Miss Lee - Cell Surface Membrane

Question 1

Phospholipid

Answer 1

Hydrophilic heads (water loving and polar) 
Hydrophobic fatty acid tails. Many phospholipids from a phospholipid bilayer

Question 2

Glycoproteins

Answer 2

Where (intrinsic) protein molecules in the membrane have a carbohydrate attached (sugar) attached. Stabilises the membrane acts on a receptor binding sites for chemical (hormones etc) this is know as cell signalling

Question 3

Glycolipids

Answer 3

Where phospholipid molecules have a carbohydrate attached. They act cell markers of antigens. They facilitate cellular recognition e.g. immune system response

Question 4

Channel proteins

Answer 4

Intrinsic proteins provide a hydrophilic channel that allows passive movement of polar molecules and ions down a concentration gradient through membranes

Question 5

Carrier proteins

Answer 5

Intrinsic proteins have important roles in both passive and active transport

Question 6

Cholesterol

Answer 6

A steroid molecule that fits between fatty acid tails and provides mechanical stability. This affects the fluidity and can reduce permeability to polar / charged molecules e.g water and ions

Question 7

Receptor cells

Answer 7

Are protein typically found on the surface of cells, which are capable of recognising and bonding to specific molecules

Question 8

Enzymes and Coenzymes

Answer 8

Some reactions take place in membranes thereby requiring enzymes. E.g. some reactions of respiration take place in the membrane of the cristae of the mitochondria

Question 9

Intrinsic proteins

Answer 9

Those which span the entire membrane. Have 4 main functions: 1) To act as channels, 2) Transporters, 3) Receptors, 4) Enzymes

Question 10

Extrinsic proteins

Answer 10

Are peripheral proteins which are present in one side of the phospholipid bilayer

Question 11

Appearance of the cell surface membrane

Answer 11

Seen using a light microscope it looks like two thin lines and is toughly 7-8nm in diameter

Question 12

Structure of the cell surface membrane

Answer 12

Lipids (phospholipid)
Proteins
Carbohydrates (attached to the proteins or lipids)

Question 13

Functions of cell surface membrane

Answer 13

Partially permeable barriers between the cell and it’s outside environment, between organelles and the cytoplasm & within organelles.
• Controls which substances enter and leave the cells.
• Membranes allow recognition by other cells e.g. cells of the
immune system.
• Sites of cell communication (‘cell signalling’).

Question 14

Functions of Membranes Within Cells

Answer 14

⚫ Keeping all cellular components inside the cell (act as a barrier)
⚫ Can form vesicles to transport substances e.g. golgi apparatus
⚫ Allowing selected molecules to move in and out of the cell ⚫ Isolating organelles from the rest of the cytoplasm, allowing
cellular processes to occur separately.
⚫ A site for biochemical reactions
⚫ Allowing a cell to change shape.

Question 15

Lipids and Water

Answer 15

• These two substances do not mix.
• Water is a polar molecule (the oxygen end is slightly negative and the hydrogen end slightly positive).
• Fats are non-polar and do not form hydrogen bonds with water.
• Fats are said to be hydrophobic and lie on the surface of the water to reduce the surface area in contact between the fat and the water

Question 16

Lipids and Water in cell surface membrane

Answer 16

When exposed to water, phospholipids form one of two structures: a micelle or a bilayer.
micelle bilayer
In each structure, the hydrophilic heads face the water, and the hydrophobic tails point inwards away from the water.
This behaviour is key to the role that phospholipids play in membranes.

Question 17

Components of the cell surface membrane

Answer 17

• Phospholipid Bilayer
• Various protein molecules (channel proteins, carrier proteins).
• Intrinsic proteins (completely embedded in bilayer)
• Extrinsic proteins (partially embedded in bilayer)
• Glycoproteins (carbohydrate attached to protein)
• Glycolipids (carbohydrate attached to phospholipid)
• Cholesterol (between fatty acid tails)
• Receptor sites (for chemical signals)
• Enzymes and coenzymes

Question 18

Proteins in the plasma membrane

Answer 18

There are two main types of protein in the plasma membrane:
 • Intrinsic proteins
Channel proteins
Carrier proteins 
Glycoproteins

• Extrinsic proteins
Peripheral proteins

Question 19

Proteins in the membrane (location and facts)

Answer 19

Proteins typically make up 45% by mass of a cell membrane, but this can vary from 25% to 75% depending on the cell type.
Intrinsic (or transmembrane) proteins span the whole width of the membrane.
Extrinsic proteins are confined to the inner or outer surface of the membrane.
Many proteins are glycoproteins – proteins with attached carbohydrate chains.

Question 20

Proteins in the membrane (what they do)

Answer 20

1) Some proteins form channels in the membrane – these allow small, uncharged or charged/polar particles through down a concentration gradient (i.e. diffusion) e.g. oxygen
2) Other proteins (called carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion e.g sodium ion
3) Proteins also act as receptors e.g. glycoproteins for molecules (e.g. hormones) in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell.

Question 21

Why is the membrane fluid?

Answer 21

All lipids can move sideways (laterally) within the membrane and exchange position with each other.
• This gives the membrane fluidity.
• This is essential for some processes within the cell e.g. phagocytosis

Question 22

What factors affect the fluidity?

Answer 22

1) The length of the fatty acid side chains (the longer the chains, the lower the fluidity).
2) The proportion of the fatty acids which are saturated (the higher the percentage of saturated fats, the lower the fluidity).
3) The steroid content (the higher the steroid content, the lower the fluidity).

Question 23

Which side faces the water solution in a phospholipid

Answer 23

Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions

Question 24

Which side faces away from the aqueous solution in a phospholipid

Answer 24

The fatty acid tails are non-polar and will move away from an aqueous environment

Question 25

What is the structure of the phospholipid (which way is facing which)

Answer 25

As both tissue fluid and cytoplasm is aqueous phospholipids form two layers with the hydrophobic tails facing inward and phosphate groups outwards interacting with the aqueous environment