Cell Membranes and Transport

Question 1

Describe the fluid mosaic model of the membrane structure

Answer 1

The membrane is described as fluid because the phospholipids and proteins move around. The phospholipids move sideways in their layer and the proteins move about within it. The word mosaic describes the scattered pattern of the proteins when viewed from above

Question 2

What is the importance of the membrane?

Answer 2

1) Controls the exchange of materials such as nutrients and waste products between the cell and its environment.
2) The surface across which chemical messages communicate

Question 3

How thick is the cell membrane?

Answer 3

10nm

Question 4

Describe the structure of the membrane

Answer 4

Made up of protein, phospholipids and a small amount of carbohydrates. When mixed with water phospholipids arrange themselves into a bilayer in which the hydrophobic tails are attracted to each other and the hydrophilic heads are attracted to water. This forms a phospholipid bilayer. This bilayer also contains proteins

Question 5

Integral proteins

Answer 5

Proteins that are partially/fully buried in the lipid layer

Question 6

Peripheral proteins

Answer 6

Proteins superficially attached on either surface of the lipid layer

Question 7

Role of phospholipids in the membrane

Answer 7

It acts as a barrier to most water-soluble substances. The non-polar fatty acids prevent polar molecules from passing across the membrane. This ensures molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted molecules cannot get in. Can be modified to act as signalling molecules

Question 8

Role of cholesterol in the membrane

Answer 8

It regulates the fluidity of the membrane. It sits between phospholipids, preventing them from packing too closely together, when temperatures are low this prevents membrane from fracturing. Increases mechanical strength and stability

Question 9

Role of glycoproteins and glycolipids in the membrane

Answer 9

They contain carbohydrate chains that exist on the surface which enables them to act as receptor molecules. It allows them to bind with certain substances at the cells surface. Some act as antigens for cell to cell recognition

Question 10

Role of proteins in the membrane

Answer 10

Transport proteins create hydrophilic channels to allow ions/polar molecules to travel through the membrane (channel proteins and carrier proteins). Each transport protein is specific to a particular ion/molecule. Allow the cells to control which substances enter or leave. They may also act as enzymes

Question 11

When are membranes less fluid?

Answer 11

At lower temperatures as molecules have less energy and are not moving freely. Increased proportion of saturated fatty acid chains, they pack together tightly causing high number of IMF between chains

Question 12

When are membranes more fluid?

Answer 12

At higher temperatures, molecules have more energy and move freely. Increased proportion of unsaturated fatty acid chains, less tightly packed together and less IMF

Question 13

Define cell sinalling

Answer 13

It is the process by which messages are sent to cells

Question 14

Describe the cell signalling pathway

Answer 14

A stimulus is received by a receptor. The stimulus is converted to a message that can be translated (transduction). The message is transmitted to a target. Appropriate response is carried out

Question 15

Describe how ligands are involved in the cell signalling pathway

Answer 15

Ligands are secreted from a cell into the extracellular space. Ligands are transported through extracellular space to target cell. The ligands bind to surface receptors on target cell. The message carried by the ligand is relayed through a chain of chemical messages inside the cell, triggering a response

Question 16

Define diffusion

Answer 16

Diffusion is the net movement, as a result of random motion of its molecules/ions, of a substance from a region of its higher concentration to a region of its lower concentration

Question 17

How does surface area affect the rate of diffusion?

Answer 17

The greater the surface area across which diffusion is taking place, the greater the number of molecules that can cross it. Increases rate of diffusion

Question 18

How properties of molecules/ions affects the rate of diffusion?

Answer 18

Large molecules diffuse more slowly than smaller ones. Non-polar molecules diffuse more quickly than polar molecules as they are soluble in the non-polar phospholipid bilayer

Question 19

Channel proteins role in facilitated diffusion

Answer 19

They allow charged substances to diffuse through the cell membrane. Part of the channel protein on the inside surface of the membrane can move in order to close and open the pore

Question 20

Carrier proteins role in facilitated diffusion

Answer 20

Carrier proteins can flip between two shapes. The binding site of the carrier protein will open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape

Question 21

Define osmosis

Answer 21

Osmosis is the net diffusion of water molecules from a region of high water potential to a region of low water potential, through a partially permeable membrane

Question 22

Define active transport

Answer 22

The movement of molecules/ions through transport proteins across a cell membrane, against their concentration gradient using energy from ATP

Question 23

Define endocytosis

Answer 23

The bulk movement of liquids (phagocytosis) or solids (pinocytosis) into a cell, by the infolding of the cell surface membrane to form vesicles. Requires ATP

Question 24

Define exocytosis

Answer 24

The bulk movement of liquids or solids out of the cell, the the fusion of vesicles containing the substance with the cell surface membrane. Requires ATP