Transport in lower organisms: diffusion and osmosis

Learning objectives

By the end of this lesson, you should be able to:

• explain why simple organisms can exchange materials directly with their surroundings
• define diffusion and osmosis accurately
• describe the role of concentration gradients and surface-area-to-volume ratio
• explain osmosis in freshwater and concentrated solutions
• distinguish diffusion, osmosis and active transport

Why lower organisms need transport

Every living cell needs substances such as oxygen, water and dissolved nutrients and must remove waste products such as carbon dioxide. Large multicellular animals require specialised transport systems because many cells are far from the body surface. Small or simple organisms, such as Amoeba and Paramecium, have a large surface area relative to their volume. Their cells are close to the surrounding water, so substances can move across the cell surface by diffusion and osmosis without a heart or blood-vessel system.

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. It does not require energy from the cell.

Oxygen dissolved in water can diffuse into an Amoeba when its concentration is higher outside than inside. Oxygen is then used in aerobic respiration, which helps maintain a lower concentration inside the cell and supports continued diffusion. Carbon dioxide produced by respiration can diffuse out when its concentration is higher inside the cell.

Diffusion is faster when the concentration gradient is steeper, the exchange surface is larger, the diffusion distance is shorter and temperature is higher (within a range that does not damage the organism). Small organisms benefit from a short distance between the external environment and the cell interior.

Osmosis

Osmosis is the net movement of water molecules through a selectively permeable membrane from a region of higher water potential (a more dilute solution) to a region of lower water potential (a more concentrated solution). Water moves in both directions, but osmosis describes the net movement.

A selectively permeable membrane allows some substances, especially water, to pass more readily than others. In a dilute external solution, water may enter a cell by osmosis. In a more concentrated external solution, water may leave the cell. The direction depends on the relative water potentials on the two sides of the membrane.

A freshwater Amoeba gains water because its cytoplasm is generally more concentrated than the surrounding water. Excess water is collected in a contractile vacuole, which periodically contracts and expels water. This osmoregulation prevents the cell from swelling excessively and bursting. Marine and freshwater organisms face different osmotic conditions, so their adaptations differ.

Effects of solutions on cells

If an animal cell is placed in a very dilute solution, water enters and the cell may swell and burst because it has no rigid cell wall. If placed in a concentrated solution, water leaves and the cell shrinks. A plant cell in a dilute solution becomes turgid because its cell wall resists expansion; in a concentrated solution, the cell membrane and cytoplasm may pull away from the wall, a process called plasmolysis.

Active transport

Active transport moves substances across a membrane against a concentration gradient, from lower concentration to higher concentration. It requires energy, usually from ATP, and carrier proteins in the membrane. It is different from diffusion and osmosis, which are passive processes. Some unicellular organisms use active transport to take up particular mineral ions or nutrients when external concentrations are low.

WASSCE examination focus

Use precise definitions. Diffusion involves particles moving down a concentration gradient; osmosis specifically involves water moving through a selectively permeable membrane down a water-potential gradient. Do not define osmosis as the movement of any liquid or dissolved substance. When explaining why Amoeba does not need a blood system, relate its small size and high surface-area-to-volume ratio to short diffusion distances.

Quick check

1. Define diffusion.
2. State the two essential conditions in the definition of osmosis.
3. Why does a freshwater Amoeba need a contractile vacuole?
4. Give one difference between active transport and diffusion.

Answers: 1. Net movement of particles from higher to lower concentration down a concentration gradient due to random motion. 2. Water moves through a selectively permeable membrane from higher to lower water potential. 3. Water continually enters by osmosis and the vacuole expels excess water. 4. Active transport uses energy to move substances against a concentration gradient; diffusion is passive and moves down a concentration gradient.