells
- can only differentiate into their associated cell type
- eg. liver stem cells can only make liver cells
Formation of embryonic stem cells
- embryos are important sources of stem cells
- once an egg has been fertilised, it starts to divide and forms totipotent cells during early stages (up until the eight-cell stage of the morula)
- each cell can still develop into a full and normal organism
- these cells continue to divide and develop to form the pluripotent cells of the blastocyst
- from here all the specialised tissues of developing embryo are generated
Stargardt’s disease
- a disease of the eye
- it’s an inherited form of juvenile macular degeneration (affects a small area near the centre of the retina)
- causes progressive loss of central vision, eventually leading to complete blindness
- it typically appears in late childhood to early adulthood
- caused by a recessive genetic mutation in gene ABCA4- causes an active transport protein on photoreceptor cells to malfunction
- ultimately causes photoreceptor cells to degenerate
Use of stem cells in Stargardt’s disease
- stem cell therapy has been shown to be effective in treating Stargardt’s disease
- patients are given retinal cells derived from human embryonic stem cells- are injected into the retina
- the inserted cells attach to the retina and become functional
- hence, it may be possible to restore sight to affected individuals using stem cells
Leukemia
a type of cancer of the blood or bone marrow
- is caused by high levels of abnormal white blood cells
- people w/ leukemia have a higher risk of developing infections, anemia and bleeding
Use of stem cells in Leukemia
- treatment involves harvesting hematopoietic stem cells (HSCs)- multipotent stem cells
- HSCs can be taken from bone marrow, peripheral blood or umbilical cord blood
- HSCs may come from either the patient or from a suitable donor
- patient then undergoes chemotherapy and radiotherapy to get rid of diseased white blood cells
- next step involves transplanting HSCs back into the bone marrow, where they differentiate to form new healthy white blood cells
Sources of stem cells
- embryo
- umbilical cord blood
- bone marrow
- adult tissues
Ethics behind embryonic stem cells
- Cells may be used in cell therapy (replacing bad cells with good ones) to eliminate serious diseases or disabilities in the human population
- Transplants can be easily obtained without requiring death of another human or inflicting any kind of pressure on normal body functioning which happens when someone donates an organ
- The stem cells are harvested from the embryo at an early stage when embryo hasn’t yet developed a nervous system and thus it is not likely to feel any pain
Prokaryotes vs. Eukaryotes
E: have a separate membrane-enclosed nucleus
P: DNA of prokaryotes is freely floating in the cytoplasm.
E: Eukaryotic cells have a complex system of membrane-bound organelles that divides cell into numerous enclosed regions – compartmentalisation
P: do not have any membrane-bound organelles
Prokaryotes
- earliest and mot primitive type of cell
- include bacteria and archaea
- simple unicellular organisms w/ no internal compartmentalisation, no nucleus and no membrane-bound organelles
- all metabolic processes occur within the cytoplasm
Function of cellular structures in prokaryotic cells
- Cell wall: Encloses cell, protecting it and helping to maintain its shape; prevents cell from bursting in hypotonic media
- Plasma membrane: surrounds cell, controlling movement of substances in and out of the cell.
- Cytoplasm: medium that fills the cell and is the site of all metabolic reactions.
- Pili: protein filaments on the cell wall that help in cell adhesion and in transferring of DNA between two cells.
- Flagella: much longer than pili; are responsible for locomotion of the organism. Their whip-like movement propels the cell along.
- 70S ribosomes: the sites of protein synthesis
- Nucleoid region: controls all cell activities and reproduction of the organism
- includes naked DNA- DNA not associated with proteins known as histones - Plasmids: small circles of DNA that carry a few genes; often these genes give cell antibiotic resistance and are used in creating genetically modified bacteria.
Ribosomes in eukaryotes vs. prokaryotes
Prokaryotic cells: 70S ribosomes, smaller than those found in eukaryotic cells
Eukaryotic cells: 80S ribosomes
NB/ 70S and 80S refer to sedimentation rate of RNA subunits
Reproduction in prokaryotes
- prokaryotes reproduce by binary fission to produce 2 genetically identical cells
- Binary fission is a means used by prokaryotes to reproduce asexually
Binary fission in prokaryotes
- The chromosome is replicated semi-conservatively, beginning at the point of origin
- Beginning with the point of origin, the two copies of DNA move to opposite ends of the cell
- The cell elongates (grows longer)
- The plasma membrane grows inward and pinches off to form two separate, genetically identical cells
Eukaryotes
- have a compartmentalised cell structure
- genetic material is isolated from cytoplasm by the nucleus
- include 4 kingdoms: Protoctista, Fungi, Plantae and Animalia
The 3 domains
- archaea
- bacteria
- eukaryota
The 4 kingdoms
- Kingdom Protoctista
- unicellular organisms; or multicellular organsims wo/ specialised tissue - Kingdom Fungi
- have a cell wall made of chitin
- obtain nutrition via heterotrophic absorption - Kingdom Plantae
- cell wall made of cellulose
- obtain nutrition autotrophically via photosynthesis - Kingdom Animalia
- no cell wall
- obtain nutrition via heterotrophic ingestion
Compartmentalisation
refers to the formation of compartments within the cell by membrane-bound organelles
Advantages of eukaryotic cells being compartmentalised
- Greater efficiency of metabolism as enzymes and substrates are enclosed- hence, much more concentrated, in the particular organelles responsible for specific functions
- Internal conditions can be differentiated in a cell to maintain optimal conditions for different enzymes
- Isolation of toxic substances away from cytoplasm eg/ storage of hydrolytic enzymes in lysosomes
- Flexibility of changing no. and position of organelles within cell based on the cell’s requirements
Function of cellular structures found in eukaryotic cells
- Plasma membrane: controls movement of substances in and out of the cell
- Cytoplasm: fills cell and holds all organelles- also contains enzymes that catalyse various reactions occurring within cytoplasm
- Mitochondria: a site of cellular respiration in which ATP is generated.
- 80S Ribosomes: sites of protein synthesis- free ribosomes produce proteins used inside the cell itself.
- Nucleus: controls all cell and reproduction of unicellular organisms.
- Nucleolus: part of the nucleus involved in production of ribosomes
- Smooth endoplasmic reticulum: responsible for producing and storing lipids, including steroids.
- Rough endoplasmic reticulum: transports protein produced by ribosomes on its surface to Golgi apparatus- usually for use outside of the cell
- Golgi apparatus: processes and packages proteins, which are ultimately released in Golgi vesicles.
- Vesicle: small sac transporting and releasing substances produced by cell by fusing w/ cell membrane
- Lysosomes (absent from plant cells): contain hydrolytic enzymes and play important roles in destruction of microbes engulfed by WBC
- Centrioles (absent from plant cells): play important role in nuclear division- help establish microtubules
- Vacuole (absent from animal cells): helps in osmotic balance of cell and in storage of substances- may also have hydrolytic functions similar to lysosomes
- Cell wall (absent from animal cells): protects cell, maintains its shape and prevents it from bursting in hypotonic media
- Chloroplast (absent from animal cells): double-membrane-bound organelles; contain pigments (chlorophyll) and are responsible for photosynthesis
Exocrine gland
- secretes enzymes into a duct
- enzymes = proteins, so exocrine cells will have a well-developed network of rER for protein synthesis
- will also have a GA that produces vesicles containing these enzymes
- vesicles merge w/ plasma membrane to release their contents into the small membrane
Palisade mesophyll cell
- contains many chloroplasts
- site of photosynthesis
- in plants, this tissue contains the greatest no. of chloroplasts per cell
- is positioned right under the upper epidermis where it’s exposed to highest amount of light
- main function is to photosynthesise: producing complex organic compounds, using CO2 and water as starting materials
Microscope resolution
the shortest distance between 2 separate points in a microscope’s filed of view that can still be distinguished as distinct objects
- higher the value, lower the resolution
Fluid mosaic model
- proposed by Singer and Nicholson in 1972
- model of the cell membrane
- according to this, biological membranes consist of phospholipid bilayers w/ proteins embedded in the bilayer, making the membrane look like a mosaic
Amphipathic
a molecule that has both a hydrophilic and a hydrophobic part
- an amphipathic phospholipid has hydrophilic and hydrophobic properties
Membrane proteins
- a group of proteins w/ diverse structures associated w/ cell membrane
- all carry out different functions
- all support the plasma membrane in carrying out its distinctive function
- are all categorised as integral or peripheral depending on their position in the membrane
Integral proteins
- are amphipathic (hydrophobic and hydrophilic)
- permanently attached to the. membrane
- are typically transmembrane
Peripheral proteins
- are polar (hydrophilic)
- temporarily attached by non-covalent interactions and associate w/ one surface of the membrane
Various functions of proteins
Proteins have these functions:
1. Channels – some proteins have a pore/channel that allows passive transport of substances between the inside and outside of the cell.
- Carriers – these proteins bind to substances on one side of the membrane and then change shape to transport them to the other side
- carrier proteins that use energy to change shape are termed protein pumps - Recognition – certain proteins help cell in differentiating between self and non-self cells- important in triggering an immune response
- Receptors – these proteins usually span whole membrane to relay information from inside or outside of the cell
- Enzymes – these are proteins that enhance rate of reactions that happens at the membrane level
Glycolipids
- are a phospholipid and carbohydrate attached together
- important in maintaining the structure of the cell membrane
- helps differentiate cells between self and non-self
Cholesterol
- a component of animal cell membranes, functions to maintain integrity and mechanical stability
- absent in plant cells, as PM is surrounded and supported by a rigid cell wall of cellulose
- an amphipathic molecule: has both hydrophilic and hydrophobic regions
- its hydroxyl group is hydrophilic and aligns towards phosphate heads of phospholipids
- remained or molecule is hydrophobic and asoociates w/ phospholipid tails
Davson-Danielli membrane model
- suggested that cell membrane comprises of a lipid bilayer
- where 2 layers of polar lipid molecules are arranged w/ their hydrophilic heads outward
- lipid bilayer itself sandwiched between 2 protein layers on either side of the membrane
- proposed following evidence from electron microscope, which showed membrane as having 3 layers
- in high magnification electron micrographs, membranes appeared as 2 dark parallel lines w/ a lighter- coloured region in between
- proteins appear dark in electron micrographs
- phospholipids appear light
- hence, it was deduced that membranes comprised two protein layers, one on either side of a phospholipid core
- model gave an explanation for the membrane’s effectiveness as barrier to movement of certain substances despite being very thin
Shortcomings of Davson-Danielli model
- assumed that all membranes had identical structures, which didn’t explain how different types of membranes could carry out different functions
- proteins are amphipathic which makes it improbable that they would only be found in the aqueous environment on either side of the membrane rather than going through the membrane
Singer and Nicholson model
Singer and Nicholson model:
- fluid mosaic model replaced D-D model
- suggested that proteins are individually embedded in phospholipid bilayer, rather than coating it on both sides
- allowed hydrophilic portions of both proteins and phospholipids to be maximally exposed to water- resulting a stable membrane structure
- it ensures that hydrophobic portions of proteins and phospholipids were in the non-aqueous environment inside the bilayer
Diffusion
movement of particles from a region of high concentration to a region of low concentration, and is the result of the random motion of particles
- two types: simple and facilitated
Factors affecting diffusion
- Temperature – molecules diffuse faster if the temperature is higher
- Surface area of membrane – an increase in surface area allows more molecules to diffuse
- Size of particles – smaller molecules diffuse faster than larger ones
- Concentration gradient of diffusing particles – the bigger the gradient the faster the diffusion
Simple diffusion
- occurs in a gas or liquid medium
- only requires a concentration gradient
- occurs in both living and non-living systems
- if a particle is too big, it can’t pass through phospholipid bilayer of membrane
- ions are also repelled by hydrophobic tails in the membrane
NB/ when simple diffusion fails, facilitated diffusion can transfer the particle across the membrane
Facilitated diffusion
- similar to simple diffusion, but it requires channel proteins or carrier proteins, which are specific the molecules being transported across plasma membrane
- size and shape of protein carriers and channels determine what substance can cross the membrane
An application of facilitated diffusion
Important application: movement of K+ ions in neurons during generation of an action potential; a key step in the propagation of nerve impulses along neurons
- helps in moving K+ ions out of the axons to cause repolarisation
- K+ channels involved only allow movement of K+
- they’re also voltage gated, they open and close w/ changes in electrical potential to control movement of K+ ions
Osmosis
the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane
- involves water molecules only
Tissues and organs in medical procedures
- in medical procedures, tissues need to be kept in a saline solution for storage
- it’s essential that osmolarity of saline solution is same as that in the cytoplasm of the cells of the tissue to prevent any osmosis that would damage the cells
Osmolarity
refers to concentration of a solution in terms of moles of solutes per litre of solution
Hypotonic solution
- solution has relatively lower osmolarity
- low solute conc. = loses water
Hypertonic solution
- solutions w/ a relatively higher osmolarity
- high solute conc. = gains water
Isotonic solution
when the concentrations of solutes are equal inside and outside of the cell, so there is no net movement of water
Active transport
- movement of particles across membranes, requiring energy in the form of ATP
- energy is used to move substances against a concentration gradient, from a region of low concentration to one of a higher concentration
Active transport in the sodium-potassium pump
- When the pump is open to the inside of the axon, 3 sodium ions (Na+) enter the pump and attach to their binding sites
- ATP donates a phosphate group to the pump
- This causes protein to change shape expelling Na+ to the outside
- 2 potassium ions (K+) from outside then enter and attach to their binding sites.
- Binding of K+ leads to release of the phosphate, causes pump to change shape again so that it’s only open to the inside of the axon
- K+ is released inside.
- Na+ can now enter and bind to the pump again
Endocytosis
a cellular process where cells take in molecules or substances from outside of the cell by engulfing them in the cell membrane
- can be divided into pinocytosis and phagocytosis
- possible due to the fluidity of membranes
Exocytosis
involves the ejection of waste products or useful substances from the inside of the cell
- can be divided into excretion and secretion
- possible due to the fluidity of membranes
Pinocytosis
the taking in of liquid substances by cells
- type of endocytosis
Phagocytosis
involves the absorption of solids
- type of endocytosis
- eg. when a WBC or phagocyte engulfs a pathogenic microbe
Division of exocytosis
Can be divided into:
1. Excretion: following phagocytosis, any undigested remains of the microbe that aren’t useful to the cell are excreted outside the cell
- Secretion: Proteins synthesised by ribosomes on rER are first passed to GA via vesicles
- here they’re processed and packaged before being released in vesicles that fuse w/ the plasma membrane for secretion outside the cell
Role of vesicles
- play an important role in exocytosis and endocytosis, allow movement of materials within the cell
- many of the organelles in the cell are membrane-bound; they can make their own vesicles
- vesicles are a common way for cell to move molecules around inside cells
Movement of substances in cell using enzymes
- Enzymes are made by bound ribosomes in rough ER
- Rough ER will package enzymes in a vesicle formed from membranes of rough ER
- Vesicles carrying enzymes will move to Golgi apparatus, and fuse w/ Golgi apparatus membrane
- Enzymes will be modified further in Golgi apparatus and then packed in a vesicle created using Golgi apparatus membrane
- Vesicle will then move to the plasma membrane and undergo exocytosis and release enzymes out of cell
Pasteur’s experiments
- Louis Pasteur provided crucial evidence to support the hypothesis that cells must come from pre-existing cells
- his experiment disproved theory of spontaneous generation
- Pasteur hypothesised that cells must come from cells and designed a falsifiable experiment
Theory of spontaneous generation
Stated that life could appear from a combination of dust, air and other factors
Procedure for Pasteur’s experiment
- Boiled nutrient broth in 3 swan-neck flasks
- He then broke the neck of one flask to allow air to enter, but left other flask unbroken
- Broth in flask where swan neck wasn’t broken remained clear
- no microbes were formed
- any microbes from the air were trapped in the curve of the swan neck - Broth in the broken-necked flask became cloudy
- microbes carried to flask in air grew and multiplied - In the third flask, Pasteur tilted the flask to expose the broth to the microbes in the curve of the swan neck
His experiments proved that spontaneous generation of cells and organisms doesn’t occur on Earth
- showed that the present conditions don’t sustain the process
Where did the first cell come from?
First cell must have come from non-living material
- this has gained support from evidence collected in the Miller-Urey experiment
Miller-Urey experiment
- Recreated conditions of early Earth in a closed system
- included a reducing atmosphere (low oxygen) w/ high radiation levels, high temp. and electrical storms - After running experiment for a week, some simple AA and complex oily hydrocarbons were found in reaction mixture
- This experiment proved that non-living synthesis of simple organic molecules was possible
Conditions for emergence of life
- Simple organic molecules, eg. amino acids, fatty acids and carbohydrates, must be formed
- Larger organic molecules eg. phospholipids, RNA and DNA, must be assembled from simpler molecules
- Organisms reproduce, so replication of nucleic acids must be possible
- Biochemical reactions require set conditions, eg. pH - hence, self-contained structures, eg. membranes are necessary.
Endosymbiotic theory
Explains origin of eukaryotic cells
- it’s highly probable that the first cells were prokaryotic in structure and appearance
- proof lies in double membrane structure of organelles eg. mitochondria and chloroplasts
- also comparison of DNA of those organelles w/ prokaryotic genomes
What is the endosymbiotic theory?
- supports idea that mitochondria and chloroplasts themselves were prokaryotes that were taken in by larger prokaryotes by endocytosis
- instead of being digested and broken down, these cells remained inside the host cells
- cells that could carry out aerobic respiration and thus provide energy to host cell, probably anaerobes who didn’t need oxygen, evolved into mitochondria
- prokaryotic cells that could convert light energy to chemical energy (probably cyanobacteria) became chloroplasts and passed on sugars produced during photosynthesis to the host cell
Evidence that supports endosymbiotic theory
Both mitochondria and chloroplasts:
- Have double membranes, as expected for cells taken in by endocytosis
- Have singular naked DNA, as in prokaryotes
- Have 70S ribosomes, as in prokaryotes
- Divide by binary fission like prokaryotic cells
- Are susceptible to some antibiotics
The 3 stages of the cell cycle
- Interphase
- Mitosis
- Cytokinesis
Interphase
- most active and the longest phase of the cell cycle
- cells spend most of their life within this stage undergoing common cell processes eg/ metabolism, endocytosis, exocytosis and using and obtaining nutrients
- it involves many processes that occur both in nucleus and cytoplasm
- It has 3 important phases of cell cycle: G1 (Gap 1), S (synthesis), and G2
Three phases of interphase
- Gap 1 (G1)
- happens in the cytoplasm
- cell grows and functions normally, undergoing everyday processes
- Rapid protein synthesis takes place allowing cell to grow in size
- Proteins required for DNA synthesis (the next phase) are made - DNA Synthesis (S)
- happens in the nucleus
- amount of DNA doubles as DNA replication takes place
- genetic material is duplicated but no chromosomes are formed yet - Gap 2 (G2)
- happens in the cytoplasm
- protein synthesis occurs to produce proteins needed for cell division, such as microtubule proteins that will make up a mitotic spindle- cell is actively preparing for cell division
- mitochondria and chloroplasts (plant cells) are replicated
Mitosis (M phase)
division of nucleus into 2 genetically identical daughter nuclei
- involves separation of sister chromatids into individual chromosomes
- these are then distributed among daughter nuclei
Cytokinesis (M phase)
- once mitosis has been completed, w/ formation of 2 nuclei w/ identical sets of chromosomes, cell enters cytokinesis
- in cytokinesis, cytoplasm of a parental cell is divided between 2 daughter cells
Cyclins
- a family of proteins that control progression of cells through the cell cycle
- cells can’t progress to next stage of cell cycle unless specific cyclin reaches a certain conc.
- cyclins bind to cyclin-dependent kinases (CDKs) and activate them
- activated CDKs then attach phosphate groups to other proteins in the cell
- attachment of phosphate triggers other proteins to become active and carry out tasks; in this case, specific to phases of the cell cycle
- cyclins and CDKs regulate the cell cycle
- depending on presence and action of these proteins, cell cycle can be fast or slow, and it may even stop altogether
Mutations in cyclins
Mutations in genes that code for cyclins can lead to problems regulating the cell cycle, which may lead to cancer
Cyclin levels at different phases of the cell cycle
G1 phase: During this phase cyclin D (red line) levels gradually increase
S phase: Cyclin E is instrumental to DNA replication and also promotes centromere duplication
G2 phase: Cyclin A helps induce DNA replication
Mitosis: Cyclin B is essential for formation of mitotic spindles and alignment of chromatids
Process of mitosis
- the division of the nucleus
- involves separating the DNA that was replicated in the S phase
- in eukaryotic cells, this is done through the formation of chromosomes
Eukaryotic chromosomes
- consist of DNA which is tightly wound around proteins called histones
- histones are basic (alkaline) proteins that form part of nucleosomes
- many nucleosomes are coiled together in a specific pater to form the chromosome
- during interphases, chromosomes are unpacked, so protein synthesis and replication can take place
- DNA is only visible during mitosis
- DNA is initially packaged by histones to form chromatin, which undergoes further condensation to form chromosome
- hence, chromatin is a lower order of DNA organisation, chromosomes are a higher order of DNA organisation
Chromosomes, sister chromatids and chromatin
- During interphase, DNA is present as chromatin
- Following prophase, phase when DNA supercoiling takes place, DNA is visible as a pair of sister chromatids that are identical to each other connected by a centromere
- After sister chromatids are separated during anaphase, they are referred to as chromosomes
The four phases of mitosis
Mitosis occurs after the G2 phases
- it involves four phases (PMAT)
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- DNA supercoils causes chromatin to condense
- Nucleolus disappears
- Nuclear membrane disintegrates
- Spindle fibres (made of microtubules) start to form (and are completely formed by end of prophase)
- Centrioles (absent from plant cells) move to opposite poles
Metaphase
- Spindle fibres bind to centromere of sister chromatids and cause their movement towards equatorial plate
- Sister chromatids are aligned at equatorial plate at end of metaphase
Anaphase
- Sister chromatids are separated (now known as chromosomes) and pulled to opposite poles by spindle fibres
Telophase
- Chromosomes have reached the poles
- A nuclear membrane starts to reform at each pole
- A nucleolus appears in each new nucleus
- The spindle fibres disintegrate
- The cell elongates in preparation for cytokinesis
- In some cases, invagination of membrane is also visible (marking the beginning of cytokinesis)
Mitotic index
the ratio of the no. of cells in a population undergoing mitosis (prophase, metaphase, anaphase or telophase) to the total no. of visible cells
- indicates how many cells in a tissue are dividing at a given time
- In a tumour, where cell division is uncontrolled, mitotic index is higher than in normal tissue
- hence, mitotic index is used to predict how quickly a cancer could spread and likely outcome in reducing cell proliferation of any treatment eg. chemotherapy
Formula for mitotic index
Mitotic index= (P+M+A+T)/ Total cells
P, M, A and T= refer to no. of cells at each phase
Cytokinesis
the division of the parental cytoplasm between the two daughter cells after mitosis (it often starts in telophase)
- for animal and plant cells, cytokinesis is very different due to the presence of a cell wall in plant cells
Cytokinesis in animals cells
- A ring of protein (microfilaments) located immediately beneath plasma membrane at the equator pulls plasma membrane inward
- The inward pull on plasma membrane produces the characteristic cleavage furrow
- When cleavage furrow reaches centre of the cells, it is pinched apart to form two daughter cells
Cytokinesis in plant cells
- Golgi apparatus forms vesicles that consist of material to build a new cell wall
- Vesicles merge and form the cell plate
- Cell plate grows and divides into two daughter cells
Tumourigenesis
formation of a tumour (or several of them)
Tumour
a mass of cells that divide uncontrollably
- there are two types: benign and malignant
- not all tumours lead to cancer
Benign tumour
- usually localised
- doesn’t spread to other parts of the body
- most benign tumour respond well to treatment
Malignant tumour
- a cancerous growth that’s often resistant to treatment
- it may spread to other parts of the body
- may also sometimes recur after it has been removed
How is a tumour formed?
When the events of the cell cycle are disrupted because of a mutation in one of the cyclins, CDKs, or a protein associated with the cell cycle
- eg/ p53 is a protein involved in regulation of the cell cycle
- a mutation in the p53 gene can lead to tumour formation
- over 50% of all tumours have a mutation in p53 gene
- a mutation in a single cell can trigger beginning of a tumour
- cell may have lost its ability to enter G1-phase, and instead continues to divide rapidly
- mutation is passed on to daughter cells and a clump of cells starts to form
- When this uncontrolled growth continues, a tumour is formed
Mutation
a change in an organism’s genetic code
- a change in base sequence of a certain gene can result in tumour formation
- some parts of a gene don’t code for anything, so a mutation in these areas won’t affect the organism
- not all gene mutations lead to uncontrolled cell division
Mutagens
- agents that cause gene mutations
- not all mutations result in cancers
- anything that causes a mutation has the potential to cause a cancer
Examples of mutagens
- Chemicals that cause mutations, referred to as carcinogens, eg/ asbestos or dioxin.
- High-energy radiation, such as X-rays
- Short-wave UV light
- Some viruses eg/ hepatitis B.
Oncogene
a mutated gene that contributes to development of a tumour
- in their normal, un-mutated state, oncogenes are called proto-oncogenes
- help in regulation of cell division
Metastasised tumour
- Once abnormal cell division has started at a particular place in the body, a malignant primary tumour begins to form
- If left untreated, this may follow a particular development pathway to form secondary tumours
- the tumour has metastasised`
Process of metastasis
- Cancerous cells detach from primary tumour
- Some cancerous cells gain ability to penetrate walls of lymph or blood vessels and so circulate around the body
- Circulating cancerous cells invade tissues at different locations and develop, by uncontrolled cell division, into secondary tumours
Metastasis
the movement of cells from a primary tumour to other parts of the body where they develop into secondary tumours
Multicellular organism
- made of more that one cell
- have properties that emerge from the interaction of their cellular components
- specialised tissues can develop by cell differentiation in multicellular organisms
- cells may be grouped together to form tissues
- functional grouping of multiple tissues forms organs
- organs interact to form organ systems, carry out specificy body functions
- organ systems collectively carry out the life function of the complete organism
Ethics of using embryonic stem cells
- Unlimited growth potential- can differentiate into any type in the body
- More risk of becoming tumour cells than adult stem cells
- Less chance of genetic damage due to accumulation of mutations than with adult stem cells
- Likely to be genetically different from an adult patient receiving the tissue
- Removal of cells from the embryo kills it
Ethics of using umbilical cord stem cells
- Easily obtained and stored- commercial collection and storage services already available
- Fully compatible with tissues of adult that grows from baby- no rejection problems
- Limited capacity to differentiate into different cells types- only naturally develop into blood cells
- Limited quantities of stem cells from one baby’s cord
- Umbilical cord is discarded whether or not stem cells are taken from it
Ethics of using adult stem cells
- Difficult to obtain as there are few of them and are buried deep in tissues
- Less growth potential than embryonic stem cells
- Less chance of malignant tumours developing than from embryonic stem cells
- Limited capacity to differentiate into different cell types
- Fully compatible with the adult’s tissue- no rejection problems
- Removal of stem cells doesn’t kill the adult from which the cells are taken
Falsification of the Davson-Danielli model
D-D model of membrane structure was accepted by cell biologists for about 30 years
- Results of many experiments fitted model including X-ray diffraction studies and electron microscopy
1950’s and 60’s: experimental evidence accumulated that didn’t fit the D-D model
Freeze-etched electron micrographs
- Involves rapid freezing of cells and then fracturing them
- Fracture occurs along lines of weakness, including centre of membranes
- Globular structures scattered through freeze-etched images of centre of membranes were interpreted as transmembrane proteins
Structure of membrane proteins
- Improvements in biochemical techniques allowed proteins to be extracted from membranes
- Found to be varied in size and globular in shape- unlike type of structural protein that would form continuous layers on membrane periphery
- Proteins were hydrophobic on part of surface so they’d be attracted to hydrocarbon tails of phospholipids in membrane centre
Fluorescent antibody tagging
- Red or green fluorescent markers were attached to antibodies that bind to membrane proteins
- Membrane proteins of some cells were tagged with red markers and others with green markers
- The cells were fused together
- Within 40 minutes red and green markers were mixed throughout membrane of fused cell
- showed that membrane proteins are free to move within membrane rather than being fixed in a peripheral layer
This experimental evidence falsified D-D model
- A replacement was needed that fitted evidence – Singer- Nicolson fluid mosaic model
Proposition of the Davson-Danielli model
1930’s: Davson and Danielli- layers of protein adjacent to phospholipid bilayer on both sides of membrane
- Proposed this as they thought it would explain how membranes, despite being very thin, are an
effective barrier to movement of substances
1950’s: high magnification electron micrographs of membranes were made
- Showed two dark lines with a lighter band between
- Proteins appear darker in electron micrographs and phospholipids appear light- this fitted Davson and
Danielli model
Potassium pump for facilitated diffusion in axons
- consists of integral proteins w/ a hydrophilic inner pore via which potassium ions may be transported
- channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
- Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage
Carrier proteins vs. channel proteins
Carrier Proteins
- integral glycoproteins that bind a solute and undergo a conformational change to translocate solute across membrane
- only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
- may move molecules against conc. gradients in presence of ATP (i.e. are used in active transport)
- much slower rate of transport than channel proteins
Channel Proteins
- Integral lipoproteins that contain a pore via which ions may cross from one side of membrane to the other
- are ion-selective and may be gated to regulate passage of ions in response to certain stimuli
- only move molecules along a conc. gradient (i.e. are not used in active transport)
- have a much faster rate of transport than carrier proteins
Estimating osmolarity of tissues
- osmolarity of a tissue is interpolated by bathing sample in solutions w/ known osmolarities
- tissue will lose water when placed in hypertonic solutions
- tissue will gain water when placed in hypotonic solutions
- water loss or gain may be determined by weighing sample before and after bathing in solution
- Tissue osmolarity is inferred by identifying conc. of solution at which there is no weight change (i.e. isotonic)
Effects of uncontrolled osmosis
Uncontrolled osmosis will have negative effects with regards to cell viability:
- Hypertonic solutions: water will leave the cell causing it to shrivel (crenation)
- Hypotonic solutions: water will enter the cell causing it to swell and potentially burst (lysis)
Effects of uncontrolled osmosis in plant tissues
In plant tissues, effects of uncontrolled osmosis are moderated by the inflexible cell wall
- Hypertonic solutions: cytoplasm will shrink (plasmolysis) but cell wall will maintain a structured shape
- Hypotonic solutions: cytoplasm will expand but is unable to rupture within the constraints of the cell wall (turgor)
Rate of metabolism and material exchange in cells
Cells need to produce chemical energy (via metabolism) to survive
- requires exchange of materials w/ the environment
Rate of metabolism of a cell:
- a function of its mass / volume
- larger cells need more energy to sustain essential functions
Rate of material exchange:
- a function of its surface area
- large membrane surface = more material movement
Increasing SA:Volume ratio
Cells and tissues specialised for gas or material exchanges will increase surface area to optimise material transfer
- Intestinal tissue of digestive tract has villi to increase surface area of inner lining
- Alveoli in lungs have membranous extensions- microvilli, that function to increase total membrane surface
Properties of stem cells
When a cell differentiates and becomes specialised, it loses its capacity to form alternative cell types
Stem cells are unspecialised cells that have two key qualities:
- Self Renewal – they can continuously divide and replicate
- Potency – have the capacity to differentiate into specialised cell types
Cellular components of prokaryotic cells
- Cytoplasm – internal fluid component of cell
- Nucleoid – region of cytoplasm where DNA is located (DNA strand is circular and called a genophore)
- Plasmid – autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)
- Ribosomes – complexes of RNA and protein responsible for polypeptide synthesis (prokaryote ribosome = 70S)
- Cell membrane – Semi-permeable and selective barrier surrounding the cell
- Cell wall – rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)
- Slime capsule – a thick polysaccharide layer used for protection against dessication and phagocytosis
- Flagella – Long, slender projections w/ a motor protein that enables movement
- Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)
Universal organelles
- found in prokaryotes and eukaryotes
- Ribosomes
Function: site of polypeptide synthesis (translation)
Structure: two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes (70S)
- Cytoskeleton
Function: Provides internal structure and mediates intracellular transport (less developed in prokaryotes)
Structure: A filamentous scaffolding within cytoplasm (fluid portion of cytoplasm = cytosol)
- Plasma membrane
Function: Semi-permeable and selective barrier surrounding the cell
Structure: Phospholipid bilayer embedded with proteins (not an organelle per se, but a vital structure)
Eukaryotic organelles- those found in animal and plants cells
- Nucleus
Structure: Double membrane structure w/ pores; has a nucleolus
Function: Stores genetic material (DNA) as chromatin; nucleolus is site of ribosome assembly
- Endoplasmic Reticulum
Structure: A membrane network that may be bare (smooth ER) or studded w/ ribosomes (rough ER)
Function: Transports materials between organelles (smooth ER = lipids ; rough ER = proteins)
- Golgi Apparatus
Structure: An assembly of vesicles and folded membranes located near cell membrane
Function: Involved in sorting, storing, modification and export of secretory products
- Mitochondrion
Structure: Double membrane structure, inner membrane highly folded into internal cristae
Function: Site of aerobic respiration (ATP production)
- Peroxisome
Structure: Membranous sac containing a variety of catabolic enzymes
Function: Catalyses breakdown of toxic substances (e.g. H2O2) and other metabolites
- Centrosome
Structure: Microtubule organising centre (contains paired centrioles in animal cells but not plant cells)
Function: Radiating microtubules form spindle fibres and contribute to cell division (mitosis / meiosis)
Organelles present in plant cells only
- Chloroplast
Structure: Double membrane structure w/ internal stacks of membranous discs (thylakoids)
Function: Site of photosynthesis – manufactured organic molecules are stored in various plastids
- Vacuole (large and central)
Structure: Fluid-filled internal cavity surrounded by a membrane (tonoplast)
Function: Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)
- Cell Wall
Structure: External outer covering made of cellulose (not an organelle per se, but a vital structure)
Function: Provides support and mechanical strength; prevents excess water uptake
Organelles present in animal cells only
Lysosome
Structure: Membranous sacs filled w/ hydrolytic enzymes
Function: Breakdown / hydrolysis of macromolecules
Electron microscopes vs. light microscopes
Advantages of electron microscopes:
- Have a much higher range of magnification (can detect smaller structures)
- Have a much higher resolution (can provide clearer and more detailed images)
Disadvantage: can’t display living specimens in natural colours`
Deducing cell function based on relative abundance of various organelles
- Mitochondria – Cells w/ many mitochondria typically undertake energy-consuming processes (e.g. neurons, muscle cells)
- ER – Cells w/ extensive ER networks undertake secretory activities (e.g. plasma cells, exocrine gland cells)
- Lysosomes – Cells rich in lysosomes undertake digestive processes (e.g. phagocytes)
- Chloroplasts – Cells w/ chloroplasts undergo photosynthesis (e.g. plant leaf tissue but not root tissue)
Structure of phospholipids
- Consist of a polar, hydrophilic head composed of a glycerol and a phosphate molecule
- Consist of two non-polar hydrophobic tails composed of fatty acid (hydrocarbon) chains
- as phospholipids have both hydrophilic and lipophilic (hydrophobic) regions, they are classed as amphipathic
Arrangement of phospholipids in membranes
- Phospholipids spontaneously arrange into a bilayer
- hydrophobic tail regions face in and are shielded from surrounding polar fluids
- the 2 hydrophilic head regions associate w/ cytosolic and extracellular fluids
Properties of phospholipid bilayer
- bilayer is held together by weak hydrophobic interactions between the tails
- Hydrophilic / hydrophobic layers restrict passage of many substances
- Individual phospholipids can move within bilayer, allowing for membrane fluidity and flexibility
- This fluidity allows for spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
Functions of membrane proteins
JET RAT
- Junctions – Serve to connect and join two cells together
- Enzymes – Fixing to membranes localises metabolic pathways
- Transport – Responsible for facilitated diffusion and active transport
- Recognition – May function as markers for cellular identification
- Anchorage – Attachment points for cytoskeleton and extracellular matrix
- Transduction – Function as receptors for peptide hormones
Cholesterol in mammalian membranes
Phospholipid bilayers are fluid= phospholipids are in constant movement relative to one another
Cholesterol interacts w/ the fatty acid tails of phospholipids to moderate properties of the membrane:
1. Cholesterol functions to immobilise outer surface of membrane, reducing fluidity
- It makes membrane less permeable to very small water-soluble molecules that would otherwise freely cross
- It functions to separate phospholipid tails and so prevent crystallisation of membrane
- It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
Energy for active transport
Active transport uses energy to move molecules against a conc. gradient
This energy may either be generated by:
- Direct hydrolysis of ATP (primary active transport)
- Indirectly coupling transport w/ another molecule that’s moving along its gradient (secondary active transport)
Use of protein pumps during active transport
- a specific solute will bind to protein pump on one side of the membrane
- hydrolysis of ATP (to ADP + Pi) causes a conformational change in protein pump
- solute molecule is consequently translocated across membrane (against gradient) and released
rough ER vs. smooth ER
Endoplasmic reticulum is a membranous network that’s responsible for synthesising secretory materials
- Rough ER: embedded w/ ribosomes and synthesises proteins destined for extracellular use
- Smooth ER is involved in lipid synthesis and plays a role in carbohydrate metabolism
