The Cell Membrane

Question 1

What is the cell membrane?

Answer 1

• Plasma membrane (plasmalemma) is the boundary that separates a cell from its surroundings and controls traffic into and out of the cells
• It exhibits selective permeability, due to its structure (lipids, carbs and proteins)
o Function is fundamental to life

Question 2

The ______ interior of the bilayer is one reason membranes are selectively permeable.

Answer 2

hydrophobic

Question 3

Eukaryotic cell membrane
• Plasma membrane (plasmalemma) is an _____ feature of a eukaryotic cell
• 8nm thick
• Ensures a ____ internal state

Answer 3

external

steady

Question 4

What is the composition of cell membranes?

Answer 4

• Lipids, proteins and carbohydrates are the staple ingredients of membranes
• Most abundant lipids—>phospholipids
o Ability to form membranes is inherent in their structure

Question 5

What are phospholipids similar to?

Answer 5

Fats

Question 6

What are the two parts of a phospholipids?

Answer 6

o Two fatty acids joined to glycerol—> hydrophobic backbone (tail) that is non-polar
o Phosphorylated alcohol (phosphate) on last hydroxyl group—> Hydrophilic head on
the molecule that is polar

Question 7

What is an amphipathic molecule?

Answer 7

> Amphipathic molecule—> has a hydrophilic and a hydrophobic region

Question 8

What is the function of the unsaturated hydrocarbons in phospholipids?

Answer 8

> Unsaturated hydrocarbons (double carbon bond) in molecular structure cause a kink in
structure to form

Question 9

What is the bilayer? (2)

Answer 9

> Two phospholipids can form a bilayer
o Stable boundary between aqueous compartments
o Molecular arrangement allows the hydrophobic tail to be sheltered from water while exposing the hydrophilic head

Question 10

Proteins like lipids are _____.

Answer 10

Amphipathic: having both hydrophilic and hydrophobic parts

Question 11

Where do proteins reside?

Answer 11

Reside in the phospholipid bilayer with hydrophilic regions protruding. Maximizes contact with water while keeping hydrophobic areas dry.

Question 12

What is the fluid mosaic model?

Answer 12

The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates.

Question 13

What does the fluid in the fluid mosaic model refer to?

Answer 13

Mosaic refers to the pattern of proteins interspersed amongst phospholipids. Fluid refers to the fact that components (proteins and phospholipids) are not static and are capable of moving in the plasma membrane
o Phospholipids move faster than proteins however due to size

Question 14

Why must membranes be fluid?

Answer 14

Membranes must be fluid to work properly as it affects cell permeability and the ability to move proteins to where they are needed.

Question 15

What are the effects of temperature on fluidity? (3)

Answer 15

• Higher temperatures would allow phospholipids to move further away from each other
• Lower temperatures would move them closer until they settle into a closely packed arrangement and the membrane solidifies
• The more kinks in the phospholipid tails the lower the solidifying temperature
- Kinks in the tails of the phospholipids prevent them from getting close together

Question 16

What is the function of cholesterol? (2)

Answer 16

Cholesterol—> fluidity buffer
• Assists in retaining integrity or structure of the phospholipid layer
• They are wedges between phospholipid molecules of membrane

Question 17

What is the effect of cholesterol in high and low temperatures?

Answer 17

• At high temperatures makes membrane less fluid by restraining movement
• Low temperatures: cholesterol hinders close packing of phospholipids—>lowers the required temp for the membrane to solidify

Question 18

There are 3 main factors that influence cell membrane fluidity:

Answer 18

• Temperature
• Cholesterol
• Saturated and unsaturated fatty acids

Question 19

How does temperature affect membrane fluidity?

Answer 19

The temperature will affect how the phospholipids move and how close together they are found. When it’s cold they are found closer together and when it’s hot they move farther apart.

Question 20

How do saturated and unsaturated fatty acids affect membrane fluidity? (4)

Answer 20

• Fatty acids are what make up the phospholipid tails.
• Saturated fatty acids are chains of carbon atoms that have only single bonds between them.
• As a result, the chains are straight and easy to pack tightly.
• Unsaturated fats are chains of carbon atoms that have double bonds between some of the carbons.
• The double bonds create kinks in the chains, making it harder for the chains to pack tightly

Question 21

How does cholesterol affect membrane fluidity?

Answer 21

The cholesterol molecules are randomly distributed across the phospholipid bilayer, helping the bilayer stay fluid in different environmental conditions. The cholesterol holds the phospholipids together so that they don’t separate too far, letting unwanted substances in, or compact too tightly, restricting movement across the membrane. Without cholesterol, the phospholipids in your cells will start to get closer together when exposed to cold, making it more difficult for small molecules, like gases to squeeze in between the phospholipids like they normally do. Without cholesterol, the phospholipids start to separate from each other, leaving large gaps.

Question 22

What do these double kinks do in terms of membrane fluidity?

Answer 22

These kinks play a role in membrane fluidity because they increase the space between the phospholipids, making the molecules harder to freeze at lower temperatures. In addition, the increased space allows certain small molecules, such as CO and O2, to cross the membrane quickly and easily.