Cell Structure

Eukaryotes and prokaryotes

All living things are made of cells, but cells fall into two broad groups. Eukaryotic cells have their genetic material enclosed in a nucleus. Animal cells, plant cells and fungal cells are all eukaryotic. Prokaryotic cells have no nucleus — their genetic material sits free in the cytoplasm as a single loop of DNA, often with small extra rings called plasmids. Bacteria are prokaryotes.

Prokaryotic cells are much smaller, typically 0.2–2 micrometres across, while eukaryotic cells are usually 10–100 micrometres. A common exam question asks you to compare the two, so learn the distinguishing features rather than just the definitions.

Animal and plant cells

Both animal and plant cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.

• Nucleus — contains the chromosomes and controls the cell's activities.
• Cytoplasm — where most chemical reactions happen.
• Cell membrane — controls what enters and leaves.
• Mitochondria — the site of aerobic respiration, releasing energy.
• Ribosomes — where proteins are synthesised.

Plant cells additionally have a cell wall made of cellulose for strength, a permanent vacuole containing sap, and often chloroplasts containing chlorophyll for photosynthesis. Note that not every plant cell has chloroplasts — root hair cells do not, because they receive no light.

Cell specialisation

Cells become specialised so they can carry out a particular function efficiently. Their structure reflects that function:

• Sperm cell — a tail for swimming and many mitochondria to release the energy for it.
• Nerve cell — long and thin to carry impulses over distance, with branched endings to connect to other cells.
• Muscle cell — packed with protein fibres that shorten, and many mitochondria.
• Root hair cell — a long projection giving a large surface area for absorbing water and mineral ions.
• Xylem and phloem — xylem cells lose their end walls to form hollow tubes for water transport; phloem cells form tubes with sieve plates for transporting dissolved sugars.

When a question asks you to explain an adaptation, always link the structural feature back to the function. "It has many mitochondria" earns little; "many mitochondria release the energy needed for the tail to move" earns the mark.

Cell differentiation

Differentiation is the process by which a cell becomes specialised. In animals, most cells differentiate early in development and thereafter cell division is mainly for repair and replacement. In plants, many cells retain the ability to differentiate throughout the plant's life.

Microscopy

Light microscopes use light and glass lenses and have a maximum useful magnification of around ×2000. Electron microscopes use a beam of electrons, giving both much higher magnification and much higher resolution — the ability to distinguish two points that are close together. Electron microscopes let us see sub-cellular structures such as ribosomes and internal mitochondrial membranes.

The magnification equation is one you must be able to rearrange:

magnification = size of image ÷ size of real object

Watch your units. Convert everything to the same unit before calculating. There are 1000 micrometres (µm) in a millimetre, and 1000 nanometres (nm) in a micrometre.

Required practical: microscopy

You need to be able to use a light microscope to observe and draw cells. The method in outline:

1. Place the slide on the stage and select the lowest-power objective lens.
2. Use the coarse focus to bring the stage and lens close, then focus until the image is sharp.
3. Switch to a higher-power objective and refine with the fine focus.
4. Draw with a sharp pencil, using clear unbroken lines, no shading, and label with straight ruled lines. Record the total magnification.

Total magnification = eyepiece lens magnification × objective lens magnification.

Culturing microorganisms

Bacteria are grown on an agar gel plate containing nutrients. To keep cultures uncontaminated:

• Sterilise petri dishes and culture media before use.
• Pass inoculating loops through a flame before transferring bacteria.
• Secure the lid with tape, but do not seal it completely — this prevents anaerobic bacteria growing.
• Store plates upside down so condensation does not drip onto the agar.

In school laboratories, cultures are incubated at a maximum of 25 °C. This reduces the likelihood of growing pathogens that are harmful to humans.

To calculate the area of an inhibition zone around an antibiotic disc, use the area of a circle, πr². Remember that the radius is half the measured diameter.