12.7 - Transport across membranes

Question 1

What is the entry and exit of molecules in cells controlled by?
What does it surround?
What is it described as?

Answer 1

The cell surface membrane
Surrounds the cytoplasm
Selectively permeable (lets certain molecules through)

Question 2

Explain the arrangement of phospholipids in a cell-surface membrane. (2)

Answer 2

1) Bilayer
2) Hydrophobic fatty acid tails point away from water and hydrophilic phosphate heads point towards the water.

Question 3

Explain why a cell membrane may be described as fluid-mosaic. (2)

Answer 3

1) The position of the molecules within the membrane is fluid - they are able to move around within the membrane. (The phospholipid molecules are constantly moving around relative to one another.)

2) Membrane is made up from a variety of different molecules (e.g proteins, glycoproteins, phospholipids and they are unevenly distributed throughout the membrane forming a mosaic.)

Question 4

Describe how an ester bond is formed in a phospholipid molecule. (2)

Answer 4

1) Condensation reaction
2) Between glycerol and fatty acid

Question 5

What is the function of this components of the fluid mosaic model:

PHOSPHOLIPIDS.

What is the bilayer made up of, which way are they oriented?

Which molecules does it let through and how, which molecules does it not let through?

Answer 5

The hydrophobic tails are attracted towards each other, hydrophilic heads orientated inwards to the cytoplasm or outwards to the extra-cellular fluid, forming the bilayer. Most abundant molecule in all membranes.

It allows lipid-soluble (non-polar) molecules to pass through by simple diffusion but prevents the passage of small polar / charged molecules (like ions) or larger molecules (like glucose).

Question 6

What is the function of this components of the fluid mosaic model:

CHOLESTEROL (CH)

What does it decrease of the membrane?

What does more CH mean for the stability of the membrane?

Answer 6

2) Decreases the permeability of the membrane, more cholestrol = less fluidity of the membrane (less movement, and lets less molecules through).

Increases the stability of the membrane. More CH = more rigid, less permeable.

Question 7

What is the function of this components of the fluid mosaic model:

CHANNEL PROTEINS

Which molecules does it allow to move across and how?

They are proteins, so what do they have?

E.g Na+ ions can only pass through…

Answer 7

Only allows specific charged ions or small molecules to move across the membrane by facilitated diffusion.

They have specific tertiary structures, so they are specific and can only transport molecules that are complementary to the shape of the channel protein.

Sodium ion channel proteins embedded in the membrane.

Question 8

What is the function of this components of the fluid mosaic model:

CARRIER PROTEINS

What do they aid the transport of and how?

Answer 8

They aid the transport of ions/polar molecules and large molecules e.g. glucose and amino acids by facilitated diffusion and active transport.

Question 9

What is the function of this components of the fluid mosaic model:

RECEPTOR PROTEINS

They act as specific receptors for what?

E.g. hormones such as insulin, may bind to the..

They are proteins so have…

Answer 9

For complementary molecules.

…insulin receptor protein, which allows a cell to respond by increasing the cells permeability to glucose.

Specific tertiary structures, only specific molecules can bind to specific receptor proteins.

Question 10

What is the function of this components of the fluid mosaic model:

ENZYMES

What is the shape of the enzyme A.S?

Answer 10

It is specific and complementary to its substrate, allowing them to bind and form enzyme-substrate complexes.

Question 11

What is the function of this components of the fluid mosaic model:

GLYCOPROTEINS

What are they composed of?

What are they important for, what do they act as?

Where are they produced?

Answer 11

Carbohydrates and proteins.

Cell recognition, they act as antigens. The immune cells detect the specific shapes of glycoproteins to identify the cells as SELF or NON-SELF.

In the golgi body within the cell that displays them.

Question 12

What is the function of this components of the fluid mosaic model:

AQUAPORINS

What type of proteins are they and what are they specific to?

A cell with a lot of aquaporins is said to be..

Answer 12

Channel proteins that are specific to water.

Said to be very permeable to water and can carry out osmosis easily.

Question 13

1) SIMPLE Diffusion is what type of process?

2)What does it not require?

3) When will it stop?

4) What molecules does it involve?

5) What is the definition?

6) Which way across the concentration gradient?

Answer 13

1) Passive process

2) Doesn’t require energy / ATP from respiration

3) It will stop when there are equal numbers of that specific molecule on either side of the membrane, when the molecules have reached equilibrium.

4) Small, non-polar, lipid-soluble molecules, e.g O2, CO2, oestrogen.

5) The net movement of molecules from an area of higher concentration to a lower concentration across a partially permeable membrane.

6) DOWN.

Question 14

What is Fick’s law for rate of diffusion?

Answer 14

Rate = Surface area x Concentration gradient / Diffusion distance

Question 15

How do these factors affect diffusion?

1) Temperature
2) Surface Area
3) Concentration gradient
4) Diffusion distance

Answer 15

1) Increased KE, faster rate of diffusion.

2) Larger SA = more space for molecules to pass through, also allow for more proteins to be present (channel/carrier)

3) As concentration difference increases (larger c.g), rate increases.

4) The shorter the diffusion distance (the fewer membranes to cross), the faster molecules will travel from one area to the next.

Question 16

1) What does facilitate mean?

2) What helps specific molecules pass through the P.bilayer?

3) What molecules can’t pass through the hydrophobic tails of the P.bilayer, why?

4) What do channel and carrier proteins have?

5) What process is facilitated diffusion, why?

6) What factor would the rate of diffusion in facilitated diffusion be limited by?

Answer 16

1) To help or aid.

2) Specific proteins (channel/carrier)

3) Hydrophilic (water soluble, charged or large) cannot pass. Because the fatty acid tails are non-polar so repel polar molecules. So they have to move via channel/carrier proteins.

4) A specific tertiary structure so can only transport specific molecules complementary to their binding site.
5) Passive, doesn’t need ATP.

6) The number of carrier proteins/channel proteins.
When their sites are full, the graph will plateau.

to speed up = add more channel/carrier proteins.

Question 17

1) What is osmosis, the definition?

2) LOWER WP =
HIGHER WP =

3) What has the highest water potential, what pressure? Why?

4) Free water molecules are able to move and collide with the membrane exerting…

5) What is the pressure, measured in?

6) Why does adding a solute (water soluble molecules e.g. glucose, ions) make the solution more negative?

Answer 17

1.) The net movement of water molecules from higher WP to a solution of lower WP.

2) Lower WP = more solute dissolved, more negative WP.
Higher WP = less solute dissolved, more positive WP.

3) Pure water = 0 kPa. As every molecule is free moving.

4) …pressure.

5) Water potential, kPa.

6) Water is a dipole, and is attracted to the charges on those molecules/ions, so the no. of free moving water molecules decreases.

Question 18

What happens if the solution type is:

1) Higher WP than the organelle/tissue

2)Lower WP than the organelle/tissue

3) Isotonic (same WP)

Answer 18

1) Animal cells swell + lysis occurs, bursts the cell - contents are lost. Plant cells swell and mass is increased, cell walls prevent lysis. WATER MOVES IN BY OSMOSIS AND HIGHER WP OUTSIDE CELL.

2) Animal cells shrivel and plant cell membrane pulls away from the cell wall. WATER MOVES OUT BY OSMOSIS, MASS IS LOST.

3) No net movement of water in or out of cells.
Same WP = no WP gradient.

Question 19

1) What is active transport used for?

2) What type of proteins does it only use?

3) What does it require?

4) What does the hydrolysis of ATP provide and do?

Answer 19

1) To transport molecules across the membrane against their concentration gradient, from lower > higher conc.

2) Carrier.

3) A source of energy, supplied by ATP produced in respiration.

4) Provides a small amount of energy / phosphorylates the carrier protein. This causes the protein to change shape and transport the molecule across the membrane.

Question 20

ANOTHER FORM OF ACTIVE TRANSPORT

What is exocytosis?

What does it transport?

How does it transport them?

What does it require?

E.g.

Answer 20

Active transport that uses Golgi vesicles to move very large quantities of molecules (like enzymes) from inside the cell to outside the cell.

Also used to move enzymes + glycoproteins from the Goldi to the C.S.M to secrete proteins.

It required ATP to move the vesicles to the C.S.M and to make the vesicle fuse with the membrane.

Question 21

ANOTHER FORM OF ACTIVE TRANSPORT

What is endocytosis?

Answer 21

The cell surface membrane is pulled inwards to create a vesicle. Any molecules next to that part of the membrane will be enclosed in the vesicle.

It requires the breakdown of ATP into ADP + Pi. The movement of the vesicle in the cell also requires this.

Question 22

Many different substances enter and leave a cell by crossing its C.S.M. Describe how substances can cross a C.S.M. (7)

Answer 22

1) Simple/facilitated diffusion from high to low conc. / down a conc. gradient.

2) Small non-polar, lipid soluble molecules pass via phospholipid bilayer OR large / polar / water soluble molecules go through proteins

3) Water moves by osmosis / from higher WP to lower WP

4) Active transport is movement from low to high conc. / against conc. gradient.

5) Active transport / facilitated diffusion involves proteins / carriers

6) Active transport requires energy / ATP

7) E.g Na+/glucose co-transport

Question 23

The movement of substances across cell membranes is affected by membrane structure. Describe how. (8)

Answer 23

1) Phospholipid bilayer allows movement of non-polar lipid soluble molecules.
2) P.Bilayer prevents movement of polar / charged / lipid-insoluble molecules - membrane proteins allow these to cross.
3) Carrier proteins allow active transport.
4) Channel/carrier proteins allow facilitated diffusion / co-transport/
5) Carrier proteins’ shape determines which substances can move across.
6) No. of channels/carriers determines how much movement.
7) Membrane SA determines how much diffusion/movement.
8) Cholesterol affects fluidity/rigidity/permeability.

Question 24

Name and describe five ways substances can move across the CSM into a cell. (5)

Answer 24

1) Simple diffusion of small/non-polar molecules DOWN a conc. gradient
2) Facilitated diffusion down a conc. gradient via carrier/channel proteins
3) Osmosis of water down a WP gradient
4) Active transport using a conc. gradient via protein carrier using ATP
5) Co-transport of 2 different substances using a carrier protein.

Question 25

Compare and contrast the processes by which water and inorganic ions enter cells. (3)

Answer 25

1) Both move down conc. gradient
2) Both move through channel proteins
3) Ions can move against a conc. gradient by active transport.

Question 26

Give two similarities in the movement of substances by diffusion and by osmosis. (2)

Answer 26

1) Movement down a conc. gradient, high > low
2) Passive process / doesn’t require energy from respiration / ATP

Question 27

Describe how substances can move across C.S.M by facilitated diffusion. (3)

Answer 27

1) Carrier / channel protein
2) Protein is specific and complementary to substance
3) Substance moves down conc. gradient

Question 28

Describe how you would use a 1.0 mol dm-3 solution of sucrose to produce 30cm3 of a 0.15 mol dm-3 solution of sucrose.

Answer 28

Add 4.5cm3 of 1.0dm-3 solution to 25.5cm3 distilled water.

Question 29

Describe how you would use the student’s results (dilution series and % change in mass) to find the WP of the potato tissue. (3)

Answer 29

1) Plot a graph with conc. on the x-axis and % change in mass on y-axis
2) Find conc. where curve crosses the x-axis / where % change = 0
3) Use another resource to find WP of sucrose conc. (where curve crosses x-axis)