Brent Cornell

    
Unicellular organisms (e.g. Euglena, amoeba) are the smallest organisms capable of independent life

All living things carry out 7 basic functions integral to survival:

• Metabolism – Living things undertake essential chemical reactions
• Reproduction – Living things produce offspring, either sexually or asexually
• Sensitivity – Living things are responsive to internal and external stimuli
• Homeostasis – Living things maintain a stable internal environment
• Excretion – Living things exhibit the removal of waste products
• Nutrition – Living things exchange materials and gases with the environment
• Growth – Living things can move and change shape or size

Mnemonic:  MR SHENG

    
As unicellular organisms are composed of a single cell, this cell must be able to carry out all the life functions

How unicellular organisms fulfil these basic functions may differ according to structure and habitat

1.  Paramecium  (heterotroph)

• Paramecia are surrounded by small hairs called cilia which allow it to move (responsiveness)
• Paramecia engulf food via a specialised membranous feeding groove called a cytostome (nutrition)
• Food particles are enclosed within small vacuoles that contain enzymes for digestion (metabolism)
• Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules (excretion)
• Essential gases enter (e.g. O₂) and exit (e.g. CO₂) the cell via diffusion (homeostasis)
• Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (reproduction)

2.  Scenedesmus  (autotroph)

• Scenedesmus exchange gases and other essential materials via diffusion (nutrition / excretion)
• Chlorophyll pigments allow organic molecules to be produced via photosynthesis (metabolism)
• Daughter cells form as non-motile autospores via the internal asexual division of the parent cell (reproduction)
• Scenedesmus may exist as unicells or form colonies for protection (responsiveness)