cell membrane and transports

Question 1

What happens when a plant and animal cell is placed in a hypertonic solution

Answer 1

what potential higher inside the cell than outside
- water moves out of the cell via osmosis
- Animal cell becomes shrivelled
- plant cell becomes plasmolysed

Question 2

plasmolysed meaning

Answer 2

cell membrane pulls away from cell wall

Question 3

Diffusion

Answer 3

net movement of non- polar molecules from an area of high to low concentration down a conc gradient across a partially permeable membrane

Question 4

How are membranes adapted to increase the movement across them?

Answer 4

Increased number of channel and carrier protein
- increases SA of membrane
- Mechanisms to increases conc gradient
- Increased number of mitochondria to supply more ATP for active transport

Question 5

Define osmosis

Answer 5

Net movement of water molecules from a higher to lower water potential, down a water potential gradient through a partially permeable membrane

Question 6

Co tranport define

Answer 6

movement of two substances simultaneously through a carrier protein one substance moves against conc gradient (active transport). One substance moves down conc gradient (facial diffusion) which has been sgtablished by aT

Question 7

What is a isotonic solution

Answer 7

A solution with the same conc of solutes and therefore the same water potential than the cell

Question 8

What happens when an animal and plant cell is placed in a hypotonic solution

Answer 8

water potential higher outside the cell than inside, water moves into the cell via osmosis, animal cell bursts, lysed (due to no cell wall)
and plant cell becomes turgid (celll membrane swell and pushes against cell wall).

Question 9

Define Active transport

Answer 9

movement of molecules against a conc gradient, from area of low to high conc, using carrier proteins and the energy from ATP

Question 10

What is water potential and how does it change

Answer 10

water potential= ability of water molecules to move freely (no IMF attaching to solute)
Pure water= 0kPa.
All solutions have -ve water potentials.
More solutes (more conc) = lower WP

Question 11

Role of cholesterol in cell membrane

Answer 11

reduces membrane fluidity by restricting the movement of other molecules

Question 12

How is facilitated diffusion different to simple diffusion

Answer 12

Facilitated diffusion uses channel and carrier proteins as it involves the movement of polar molecules whereas simple diffusion involves non - polar molecules that move straight through the membrane

Question 13

How do phospholipids form a bilayer

Answer 13

hydrophobic fatty acids repel water in middle and hydrophobic phosphate heads attracts water

Question 14

Structure and role of glycolipids in the cell membrane

Answer 14

phospholipid with short carbs chain.
cell recognition and attachment

Question 15

Role of proteins in cell membrane

Answer 15

Channel and carrier (transmembrane) for movement of substance across membrane.
Other transmembrane and and peripheral proteins - many functions, including receptors, signalling molecules, increasing membrane stability

Question 16

how does water move through the membrane

Answer 16

through channel proteins called aqauporins

Question 17

structure and role of glycoproteins in the cell membrane

Answer 17

protein with short carb chain.
Cell recognition and signalling
cell attachment

Question 18

What is a hypertonic solution

Answer 18

A solution that ha s a higher solute conc and therefore lower water potential that cell

Question 19

What is a hypotonic solution

Answer 19

A solution that has a lower solute conc and therefore a higher water potential that the cell

Question 20

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Answer 20

1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
   If no reference to ‘small/ non-polar’ for 1.
   accept this idea from ‘large/charged’ given in description of 2.
   2. Facilitated diffusion down a concentration gradient via protein carrier/channel;
   Reject if active rather than passive
   3. Osmosis of water down a water potential gradient;
   4. Active transport against a concentration gradient via protein carrier using ATP;
   5. Co-transport of 2 different substances using a carrier protein;
   For any answer accept a correct example
   For ‘carrier protein’ accept symport OR cotransport protein

Question 21

The movement of substances across cell membranes is affected by membrane structure. Describe how.

Answer 21

1.   Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
1. and 2. Accept correct named examples
1. and 2. Ignore water
Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2
Accept water-insoluble
2.   Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances
OR
(Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
Accept water-soluble
3.   Carrier proteins allow active transport;
4.   Channel/carrier proteins allow facilitated diffusion/co-transport;
Accept aquaporins allow osmosis
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;
6. and 7. Accept correct reference to faster/slower/rate for ‘how much movement’
Accept microvilli / Golgi (apparatus) / ER / rER
Accept surface area to volume for ‘surface area’
8.   Cholesterol affects fluidity/rigidity/permeability;

Question 22

Glucose is absorbed from the lumen of the small intestine into epithelial cells.
Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells.

Answer 22

1. Na+ ions leave epithelial cell and enter blood;
   Penalise for Na without ions once.
   2. (Transport out is by) active transport / pump / via carrier protein using ATP;
   Reject channel protein
   3. So, Na+ conc. in cell is lower than in lumen (of gut);
   Maintains diffusion gradient for Na+ from lumen/into cells;
   4. Sodium/Na+ ions enter by facilitated diffusion;
   Accept diffusion/from high to low concentration through a symport/cotransport protein
2. Glucose absorbed with Na+ ions against their concentration/diffusion gradient / glucose absorbed down an electrochemical gradient;

Question 23

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.

Answer 23

1. Chloride ions water soluble/charged/polar;
   Penalise chloride molecules only once
   Ignore ref to size
   Accept not lipid soluble
   2. Cannot cross (lipid) bilayer (of membrane);
   3. Chloride ions transported by facilitated diffusion OR diffusion involving channel/carrier protein;
   4. Oxygen not charged/non-polar;
   Accept oxygen lipid soluble
   5. (Oxygen) soluble in/can diffuse across (lipid) bilayer;

Question 24

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.

Answer 24

By osmosis (no mark)
No mark awarded for naming terms e.g. osmosis, facilitated diffusion, active transport, co-transport etc.
1. From a high water potential to a low water potential / down a water potential gradient;
2. Through aquaporins / water channels;
QWC ignore large / small WP
By facilitated diffusion (no mark)
QWC ignore reference to high / low concentrations of water or high / low concentration of solution
3. Channel / carrier protein;
4. Down concentration gradient;
By active transport (no mark)
QWC ignore ‘ along’ concentration gradients
5. Carrier protein / protein pumps;
6. Against concentration gradient;
7. Using ATP / energy (from respiration);
Co-transport subsumed into mark scheme for active transport and facilitated diffusion
By phagocytosis / endocytosis (no mark)
Can award MP2, 3, 5 for 3 marks with no context given
8. Engulfing by cell surface membrane to form vesicle / vacuole;
Ignore lipid diffusion as in stem of question
By exocytosis / role of Golgi vesicles (no mark)
9. Fusion of vesicle with cell surface membrane;

Question 25

The epitheilal cells that line the small intestine are adapted for the abosrption of glucose. Explain how.

Answer 25

1. Microvilli provide a large / 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 or symport / carrier protein for sodium (ions) and glucose;
   6. Membrane-bound enzymes digest disaccharides / produce glucose;

Question 26

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.

Answer 26

1. (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
   Q Do not allow across / along / with concentration gradient
   2. Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
   Reject: named molecule passing through membrane by an incorrect route
   Accept: diagrams if annotated
   OR
   Large / polar / water-soluble molecules go through proteins;
   3. Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
   4. Active transport is movement from low to high concentration / against concentration gradient;
   Only penalise once if active transport is not named
   e.g. ‘movement against the concentration gradient involves proteins and requires ATP’ = 2 marks
   5. Active transport / facilitated diffusion involves proteins / carriers;
   Accept: facilitated diffusion involves channels
   Reject: active transport involves channels
   6. Active transport requires energy / ATP;
2. Ref. to Na+ / glucose co-transport;

Question 27

Describe the processes involved in the absorption of the products of starch digestion.

Answer 27

Glucose moves in with sodium (into epithelial cell);
Via (carrier / channel) protein / symport;
Sodium removed (from epithelial cell) by active transport / sodium- potassium pump;
Into blood;
Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium concentration gradient (between lumen and epithelial cell);
Glucose moves into blood;
By (facilitated) diffusion;