1.1.2 - Cell Membranes (The Fluid Mosaic Model) Part 7 & 8

Question 1

What is the difference between peripheral proteins and integral proteins?

Answer 1

Peripheral - Goes through part of the bilayer.

Integral - Goes through the bilayer.

Question 2

Describe the role of phospholipids in the membrane.

Answer 2

Phospholipids form a layer (a bilayer in this case) where they act as a partially permeable barrier where only some substances can penetrate through but charged ions cannot.

Question 3

List at least 4 major roles of membranes within cellls and at the surface of cells.

Answer 3

Major roles include:

• To act as a selectively permeable barrier where it stops substances such as water-soluable molecules from entering.
• Separating cell components from cytoplasm.
• Cell recognition of other external substances.
• Cell signalling.
• Membranes are what organelles are made of.
• Is the site at which various chemical reactions take place eg. enzyme activity.

Question 4

Describe the role of cholesterol in the cell membrane.

Answer 4

Cholesterol fit between the fatty acid tails in order to help stabilise the membrane as this stops substances such as water molecules (and ions) from diffusing through the membrane as easily.

Question 4

Describe how the effect of changing temperature can have on the structure and permeability of the membrane.

Answer 4

As the temperature increases, there will be more gaps inbetween the phospholipid, making it more permeable. This happens because phospholipids gain more kinectic energy. This causes them to move around more, creating more gaps in between them.

Question 5

Plasma membranes are ________ ________ barriers.

Answer 5

Plasma membranes are partially permeable barriers.

Question 7

Describe how the term ‘fluid mosaic’ refers to the molecular arrangement in membranes.

Answer 7

The fluid mosiac model refers to the membranes that are composed of:

• A phospholipid bilayer
• Various protein molecules in the layer give a mosaic appearance eg. glycoproteins and channel proteins.
• The term ‘fluid’ refers to the fact that some of these proteins can move freely or ‘float’ around.
• Some of these proteins are partially embedded in the bilayer (extrinsic/ peripheral) and some go straight through the layer (intrinsic/ integral).

Question 8

1. Describe what a glycoprotein is and how it differs from a glycolipid.
2. What is the role of the glycoprotein and glycolipid in the cell membrane?

Answer 8

1. Glycoproteins are proteins with a carbohydrate chain attached to it. This differs from glycolipids because there are phospholipids with a carbohydrate chain attached to it.

• Glycoproteins and glycolipids have a carbohydrate chain that can act as a receptor site where hormones can bind on to this, carrying out a cell ‘response.’
• The carbohydrate part may also act as antigens in order to identify damaged cells.
• Glycoproteins may also bind cells together in tissues.

Question 9

True or false?

The phosphate heads of phospholipids are hydrophillic.

Answer 9

True - The phosphate heads of phospholipids are hydrophillic.

Question 10

1. Describe the role of channel proteins.
2. How do channel proteins differ from carrier proteins in terms of how they carry out their role in the cell membrane.

Answer 10

1. Channel proteins are integral proteins that go straight through the bilayer. This allows a pathway for the movement of some substances like ions (which are too big to pass through directly) to enter the cell.
2. Carrier proteins also allow movement across the membrane but may need ATP in order to do this. This is because the substances needed may go against the concentration gradient or are too big to directly pass through.