Cell organelles: the factory inside a cell

Think of a cell as a tiny living factory with many departments, each with one job, all working at once. A eukaryotic cell runs many life processes at the same moment, each in its own compartment, its organelles. Between them they supply energy, make new materials and get rid of waste, and together they keep the cell alive.

Explore the factory below. Turn it, switch between a plant cell and an animal cell, and tap any organelle.

The nucleus is wrapped in a double-layered nuclear membrane with pores in it, so materials can move between the nucleus and the cytoplasm. Inside sits a dense, round nucleolus, where the two parts (subunits) of ribosomes are made. These subunits leave through the pores, and in the cytoplasm one large and one small subunit join to make a ribosome.

The nucleus contains chromosomes, made of DNA (deoxyribonucleic acid) and special proteins. DNA carries the information for passing characters from parents to children, and its working segments are called genes. In a cell that is not dividing, the DNA is spread out as chromatin, a tangled mass of threads. Only when the cell is about to divide does chromatin pack up into rod-shaped chromosomes you can see.

Prokaryotic cells have no proper nucleus. Their DNA is a single circular molecule with some proteins, lying in a region called the nucleoid.

Ribosomes are the protein factories: tiny structures where proteins are made. Some float freely in the cytoplasm and others sit on the endoplasmic reticulum.

The endoplasmic reticulum (ER) is a large network of membranes spread through the cytoplasm, joined to the outer membrane of the nuclear envelope. It makes and transports proteins, fats (lipids) and, in some special cells, hormones. It comes in two forms:

Rough ER (RER) looks rough under an electron microscope because ribosomes stud its surface. It mainly makes and secretes proteins, so it is plentiful in gland cells such as those of the pancreas.

Smooth ER (SER) has no ribosomes on it, so it looks smooth. It makes and stores fats and hormones.

In a cell diagram you can spot stacks of flattened, sac-like pouches. Together they form the Golgi apparatus, the cell's post office. Working with the ER, the cell membrane and other organelles, it modifies, sorts and packs proteins and lipids into small bubbles called vesicles, which carry them to where they are needed, out of the cell, or into new lysosomes.

Lysosomes are the clean-up system. A busy cell keeps making waste and wearing out its own parts. Each lysosome is a sac of digestive enzymes inside a single membrane. Those enzymes can take apart unwanted proteins, carbohydrates and fats, and even worn out parts of the cell itself. What they break down goes back into the cytoplasm to be used again. (The book's figure of a typical plant cell does not draw lysosomes, and the explorer above follows that figure, so you will find them only in its animal cell.)

Almost everything a cell does needs energy, and most of it comes from mitochondria, which is why they are called the powerhouses of the cell. A mitochondrion is wrapped in two membranes. The outer one is smooth and porous. The inner one is pleated into finger-like folds called cristae, and the folds pack in far more surface for the energy releasing chemical reactions.

The process inside is cellular respiration: glucose and other food molecules are split to free their energy. That energy is parked in a molecule called ATP (adenosine triphosphate), the energy currency the cell spends on almost everything it does.

Animals take in food; plants make their own using sunlight. They do it in organelles called plastids, which make and store food.

Photosynthesis runs on sunlight, and the light is caught by a green pigment, chlorophyll, inside a type of plastid called the chloroplast. A chloroplast has a double membrane, just as a mitochondrion does. It is filled with a semi-fluid stroma, and in the stroma sit stacks of disc-shaped membranes: these hold the chlorophyll and do the light catching. Sugar from photosynthesis is kept in the stroma, alongside grains of starch.

Mitochondria and plastids also hold a clue to their past. They resemble certain bacteria: each carries its own DNA and ribosomes, so it can build some of its own proteins. The likely explanation is a shared evolutionary history with those single-celled organisms.

Not all plastids are green. The yellow, orange and red of many petals and fruits come from chromoplasts (chroma means colour), plastids that carry pigments other than chlorophyll. The colours work as advertising: they bring in pollinators, and the fruit-eating animals that carry seeds away.

Other plastids have no pigment and look colourless. These leucoplasts (leukos means white) are food stores, holding starch, oils or proteins. In potato and taro (Colocasia) they are packed with starch.

Where does a cell keep water, minerals and waste? Look inside a mature plant cell and you will usually find one large central vacuole. It is a store for water, minerals, sugars and waste, held in a watery liquid called cell sap and closed off by one selectively permeable membrane.

Because it holds so much water, the vacuole presses outwards and keeps the plant cell firm. A thirsty plant shows what happens without that water: the vacuoles give it up, the cells go limp and the plant wilts. Animal cells sometimes have vacuoles too, but they are much smaller and only store materials for a short time.