TOPIC 2A 1 CELL MEMBRANES

What is the job of membranes?

1. Membranes act as barriers, controlling what goes in or out, allowing the fluid on the inside to have a different composition than the outside.

2. Allowing certain conditions for certain reactions to occur.

Asides from acting as a barrier, what other functions do membranes serve?

1. Many chemical processes take place on membrane surfaces.

2. For example, some respiration reactions occur on the surface of mitochondria membranes.

What is a vesicle?

1. A vesicle is a membrane bag that contains chemical secretions, that can fuse with the cell membrane to release their contents.

What is the cell surface membrane composed of?

1. The cell surface membrane is composed of phospholipids and proteins.

What are the lipids in a cell surface membrane called?

1. They are called polar lipids.

2. These are lipid molecules with one end joined to a polar group, the polar group could be a phosphate group.

Elaborate on the solubility of the phospholipid.

1. The fatty acid chains of a phospholipid are neutral and insoluble in water,

2. The phosphate head carries a negative charge and is soluble in water.

Describe lightly the structure of a phospholipid.

1. A phosphate group attached to glycerol, attached to two fatty acid chains.

Describe how a phospholipid behaves in water.

1. The polar phosphate part is hydrophilic and dissolves easily in water.

2. The lipid tails are hydrophobic and insoluble in water.

3. If the phospholipid molecules are tightly packed in water, they either form a monolayer where the tails stick out in the air and the heads are in the water.

4. Or they may form micelles. In the micelles, the hydrophilic heads point outwards, and the hydrophobic tails are hidden inwards.

Describe why a phospholipid bilayer is formed within living organisms.

1. In living organisms, there are usually water-based solutions inside and outside the cell. So two monolayers of phospholipids form a bilayer, with the hydrophilic heads pointing outwards and the hydrophobic tails pointing inwards.

2. The simple structure of a phospholipid bilayer is a unit membrane, which is the basis for all cell surface membranes.

Decribe what things can pass through the phospholipid bilayer.

1. The unit membrane allows fat soluble organic molecules to pass through it.

2. But ionic molecules cannot pass through it alone, even polar lipids. Proteins and other molecules present in the membrane allow the ionic molecules to pass through.

Describe the fluid mosaic model lightly.

1. In the fluid mosaic model, the phospholipid bilayer is a fluid system, where many proteins and other molecules are floating within it like icebergs, with others that are fixed in place.

2. The proportion of phospholipids containing unsaturated fatty acids in the bilayer affects how freely the proteins move within the membrane.

3. Cholesterol is a more rigid molecule than many phospholipids, it makes the cell surface membrane stronger, more stable and more effective barrier.

4. Many proteins have a hydrophilic part which sticks out of the membrane and can be involved in a variety of activities. The hydrophobic part is hidden within the phospholipid bilayer.

What are some functions of the proteins in the phospholipid bilayer?

1. Proteins help substances move through the phospholipid bilayer, either by creating channels or pores, some of which are permanent and some temporary. These are known as gated channels. They can be opened or shut depending on the conditions of the cell.

2. Some proteins may act as specific receptor molecules, Eg, making cells sensitive to a hormone.

3. Some proteins may be enzymes.

4. Some proteins may be glycoproteins, which are proteins with a carbohydrate component added to the molecule. It is how cells recognise each other.

Which kinds of molecule are in the structure of a membrane and how do their properties affect the properties of the membrane itself?

1. Phospholipids make up the main bilayer of the membrane, separating an aqueous layer on one side from an aqueous layer on the other, by a hydrophobic region that makes diffusion difficult from substances that are not lipid soluble or very small.

2. Large proteins that sit within or across the membrane act as controls or gates for the passage of ionic and larger molecules across the membrane.

3. Glycoproteins on the outside of the membrane are part of the cell recognition system.

explain why a membrane may be more fluid when it contains more unsaturated fatty acids.

1. Unsaturated fatty acids contain at least one double carbon covalent bond.

2. The nature of any lipid is based on the balance of unsaturated and saturated fatty acids.

3. Saturated fatty acids make compounds more solid, unsaturated fatty acids are more flexible molecules and make substances more fluid or even liquid.