Membrane Structure And Function.

Question 0

What was the first model of the membrane?

Answer 0

Produced in the 1970’s it presented the membrane as a static structure.

Question 1

Where are membranes found?

Answer 1

Surrounding all cells (the plasma membrane). And surrounding organelles within eukaryotic cells.

Question 2

What is the new membrane model?

Answer 2

The fluid mozaique model.

Question 3

Describe the membrane Bilayer.

Answer 3

This consists of a bimolecular layer of phospholipids which is about 6nm wide and forms the centre of the membrane. In this structure the hydrophobic tails point inwards (away from the cytoplasm or tissue fluid) and the hydrophilic heads point outwards creating a hydrophobic centre to the bilayer.

Question 4

What is the purpose of the arrangment of the bilayer?

Answer 4

To prevent water and water soluble like glucose from passing

Directly through the general membrane.

Question 5

How does the membrane control the movement of water and its solutes?

Answer 5

By forcing them to move only through specific channels.

Question 6

How is an intrinsic protein positioned in the membrane?

Answer 6

There are embedded in the membrane but do not pass right the way through it.

Question 7

How is an extrinsic protein positioned in the membrane?

Answer 7

These are proteins found on the outer or inner surface of the membrane.

Question 8

Where are trans-membrane proteins positioned on the membrane?

Answer 8

They pass straight the way through the membrane.

Question 9

What are channel protiens?

Answer 9

These are tubular trans-membrane proteins which are lined on the inside with hydrophilic side groups creating a 1nm hydrophilic channel through which water and its solutes can pass.

Question 10

Where are carbohydrates found in the plasma membrane?

Answer 10

These are found only on the outside of the membrane and consist of short, branched monosaccharide chains called oligosaccharides which can be attached to either proteins (glycoproteins) or lipids (glycolipids).

Question 13

Describe the selectively permeable nature of the membrane.

Answer 13

Because of its structure it allows very small molecules like CO2 and O2 to pass straight through the membrane whilst larger molecules like glucose which are water soluble and smaller than 1nm must pass through channel proteins to diffuse. Any molecule larger I.e disaccharides cannot pass through the membrane as they are too large.

Question 14

What is facilitated diffusion?

Answer 14

The process by which large molecules are helped throughout the membrane by binding to membrane proteins which carry them through the membrane and release them on the other side using the kinetic energy of the molecules themselves. I.e no energy is required.

Question 15

In what “direction” does facilitated diffusion occur?

Answer 15

In the direction of the diffusion gradient.

Question 16

What is the rate of facilitated diffusion limited by?

Answer 16

The factors that limit diffusion (temprature, pressure, diffusion gradient) and most importantly by the availability of membrane protiens. I.e when they are all being used the process cannot go any faster.

Question 17

What is active transport?

Answer 17

This is the movement of molecules against the diffusion gradient using carrier proteins called molecular pumps which are built into the membrane. Requiring energy in the form of A.T.P.

Question 18

How may ions pass through the membrane?

Answer 18

Anions can pass straight through the positive centre of the phospholipid bilayer whilst cations have to use channel protiens.

Question 19

How is the membrane involved in communication?

Answer 19

The branched carbohydrates on its surface act as receptor sites for external molecules (e.g insulin) which have a shaped fit with the carbohydrates.

Question 20

What happens when a molecule slots into a glycoprotein or glycolipid?

Answer 20

A change occurs within the cell.

Question 21

What are the two main factors in the degree of membrane permeability and how do they both work (in general)?

Answer 21

Temperature and the presence of organic solvents e.g ethanol.

By disrupting the phospholipid bilayer.

Question 22

What phase is the bilayer in at temperatures under 35°C and describe the movement of the molecules.

Answer 22

The gel phase where the phospholipids are moving very little.

Question 23

What happens at temperatures above 35°C?

Answer 23

The membrane enters a more fluid phase with the phospholipids moving faster and faster.

Question 24

Describe the degree of the effect of temprature on the membrane.

Answer 24

It is progressive, as the temperature goes up, so does the membrane permeability.

Question 25

What experiment is there to show the effect of temperature on membranes?

Answer 25

If a cube of betrothal is winced and placed in water at temperatures around 35°C none of the purple pigment will leak out as the membrane is in the gel phase.

Place it in water with a temperature of 50°C or above and the pigment will pass through the now fluid membrane and into the surrounding water.

Question 26

What happens when a cell is exposed to an organic solvent e.g ethanol?

Answer 26

The solvent disrupts the bilayer by progressively dissolving it away so the higher the ethanol concentration outside, the more permeable the membrane becomes.

Question 27

What a the six main ways in which substances can move across membranes?

Answer 27

Diffusion
Facilitated diffusion
Osmosis
Active transport
Phagocytosis
Pinocytosis.

Question 28

What is diffusion?

Answer 28

The random movement of particles from an area of high concentration to an area of low concentration due to latent heat.

Question 29

What are the factors that control the rate of diffusion?

Answer 29

The concentration gradient
Temperature
Pressure

Question 30

Why do all living things prefer to use diffusion when possible?

Answer 30

Because it requires no energy from the cell.

Question 31

Compare the rate of diffusion to the rate of facilitated diffusion and give an explanation.

Answer 31

Facilitated diffusion is faster because the carrier proteins pull the substance through the membrane. but it will level off once the carrier proteins are saturated.

Question 32

In what “direction” does facilitated diffusion occur?

Answer 32

In the same direction as the diffusion gradient.

Question 33

Thwart is the rate of active transport control by?

Answer 33

The need of the cell for that particular substance. I.e active transport only goes as fast as it needs to to save A.T.P.

Question 34

How do lipid soluble substances pass through the membrane?

Answer 34

By passing straight through the phospholipid bilayer through diffusion by dissolving in the phospholipid bilayer. E.g ethanol .

Question 35

What is active transport?

Answer 35

The movement of particles against the diffusion gradient by means of molecular pumps, requiring energy in the form of A.T.P

Question 36

Can a molecular pump draw in more than one type of substrate?

Answer 36

No because they are molecule specific as they have a specific configuration containing a receptor site which has a shaped fit with the molecule being pumped.

Question 37

What will cells carrying out active transport have?

Answer 37

Large numbers of mitochondria a high metabolic rate and a high demand for oxygen.

Question 38

What is the rate of active transport controlled by?

Answer 38

The rebate of active transport may respond to the availability of oxygen and glucose. (THESE THINGS HAVE O EFFECT ON THE REATE OF DIFFUSION AND FACILITATED DIFFUSION) However rate will only increase so long as the requirement of the cell for the molecule being pumped is not yet met.

Question 39

What could cause the rate of active transport to fall?

Answer 39

A respiratory poison such as cyanide which stops respiration will also stop active transport by halting the production of A.T.P.

Question 40

What is osmosis?

Answer 40

The movement of water from an area of high water potential to an area of low water potential through a partially permeable membrane.

Question 41

What is the rate of osmosis controlled by?

Answer 41

The same actors that control diffusion I.e water potential gradient, temperature and pressure.

Question 42

What is the water potential of pure water (highest water potential)?

Answer 42

0

Question 43

What happens to the water potential as more solutes are added?

Answer 43

I’d decreases from zero.

Question 44

Why does the addition of solutes lower water potential?

Answer 44

Because the molecules in solution take up space which could be filled by more water molecules decreasing the potential for water to move.

Question 45

What is phagocytosis?

Answer 45

This refers to the cell engulfing a solid material by an infolding of the plasma membrane to form a phagocytotic vesicle. This is used by single felled animals to feed and by white blood cells called phagocytes to defend the body.

Question 46

What is pinocytosis?

Answer 46

This is the unfolding of the plasma membrane to engulf fluid only from the outside of the cell to create pinocytotic vesicles.

Question 47

What does endocytosis refer to?

Answer 47

This is a collective term for both phagocytosis and pinocytosis.