B2.1 Membrane and Membrane Transport

Question 1

State the difference between a lipid bilayer and the double membrane of any organelle

Answer 1

A lipid bilayer has 2 layers but in organelles there is only 1

Question 2

B2.1 5 Movement of water molecules across membranes by osmosis and the role of aquaporins

Define osmosis (1m)

Answer 2

Osmosis is the passive movement of water molecules across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Water molecules move, through random movement, from region of lower solution concentration to higher solute concentration

Even though water molecules are hydrophilic, they are small enough to pass through the phospholipid bilayer.

Question 3

B2.1 5 Movement of water molecules across membranes by osmosis and the role of aquaporins

How do protein channels help osmosis? (2m)

Answer 3

There are also protein channels specialized in transporting water – aquaporins
Water is thus transported by facilitated diffusion, from lower to higher solute concentration
Aquaporins greatly increase membrane permeability to water

(The aquaporin pore is narrow, only allowing a single water molecule to go through at one time, preventing other molecules from moving through.)

Question 4

B2.1 5 Movement of water molecules across membranes by osmosis and the role of aquaporins

What is an aquaporin and what does it do?

Answer 4

There are also protein channels specialized in transporting water – aquaporins

Aquaporins greatly increase membrane permeability to water

Water is thus transported by facilitated diffusion, from lower to higher solute concentration

Question 5

B2.1.6 Channel proteins for facilitated diffusion

Explain what are channel proteins and the roles they play in facilitating diffusion. Hint: Facilitate

Answer 5

Channel proteins provide hydrophilic channels for polar and charged molecules to pass through. E.g. for glucose to pass through.
Channel proteins are usually specific - only one type of particle passes through.
Eg. sodium channels only allow Na+ to pass through, but not K+

Question 6

B2.1.6 Channel proteins for facilitated diffusion

How do channel proteins exercise their specificity?

Answer 6

By controlling
1) the diameter of its pore
2) the chemical properties of amino acids lining the pore

Question 7

B2.1.6 Channel proteins for facilitated diffusion

Only integral proteins can be transmembrane proteins. With an example, explain its function.

Answer 7

e.g. Ion channels. They are transmembrane proteins which enclose a hydrophilic pore. When a channel opens, it provides an aqeuous pathway for ions to cross between the extracellular fluid and intracellular fluid.

Question 8

B2.1.7 Pump proteins for active transport

Explain , with the use of an example, how are pump proteins used in active transport?

Answer 8

Active transport requires additional energy as the molecules or ions are moved against their concentration gradients.

Hence energy from ATP is used to move substances against their concentration gradient across cell membranes. This is done with the help of pump proteins (integral membrane proteins)

Energy from hydrolysis of ATP is used to change the conformation of the protein – this allows the ion or molecule to pass out on the opposite side of the membrane

Membrane pumps are specific to particular molecules or ions
E.g. sodium-potassium pump

Question 9

B2.1.7 Pump proteins for active transport

Explain how does a sodium potassium pump carry out active transport? (3-4 m)

Answer 9

Firstly, 3 Na+ and 1 ATP bind to the protein pump.

Secondly, Hydrolysis of ATP occurs and phosphorylates the pump protein and changes its shape.

Thirdly, the shape change releases Na+ outside the cell and enables K+ to bind to the cell.

Lastly, the release of phosphate returns the pump to its original shape, releasing K+ into the cell’s interior and once again exposing Na+ to binding sites. The cycle repeats

Question 10

B2.1.8 Selectivity in membrane permeability

Compare and contrast between simple, diffusion, facilitated diffusion and active transport (3m)

Answer 10

Selective permeability

Facilitated diffusion and active transport allow selective permeability in membranes. (e.g. each proetin channel in facilitated diffusion is specific to each molecule) Simiarly, protein pumps in active transport are specific to a type of element eg sodium-potassium pump only pumps sodium or potassium.

However, Permeability by simple diffusion is not selective and depends only on the size and hydrophilic or hydrophobic properties of particles.

Substance transported

Simple diffusion only transports small, non-polar hydrophobic molecules. However, facilitated diffusion and active transport will carry hydrophilic substances e.g. polar molecules or charged ions.

Part of the membrane that the substance moves through

Simple diffusion moves through the hydrophobic fatty acid tails of the lipid bilayer. However in faciitated diffusion, substance moves through protein carrier or channel. Lastly in active transport, substance only moves through the protein pump.

Concentration gradient

In simple and faciliatted diffusion, the substance moves down the concentratio gradient. However, in active transport, it moves against the concentration gradient

Under Category as an Active or passive process

Facilitated and simple diffusion are passive transport. Only active transport requires ATP, which is used to create energy to change the conformation of the protein to allow hydrophilic molecules or ions to pass through.

Involving phospholipids

Endocytosis and exocytosis

Question 11

General points to note when describing membrane transport (other than endo/exocytosis)

Answer 11

-Substance being transported
-Direction of movement
-Gradient (along or against / specific chemical)
-Involvement of membrane protein or phospholipids
-Passive or active?
-Named example (name of substance, name of protein if applicable)

Question 12

B2.1.9 Structure and function of glycoproteins and glycolipids

Define endocytosis and exocytosis

Answer 12

Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane, and bringing it into the cell.

Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell.

They are also involved in cell-cell recognition and adhesion, the latter is the binding of cells to the cell surface.

Question 13

B2.1.9 Structure and function of glycoproteins and glycolipids

What are the some shared functions of glycoprotein and glycolipids?

Answer 13

Cell-cell recognition

Cell-cell recognition helps the immune system to distinguish between self and non-self cells, so pathogens and foreign tissue can be recognized and destroyed.

Receptor sites
Glycoproteins or lipids bind with substances at the cell surface. Receptor types include:
Signalling receptors , which bind to hormones & neurotransmitters
Receptors involved in endocytosis,

Cell identification
This allows the immune system to determine whether a cell belongs in the body, or whether it is a pathogen eg. antigen for ABO blood group

Cell-cell adhesion
to form tissues

Question 14

B2.1.10 Fluid mosaic model of membrane structure

Draw a two-dimensional representation of the fluid mosaic model and include peripheral and integral proteins, glycoproteins, phospholipids and cholesterol. Also should also be able to indicate hydrophobic and hydrophilic regions.

Answer 14

Describe from micro 🡪 macro
Components: phospholipids, proteins and cholesterol
Phospholipids: phosphate, glycerol + 2 fatty acids
Amphipathic: hydrophilic phosphate head + hydrophobic hydrocarbon/fatty acid tail
Proteins: different ways of referring to them
Arrangement in membrane – integral and peripheral proteins
Chemical composition – glycoproteins (protein + carbohydrate)
Function of glycoproteins (and glycolipids) – cell-cell recognition, receptor, cell identification, cell adhesion
(extra for SL) Cholesterol: a type of steroid
Maintains membrane fluidity

Question 15

Compare and contrast the structure and function of peripheral and integral proteins.

Question 16

When a concentrated solution of glucose is separated from a dilute solution of glucose by a partially permeable membrane, determine which solution will show a net gain of water molecules.

Answer 16

The dilute solution of glucose will show a net gain of water molecules.

This is due to osmosis, the process where water molecules move from an area of higher water potential (the dilute solution) to an area of lower water potential (the concentrated solution) across a partially permeable membrane. The concentrated glucose solution has a lower water potential because of its higher solute concentration, so water moves toward it. Therefore, the dilute solution will lose water, and the concentrated solution will gain water until equilibrium is reached.

Question 17

Explain what happens to a fungal spore that germinates after landing on jam made from fruit and its own weight of sucrose.

Question 18

Draw a diagrammatic cross-section of a fluid mosaic membrane. Label the phospholipid bilayer, hydrophobic region, hydrophilic region. cholesterol, glycoprotein, integral protein, peripheral protein.

Question 19

Describe the role of membranes in cells