NSB31 Recap

Cells: plant and animal cells

Parts of a cell

The structures inside a cell are called organelles — “little organs”, each with a job. All animal and plant cells share these:

  • Cell membrane — thin, flexible layer around the cell. It controls what enters and leaves.
  • Cytoplasm — jelly-like fluid where most chemical reactions happen. Organelles float in it.
  • Nucleus — the control centre. It holds the DNA (genetic material) and directs everything the cell does.
  • Mitochondria — the powerhouses. Respiration happens here, releasing energy from glucose. Cells that need lots of energy, such as muscle cells, have many.

Plant-only organelles

Plant cells have three extra structures that animal cells never have:

  • Cell wall — a rigid outer layer made of cellulose that sits outside the membrane. It gives the cell a fixed, box-like shape and stops it bursting.
  • Chloroplasts — green organelles containing chlorophyll. Photosynthesis happens here, making glucose from carbon dioxide and water using light.
  • Large permanent vacuole — a big sac of watery cell sap that stores substances and keeps the cell firm. Animal cells may have small temporary vacuoles, but never a large central one.

Comparing the two

Under a microscope, plant cells such as onion skin look like neat bricks in a wall because the cell walls hold them in regular shapes. Cheek cells are rounder, more irregular and scattered. Only green parts of a plant have chloroplasts — onion bulb cells grow underground, so they have a cell wall but no chloroplasts.

A useful way to remember the difference: plants cannot move to find food, so they need chloroplasts to make it, a wall to stand up straight, and a vacuole to stay firm.

Drawing cells

Exam drawings follow strict rules:

  • Use a sharp pencil and draw clear, single, unbroken lines — no shading or colouring.
  • Draw large enough to show detail, and keep the proportions of what you see.
  • Label with straight ruled lines that touch the structure and do not cross each other.
  • Give the drawing a title and the total magnification, e.g. “Onion cell, ×100\times 100”.