STUDYING CELLS AND TRANSPORT

Question 1

describe how you would make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using a light microscope (4)

Answer 1

1.Add drop of water to glass slide;
2.Obtain thin section of plant tissue and place on slide;
3.Stain with iodine in potassium iodide.
4.Lower cover slip using mounted needle

Question 2

describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells

Answer 2

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;
4. Keep cold to reduce damage by enzymes/ use buffer to prevent enzyme denaturation;
5. Centrifuge at lower speed to separate nuclei / cell fragments / heavy organelles;
6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get
   mitochondria at bottom.

Question 3

describe the principles and limitations of using a TEM to investigate cell structure (5)

Answer 3

Principles:
1. Electrons pass through thin specimen;
2. Denser parts absorb more electrons;
3. So denser parts appear darker;
4. Electrons have short wavelength so give high resolution;
Limitations:
5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be very thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced

Question 4

scientists use optical microscopes and TEMs to investigate cell structure. explain the advantages and the limitations of using a TEM to investigate cell structure

Answer 4

Advantages:
1 Small objects can be seen;
2 TEM has high resolution as wavelength of electrons shorter;
Limitations:
3 Cannot look at living cells as cells must be in a vacuum / must cut section / thin specimen;
4 Preparation may create artefact
5 Does not produce colour image;

Question 4

oxygen and chloride ions can diffuse across a cell surface membrane. the diffusion of chloride ions involves a membrane protein. the diffusion of oxygen doesnt involve a membrane protein. explain why the diffusion of chloride ions involves a membrane protein but the diffusion of oxygen doesnt (5)

Answer 4

1. Chloride ions water soluble/charged/polar;
2. Cannot cross lipid bilayer;
3. Chloride ions transported by facilitated diffusion using channel/carrier
   protein;
4. Oxygen not charged/non-polar;
5. Oxygen soluble in/can diffuse across lipid bilayer;

Question 5

contrast how an optical microscope and a TEM work and contrast the limitations of their use when studying cells (6)

Answer 5

1.TEM use electronsandoptical use light;
2.TEM allows a greaterresolution;
3.(So with TEM) smallerorganelles/named cell structurecan be observed
OR
greaterdetail inorganelles/named cell structurecan be observed;
4.TEM view only dead / dehydrated specimensandoptical (can) view live specimens;
5.TEM does not show colourandoptical (can);
6.TEM requires thinnerspecimens;
7.TEM requires amorecomplex/time consuming preparation;
8.TEM focuses using magnetsandoptical uses (glass) lenses;

Question 5

many different substances enter and leave a cell by crossing its cell surface membrane. describe how substances can cross a cell surface membrane (5)

Answer 5

1.Simple / facilitated diffusion from high to low concentration;
2.Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
OR
Large / polar / water-soluble molecules go through proteins;
3.Water moves by osmosis from high water potential to low water potential;
4.Active transport against concentration gradient;
5.Active transport/facilitated diffusion involves proteins /carriers;
6.Active transport requires energy / ATP;
7.Ref. to Na + / glucose co-transport

Question 6

the epithelial cells that line the small intestine are adapted for the absorption of glucose. explain how (6)

Answer 6

1.Microvilli provide an increased surface area;
2.Many mitochondria produce ATP / release or provide energy for active transport;
3.Carrier proteins for active transport;
4.Channel/carrier proteins for facilitated diffusion;
5.Co-transport of sodium ions and glucose using a symport / carrier protein;
6.Membrane-bound enzymes digest disaccharides to produce glucose;

Question 7

the movement of substances across cell membranes is affected by membrane structures. explain how (5)

Answer 7

1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances
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 channels/carriers determines how much movement;
   7.Membrane surface area determines how much diffusion/movement;