2-3 Transport across membranes

Question 1

What is the structure of the cell surface membrane?

Answer 1

• Partially permeable consisting of phospholipids.
• Phospholipids have hydrophilic heads which point outwards and hydrophobic tails which point inwards.
• This structure allows lipid soluble molecules to pass through the membrane, but not water-soluble molecules.

Question 2

What is the function of the cell surface membrane?

Answer 2

• Controlling the movement of substances in and out of the cell.

Question 3

What is the fluid mosaic model?

Answer 3

• Fluidity of the membrane and the mosaic arrangement of the proteins.

Question 4

What are the components of the cell surface membrane?

Answer 4

• Proteins, intrinsic or extrinsic. Aid the movement across the membrane, provide mechanical support and act in conjunction with glycolipids as receptors.
• Cholesterol. Makes the membrane more rigid and reduce lateral movement of the phospholipids. Prevents leakage of water and dissolved ions.
• Glycolipids. Made up of carbohydrate that is bound to lipids. Act as cell surface receptors. Allow cells to adhere to other cells to form tissues.
• Glycoproteins. Carbohydrates that attach to extrinsic proteins and act as cell surface receptors and neurotransmitters. Allow cells to recognise one another. Allow cells to adhere to other cells to form tissues.

Question 5

What is diffusion?

Answer 5

• The passive movement of small, non-polar, lipid soluble molecules such as CO2 and O2 from an area of high concentration to an area of low concentration.
• The molecules move directly through the phospholipid bilayer.

Question 6

What is facilitated diffusion?

Answer 6

• Requires a channel protein in the cell membrane to transport polar molecules, charged and water-soluble molecules across the membrane.

Question 7

What is osmosis?

Answer 7

• The diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane.

Question 8

What is active transport?

Answer 8

• Can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration using energy in the form of ATP.

Question 9

What is Endocytosis?

Answer 9

• The large particles are enclosed in vesicles made from the cell surface membrane and transported into the cell.

Question 10

What is Exocytosis?

Answer 10

• Vesicles containing large particles are fused with the cell surface membrane and released from the cell.

Question 11

What is co-transport?

Answer 11

• Uses ions to move substances into and out of cells.
• Occurs particularly in epithelial cells of the ileum.
• Sodium and potassium ions are pumped out of the epithelial cell by active transport into the blood leaving a lower concentration in the cell.
• This causes these ions to move in from the lumen by facilitated diffusion, which at the same time brings glucose and amino acids into the cell.
• These then diffuse from a high concentration in the epithelial cell to a low concentration in the blood.

Question 12

What affects the rate of gas exchange by diffusion?

Answer 12

• Surface area
• Diffusion gradient
• Diffusion distance
• Temperature