T4

Question 1

Each phospholipid consists of

Answer 1

1. Polar head - hydrophilic (water-loving)

2. Hydrocarbon tail - hydrophobic (water-hating)

Question 2

Why is it called Fluid Mosaic Model?

Answer 2

Fluid because the phospholipid molecules and some membrane proteins are able to move laterally by diffusion within their own monolayer.

Mosaic because if viewed from the surface, the membrane proteins are scattered throughout the phospholipid layer in a mosaic arrangement.

Model because no-one has seen a membrane looking like the diagram - the molecules are too small to see even with the most powerful microscope.

Question 3

Several types of molecules in the cell membrane

Answer 3

1. Phospholipid
2. Proteins
3. Glycolipids
4. Glycoproteins
5. Cholestrol

Question 4

Functions of phospholipid

Answer 4

Affect Fluidity and Permeability of the membranes

Question 5

Functions of Proteins

Answer 5

1. Provide structural support for the membrane
2. Act as a transport proteins namely the channel proteins and carrier proteins
3. They sometimes act as enzymes

Question 6

Functions of glycolipids and glycoproteins

Answer 6

1. The carbohydrate chains form hydrogen bonds with the water molecules outside the cell and so help to stabilise the membrane structure.
2. Act as antigens or cell identity markers and thus are involved in cell-to-cell recognition.
3. Act as receptor molecules.

Question 7

Functions of cholesterol

Answer 7

1. Limit the close and orderly packing of the phospholipids and thus increase the fluidity of the membrane.
2. They are important for the mechanical stability of membranes, as without them membranes quickly break and cells burst open.
3. Cholesterol molecules have hydrophobic regions which further help to prevent ions or polar molecules from passing through the membrane.

Question 8

Roles of membranes in cells

Answer 8

1. Act as a physical barrier separating the contents of a cell and its external environment. This helps to maintain a constant internal environment within the cell.
2. Act as partially/selectively permeable membrane which regulates the movement of solutes into and out of the cell.
3. Enables the cell to communicate with external environment
4. Helps cell to maintain structural relationships with neighbouring cells.
5. Separate regions within a cell and within organelles inside of the cell.

Question 9

Cell signalling

Answer 9

Cells interact with their environment and communicate with other cells by sending signals and by receiving signals.

Question 10

Signalling pathway

Answer 10

1. Receiving a stimulus
2. Transmitting the message/signals
3. Making appropriate response

Question 11

Membrane receptors

Answer 11

1. Receptor acts as an ion channel
2. Receptor activates a G-protein
3. Receptor acts as an enzyme

Question 12

Signalling pathway (with second messenger)

Answer 12

1. Signal arrives at protein receptor
2. transduction to inside the cell, or G protein 
3. G protein activate enzyme
4. Enzyme makes second messenger (small, soluble, amplifies signals)
5. Second messenger activate an enzyme, which further activates others, increasing amplification (this is called signalling cascade)
6. Response formed

Question 13

Mechanisms which transport occur across cell surface membrane

Answer 13

1. Diffusion (Simple & facilitated)
2. Osmosis
3. Active transport
4. Bulk transport (endocytosis and exocytosis)

Question 14

Simple diffusion

Answer 14

The net movement of particles such as molecules from a region where they are at a higher concentration to a region where they are at a lower concentration. It is the net movement of particles down a concentration gradient.

Question 15

Rate of diffusion is affected by

Answer 15

1. Steepness if the concentration gradient
2. The distance over which diffusion occurs
3. The surface area across which diffusion occurs
4. The size and nature of the diffusing molecule

Question 16

Facilitated diffusion

Answer 16

The diffusion of ions and polar molecules through the cell surface membranes using specific protein channels or carriers, down a concentration gradient.

(energy is required)

Question 17

Osmosis

Answer 17

The net movement of water molecules from a region where water is at a higher water potential to a region with lower water potential through a partially/selectively permeable membrane. It is the net movement of water molecules down a water potential gradient.

Question 18

Components of water potential

Answer 18

1. Solute potential

2. Pressure potential

Question 19

Solute potential

Answer 19

The measure of the reduction in water potential due to the presence of solute molecules. The value is always negative

Question 20

Pressure potential

Answer 20

The measure of the hydrostatic pressure to which water is subjected. The value is usually positive.

Question 21

Active transport

Answer 21

The movement of ions or polar molecules across a membrane from a region where they are at a lower concentration to a region where they are at a higher concentration by means of specific transport proteins.
This process requires energy because the movement of ions or molecules occurs up or against a concentration gradient.
The energy is supplied in the form of ATP which is produced during respiration inside the cell

Question 22

Bulk transport

Answer 22

1. Endocytosis

2. Exocytosis

Question 23

Endocytosis

Answer 23

Involves the bulk uptake of materials into the cytoplasm of a cell.

Question 24

Exocytosis

Answer 24

Involves the bulk transport of materials out of a cell