Transport across cell membranes

Question 1

What are the molecules which form the structure of the cell membrane?

Answer 1

Phospholipids
Proteins
Cholesterol 
Glycolipids 
Glycoproteins

Question 2

What is the structure of phospholipids? (in relation to membranes)

Answer 2

Form a phospholipid bilayer
Hydrophilic head point outwards (outside) the membrane which attracts water
Hydrophobic tails face inwards and point to the centre which repel water

Question 3

What are the functions of phospholipids?

Answer 3

Lipid soluble molecules move across the membrane via the phospholipids (enter/leave)
Prevent water soluble substances entering/leaving
Makes membrane flexible/self-sealing

Question 4

Where are proteins located in phospholipid bilayer?

Answer 4

Interspersed throughout
Some don’t extend completely across
Some span completely across, forming protein channels to allow water-soluble ions to diffuse across + some are carrier proteins that bind to ions/molecules like glucose and change shape to allow molecules across the membrane

Question 5

What are the functions of proteins?

Answer 5

Provide structural support
Act as channels to transport water soluble molecules
Allow active transport through carrier proteins across the membrane
Form cell surface receptors for identifying cells
Help cells join together

Question 6

What is the function of cholesterol in the phospholipid bilayer?

Answer 6

Reduce movement of molecules -> e.g phospholipids
Make membrane less fluid at high temperatures
Prevent leakage of water and dissolved ions from cell
Provide strength

Question 7

What is the structure of glycolipids?

Answer 7

Carbohydrate covalently bonded with a lipid

Question 8

What is the function of glycolipids in the phospholipid bilayer?

Answer 8

Act as recognition sites
Maintain stability
Form tissues by helping cells attract to one another

Question 9

What is the structure of glycoproteins?

Answer 9

Carbohydrates attached to proteins on outer surface of membrane

Question 10

What is the function of glycoproteins?

Answer 10

Act as recognition sites

Allow cells to recognise each other -> lymphocytes can recognise own cells

Question 11

Why can’t molecules freely diffuse across the cell membrane?

Answer 11

Not soluble in lipids so cannot pass through phospholipid bilayer
Too large to pass through channels
Same charge as protein channels so are repelled
Polar so cannot pass through the non polar hydrophilic tails in bilayer

Question 12

What is the fluid mosaic model?

Answer 12

How molecules are combined to form the phospholipid bilayer

Fluid- phospholipid molecules can freely move + has a flexible structure which changes shape constantly

Mosaic- proteins within the membrane are different shapes, sizes

Question 13

What is the definition of simple diffusion? + what type of process is it?

Answer 13

Net movement of molecules or ions from a region of high concentration to a region of low concentration until evenly distributed

Passive
Down a concentration gradient

Question 14

What factors affect the rate of diffusion?

Answer 14

Concentration gradient
Size of molecule
Temperature

Question 15

How do protein channels work?

Answer 15

Allow specific water-soluble molecules to pass through
Therefore there is control over the entry and exit of ions
Ions bind to the protein which causes it change shape, allowing it to close one end of the protein but open the other

Question 16

How do carrier proteins work?

Answer 16

When a molecule that is specific to the protein present, it binds to the protein causing it to change shape so the molecule is released to the inside of the membrane

Question 17

What are the differences between simple and facilitated diffusion?

Answer 17

Simple

1. Passes directly through phospholipid bilayer
2. Protein channels are not required
3. Small/non polar molecules
4. Slower

Facilitated

1. Does not pass directly through phospholipid bilayer
2. Protein channels are required
3. Large/polar molecules
4. Faster

Question 18

What is the definition of osmosis?

Answer 18

The passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane

Question 19

What is the highest value of water potential?

Answer 19

Pure water = 0

More negative the value -> lower water potential

Question 20

What factors affect the rate of osmosis?

Answer 20

Surface area
Water potential gradient
Thickness of exchange surface
Diffusion distance

Question 21

What factors affect the rate of facilitated diffusion?

Answer 21

Surface area
Concentration gradients (until the number of proteins is the limiting factor as all are in use/saturated)
Number of channel/carrier proteins

Question 22

What is the definition of active transport?

Answer 22

The movement of molecules or ions into or out of the cells from a low to high concentration using ATP or carrier proteins

Question 23

Describe the direct active transport of a single molecule

Answer 23

Molecule binds to receptor site of carrier protein
Inside of cells/organelle, ATP binds to protein causing it to split into ADP and a phosphate - this results in the protein changing shape to open on the other side of membrane + release molecule
Phosphate molecule is released so protein reverts back to original shape + phosphate and ADP recombine to from ATP in respiration

Question 24

What is a sodium potassium pump?

Answer 24

Sodium ions are actively transported out of cell + potassium ions are actively transported in from surroundings

Question 25

How is the rate of movement across membranes increased?

Answer 25

Microvilli-> Larger surface area

Increase number of protein channels/carrier proteins

Question 26

What is the role of diffusion in absorption?

Answer 26

Greater concentration of glucose/amino acids as carbs and proteins are being digested continuously

• > this creates a concentration gradient where glucose moves down (by facilitated diffusion) from ileum to blood
• > concentration gradient between ileum + blood which increases rate of facilitated diffusion
• > glucose is constantly moving around the body through the blood to supply cells

Question 27

How does co-transport of glucose take place?

Answer 27

1. Sodium ions are actively transported out of epithelial cell by sodium potassium pump into blood using a carrier protein
2. This creates a higher conc of sodium ions in lumen of small intestine than inside epithelial cells
3. Sodium ions diffuse into epithelial cells through co-transport protein down conc gradient + also carry amino acid/glucose molecules with them
4. Glucose/amino acids pass into the blood plasma by facilitated diffusion