TRANSCRIPT

Unit 1

Unit 2

Unit 4

Unit 5

Microscopes

Prokaryotic cells

Cell fractionation

Scientists use optical microscopes and transmission electron microscopes (TEMs) to

investigate cell structure. Explain the advantages and the limitations of using a TEM

to investigate cell structure.

Advantages:

1 Small objects can be seen;

2 TEM has high resolution;

3 Wavelength of electrons shorter;

Limitations:

4 Cannot look at living cells;

5 Must be in a vacuum;

6 Must cut section / thin specimen;

7 Preparation may create artefact

8 Does not produce colour image;

The structure of a cholera bacterium is different from the structure of an epithelial cell from the small intestine. Describe how the structure of a cholera bacterium is

Different.

1 Cholera bacterium is prokaryote;

2 Does not have a nucleus/nuclear envelope/ has DNA free in cytoplasm/has loop of DNA;

3 and 4 Any two from

No membrane-bound organelles/no mitochondria / no golgi/

no endoplasmic reticulum/etc;

5 Small ribosomes only;

6 and 7 Any two from

Capsule/flagellum/plasmid / cell wall/etc;

Measuring the size of an object under a microscope

Measure with an eyepiece graticule

Calibrate with the stage mcirometer (an object of a known size)

Repeat and calculate an average

Explain the advantages and limitations of using a transmission electron microscope to study cells.

1 TEM uses (beam of) electrons;

2 These have short wavelength;

3 Allow high resolution/greater resolution/Allow more detail tobe seen/greater useful magnification;

4 Electrons scattered (by molecules in air);

5 Vacuum established;

6 Cannot examine living cells;

7 Lots of preparation/procedures used in preparing specimens/ fixing/staining/sectioning;

8 May alter appearance/result in artefacts;

Starting with some lettuce leaves, describe how you would obtain a sample of undamaged chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question.

1. Chop up (accept any reference to crude breaking up); 2. Cold; (reduces enzyme activity)

3. Buffered solution; (prevents pH affecting enzymes)4. Isotonic / same water potential; (prevents osmosis and possible lysis or shrinkage of organelles)

5. Filter and centrifuge filtrate;6. Centrifuge supernatant;

7. At higher speed;

8. Chloroplasts in (second) pellet;

Prokaryotic cells and fractionation

The bacteria in the intestine are prokaryotic cells. The epithelial cells which line the small intestine are eukaryotic cells. Describe the ways in which prokaryotic cells and eukaryotic cells differ

1 Prokaryotic cells do not have a nucleus / have genetic material in cytoplasm;2 DNA in loop / ring;3 Not associated with proteins / do not have chromosomes / chromatin / do not divide by mitosis;4 Smaller ribosomes;5 No membrane-bound organelles;6 Such as mitochondria / lysosomes / endoplasmic reticulum / Golgi / chloroplasts;7 Prokaryotic cells may have mesosomes;8 Prokaryotic cells smaller;9 May be enclosed by capsule;

How prokaryotic cell is the same and different to a eukaryotic cell

cytoplasm;ribosomes;phospholipid membranes / cell membrane / semipermeablemembrane;2 max

(accept folded membrane for two marks)

(ii)(it = bacterium)cell wall;capsule;flagellum;mesosome;no nucleus / nuclear membrane / DNA free;no mitochondria;

(accept no membrane-bound organelles if neither nucleus normitochondria mark scored)

no microvilli;no Golgi;no ER;70S/smaller ribosomes;

Parts of the prokaryotic cell

cell (surface) membrane,regulates entry/exit/selectively permeable;

Bmesosome,respiration/cell division;

Ccell wall,(mechanical) protection/prevents (osmotic) lysis;

Dslime layer/capsule,protection (against e.g. antibiotics);

Eflagellum,movement of cell;

F DNA molecule/bacterial chromosome,genetic information;

Give two factors which affect the ability of bacteria to cause a disease.

pathogenicity / toxicity of products;site of infection;invasiveness;

Microscopes

Fractionation

Labelled antibodies and an electron microscope can be used to produce images locating proteins on the surface of organelles, but cannot be used to observe cross bridge cycling in muscle cells. Explain why.

1.e.m. gives high resolution;2.due to short wavelength of electrons;3.antibodies attach specifically to target proteins;4.gold particles are electron dense;5.electrons must pass through a vacuum;6.material must be dead / fixed for e.m.;7.cross-bridge cycling requires living cells / metabolism / namedaspect-e.g. ATP synthesis;

Starting with some lettuce leaves, describe how you would obtain a sample of undamaged chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question.

1. Chop up (accept any reference to crude breaking up);2. Cold;3. Buffer solution;4. Isotonic / same water potential;5. Filter and centrifuge filtrate;6. Centrifuge supernatant;7. At higher speed;8. Chloroplasts in (second) pellet;

Explain the advantages and the limitations of using a TEM to investigate cell structure.

Advantages:

1Small objects can be seen;

2TEM has high resolution;

Accept better

3Wavelength of electrons shorter;

Advantages: allow maximum of 3 marks.

Limitations:

4Cannot look at living cells;

5Must be in a vacuum;

6Must cut section / thin specimen;

7Preparation may create artefact

8Does not produce colour image;5 max

Limitations: allow maximum of 3 marks.

Describe the ways in which prokaryotic cells and eukaryotic cells differ

1 Prokaryotic cells do not have a nucleus / have genetic material in cytoplasm;2 DNA in loop / ring;3 Not associated with proteins / do not have chromosomes /chromatin / do not divide by mitosis;4 Smaller ribosomes;5No membrane-bound organelles;6 Such as mitochondria / lysosomes / endoplasmic reticulum / Golgi / chloroplasts;7 Prokaryotic cells may have mesosomes;8 Prokaryotic cells smaller;9 May be enclosed by capsule;

Prokaryotic cells and viruses and microscopes

Describe the ways in which prokaryotic cells and eukaryotic cells differ

Prokaryotic cells do not have a nucleus / have genetic materialin cytoplasm;DNA in loop / ring;Not associated with proteins / do not have chromosomes /chromatin / do not divide by mitosis;Smaller ribosomes;No membrane-bound organelles;Such as mitochondria / lysosomes / endoplasmic reticulum /Golgi / chloroplasts;Prokaryotic cells may have mesosomes;Prokaryotic cells smaller;May be enclosed by capsule;

Define resolving power and state why it is bteter for electron microscopes than light

(i)Ability to distinguish points (close together);1

(ii)Electrons have a shorter wavelength;

Explain how viruses cause damage to cells.

uses / breaks up / digests host nuclear / genetic material (allow referencesmade to DNA /RNA instead of nuclear /genetic);virus DNA / genetic material inserted into hosts DNA / chromosome/ genetic material;host cells amino acids are used to synthesize viral proteins;cell lysis;by enzyme (produced by expressing a virus gene);toxin production;

Cell membranes

Describe the fluid-mosaic structure of a cell surface membrane.(5)

Phospholipids and proteins;Phospholipid bilayer;Arrangement of phospholipid molecules Tails to tails;Floating(protein) molecules / molecules can move in membrane;Intrinsic proteins extend through bilayer;Extrinsic proteins in outer layer only;(Ref. to intrinsic and extrinsic, unqualified, gains 1 mark);Detail of channel proteins / protein shapes / glycoproteins;Presence of cholesterol.

Some substances pass through the plasma membrane of a milk-producing cell by diffusion. Describe the structure of a plasma membrane and explain how different substances are able to pass through the membrane by diffusion (6)

1 Phospholipids forming bilayer/two layers;2 Details of arrangement with heads on the outside;3 Two types of protein specified; e.g. passing right through or confined to one layer / extrinsic or intrinsic / channel proteins and carrier proteins / two functional types4 Reference to other molecule e.g. cholesterol or glycoprotein;5 Substances move down concentration gradient/from high to low concentration;Reject references to across or along a gradient6 Water/ions through channel proteins/pores;7 Small/lipid soluble molecules/examples pass between phospholipids/through phospholipid layer;8 Carrier proteins involved with facilitated diffusion;Ignore references to active transport.Credit information in diagrams.

Describe how proteins are arranged in a plasma membrane and the part they play in transporting substances into and out of cells.(6)

1 Some proteins pass right through membrane;

2 Some proteins associated with one layer;

3 Involved in facilitated diffusion;

4 Involved in active transport;

5 Proteins act as carriers;

6 Carrier changes shape / position;

7 Proteins form channels / pores;

8 Protein allows passage of water soluble molecules /charged particles / correct named example;

Explain how three features of a plasma membrane adapt it for its functions.

1. phospholipid bilayer (as a barrier);

2.forms a barrier to water soluble / charged substances / allows non-polar substances to pass

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