Why a Green Tomato Turns Red as It Ripens

A cell needs a power supply, a framework to hold its shape and move, and a control centre that stores its instructions.

This part covers mitochondria, plastids, ribosomes and the cytoskeleton with cilia and centrioles, and the nucleus and chromosomes.

A mitochondrion has a smooth outer membrane and an inner membrane folded into cristae around a matrix containing circular DNA and 70S ribosomes; it carries out aerobic respiration and produces most of the cell's ATP, which is why it is called the powerhouse of the cell.

Structure:

- Shape — sausage-shaped or cylindrical; seen clearly only after special staining
- Outer membrane — smooth, forming the boundary
- Inner membrane — folded into cristae, which greatly increase its surface area
- Two compartments — the space between the membranes, and the matrix inside
- Matrix — contains a single circular DNA, a few RNA molecules, 70S ribosomes and enzymes

Function. Mitochondria are the sites of aerobic respiration, releasing energy from food and storing it as ATP. They multiply by fission.

An everyday example. Leg muscles of a regular runner develop more mitochondria, helping them keep working longer without tiring.

The substance. Having their own DNA and ribosomes makes mitochondria semi-autonomous — they can make some of their own proteins.

Plastids are organelles of plant cells and euglenoids: chloroplasts contain chlorophyll for photosynthesis, chromoplasts contain carotenoid pigments that give yellow, orange and red colours, and leucoplasts are colourless stores of food; inside a chloroplast, stacks of thylakoids called grana capture light, while the surrounding stroma makes sugars.

Types of plastids:

- Chloroplasts — chlorophyll and carotenoids; trap light energy
- Chromoplasts — fat-soluble carotenoids such as carotene and xanthophylls
- Leucoplasts — colourless storage plastids: amyloplasts store starch, elaioplasts store oils and fats, aleuroplasts store proteins

Chloroplast structure:

- Double membrane, the inner one less permeable
- Stroma — the fluid inside, with enzymes for making carbohydrates and proteins, circular DNA and 70S ribosomes
- Thylakoids — flattened membrane sacs holding chlorophyll
- Grana — stacks of thylakoids, joined by flat stroma lamellae

Link to photosynthesis. Light is captured in the thylakoid membranes; sugars are built in the stroma.

Worked example — name the plastid. Starch grains in potato cells are stored in amyloplasts; the orange colour of a carrot comes from chromoplasts.

An everyday example. A green tomato turns red as it ripens because its chloroplasts change into pigment-rich chromoplasts.

The substance. Plastids can change from one type to another, as ripening fruits show.

Ribosomes are membrane-less particles of RNA and protein that make proteins — 80S in the eukaryotic cytoplasm and 70S in prokaryotes, mitochondria and chloroplasts; microbodies are small enzyme-filled vesicles, the cytoskeleton gives support and movement, cilia and flagella move cells or fluids using a 9 + 2 arrangement of microtubules, and centrioles organise cilia, flagella and spindle fibres.

Ribosomes:

- 80S — made of 60S and 40S subunits
- 70S — made of 50S and 30S subunits
- S (Svedberg unit) reflects how fast a particle settles, an indirect measure of size and density

Microbodies. Tiny membrane-bound vesicles containing enzymes, found in plant and animal cells.

Cytoskeleton. A network of protein filaments — microtubules, microfilaments and intermediate filaments — for mechanical support, movement and maintaining cell shape.

Cilia and flagella:

- Cilia are short and beat like oars; flagella are longer and move the whole cell
- Their core, the axoneme, has nine doublets of microtubules around a central pair — the 9 + 2 array
- They grow from a basal body

Centrosome. Usually two centrioles lying at right angles; each has nine triplets of tubulin arranged like a cartwheel around a central hub. Centrioles form basal bodies and the spindle in animal cell division.

An everyday example. Cilia lining your windpipe sweep mucus and trapped dust up and away from the lungs.

The substance. Centrioles are absent from most higher plant cells, which still manage to divide.

The nucleus is bounded by a double nuclear envelope with pores and contains chromatin and one or more nucleoli; loosely packed euchromatin is active while tightly packed heterochromatin is inactive, and chromosomes are classed as metacentric, submetacentric, acrocentric or telocentric by the position of their centromere.

Parts of the nucleus:

- Nuclear envelope — two membranes with a space between; the outer one continues into the ER and bears ribosomes
- Nuclear pores — passages for RNA and proteins moving between nucleus and cytoplasm
- Nucleolus — not bound by a membrane; site of ribosomal RNA synthesis; larger and more numerous in cells making lots of protein
- Chromatin — DNA with histone and non-histone proteins and some RNA

Chromatin types:

- Euchromatin — loosely packed, lightly stained, transcriptionally active
- Heterochromatin — densely packed, darkly stained, inactive

Chromosome types by centromere position:

- Metacentric — centromere in the middle, two equal arms
- Submetacentric — slightly away from the middle, one arm shorter
- Acrocentric — close to one end, one very short arm
- Telocentric — centromere at the tip

The centromere carries disc-shaped kinetochores, and a few chromosomes have a secondary constriction forming a satellite.

An everyday example. In a library, books kept on open shelves are in regular use, while those boxed in storage are not — like euchromatin and heterochromatin.

The substance. Mature mammalian red blood cells have no nucleus, which leaves more room to carry oxygen.