4.1 Transport across cell membranes

Question 1

What is the structure of phospholipids?

Answer 1

Form a bilayer in the cell membrane
Hydrophilic heads point to the outside of the cell-surface membrane attracted by water
Hydrophobic tails point into the centre of the cell membrane repelled by water

Question 2

What are the functions of phospholipids in the membrane?

Answer 2

-Allow lipid-soluble substances to enter and leave the cell
-Prevent water-soluble substances entering and leaving the cell
-Make the membrane flexible and self-sealing

Question 3

How are proteins in the membrane?

Answer 3

• Surface of bilayer which give mechanical support to the membrane as cell receptors for molecules
• Span the bilayer
  –> protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane
  –> carrier proteins that binds to ions or molecule then change shape to move them

Question 4

What are the functions of the proteins in the membrane?

Answer 4

• Provide structural support
• Act as channels transporting water-soluble substances across the membrane
• Allow active transport across the membrane through carrier proteins
• Form cell-surface receptors for identifying cells
• Help cells adhere together
  -Act as receptors e.g. hormones

Question 5

Where are cholesterol molecules and what do they do?

Answer 5

Within the phospholipid bilayer
Add strength to the membranes
Very hydrophobic so prevent loss of water and dissolved ions from the cell
Pull together fatty acid tails of the phospholipid molecules, limiting their movement

Question 6

What are the functions of cholesterol in the membrane?

Answer 6

• Reduce lateral movement of other molecules including phospholipids
  -Make the membrane less fluid at high temperatures
• Prevent leakage of water and dissolved ions from the cell

Question 7

What is the structure of a glycolipid?

Answer 7

Made up of a carbohydrate covalently bonded with a lipid
Carbohydrate portion extends from bilayer to outside cell as a cell-surface receptor for chemicals

Question 8

What are the functions of glycolipids in the membrane?

Answer 8

Act as recognition sites
Help maintain the stability of the membrane
Help cells to attach to one another and so form tissues

Question 9

What is the structure of glycoproteins?

Answer 9

Carbohydrate chains attached to many extrinsic proteins on the outer surface of the cell membrane
These glycoproteins also acts as cell-surface receptors

Question 10

What are the functions of glycoproteins?

Answer 10

Act as recognition sites
Help cells to attach to one another and so form tissues
Allows cells to recognise one another

Question 11

Why do most molecules not freely diffuse across the cell-surface membrane?

Answer 11

• Not soluble in lipids and therefore cannot pass through the phospholipid layer
• Too large to pass through the channels
  -Same charge as the charge on the protein channels so are repelled
• Polar so have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 12

What is the name of the model of the cell surface membrane and what does it do?

Answer 12

Fluid-mosaic model
Shows the way in which all the various molecules are combined into the structure of the cell-surface membrane

Question 13

Why is the cell surface membrane described as fluid?

Answer 13

The individual phospholipid molecules can move relative to one another
Gives the membrane a flexible structure that is constantly changing in shape

Question 14

Why is the cell surface membrane described as a mosaic?

Answer 14

The proteins embedded in the bilayer vary in shape, size and pattern in the same way as the stone or tiles of a mosaic

Question 15

Define simple diffusion

Answer 15

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed

Question 16

What are 3 things to remember for diffusion and passive transport?

Answer 16

-All particles are constantly in motion due to their KE
- Random motion
- Particles are constantly bouncing off each other and other objects

Question 17

What type of molecules diffuse by facilitated diffusion and why?

Answer 17

Charged ions, large and polar molecules - cannot diffuse easily because of the hydrophobic nature of the fatty-acid tails

Question 18

What is facilitated diffusion and what is involved?

Answer 18

Passive
Occurs at specific points on the plasma membrane where there are special protein molecules
Protein channels and carrier proteins

Question 19

How do protein channels work?

Answer 19

These proteins form water-filled hydrophilic channels across the membrane
Allow specific water-soluble ions to pass - otherwise channel is closed - so control entry and exit of ions

Question 20

What happens where ions bind with a protein channel?

Answer 20

Causes it to change shape in a way that closes it to one side of the membrane and opens it to the other side

Question 21

How do carrier proteins work?

Answer 21

When a molecule (e.g. glucose) that is specific to the protein, it binds with the protein - changes shape so the molecule is released to the inside of the membrane
No external energy needed

Question 22

Define osmosis

Answer 22

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane

Question 23

What rules does water potential follow?

Answer 23

• Addition of a solute to pure water will lower its water potential
• The WP of a solution must always be less than zero
• Water will move by osmosis from a region of higher water potential (less negative) to lower (more negative)

Question 24

Define active transport

Answer 24

The movement of molecules or 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 25

What is ATP used for in active transport?

Answer 25

-Directly move molecules
- Individually move molecules using a concentration gradient which has already been set up by active transport –> co transport

Question 26

How does active transport differ from passive forms of transport?

Answer 26

-ATP needed
-Against concentration gradient
-Carrier proteins act as pumps
-Selective process

Question 27

Describe the direct active transport of a single molecules or ion

Answer 27

• The molecule or ion binds to receptor sites on a carrier protein in the plasma membrane
• On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and a phosphate –> the protein molecule changes shape and opens to the opposite side of the membrane
• The molecule/ion is released to the other side of the membrane
  -The phosphate molecule is released from the protein so the protein goes back to its original shape (so the process can be repeated)

Question 28

What is the difference between facilitated diffusion and active transport?

Answer 28

Similarities
- Both use carrier protein

Differences
-FD occurs down a concentration gradient whereas active transport occurs against a concentration gradient
-FD does not require metabolic energy whereas AT does (ATP)

Question 29

When is more than one molecule/ion moved by active transport?

Answer 29

-Sodium ions are removed from the cell/organelle WHILE K ions are actively taken in from the surroundings

Question 30

How is the rate of movement across membranes increased?

Answer 30

Epithelial cells lining the ileum, they have microvilli
More SA for insertion of carrier proteins for diffusion, facilitated diffusion and active transport
Another way - Increase the number of protein channels and carrier proteins in any given area of membrane

Question 31

What is diffusion’s role in absorption in the ileum?

Answer 31

-As carbohydrates and proteins are digested, there is a greater conc of glucose and amino acid within the ileum than the blood
-Hence, there is a concentration gradient down which glucose moves by facilitated diffusion from the ileum to the blood
-Blood constantly circulated, glucose absorbed is constantly removed by the cells for respiration
-The microvilli help to maintain the conc gradient between ileum and the blood
-Rate of movement by FD across epithelial cell-surface membranes is increased

Question 32

What is the role of active transport in absorption in the ileum?

Answer 32

-Diffusion results in equilibrium –> not all the glucose and amino acids can be absorbed
-Co-transport causes glucose or amino acids to be draw into the cells with the sodium ions that have been actively transported out by the sodium-potassium pump

Question 33

Describe the sodium-potassium pump

Answer 33

1. Sodium ions actively transport out of epithelial cells into the blood, in a protein-carrier molecule in the cell-surface membrane of the epithelial cells
2. This maintains a higher conc of Na+ in the lumen of the intestine than inside the epithelial cells
3. Na+ ions diffuse into the epithelial cells down this concentration gradient through a co-transport protein, during this they carry amino acid/glucose molecules with them into the epithelial cells
4. Glucose/amino acids pass into the blood plasma by facilitated diffusion using another tube of carrier