What is a membrane?

Question 1

Why are membranes important?

Answer 1

Cell (plasma) membrane forms an outer boundary of every cell.
Selectively permeable.
Controls the entry of nutrients and exit of waste (and secretory) products.
Maintains differences in ion concentration inside and outside the cell.
Participates in joining of cells to form tissues and organs.
Enables a cell to respond to changes (signals) in the cell’s environment.
membrane functions are crucial for cell survival.
many functional differences between cell types are due to variations in the composition of their plasma membranes
- Different cells interact in different ways with essentially the same ECF.
Biochemical analysis reveals that the cell membrane is composed of 2 principle constituents: lipid and protein.

Question 2

The cell membrane is composed primarily of phospholipids, what are the phospholipids composed of?

Answer 2

Head (negatively charged, polar hydrophilic)

Tails (uncharged nonpolar, hydrophobic). The tails are made of a fatty acid chain.

Question 3

phospholipids form complex structures in aqueous solution. What structure do they form?

Answer 3

In an aqueous environment, polar hydrophilic head groups orient toward the polar water and nonpolar (hydrophobic) tails orient away from the water.
Thus a phospholipid bilayer is formed.

Question 4

What are the structures of some common membrane lipids
- Phosphatidylinositol
- Phosphatidylserine
- Phosphatidylcholine
- Sphingomyelin
- Galactocerebroside
- Cholesterol
?

Answer 4

Phosphatidylinositol
- Inositol, Phosphate, Glycerol, Fatty acid.
Phosphatidylserine
- Serine, phosphate, glycerol, fatty acid
Phosphatidylcholine
- Choline, Phosphate, Glycerol, Fatty acid.
Sphingomyelin
- Choline, phosphate, Sphingosine, fatty acid.
Galactocerebroside
- Galactose, Sphingosine, fatty acid.
Cholesterol
- Cholesterol

Question 5

How do phospholipid bilayer membranes act as semi permeable membranes?

Answer 5

The lipid bilayer is ideally suited to separate two aqueous compartments.
Pure phospholipid bilayer membranes are extremely impermeable to almost any water - soluble substance
- e.g. ions, proteins, and sugars are insoluble in the hydrophobic membrane core.
In contrast, small uncharged polar molecules can cross fairly freely
- E.g. O2, CO2, NH3 and water itself

Question 6

On a transmission electron micrograph, a cell membrane has what appearance?

Answer 6

Trilaminar appearance

Question 7

What are the 3 important functions of the lipid bilayer?

Answer 7

It forms the basic structure of the membrane.
Its hydrophobic interior serves as a barrier
- The cell can maintain differences in solutes composition and concentrations inside/outside the cell.
It is responsible for the fluidity of the membrane
- Enables cells to change shape (e.g. RBC or a skeletal muscle cell).
(The lipid bilayer forms the basic structural barrier that encloses the cell).

Question 8

Membrane proteins can belong to either of two broad classes, what are these?

Answer 8

peripheral or integral.

Question 9

What are the characteristics of peripherally associated membrane proteins?

Answer 9

Not embedded within the membrane.
Instead they adhere tightly to the cytoplasmic or extracellular surfaces of the PM.
Peripheral proteins are noncovalently bonded with integral proteins.

Question 10

Integral membrane proteins are intimately associated with the lipid bilayer in any of 3 ways, what are these?

Answer 10

Some proteins span the lipid bilayer once or several times - transmembrane proteins.
Some are embedded but do not cross the bilayer.
Some are linked to a lipid component of the membrane or a fatty-acid derivative that intercalates into the membrane.

Question 11

What are some characteristics of integral proteins?

Answer 11

Mos integral membrane proteins have membrane-spanning alpha-helical domains of about 20 amino acids.
Some have multiple membrane-spanning domains.
Some proteins are linked to membrane phospholipids via an oligosaccharide.
Or are linked directly to fatty acids or prenyl groups.

Question 12

What are the functions of integral membrane proteins?

Answer 12

Ligand-binding receptors
- e.g. hormone receptors.
Adhesion molecules
- Form physical contacts with the surrounding extracellular matrix or with cellular neighbours.
- Important in regulating cell shape, growth and differentiation, allowing the cell to adapt to its immediate surroundings.
- e.g. integrins: cell-matrix adhesion molecules.
- e.g. cell adhesion molecules: incl. cadherins (ca2+ - binding glycoproteins).
Integral membrane proteins can carry out the transmembrane movement of water-soluble substances.
Pores and channels
- Sere as conduits that allow water or specific ions to flow passively through the lipid bilayer.
Carriers
- Either facilitate the transport of a specific molecule or couple the transport of a molecule to that of other solutes.
Pumps
- Use energy that is released through the hydrolysis of ATP to drive the transport of substances into or out of cells against energy gradients.
Integral membrane proteins can also be enzymes
- e.g. membrane-bound enzymes in small intestine.
- e.g. carbonic anhydrase
Integral membrane proteins can participate in intracellular signalling
- Associated with the cytoplasmic surface of the PM.
- E.g. GTP-binding proteins, kinases.

Question 13

What are Docking-marker acceptors?

Answer 13

Docking-marker acceptors are located on the inner membrane surface.
They interact with secretory vesicles leading to exocytosis of the vesicle contents

Question 14

What is the process of exocytosis of the vesicle contents?

Answer 14

Secretory vesicle formation.
Budding from Golgi.
Uncoating.
Docking at plasma membrane.
Exocytosis.

Question 15

What are membrane carbohydrates?

Answer 15

A small amount of membrane carbohydrate is located on the outer surface of cells (“sugar coating”).
Short carbohydrate chains are often bound to membrane proteins and to a lesser extent lipids
- Glycoproteins and glycolipids
- Together form a layer called the glycocalyx

Question 16

How do membrane carbohydrates serve as self-identity markers?

Answer 16

Short carbohydrate chains on the outer membrane surface serve as self-identity markers that enable cells to identify and interact with one another.
Different cell types have different markers.
- Important in cell-to-cell interactions, especially during embryonic development.
Role is tissue growth
- Cells do not overgrow their own territory
- Exception: cancer cell growth - abnormal surface markers.

Question 17

What are the different specialised cell junctions?

Answer 17

Some cells are directly linked by one of three types of specialised cell junctions:
Tight junctions
- Join the lateral edges of epithelial cells near their luminal (apical) membranes. (tight or leaky).
Desmosomes
- Adhering junctions that anchor cells together, especially in tissues subject to stretching, (e.g. skin, heart, uterus).
Gap junctions
- “Communicating” junctions that allow the movement of charge carrying ions and small molecules between 2 adjacent cells.

Question 18

How do tight junctions stop the movement of dissolved materials through the space between epithelial cells?

Answer 18

The proteins of tight junctions form a “quilted” seal, barring the movement of dissolved materials through the space between epithelial cells.

Question 19

What is the role of desmosomes?

Answer 19

Desmosomes link adjacent cells tightly but permit materials to move around them in the intercellular space

Question 20

What is the role of gap junctions?

Answer 20

Gap junctions let adjacent cells communicate.

Question 21

What does the plasma membrane consist of?

Answer 21

The plasma membrane consists of

• A lipid bilayer (primary barrier to diffusion).
• Proteins (Integral and peripheral - perform most of the membrane-specific functions).
• carbohydrates (role in “self-recognition” and cell-to-cell interactions).