3 - EXCHANGE OF SUBSTANCES

Question 1

Explain the process of lipid digestion and absorption

Answer 1

• bile emulsifies lipids to form smaller molecules
• these smaller molecules are broken down from triglycerides into monoglycerides and fatty acids
• monoglycerides and fatty acids combine with bile to form micelles
• micelles travel to cell lining in epithelium
• monoglycerides and fatty acids released and absorbed into cell by diffusion (not the micelles)
• triglycerides reformed in the cell by smooth ER and form chylomicrons
• chylomicrons packaged in vesicles
• chylomicrons eventually released into lymphatic system by lacteal and then into the blood

Question 2

What are chylomicrons?

Answer 2

Lipoproteins (combine with lipids) that are transported in the lymphatic system to enter the blood in order to deliver lipids to various tissues for storage or energy utilization.

Question 3

How many oxygen molecules can each haemoglobin molecule carry?

Answer 3

up to 4

Question 4

Describe the transport of carbohydrates in plants (mass flow hypothesis)

Answer 4

• sucrose actively transported into phloem by companion cells
• this lowers the water potential in the phloem
• water enters from the xylem into phloem by osmosis
• creates hydrostatic pressure gradient
• mass flow of sucrose to respiring cells

Question 5

Describe cotransport of glucose and sodium in the ileum

Answer 5

• sodium actively transport from epithelial cells into blood by a sodium potassium pump
• means that there is now more sodium ions in the lumen of the intestine than in the epithelial cell
• sodium enters epithelial cell down its concentration gradient whilst carrying glucose via a co transport protein
• glucose then moves down its concentration gradient into blood via facilitated diffusion

Question 6

How does sodium potassium pumps work?

Answer 6

• powered by ATP
• The pump moves sodium and potassium ions in opposite directions, each against their concentration gradient
• 3 sodium ions are moved out while 2 potassium ions are moved in to cell

Question 7

Explain the cohesion tension theory of water transport in the xylem

Answer 7

1. Water lost from leaf by transpiration (evaporation) from Mesophyll cells into air spaces, water then diffuses through open stomata
2. Reduces water potential of mesophyll cells, so water drawn out of xylem by osmosis
3. Creates tension or ‘pull’ in xylem
4. Cohesion of water molecules due to hydrogen bonds, so water is pulled up as a continuous column
5. Water enter roots via osmosis