• Cell theory says all living things are made of cells, cells are the basic unit of life, and new cells only come from existing cells.
• Cells organise into a hierarchy: organelles → cells → tissues → organs → organ systems.
• There are two broad types of cell: eukaryotic (complex, membrane-bound organelles — animals, plants, fungi) and prokaryotic (simple, no membrane-bound organelles — bacteria).
• Eukaryotic cells share a "protein production line": nucleus → ribosomes → rough ER → Golgi apparatus → cell membrane — this is how proteins are made, processed, and exported.
• Prokaryotic cells are smaller, have circular DNA free in the cytoplasm, smaller (70S) ribosomes, and a peptidoglycan cell wall.
• Light microscopes use light and glass lenses (lower resolution, can view living specimens). Electron microscopes use electron beams (much higher resolution, specimen must be dead).
• You need to be able to calculate magnification and use an eyepiece graticule + stage micrometer to measure real specimen sizes.
• Practical skills: preparing slides (dry mount, wet mount, squash, smear), staining, and making correctly-formatted biological drawings.

Where the idea came from

Before microscopes were powerful enough, nobody knew what living things were actually made of. Robert Hooke was the first person to view cells (in cork) and gave them the name "cell" because the boxy compartments reminded him of the small rooms (cells) monks lived in. Later, Matthias Schleiden (studying plants) and Theodor Schwann (studying animals) put the pieces together in 1837 into what we now call cell theory — a "unifying concept," meaning it applies to every living thing on Earth and is universally accepted by biologists.

The three core ideas

1. All living organisms are made up of one or more cells.
2. Cells are the basic functional unit of living organisms (the smallest unit that can be said to be "alive").
3. New cells are only produced from pre-existing cells (cells don't just spontaneously appear).

Features every cell shares

Regardless of whether a cell is a bacterium or part of your brain, it will always have these four things:

• Cell surface membrane — the boundary between inside and outside
• Cytoplasm — the jelly-like substance everything sits in
• DNA — the genetic instructions
• Ribosomes — the protein-making machinery

Beyond these four "universal" features, different cell types have very different extra structures — this is exactly what separates prokaryotes from eukaryotes (covered below). When you look at a cell under a microscope and study its internal structures, you're studying its ultrastructure.

Why cells specialise

A single cell can't do everything well — a cell built to absorb food can't also be great at transporting oxygen. So in multicellular organisms, cells become specialised, meaning they develop a particular structure suited to one particular job.

• Epithelial cells in the small intestine — specialised to absorb food efficiently (often have microvilli to increase surface area)
• Red blood cells — specialised to transport oxygen (packed with haemoglobin, no nucleus to make more room)
• Xylem cells in plants — specialised to transport water (hollow, dead, reinforced tubes)

The organisation hierarchy

Once cells are specialised, biology builds upward in a strict hierarchy. Each level is made from several units of the level below it, and each level exists to perform a bigger, more complex function than the level before:

What makes a cell "eukaryotic"?

Eukaryotic cells (animal, plant, and fungal cells) have a more complex ultrastructure than prokaryotic cells. The defining feature is that their cytoplasm is divided up into membrane-bound compartments called organelles — think of these as separate rooms inside the cell, each with a specific job, so that chemical reactions can happen in the right place without interfering with each other. Eukaryotic cells are also much bigger (10–100 μm) than prokaryotic cells (0.1–5 μm).

Nucleus

The control centre of the cell, and the largest organelle in most eukaryotic cells. It's surrounded by a double membrane called the nuclear envelope, which is riddled with tiny holes called nuclear pores. These pores are the "doorways" of the nucleus — mRNA and ribosomes travel out through them, while enzymes like DNA polymerase and signalling molecules travel in.

Inside the nucleus is chromatin — the material chromosomes are made from. Chromosomes are lengths of linear DNA tightly wound around proteins called histones (this winding is what lets metres of DNA fit inside a microscopic nucleus). You'll usually also see one or more darkly-stained regions called the nucleolus (plural: nucleoli) — this is where ribosomes are manufactured.

Mitochondria

The "powerhouse" of the cell — the site of aerobic respiration, which produces ATP (the energy currency cells use for everything). Mitochondria (singular: mitochondrion) are just visible under a light microscope.

They have a double membrane, and the inner membrane folds inward to create structures called cristae — folding massively increases the surface area available for the respiration reactions to occur on. Inside, the fluid-filled matrix contains the enzymes needed for aerobic respiration, plus its own small circular piece of mitochondrial DNA and ribosomes — these let the mitochondrion replicate itself before cell division.

Ribosomes

Ribosomes are the site of translation (where proteins are built from mRNA instructions). They are not surrounded by a membrane, and each one is a complex of ribosomal RNA (rRNA) and protein. They can float freely in the cytoplasm, or be attached to the rough endoplasmic reticulum.

Endoplasmic reticulum (ER)

A network of folded membranes continuous with the nuclear envelope. There are two types, and the difference between them comes down to one visible feature — ribosomes:

Golgi apparatus

A stack of flattened membrane sacs — visually similar to SER, but distinguishable by its regular, stacked appearance (often described as looking like a Wi-Fi symbol). Its job is to modify proteins and lipids arriving from the RER, then package them into Golgi vesicles which transport them to their final destination. Proteins that pass through the Golgi apparatus can be:

• Exported out of the cell (e.g. hormones like insulin)
• Packaged into lysosomes (e.g. hydrolytic enzymes)
• Delivered to other membrane-bound organelles

Lysosomes

Specialist vesicles containing hydrolytic enzymes (enzymes that break things down using water). Their role is to break down waste materials such as worn-out organelles. They're heavily used by immune system cells (to digest invaders) and during apoptosis — programmed cell death, where a cell deliberately self-destructs in a controlled way.

Centrioles

Made of hollow protein fibres called microtubules, which can move substances around inside a cell and support the cell's shape from within. Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division. Centrioles are not found in plant or fungal cells.

The six-step journey of an extracellular enzyme

This is one of the most commonly examined processes in this chapter — it links together five organelles into one connected pathway. Picture it like a factory assembly line, where the "product" is a protein destined to leave the cell:

Note the key distinction: free ribosomes floating in the cytoplasm make proteins that stay inside the cytoplasm. Only ribosomes attached to the RER make proteins destined for secretion or the cell membrane. This single fact explains why cells that secrete a lot of protein (like pancreas cells producing digestive enzymes) are absolutely packed with RER.

Core differences from eukaryotic cells

Prokaryotic cells (bacteria) are much smaller than eukaryotic cells and lack the internal compartmentalisation eukaryotes have. Four features you must know as "always different":

• Cytoplasm lacks membrane-bound organelles
• 70S ribosomes (smaller than the 80S in eukaryotes)
• No nucleus — instead, a single circular bacterial chromosome free in the cytoplasm, not associated with proteins (no histones)
• A cell wall made of the glycoprotein murein (also called peptidoglycan) — chemically completely different from a plant cell wall (cellulose) or fungal cell wall (chitin)

Structures found in SOME (not all) prokaryotes

Full comparison table

Two types of electron microscope

Transmission Electron Microscope (TEM): fires a beam of electrons through a very thin specimen. Denser parts absorb more electrons and appear darker, creating contrast. This reveals internal structures as a flat 2D image, at very high resolution.

Scanning Electron Microscope (SEM): scans a beam of electrons across the surface of a specimen; the bounced-off electrons are detected to build the image. This produces 3D images of surfaces, and because it only scans the surface, the specimen doesn't need to be thin. Resolution is lower than a TEM.

Magnification vs Resolution — the crucial distinction

Magnification is how many times bigger the image is compared to the real, actual size of the specimen. A light microscope has two lenses that multiply together: the eyepiece lens (usually x10) and one of a set of objective lenses.

Resolution (or resolving power) is the ability to distinguish between two separate points as genuinely separate — rather than as one single blur. This is a completely different idea from magnification, and it's one of the most commonly misunderstood concepts in this whole topic.

The resolution of a light microscope is limited by the wavelength of light (light's wavelength is too long to resolve very small structures). Its maximum resolution is about 200 nm — so anything closer together than 200 nm (like the 10 nm-wide phospholipid bilayer of a cell membrane) simply cannot be seen as separate points. Electron microscopes use electron beams, which have a much shorter wavelength than light, giving them far higher resolution (down to about 0.5 nm) — and therefore allowing much higher useful magnification.

The core formula

This can be rearranged depending on what you're asked to find:

• To find actual size: A = I ÷ M
• To find image size: I = A × M

Eyepiece graticule & stage micrometer

A graticule is a tiny disc engraved with a ruler scale that sits inside the eyepiece, acting like a ruler over whatever you're viewing. The problem is that a graticule has no fixed real-world units — its scale is arbitrary until it's calibrated.

Calibration is done against a stage micrometer — a microscope slide with an accurately known engraved scale. By lining the graticule scale up against the stage micrometer scale, you work out how many real micrometres (μm) each graticule division represents — and because the graticule itself doesn't change size when you change magnification, you must recalibrate every time you switch objective lens.

Why we stain specimens

Cytoplasm and many cell structures are naturally almost transparent, so stains are used to make structures visible and distinguishable. Note: most of the colour you see in a light micrograph comes from added stain — chloroplasts are the exception, since their green colour is natural.

Using several stains together on one specimen so that different tissues show up distinctly is called differential staining.

Slide preparation methods

Rules for a proper biological drawing

• Must have a title and the magnification recorded
• Use a sharp pencil on plain white paper
• Clear, single lines — no sketching, no shading
• Drawing should be large — fill as much of the page as possible
• Structures must be in proper proportion
• Label lines — ruled, don't cross, no arrowheads, connect directly to the structure, kept to one side and parallel to the top of the page
• Only draw what is actually visible — never what you "know should be there"

There are two types of drawing you'll be asked to produce:

• Cellular drawings — made at high magnification, show individual cell detail
• Plan drawings — made at low magnification, show the arrangement/layout of tissues within an organ. Individual cells are never drawn in a plan diagram — only tissue boundaries and outlines.