transport across cell membranes?

Question 1

describe the structure off a cell membrane?

Answer 1

made of a double layer of phospholipid molecules. You will remember that the ‘head’ of a phospholipid molecule carries a charge because of its phosphate group, so it is hydrophilic. The fatty acid ‘tail’ is hydrophobic. Therefore, in water phospholipid molecules arrange themselves with the ‘heads’ towards water and the ‘tails’ away from water, forming a bilayer

Question 2

describe the function of a protein molecule?

Answer 2

Protein molecules float in the phospholipid bilayer. Some of these are extrinsic proteins, which float in just one half of the bilayer. Others are intrinsic and pass through the whole bilayer.

Question 3

where are cholesterol molecules found?

Answer 3

Cholesterol molecules are found among the phospholipids.

Question 4

what is the function of cholesterol molecules?

Answer 4

These stabilise the membrane by restricting the movement of other molecules within the membrane. Many of the proteins and phospholipids have short carbohydrate chains attached to them, on the outside surface of the membrane. They are known as glycoproteins and glycolipids.

Question 5

cheeky fact

Answer 5

Many of the proteins and phospholipids have short carbohydrate chains attached to them, on the outside surface of the membrane. They are known as glycoproteins and glycolipids.

Question 6

why is the structure described as fluid mosaic?

Answer 6

■ the phospholipids move around within their layer, making the membrane fluid
■ the proteins are scattered among the phospholipids, creating a mosaic

Question 7

proteins in the cell membrane can function as? 5 marks

Answer 7

■ channels
■ carriers
■ antigens
■ receptors
■ enzymes

Question 8

define diffusion?

Answer 8

Diffusion is the movement of molecules from a region of high concentration to a region of lower concentration down a concentration gradient.

Question 9

give an example of diffusion?

Answer 9

Oxygen, for example, is a small molecule and it is not charged, so it is able to pass freely through the phospholipid bilayer. It diffuses from outside the cell where it is in higher concentration, across the membrane into the cell, down a concentration gradient.

Question 10

why is simple diffusion described as a passive process?

Answer 10

because it does not require additional energy.

Question 11

define facilitated diffusion? 4 points

Answer 11

Facilitated means ‘helped’.
Ions or molecules that carry a charge cannot pass through the phospholipid bilayer.
Also, large molecules cannot pass between the phospholipids.
These ions and molecules can only pass through the membrane with the help of proteins, which is why this kind of diffusion is described as ‘facilitated’.

Question 12

describe a channel protein?

Answer 12

channel proteins have a hydrophilic channel through them that allows specific molecules through.

Question 13

describe a carrier protein?

Answer 13

These are also specific to certain kinds of ion
or molecule. The protein carrier changes shape when the molecule binds, and releases it on the other side of the membrane.

Question 14

is facilitated diffusion a passive process?

Answer 14

is a passive process because no additional energy is needed.

Question 15

define osmosis?

Answer 15

movement of water from an area of high water potentials to an area of low water potentials across a partially permeable membrane

Question 16

define water potential?

Answer 16

the pressure created by water molecules measured in kPA and represented with the fork symbol

Question 17

describe the water potential of water? 2 marks

Answer 17

pure water has a water potential of zero so when solutes are dissolved in water the water potential will become negative
the more negative the water potential the more solute dissolved in it

Question 18

define an isotonic solution?

Answer 18

the water potential is the same in the solution and the cell within the solution

Question 19

define a hypotonic solution?

Answer 19

the water potential of a solution is more postive [closer to zero] than the cell

Question 20

define hypertonic?

Answer 20

when the water potential of a solution is more negative than the cell

Question 21

define active transport?

Answer 21

Active transport, like facilitated diffusion, uses carrier proteins that are specific to the molecule being carried.

Question 22

is active transport an active process?

Answer 22

active transport uses additional energy because the molecule is being moved against its concentration gradient. This is why it is called an active process.

Question 23

where does the energy for active transport come from?

Answer 23

The energy for active transport comes from the hydrolysis of ATP:
ATP → ADP + energy

Question 24

describe the 3 steps of co transport of glucose and sodium ions in the ileum?

Answer 24

■ Protein A actively transports sodium ions out of the cell, creating a low concentration of sodium ions inside the cell.
■ Protein B is a co-transport protein. It carries glucose into the cell, alongside a sodium ion, which diffuses into the cell down its concentration gradient. It is called a co-transport protein because it has to have both a sodium ion and a glucose molecule to bring into the cell.
■ Protein C is a carrier protein that transports glucose out of the cell into the blood capillary by facilitated diffusion.

Question 25

define co transport?

Answer 25

co-transport, which involves a carrier protein moving two molecules or ions at the same time