4. CELL MEMBRANES AND TRANSPORT

Question 1

Examples of exocytosis

Answer 1

• secretion of digestive enzymes from the cells of the pancreas
• plants use exocytosis to get their cell wall building materials to the outside of the cell surface membrane

Question 2

Why is ATP required for active transport

Answer 2

because substances are moved against a concentration gradient

Question 3

How is ATP required for active transport

Answer 3

• the energy is used to make the carrier protein change shape
• transferring the molecules or ions across the membrane

Question 4

Ions in high concentration in cells than outside

Answer 4

potassium and chloride ions

Question 5

Role of the sodium-potassium pump

Answer 5

• for each molecule of ATP used

- three sodium ions are pumped out of the cell and two potassium ions into the cell

Question 6

Result of the sodium-potassium pump

Answer 6

• the inside of the cell becomes more negative than the outside
• a potential difference is created across the membrane
• significant in nerve cells

Question 7

Sites where active transport takes place

Answer 7

• reabsorption of certain molecules or ions in the kidneys after filtration
• absorption of some products of digestion in the gut
• in plants, active transport is used to load sugar from photosynthesising cells into the phloem tissue
• in plants, active transport is used to load inorganic ions from the soil into root hairs

Question 8

Why is it important to maintain a constant water potential inside the bodies of animals

Answer 8

• if the water potential of the solution surrounding the cell is too high, red blood cells bursts (haemolysis)
• if the water potential is too low:
  red blood cell gets crenated (shrinks)
  plant cell gets plasmolysed (cytoplasm pulls away)

Question 9

What happens when a plant cell is placed in a hypotonic solution

Answer 9

• water enters the cell by osmosis and the volume of the cell increases
• the cell wall pushes against the expanding protoplast and pressure starts to build up rapidly
• this pressure potential increases until the water potential inside the cell equals the water potential outside the cell and equilibrium is reached

Question 10

Method of preparing membranes to split open the bilayer

Answer 10

freeze-fracturing

Question 11

Why is the membrane more fluid when there are more unsaturated fatty acid tails

Answer 11

• the unsaturated fatty acid tails are bent

- so they fit together more loosely

Question 12

Factors that increase fluidity of the membrane

Answer 12

• more unsaturated fatty acids
• shorter tail
• higher temperature
• more cholesterol

Question 13

How do organisms respond at a lower temperature to live

Answer 13

• membranes become less fluid
• so bacteria and yeasts increase the proportion of unsaturated fatty acids in their membranes
• so the membrane becomes more fluid
• can also increase the amount of cholesterol in the membrane
• this prevents close packing of the phospholipid tails so increases fluidity

Question 14

In transmembrane proteins, what are the hydrophobic regions that cross the membrane made of

Answer 14

alpha-helical chains

Question 15

Why do intrinsic proteins stay in the membrane

Answer 15

• because the hydrophobic regions, made from hydrophobic amino acids, are next to the hydrophobic fatty acid tails
• and are repelled by the watery environment either side of them
• because the hydrophilic regions, made from hydrophilic amino acids, are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment inside or outside the cell
• or line hydrophilic pores which pass through the membrane

Question 16

Types of lipid present in the cell membrane

Answer 16

phospholipids
cholesterol
glycolipids

Question 17

Types of protein in the cell membrane

Answer 17

protein

glycoprotein

Question 18

Why can polar (charged) molecules not pass through the cell membrane

Answer 18

• because the fatty acid tails of phospholipids that make up the membrane are non-polar (hydrophobic)
• so they act as a barrier to most water-soluble substances

Question 19

How can phospholipids act as signalling molecules

Answer 19

• some phospholipids can be modified chemically to act as signalling molecules
• they can move about in the phospholipid bilayer, activating other molecules such as enzymes
• they can be hydrolysed to produce small, water-soluble glycerol related molecules that can diffuse into the cytoplasm and bind to specific receptors

Question 20

How do cholesterol molecules fit in the phospholipid bilayer

Answer 20

like phospholipids, they have a hydrophilic head and hydrophobic tail so they fit in with their heads at the membrane surface

Question 21

Presence of cholesterol in cell membranes

Answer 21

• almost as much as phospholipids in animal cells
• much less common in plant cells
• absent from prokaryotic cells (compounds similar to cholesterol serve the same function)

Question 22

How does more cholesterol increase the fluidity of the membrane

Answer 22

• prevents close packing of the phospholipid tails

Question 23

Functions of cholesterol

Answer 23

• prevent the membrane from becoming too rigid at low temperatures by preventing close packing of the phospholipid tails
• interactions between phospholipid tails and cholesterol molecules prevent the membrane from becoming too fluid at high temperatures
• important for the mechanical stability of membranes, as without it membranes quickly break and cells brust open
• hydrophobic regions of cholesterol prevent polar molecules or ions fro escaping. particularly useful in the myelin sheath around nerve cells

Question 24

Functions of glycolipids/glycoproteins

Answer 24

• carbohydrate chains project into the watery fluids surrounding the cell and form hydrogen bonds with water molecules, stabilising the membrane structure
• carbohydrate chains form a sugary coating outside the cell called the glycocalyx
• carbohydrate chains help the glycoproteins and glycolipids to act as receptor molecules, which bind with particular substances at the cell surface membrane
• some glycolipids and glycoproteins act as antigens, allowing cell-cell recognition

Question 25

Molecules responsible for the stability of the membrane structure

Answer 25

cholesterol: without it, membranes quickly break and cells burst open
carbohydrate chains on lipids/proteins: form hydrogen bonds with water molecules

Question 26

What is the glycocalyx

Answer 26

• carbohydrate chains on lipids/proteins form a sugary coating outside the cell
• animals: mainly glycoproteins
• plants: mainly glycolipids

Question 27

What determines the types of receptors on cells

Answer 27

the function of the cell

Question 28

Functions of proteins in the cell membrane

Answer 28

• transport proteins
• others may be enzymes
• some proteins on the inside of the cell surface membrane may be attached to a system of protein filaments inside the cell, known as the cytoskeleton
• involved in processes in membranes such as respiration and photosynthesis in mitochondria and chloroplasts