Every living thing — a bacterium, an oak tree, you — is built from cells. The cell is the smallest unit that can carry out all the processes of life, and inside it a set of tiny "organs" (organelles) each do a specific job.
1 · Cell theory
Centuries of microscope work are summed up in the cell theory:
All living organisms are made of one or more cells.
The cell is the basic unit of structure and function in living things.
All cells arise from pre-existing cells (by cell division).
An organism may be unicellular (one cell does everything — e.g. a bacterium, Amoeba) or multicellular (cells specialise and work together as tissues, organs and systems).
2 · Two kinds of cell: prokaryotic & eukaryotic
Feature
Prokaryotic (e.g. bacteria)
Eukaryotic (plants, animals, fungi, protists)
Nucleus
No true nucleus — DNA loose in the cytoplasm
True nucleus enclosed by a membrane
Membrane-bound organelles
Absent
Present (mitochondria, ER, etc.)
Size
Small (~1–10 µm)
Larger (~10–100 µm)
DNA
Single circular loop
Linear, in chromosomes
This chapter focuses on the eukaryotic cell and its organelles.
3 · Seeing cells — the microscope
Cells are far too small for the naked eye, so we use a microscope. Two ideas matter:
Magnification — how many times bigger the image is than the object (total magnification = eyepiece × objective).
Resolution — the smallest distance at which two points can still be seen as separate. A light microscope resolves down to ~0.2 µm; an electron microscope resolves far smaller detail (organelle ultrastructure) but cannot show living cells.
Magnification vs resolution A blurry photo blown up huge is highly magnified but has poor resolution. Good biology needs both — that's why the electron microscope revealed the organelles a light microscope only hinted at.
4 · The organelles and what they do
Cell (plasma) membrane — a selectively permeable boundary controlling what enters and leaves.
Cytoplasm — the jelly-like fluid where many reactions occur and organelles sit.
Nucleus — the control centre; holds the DNA (chromatin) and directs the cell. The nucleolus inside makes ribosomes.
Mitochondria — the "powerhouses"; site of aerobic respiration, releasing energy as ATP.
Ribosomes — tiny factories that build proteins (free in cytoplasm or on the rough ER).
Endoplasmic reticulum (ER) — membrane network; rough ER (with ribosomes) makes/transports protein, smooth ER makes lipids.
Golgi apparatus — modifies, packages and ships proteins in vesicles.
Lysosomes — vesicles of digestive enzymes that break down waste and worn-out parts (mainly animal cells).
Vacuole — fluid-filled sac; large and permanent in plant cells (stores cell sap, keeps the cell turgid).
5 · Plant cells vs animal cells
Plant cells have three structures animal cells lack:
Cell wall — a rigid layer of cellulose outside the membrane; gives shape and support and stops the cell bursting.
Chloroplasts — contain chlorophyll and carry out photosynthesis.
A large central vacuole — keeps the cell firm (turgid).
Animal cells are typically rounded with only a thin membrane; plant cells are boxy because of the wall. Both share the membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER and Golgi.
Levels of organisation
Cells of one type form a tissue; tissues form an organ; organs form an organ system; systems form the organism. Understanding the cell is the foundation for every later chapter — nutrition, gas exchange, transport, bioenergetics.
In one minute
Cell theory: living things are made of cells; the cell is the unit of life; cells come from cells.
Prokaryotic (no nucleus/organelles) vs eukaryotic (true nucleus + membrane-bound organelles).
Microscopes give magnification and resolution; electron > light for detail.
Organelles each have a job: nucleus (control/DNA), mitochondria (ATP), ribosomes (protein), ER/Golgi (make & ship), membrane (boundary).
Plant-only: cell wall, chloroplasts, large vacuole.