Active Transport, Bulk Transport & Osmosis

Question 1

What is the definition of active transport?

Answer 1

The movement of molecules or ions into or out of a cell from an area of low concentration to an area of high concentration (against a concentration gradient) across a partially permeable membrane.

Question 2

What does active transport require?

Answer 2

• energy (ATP from respiration). It is an active process.
• transport / carrier protein in the cell surface membrane - acts as a “pump” and is complementary in shape to the molecule they transport. Molecules are pumped in one direction across the membrane.

Question 3

What is active transport important for?

Answer 3

• reabsorption of useful molecules and ions into the blood after filtration into kidney tubules
• absorption of some digestive products from digestive tract
• loading of sugar from photosynthetic cells into phloem tissue
• loading of inorganic ions from soil into root hairs

Question 4

How do molecules move by active transport?

Answer 4

1. Molecule / ion binds to receptors with a complementary shape on the carrier protein.
2. ATP binds to the carrier protein on inside of the cell. It is hydrolysed to ADP + P with the release of energy. The phosphate molecule remains bound to the protein.
3. The energy provided allows the carrier protein to change shape (conformational change).
4. Molecule / ion released inside cell.
5. Phosphate is released from the carrier protein and recombines with ADP.
6. Carrier protein returns to it’s original shape.

Question 5

What factors affect the rate of active transport?

Answer 5

• speed of individual carrier proteins
• number of carrier proteins
• rate of respiration and release of ATP molecules

Question 6

What is bulk transport?

Answer 6

Another form of active transport - moves substances that are too large to move through channel or carrier proteins.

• exocytosis
• endocytosis

Question 7

What is exocytosis?

Answer 7

Bulk transport of material out of a cell using a vesicle which fuses with the cell surface and the contents are released.

Question 8

What is endocytosis?

Answer 8

Bulk transport of material into a cell by the fusing of a vesicle with the cell surface membrane.

• phagocytosis (solids)
• pinocytosis (liquids)

Question 9

Why is energy required in bulk transport?

Answer 9

• transporting vesicles along the cytoskeleton
• fusing vesicles to the membrane

Question 10

What is water potential?

Answer 10

The pressure exerted by water molecules as they collide with a membrane or container. It is measured in Pa or kPa.

Question 11

What is the definition of osmosis?

Answer 11

Osmosis is the net random movement of water molecules from a region of high water potential to a region of low water potential (down a concentration gradient) across a a partially permeable membrane.

Osmosis is a passive process.

Question 12

How does water potential work in pure water and solutions?

Answer 12

Pure water has a water potential of 0. Solutions have a value below 0.

Question 13

Which factors affect the rate of osmosis?

Answer 13

• water potential gradient
• thickness of exchange surface
• SA of exchange surface

Question 14

What is meant by hypotonic solution?

Answer 14

A solution with more water in it than solute, compared to another solution.

Question 15

What is meant by hypertonic solution?

Answer 15

A solution with more solute in it than water, compared to another solution.

Question 16

What is meant by isotonic solution?

Answer 16

Two solutions / cells that have the same water potential.

Question 17

How does osmosis work in plant cells?

Answer 17

• Plasmolysed - hypertonic - lower water potential outside cell
• Flaccid - isotonic - equal water potential inside and out
• Turgid - hypotonic - higher water potential outside cell

Question 18

How does osmosis work in animal cells?

Answer 18

• Crenated - hypertonic - lower water potential outside cell
• Normal - isotonic - equal water potential inside and out
• Cytolysis - hypotonic - higher water potential outside cell

Question 19

What happens to animal cells in a hypotonic solution?

Answer 19

• Outside environment has a higher water potential, water enters the cell by osmosis
• Hydrostatic pressure inside the cell increases
• Cell surface membrane cannot withstand pressure increase, animal cell bursts (cytolysis)

Question 20

What happens to animal cells in a hypertonic solution?

Answer 20

• Outside environment has a lower water potential, water leaves the cell by osmosis
• Hydrostatic pressure inside the cell decreases
• Cell shrinks and shrivels (crenation)

Question 21

What happens to plant cells in a hypotonic solution?

Answer 21

• Outside environment has a higher water potential, water enters the cell by osmosis
• Hydrostatic pressure inside the cell increases
• Cell swells (turgidity) but the cellulose cell wall prevents bursting - cytoplasm pulled towards from cell wall

Question 22

What happens to plant cells in a hypertonic solution?

Answer 22

• Outside environment has a lower water potential, water leaves the cell by osmosis
• Hydrostatic pressure inside the cell decreases
• Cell shrinks and shrivels - cytoplasm pulled away from cell wall (plasmolysis)