Cell theory: every cell from a cell
A cell is the smallest part of a living thing that can live on its own. Anton von Leeuwenhoek first saw a live cell. Robert Brown found the nucleus. In 1838 Matthias Schleiden said all plants are made of cells. In 1839 Theodor Schwann said the same for animals, and noticed that plant cells have an extra wall. In 1855 Rudolf Virchow added the missing idea: omnis cellula-e cellula – every cell comes from a cell that already existed.
Modern cell theory (two points)
- All living things are made of cells and products of cells.
- All cells arise from pre-existing cells.
Cells vary a lot: a mycoplasma is about 0.3 µm, a bacterium 3–5 µm, a human red blood cell about 7 µm, and an ostrich egg is the biggest single cell. Nerve cells are among the longest.
Prokaryotic and eukaryotic cells
Prokaryotic means "before a nucleus". Bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO are prokaryotes.
- DNA is one circular chromosome lying bare in the nucleoid; no nuclear envelope.
- Many have small extra DNA rings called plasmids, which can carry genes such as antibiotic resistance.
- No membrane-bound organelles. Ribosomes are 70S.
- Most have a three-layer envelope: glycocalyx (loose slime layer or tough capsule), cell wall and cell membrane.
- The membrane folds inward to make mesosomes, which help in wall formation, DNA copying, respiration and secretion.
- Some have flagella (filament, hook, basal body), pili and fimbriae for sticking to surfaces. Several ribosomes on one mRNA form a polysome. Stored food lies in inclusion bodies (phosphate granules, glycogen granules, gas vacuoles).
Gram staining: bacteria that keep the stain are Gram positive; those that lose it are Gram negative.
Eukaryotic cells (protists, fungi, plants, animals) have a true nucleus with a nuclear envelope, many membrane-bound organelles, a cytoskeleton and 80S ribosomes.
Plant cell vs animal cell
| Plant cell | Animal cell |
|---|---|
| Has a cellulose cell wall | No cell wall |
| Plastids (chloroplasts) present | No plastids |
| One large central vacuole | Small or no vacuoles |
| No centrioles (in most) | Centrioles present |
| Usually box-shaped | Shape can change |
Both have a cell membrane, nucleus, mitochondria, ER, Golgi, ribosomes and cytoskeleton.
Cell membrane and cell wall
The cell membrane is made mainly of lipids (mostly phospholipids) arranged in two layers, with proteins and some carbohydrates. In a human red blood cell membrane, proteins are about 52% and lipids about 40%. Each lipid has a water-loving (polar) head facing outwards and water-fearing (non-polar) tails facing inwards.
Fluid mosaic model
Proposed by Singer and Nicolson (1972). The lipid layer behaves like a thick liquid, so proteins can drift sideways – "fluid". Proteins sit scattered in it like tiles – "mosaic". Integral proteins are buried in the layer; peripheral proteins sit on the surface. The fluid nature helps the cell grow, divide, form junctions and secrete.
How things cross
- Passive transport: along the gradient, no energy – simple diffusion; water moves by osmosis.
- Polar molecules need carrier proteins.
- Active transport: against the gradient, uses ATP – for example the Na⁺/K⁺ pump.
Cell wall
A non-living, stiff layer outside the membrane of plants, fungi and bacteria. It gives shape, protects from damage and germs, and lets cells talk to each other. Plant wall: cellulose, hemicellulose, pectins, proteins. Algal wall: cellulose, galactans, mannans, calcium carbonate. Layers: middle lamella (calcium pectate, glues neighbours) → primary wall (young, can stretch) → secondary wall (inner, thick). Plasmodesmata are cytoplasm threads that pass through walls and join neighbour cells.
Endomembrane system: ER, Golgi, lysosomes, vacuoles
These four work together, so they form the endomembrane system. Mitochondria, chloroplasts and peroxisomes are not part of it because their work is not linked with these.
- Endoplasmic reticulum (ER): a network of tiny tubes that divides the cytoplasm into inside (luminal) and outside parts. Rough ER (RER) has ribosomes and makes and secretes proteins – common in cells that secrete. Smooth ER (SER) has no ribosomes and makes lipids; in animal cells it makes steroid hormones.
- Golgi apparatus (Camillo Golgi, 1898): stacks of flat discs called cisternae, 0.5–1.0 µm across. The cis (forming) face receives vesicles from ER; the trans (maturing) face sends them out. It packs materials, and is where glycoproteins and glycolipids are formed.
- Lysosomes: single-membrane vesicles made by the Golgi, full of hydrolytic enzymes (lipases, proteases, carbohydrases) that work best at acidic pH. They digest carbohydrates, proteins, lipids and nucleic acids.
- Vacuoles: spaces bounded by one membrane, the tonoplast. In plants they can fill up to 90% of the cell. The tonoplast pumps ions into the vacuole, so their level inside is much higher than in the cytoplasm. Amoeba has a contractile vacuole for removing extra water; protists form food vacuoles.
Mitochondria, ribosomes, plastids and microbodies
- Mitochondria: sausage- or cup-shaped, 0.2–1.0 µm wide and 1.0–4.1 µm long. Two membranes; the inner one folds into cristae to give more surface. Inside is the matrix with a circular DNA, a few RNA molecules, 70S ribosomes and enzymes. Site of aerobic respiration; they make ATP, so they are called the power houses. They divide by fission.
- Plastids (plants, euglenoids): chloroplasts (chlorophyll and carotenoids, photosynthesis), chromoplasts (carotene, xanthophyll: yellow, orange, red colours) and leucoplasts (colourless stores: amyloplasts – starch, e.g. potato; elaioplasts – oils and fats; aleuroplasts – proteins). A chloroplast is lens-shaped, has two membranes, and inside the stroma are flat sacs called thylakoids, stacked like coins into grana, joined by stroma lamellae. The stroma holds enzymes, a circular DNA and 70S ribosomes.
- Ribosomes (George Palade saw them first): granules of RNA and protein with no membrane. Eukaryotic 80S = 60S + 40S; prokaryotic 70S = 50S + 30S. "S" is the Svedberg unit, a measure of how fast a particle settles in a centrifuge.
- Microbodies: small single-membrane vesicles full of enzymes, found in both plants and animals.
Cytoskeleton, cilia, flagella and centrioles
- Cytoskeleton: a network of protein fibres – microtubules, microfilaments and intermediate filaments. It gives support and shape, and helps movement of the cell and its parts.
- Cilia (small, many, beat like oars) and flagella (long, few, wave like a whip) are hair-like outgrowths covered by the cell membrane. Their core, the axoneme, has 9 pairs of peripheral microtubules and 2 central single microtubules: the 9 + 2 array. The central tubes are joined by bridges and covered by a central sheath, joined to each peripheral pair by radial spokes. Both grow from a basal body. The prokaryotic flagellum is built differently.
- Centrosome: two cylindrical centrioles lying at right angles. Each is made of 9 evenly spaced triplets of tubulin (a cartwheel), with a central hub: the 9 + 0 plan. Centrioles form the basal body of cilia and flagella, and the spindle in animal cell division.
The nucleus and chromosomes
Robert Brown described the nucleus in 1831. Flemming named the stainable material chromatin.
- Nuclear envelope: two membranes with a gap (perinuclear space, 10–50 nm). Nuclear pores let RNA and proteins move in and out.
- Nucleoplasm holds the nucleolus (no membrane, a busy site of rRNA making) and chromatin (DNA + basic proteins called histones + some non-histone proteins + RNA).
- A human cell has about 2 metres of DNA in 46 chromosomes (23 pairs).
- Mature red blood cells of mammals and sieve-tube cells of plants have no nucleus.
Chromosome types by centromere position
The centromere (primary constriction) holds the two chromatids; disc-shaped kinetochores sit on its sides. By where it lies:
- Metacentric: in the middle – two equal arms.
- Sub-metacentric: a little off the middle – one short arm, one long arm.
- Acrocentric: very close to one end – one very short, one very long arm.
- Telocentric: at the very tip.
Some chromosomes have a non-staining secondary constriction that cuts off a small piece called the satellite.
Try it at home: see real cells
Peel the thin skin from the inside of an onion layer, put it on a glass slide with a drop of water and a drop of iodine or red ink, and look with a toy microscope or a phone macro lens. You will see brick-shaped cells with walls and a dark dot (the nucleus) in each. Then do the same with a gentle scrape from inside your cheek using a clean ice-cream stick: the cells are rounder and have no wall. Compare with step 3 of the 3D.
Key formulas and definitions
- Cell theory: all living things are made of cells; all cells arise from pre-existing cells (Virchow)
- Prokaryote: nucleoid, no nuclear envelope, 70S ribosomes (50S + 30S), no membrane-bound organelles
- Eukaryote: true nucleus, membrane-bound organelles, 80S ribosomes (60S + 40S)
- Fluid mosaic model: lipid bilayer + integral and peripheral proteins (Singer & Nicolson, 1972)
- Endomembrane system = ER + Golgi + lysosomes + vacuoles
- Cilia and flagella: 9 + 2 microtubules; centriole: 9 triplets + 0 (cartwheel)
- Double-membrane organelles: nucleus, mitochondria, chloroplast
- Chromosome types: metacentric, sub-metacentric, acrocentric, telocentric
Worked examples
1. A cell has 70S ribosomes, a circular DNA in its cytoplasm and no nuclear envelope. What kind of cell is it?
No nuclear envelope + DNA in a nucleoid + 70S ribosomes = prokaryotic cell, for example a bacterium.
2. A cell sample has a cell wall, a large central vacuole and chloroplasts but no centrioles. Plant or animal?
Wall + large vacuole + plastids and no centrioles → plant cell.
3. Trace the path of a digestive enzyme that a pancreas cell sends out.
It is made on ribosomes of the rough ER → carried in a vesicle to the cis face of the Golgi → modified and packed → leaves from the trans face in a secretory vesicle → the vesicle fuses with the cell membrane and releases it outside.
4. A chromosome has its centromere very close to one end, giving one very short and one very long arm. Name the type.
Acrocentric. (Middle = metacentric, slightly off = sub-metacentric, at the tip = telocentric.)
5. How many microtubules in total are there in a cross-section of the axoneme of a cilium?
9 pairs = 18 peripheral + 2 central = 20 microtubules (the 9 + 2 array).
6. Why are mitochondria and chloroplasts called semi-autonomous?
They have their own circular DNA and 70S ribosomes, so they can make some of their own proteins and can divide by themselves – but they still need many proteins coded by the nucleus.
Common mistakes
- Saying Schleiden and Schwann gave the full cell theory. They missed how new cells form; Virchow added "every cell from a cell".
- Putting mitochondria or chloroplasts in the endomembrane system. Only ER, Golgi, lysosomes and vacuoles are in it.
- Adding Svedberg units: 50S + 30S is not 80S. The prokaryotic ribosome is 70S; S depends on shape and size, not just mass.
- Mixing up the ciliary 9 + 2 (pairs, with a centre) and the centriole 9 + 0 (triplets, no central tubes).