UNIT 2 LAQS

Contrast the structure of a bacterial cell and the structure of a human cell (5 marks)

Bacterial cell:

Much smaller than human cell

Has cell wall, human cell doesn’t

Lacks nucleus, human cell has nucleus

Lacks membrane bound organelles, human cell has them

Bacterial DNA is circular, human DNA is linear

Bacterial DNA is ‘naked’, human DNA is bound to proteins/histones

Bacterial ribosomes are smaller than human ribosomes/ bacteria have 70s ribosomes, whereas humans have 80s ribosomes

Describe the appearance and behaviour of chromosomes during mitosis (5 marks)

Prophase:

Chromosomes coil/condense/shorten/thicken/become visible

(Chromosomes) appear as (two sister) chromatids joined at centromere

Metaphase:

Chromosomes line up on the equator/centre of cell

(Chromosomes) attached to spindle fibres

By their centromere

Anaphase:

Centromere divides/splits

Sister chromatids/chromosomes pulled to oppsite poles/ends of cell

Telophase:

Chromosomes unwind/become thinner/longer

Movement of substances across cell membranes is affected by membrane structure. Explain how (5 marks)

1) Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid- soluble substances/ O2/CO2

2) Phospholipid (bilayer) prevents movement/diffusion or polar/charged/lipid-insoluble substances/water soluble

3) Carrier proteins allow active transport

4) Channel/carrier proteins allow facilitated diffusion/co-transport

5) Shape/charge of channel/carrier determines which substances move

6) Number of channel/carriers determine how much movement

7) Membrane surface area determines how much diffusion/movement

8) Cholesterol affects fluidity/rigidity/permeability

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspense of animal cells (5 marks)

1) Cell homogenisation to break open cells

2) Filter to remove (large) debris/whole cells

3) Use isotonic solution to prevent damage to mitochondria/organelles/cells

4) Keep cold to prevent/reduce damage by enzymes

Use buffer to prevent protein/enzyme denaturation

5) Centrifuge (at lower speed) to separate nuclei/cell fragments/heavy organelles

6) Re-spin (supernatant/after nuclei or pellet removed) at higher speed to get mitchondria in pellet/at bottom

Describe binary fission in bacteria (3 marks)

1) Replication of (circular) DNA/nucleiod

2) Replication of plasmids

3) Division of cytoplasm (to produce daughter cells)

Describe how you could make a temporary mount of a piece of tissue to observe the position of starch grains in the cell when using an optical microscope (4 marks)

1) Add drop of water to (glass) slide

2) Obtain thin section (of plant tissue) and place on slid/float on drop of water

3) Stain with/add iodine in potassium iodide

4) Lower cover slip using mounted needle

Describe how HIV is replicated once inside T-helper cells (4 marks)

1) RNA converted into DNA using reverse transcriptase

2) DNA incorporated/inserted into (helper T cell)/nucleus/ribosome/genome

3) DNA transcribed into (HIVm)RNA

4) (HIV mRNA) translated into (new) HIV/viral proteins/capsid (for assembly into virus particles)

when a vaccine is given to a person, it leads to the production of antibodies against a disease causing organism. describe how (5 marks)

1) Vaccine contains antigen from pathogen

2) Macrophage presents antigen on its surface

3) T cell with complementary receptor protein binds to antigen

4) T cell stimulates b cell

5) (With complementary antibody on its surface)

6) B cell secretes large amounts of antibody

7) B cell divides to form clone all secreting/producing same antibody

Describe viral replication (5 marks)

1) Attachment proteins attach to receptors

2) (Viral) nucleic acid enters cell

3) Nucleic acid replicated in cell OR reverse transcriptase makes DNA from RNA

4) Cell produces (viral) protein/capsid/enzymes

5) Virus assembled and released (from cell)

Describe how vaccination can lead to protection against bacterial meningitis (5 marks)

1) Antigen on surface on meningitis/bacterium to surface protein/surface receptor on a specific b cell OR antigen binds to (specific) t cell

2) (Activated) b cell divides by mitosis/produces clone OR (activated) t cell release cytokine

3) (Division) stimulated by cytokines/t cells OR (cytokine) stimulates production of plasma cells

4) B cells/plasma cells release antibodies

5) (Some) b cells become memory cells

6) Memory cells produce plasma/antibodies faster

Describe the difference between active and passive immunity (5 marks)

1) Active involves memory cells, passive doesn’t

2) Active involves production of antibodies by plasma cells/memory cells

3) P involves antibodies introduced into body from outside/named source

4) Active- long term- antibodies are produced in response to the antigen

5) Passive- short term- antibodies (given) are broken down

6) Active- takes time to develop/work, passive- fast acting

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells (5 marks)

TEM:

1) Uses electrons; optical uses light

2) Allows greater resolution

3) Smaller organelles can be observed OR greater detail in organelles

4) View only dead/dehydrated specimens and optical- view live specimen

5) Doesn’t show colour and optical can

6) Required thinner specimens

7) More complex/time consuming preparation

8)Focuses using magnets or optical uses (glass) lenses

Describe the limitations and principles of transmission electron microscope (5 marks)

1. electromagnets focus a beam of electrons through a thin specimen.

2. denser parts of specimen absorb more electrons and therefore appear darker on final image produced.

3. image has a high resolution.

4. a limitation of transmission electron microscopes (TEMs) is they do not produce colour images, unlike an optical microscope for example.

5. TEMs require thin specimen as thick specimen would absorb more electrons and produce a low-quality image, limiting type of specimen that can be observed.

6. TEMs can only be used on dead or dehydrated specimen as interior of microscope is within a vacuum

Eukaryotic cells produce and release proteins. Outline the role of organelles in the production, transport and release of proteins from eukaryotic cells. Do not include details of transcription and translation in your answer. (4 marks)

1. DNA in nucleus is code (for protein);

2. Ribosomes/rough endoplasmic reticulum produce (protein);

3. Mitochondria produce ATP (for protein synthesis);

4 Golgi apparatus package/modify;

OR

Carbohydrate added/glycoprotein produced by Golgi apparatus;

5 Vesicles transport

OR

Rough endoplasmic reticulum transports;

6. (Vesicles) fuse with cell(-surface) membrane;

Describe the structure and function of the nucleus (4 marks)

containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli

the nucleus contains the organisms genetic material and controls the cells activities

nuclear envelope - double membrane that surrounds the nucleus

nuclear pores - allow the passage of large molecules out of the nucleus

nucleoplasm - granular, jelly like material that makes up bulk of the nucleus

chromosomes - consist of protein bound linear DNA

nucleolus - small spherical region within nucleoplasm

manufactures ribosomal RNA and assembles the ribosomes

Describe how a phagocyte destroys a pathogen present in the blood (3 marks)

1. Engulf the pathogen

2. Form vesicle( phagosome), fuse with Lysosome

3. Enzyme hydrolysis

Describe the role of antibodies in producing a positive result in an ELISA test (4 marks)

1. First antibody binds/ attachés/ complementary in shape to antigen

2. Second antibody with enzyme attached is added

3. Second antibody attaches to antigen OR first antibody

4. Substrate/solution added and colour changes

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein.

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 marks)

1. Chloride ions water soluble/charged/polar;

2. Cannot cross (lipid) bilayer (of membrane);

3. Chloride ions transported by facilitated diffusion OR

4. diffusion involving channel/carrier protein;

5. Oxygen not charged/ non-polar;

6. (Oxygen) soluble in/ can diffuse across (lipid) bilayer;

Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer.

Describe other ways by which substances cross this membrane. (5 marks)

By osmosis (no mark)

1. From a high water potential to a low water potential / down a water potential gradient;

2. Through aquaporins / water channels;

By facilitated diffusion (no mark)

1. Channel / carrier protein;

2. Down concentration gradient;

By active transport;

1. Carrier protein / protein pumps;

2. Against concentration gradient;

3. Using ATP / energy (from respiration);

[Co-transport subsumed into mark scheme for active transport and facilitated diffusion]

By phagocytosis / endocytosis (no mark)...