Chapter Five: Plasma Membranes

Question 1

What is a biological membrane composed of, and what are the three features of a biological membrane?

Answer 1

• they are composed of a phospholipid bilayer
• the features are:
  1. provides a partially permeable membrane
  2. are a site for chemical reactions
  3. play a role in cell communication

Question 2

What is a fluid-mosaic model? why is it referred to as fluid?

Answer 2

• the mixture and movement of the phospholipids, proteins, glycoproteins, and glycolipids the membrane is made of
• described as fluid due to lateral movement in the molecules

Question 3

Why do the phospholipids align as a bilayer?

Answer 3

• hydrophilic heads are attracted to water and the hydrophobic tails repel water

Question 4

What is the role of extrinsic proteins? what is the function of glycolipids and glycoproteins?

Answer 4

• to provide mechanical support, or make glycolipids and glycoproteins; their function is cell recognition, as receptors

Question 5

What is the role of intrinsic (integral) proteins?

Answer 5

• to be protein carriers or channel proteins the are involved in the transport of molecules across the membrane

Question 6

What is the difference between extrinsic and intrinsic proteins?

Answer 6

• intrinsic proteins form tubes that fill with water to enable water-soluble ions to diffuse
• carrier proteins bind with other ions and larger molecules, and change shape to transport them to the their side of the membrane

Question 7

What is the role of cholesterol? why is its role important?

Answer 7

• to restrict the lateral movement of other molecules in the membrane, so at high temps the membrane is less fluid to prevent water and dissolved ions from leaking out of the cell

Question 8

What two factors affect membrane structure?

Answer 8

• temperate and solvents

Question 9

How does temperature and solvents affect membrane structure?

Answer 9

• temperature: high temp increases the kinetic energy of the phospholipids, making them move more. this increases membrane fluidity which increases permeability and the structure can start to break, making it easier for particles to cross the membrane
• high temps can also cause carrier and channel proteins in the membrane to denature
• solvents: organic solvents dissolve the phospholipid bilayer in membrane which causes the fluidity of the membrane to increase, making it more permeable

Question 10

What are the six key modes of transport in and out of cells?

Answer 10

1. simple diffusion
2. facilitated diffusion
3. osmosis
4. active transport
5. endocytosis
6. exocytosis

Question 11

What is simple diffusion and what is required of the molecules in simple diffusion?

Answer 11

• simple diffusion: the net movement of molecules from an area of higher concentration to and area of lower concentration until equilibrium is reached. this process doesn’t require ATP
• molecules have to be lipid soluble and small to diffuse across the membrane

Question 12

What is facilitated diffusion? what ions are moved using this transport method and why?

Answer 12

• facilitated diffusion: a passive process, down the concentration gradient, through proteins
• ions and polar molecules are transported using facilitated diffusion using protein channels and carrier proteins because they just can’t diffuse

Question 13

What is osmosis?

Answer 13

• osmosis: the movement of water from an area of higher water potential to an area of low water potential (more negative) across a partially permeable membrane

Question 14

What are the three types of solutions and what do they mean?

Answer 14

1. isotonic: when the water potential of the solution is the same in the solution and the cell within the solution
2. hypotonic: when the water potential of a solution is more positive (closer to zero) than the cell
3. hypertonic: when the water potential of a solution is more negative that the cell

Question 15

What is active transport? why is it selective? why does the protein change shape?

Answer 15

• active transport: the movement of molecules and ions from an area of lower concentration to an area of higher concentration, against the concentration gradient, using ATP and carrier proteins
• its selective because only certain molecules can bind to receptor site on carrier proteins; ATP will bind, it is then hydrolysed into ADP and Pi
• this reaction causes the protein to change shape, opening towards the the inside of the membrane which causes the molecule to be released on the other side. the Pi molecule is then released from the protein, the protein then reverts back to its original shape

Question 16

What is endocytosis? how does it occur? what are the two classifications of endocytosis and when are they used?

Answer 16

• endocytosis: a type of active transport which is the bulk transport of molecules into a cell
• occurs by the cell surface membrane bending inwards around the molecule surrounding it to form a vesicle that pinches off and moves within the cytoplasm
• phagocytosis when its a solid particle being taken in
• pinocytosis when its a liquid being taken in

Question 17

What is exocytosis? how does it occur? why is energy required?

Answer 17

• exocytosis: the bulk transport of molecules out of a cell
• occurs by vesicles moving toward the cell surface membrane fuse with the membrane and the content of the vesicle is released outside the cell
• it requires energy because ATP is needed to move the vesicle along the cytoskeleton