2.5 Biological membranes

Question 1

What are the roles of the cell surface membrane?

Answer 1

• separates the inside from the outside
• regulates transport in and out
• may contain enzymes involved in specific metabolic pathways
• has antigens for the immune system
• release chemical signals to other cells
• site of chemical reactions

Question 2

What is the fluid mosaic model?

Answer 2

The model which aims to outline the structure and function of the membrane

Question 3

Describe the fluid mosaic model:

Answer 3

• phospholipid bilayer (7nm)
• carrier proteins
• channel proteins
• glycoproteins
• peripheral proteins

Question 4

What is the purpose of cholesterol in eukaryotic membranes?

Answer 4

To regulate membrane fluidity

Question 5

How is a neuron a specialized cell?

Answer 5

• carrier and channel proteins span the long axis to allow electrical impulses
• myelin sheath is 20% protein and 76% lipid

Question 6

What difference do root hair cells have that make them optimal?

Answer 6

They have lots of carrier and channel proteins to allow easy uptake of ions from soil

Question 7

What is diffusion?

Answer 7

the movement of molecules down a concentration gradient

Question 8

How do steroid hormones move in and out of the cell?

Answer 8

Using simple diffusion to dissolve in the bilayer

Question 9

What are specific water channel proteins called?

Answer 9

Aquarporins, to allow the water to move across the membrane despite being a polar molecule

Question 10

How is the concentration gradient maintained?

Answer 10

• oxygen is used in mitochondria during aerobic respiration
• carbon dioxide used in the chloroplasts during photosynthesis

Question 11

What are the factors that affect rate of diffusion?

Answer 11

• Temperature
• Distance (of diffusion)
• surface area
• size of diffusing molecule
• concentration gradient

Question 12

What molecules cannot pass through the membrane?

Answer 12

charged molecules interact with the hydrophobic tails badly, some molecules are just too large.

Question 13

How do molecules, that cannot pass through the membrane via simple diffusion, enter the cell?

Answer 13

Facilitated diffusion - using carrier proteins across the membrane to be transported

Question 14

What and why are the neuron and epithelial channels specific?

Answer 14

They are specific to sodium or potassium ions which is crucial within the transmission of electrical impulses
They are specific to chloride ions to maintain mucus composition

Question 15

What is the definition of osmosis?

Answer 15

the net movement of water from a high water potential to a low water potential through a partially (semi) permeable membrane

Question 16

What liquid has the highest water potential?

Answer 16

distilled water

Question 17

What is the correlation between solute molecules and water potential?

Answer 17

more solute molecules = lower water potential

Question 18

What is water potential measured in?

Answer 18

kilopascals (kPa), starting at 0 at the highest and decreasing to negative for lower water potential

Question 19

What is the measurement for the water potential in distilled water?

Answer 19

0 kPa

Question 20

What happens to cells placed in a solution of higher water potential?

Answer 20

The wp in the cell is lower than the solution, so the water moves down the water potential gradient by osmosis through a semi-permeable membrane into the cell, gaining mass

Question 21

What is it called when an animal cell bursts?

Answer 21

cytolysis

Question 22

What happens when lots of water enters a plant cell?

Answer 22

The cell becomes turgid but due to the strong nature of the cell wall it does not burst

Question 23

What happens to cells placed in a solution of lower water potential?

Answer 23

The water potential in the cell is higher, so the water moves down the water potential gradient into the solution, losing mass

Question 24

What is it called when animal cells shrivel?

Answer 24

crenation

Question 25

What happens when a plant cell loses water?

Answer 25

The cytoplasm shrivels, pulling the cell membrane away from the cell wall, known as plasmolysation

Question 26

What is a plant tissue with plasmolysed cells called?

Answer 26

flaccid

Question 27

Why can’t metabolism take place in plasmolysed cells?

Answer 27

They are too dehydrated, the enzyme-catalysed reactions need a solution to work in

Question 28

What are key characteristics of active transport?

Answer 28

• Requires ATP
• Moves against the concentration gradient

Question 29

What are the main active processes of substance movement?

Answer 29

• active transport
• carrier proteins
• bulk transport

Question 30

What are the 2 types of bulk transport?

Answer 30

endocytosis and exocytosis

Question 31

What is endocytosis?

Answer 31

The intake of a substance, also known as phagocytosis for solid matter, and pino(endo)cytosis for liquids

Question 32

Why does endocytosis require ATP?

Answer 32

To form vesicles and transport using motor proteins

Question 33

What is exocytosis?

Answer 33

The vesicle fuses with the plasma membrane

Question 34

What is an example of exocytosis?

Answer 34

Synapses use exocytosis by fusing the vesicle with the plasma membrane to release the chemicals into the synaptic cleft

Question 35

Why is ATP needed in exocytosis?

Answer 35

One molecule of ATP is created for each step of the motor proteins on the cytoskeleton. ATP is also used for fusing the vesicle with the membrane

Question 36

What happens to unsaturated fatty acids at lower temperatures?

Answer 36

The acids become compressed and the kinks in their tails push the phospholipid molecules away, maintaining membrane fluidity.

Question 37

How does cholesterol help maintain membrane fluidity at low temperatures?

Answer 37

the molecules fill the gap between phospholipids so prevent total compression

Question 38

What happens to a membrane when the temperature increases?

Answer 38

A higher temp means more kinetic energy so the permeability of the membrane increases. Phagocytosis and cell signals will be affected.

Question 39

What does temperature do to the overall structure of the membrane?

Answer 39

hydrogen and ionic bonds break at high temperatures due to vibrations. Tertiary structure is affected so cells become denatured.