Topic 1 Cell Biology

Question 1

State the cell theory.

1.1

Answer 1

1. All living things are composed of cells (or cell products)
2. The cell is the smallest unit of life
3. Cells only arise from pre-existing cells

Understanding: Living organisms are composed of cells.

Question 2

State how magnification can be calculated.

1.1

Answer 2

Magnification = Size of image/ actual size

Skill: Calculation of the magnifcation of drawings and the actual size of structures shown in drawings or micrographs.

Question 3

Outline how striated muscle fibres are atypical to the cell theory.

1.1

Answer 3

• Muscle cells fuse to form fibres that may be very long (>300mm)
• Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane
• Challenges the idea that cells always function as autonomous units

Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.

Question 4

Outline how aseptate fungal hyphae
is atypical to the cell theory.

1.1

Answer 4

• Fungal hyphae are very large with many nuclei and a continuous cytoplasm
• The cytoplasm is continuous along the hyphae with no end cell wall or membrane
• challenges the idea that a cell is a single unit

Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.

Question 5

Outline how giant algae
is atypical to the cell theory.

1.1

Answer 5

• Gigantic in size
• Complex in form (compartmentalized)
• The single nucleus is located in the rhizoid
• Challenges the idea that larger organisms are always made of many microscopic cells

Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.

Question 6

Outline the seven functions of life carried out by unicellular organisms.

1.1

Answer 6

1. Nutrition (obtaining food, to provide energy and the materials needed for growth.)
2. Metabolism (chemical reactions inside the cell, including cell respiration to release energy.)
3. Growth (an irreversible increase in size.)
4. Response (the ability to react to changes in the environment.)
5. Excretion (getting rid of the waste products of metabolism.)
6. Homeostasis (keeping conditions inside the organism within tolerable limits.)
7. Reproduction (producing offspring either sexually or asexually.)

Understanding: Organisms consisting of only one cell carry out all
functions of life in that cell.

Question 7

Outline the importance of a large SA:V ratio.

1.1

Answer 7

Large SA:V ratio
* Cell can be more efficient
* More SA to supply each unit of volume
* Shorter diffusion pathway

Understanding: Surface area to volume ratio is important in the limitation
of cell size.

Question 8

Outline how the SA:V ratio is maximised in cells.

1.1

Answer 8

• Cells divide rather than grow bigger (also allows cells to specialise)
• Folded membranes are used to increase SA (recall the folds in mitochondria)
• Folded tissues to increase SA (recall the villi in the small intestine)

Understanding: Surface area to volume ratio is important in the limitation
of cell size.

Question 9

Outline how the seven functions of life are carried out in a paramecium.

1.1

Answer 9

• Nutrition: feeds on smaller organisms by ingesting and digesting them in vesicles by endocytosis
• Growth: Increases in size by accumulating minerals and organic matter from food
• Response: reacts to stimuli with cillia to move
• Excretion: expels waste products of metabolism through plasma membrane
• Metabolism: produces enzymes for chemical reactions in cytoplasm
• Homeostasis: keeps internal conditions within limits (contractile vacuoles)
• Reproduction: mitosis

Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.

Question 10

Outline how the seven functions of life are carried out in a chlamydomonas.

1.1

Answer 10

• Nutrition: photosynthesis using chloroplast
• Growth: Increases in size by photosynthesis
• Response: reacts to light and moves towards it (uses flagella)
• Excretion: expels waste products of metabolism (ex. oxygen from photosynthesis diffused out of cell)
• Metabolism: produces enzymes for chemical reactions in cytoplasm
• Homeostasis: keeps internal conditions within limits (contractile vacuoles)
• Reproduction: mitosis

Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.

Question 11

Define “emergent properties”.

1.1

Answer 11

The characteristics of the whole organism, including the fact that it is alive, are known as emergent properties.
* Emergent properties arise from the interaction of the component parts of a complex structure.
* We sometimes sum this up with the phrase: the whole is greater than the sum of its parts.

Understanding: Multicellular organisms have properties that emerge from
the interaction of their cellular components.

Question 12

Explain why cells become specialised.

1.1

Answer 12

**By becoming specialized, the cells in a tissue can carry out their role more effciently **than if they had many different roles.
* They can develop a specialized structure/enzymes to support their function.

Understanding: Specialized tissues can develop by cell differentiation in
multicellular organisms.

Question 13

Outline how differentiation occurs in terms of the expression of genes.

1.1

Answer 13

When a gene is “switched on” the information is used to create a protein or other gene product.
* Development of a cell involves switching on particular genes and expressing them, but not others.
* Cell differentiation happens because a different sequence of genes is expressed in different cell types.
* The control of gene expression is the key to development.

Understanding: Differentiation involves the expression of some genes and
not others in a cell’s genome.

Question 14

Define “stem cells.”

1.1

Answer 14

Stem cells are unspecialised cells that have two key qualities:

1. Self Renewal – They can continuously divide and replicate
2. Potency – They have the capacity to differentiate into specialised cell types

Understanding: The capacity of stem cells to divide and diferentiate along diferent pathways is necessary in embryonic development. It also makes stem cells suitable for therapeutic uses.

Question 15

Outline the levels of potency in stem cells.

1.1

Answer 15

• Totipotent = can differentiate into any type of cell.
• Pluripotent = can differentiate into many types of cell.
• Multipotent = can differentiate into a few closely-related types of cell (e.g. blood stem cells can make RBCs, WBCs, platelets)
• Unipotent = can regenerate but can only differentiate into their associated cell type (e.g. liver stem cells can only make liver cells).

Understanding: The capacity of stem cells to divide and diferentiate along diferent pathways is necessary in embryonic development. It also makes stem cells suitable for therapeutic uses.

Question 16

Outline the use of stem cells to treat Stargardt’s disease.

1.1

Answer 16

• An inherited form of juvenile macular degeneration that causes progressive vision loss to the point of blindness
• Caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate
• Treated by replacing dead cells in the retina with functioning ones derived from stem cells

Application: Use of stem cells to treat Stargardt’s disease and one other named condition.

Question 17

Outline the use of stem cells to treat Leukemia.

1.1

Answer 17

• Cancer of the blood or bone marrow, resulting in abnormally high levels of poorly-functioning white blood cells.
• Needle is used to remove fluid from bone marrow.
• Adult stem cells are extracted from this fluid and are kept frozen.
• Chemotherapy is performed to kill all cancer cells.
• Stem cells are re-established in bone marrow and produce blood cells again.

Application: Use of stem cells to treat Stargardt’s disease and one other named condition.

Question 18

Outline embryonic stem cells as source of stem cells.

1.1

Answer 18

• Almost unlimited growth potential.
• Can differentiate into any type in the body.
• More risk of becoming tumour cells than with adult stem cells
• Less chance of genetic damage due to the accumulation og 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, unless only one or two cells are taken.

Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.

Question 19

Outline adult stem cells as source of stem cells.

1.1

Answer 19

• Difficult to obtain as there are very few of them and they 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 tissues
• Removal of stem cells does not kill the adult from which the cells are taken.

Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.

Question 20

Outline cord blood stem cells as source of stem cells.

1.1

Answer 20

• Easily obtained and stored.
• Commercial collection and storage services already available.
• Fully compatible with the tissues of the adult that grows from the baby, so no rejection problems occur.
• Limited capacity to differentiate into different cell types (only naturally develop into blood cells)
• Limited quantities of stem cells from one baby’s cord.
• The umbilical cord is discarded whether or not stem cells are taken from it.

Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.

Question 21

Outline the benefits of electron microscopes.

1.2

Answer 21

• They have a much higher range of magnification (can detect smaller structures)
• They have a much higher resolution (can provide clearer and more detailed images)

Understanding: Electron microscopes have a much higher resolution
than light microscopes.

Question 22

Outline the ultrastructures of prokaryotes.

1.2

Answer 22

Prokaryotes are organisms whose cells lack a nucleus and are not compartmentalised.
* * Cytoplasm
* Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore)
* Plasmids
* 70S Ribosomes
* Cell membrane
* Cell wall
* Flagella – enables movement (singular: flagellum)
* Pili – enable adherence to surfaces or mediate bacterial conjugation (sex pili)

Understanding: Prokaryotes have a simple cell structure without compartments.

Question 23

Outline the process of binary fission in prokaryotes.

1.2

Answer 23

Cell division in prokaryotic cells is called binary fssion and it is used for asexual reproduction.
1. The single circular chromosome is replicated
2. The two copies of the chromosome move to opposite ends of the cell
3. Division of the cytoplasm of the cell
4. Each of the daughter cells contains one copy of the chromosome so they are genetically identical.

Understanding: Prokaryotes divide by binary fssion.

Question 24

Outline the advantages of compartmentalized.

1.2

Answer 24

1. Efficiency of metabolism - enzymes and substrates can localized and much more concentrated
2. Localised conditions - pH and other such factors can be kept at optimal levels. The optimal pH level for one process in one part of the cell
3. Toxic / damaging substances can be isolated, e.g. digestive enzymes (that could digest the cell itself) are stored in lysosomes
4. Numbers and locations of organelles can be changed dependent on the cell’s requirements.

Understanding: Eukaryotes have a compartmentalized cell structure.

Question 25

Outline the difference between eukaryotic and prokaryotic cells.

1.2

Answer 25

• Cytoplasm is divided into compartments by single or double membranes to form organelles in eukaryotic cells
• eukaryotes have a nucleus whereas prokaryotes do not

Understanding: Eukaryotes have a compartmentalized cell structure.

Question 26

Outline the nucleus.

1.2

Answer 26

• Structure: Double membrane structure with pores; contains an inner region called a nucleolus
• Function: Stores genetic material (DNA) as chromatin, where DNA is replicated and transcribed to form mRNA

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 27

Outline the Endoplasmic Reticulum.

1.2

Answer 27

• Structure: consists of flattened membrane sacs, called cisternae. Attached to the outside of these cisternae are ribosomes
• Function: synthesize protein for secretion from the cell

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 28

Outline the golgi apparatus.

1.2

Answer 28

• Structure: An assembly of vesicles and folded membranes located near the cell membrane
• Function: processes proteins brought in vesicles from the rER

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 29

Outline the lysosome.

1.2

Answer 29

• Structure: spherical with a single membrane. They are formed from Golgi vesicles. They contain high concentrations of protein, which makes them densely staining in electron micrographs.
• Function: contain digestive enzymes, used to break down ingested food in vesicles or break down organelles in the cell

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 30

Outline the mitochondrion/mitochondria.

1.2

Answer 30

• Structure: double membraned, inner membrane forms cristae.
• Function: They produce ATP for the cell by aerobic cell respiration.

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 31

Outline free ribosomes.

1.2

Answer 31

• Structure: appear as dark granules in the cytoplasm (80S)
• Function: synthesize protein

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 32

Outline the chloroplast.

1.2

Answer 32

• Structure: double membraned, stacks of thylakoids, which are fattened sacs of membrane.
• Function: produce glucose, Starch grains may be present inside chloroplasts if they have been photosynthesizing rapidly.

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 33

Outline vacuoles and vesicles.

1.2

Answer 33

• Structure: single membrane with fluid inside
• Function: vacuoles (absorb foods from outside and digest them inside vacuoles), vesicles (small vacuoles used to transport materials inside the cell)

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 34

Outline microtubules and centrioles.

1.2

Answer 34

microtubules
* Structure: small cylindrical fibres
* Function: moving chromosomes during cell division

centrioles
* Structure: two groups of nine triple microtubules
* Function: form an anchor point for microtubules during cell division

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 35

Outline cilia and flagella.

1.2

Answer 35

• Structure: whip-like structures projecting from the
  cell surface, flagella (one and larger), cilia (many and smaller)
• Function: used for locomotion

Draw the ultrastructure of eukaryotic cells based on electron micrographs.

Question 36

Outline the structure and function of exocrine gland cells.

1.2

Answer 36

Exocrine gland cells in the pancreas secrete digestive enzymes into a duct that carries them to the small intestine where they digest foods.
* a lot of ribosomes, mitochondria, and rER
* needed to created proteins (enzymes) for its function
* plasma membrane, mitochondrion, nucleus, rough ER, golgi apparatus, vesicles, lysosomes

Application: The structure and function of organelles within exocrine gland cells of the pancreas.

Question 37

Outline the structure and function of a palisade mesophyll cell.

1.2

Answer 37

The cell type that carries out most photosynthesis in the leaf is palisade mesophyll.
* chloroplast, cell wall, plasma membrane, mitochondrion, vacuole, nucleus
* photsynthesizes thus needs chloroplast

Application: The structure and function of organelles within palisade mesophyll cells ofthe leaf.

Question 38

Outline the structure of phospholipids.

1.3

Answer 38

• Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
• Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
• Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic

Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Question 39

Outline the arrangement in membranes of phospholipids.

1.3

Answer 39

• When phospholipids are mixed with water the phosphate heads are attracted to the water but the hydrocarbon tails are attracted to each other, but not to water.
• Become arranged into double layers:
• Hydrophobic hydrocarbon tails facing inwards towards each other and the hydrophilic heads facing the water on either side

Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Question 40

Outline the evidence that led to theDavson-Danielli model.

1.3

Answer 40

This model showed a bilayer of phospholipids in the center with proteins on both sides.
1. Calculated that there were twice as many phospholipids than would be needed for monolayer (must be a bilayer)
2. electron micrographs showed “railroad track” appearance (Proteins appear dark and phospholipids appear light)

Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

Question 41

Outline the problems of the Davson-Danielli model.

1.3

Answer 41

Assumptions:
* All membranes were the same thickness and would have a constant lipid-protein ratio
* All membranes would have symmetrical internal and external surfaces

Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model

Question 42

Outline the evidence that led to the falsification of the Davson-Danielli model.

1.3

Answer 42

1.Freeze-etched electron micrographs
* Fracture reveals an irregular rough surface inside the phospholipid bilayer
* Refutes the point that they are only found on the outside of the membrane

2.Structure of membrane proteins
* Proteins extracted found to be different in size and shape
* Refutes uniform/continuous layer hypothesis
* Proteins found to all have hydrophobic section )would embed in “tails”)
* Refutes proteins found only on outer layer

3.Fluorescent antibody tagging
* The membrane proteins of some cells were tagged with red markers and other cells with green markers.
* The cells were fused together. Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.
* Membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer.

Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model

Question 43

Outline the functions of membrane proteins.

1.3

Answer 43

1. Hormone binding sites
2. Immobilized enzymes with the active site on the outside
3. Cell adhesion to form tight junctions between groups of cells in tissues and organs.
4. Cell-to-cell communication (receptors for neurotransmitters)
5. Channels for passive transport to allow hydrophilic particles across by facilitated difusion.
6. Pumps for active transport which use ATP to move particles across the membrane.

Understanding: Membrane proteins are diverse in terms of structure,
position in the membrane and function.

Question 44

Outline the structures of membrane proteins.

1.3

Answer 44

Integral proteins
* hydrophobic on at least part of their surface
* embedded in the membrane
* can be transmembrane

Peripheral proteins
* hydrophilic
* not embedded in the membrane
* Most of them are attached to the surface of integral proteins
* Some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the membrane surface.

Glycoproteins
* have sugar units attatched to the outer surface of the membrane

Understanding: Membrane proteins are diverse in terms of structure,
position in the membrane and function.

Question 45

Outline how the protein content varies depending on the function of the cell.

1.3

Answer 45

• Myelin sheath around nerve cells insulate (18% protein content)
• Most plasma membranes (50%)
• Mitochondria (cell respiration) and chloroplasts (photosynthesis) –>75%

Understanding: Membrane proteins are diverse in terms of structure,
position in the membrane and function.

Question 46

Outline where cholesterol is located in animal cell membranes.

1.3

Answer 46

• Cholesterol is an amphipathic molecule (like phospholipids), meaning it has both hydrophilic and hydrophobic regions
• Cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids
• The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails

Understanding: Cholesterol is a component of animal cell membranes.

Question 47

Outline the importance of membrane fluidity.

1.3

Answer 47

• Fluid enough that the cell can move
• Fluid enough that the required substances can move across the membrane
• If too fluid however the membrane could not effectively restrict the movement of substances across itself

Application: Cholesterol in mammalian membranes reduces membrane fuidity and permeability to some solutes.

Question 48

Outline cholesterol’s role in membrane fluidity.

1.3

Answer 48

• Reduces membrane fluidity by restricting motion of phospholipids molecules
• Disrupts tight packing of hydrophobic tails in the bilayer, increase flexibility by preventing the tails from crystallising and hence behaving like a solid.
• Reduces permeability to some solutes (hydrophilic)
• Helps membranes curve into a concave shape (formation of vesicles during endocytosis)

Application: Cholesterol in mammalian membranes reduces membrane fuidity and permeability to some solutes.

Question 49

Outline how the fluidity of the membrane allows for endocytosis and exocytosis.

1.4

Answer 49

The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids

This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent

Understanding: The fuidity of membranes allows materials to be taken
into cells by endocytosis or released by exocytosis.

Question 50

Outline the process of endocytosis.

1.4

Answer 50

1. Part of the plasma membrane is pulled inwards
2. A droplet of fluid becomes enclosed when a vesicle is pinched off
3. Vesicles can then move through the cytoplasm carrying their contents

Understanding: Vesicles move materials within cells.

Question 51

Outline the process of exocytosis.

1.4

Answer 51

1. Proteins are synthesized by ribosomes and then enter the rough endoplasmic reticulum
2. Vesicles bud off from the rER and carry the proteins to the Golgi apparatus
3. The golgo apparatus modifies the proteins
4. Vesicles bud off from the Golgi apparatus and carry the modified proteins to the plasma membrane
5. Vesicles fuse with the plasma membrane
6. The contents of the vesicle are expelled
7. The membrane then flattens out again

Understanding: Vesicles move materials within cells.

Question 52

Outline the process of endocytosis.

1.4

Answer 52

1. Part of the plasma membrane is pulled inwards
2. A droplet of fluid becomes enclosed when a vesicle is pinched off
3. Vesicles can then move through the cytoplasm carrying their contents

Understanding: Vesicles move materials within cells.

Question 53

Outline simple diffusion.

1.4

Answer 53

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration
* This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
* Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol)

Understanding: Particles move across membranes by simple diffusion,
facilitated diffusion, osmosis and active transport.

Question 54

Outline factors affecting the rate of diffusion.

1.4

Answer 54

• Temperature (affects kinetic energy of particles in solution)
• Molecular size (larger particles are subjected to greater resistance within a fluid medium)
• Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

Understanding: Particles move across membranes by simple diffusion,
facilitated diffusion, osmosis and active transport.

Question 55

Outline facilitated diffusion.

1.4

Answer 55

Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein
* from an area of high concentration to an area of low concentration through a selectively permeable membrane with the help of protein channels.

Understanding: Particles move across membranes by simple difusion,
facilitated difusion, osmosis and active transport.

Question 56

Outline osmosis.

1.4

Answer 56

The passive net movement of water molecules from an area of low [SOLUTE] to high [SOLUTE] through a selectively permeable membrane.

Understanding: Particles move across membranes by simple difusion,
facilitated difusion, osmosis and active transport.

Question 57

Outline the three types of solutions.

1.4

Answer 57

• Solutions with a relatively higher osmolarity are categorised as hypertonic (high solute concentration ⇒ gains water)
• Solutions with a relatively lower osmolarity are categorised as hypotonic (low solute concentration ⇒ loses water)
• Solutions that have the same osmolarity are categorised as isotonic (same solute concentration ⇒ no net water flow)

Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

Question 58

Outline how osmolarity can be estimated.

1.4

Answer 58

Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

Question 59

Outline the process of active transport.

1.4

Answer 59

• Active transport uses energy to move molecules against a concentration gradient
• This energy may either be generated by the hydrolysis of ATP
• Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

1. A specific solute will bind to the protein pump on one side of the membrane
2. The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
3. The solute molecule is consequently translocated across the membrane (against the gradient) and released

Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport

Question 60

Outline the sodium-potassium pump.

1.4

Answer 60

1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
2. A phosphate group is transferred to the pump via the hydrolysis of ATP
3. The pump undergoes a conformational change, translocating sodium across the membrane
4. The conformational change exposes two potassium binding sites on the extracellular surface of the pump
5. The phosphate group is released which causes the pump to return to its original conformation
6. This translocates the potassium across the membrane, completing the ion exchange

Application: Structure and function of sodium-potassium pumps for active transport.

Question 61

Outline the potassium channels in axons.

1.4

Answer 61

1. At one stage during a nerve impulse there are relatively more positive charges inside.
2. This voltage change causes potassium channels to open, allowing potassium ions to diffuse out of the axon.
3. Once the voltage conditions change the channel rapidly closes again.

Application: Structure and function of sodium-potassium pumps for active transport and potassium channels for facilitated difusion in axons.

Question 62

Outline the importance of bathing organs and tissues in solutions of same osmolarity.

1.4

Answer 62

Uncontrolled osmosis will have negative effects with regards to cell viability:
* In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
* In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)

Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

Question 63

Explain the origin of cells.

1.5

Answer 63

If we accept that humans evolved from pre-existing ancestral species, we can trace the origins of cells back through hundreds of millions of years to the earliest cells on Earth.

Understanding: Cells can only be formed by division of pre-existing cells.

Question 64

Outline Pasteur’s experiments and the conclusions.

1.5

Answer 64

1. Broths were stored in vessels that contained long tubings (swan neck ducts) that did not allow external dust particles to pass
2. The broths were boiled to kill any micro-organisms present in the growth medium (sterilisation)
3. Growth only occurred in the broth if the flask was broken open, exposing the contents to contaminants from the outside
   From this it was concluded that emergent bacterial growth came from external contaminants and did not spontaneously occur

Application: Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.

Question 65

Outline how living cells arose from non-living matter.

1.5

Answer 65

1. Non-living synthesis of simple organic molecules (Miller and Urey experiment showed that early earth conditions could form organic molecules)
2. Assembly of these organic molecules into polymers (deep sea vents have the right condition to form organic polymers)
3. Formation of membranes to package the organic molecules (if there were phospholipid like coumpounds, they could form an internal environment similar to the cell)
4. Formation of polymers that can self-replicate (RNA may have been the genetic material before DNA)

Understanding: The frst cells must have arisen from non-living material.

Question 66

Outline the endosymbiotic theory.

1.5

Answer 66

Mitochondria:
* Cell that respired anaeoribcally took in a bacterium that respired aerobically
* bacterium supplied itself and the larger cell with ATP
* Larger cell gets an advantage of respiring aerobically
* Symbiotic relationship: The smaller cell gets supplied with food, larger cell gets energy from aerobic respirationfrom smaller cell.
* Aerobic bacterium evolved into the mirochondria and larger cell evolved into heterotrophic eukaryotes

Chloroplast:
* Heterotrophic cell took in smaller photosynthetic bacterium
* photosynthetic prokaryote eveolved into chloroplast and larger cell evolved into photosynthetic eukaryotics

Understanding: The origin of eukaryotic cells can be explained by the
endosymbiotic theory.

Question 67

Outline the evidence for the endosymbiotic theory of mitochondria and chloroplasts.

1.5

Answer 67

• They have their own genes, on a circular DNA molecule like that of prokaryotes.
• They have their own 70S ribosomes of a size and shape typical of some prokaryotes.
• They transcribe their DNA and use the mRNA to synthesize some of their own proteins.
• They can only be produced by division of pre-existing mitochondria and chloroplasts.

Understanding: Understanding: The origin of eukaryotic cells can be explained by the endosymbiotic theory.

Question 68

Define “cell cycle”.

1.6

Answer 68

The cell cycle is an ordered set of events which culminates in the division of a cell into two daughter cells.

Understanding: Mitosis is division of the nucleus into two genetically
identical daughter nuclei.

Question 69

Outline when mitosis is involved.

1.6

Answer 69

Mitosis is involved whenever cells with genetically identical nuclei are required in eukaryotes: during embryonic development, growth, tissue repair and asexual reproduction.

Understanding: Mitosis is division of the nucleus into two genetically
identical daughter nuclei.

Question 70

Outline interphase.

1.6

Answer 70

Interphase - part of the cell cycle that does not involve division; majority of life cycle
G1 (Gap 1):
* Metabolic reactions
* Increase volume of cytoplasm
* Organelles produced and replicated
* Proteins synthesised

S (Synthesis)
* DNA replicated

G2 (Gap 2)
* Metabolic reactions
* Increase the volume of the cytoplasm
* Organelles produced and replicated
* Proteins synthesised

G0 (Gap 0)
* “Resting phase”
* Temporary or permanent phase
* Cells that do not divide
* Carry out regular functions
* Metabolic reactions

Understanding: Interphase is a very active phase of the cell cycle with
many processes occurring in the nucleus and cytoplasm.

Question 71

Define “condensation” of chromosomes.

1.6

Answer 71

Condensation occurs by means repeatedly coiling the DNA molecule to make the chromosome shorter and wider. This process is called supercoiling.

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 72

Define “centromere”.

1.6

Answer 72

the part of a chromosome that links sister chromatids

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 73

Define “sister chromatids”.

1.6

Answer 73

Sister chromatids are duplicated chromosomes attached by a centromere

After anaphase when the sister chromatids separate they should then be referred to as daughter chromosomes

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 74

Define “centrioles”.

1.6

Answer 74

centrioles organise spindle microtubules

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 75

Define “spindle microtubules”.

1.6

Answer 75

Spindle microtubules (also referred to as spindle fibres)

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 76

Distunguish between chromatin and chromosomes.

1.6

Answer 76

Chromatin:
* loosely packed DNA within the nucleus as unravelled chromatin
* DNA is organised as chromatin in interphase

Chromosome:
* DNA is condensed prior to division (via supercoiling)
* DNA is able to be easily seperated and moved
* During the process of mitosis (condense in prophase, decondense in telophase)

Understanding: Chromosomes condense by supercoiling during mitosis.

Question 77

Outline Prophase

1.6

Answer 77

• DNA supercoils and chromosomes condense (becoming visible under microscope)
• Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere)
• Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibres
• The nuclear membrane breaks down and the nucleus dissolves

Skills: Identifcation of phases of mitosis in cells viewed with a microscope.

Question 78

Outline metaphase

1.6

Answer 78

• Microtubule spindle fibres from both centrosomes connect to the centromere of each chromosome
• Chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)

Skills: Identifcation of phases of mitosis in cells viewed with a microscope.

Question 79

Outline anaphase.

1.6

Answer 79

• Contraction of the spindle fibres causes genetically identical sister chromatids to separate
• Sister chromatids seperate to become chromosomes
• The genetically identical chromosomes move to the opposite poles of the cell

Skills: Identifcation of phases of mitosis in cells viewed with a microscope.

Question 80

Outline telophase.

1.6

Answer 80

• Once the two chromosome sets arrive at the poles, spindle fibres dissolve
• Chromosomes decondense (no longer visible under light microscope)
• Nuclear membranes reform around each chromosome set
• Cytokinesis occurs concurrently, splitting the cell into two

Skills: Identifcation of phases of mitosis in cells viewed with a microscope.

Question 81

State the formula to determine the mitotic index.

1.6

Answer 81

mitotic index= # of cells in mitosis/ total # of cells

Skill: Determination of a mitotic index from a micrograph.

Question 82

Compare and contrast cytokinesis in plants and animal cells.

1.6

Answer 82

Animal cells:
* Plasma membrane is pulled inwards around the equator
* Forms a cleavage furrow (using proteins)
* When the cleavage furrow reaches the centre, the cell is pinched apart into two daughter cells.

**Plant cells: **
* Vesicles form in a row at the centre of the cell (equatorial plane)
* The vesicles fuse together and an early cell plate begins to form within the middle of the cell
* The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells

Understanding: Cytokinesis occurs after mitosis and is diferent in plant
and animal cells.

Question 83

Define “cyclin”.

1.6

Answer 83

Cyclins are a group of proteins that control the progression of the cell cycle.

1. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold.
2. Cyclins bind to enzymes called cyclin-dependent kinases
3. These kinases then become active and attach phosphate groups to other proteins in the cell.
4. The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).

Understanding: Cyclins are involved in the control of the cell cycle.

Question 84

Outline the function of specific cyclins.

1.6

Answer 84

• Cyclin D: Triggers cells to move from G0 to G1 and from G1 into S phase.
• Cyclin E: prepares the cell for DNA replication in S phase.
• Cyclin A: activates DNA replication inside the nucleus in S phase.
• Cyclin B: promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.

Understanding: Cyclins are involved in the control of the cell cycle.

Question 85

Define “oncogene”.

1.6

Answer 85

An oncogene is a gene that has the potential to cause cancer
* In a normal cell oncogenes are involved in the control o the cell cycle and cell division.
* This is why mutations in oncogenes can result in uncontrolled cell division and therefore tumour formation.

Understanding: Mutagens, oncogenes and metastasis are involved in the
development of primary and secondary tumours.

Question 86

Define “tumor”.

1.6

Answer 86

Tumours are abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ
* Diseases caused by the growth of tumours are collectively known as cancers

Understanding: Mutagens, oncogenes and metastasis are involved in the
development of primary and secondary tumours.

Question 87

Define “metastasis”.

1.6

Answer 87

• Tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant)
• Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour

Understanding: Mutagens, oncogenes and metastasis are involved in the
development of primary and secondary tumours.

Question 88

Evauluate the chances of a tumored cell.

1.6

Answer 88

Several mutations must occur in the same cell for it to become a tumour causing cell. The probability of this happening in a single cell is extremely small.

Factors (other than exposure to mutagens) that** increase the probability of tumour** development include:
* The vast number of cells in a human body – the greater the number of cells the greater the chance of a mutation.
* The longer a life span the greater the chance of a mutation.

Understanding: Mutagens, oncogenes and metastasis are involved in the
development of primary and secondary tumours.

Question 89

Outlie the correlation between smoking and cancer.

1.6

Answer 89

• Cigarette smoke contains chemical compounds known to be carcinogenic
• There appears to be a strong positive correlation between the frequency of smoking and the development of cancer
• The risk of lung cancer is strongly correlated with smoking, with ~90% of lung cancers attributable to tobacco use
• Smoking also increases the risk of over a dozen other cancers, including mouth, stomach, liver, panceas and bowel

Application: The correlation between smoking and incidence of cancers.