4 - Transport across cell membranes

Question 1

What are the membranes around and within cells?

Answer 1

Plasma membranes

Question 2

Give 5 molecules that make up the cell-surface membrane.

Answer 2

Phospholipids
Proteins
Cholesterol
Glycolipids
Glycoproteins

Question 3

What are three functions of phospholipids in the membrane?

Answer 3

Allows lipid-soluble substances to enter and leave cell

Prevent water-soluble substances entering and leaving cell

Make membrane flexible and self-sealing

Question 4

What are the two types of transport proteins?

Answer 4

Protein channels

Carrier proteins

Question 5

How do protein channels work?

Answer 5

They are water-filled tubes that allow water-soluble ions to diffuse across the membrane.

The channel remains closed until a specific ion is present. It then opens, and the ion binds with it to change its shape, causing it to open on the other side.

Question 6

How do carrier proteins work?

Answer 6

They bind to ions or molecules ie glucose, then change shape to release the molecules across the membrane.

Question 7

What does cholesterol do in the membrane?

Answer 7

Add strength, by limiting the movement of other molecules.

Make membrane less fluid at high temperatures.

Question 8

What are two functions of glycolipids in the membrane?

Answer 8

Act as recognition sites

Help maintain stability of membrane

Question 9

What are two functions of glycoproteins in the membrane?

Answer 9

Act as recognition sites

Helps cells attach to one another

Allows cells to recognise each other

Question 10

Why can most molecules not pass through the cell-surface membrane? (4)

Answer 10

Not soluble in lids, so cannot pass through phospholipid bilayer

Too large to pass through channels

Of the same charge as protein channels

Electrically charged, so cannot pass through non-polar tails in phospholipid bilayer

Question 11

Why is the cell-surface membrane referred to fluid-mosaic?

Answer 11

Mosaic - the embedded proteins vary in shape, size and pattern

Fluid - individual phospholipid molecules can move, so the membrane is constantly changing in shape

Question 12

What is simple diffusion defined as?

Answer 12

The net movement of molecules or ions from a region of high concentration to one of low concentration, until evenly distributed.

Question 13

What are the two types of facilitated diffusion in membranes?

Answer 13

That using carrier proteins, and that using channel proteins.

Question 14

What is osmosis defined as?

Answer 14

The net movement of water from a region of high water potential so a region of lower water potential, through a partially permeable membrane.

Question 15

What is meant when two solutions are isotonic?

Answer 15

They have the same water potential.

Question 16

What is the water potential of pure water?

Answer 16

0

Question 17

How does increasing solute concentration in a solution affect its water potential?

Answer 17

It reduces it, so it becomes negative.

Question 18

Why are animal cells in a liquid of the same water potential?

Answer 18

To prevent water entering or leaving the cell, as this would disrupt its shape and could burst it, or cause it to shrivel.

Question 19

How is active transport defined?

Answer 19

The movement of molecules/ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Question 20

How do microvilli increase transport across cell membranes?

Answer 20

Increase surface area.

Question 21

How is glucose moved from the small intestine into the blood? (1)

Answer 21

Co-transport

Question 22

How is glucose moved by co-transport from the small intestine to blood?

Answer 22

Sodium ions actively transported out of epithelial cells into blood, causing a higher concentration of sodium in the small intestine to the epithelial cells.

Sodium ions diffuse by carrier proteins into the epithelial cells, bringing glucose molecules with them (co-transport)

Glucose then diffuses from epithelial cells into blood.

Question 23

Give five methods of transport across a membrane.

Answer 23

• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport
• Co-transport