Cells and Microorganisms: the structure of plant, animal and bacterial cells, the microbes used as biological tools, the microscope and magnification calculations, microbial growth, and aseptic technique

What this module is about

Cells are the basic unit of life and the working material of biotechnology, and microorganisms are the field's favourite living tools. This module gives you the structures you will manipulate, the organisms you will grow, the instrument that lets you see them, the pattern of their growth, and the discipline that keeps a culture pure. Master these and the practical work in later modules becomes routine rather than risky.

The full set of dot points for this module is at /sg-o-level/biotechnology/syllabus/cells-and-microorganisms.

The structure of cells

Everything starts with the cell. The dot point on the structure of cells asks you to describe plant, animal and bacterial cells and the functions of their main parts. The crucial division is between prokaryotic and eukaryotic cells. Bacteria are prokaryotic: no true nucleus, no membrane-bound organelles, DNA free in the cytoplasm as a loop, and often small extra rings called plasmids. Plant and animal cells are eukaryotic, with a true nucleus and organelles such as mitochondria. Plant cells add a cellulose cell wall, chloroplasts and a large permanent vacuole.

The plasmid is the structure to watch: it is small, separate from the main chromosome, and easily taken up, which is exactly why it becomes the gene-carrying vector you meet in later modules.

Microorganisms as biological tools

The dot point on microorganisms in biotechnology explains why bacteria, yeasts and fungi dominate the field. They grow fast, are cheap and easy to grow in large numbers, can use simple or even waste materials, and can be genetically modified to make new products. A single cell can multiply into millions overnight, so a useful gene placed in a microbe becomes a large, low-cost supply of product. This is the economic engine behind recombinant insulin, industrial enzymes and fermented foods alike.

Seeing and measuring cells

Because cells are far too small to see, you need a microscope and the maths to interpret it. The dot point on microscopy and cell measurement covers using the light microscope and calculating magnification and actual size. The central relationship is:

which rearranges to give actual size when you know the magnification:

The most common slip is mixing units, so always convert image and actual sizes to the same unit (usually micrometres) before dividing.

How microbial cultures grow

When you grow microbes in a flask their numbers follow a predictable curve. The dot point on the growth of microbial cultures describes the four phases: the lag phase (cells adjust, few divide), the log or exponential phase (numbers double rapidly at a steady rate), the stationary phase (growth balances death as nutrients run low and waste builds up), and the death or decline phase (cells die faster than they divide). It also covers what microbes need to grow: nutrients, the right temperature and pH, water, and (for many) oxygen. Understanding this curve is essential for timing when to harvest a product.

Keeping cultures pure

A culture is only useful if it contains the organism you want and nothing else. The dot point on aseptic technique and sterilisation covers the practices that keep contaminants out and the worker safe: flaming the necks of bottles and loops, working close to a Bunsen flame, and sterilising media and equipment by autoclaving. A single contaminating microbe can outgrow your culture, spoil the product and create a health hazard, which is why aseptic technique is treated as a non-negotiable habit rather than an optional extra.

How the module fits together

• Structure underpins function. The plasmid you note here becomes the vector later; the cell wall and membrane explain how DNA is extracted.
• Microbes are the workhorses. Their fast, cheap growth is why biotechnology scales.
• Maths makes microscopy meaningful. Magnification and actual size are reliable marks if you keep units consistent.
• Growth and asepsis go together. You can only ride the log phase to a good harvest if the culture stays pure.

Check your knowledge

A mix of recall, calculation and reasoning across the five dot points. Attempt them timed, then check the solutions.

1. State two structures found in a bacterial cell that are not found in an animal cell. (2 marks)
2. Give two reasons why microorganisms are useful as biological tools. (2 marks)
3. A cell is 0.05 mm wide and appears 25 mm wide in an image. Calculate the magnification. (2 marks)
4. Name the four phases of microbial growth in order. (2 marks)
5. Explain why aseptic technique is essential when growing a culture. (2 marks)