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Cell structure: what each organelle is shaped for

A eukaryotic cell is a set of compartments, and every one of them is built the way it is because of the job it does. Folded inner membrane and respiration. Stacked sacs and modification. A single membrane and a bag of enzymes that would digest the cell if it escaped. Learn the pairs and the list stops being a list.

Before this The animal and plant cells of GCSE biology · Standard form and the SI prefixes down to nanometres

What you should be able to do

• Describe the ultrastructure of an animal cell and of a plant cell, and say which structures belong to which.
• Link the structure of the nucleus, mitochondrion, chloroplast, Golgi apparatus, lysosome and both kinds of endoplasmic reticulum to what each does.
• Put the organelles of the secretory pathway in the order a protein meets them.
• State where 80S and 70S ribosomes are found, and what a ribosome is made of.
• Name the structures a plant cell has and an animal cell does not, and give a function for each.

What the electron microscope added

Under a light microscope a cell is a bag with a dark spot in it. You can make out the nucleus, the cytoplasm, and in a plant the wall and the chloroplasts, and that is close to the limit. Nearly everything in this lesson was invisible until the 1950s, because light cannot separate two points closer together than about 0.2 µm and most organelles are smaller than that. What the electron beam revealed is the cell's ultrastructure, and the word is worth using in an answer: it tells the examiner you mean the detail no light microscope could reach.

Two habits will carry you through the topic. Count membranes: two, one and none are three different kinds of thing, and the count is often the mark. And say the job in the same sentence as the shape — nobody wants "the mitochondrion has cristae" on its own.

Organelle

A structure inside a cell with a distinct function, usually but not always surrounded by its own membrane.

Ultrastructure

The detail of a cell's internal structure as revealed by an electron microscope.

Cytoplasm

Everything inside the cell-surface membrane except the nucleus: the cytosol and the organelles suspended in it.

The nucleus, and two kinds of endoplasmic reticulum

The nucleus is wrapped in two membranes, and the pair together is the nuclear envelope. It is punched through with nuclear pores — a few thousand of them in a typical cell, each around 40 to 100 nm across. Those pores are the reason the arrangement works: messenger RNA is small enough to leave through one, and the chromosomes are not, so transcription and translation happen in separate rooms.

Inside, the DNA is wound round histone proteins into chromatin. One patch is denser than the rest and takes up stain more heavily: the nucleolus, where ribosomal RNA is made and the two ribosomal subunits are assembled. A cell with a large nucleolus is building a lot of ribosomes, which usually means it is about to make a lot of protein.

The outer membrane of the nuclear envelope does not stop at the nucleus. It runs straight on into the endoplasmic reticulum, a system of flattened sacs called cisternae folded through the cytoplasm. There are two versions, and the only structural difference between them is what is stuck to the outside.

Rough endoplasmic reticulum is studded with ribosomes. A protein made on one of those ribosomes is threaded into the sac as the chain grows, which puts it inside the membrane system straight away — the start of the journey out of the cell. Proteins for secretion, for lysosomes and for the cell-surface membrane are all made here. A plasma cell pouring out antibody is almost solid rough ER.

Smooth endoplasmic reticulum has no ribosomes on it and a completely different job list: it synthesises lipids, phospholipids and steroids. Liver cells use theirs to break down drugs and alcohol, which is why a liver cell has an unusual amount. In a muscle fibre a specialised version stores calcium ions and releases them to trigger contraction. Told that a cell is rich in smooth ER, think lipids before you think anything else.

Nuclear envelope

The double membrane surrounding the nucleus, continuous with the endoplasmic reticulum and perforated by nuclear pores.

Nucleolus

A dense region within the nucleus where ribosomal RNA is made and ribosomes are assembled.

Rough endoplasmic reticulum

Flattened membrane sacs with ribosomes attached, which make and transport proteins for export or for membranes.

Smooth endoplasmic reticulum

Membrane sacs without ribosomes, which synthesise lipids and steroids and store calcium ions.

Making something, then sending it out

The organelles of a cell are not a collection of separate gadgets. Several of them are stations on one route, and questions about that route are common because they test whether you know the order rather than the names.

The Golgi apparatus is a stack of flattened sacs, usually four to eight of them, curved and with vesicles budding off one face. Vesicles arrive at one side carrying newly made protein, the contents move through the stack, and finished product leaves the other side. What happens in between is modification: sugars are added to make glycoproteins, chains are trimmed, phosphate groups are attached, and each product is sorted and addressed to wherever it belongs. Folding is not on that list, and it is a common answer that costs a mark. A polypeptide folds in the endoplasmic reticulum, as it is being made and with chaperone proteins holding it while it does; it reaches the Golgi already in shape. The stack is what allows a sequence of steps to happen in order, which a single bag could not do.

Lysosomes come off the Golgi as well, which is a neat detail worth knowing — the enzymes inside them are proteins, made on rough ER, packaged like any other export and then simply kept.

The two organelles that handle energy

Mitochondria are usually between 1 and 10 µm long — about the size of a whole bacterium, which is not a coincidence. Each has two membranes. The outer one is smooth. The inner one is folded into cristae, and the folding matters because the electron transport chain and ATP synthase sit in that inner membrane: more folding, more surface area, more of the machinery, more ATP. The fluid inside is the matrix, where the Krebs cycle runs, and it contains enzymes, a small loop of DNA and 70S ribosomes of its own.

The number of mitochondria in a cell tells you how much ATP that cell needs. Muscle fibres, sperm cells and the epithelial cells lining the small intestine are all crowded with them; a mature red blood cell has none at all, along with no nucleus.

Chloroplasts are the plant equivalent and are built on the same plan: a double envelope, a fluid interior called the stroma, and an internal membrane system. Here the membranes form flattened discs called thylakoids, stacked into piles called grana, and the chlorophyll sits in the thylakoid membranes. Stacking gives a large area of pigment-bearing membrane in a small volume, so a lot of light gets absorbed. The stroma holds the enzymes of the light-independent reactions, along with starch grains, a loop of DNA and — again — 70S ribosomes.

Those loops of DNA and small ribosomes are the evidence behind the endosymbiotic theory, that both organelles descend from free-living prokaryotes taken into an early eukaryotic cell. The theory is optional on most boards; the 70S ribosomes are not, because they make the 70S-versus-80S question harder than it first looks.

Ribosomes, lysosomes and the rest of the small print

Ribosomes are the exception to everything else on this page: they have no membrane at all. Each is built from ribosomal RNA and protein in two subunits that clamp round a strand of mRNA to translate it. In the cytoplasm of a eukaryotic cell they are 80S and about 25 nm across. In prokaryotes they are 70S and smaller — and so are the ones inside mitochondria and chloroplasts. The S is a Svedberg unit, which measures how fast a particle settles in a centrifuge rather than its mass, which is why a 60S and a 40S subunit make an 80S ribosome instead of a 100S one.

Lysosomes are spherical sacs, roughly 0.1 to 1 µm across, each holding up to fifty kinds of hydrolytic enzyme behind a single membrane. They digest worn-out organelles, break down material taken in by phagocytosis, and in a white blood cell they empty their contents onto engulfed bacteria. The membrane is the whole design. Loose in the cytoplasm those enzymes would hydrolyse the cell that made them, so they are kept in a compartment and released only where they are wanted.

Centrioles finish the animal cell off: pairs of hollow microtubule cylinders that organise the spindle during nuclear division, and absent from the cells of flowering plants.

Ribosome

A structure of ribosomal RNA and protein, in two subunits, with no membrane, which is the site of translation. 80S in eukaryotic cytoplasm, 70S in prokaryotes and in mitochondria and chloroplasts.

Lysosome

A vesicle containing hydrolytic enzymes, bounded by a single membrane, which digests worn-out organelles and engulfed material.

Crista

A fold of the inner mitochondrial membrane, giving a large surface area for the electron transport chain.

What a plant cell has that yours does not

Every organelle so far appears in a plant cell too. Plant cells respire, so they have mitochondria; they make and export protein, so they have rough ER and Golgi. Four structures are added, and one is taken away.

The cell wall is cellulose, not membrane, and that matters twice over. It is freely permeable, so it is no barrier to water or dissolved substances and cannot control what enters the cell — the cell-surface membrane still does that. And it is strong in tension, so when water enters by osmosis the wall pushes back and the cell becomes turgid rather than bursting. Turgid cells packed against each other are what holds a non-woody plant up.

The permanent vacuole is a large sac of cell sap behind a single membrane called the tonoplast. It stores water, sugars, ions and sometimes pigments, and by filling with water it keeps the cell pressed against its wall. Animal cells have vacuoles too, but small and temporary ones.

Chloroplasts you have already met. Plasmodesmata are the fourth: narrow channels through the walls of neighbouring cells, lined with membrane, through which cytoplasm is continuous from one cell to the next. They are how a plant moves substances and signals between cells without crossing a wall.

In the exam

• Count the membranes and say the number. Nucleus and mitochondrion: two. Golgi sac, lysosome, vacuole, vesicle, both kinds of ER: one. Ribosome: none. Writing that a ribosome is 'surrounded by a membrane' loses the mark and makes the rest of the answer look shaky.
• Pair every structure with a function. 'Cristae give a large surface area for the enzymes of the electron transport chain' scores; 'the inner membrane is folded' on its own does not.
• For the secretory pathway, give the order and name the vesicles. Rough ER, transport vesicle, Golgi, secretory vesicle, cell-surface membrane, exocytosis. Questions that scramble the order are testing exactly this.
• 80S is the eukaryotic cytoplasmic ribosome and 70S the prokaryotic one — but mitochondria and chloroplasts also carry 70S, so a question asking where 70S ribosomes are found in a plant cell has more than one right answer.
• The cell wall does not control what enters the cell. It is fully permeable; the cell-surface membrane is the partially permeable one. This is a favourite one-mark trap and the answer is always the membrane.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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