Topic 2---B: Cell membranes- 1. Cell membranes- The basics Flashcards
1
Q
What are cell-surface membranes?
A
- They surround cells
- They are a barrier between the cell and it’s environment
- They control which substances enter and leave the cell
- They’re partially permeable so they let some molecules through but not others
- Sometimes called the plasma membrane
2
Q
How can substances move across the cell-surface membrane?
A
By
- Diffusion
- Osmosis
- Active transport
3
Q
How do membranes around organelles divide the cell into different compartments?
A
- They act as a barrier between the organelle and the cytoplasm
E.g. The substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion.
4
Q
What components are in a membrane?
A
- Phospholipids (7nm thick)
- Proteins
- Carbohydrates
- Cholesterol
5
Q
What was the ‘Fluid Mosaic Model’?
A
- In 1972 Singer and Nicholson suggested it to describe the arrangement of molecules in the membrane.
- In the model phospholipids molecules form a continuous, double layer (bilayer) which is ‘fluid’ because the phospholipids are constantly moving.
- Proteins are scattered through the bilayer like tiles in a ‘mosaic’ (including channel and carrier proteins).
6
Q
What do channel and carrier proteins do?
A
They allow large molecules and ions to pass through the membrane.
7
Q
Phospholipids
A
- Phospholipid molecules form a barrier to dissolved (water-soluble) substances.
- They have a head (hydrophilic) which attracts water
- They have a tail (hydrophobic) which repels water
- Heads face out towards the water on either side of the membrane
- Centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances to diffuse through it
e.g (ions& polar molecules) - Small, non-polar substances can diffuse through the membrane
e.g (carbon dioxide and water)
8
Q
Cholesterol
A
- Present in all cell membranes apart from bacterial cell membranes
- It gives the membrane stability
- Cholesterol fits between the phospholipids
- Cholesterol binds to the hydrophobic tails of the phospholipids causing them to pack more closely together
- This restricts movement of the phospholipids making the membrane less fluid and more rigid
- Cholesterol has hydrophobic regions so it’s able to create a further barrier to polar substances moving through the membrane
9
Q
What does cholesterol help with?
A
It helps to maintain the shape of animal cells that don’t have cell walls
e.g (red blood cells)
10
Q
Protein
A
- Scattered through the bilayer
- They allow molecules and ions to pass through the membrane
- Receptor proteins on cell-surface membrane allow the cell- surface membrane to detect chemicals released from other cells
11
Q
Carbohydrates
A
- Glycoprotein (attatched to a protein)
- Glycolipid (attatched to a lipid)
12
Q
Functions (protein)
A
- Act as a hydrophilic pore where water soluble substances can pass in or out of the cell
13
Q
Phospholipid bilayer (function)
A
- Allows water to pass through with relative ease
- Acts as a barrier to the passage of water-soluble substances
14
Q
Carbohydrate function
A
- Helps stabilize the membrane structure by forming hydrogen bonds with water molecules outside the membrane
15
Q
How does temperature affect membranes?
A
- It affects how much the phospholipids in the bilayer can move
- Affecting membrane structure and permeability
