Chap 4 Plasma Membrane

Question 1

Give some characteristics of the plasma membrane.

Answer 1

1. Seperates cell from external environment
2. Is a thin fragile structure (5-10nm)
3. Built from lipids and proteins
4. Dynamic and ever changing (its overall shape can change during locomotion or cell division)
5. Semi Permeable (allows movement of solvent molecules and solutes across a membrane)

Question 2

Name the components of the cell membrane.

Answer 2

• Lipids (phospholipids,glycolipids)
• Proteins (integral proteins,peripheral proteins,lipid anchored proteins)
• Sterols (cholesterol)

Question 3

What are the functions of cholesterol in the plasma membrane?

Answer 3

1. Stabilizes phospholipids at room temperature
2. Helps keep membrane fluid at low temperatures
3. Makes the cell membrane more rigid at high temperatures

Question 4

Where in the cell membrane is cholesterol found at and why?

Answer 4

Sterols are hydrophobic and are thus found between the hydrophobic lipid tails of phospholipids in the membrane.

Question 5

How does cholesterol affect the fluidity of the membrane at high and low temperatures?

Answer 5

At low temperatures,phospholipid molecules come closer together reducing fluidity.Cholesterol increases the distance between phospholipid molecules at low temperatures increasing membrane fluidity.

At high temperatures,phospholipid molecules are further apart increasing membrane fluidity.Cholesterol decreases distance between phospholipids at high temperatures decreasing fluidity.

Question 6

Why do cell membranes have to control their fluidity?

Answer 6

Low fluidity causes a membrane to be strong but poor functioning as transport of compounds through the membrane is reduced.
High fluidity causes a membrane to be weak and potentially break.

Question 7

What are the two factors that control membrane fluidity?

Answer 7

Presence of Cholesterol

Type of fatty acids in phospholipid

Question 8

How does the type of fatty acid present in the cell membrane’s phospholipid affect membrane fluidity?

Answer 8

Saturated fatty acids cause the cell membrane to be less fluid then unsaturated fatty acids.Unsaturated fatty acids have “kinks” due to their carbon double bonds while saturated fatty acids are straight.This kink causes unsaturated fatty acid tails to take up more space then saturated fatty acids thus increasing distance distance between phospholipid molecules and increasing fluidity.

Question 9

Explain how the chemistry of phospholipids causes them to form a phosphobilipid layer.

Answer 9

The phosphate head of the phospholipid is hydrophilic and polar while the fatty acid tail of the phospholipid tail is non polar and hydrophobic. This causes the heads to face the water or cytoplasm as they are hydrophilic while the tails face towards each other as they are hydrophobic forming the bi-layer.

Question 10

Explain the structure of glycolipids.

Answer 10

Glycolipids are made up of a hydrophilic polysaccharide (sugar head) and hydrophobic lipid tails.

Question 11

What are the functions of glycolipids?

Answer 11

1. May help protect membrane from damage
2. Binding sites for signal molecules
3. Useful marker for identifying cells.

Question 12

Explain the Fluid Mosaic Model of membrane structure.

Answer 12

The cell surface membrane is fluid, dynamic and ever changing. The cell membrane is also like a mosaic due to the scattered arrangement of many transmembrane and peripheral membrane proteins.

Question 13

What is the difference between a prokaryotic and eukaryotic cell membrane?

Answer 13

Eukaryotic cell membrane contains sterols while most prokaryotic cell membranes do not. (5 to 25% of eukaroytic cell membrane is sterols)

Question 14

Briefly describe and explain the 3 classes of membrane proteins.

Answer 14

1. Integral Protein
   - penetrates the lipid bi layer
   - either spans part of the cell membrane or the entire cell membrane (transmembrane protein)
2. Peripheral Protein
   - located entirely outside the lipid bi-layer
   - linked to the surface membrane by non covalent bonds (hydrophobic,electrostatic etc.)
3. Lipid Anchored Proteins
   - located outside of the lipid bi-layer
   - covalently bonded to a lipid molecule that is situated within the bi-layer.

Question 15

Name the 6 Functions of Membrane Proteins

Answer 15

Transporter/Carriers
Enzymatic Activity
Receptors/ Cell Surface Receptors
Cell Surface Markers
Cell Adhesion Proteins
Interior Protein Network (cytoskeleton)

Question 16

Describe Transport/Carrier Proteins and give an example.

Answer 16

They are transmembrane proteins that transport molecules across the membrane. (e.g. GLUT4 glucose transporter an insulin regulated transporter protein found in adipose tissue and striated muscle cells)

Question 17

Describe Receptor Proteins and give an example.

Answer 17

They are transmembrane proteins that transmit information to a cell via binding of a ligand to receptor. (e.g. signal molecule insulin binds to cell surface receptors altering receptor protein portion within the cell inducing glucose to be stored as glycogen.)

Question 18

Describe Proteins that help in Enzymatic Activity.

Answer 18

Enzymes attached to the interior surface of the membrane can help to catalyze reactions.

Question 19

Describe Proteins that act as Cell Surface Markers and give an example.

Answer 19

Cell surface markers on membranes are glycoproteins. These help in cell-cell recognition, “self” recognition. (e.g major histocompatibillity complex proteins. MHC proteins are proteins found on the surface of cells that help the immune system recognize foreign substances from its own cells.

Question 20

Describe Proteins that act as Interior Protein Network and give an example.

Answer 20

They are intracellular proteins that support the shape of membranes. (e.g. the biconcave shape of RBCs are due to spectrin a scaffold of proteins, linking membrane with actin filaments of cytoskeleton.)

Question 21

Describe Cell Adhesion Proteins and give an example.

Answer 21

Special proteins that glue cells to one another, they are also termed receptors and ligand as they bind to one another in the same fashion. (e.g. outer surface of cell membrane in developing nervous system [embryo])

Question 22

What are cell junctions and what are their purpose?

Answer 22

Cell junctions are connection between neighboring cells or between a cell and the extracellular matrix. Cell junctions helps cell reduce stress and allows neighboring cells to communicate with each other.

Question 23

Describe tight/occluding junctions and their function. (give examples)

Answer 23

They bind adjacent cells together (usually on the surface of cell) forming a leak proof sheet.This prevents the leakage of contents into surrounding tissues. (e.g. lining of digestive tract,lungs,capillaries in the brain)

Question 24

What is the apical and basolateral surface?

Answer 24

Apical surface is the surface that is exposed to the lumen. Basolateral surface is the surface that make up the rest of the sides (sides and base of cell).

Question 25

Describe and explain the function of Anchoring/Adhesion/Adherens junctions. (give an example)

Answer 25

They attatch cell to one another which help prevent lateral tearing of tissues. They do not prevent substances from passing between cells.They usually have portions within the cell and portions outside the cell. (e.g. skin cells)

Question 26

Describe the difference between adherens junctions,spot desmosomes and hemidesmosomes.

Answer 26

Spot desomosomes attach cells to one another, usually on the sides of cells not the base. Hemidesmosomes attach cell to ECM/attach basal epithelial cells to the lamina .Adherens junctions are found below the tight junctions and surround the cell completely. They reinforce connections done by the tight junctions.

Question 27

Describe the structure and function of gap junctions/communicating junctions. (give examples)

Answer 27

Gap junctions are made of a protein called conexins that form a group of tunnels called connexons between adjacent cells. Connexons pierce through the lipid bi-layer of both cells allowing electrolytes and other molecules to pass between cells. (e.g. heart muscles and uterine muscles)

Question 28

Briefly describe the structure and function of bacteria cell wall.

Answer 28

Contains peptidoglycan – structural component, holds cell wall together.Prevent cells from bursting due to hypotonic environment

Question 29

Describe the structure of the plant cell wall and explain the purpose of plasmodesmata.

Answer 29

Plant cell wall is made up of cellulose,hemicellulose and pectin. Plasmodesmata are holes in the plant cell walls that connect cytoplasm of neighboring cells, enabling transport and communication between them.