Transport Across the Plasma Membrane

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Passive transport

Active transport

The movement of substances follows the concentration gradient.

Substances move against the concentration gradient.

Cellular energy is not required.

Cellular energy (ATP) is required.

The process continues until a dynamic equilibrium is reached

The process results in the accumulation of or elimination of substances from the cell.

The processes of passive transport and active transport in living organisms:

Simple Diffusion

- Gaseous exchange through the stomata of leaves during photosynthesis.

- Evaporation of water from leaf cells through stomata during transpiration.

- Gaseous exchange in unicellular organisms such as Amoeba sp.

- Gaseous exchange between alveoliand blood capillaries during respiration:

- Gaseous exchange at the alveoli occurs by simple diffusion.

- Oxygenin the alveoli diffuses into the blood capillaries of the lungs because the concentration of oxygen in the alveoli is higher than of the blood capillaries.

- On the other hand, carbon dioxide diffuses from the blood capillaries of the lungs into the alveoli because the concentration of carbon dioxide in the blood capillaries is higher than that of the alveoli.

Facilitated Diffusion

Movement of small molecules such as amino acids and glucose into the blood capillaries of the villi.

Osmosis

- Osmoregulationin aquatic organisms.

- The movement of water in the cortex of plant stems.

- Transport of water in the cortex of plant stems.

- Water uptake by plant roots. Soil normally has higher concentration of water when compared to the cell sap of root hairs. Therefore, water moves into the vacuoles of the root hair by osmosis.

Active Transport

- Uptake of mineral salts and ions by root hairs. The concentration of ions and mineral salts is higher in the cell sap than in the soil. These substances are transported into the cells via active transport.

- Accumulation and elimination of substances by organisms which live in the sea. The alga Nitellasp. Accumulates iodine ions although the concentration of iodine in its protoplasm is higher than that of the sea. The alga Halicystissp. Eliminates sodium ions from its cells through active transport.

- Examples of active transport in cells include:

- The accumulation of iodine in the cells of the thyroid gland.

- The movement of sodium ions into the cells lining the kidneys tubules.

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