2.1.5 Biological Membranes

Question 1

What are the 5 functions of membranes?

Answer 1

• Partially permeable- controls what goes through them.
• Cell signalling.
• Provide attachment sites to enzymes and other molecules.
• Allow electrical signals to pass along them.
• Produce compartments inside the cell.

Question 2

What are membranes made of?

Answer 2

• The hydrophilic head (phosphate group)
• The hydrophobic tail (glycerol and 2 fatty acids)

Question 3

What parts of the membrane are polar and water soluble?

Answer 3

Tail is non-polar, hydrophobic and water insoluble.
Head is polar, hydrophilic and water soluble.

Question 4

Why do phospholipids form a bilayer in the membrane?

Answer 4

Phospholipids have a polar phosphate group which is hydrophilic and faces aqueous solutions.
Fatty acid tails are non-polar and will move away from aqueous environments.
As both tissue fluid and cytoplam is aqueous, phospholipids form 2 layers with the hydrophobic tails facing inwards and the hydrophilic heads facing outwards.

Question 5

What is the difference between integral and peripheral proteins?

Answer 5

Integral/intrinsic go through the whole membrane whereas peripheral/extrinsic proteins go through one layer only.

Question 6

How do polar and non-polar molecules pass through the bilayer?

Answer 6

Non-polar molecules diffuse straight through.
Polar molecules require proteins to enable them to pass through.

Question 7

What are glycolipids and glycoproteins?

Answer 7

Lipids and proteins in the membrane which have short carbohydrate chains protruding out from them.

Question 8

What is the glycocalyx and what is its function?

Answer 8

The glycocalyx is the region around the cell membrane consisting of glycoproteins and glycolipids.
Used in cell signalling and cell adhesion.

Question 9

What proteins do membranes contain?

Answer 9

• Glycoproteins
• Peripheral/extrinsic proteins
• Integral/intrinsic proteins
• Channel proteins

Question 10

Why is the model for membrane structure known as the fluid mosaic model.

Answer 10

The phospholipid molecules can move freely laterally, making the membrane fluid.
Proteins are distributed throughout the membrane unevenly in a mosaic pattern.

Question 11

What are channel proteins?

Answer 11

Pores in the membrane which allow large, polar molecules to pass through. The may be ‘gated’ or only allow one type of ion through.

Question 12

What are carrier proteins?

Answer 12

Proteins which have a specific shape for molecules to fit then this changes to allow the molecules through. Often actively move substances (using energy)

Question 13

What are the functions of membrane bound proteins?

Answer 13

• Receptors
• Enzymes
• Structural (attached to microtubules)
• Transport

Question 14

How does cholesterol affect membranes?

Answer 14

Regulates the fluidity of the membrane.
- High temps becomes less fluid/more stable.
- Low temps becomes more fluid (prevents freezing).

Question 15

What are the different distances of cell signalling called?

Answer 15

Endocrine- other cells across large distances.
Paracrine- other cells locally.
Autocrine- within cell or cells of same type.

Question 16

What are the 5 main steps in cell signalling?

Answer 16

1. Stimuli
2. Receptors
3. Transducers
4. Amplifiers
5. Intracellular responses

Question 17

How do hormone receptors work?

Answer 17

Hormones travel in blood and target any cell with receptors for that hormone.
Hormone and receptor on target cell bind due to complementary shapes.
The binding causes the cell to respond.

Question 18

What is insulin and what is it’s function?

Answer 18

Insulin is a hormone that responds to high blood glucose levels. When the hormone binds to the receptor, more glucose channels become present in the membrane (vesicles move to the cell surface membrane from inside the cell) so cells take up more glucose and blood glucose levels are reduced.

Question 19

How do some medicinal drugs interfere with receptors?

Answer 19

They have been developed so are complementary to certain receptors.
E.g. Beta blockers- block receptors and prevent the heart muscle from increasing the heart rate so extremely high levels.

Question 20

What happens to the membrane in the beetroot practical?

Answer 20

• Molecules spin and vibrate faster.
• Lipids become more fluid as temp increases so membrane becomes more fragile.
• The H+ bonds and disulphide bridges in the proteins may break apart
• This creates holes in the membrane and allowing pigment to spill out.

Question 21

What is diffusion?

Answer 21

The net movement of particles from an area of high concentration to low concentration, down the concentration gradient, not requiring energy.

Question 22

How can diffusion across membranes differ?

Answer 22

Some molecules that are small and non-polar can diffuse straight through.
Other very small polar molecules can diffuse straight through (slowly).
Larger or charged particles (like glucose or amino acids) must pass through membrane proteins to cross the membrane. This is called facilitated diffusion and is still a passive process.

Question 23

What are the two types of proteins used in facilitated diffusion?

Answer 23

• Channel proteins
• Carrier proteins

Question 24

How do channel proteins work?

Answer 24

Pores in the membrane with a specific shape that only allow a certain shaped ion through. Can be gated to open and close.

Question 25

How do carrier proteins work?

Answer 25

Specific shape so that only a certain molecule fits into it at the membrane surface. The molecule binding causes a change in shape of the protein that delivers the molecule to the other side of the membrane.

Question 26

What factors affect the rate of diffusion?

Answer 26

• Temperature
• Concentration gradient
• Stirring/moving
• Surface area
• Distance/thickness
• Size of molecule (small diffuse fastest)

Question 27

What are the differences between active and passive transport?

Answer 27

Active requires energy in the form of ATP and is used for bulk transport (endocytosis and exocytosis) whereas passive does not require energy and is diffusion, osmosis and facilitated diffusion.

Question 28

What is active transport?

Answer 28

Movement of particles against the concentration gradient requiring ATP. It is much faster and allows for accumulation of substances.

Question 29

Why is active transport used in the intestines instead of only diffusion?

Answer 29

• Glucose is absorbed from the small intestine to the blood. So can only be absorbed when there is more glucose in small intestine.
• Active transport allows glucose to be absorbed irrespective of the concentration gradient.

Question 30

How does active transport work through a membrane?

Answer 30

• Molecule binds to carrier protein and ATP attached to the membrane protein on the inside of the cell.
• Binding of ATP ion causes protein to change shape so that access for the molecules is open to the inside of the membrane but closed to the outside.

Question 31

What is bulk transport?

Answer 31

Moving large amounts of substances in and out (endo- and exo-cytosis). Involves fusing and pinching off of membranes. Is an active process. Energy is used to from vesicled and to move vesicles around the cell.

Question 32

What is osmosis?

Answer 32

The net movement of water particles from an area of high water potential to an area of low water potential, across a partially permeable membrane.

Question 33

What is water potential?

Answer 33

The measure of the water molecule potential for movement in a solution.
It is the pressure created by water molecules.
At rtp, pure water has a potental of 0.

Question 34

How does adding and removing solute affect water potential?

Answer 34

• Adding solute makes it more negative
• Removing it makes it less negative (more positive).
  This is because when substances dissolve in water, the molecules cluster around the solute and can’t move so has decreased water potential.

Question 35

What is hydrostatic pressure?

Answer 35

A pushing pressure.

Question 36

What is osmotic pressure?

Answer 36

A pulling pressure.
When there is an area of low water potential (high solute conc) water is pulled into it by osmosis.

Question 37

What happens when water moves in to a cell by osmosis down a water potential gradient?

Answer 37

In a plant cell, the cell wall prevents it bursting but the membrane pushes against the wall- the cell is turgid.
An animal cell will burst open- it is cytolysed.

Question 38

What happens when water moves out of a cell by osmosis down a water potential gradient?

Answer 38

Plant cell membrane will pull away from the cell wall as water leaves- the cell is plasmolysed.
Animal cell will shrink and appear wrinkled- it is crenated.

Question 39

What names are given to solutions with a high/low/same salt concentration?

Answer 39

Low salt solution- hypotonic
High salt solution- hypertonic
Same concentration- isotonic

Question 40

What is incipient plasmolysis?

Answer 40

The point at which the cell membrane of a plant cell begins to pull away from it’s cell membrane. This occurs when it is in a hypertonic solution, causing water to move out of the cell.