Topic 2: Cells Flashcards
1
Q
What is the mitochondria?
A
The powerhouse of the cell 💪😡🧬
if u see this, keep slaying <3
Doing an amazing job i believe in u :>
Remember to eat some food, drink some water, take breaks, and get some sleep xx
2
Q
What are cells?
A
Cells are the structural unit of all living things, the basic functional unit of life, not only making up the bodies of living things but they also carry out the ‘life processes’.
3
Q
What is the cell theory?
A
The cell theory is a concept that unifies all living things.
4
Q
what does the cell theory state?
A
The cell theory states that:
All organisms consist of one or more cells
The cell is the basic structural and functional unit of all living things
All cells arise from pre-existing cells
(Most) cells contain hereditary information (DNA) which is passed on to daughter cells.
Cells use energy
5
Q
What are some features that distinguish living things from non-living things?
A
Living things:
Are complex and have an organised structure
Take in energy from their surroundings and use it
Preserve a composition that is chemically different from that of their external environment
Respond to stimuli
Are able to reproduce themselves
Grow and develop
6
Q
What encloses the cytoplasm (internal fluid) of the cell?
A
All cells are made up of an outer membrane called the cell membrane which encloses the cytoplasm.
7
Q
What is cytoplasm and what does it contain?C
A
Cytoplasm is a fluid inside cells made up of mainly water and contains many smaller structures called organelles (small organs).
8
Q
What can a cell be defined as?
A
A cell can be defined as a unit of living matter (protoplasm) that is separated from its external surroundings by a membrane which regulates the passage of materials into and out of the cell.
9
Q
What are the main functions of the cell membrane?
A
Separate the contents of the cell from the external environment
Regulate the passage of substances into and out of the cell
Enable cells to recognise each one other, and to recognise certain hormones
Enable attachment of the cytoskeleton
10
Q
Describe current model of the cell membrane.
A
The cell membrane is made up of two layers of phospholipid, a phospholipid bilayer.
It is a dynamic living structure, where proteins and lipids penetrating it at various points can move around, known as the fluid mosaic model.
This is able to explain how substances move through the permeable membrane, and hence, into and out of cells.
11
Q
What are lipids/phospholipids?
A
Lipids are molecules that contain only carbon, hydrogen, and oxygen making up fatty acids that join together to form a lipid molecule.
Phospholipids are lipids with a phosphate group. They make up the bilayer of the cell membrane.
The heads (phosphates) are hydrophilic and align to be facing outside, in contact with water.
The tails (lipids) are hydrophobic and form the interior of the membrane, reducing exposure to the water
12
Q
What does cholesterol do to a cell membrane?
A
Cholesterol provides structural support so that the bilayer is more stable and fluidity is reduced - “regulates fluidity of the cell membrane”.
13
Q
There are various types of proteins that are a part of the cell membrane, what are they?
A
Integral proteins - span across both layers of the membrane and are permanently there, used in transport across the membrane.
Peripheral proteins - in one of the two layers and can move and are temporary, used in enzymic reactions.
Glycoproteins - proteins attached to carbohydrates on the surface, allow other molecules (e.g., hormones) to bind.
14
Q
What are glycolipids and what are they for?
A
Glycolipids are lipids with a carbohydrate attached, they recognise other cells of the body.
15
Q
Compare the roles of lipids and proteins in the cell membrane.
A
While lipids help to give membranes their flexibility, proteins monitor and maintain the cell’s chemical climate and assist in the transfer of molecules across the membrane.
16
Q
What are some of the functions of proteins in the cell membrane?
A
Transport materials in and out i.e., channel proteins.
Enzymes speed up reactions along a membrane.
Receptors receive information.
Cell adhesion - can allow cells to stick together.
Cell-cell recognition - allows cells to communicate with each other by proteins attaching.
17
Q
Describe the structure of prokaryotic cells.
A
Very small, <10um
Have simple internal organisation
Have circular and unbound DNA found free-floating in the cytosol in the nucleoid region
Do not have membrane-bound organelles (only have ribosomes which are smaller than those in eukaryotic cells)
Often have flagella (like a tail) for movement and pili to stick to other cells
All prokaryotes are unicellular organisms.
18
Q
Describe the structure of eukaryotic cells.
A
The DNA is condensed around proteins that form linear chromosomes located in the nucleus
Have more complex internal organisation and are generally bigger, 10-100um
Contain membrane-bound organelles, ‘compartmentalised’
19
Q
What do bacteria, archaea, and plant cells have in common?
A
Bacteria, archaea, and plant cells all have outer cell walls along their cell membranes, each have cell walls made of different substances.
20
Q
Describe eukaryotic organisms.
A
Eukaryotic organisms differ greatly from one another in size, shape, colour and complexity
Whilst there is a great diversity among eukaryotic organisms, they are made up of cells that have similar features, including membrane-bound organelles
Eukaryotic organisms can be both unicellular and multicellular
21
Q
Prokaryotic and eukaryotic cells have many features in common, what are some and what does this reflect?
A
They both have a phospholipid cell membrane
A fluid-like matrix filling the cell (cytoplasm and cytosol)
A cytoskeleton structure within the cell
Similar protein synthesis mechanisms (using ribosomes)
The same genetic code system (DNA and RNA)
This is a reflection of their common evolutionary past, showing they have similar ancestors to have similar features.
22
Q
Compare the structure of animal and plant cells.
A
Animal cells are typically round or irregular in shape, where plant cells are typically rectangular or cubic
Animal cells are typically 10-30um, plant cells are typically 10-100um
Plant cells contain larger vacuoles than animal cells
Both have cell membranes, but plant cells also have a cell wall made of peptidoglycan
Due to this, plant cells do not have cholesterol in the cell membrane, where animal cells do
Plant cells have chloroplasts
23
Q
Describe the structure of fungal cells
A
Fungi are saprophyte heterotrophs, they secrete enzymes to breakdown and digest dead or decomposing matter
They do not need chloroplasts to produce energy
Grow as a tangle of long, threadlike hyphae, a branching structure
As more hyphae grow they divide into separate cells by forming septa
Cell wall made from glucans and chitin instead of peptidoglycan (plant cells)
Ergosterol instead of cholesterol (animal cells) in the cell membrane for stabilisation of the membrane
24
Q
Provide some examples of fungal organisms.
A
Yeasts - unicellular fungi
Moulds and mushrooms - multicellular fungi
25
Describe the structure of the nucleus and its function.
The nucleus controls the activities of the cell.
Composed of a double membrane boundary called the nuclear envelope, containing many nuclear pores lined with proteins
It is able to determine which enzymes are made and thus determines which chemical reactions take place, and even what structure the cell will have
When a cell is not dividing, the nucleus has a grainy appearance and its contents are referred to as chromatin (which has no distinctive shape)
26
Describe the structure of the nucleolus and its function.
Inside the nucleus ,there may be one or more circular regions that are very distinguishable from the rest
These are nucleoli, and are not bounded by a membrane
They are composed of DNA and protein and are the site of ribosomal RNA synthesis
27
Describe the structure of the mitochondria and its function.
Sausage-shaped organelle roughly 10um containing an outer and inner membrane
The inner membrane is folded to form structures called cristae
The latter stages of aerobic respiration are carried out inside the mitochondria and a great deal of energy is released from this reaction
This explains why it is often referred to as the powerhouse of the cell, and explains why some cells, such as muscle cells, which require a lot of energy, have abundant mitochondria
Contain their own circular mitochondrial DNA and ribosomes, both of which work to produce proteins for the mitochondria
28
Describe the structure of the chloroplast and its function
They are a type of plastid.
Inside the double membrane there are membranous flattened sacs called thylakoids, some of which are arranged into stacks called grana (singular granum)
The fluid which surrounds the thylakoids is called stroma
The thylakoid membranes contain photosynthetic pigments such as chlorophyll, and their function is to carry out photosynthesis
The first stage requires light energy and occurs in the grana
Enzymes that catalyse the many steps of the process are located in the grana and stroma
Chloroplasts contain their own circular DNA and ribosomes
29
What is so special about mitochondria and chloroplasts?
Both mitochondria and chloroplasts contain their own circular DNA and ribosomes, both of which work to produce necessary proteins
Evidence that they were possibly their own bacteria that functioned on their own and were incorporated into cells at some stage
30
Describe the structure and function of the vacuole
A vacuole is a fluid filled space bounded by a membrane
In it there is a watery solution, with solutes such as salts, simple sugars, and amino acids
It maintains the water and salt balance of the cell
Most mature plant cells have a large, central vacuole that provides structural support
Animal cells tend to have many smaller vacuoles
They may also be used to store the cell's waste products, and some contain pigments
31
Compare vacuoles in plant cells and other eukaryotic cells
Unlike plant cells, other eukaryotic cells do not contain large central vacuoles
Vacuoles found in animal cells are usually much smaller and more numerous
Other cells can have vacuoles such as food vacuoles and contractile vacuoles
Food vacuoles are formed when a cell engulfs a particle by phagocytosis
32
Describe the structure of a vesicle and its function.
A small vacuole is called a vesicle
May be involved in phagocytosis, and with Golgi bodies in exocytosis
There are also specialised vesicles, such as lysosomes
33
Describe the structure of the Golgi body and its function.
The Golgi body is a stack of flattened sacs made of smooth membrane
They are involved in the packaging and secretion of proteins and carbohydrates manufactured by the cell
They are abundant in cells involved in secretion such as gland cells like salivary glands, which secrete saliva
It also manufactures some carbohydrates for use outside the cell
These are packaged into vesicles that bud off from the Golgi body and migrate to the cell membrane where they fuse with it and release their contents
This secretion is an example of exocytosis (exo=out of, cytosis=the cell)
34
What is the endoplasmic reticulum?
The endoplasmic reticulum (ER) is a system of membranes that extends throughout the cytoplasm from the nuclear envelope to the cell membrane
It forms an intricate network of passageways throughout the cell and is important in the transport of materials from one part of the cell to another
35
What is the rough endoplasmic reticulum?
Some endoplasmic reticulum has ribosomes attached to it and this is referred to as rough endoplasmic reticulum, due to its appearance
Rough ER is mainly the site of protein synthesis (and for transport)
36
What is the smooth endoplasmic reticulum?
Endoplasmic reticulum without ribosomes attached to it is called smooth endoplasmic reticulum, and it tends to be involved in metabolic processes such as lipid synthesis and carbohydrate metabolism
It can be thought of as a 'workbench' for many metabolic processes, since certain enzymes are embedded in it
37
Describe the structure of the ribosome and its function
A ribosome is made of RNA and protein
The RNA in ribosomes is a special type, produced in the nucleolus, ribosomal RNA (rRNA)
It is not enclosed by a membrane
It is the site of protein synthesis
38
Describe the structure of the lysosome and its function
A vesicle containing digestive enzymes
In some cells, lysosomes fuse with food vacuoles and release digestive enzymes, which break down the food
Certain cells of our immune system that are involved in engulfing and destroying foreign particles like bacteria also use lysosomes
Lysosomes are also involved in programmed cell death (apoptosis)
39
What is the cytoskeleton? What is it made up of?
The cytoskeleton is a structure that helps cells maintain their shape and internal organisation, whilst also helping them carry out certain functions
All eukaryotic cells have a cytoskeleton which is made up of three main components:
Microfilaments
Intermediate filaments
Microtubules
The filaments and microtubules of the cytoskeleton are made up of subunits which can be rapidly removed or inserted into the existing structure to change its shape - much like the temporary scaffolding at a building site
The cytoskeleton is therefore a dynamic structure
40
What are the main functions of the cytoskeleton?
To give cells their shape
To be involved in cell movement
To hold organelles in place
To strengthen cells
41
Describe the structure and function of microfilaments (cytoskeleton).
Microfilaments, made of the globular protein actin, are involved in intracellular movement like cytoplasmic streaming, chloroplast orientation, the pinching in of the cell membrane to form daughter cells after cell division, and the formation of a food vacuole by the process of phagocytosis
The contraction of muscle is due to one group of microfilaments interacting with one another
Actin proteins are also found in some prokaryotes
42
Describe the structure and function of microtubules (cytoskeleton).
Microtubules, made of the protein tubulin, are an essential part of the flagella and cilia, fine hair-like projections found on many cell membranes
The coordinated beating of these structures causes fluid movement, so that either the cell moves of fluid moves past the cell
Unicellular organisms often use cilia to move around its environment, while epithelial cells in the airways of our lungs use cilia to move mucus out of the lungs
43
Describe the structure and function of intermediate filaments (cytoskeleton).
Intermediate filaments are made up of strong fibrous proteins and they are found in cells, such as skin cells, that are subject to wear and tear
The function of the intermediate filaments is to strengthen these cells and their tissues
44
How does the cytoskeleton affect cell structure?
A human red blood cell is formed into a bi-concave shape, rather than a sphere, by its cytoskeleton, and this gives it a high surface area to volume ratio suitable for transporting oxygen
A nerve cell, with its very long axon, is another example of a cell shape that would be impossible without a cytoskeleton
It enables some cells to have highly specialised structures
45
How are organelles in a cell held in place?
The organelles in the cell are held in place by the cytoskeleton
For example, the chloroplasts in plant cells are held near the surface of the cell with the correct orientation to receive maximum light
46
How is movement in a cell possible?
Movement within a cell is made possible by the cytoskeleton
For example, the spindle apparatus formed to move chromosomes in cell division is a specialised microtubule structure
47
How is the abstract concept of energy defined?
Energy is defined as 'the capacity to do work'
The more energy an animal or plant has, the more work it can do
48
What needs energy and why?
All living things need energy to carry out essential life processes such as movement involving the whole cell or movement within the cell, and synthesis of compounds such as proteins, carbohydrates, lipids and nucleic acids
By performing these tasks cells are able to grow, repair, reproduce, and function normally
49
Where can cells obtain energy from to carry out their life processes?
Cells can obtain energy to carry out their life processes in either physical or chemical form
Some cells can use sunlight, a physical form of energy, while others must take in energy-rich compounds, a chemical form of energy
In both cases the energy that enters the cell is then transformed so that it is useful to the cell
All cells produce heat as part of their chemical activities
50
What are autotrophs?
Autotrophs are organisms (autotrophic cells) that are able to make all the energy-rich compounds they need from inorganic substances
Most use sunlight, a physical form of energy, for the synthesis reaction of photosynthesis, as the Sun is the main source of energy for life on earth
Organisms that photosynthesise are called photosynthetic autotrophs
Chemosynthetic reactions are carried out by some rare single-celled organisms to make food molecules from simple raw materials
51
What are heterotrophs? Example?
Heterotrophs are organisms (heterotrophic cells) that cannot produce all of their organic compounds from simple inorganic substances, and so they rely on other organisms, or their products or remains, for 'food'
An example is the snail obtaining its organic compounds from grass, and the lizard obtains some of its organic compounds from snails. The kookaburra, in turn, obtains some of its organic compounds from the lizard. Ultimately, the kookaburra's organic compounds have come from the grass (an autotroph)
52
Simply compare autotrophs and heterotrophs
Autotrophs make their own organic compounds, whereas heterotrophs must obtain at least some of their organic compounds by feeding
53
What is the formula for photosynthesis?
6CO2 + 6H2O sunight,chlorophyll-> C6H12O6 + 6O2
Carbon dioxide + water sunlight,chlorophyll-> glucose + oxygen
54
What is a transformation of energy?
A transformation of energy is the conversion of a form of energy to another through the formation and breaking of chemical bonds
55
What is photosynthesis and where does it occur?
In photosynthetic eukaryotic cells (plant cells) photosynthesis occurs in chloroplasts, where light energy is used to convert carbon dioxide and water into glucose, a simple sugar
Chlorophyll, the green pigment that absorbs light, is located in the thylakoid membranes of the grana
Enzymes that catalyse the many steps of photosynthesis are found in the grana and stroma
In photosynthetic prokaryotic cells (cyanobacteria) photosynthesis occurs in the cytoplasm which contains the chlorophyll and necessary enzymes
56
When is there a net output of energy in a chemical reaction?
There is a net output of energy in a chemical reaction when the energy in the reactants is greater than the energy in the products
57
How does glucose release energy?
The breakdown of glucose in the presence of oxygen releases energy, as the energy in the reactants is greater than the energy in the products.
This is because the amount of energy required to break the bonds in the glucose and oxygen molecules (reactants) is less than the amount of energy released when the bonds in carbon dioxide and water (products) are formed
Some of this energy released can be used by the cell to do work, while the rest is lost as heat
58
What is ATP? Describe its structure and its formation
ATP, adenosine triphosphate, is one of the most important energy storage compounds in cells, and is a short-term energy storage compound
ATP consists of the nitrogen base adenine bonded to the sugar ribose, which in turn is bonded to three phosphate groups, called a triphosphate
The third phosphate group is held to the others by an unstable bond
The energy needed to produce ATP from ADP and Pi comes from cellular respiration
ADP + Pi -> ATP + water
59
Describe the conversion of ATP to ADP and Pi
When ATP is broken down to form ADP (adenosine diphosphate) and Pi (an inorganic phosphate group), energy is released
This energy can be used by the cell to drive many processes that require energy, such as synthesis, cell movement, endocytosis and exocytosis
ATP -> ADP + Pi + energy
60
Why do cells/organisms need to respire?
All living cells, and hence organisms, need to respire in order to obtain the energy to carry out essential life processes
Cellular respiration provides this energy by breaking down energy-rich organic molecules into lower energy products
61
What are the two types of respiration? Compare them.
The two types of respiration are aerobic respiration and fermentation (anaerobic respiration)
Both reactions use glucose as their starting point, but produce different products
The most efficient of the two is aerobic respiration, which, like many other chemical processes in the cell, consists of a series of small steps - a metabolic pathway
Each step in the pathway is catalysed by a specific enzyme
62
What is the formula for aerobic respiration?
C6H12O6 + 602 -> 6CO2 + 6H20
Glucose + oxygen -> carbon dioxide + water
The reactants and products for photosynthesis and aerobic respiration are the same but reversed, but both undergo different processes to achieve them
63
Describe the process of aerobic respiiration
Aerobic respiration begins with the conversion of glucose to an intermediate compound called pyruvic acid
This metabolic pathway is called glycolysis and it occurs in the cytoplasm
One glucose molecule breaks down to produce two pyruvic acid molecules. The energy of this breakdown produces a net gain of two ATPs
The next two stages occur inside the mitochondria
First, the two molecules of pyruvic acid produced by glycolysis are converted to acetyl coenzyme A (acetyl CoA) which enters the Krebs cycle
The acetyl CoA molecules are then broken down to carbon dioxide and water
The final stage is called phosphorylation, and its function is to add a phosphate group to ADP using the energy released from the pyruvic acid breakdown, thus making the energy-rich molecule ATP
The energy for phosphorylation is obtained from glycolysis and the Krebs cycle in which electrons are transferred from one molecule to another
64
Compare glycolysis in other living things.
All organisms carry out glycolysis in exactly the same way using the same enzymes to catalyse the same steps, evidence that all living things evolved from a common ancestor
65
Summarise aerobic respiration
In summary, glycolysis and the Krebs cycle break down glycose and are energy releasing processes, whilst phosphorylation synthesises ATP and is an energy transferring process
As a result of the completion of the two stages in the mitochondria, enough energy is released to make about 34 more ATPs, thus about 36 ATPs are produced from the aerobic respiration of one glucose molecule
66
What does the mitochondria do in aerobic respiration?
The role of mitochondria is vital in the later stages of aerobic respiration, providing four separate regions for the different reactions to occur:
The outer membrane
The space between the outer and inner membrane
The inner membrane
The space inside the inner membrane
Each of the proteins and enzymes that are necessary is located in one particular region
67
What is fermentation?
Many cells are also able to respire in the absence of oxygen, a process called fermentation or anaerobic respiration
It also uses a fuel such as glucose, but the harvest of ATP is much less than for aerobic respiration, as the fuel is not completely broken down
The products of fermentation depend on the type of cell involved
In plant cells and yeasts, ethanol and carbon dioxide are the products and the process is called alcoholic fermentation
In animals the sole product is lactic acid and the process is called lactic acid fermentation
68
What is the formula of fermentation in plants and yeast?
Ethanol fermentation
C6H12O6 -> 2C2H5OH + 2CO2
Glucose -> ethanol + carbon dioxide
69
What is the formula of fermentation in animals?
Lactic acid fermentation
C6H12O6 -> 2C3H6O3
Glucose -> lactic acid
70
Why is fermentation so significant?
It is of significant commercial importance
Under anaerobic conditions sugars are converted by yeast into ethanol and carbon dioxide
Yeast is also used in baking - the carbon dioxide gas produced causes the product to 'rise', while the oven kills the yeast cells and evaporates the ethanol
71
Compare the processes of aerobic respiration and fermentation
The initial steps of aerobic respiration and fermentation are identical - both begin with glycolysis
However, in fermentation, instead of the pyruvic acid being converted to acetyl coenzyme A, it is converted to either ethanol and carbon dioxide in yeast/some plant cells, or to lactic acid in animal cells
The final conversion occurs in the cytoplasm. Mitochondria are not involved in fermentation, nor is oxygen consumed and there is a net gain of two ATP molecules per glucose molecule
In the presence of oxygen, aerobic respiration occurs and converts the pyruvic acid to acetyl CoA, which is then completely broken down to carbon dioxide and water
Aerobic respiration is about 18 times more effective than fermentation at producing ATP molecules, producing 36 net ATP compared to 2
72
What materials do cells need to survive?
The materials that a cell needs depend on its function
All heterotrophic cells require a source of energy, generally glucose, and most will require an input of oxygen in order to respire aerobically
In order to produce the range of materials needed to sustain life, all cells need a supply of simple inorganic nutrients, such as ions
73
Why must waste be removed from a cell? Examples of substances that are removed?
Metabolic processes in cells produce wastes that have no use and may become toxic to the cell in high concentrations, so must be removed
Depending on the type of cell and their circumstances, substances that need to be removed may include oxygen, carbon dioxide, lactic acid, ethanol, and a variety of ions
74
Inputs and outputs of materials differ between organisms, why?
Inputs and outputs of materials differ between autotrophs and heterotrophs due to variations in their metabolic processes
75
Compare the inputs of autotrophic and heterotrophic cells
Oxygen
AC - for aerobic respiration when rate of respiration exceeds rate of photosynthesis
HC - for aerobic respiration
Carbon dioxide
AC - for photosynthesis when rate of photosynthesis exceeds the rate of respiration
HC - not required
Organic compounds
AC - not required, as they are manufactured by the cell
HC - some are required, as not all can be manufactured by the cell (e.g. glucose)
76
Compare the outputs of autotrophic and heterotrophic cells
Oxygen
AC - from photosynthesis when the rate of photosynthesis exceeds the rate of respiration
HC - no output
Carbon dioxide
AC - from respiration and fermentation when their rate exceeds rate of photosynthesis
HC - from aerobic respiration
Lactic acid
AC - not normally produced
HC - a waste product of fermentation
Ethanol
AC - a product of fermentation
HC - not normally produced
77
The cell membrane must be...
The cell membrane must be selectively permeable in order for the cell to maintain a composition chemically different from the exterior
A cell needs to maintain concentrations within a narrow range if it is to function normally
The plasma (cell) membrane restricts the passage of substances into and out of the cell
It is not equally permeable to all substances
78
What are important factors of a substance to determine its permeability through the cell membrane?
The size and charge of the particle, and whether or not it is lipid-soluble, are important factors
Small uncharged molecules (such as water) are able to pass through the membrane easily, whereas even small charged ions find it difficult to penetrate
79
What plays a part in the selectivity of the plasma membrane?
Transport proteins span the membrane and provide pathways for specific materials
The selectivity of the membrane is due in large part to the types of proteins in the membrane
80
Transport of substances is either ___ or ___. Define them.
Transport of substances is either passive or active
Passive transport is the spontaneous movement of a substance and can take place without an input of energy
Active transport is the movement of a substance in response to an input of energy
81
Categorise the 6 main methods of transport across a membrane.
Passive: diffusion, osmosis, facilitated diffusion
Active: active transport, endocytosis, exocytosis
82
Define diffusion
Diffusion is the overall movement of a substance in a fluid from a region of high concentration of the substance towards regions of lower concentration of the substance
The particles are moving along the concentration gradient
83
When particles move, they tend to spread out. When does this stop?
When particles move they tend to spread out, or diffuse, until they take up all the available space.
When all the particles of a substance in a space are equally dispersed, equilibrium is reached and there is no concentration gradient
84
How does diffusion relate to the cell membrane?
Provided the cell membrane is permeable to a substance, that substance will diffuse across the membrane if a concentration gradient exists.
Examples include the diffusion of oxygen into cells and carbon dioxide out of cells
This is due to the concentration gradient being maintained by the activities of the cell: they continuously use up oxygen and produce carbon dioxide as a result of respiration
It is a passive process that does not require energy
85
Explain facilitated diffusion
Some of the proteins in the cell membrane form special 'channels'
These channel proteins bind to certain ions or molecules and assist them across the membrane, whilst others have no specific protein to help them and so cannot move across
They assist the movement of substances, such as glucose, along the concentration gradient ,from a region of high concentration to a region of low concentration
No energy is required for this passive process.
Examples of substances include ions and proteins (large/charged)
86
What are factors affecting diffusion?
Temperature - higher temperature -> diffuse faster
Surface area - larger surface -> diffuse faster
Concentration gradient - higher gradient -> diffuse faster
Size of particles - smaller particles -> diffuse faster
Diffusion medium - solid slowest, liquid faster, gas fastest
87
What is osmosis (in organisms)?
Osmosis in organisms refers to the net movement (diffusion) of water across a semi-permeable membrane from a region of lower solute concentration towards a region of higher solute concentration.
Cell membranes contain proteins called aquaporins that act as channels for the movement of water molecules
It is a special case of diffusion (hence a passive process)
88
What is the role of the membrane in osmosis?
The role of the membrane in osmosis is to allow water to move across more easily than the solute
89
What is significant about osmosis?
The significance of osmosis to cells is great, since it is the osmotic pressure that maintains the shape of an animal cell and provides support in plant cells
This pressure needs to be just right, for example a human red blood cell will burst if placed in distilled water, or it will shrivel if placed in a concentrated salt solution
90
What solution perfectly balances the concentration of solutes in the cell's cytoplasm?
A solution of 0.9% sodium chloride will exactly balance the concentration of solutes in the (human) cell's cytoplasm, resulting in no net movement of water, no osmosis
Such a solution is known as physiological saline
91
Compare the effect of losing/gaining excess water in animal and plant cells
The result of excess uptake in water in plant cells is slightly less severe than in animal cells, due to the presence of the cell wall
When placed in distilled water, plant cells will not burst, but will swell with water, they are fully turgid
If plant cells lose water, they become flaccid or limp as opposed to shriveling - the plant will wilt and die if not given water
92
Why do diffusion and osmosis not require energy?
Both diffusion and osmosis are passive processes and do not require an input of energy from the cell
This is because materials are moving with the concentration gradient
Active transport, endocytosis, and exocytosis require the expenditure of energy, usually supplied in the form of ATP
93
Describe hyper/hypo/isotonic solutions.
A hypertonic solution is one that contains more dissolved solute than in a cell, and so when a cell is placed in it, water will flow out of the cell.
A hypotonic solution is one that contains less dissolved solute than in a cell, and so when a cell is placed in it, water will flow into the cell.
An isotonic solution is one that contains the same dissolved solute as in the cell
These terms are relative to the cell in the solution
94
Describe active transport
Active transport is a process that is the opposite of diffusion, since substances are moved across the membrane against the concentration gradient and requires energy, usually in the form of ATP
Membrane carrier proteins are involved
Examples of substances transported via active transport are ions and proteins
95
Describe the structure and role of CARRIER/TRANSPORT proteins
The cell is selective about what it transports in or out
Transport proteins are located in the cell membrane and their function is to speed up the movement of a substance across the membrane
They are globular proteins with specific shapes for the substances being transported
In facilitated diffusion CHANNEL proteins speed up the normal process of diffusion, moving with the concentration gradient
In active transport, CARRIER/TRANSPORT proteins move the substance against the concentration gradient
96
What is the general process of endocytosis?
Some cells are able to take in particles or large molecules by enclosing them in a membranous vacuole
97
Describe phagocytosis
One of the types of endocytosis
Phagocytosis involves the intake of solid particles, where the membrane creates a pouch around the particle and encloses it in a vacuole which then breaks away from the cell membrane and enters the cytoplasm
Lysosomes then fuse with the 'food vacuole', which release digestive enzymes which break down the particle
Cells that carry out phagocytosis are selective and will not engulf just any particle
98
What are examples of phagocytic cells?
Examples of cells that are phagocytic include unicellular amoebae, which feed in this way, engulfing specific particles of nutritional value, and certain cells of our immune system that are involved in engulfing and destroying foreign particles like bacteria
99
Explain the process of pinocytosis
Pinocytosis is a similar process on a much smaller scale, and involves the intake of liquids into tiny vesicles that form at the surface of the cell
Pinocytosis may be non-selective, and involve the intake of extracellular fluid, or may be more specific, such as fat droplets in the small intestine
100
Explain the process of exocytosis
Exocytosis is essentially the opposite of endocytosis
Secretion of materials produced by the cell usually involves packing the material into a vesicle, which migrates to the plasma membrane with which it fuses, and then releases its contents to the outside
The manufacture of the material would most likely have occurred in the endoplasmic reticulum, and the packaging is a function of the Golgi body
Example of substance secreted via exocytosis is hormones
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What factors can affect the exchange of materials across membranes?
SA to V ratio, concentration gradients, the physical and chemical nature of the materials being exchanged
102
Why is it thought that cells are so small?
The reason for the microscopic size of most cells is thought to be due to the fact that as their size increases, their surface area to volume ratio decreases.
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Why does the surface area to volume ratio of a cell matter?
As a cell gets bigger, there is proportionally less surface area, and as a result there is a decrease in efficiency with which the cell can exchange materials with its surroundings (engulf surrounding nutrients, secrete waste products).
It may be that the ability of a cell to perform these functions efficiently limits its size
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Define the term concentration gradient
the difference in concentration between two different regions
105
Aside from the SA:V ratio, why else is it thought that cells are so small?
There is also the notion that the DNA, which controls the activities of the cell, may only be able to exert an influence over a finite volume.
This factor would also limit the size to which most cells can grow
106
Compare the concentration gradients of passive and active transport
In diffusion and osmosis, there is no input of energy and the particles move with the concentration gradient from a region of higher concentration of the substance towards a region of lower concentration of the substance, until equilibrium is reached
If particles move across membranes against the concentration gradient from a region of lower concentration of the substance towards a region of higher concentration of the substance, energy is required, such as active transport, endocytosis, and exocytosis
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Describe the nature of exchange materials
Small, uncharged particles diffuse easily through cell membranes while particles that are large or charged require assistance from channel/transport proteins, or even whole sections of the membrane
108
Where do all cells come from?
All cells arise from pre-existing cells.
All cells in a human can be traced back to one cell, the zygote, which divides into many more cells to form organs and systems
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Why do cells divide?
Reproduction - some organisms asexually reproduce
Growth - growing from a fertilised egg to a multi-cellular organism
Repair & renewal - replacing cells that die from normal wear & tear or from injury
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Why does the amount of DNA in a cell double before cell division?
In cell division, each new cell needs to have an exact copy of the DNA in the parent cell
For this to happen, the DNA must replicate itself before the cell divides
This results in double the amount of DNA from before it replicated, ensuring that each new cell receives the same amount of DNA containing the same information
111
Discuss the chromosomes in body cells
The body cells, somatic cells, contain 2 sets of DNA, one set from each parent (two of each chromosome), and is called diploid
Humans have 46 chromosomes, meaning 32 homologous pairs, so our diploid number is 46, but this varies from species to species
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Discuss homologous chromosomes/pairs
Two chromosomes of the same type make up a homologous pair
A cell that contains pairs of homologous chromosomes is called a diploid cell (2n chromosomes)
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Discuss the chromosomes in sex cells
The sex cells (sperm and egg cells which are derived from germ-line cells) only have one chromosome from each homologous pair
These cells are called haploid (n chromosomes)
Two haploid cells fuse to form one diploid cell
To have only half the genetic information, they are formed from diploid cells dividing in meiosis
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How do prokaryotic cells reproduce?
Prokaryotic cells asexually reproduce through binary fission
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What is asexual reproduction?
Asexual reproduction is the process of forming offspring without fertilisation
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Describe the process of binary fission
The DNA is replicated, forming two circular chromosomes, which are attached separately by proteins to the cell membrane near the middle of the cell
The cell then elongates, increasing the amount of cell membrane between the chromosomes, separating them
The cell then pinches across its equator, dividing to form two new and identical cells
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Explain the role of mitosis in asexual reproduction
Many eukaryotic organisms reproduce asexually
Asexual reproduction in eukaryotic organisms involves mitotic division (as opposed to meiotic), since the genetic material needs to be replicated for each new cell to have the same amount
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Where is mitosis in the cell cycle?
Mitosis is part of the cell cycle of eukaryotic cells, and is where the DNA of the parent cell is divided equally between two daughter cells
Before mitosis, the cell cycle includes preparation to grow and DNA replication (interphase)
Following mitosis, cytokinesis occurs, where the cytoplasm and its contents split evenly between the two daughter cells
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State the four phases of mitosis
Prophase, metaphase (ZUCKERBERGPHASE), anaphase, telophase (PMAT)
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Explain what occurs in prophase
In prophase, the chromatin condense into visible chromosomes so that they can be easily separated
Each DNA strand has been replicated so that each chromosome consists of two identical sister chromatids, joined together at the centromere
The nuclear envelope also disintegrates
The spindle apparatus also begins to form, with a centrosome at each pole of the cell (made up of two centrioles), extending spindle fibres, both of which are made of microtubules
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Explain what occurs in metaphase
During metaphase, the chromosomes line up along the equator of the cell, and spindle fibres attach to their centromeres
The sister chromatids are arranged so that they face opposite poles
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Explain what occurs in anaphase
In anaphase, the sister chromatids are pulled apart to opposite poles using the spindle fibres and protein 'motors' called kinesins
Their centromeres are divided and the sister chromatids appear V-shaped
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Explain what occurs in telophase
In telophase, the DNA begins to decondense back into chromatin
Two new nuclear envelopes form around the two sets of DNA
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What occurs after mitosis?
After mitosis is cytokinesis, where the cytoplasm and its contents are divided between the two new cells
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Compare the products of binary fission and mitosis
The cells formed through binary fission and mitotic division are genetically identical to each and their parent
This is because the genetic material is replicated and each daughter cell receives an identical copy
126
What is the cell cycle?
The cell cycle refers to the alternating stages of cell division and enlargement
it is comprised of three stages:
Interphase - which is broken down into 3 phases
Mitosis - which has 4 phases
Cytokinesis
It also includes 3 checkpoints
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What is the cell cycle control system?
The cell cycle control system is a cyclically operating set of molecules that trigger key events
128
When will cells divide?
Cells will only divide when environmental conditions are favourable
These five conditions must be met for a cell to divide: contact inhibition, anchorage dependence, nutrient dependence, surface area to volume ratio, and presence of growth factors
129
Explain the term contact inhibition
An external factor that determines cell division is contact inhibition.
Normal cells stop dividing when they come in contact with other cells, such as a cut, where skin cells will divide until the wound it closed and they touch
Cancer cells will continue to grow, piling up on each other
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Explain the term anchorage dependence
An external factor that determines cell division is anchorage dependence
Cells must be anchored to a surface - either to the surface of a culture dish or the extracellular matrix of a tissue
Cancer cells can lose their anchorage dependence and can travel to new locations
131
Explain the term nutrient dependence
An external factor that determines cell division is nutrient dependence
Cells must have enough nutrient reserves to undergo mitosis
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How is the surface area to volume ratio relevant to cell division?
The SA:V ratio of a cell is an external factor that determines cell division
A small SA:V ratio stimulates division
Cells divide to maintain a workable ratio
133
What are growth factors?
Growth factors are external factors that determine cell division, as a cell will divide only when told to do so by growth factors
They are molecules such as proteins and hormones secreted by other cells and they bind to specific cell receptors, inducing a change in proteins in the cytoplasm
These are relay proteins, which relay the signal from the growth factors to the cell cycle control mechanism
134
What are the phases of interphase?
Interphase is not a period of resting, but rather preparation for mitosis
It can be divided into 3 phases: G1, S, and G2
G1 and G2 are growth phases, where the cell gets bigger, produces proteins and organelles, and stores up nutrients for cell division
S stands for synthesis, where DNA is replicated
G0 is a phase were a cell does not prepare for cell division, which many differentiated cells remain in such as mature muscle and nerve cells
135
What are the cell cycle checkpoints?
The cell cycle includes 3 checkpoints, two in interphase and one in mitosis (restriction point R, G2 checkpoint, and M checkpoint)
The cell cycle is halted and cannot proceed until a specific signal is received by the cell cycle control mechanism
136
What is the first checkpoint in the cell cycle?
The first checkpoint is checkpoint 1, or restriction point R
It occurs at the end of the G1 phase
The cell is checked for damage to DNA, cell size, nutrients and growth factors
If conditions are inadequate, the cell will move to the G0 phase, a non-dividing state
137
What is the second checkpoint of the cell cycle?
The second checkpoint is the G2 checkpoint
It occurs during the G2 phase
In it, it is made sure that the DNA has been replicated properly during the S phase, and that there are two complete sets (no more, no less)
It also checks that all proteins and growth factors and resources are in place for mitosis
138
What is the third checkpoint of the cell cycle
The third checkpoint is the M checkpoint, or mitosis checkpoint
It occurs at the end of metaphase, checking the integrity of the spindle apparatus and making sure the sister chromatids are attached and correctly oriented before being pulled apart in anaphase
139
What determines movement through the cell cycle?
Movement through the cell cycle is due to internal factors and external factors (i.e., DNA damage & protooncogenes internal, hormones and growth factors external)
140
What are cyclins and when are they produced?
Cyclins are regulatory proteins who level cycles (goes up and down) in the cell
They are produced during the S and G2 phases, but fall after mitosis (specifically after metaphase, before M checkpoint) as they are used up
141
What are CDKs?
CDKs - cyclin-dependent-kinases
Enzymes which have a constant level in the cell (as they can be reused)
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How do cyclins and CDKs interact?
Cyclin binds to CDK, activating it and forming a CDK-cyclin complex, or MPF, mitosis promoting factor
CDK + cyclin -> MPF
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When does MPF activity peak?
MPF activity peaks when cyclin concentration peaks, as this is when the most MPF complexes are formed
Hence, this is at mitosis, when cyclin concentration peaks
144
What do MPFs do?
MPFs activate proteins required for mitosis, allowing the cell to pass through the cell cycle
The presence of MPF controls the cell's passage through the cell cycle
145
What is the life cycle?
The life cycle is the sequence of stages leading from the adults in one generation to the adults in the next generation
146
What is sexual reproduction? How many organisms does it involve?
Sexual reproduction is reproduction involving the fusion of a male and female gamete in a process called fertilisation, forming a zygote, containing two sets of chromosomes
Sexual reproduction can involve just one organism, for example the pollen from a flower fertilising an ovum of the same flower
In many species, it involves two parents that produce gametes that fuse
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What are the advantages of sexual reproduction? (As opposed to reproducing asexually)
Sexual reproduction produces offspring that show genetic variability, so if environmental conditions were to suddenly change, a species would have an evolutionary advantage, increasing its chance of survival
Another advantage of sexual reproduction is that it masks the effect of harmful recessive genes by obtaining DNA from two sources
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What is the purpose of meiosis? What does it involve?
The purpose of meiosis is to produce haploid gametes
When these gametes combine in fertilisation, the diploid number is restored (n + n=2n)
If they were formed by mitosis, they would be diploid, fusing to produce a cell with double the diploid number, which would increase with each successive generation
Meiosis involves two consecutive cell divisions, called meiosis I and meiosis II
Both follow the same steps of nuclear division as mitosis
149
State the steps of meiosis I and II
Meiosis I:
Prophase I, metaphase I, anaphase I, telophase I (then cytokinesis)
Meiosis II:
Prophase II, metaphase II, anaphase II, telophase II (then cytokinesis)
150
Describe what occurs in prophase I
The chromatin condenses to form visible chromosomes
The nuclear membrane breaks down
The centrioles move to the poles and extend their spindle fibres
Unlike mitosis, the chromosomes pair up in homologous pairs in a process called synapsis and begin crossing over
151
What happens when chromosomes pair up in homologous pairs in prophase I? What is the importance of this?
Unlike mitosis, the chromosomes in prophase I pair up in homologous pairs in a process called synapsis
These structures are called bivalents and contain four chromatids, since each chromosome has two sister chromatids
Corresponding sections of non-sister chromatids then swap sections as they touch, break, and rejoin
These locations at which it occurs are called chiasmata (singular chiasma)
Crossing over plays an important role in increasing the genetic variability of the offspring by alternating the combination of genes that is passed on
152
Describe what occurs in metaphase I
The chromosomes line up at the equator in homologous pairs
These pairs line up independently of other pairs, orienting themselves randomly with some maternal chromosomes facing one way and the rest the other
This is known as independent assortment and increases the genetic variability in the offspring further by increasing the number of possible combinations of maternal and paternal chromosomes in gametes
Spindle fibres attach to their centromeres
153
Describe what occurs in anaphase I
The homologous pairs are separated, pulling the individual double-stranded chromosomes to opposite poles of the cell via spindle fibres and protein 'motors' called kinesins
154
Describe what occurs in telophase I
The spindle fibres break down
The chromosomes uncoil
Two nuclear envelopes begin to form
The cell then divides (cytokinesis), forming two haploid cells with double-stranded chromosomes, known as the reduction division as the number of chromosomes is reduced
155
Describe what occurs in prophase II
After cytokinesis of meiosis I, spindle apparatus begin to form and the chromosomes condense
This division looks like mitosis but there are no homologous pairs
156
Describe what occurs in metaphase II
The haploid number of chromosomes line up at the equator
The spindle fibres attach to their centromeres
157
Describe what occurs in anaphase II
The sister chromatids are pulled apart at the centromere by spindle fibres and protein 'motors' called kinesins
The chromatids are pulled to opposite poles of the cell
158
Describe what occurs in telophase II
Nuclear membranes form around the four new cells
Cytokinesis occurs, resulting in four haploid cells, each with one set of chromosomes
159
Compare the products of mitotic and meiotic division
Mitosis produces diploid cells with 2 sets of chromosomes (2n), meiosis haploid cells with 1 set (n)
Mitosis produces chromosomes that are identical, meiosis chromosomes are different
Mitosis products have variation only due to mutation, whereas
products of meiosis experience variation due to mutation, crossing over, and independent assortment
Mitosis products can divide further, meiosis products cannot
Mitosis produces 2 cells, meiosis 4
160
Compare the sources and degree of genetic variation of the products of asexual and sexual reproduction
In asexual reproduction, which involves mitotic division, the only source of genetic variation is mutation
In sexual reproduction, genetic variation not only results from mutation, but also crossing over and independent assortment during meiosis, and the random fertilisation of gametes
Any male gamete can fuse with any female gamete of the same species
There is much greater variation in the products of sexual reproduction than in the products of asexual reproduction
161
What is cancer?
Cancer is the uncontrolled division of cells caused by a breakdown of the mechanisms which regulate the cell cycle
162
When is the cell cycle no longer controlled (cancer)?
When normal genes which control the cell cycle mutate, the cell cycle is no longer controlled
Proto-onco genes are genes which code for proteins which control cell division, and can be converted into cancer-promoting oncogenes
Oncogenes allow for cell cycle checkpoints to be overcome by removing the need for growth factors or producing their own, resulting in the state of uncontrolled cell division (cancer)
Tumour-suppressing genes also regulate the cell cycle and can be mutated
163
What are factors that promote mutations which result in oncogenes?
Factors that promote mutations which result in oncogenes are called carcinogens
Carcinogens include:
Chemical carcinogens which cause changes to DNA sequences
Radiation, especially X-rays and UV radiation which causes breaks in DNA
Viruses that add foreign DNA sequences to the host DNA
164
What do carcinogens promote?
Carcinogens are factors that promote mutations which result in oncogenes
165
What do all cancers do?
All cancers shut down p53 activity
The p53 gene is a tumour suppressing gene
The p53 gene produces p53 proteins, which halt cell division if it detects damaged DNA
It forces the cell into the G0 phase until the damage is repaired
If the DNA is beyond repair, it triggers programmed cell death, apoptosis
166
What is cell culturing?
Cell culturing involves the growing of cells and tissues in a nutrient medium
167
Cell cultures are used for various reasons, such as?
Research into cells without the surrounding tissue
Reduce animal usage for drug and cosmetic testing
Production of goods such as vaccines and hormones (i.e., insulin)
Studying the effects of drugs and toxins
Studying mutations and carcinogens
Genetics: study many generations in a short time
Producing plant clones/hybrids
168
How are animal cell cultures generally produced?
The tissue of interest is dissected to expose cells of the desired type
A protein digesting enzyme is used to separate the cells from each other
The desired cells are placed onto a nutrient medium, agar, which contains the necessary sugars, minerals etc. for growth and provides anchorage
The number of cells put on the plate must be optimised
Hormones or growth factors are added to help the cells move through the cell cycle
Suitable conditions must be maintained too, including sterility, pH, temperature, and aeration
169
How are plant cell cultures generally produced?
A sample of cells is dissected and washed with alcohol or bleaching agent to remove any contaminating microbes such as bacteria
These cells are then placed in a solution containing minerals, plant growth hormones, and a supply of an energy-rich chemical such as glucose
Suitable conditions must be maintained too, including sterility, pH, temperature, and aeration
170
What is accuracy in an experiment?
Accuracy refers to how close a measurement is to the true or accepted value
171
What is precision in an experiment?
Precision refers to how close measurements are to each other
172
What is validity in an experiment?
Validity refers to the accuracy of a measure, whether the results represent the true value or not
173
What is reliability in an experiment?
Reliability refers to the consistency (precision) of a measure, whether the results can be reproduced under the same conditions
174
What are random errors?
Random errors usually result from human and accidental errors and affect the data differently every time
Accidental errors are changes in environmental condition such as temperature and humidity fluctuating
Human errors include miscalculations, improper techniques, etc
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What are systematic errors?
Systematic errors often result from defects in equipment and affect all the data in the same way
