1.4 movement of substances

define diffusion

the net movement of particles down a concentration gradient (until equilibrium is reached)

describe the role of diffusion in nutrient uptake and gaseous exchange in plants and humans

nutrient uptake (plants):

• helps mineral salts / ions and water move short distances between cells in the roots and throughout the plant

nutrient uptake (humans):

• allows digested food molecules (eg glucose and amino acids) to move from the small intestine into the bloodstream for transport to cells

gaseous exchange (plants):

• allows CO₂ to enter the leaves for photosynthesis and O₂ to leave as a waste product

• enables movement of gases within the leaf through the stomata and air spaces

gaseous exchange (humans):

• allows O₂ to move from alveoli in lungs into blood and CO₂ to move from blood into alveoli for exhalation

define osmosis

the net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane

define water potential (WP)

a measure of the tendency of water to move from one place to another

describe the effects of osmosis on plants (plant cell in solution with higher WP)

1. cell sap has lower WP than surrounding solution

2. net movement of water into the cell through selectively permeable cell surface membrane via osmosis

3. cell wall prevents cell from lysing as cytoplasm volume increases

4. cell expands and becomes turgid

describe the effects of osmosis on plants (plant cell in solution with lower WP)

1. cell sap has higher WP than surrounding solution

2. net movement of water out of the cell through selectively permeable cell surface membrane via osmosis

3. cell surface membrane pulls away from cell wall and cell becomes plasmolysed

4. cell decreases in size and becomes flaccid

describe the effects of osmosis on animal tissues (animal cell in solution of higher WP)

1. cytoplasm has lower WP than surrounding solution

2. net movement of water molecules into the cell through selectively permeable cell surface membrane via osmosis

3. cell expands and bursts

describe the effects of osmosis on animal tissues (animal cell in solution of lower WP)

1. cytoplasm has higher WP than surrounding solution

2. net movement of water molecules out of the cell through selectively permeable cell surface membrane via osmosis

3. cell shrinks in size (crenated) — with spikes

define active transport

the process where cellular energy, in the form of adenosine triphosphate (ATP), is used to move the particles of a substance across a selectively permeable cell surface membrane against its concentration gradient

discuss the importance of active transport

it is an energy-consuming process by which substances are transported against a concentration gradient, as in ion uptake by root hairs and uptake of glucose by cells in the villi

describe the key features of active transport

1. occurs against a concentration gradient

• the cell holds some reserves of molecules valuable in metabolism (eg nitrates ions in plant cells / calcium ions in muscle fibres)

2. utilises metabolic energy in the form ATP

3. active uptake is a highly selective process

• only substances that are more needed by the cell will more likely to be taken up

4. involves protein “pumps” at the selectively permeable cell surface membrane

• most membrane pumps are specific to particular substances, so only selected substances are taken up

give 2 examples on where active transport takes place in plants and humans/animals

• selectively permeable cell surface membrane of root hair cells to absorb mineral ions needed

• selectively permeable cell surface membrane of intestinal epithelial cell to absorb nutrients (eg glucose and amino acids) from the small intestine into the body