transport and exchange

What is diffusion?

The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

What three factors increase the rate of diffusion?

1. Steeper concentration gradient. 2. Higher temperature (more kinetic energy). 3. Larger surface area of the membrane.

Why do large multicellular organisms need specialised exchange surfaces and transport systems?

They have a small surface area to volume ratio, so diffusion distances are too large for oxygen/nutrients to enter and for waste (CO2, urea) to leave quickly enough, so they need specialised exchange surfaces and a transport system.

How do you calculate Surface Area to Volume Ratio (SA:V) for cubes vs cuboids?

For a cube: SA = area of one face x 6. For a cuboid: SA = (2 x length x width) + (2 x length x height) + (2 x width x height). Then divide SA by Volume (length x width x height) to simplify.

How is the small intestine adapted for exchange of materials?

1. Covered in villi to provide a huge surface area. 2. Villi have microvilli for extra surface area. 3. Wall is one cell thick for a short diffusion pathway. 4. Good blood supply to maintain steep concentration gradient.

How are alveoli in lungs adapted for gas exchange?

1. Millions of alveoli present a huge surface area. 2. Walls are one cell thick for short diffusion distance. 3. Moist lining allows gases to dissolve. 4. Rich capillary network maintains high concentration gradient.

How are plant leaves adapted for gas exchange?

1. Flattened shape increases surface area. 2. Stomata allow CO2 to enter and O2/water vapour to exit. 3. Spongy mesophyll air spaces increase surface area inside leaf.

How are fish gills adapted for gas exchange?

1. Gill filaments covered in lamellae create large surface area. 2. Thin surface layer for short diffusion path. 3. Countercurrent flow of blood and water maintains steep concentration gradient.

What is osmosis?

The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

What happens to animal cells in dilute vs concentrated solutions?

In a solution more dilute than the cell contents: cell gains water, swells, and bursts (lysis). In a solution more concentrated than the cell contents: cell loses water and shrivels (crenation).

What happens to plant cells in dilute vs concentrated solutions?

In a solution more dilute than the cell contents: cell gains water, vacuole swells, pushing against cell wall making it turgid. In a solution more concentrated than the cell contents: cell loses water, cell membrane pulls away from cell wall (plasmolysis).

What is active transport?

The movement of substances from a dilute solution to a more concentrated solution against a concentration gradient, requiring energy from respiration.

Where is active transport used in plants?

Absorption of mineral ions from dilute soil solution into root hair cells.

Where is active transport used in humans?

Absorption of sugar molecules (glucose) from lower concentrations in the lumen of the gut into the blood.

Name the functions of the following plant tissues: Epidermal, Palisade mesophyll, Spongy mesophyll, Meristem?

Epidermal: Covers plant, waxy cuticle reduces water loss. Palisade: Packed with chloroplasts near top of leaf for maximum photosynthesis. Spongy: Has air spaces to allow rapid diffusion of gases. Meristem: Contains stem cells at root/shoot tips for growth.

What is the xylem and what does it transport?

Dead, hollow tubes reinforced with lignin that transport water and mineral ions from roots to leaves in one direction (Transpiration stream).

What is the phloem and what does it transport?

Living sieve tubes with companion cells that transport dissolved sugars (sucrose) up and down the plant to growing/storage tissues (Translocation).

What is transpiration?

The evaporation and loss of water vapour from the surface of plant leaves through the stomata.

What four factors increase the rate of transpiration?

1. Increased light intensity (stomata open wider). 2. Increased temperature (water molecules have more kinetic energy). 3. Increased air movement/wind (removes water vapour, maintaining steep concentration gradient). 4. Decreased humidity (steeper concentration gradient between inside leaf and air).

How do guard cells control stomata?

In high light/water availability, guard cells absorb water by osmosis, become turgid, and bow outwards to open stomata. In drought/darkness, they lose water, become flaccid, and close stomata to prevent water loss.

Why can a single-celled organism get enough substances by diffusion alone?

A single-celled organism has a relatively large surface area to volume ratio, which allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

What increases the effectiveness of an exchange surface?

1. A large surface area. 2. A membrane that is thin, to provide a short diffusion path. 3. (In animals) an efficient blood supply. 4. (In animals, for gaseous exchange) being ventilated.