Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), as a result of their random movement.

• The energy for diffusion comes from the kinetic energy of the random movement of molecules and ions — no extra energy from the cell.
• Substances such as oxygen, carbon dioxide and urea move in and out of cells by diffusion through the cell membrane.

Factors that increase the rate of diffusion:

• a larger surface area;
• a higher temperature (faster-moving particles);
• a steeper concentration gradient;
• a shorter distance to diffuse across.

Watch out: diffusion is the net movement. Particles move both ways randomly, but overall more move from high to low concentration until evenly spread.

Speeding up diffusion

Osmosis

Water diffuses through partially permeable membranes by osmosis. Water moves into and out of cells by osmosis through the cell membrane. Water is a vital solvent in digestion, excretion and transport.

Supplement definition: osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.

Which way does water go?

Turgor, flaccid & plasmolysis

Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. Osmosis changes this pressure:

• Turgid — in a dilute solution, water enters by osmosis; the cell swells and pushes on the wall. The wall stops it bursting. This turgor pressure supports the plant.
• Flaccid — when a cell loses water, turgor pressure falls and the cell becomes floppy; the plant wilts.
• Plasmolysis — in a very concentrated solution, so much water leaves that the membrane pulls away from the cell wall.

Supplement: use the terms turgid, turgor pressure, flaccid and plasmolysis, and explain the importance of water potential and osmosis in the uptake and loss of water by organisms.

Which process is it?

Tap a statement, then tap the process it describes.

Active transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration.

• Because it goes against the gradient, the cell must supply energy from respiration — unlike diffusion and osmosis.
• An important example is the uptake of mineral ions by root hair cells, where the ions are more concentrated inside the root than in the soil water.

Supplement: active transport uses protein carriers in the membrane to move molecules or ions across. Explain its importance, including ion uptake by root hairs.

Against the gradient

Investigating osmosis

Two classic investigations:

• Dialysis (Visking) tubing acts as a model partially permeable membrane. Filled with sugar solution and placed in water, water moves in by osmosis, so the level in an attached tube rises.
• Potato cylinders — cut equal-sized cylinders, record starting mass, leave each in a different sugar concentration, re-weigh and find the percentage change in mass.

A potato in pure water gains mass (water enters by osmosis); in a concentrated solution it loses mass. Using % change fairly compares cylinders that began at slightly different masses.

Your turn — percentage change

Your turn — a cylinder that shrinks

Match each statement to its process

Tap a statement on the left, then its matching process on the right.

The big ideas to know

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