5.1 Structure and Function of Membranes

Question 1

What are membranes?

Answer 1

Structures that separate the contents of cells from their environment.

Question 2

What are the functions of membranes?

Answer 2

-Separate the different organelles within cells
-Cell communications
-Acts as a partially permeable barrier
-Site of chemical reactions
-Compartmentalisation.

Question 3

Describe the features of compartmentalisation.

Answer 3

-The formation of separate membrane-bound areas in a cell
-Contains reactions in separate parts of the cell
-Allows the specific conditions required for cellular reactions to be maintained
-Protects vital cell components

Question 4

Describe the basic membrane structure

Answer 4

-Membranes are formed from a phospholipid bilayer
-The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids, to form a hydrophobic core inside the membrane.

Question 5

What is a plasma membrane?

Answer 5

The cell surface membrane that separates the cell from its external environment

Question 6

What kinds of environments do cells normally exist in?

Answer 6

Aqueous environments

Question 7

Why are phospholipid bilayers perfectly suited as membranes?

Answer 7

Because the outer surfaces of the hydrophilic phosphate heads can interact with water.

Question 8

What is the name of the current model of the plasma membrane, and why?

Answer 8

-Fluid-mosaic model
-Because the phospholipids are free to move within the layer, and because the proteins embedded in the bilayer vary in shape, size and position.

Question 9

What are membrane proteins?

Answer 9

Proteins that have important roles in the various functions of membranes.

Question 10

What are the two types of proteins in the cell surface membrane?

Answer 10

Intrinsic and extrinsic proteins

Question 11

What are intrinsic proteins?

Answer 11

-Transmembrane proteins that are embedded through BOTH layers of a membrane

Question 12

What is the key structure of an intrinsic protein, and why?

Answer 12

Have amino acids with HYDROPHOBIC R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.

Question 13

What type of protein is a carrier protein?

Answer 13

Intrinsic protein

Question 14

What type of protein is a channel protein?

Answer 14

Intrinsic protein

Question 15

What do channel proteins do?

Answer 15

-Provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient, through membranes.

Question 16

How is a channel protein held in place?

Answer 16

Because it is an intrinsic protein, by interactions between the hydrophobic core of the membrane, and the hydrophobic R-groups on the outside of the proteins.

Question 17

What do carrier proteins do?

Answer 17

Have an important role in both passive and active transport into cells, often involving the shape of the protein changing.

Question 18

What type of proteins are glycoproteins?

Answer 18

Intrinsic proteins.

Question 19

How are glycoproteins connected to the membrane?

Answer 19

Embedded in the cell-surface membrane, with attached carbohydrate chains of varying lengths and shapes

Question 20

What is the function of a glycoprotein?

Answer 20

They play a key role in cell adhesion, and as receptors for chemical signals.
-When the chemical binds to the receptor, it elicits a response from the cell.

Question 21

Describe the structure of a glycolipid.

Answer 21

Similar to a glycoprotein, a lipid with an attached carbohydrate chain.

Question 22

Are glycolipids intrinsic or extrinsic proteins?

Answer 22

Neither, because they are not proteins.

Question 23

What are alternate names for glycolipids?

Answer 23

Cell markers and antigens

Question 24

What are extrinsic proteins?

Answer 24

Present in only one side of the bilayer

Question 25

Describe the structure of extrinsic proteins.

Answer 25

Normally have HYDROPHILIC R-groups on their outer surfaces, and interact with the polar heads or intrinsic proteins of the phospholipids.

Question 26

Describe the movement of extrinsic proteins.

Answer 26

Can be present in either layer, and some move between layers.

Question 27

Is cholesterol intrinsic or extrinsic?

Answer 27

It is neither, because it is not a protein.

Question 28

Describe the structure of cholesterol.

Answer 28

A lipid with a hydrophilic end and a hydrophobic end

Question 29

Describe the functions of cholesterol.

Answer 29

-Regulates the fluidity of membranes

Question 30

Where is cholesterol positioned in the membrane?

Answer 30

Positioned between phospholipids in a membrane bilayer. Hydrophobic ends interact with the tails, hydrophilic ends interact with the heads, pulling them together.

Question 31

How does cholesterol add stability to membranes?

Answer 31

By their hydrophobic heads interacting with the phospholipid tails, and the hydrophilic tails interacting with the phospholipid heads. These interactions pull the membrane together, giving it stability without making it too rigid.