Transport Across Membranes

Question 1

What are the 4 key types of transport across membranes?

Answer 1

• simple diffusion
• facilitated diffusion
• active transport
• osmosis

Question 2

Describe simple diffusion.

Answer 2

• the net movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached
• no atp required: down conc. gradient
• molecules must be lipid soluble and small

Question 3

Describe facilitated diffusion.

Answer 3

• the net movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached through the use of proteins to transport the molecule
• movement of ions + polar molecules
• channel/ carrier proteins

Question 4

Describe osmosis.

Answer 4

• the movement of water from an area of high water potential to an area of low water potential across a partially permeable membrane

Question 5

What is water potential?

Answer 5

• pressure created by water molecules

Question 6

What is the water potential of pure water? What happens when solutes are dissolved?

Answer 6

• water potential of pure water is 0
• the more solutes that are dissolved, the more negative the water potential becomes

Question 7

What is an isotonic solution?

Answer 7

• a solution where the water potential is the same in the solution as the cell

Question 8

What is a hypotonic solution?

Answer 8

• a solution where water potential is more positive (closer to 0) than the cell

Question 9

What is a hypertonic solution?

Answer 9

• a solution where the water potential is more negative than the cell

Question 10

What is active transport?

Answer 10

• the movement of substances from a low concentration to a high concentration using metabolic energy and a carrier protein
• requires ATP

Question 11

Describe the process of active transport.

Answer 11

1. Transport is through only carrier proteins
2. Molecule binds to a receptor complementary to its shape on the protein
3. ATP binds to the carrier protein from the inside of the cell and is hydrolysed into ADP + Pi
4. This causes the carrier protein to change shape + release the molecule on the other side
5. The phosphate ion is the released as the protein returns to its original shape

Question 12

Why is co-transport needed in the iluem?

Answer 12

a high concentration of glucose is needed in the lumen compared to epitheleal cells for fascilitated diffusion to absorb glucose

but

there is usually more glucose in the epitheleal cellls; active transport and co-transport are needed

Question 13

Describe co-transport of glucose + sodium ions in the iluem.

Answer 13

1. Sodium ions are actively transported out of the epithelial cell into the capillaries
2. Causes conc. of sodium ions to decrease in epithelial cells
3. Creates conc. gradient from the ileum to the epithelial cells ; sodium ions can diffuse down the conc. gradient into epithelial cells
4. Sodium ions diffuse through co-transporter proteins (so glucose/ amino acids must also attach) against their concentration gradient
5. Glucose diffuses by facilitated diffusion from the epithelial cell into the bloodstream.

Question 14

How can cells be adapted for rapid transport across internal/ external membranes?

Answer 14

• big surface area (eg having microvilli)
• high number of channel/ carrier proteins in their membranes