Processes of life in unicellular organisms

Cells (Unity and diversity) · Cell structure · note 5 of 7

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Processes of life in unicellular organismsSpec A2.2.7

In short

A unicellular organism must carry out all the functions of life within one cell: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. In Paramecium, for example, contractile vacuoles expel excess water for homeostasis, cilia provide movement, food vacuoles digest food, and the cell reproduces by dividing in two.

A unicellular organism consists of one cell, so that one cell must carry out all the functions of life.

Homeostasis: Keeping internal conditions within narrow limits
Metabolism: All the enzyme-catalysed reactions in the cell
Excretion: Removal of the waste products of metabolism

Functions of life in two unicellular organisms
Function	Paramecium (heterotroph)	Chlamydomonas (autotroph)
Homeostasis	Contractile vacuoles collect and expel excess water that enters by osmosis	Contractile vacuoles expel excess water
Metabolism	Enzyme-catalysed reactions in the cytoplasm and mitochondria, including respiration	Reactions in the cytoplasm, mitochondria and chloroplast, including photosynthesis and respiration
Nutrition	Ingests bacteria and other food into food vacuoles, where it is digested	Makes its own food by photosynthesis in a large cup-shaped chloroplast
Movement	Beating cilia over the cell surface	Two flagella pull the cell through the water
Excretion	Wastes such as carbon dioxide and ammonia diffuse out across the plasma membrane	Oxygen from photosynthesis and other wastes diffuse out
Growth	Increases in size and dry mass by making new molecules	Increases in size and dry mass by making new molecules
Response to stimuli	Reverses direction when it meets an obstacle or harmful chemical	An eyespot detects light, so it swims towards light
Reproduction	Asexually, by dividing in two (binary fission)	Asexually by mitosis, or sexually when gametes fuse

Paramecium annotated with functions: cilia for movement, oral groove for nutrition, food vacuoles for digestion, contractile vacuoles for homeostasis by expelling excess water, plasma membrane for excretion of CO₂ and ammonia by diffusion, macronucleus controlling the cell and micronucleus for reproduction. — *Paramecium*, a heterotroph, carries out all the functions of life in one cell

Chlamydomonas annotated with functions: two flagella for movement, eyespot for response to light, cup-shaped chloroplast for nutrition by photosynthesis, contractile vacuoles for homeostasis, nucleus and cell wall. — *Chlamydomonas*, an autotroph, makes its food by photosynthesis in a cup-shaped chloroplast.

Exam tip:

Learn the eight functions as a list: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. Questions often ask how a named organism carries out each one.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells have no nucleus: their DNA is a naked loop in the cytoplasm, and they have 70S ribosomes and no membrane-bound organelles. Eukaryotic cells have a nucleus with a double membrane and pores, chromosomes of DNA bound to histones, 80S ribosomes, membrane-bound organelles and a cytoskeleton.

How do you calculate magnification in biology?

Magnification equals the size of the image divided by the actual size of the specimen. Measure the image with a ruler, convert both values to the same unit, usually micrometres (1 mm = 1000 µm), then divide. Rearranged, actual size equals image size divided by magnification, which is how cell sizes are found from micrographs.

How do you calculate magnification using a scale bar?

Measure the length of the scale bar on the image with a ruler, convert it to the same unit as the value written on the bar, then divide the measured length by that value. For example, a 20 mm bar labelled 0.5 µm gives 20 000 µm divided by 0.5 µm, a magnification of ×40 000.

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