Chapter 2 - Transport Across Membranes

Question 1

What is diffusion?

Answer 1

The net movement of particles (ions or molecules) down a concentration gradient from a region of higher concentration to a region of lower concentration.

Question 2

What is the relationship between concentration gradient and rate of diffusion?

Answer 2

The steeper the concentration gradient for a substance, the faster the rate of diffusion is for that substance

Question 3

Why do particles diffuse?

Answer 3

• The particles of fluids possess kinetic energy and are continually moving about
• As their movement is random, the particles will diffuse down the concentration gradient and become evenly spaced out over time

Question 4

A permeable membrane allows ________ to pass through it

Answer 4

both the solvent (water) and the solutes (the dissolved substances)

Question 5

The cell membrane is ________

Answer 5

partially permeable

Question 6

Oxygen and carbon dioxide move into and out of cells by ________

Answer 6

diffusion

Question 7

What is facilitated diffusion?

Answer 7

The process of passive transport of molecules or ions passing through transmembrane/integral/transport proteins across a biological membrane.

Question 8

Why is there a need for facilitated diffusion?

Answer 8

Polar or large molecules cannot easily pass through the hydrophobic region of the phospholipid bilayer by diffusion

Question 9

How does a channel protein work?

Answer 9

Through a channel that can open or close

- transports ions/very small molecules

Question 10

How does a carrier protein work?

Answer 10

By changing shape

• molecule attaches to the binding site of the protein
• protein changes configuration and releases molecule to the other side of the membrane

Question 11

What is osmosis?

Answer 11

The net movement of water molecules from a region (solution) of less negative water potential to a region (solution) of more negative water potential, through a partially permeable membrane.

Question 12

What is meant by “hypertonic”?

Answer 12

more negative water potential

Question 13

What is meant by “hypotonic”?

Answer 13

less negative water potential

Question 14

What is meant by “isotonic”?

Answer 14

same water potential

Question 15

What happens to a plant cell in hypotonic solution?

Answer 15

1. Cell sap has more negative water potential than that of the solution outside the cell.
2. By osmosis, water enters the cell through the partially permeable cell membrane.
3. Cell expands and becomes turgid.
4. The vacuole increases in size and pushes the cytoplasm against the cell wall. The cell does not burst because it is protected by the inelastic cell wall.

Question 16

How does the cell wall prevent the plant cell from bursting?

Answer 16

Cell wall is strong and relatively inelastic

- prevents over-expansion of cell by exerting an opposing pressure as water enters, preventing entry of more water

Question 17

What happens to a plant cell in hypertonic solution?

Answer 17

1. Cell sap has less negative water potential than that of the solution.
2. By osmosis, water from the vacuole and cytoplasm leaves the cell through the partially permeable cell membrane.
3. Cell decreases in size and becomes flaccid.
4. The vacuole decreases in size and the cytoplasm and cell membrane shrinks away from the cell wall (plasmolysed).
   (Can be restored to its original state by placing it in water or in a solution with less negative water potential.)

Question 18

What happens to an animal cell in hypotonic solution?

Answer 18

1. Cytoplasm has lower water potential than that of the solution.
2. By osmosis, water enters the cell through the partially permeable cell membrane.
3. Cell swells/expands and bursts (as it has no cell wall to protect it).

Question 19

What happens to an animal cell in hypertonic solution?

Answer 19

1. Cytoplasm has higher water potential than that of the solution.
2. By osmosis, water from the vacuole and cytoplasm leaves the cell through the partially permeable cell membrane.
3. Cell shrinks and little spikes appear on the cell membrane (crenated).
4. Cell will become dehydrated and eventually die.

Question 20

What happens to a plant/animal cell in isotonic solution?

Answer 20

Will not change in size or shape

Question 21

What is the importance of turgor in plant cells?

Answer 21

Maintains the shape of soft tissues in plants

• Most leaves and young stems are able to remain firm and erect because of the turgor pressure within their cells
• Plants wilts: due to high rate of evaporation of water from the cells and they lose their turgidity
• Change in turgor of the guard cells cause the opening and closing of the stomata

Question 22

Why is it not advisable to add too much fertiliser around the roots of plants?

Answer 22

• Soil solution will become very concentrated → cell sap of root hair cells has less negative water potential than soil.
• Water will leave the roots by osmosis.
• Inability of the roots to absorb water + continued evaporation of water from the leaves → plant will wilt.

Question 23

Example of osmosis: water entering root hair cell

Answer 23

• Thin film of fluid surrounding each soil particle: dilute solution of mineral salts (less negative WP)
• Water enters cell by osmosis
• Water dilutes cell sap in fist cell (less negative WP compared to next cell)
• Water moves from one cell to the next by osmosis

Question 24

Similarities between diffusion and osmosis

Answer 24

• Passive processes (no energy required)

- Movement of substances down a concentration gradient

Question 25

Differences between diffusion and osmosis

Answer 25

- Diffusion can be the movement of any substance whereas osmosis refers specifically to the movement of water molecules. - A partially permeable membrane is not required in diffusion but is required in osmosis.

Question 26

What is active transport?

Answer 26

The process in which energy (ATP molecules) is used to move particles against a concentration gradient (from a region of lower concentration to a region of higher concentration)

Question 27

Active transport occurs only in ________, energy used comes from ________

Answer 27

living cells | respiration

Question 28

Example of active transport (in plants)

Answer 28

Absorption of mineral salts by root hair cells - When concentration of ions/dissolved mineral salts in soil is lower than that in the cell sap - They are pumped into the cells

Question 29

Example of active transport (in humans)

Answer 29

Absorption of glucose and amino acids in the lumen by epithelial cells in the ileum (small intestine)

Question 30

Movement of glucose from ileum epithelial cells to blood capillaries in the villus is by ________

Answer 30

facilitated diffusion

Question 31

Similarities between facilitated diffusion and active transport

Answer 31

- Transports solutes across cell membranes | - Involve transport proteins

Question 32

Differences between facilitated diffusion and active transport

Answer 32

- Facilitated diffusion is the movement of substances from higher to lower concentration whereas active transport is from lower to higher concentration - Facilitated diffusion is a passive process (energy not required) whereas active transport is an active process (energy required)

Question 33

Surface area to volume ratio ________ with increasing cell size

Answer 33

decreases

Question 34

The greater the area of cell surface membrane per unit volume, the ________ the rate of diffusion of a substance for a given concentration gradient.

Answer 34

faster

Question 35

Surface area to volume ratio: why are cells so small?

Answer 35

Cells need to move waste materials and nutrients across their cell membrane quickly to stay alive

Question 36

Define "endocytosis"

Answer 36

A cellular process where cells absorb large molecules or ‘bulk materials’ from outside the cell by engulfing it via a membranous vesicle

Question 37

Examples of endocytosis

Answer 37

- feeding by amoeba or paramecium | - phagocytosis and destruction of a bacterial cell by a white blood cell

Question 38

Why is endocytosis important to the cell?

Answer 38

Most molecules important to the cell cannot normally pass through the cell's hydrophobic plasma membrane

Question 39

Define "exocytosis"

Answer 39

A cellular process where cells eject large molecules––waste products or cell products (such as enzymes and hormones)––from the interior of the cell via a membranous vesicle

Question 40

Examples of exocytosis

Answer 40

- secretion of protein from cells | - secretion of waste material from cells

Question 41

Differences between endocytosis and exocytosis

Answer 41

In endocytosis, the cells absorb large molecules or 'bulk materials' from outside the cell to inside the cell whereas in exocytosis, the cells eject waste products or cell products from the interior of the cell to outside the cell

Question 42

Exocytosis is ________ in function to endocytosis but working in the ________ direction

Answer 42

similar | opposite