2.2 - Living Processes in Unicellular Organisms

Unicellular organisms consist of only one cell. This cell is a complete unit of life and performs all living processes such as movement, response to stimuli, nutrition, respiration, growth, excretion and reproduction.

Living Processes of Amoeba sp. and Paramecium sp.

Movement

• Amoeba sp. extends its pseudopodium, followed by the flow of cytoplasm into the extended pseudopodium.
• Paramecium sp. moves by the rhythmic beating of cilia.

Response to Stimuli

• Both respond to chemicals, touch and bright light by moving away from the stimuli.

Respiration

• Oxygen and carbon dioxide are exchanged through the plasma membrane by simple diffusion over the entire cell surface.

Nutrition

• Amoeba sp. extends pseudopodia to surround food particles and carries out phagocytosis. The food particles are enclosed in a food vacuole and hydrolysed by enzymes. Nutrients are absorbed into the cytoplasm, while undigested food is expelled when the Amoeba sp. moves.
• Paramecium sp. uses the rhythmic beating of cilia to sweep food particles into the oral groove. A food vacuole is formed, food is hydrolysed by enzymes and nutrients are absorbed. Undigested food is discharged through the anus.

Growth

• Both grow by synthesising new cytoplasm.

Excretion and Osmoregulation

• Waste products such as carbon dioxide and ammonia are eliminated by diffusion.
• In freshwater, water diffuses into the cell by osmosis and fills the contractile vacuole.
• When the contractile vacuole reaches its maximum size, it contracts to expel excess water. This process is called osmoregulation.

Reproduction

• When conditions are suitable and food is plentiful, Amoeba sp. and Paramecium sp. reproduce asexually by binary fission.
• Under unsuitable conditions, Amoeba sp. forms spores that germinate when conditions become suitable.
• Under unsuitable conditions, Paramecium sp. carries out conjugation, in which genetic materials are exchanged between two individuals.