Living Processes in Unicellular Organisms
A unicellular organism is made of a single cell that alone carries out all life processes, nutrition, respiration, excretion, growth, reproduction and response to stimuli, without help from other cells.
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What a unicellular organism is
Content standard 2.2 examines organisms whose entire body is a single cell, such as Amoeba, Paramecium, Chlamydomonas, yeast and bacteria. Because there is no division of labour between cells, that one cell and its organelles must independently perform every life process needed for survival.
How life processes occur in one cell
- Nutrition: Amoeba engulfs food particles by phagocytosis using pseudopodia; Chlamydomonas photosynthesises using its single chloroplast; Paramecium sweeps food into its gullet using cilia.
- Respiration and gas exchange: oxygen diffuses in and carbon dioxide diffuses out directly across the cell membrane, since the cell is small enough for diffusion alone to meet its needs.
- Excretion: metabolic waste diffuses out across the cell membrane; in freshwater species like Amoeba, a contractile vacuole also pumps out excess water that enters by osmosis.
- Response to stimuli: Paramecium's cilia beat to move it toward food or away from a harmful stimulus; Chlamydomonas has a light-sensitive eyespot that guides it toward light for photosynthesis.
- Reproduction and growth: most unicellular organisms reproduce asexually by binary fission, where one cell grows and splits into two identical daughter cells.
Why a single cell is enough
A unicellular organism has a very large surface-area-to-volume ratio compared with its size, so diffusion across the cell membrane is fast enough to supply oxygen and nutrients and remove waste without needing specialised transport structures. This is why these organisms remain functional and efficient despite having only one cell.
How it is examined
Questions typically show a labelled diagram of Amoeba, Paramecium or Chlamydomonas and ask you to identify a structure (such as a contractile vacuole, pseudopodium or eyespot) and state the life process it carries out. You may also be asked to explain why a single cell is sufficient to carry out all life processes in these organisms.
Worked exam-style question
Question. Two cube-shaped cells, X and Y, have side lengths of 1 mm and 3 mm respectively. (a) Calculate the surface area, volume, and surface-area-to-volume ratio of each cell.
(b) State which cell has the larger surface-area-to-volume ratio. (c) Using your answer to (b), explain why a unicellular organism the size of cell X can survive using diffusion alone, while an organism the size of cell Y generally cannot.
Model answer. (a) Cell X: surface area = 6 × 1² = 6 mm²; volume = 1³ = 1 mm³; ratio = 6 : 1. Cell Y: surface area = 6 × 3² = 54 mm²; volume = 3³ = 27 mm³; ratio = 2 : 1.
(b) Cell X has the larger surface-area-to-volume ratio. (c) A larger surface-area-to-volume ratio means every unit of volume (cytoplasm) in cell X has more membrane surface available for exchange, so diffusion of oxygen in and waste out is fast enough to meet the cell's needs.
In cell Y, the smaller ratio means the interior is relatively far from the membrane, so diffusion alone would be too slow to supply the whole volume, which is why larger organisms need specialised transport and exchange structures instead of relying on diffusion alone.
Practice question
Try this. A spherical unicellular organism has a radius of 2 μm. If the organism grew to twice this radius (4 μm) without changing shape, state what would happen to its surface-area-to-volume ratio, and explain the consequence for its efficiency in obtaining oxygen by diffusion.
(You do not need to calculate exact values, reason from how surface area and volume scale with radius.)
Exam tip
Key terms
These terms are essential for explaining why a single cell is enough in a unicellular organism:
- Cell, the entire body of a unicellular organism, performing every life process alone.
- Organelle, a specialised structure within the cell, such as the contractile vacuole.
- Diffusion, the passive movement that supplies oxygen and removes waste across the cell membrane.
- Osmosis, the movement of water into the cell that the contractile vacuole must counteract.
Source:SRC-DSKP-EN
Frequently asked questions
How does Amoeba carry out excretion and osmoregulation?
Why can a unicellular organism rely on diffusion alone for gas exchange?
Why don't unicellular organisms simply keep growing larger?
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