Cells: Cell Membranes

Question 1

Describe the similarity of structure of different cell membranes

Answer 1

• The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same

Question 2

Function of Cell Membranes

Answer 2

• Cell surface membrane is a barrier between the cell and its environment
• It controls what enters and leaves the cell
• They’re partially permeable - they let some molecules through but not others

Question 3

Overview of Cell Membrane Structure

Answer 3

• Composed of lipids (mainly phospholipids), proteins and carbohydrates (attached to proteins or lipids)
• Fluid-mosaic structure

Question 4

Why are cell membranes described to have fluid mosaic structures?

Answer 4

• Cell membrane is composed of 2 layers of phospholipids - phospholipid bilayer
• This bilayer is ‘fluid’ because the phospholipids are constantly moving
• Proteins are scattered through the bilayer
• They vary in shape and size like tiles in a mosaic

Question 5

Cell Membrane Structure

Answer 5

• Proteins are scattered through the bilayer
• Includes channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane
• Receptor proteins are for cell recognition and hormones
• Some proteins have a carbohydrate attached - glycoproteins
• Some lipids have a carbohydrate attached - glycolipids
• Cholesterol molecules are present within bilayer

Question 6

Membrane Components: Phospholipids

Answer 6

• Phospholipid molecules form a barrier to dissolved (water-soluble) substances
• They have a hydrophilic head and hydrophobic tails
• This forms a bilayer - heads face out towards water on either side of membrane
• Hydrophobic bilayer doesn’t allow water-soluble substances (ions and polar molecules) to diffuse through membrane
• Small, non-polar substances (CO2) and water can diffuse through membrane
• Water is polar but it’s so small it can diffuse through

Question 7

Membrane Components: Cholesterol

Answer 7

• Cholesterol gives membrane stability
• Cholesterol binds to hydrophobic tails of phospholipids, causing them to pack more closely together
• Restricts lateral (horizontal) movement of phospholipids and makes membrane less fluid and more rigid
• Cholesterol also has hydrophobic regions, acts as further barrier to polar substances
• Cholesterol helps maintain shape of animal cells with no walls
• Important for cells that aren’t supported by other cells e.g red blood cells

Question 8

How does temperature affect cell membranes?

Answer 8

• Temperature affects how much the phospholipids in the bilayer can move
• This affects membrane structure and permeability

Question 9

Temperatures below 0°C

Answer 9

• Phospholipids don’t have much energy - don’t move much
• They’re packed closely together and membrane is rigid
• Channel and carrier proteins denature, increasing permeability
• Ice crystals may form and pierce membrane, making it highly permeable when it thaws

Question 10

Temperatures between 0 and 45°C

Answer 10

• Phospholipids can move around and aren’t packed as tightly together - membrane is partially permeable
• As temperature increases, phospholipids have more energy and move more - increasing permeability of membrane

Question 11

Temperatures above 45°C

Answer 11

• Phospholipid bilayer starts to melt and membrane becomes more permeable
• Proteins denature so they can’t control what enter or leaves the cells - increases permeability