Transport Across Cells Membranes

Question 1

What are the functions of cell-surface membranes?

Answer 1

• Forms the boundary between the cell and its environment.
• Controls the movement of substances in and out of the cell.

Question 2

How can substances move across the cell-surface membrane?

Answer 2

• Diffusion
• Osmosis
• Active transport

Question 3

What are all cell membranes composed of?

Answer 3

Proteins, carbohydrates and lipids?

Question 4

What are the functions of the membranes surrounding organelles? Check wording

Answer 4

• Divide the cell into different compartments, acting as a barrier between the organelle and the cytoplasm.
• Partially permeable and control what substances enter and leave the organelle.

Question 5

What is the structure of cell membranes known as?

Answer 5

The fluid mosaic model

Question 6

What does the fluid mosaic model describe?

Answer 6

The arrangement of molecules in the membrane

Question 7

Describe the fluid mosaic model of a cell membrane and explain why the model holds this name.

Answer 7

• Phospholipid molecules form a continuous double layer (bilayer).
• The bilayer is fluid as the phospholipids are constantly moving.
• Cholesterol molecules are present within the bilayer.
• Proteins are scattered through the bilayer, like tiles in a mosaic.

Question 8

What are the names of the proteins present in cell membranes?

Answer 8

• Channel proteins
• Carrier proteins
• Receptor proteins

Question 9

What is the role of the channel proteins?

Answer 9

Form pores in the membrane for charged particles to diffuse through down their concentration gradient.

Question 10

What is the role of the carrier proteins?

Answer 10

• They move large molecules across membranes, down their concentration gradient.
• First a large molecule attached to a carrier protein in the membrane.
• Then the protein changes shape.
• This releases the molecule on the opposite side of the membrane.

Question 11

What is the role of the receptor proteins?

Question 12

What are glycoproteins?

Answer 12

Proteins that have a polysaccharide (carbohydrate) chain attached

Question 13

What are glycolipids?

Answer 13

Lipids that have a polysaccharide (carbohydrate) chain attached

Question 14

What is the role of the cholesterol?

Answer 14

• Gives the membrane stability
• They help to maintain the shape of animals cells (which don’t have cell walls) which is particularly important for cells that aren’t supported by other cells, e.g. red blood cells which float free in the blood.

Question 15

Where is cholesterol in a cell membrane?

Answer 15

• Cholesterol molecules fit between the phospholipids, binding to the hydrophobic tails which causes them to pack more closely together.
• This restricts the movement of the phospholipids, making the membrane more rigid and less fluid.

Question 16

What is the role of the phospholipids?

Answer 16

• Form a barrier to dissolved substances.
• Automatically arrange themselves into a bilayer (the head face out towards the water either side of the membrane and the tail face into the bilayer).

Question 17

What are the two modes of transport across a membrane?

Answer 17

• Passive transport
• Active transport

Question 18

Define passive transport.

Answer 18

Does not require any external energy, due to the kinetic energy of particles.

Question 19

Define active transport.

Answer 19

Requires energy in the form of the energy molecule ATP (an immediate energy source for cells).

Question 20

Define diffusion.

Question 21

Define simple diffusion.

Answer 21

When molecules diffuse directly through a cell membrane.

Question 22

What does facilitated diffusion

Question 23

What factors affects the rate of simple diffusion?

Answer 23

• Concentration gradient
• The thickness of the exchange surface
• The surface area

Question 24

What factors affects the rate of facilitated diffusion?

Answer 24

• Concentration gradient
• Number of channel or carrier proteins

Question 25

Define osmosis.

Answer 25

The passive movement (diffusion) of water from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane.

Question 26

What is water potential?

Answer 26

- A measure of how free water molecules are to ‘do work’ I.e. to move (their potential to do osmosis). - It is measured in units of pressure (e.g. Pascals).

Question 27

What is the water potential of pure water?

Answer 27

0kPa

Question 28

Does water potential increase or decrease with the addition of solutes?

Answer 28

The addition of solutes reduces water potential (to a negative value) so you can never had a positive water potential.

Question 29

What has the highest water potential.

Answer 29

Pure water

Question 30

What factors affect the rate of osmosis?

Answer 30

- Water potential gradient - Thickness of the exchange surface - Surface area of the exchange surface