Eukaryotic and prokaryotic cells
- Animal and plant cells are eukaryotic: their genetic material is enclosed in a nucleus.
- Bacterial cells are prokaryotic: they are much smaller and their DNA is a single loop in the cytoplasm, not in a nucleus.
- Bacteria may also have small rings of DNA called plasmids.
Parts of an animal cell
- Nucleus: contains the genetic material that controls the cell's activities.
- Cytoplasm: a gel where most chemical reactions happen, controlled by enzymes.
- Cell membrane: controls what enters and leaves the cell.
- Mitochondria: where aerobic respiration happens, releasing energy.
- Ribosomes: where proteins are made (protein synthesis).
Extra parts in plant cells
- Cell wall: made of cellulose; strengthens and supports the cell.
- Chloroplasts: contain chlorophyll, which absorbs light for photosynthesis.
- Permanent vacuole: filled with cell sap; keeps the cell rigid.
Specialised cells
- Sperm cells have a tail to swim and lots of mitochondria to release energy.
- Nerve cells are long, with branched endings, to carry electrical impulses around the body.
- Muscle cells contain protein fibres that contract, and many mitochondria.
- Root hair cells have a large surface area to absorb water and mineral ions.
- Xylem cells form hollow tubes that carry water; phloem cells carry dissolved sugars.
Size and scale
- Cells are measured in micrometres (µm): 1 mm = 1000 µm.
- Bacteria are about 1–10 µm long; animal cells are about 10–100 µm.
- Magnification = image size ÷ actual size. Use the same units for both.
Culturing microorganisms (separate sciences)
- Bacteria multiply by simple cell division called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
- Bacteria are grown in a nutrient broth or as colonies on an agar gel plate. Uncontaminated cultures are needed to study the effect of disinfectants and antibiotics.
- Aseptic technique: sterilise Petri dishes and culture media before use, and pass inoculating loops through a flame. Tape the lid on lightly (not sealed all round) and store the dish upside down.
- In school laboratories, cultures are incubated at no more than 25 °C, which reduces the chance of growing pathogens that could harm humans.
- Number of bacteria = starting number × 2ⁿ, where n is the number of divisions.
- Required practical: put paper discs soaked in different antibiotics or antiseptics on an agar plate spread with bacteria. The bigger the clear zone of inhibition around a disc, the better it stops the bacteria. Area of a clear zone = πr².
Key terms
- Eukaryotic cell
- A cell whose genetic material is enclosed in a nucleus, e.g. animal and plant cells.
- Prokaryotic cell
- A cell with no nucleus; its DNA is a single loop in the cytoplasm, e.g. bacteria.
- Nucleus
- Contains genetic material that controls the activities of the cell.
- Mitochondria
- Where aerobic respiration happens, releasing energy for the cell.
- Ribosome
- The site of protein synthesis.
- Chloroplast
- Contains chlorophyll to absorb light for photosynthesis.
- Cell wall
- Made of cellulose in plants; strengthens and supports the cell.
- Plasmid
- A small ring of DNA found in bacterial cells.
- Specialised cell
- A cell with features that help it carry out a particular job.
- Magnification
- How many times bigger an image is than the real object: image size ÷ actual size.
- Binary fission
- The simple cell division by which bacteria reproduce.
- Zone of inhibition
- The clear area around an antibiotic or antiseptic where bacteria have not grown.