Biological Membranes

Question 1

What are the roles of membranes at the surfaces of cells?

Answer 1

• separates the cell’s components from its external environment
• regulates transport of materials into and out of the cell
• may contain enzymes involved in specific metabolic pathways
• has antigens so organism’s immune system can recognise the cell as being self and not attack
• site for cell communication or signalling
• site of chemical reactions

Question 2

What are the roles of membranes within cells?

Answer 2

• separate organelle contents from the cell cytoplasm
• folded membranes in mitochondria increase surface area and localise enzymes needed for respiration
• some reactions occur on the inner membrane of chloroplasts (thylakoid)
• digestive enzymes on plasma membrane of epithelial cells catalyse final stages in the breakdown of some sugars

Question 3

What is the fluid mosaic model?

Answer 3

• theory of cell membrane structure that proposes the membrane consists of a phospholipid bilayer with proteins embedded in a mosaic pattern.

Question 4

Describe the fluid mosaic model

Answer 4

• fluid- phospholipid bilayer where individual phospholipids can move. Consists of phospholipids with hydrophobic tails and hydrophilic heads
• mosaic- embedded with glycoproteins, carrier proteins, peripheral proteins, channel proteins, cholesterol and glycolipids

Question 5

Explain the role of cholesterol in cell membranes

Answer 5

• steroid molecule that reduces fluidity to make bilayer more stable
  -important for regulating the fluidity of the membrane and maintaining mechanical stability
• resists effects of temperature changes on the structure of the membrane

Question 6

Explain the role of glycolipids in cell membranes

Answer 6

important in cell signalling and cell recognition

Question 7

Explain the functions of extrinsic proteins

Answer 7

• binding sites/receptors
• antigens (glycoproteins)
• cell signalling
• bind cells together

Question 8

Explain the functions of integral proteins such as carrier and channel proteins

Answer 8

• carrier proteins carry specific molecules across the membrane. They are involved in active transport and facilitated diffusion.
• Channel proteins are involved in facilitated diffusion and allow ions and similar substances to pass through
• channel and carrier poteins membrane spanning regions interact with the hydrophobic region of the phospholipid bilayer

Question 9

Give one example of an organelle with a different cell membrane composition and explain why

Answer 9

• Neurons
• protein channels and carriers in plasma membrane along axon allow entry and exit of ions, which results in the conduction of electrical impulses along their length
• they have a myelin sheath formed by flattened cells wrapped around them several times which results in several layers of cell membrane

Question 10

Name 2 types of diffusion

Answer 10

• simple
• facilitated

Question 11

Define simple diffusion

Answer 11

movement of molecules from an area of high concentration to an area of low concentration

Question 12

Define facilitated diffusion

Answer 12

movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels or carriers

Question 13

explain how an equilibrium is reached during diffusion

Answer 13

if there is a high concentration of molecules then they will bump into each other (kinetic energy) and eventually spread further from each other. More will move to an area where they are in lower concentration until they become evenly dispersed. Once the molecules have moved down their conc gradient they remain evenly dispersed and so there is no net diffusion. They have reached equilibrium