2.3 TRANSPORT ACROSS CELL MEMBRANES

Question 1

What is a cell membrane primarily made up of?

Answer 1

A barrier of phospholipids

Question 2

How are phospholipid arranged in the membrane?

Answer 2

• Hydrophilic phosphate heads point out towards the water.
• Hydrophobic tails point inwards, away from the water.
• This creates a double layer of phospholipids, with the heads protecting the tails from the water.

Question 3

What other things are in the phospholipid bilayer (except phospholipids)?

Answer 3

• Glycolipids
• Glycoproteins
• Cholesterol
• Intrinsic & Extrinsic proteins

Question 4

What is the function of cholesterol in the bilayer?

Answer 4

• Provides strength.
• Extremely hydrophobic so prevents water loss.
• Reduces fluidity and lateral movements at high temperatures.

Question 5

What is the function of glycolipids in the bilayer?

Answer 5

• A carb bonded to a lipid
• Acts as a recognition site for other molecules.
• Helps keep the membrane strong and stable.
• Acts as an attachment site for other cells, to form tissues.

Question 6

What is the function of glycoproteins in the bilayer?

Answer 6

• A carb bonded to extrinsic proteins.
• Act as recognition sites for other molecules.
• Acts as an attachment site for other cells, to form tissues.
• Has receptors for hormones and neurotransmitter.

Question 7

What is the function of extrinsic proteins in the bilayer?

Answer 7

Provide mechanical support

Question 8

What is the function of intrinsic proteins in the bilayer?

Answer 8

Span across the bilayer, and are used in facilitated diffusion and active transport.

Question 9

What type of molecules can pass directly through the phospholipid bilayer?

Answer 9

• Small
• Non-polar
• Lipid soluble

Question 10

What are the main functions of the cell membrane?

Answer 10

• Separate cell contents from outside environments.
• Separate organelles from cytoplasm.
• Regulate transport of materials in and out of cell/ organelles.
• Involved in cell recognition and signalling

Question 11

Why is the phospholipid bilayer called ‘fluid’?

Answer 11

Phospholipids can move around in relation to one another as the membrane is flexible.

Question 12

Why is the phospholipid bilayer called ‘mosaic’?

Answer 12

The proteins in the bilayer are different shapes and sizes, like tiles in a mosaic.

Question 13

What is simple diffusion?

Answer 13

The net passive movement of molecules down the concentration gradient through a partially permeable membrane, until equilibrium is reached.

Question 14

Which molecules can diffuse simply across the bilayer?

Answer 14

• Small
• Non-polar
• Lipid soluble

Question 15

What is facilitated diffusion?

Answer 15

The net passive movement of molecules down the concentration gradient, through protein channels or carrier proteins, until equilibrium is reached.

Question 16

How do carrier proteins work in facilitated diffusion?

Answer 16

1. A specific molecule bonds to the carrier protein.
2. This causes it to change shape so the molecule can be passed through the bilayer.
3. Once the molecule is released, the carrier protein returns to its original shape.

Question 17

How do protein channels work in facilitated diffusion?

Answer 17

1. A specific water soluble ion binds to the channel.
2. This causes it to change shape and open up.
3. Once the molecule is released, the carrier protein returns to its original shape.

Question 18

Which molecules travel across the bilayer by facilitated diffusion?

Answer 18

• Polar
• Large
• Non-lipid soluble

Question 19

What is osmosis?

Answer 19

The net passive movement of water molecules down the water potential gradient, through a partially permeable membrane (aquaporin channels), until equilibrium has been reached.

Question 20

What is water potential?

Answer 20

The pressure created by water molecules.

Question 21

What is the water potential of pure water (highest WP)

Answer 21

0 kPa

Question 22

What symbol denotes water potential?

Answer 22

ᴪ (psi)

Question 23

What will the water potential always be if it has some solute dissolved in it?

Answer 23

<0 kPa

Question 24

What happens if the ᴪ outside an animal cell is greater than inside?

Answer 24

Water moves in, cell swells and bursts.

Question 25

What happens if the ᴪ outside an animal cell is less than inside?

Answer 25

Water moves out, cell shrinks.

Question 26

What happens if the ᴪ outside a plant cell is greater than inside?

Answer 26

Water moves in, protoplast swell, plant becomes turgid.

Question 27

What happens if the ᴪ outside a plant cell is less than inside?

Answer 27

Water moves out, protoplast shrinks and pulls away from the cell way, cell become plasmolysed.

Question 28

What is the definition of active transport?

Answer 28

The net active movement of molecules, against the concentration gradient, using carrier proteins and ATP, until equilibrium has been reached.

Question 29

Outline the mechanism active transport uses to transport molecules?

Answer 29

1. Specific molecules bind to receptors on carrier protein on cell surface membrane.
2. This cause ATP to bond to the same protein from inside the cell.
3. This ATP is then hydrolysed, to form ADP & Pi.
4. The inorganic phosphate stays bonded to the carrier protein, causing it to change shape.
5. Once the molecule has been released, the Pi is also released to form ATP under condensation.
6. This returns the carrier proteins back to their original shape, ready for the process to repeat.

Question 30

What is the sodium-potassium pump?

Answer 30

A mechanism that uses active transport to remove sodium ions from a cell, and supply potassium ions to the cell.

Question 31

What is co-transport?

Answer 31

The movement of small molecules (glucose/amino acids), couple with sodium ions during active transport.

Question 32

Outline the process of the co-transport of glucose with sodium ions, supplying glucose to the bloodstream.

Answer 32

1. Sodium ions are pumped out of the cell into the bloodstream by the sodium-potassium pump.
2. There is now a lower concentration of sodium in the cell than in the ileum.
3. This causes sodium ions to move into the cell from the ileum.
4. This brings glucose molecules, and they both pass through a specific co-transport carrier protein.
5. Now glucose is in the cell, it moves out into the blood stream by facilitated diffusion and another specific protein.

Question 33

How do you maximise the transport of molecules across the membrane?

Answer 33

• More protein channels
• Bigger concentration difference
• Bigger surface area