1.2 Cells as the Basic Units of Living Organisms

Cambridge International AS Level Biology | 9700

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recognise organelles and other cell structures in eukaryotic cells, and describe their structure and function
  2. Describe and interpret photomicrographs, electron micrographs, and drawings of typical plant and animal cells
  3. Compare the structure of typical plant and animal cells
  4. State that cells use ATP from respiration for energy-requiring processes
  5. Outline the key structural features of a prokaryotic cell (typical bacterium)
  6. Compare the structure of a prokaryotic cell with typical plant and animal (eukaryotic) cells
  7. State that viruses are non-cellular structures with a nucleic acid core and a protein capsid, and that some have a phospholipid envelope

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1. Eukaryotic Cell Structures and Functions

A eukaryotic cell is a cell that contains a true nucleus (a compartment holding its genetic material) and many other membrane-bound structures called organelles (specialised parts of the cell, each with a specific job). Plants, animals, and fungi all have eukaryotic cells.

Below is every organelle and structure you need to know, with its structure and function explained clearly.

Cell Surface Membrane

  • A very thin layer (about 7–10 nm thick — that's just millionths of a millimetre) that surrounds every cell.
  • It is made of a phospholipid bilayer — two layers of fat-like molecules.
  • It is partially permeable, meaning it allows some substances to pass through but blocks others.
  • Its main job is to control what enters and leaves the cell, keeping the internal environment stable.

Nucleus, Nuclear Envelope, and Nucleolus

The nucleus is the largest organelle in most cells. Think of it as the cell's control centre.

  • It contains the cell's DNA, which is organised into structures called chromosomes. DNA carries instructions for all the cell's activities.
  • When the cell is not dividing, DNA exists as loosely coiled threads called chromatin (DNA combined with proteins).
  • The nucleus is surrounded by a double membrane called the nuclear envelope — two membranes stacked closely together.
  • The nuclear envelope is dotted with nuclear pores — tiny openings that control the movement of materials (like messenger molecules and ribosome parts) between the nucleus and the rest of the cell.
  • Inside the nucleus is a dense, rounded structure called the nucleolus. Its job is to make ribosomes using its own DNA. Ribosome parts are then exported through the nuclear pores to the cytoplasm.

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