What is inside a cell? Click your way through every organelle

How to use this simulation

1. The picture above shows two cells side by side: the animal cell on the left, the plant cell on the right. The small blue dots are vesicles carried along in the cytoplasm. In the plant cell they circle the big vacuole; in the animal cell they drift about.
2. Click any part, say a green chloroplast or the purple nucleus. The chosen part glows, the rest fade, and a line under the picture gives its name and job. Click an empty patch to bring everything back.
3. No mouse? Use the "Pick a part" list for the same effect. The readings panel on the right tells you which cell the part belongs to and its typical size.
4. Switch "Cell" to plant only or animal only for a bigger view. Then pick a plant-only part, such as the cell wall, while the animal cell is showing; the caption will tell you the part is not present in this cell.
5. Drag the magnification slider. The real width of a cell stays fixed, but the width of its image in a microscope is simply the real width multiplied by the magnification. That one calculation is a regular exam question.

Peel an onion and find a brick wall

Take an onion from the kitchen. Strip off the dry outer skin, peel the thin wet membrane from the inside of a layer, and lay it in a drop of water under a microscope. What you see is surprising: the membrane is a tidy grid of tiny rooms, like a brick wall. Each room has a clear wall and a round dot inside. Those rooms are cells.

An animal cell is about 20 µm across and a plant cell about 50 µm. That is far too small to see with the naked eye, yet your own body runs on trillions of them. Each one is a tiny factory: it makes energy, builds proteins, throws out waste and copies itself. This page walks you through every machine in that factory.

Textbooks ask you to memorise organelle names and jobs, and most students cram the list and forget it a week after the exam. We will do the opposite. First look at the picture, then match each part to something familiar, like a town office, a power station or a post office. Once the picture and the comparison sit in your head, the names come back by themselves.

Cell theory, from zero

In 1665 Robert Hooke looked at a thin slice of cork through a microscope he had built and saw rows of small boxes. They reminded him of the little rooms (cells) of a monastery, so he called them cells. He was really seeing the walls of dead cells. A few years later Antonie van Leeuwenhoek became the first person to see living cells, such as bacteria and sperm.

In 1838-39 Matthias Schleiden (working on plants) and Theodor Schwann (on animals) said that all living things are made of cells. In 1855 Rudolf Virchow added that new cells arise only from existing cells; they do not appear out of nothing. Together these ideas form the cell theory.

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in living things.
• New cells always come from existing cells.

New words at a glance

Let us clear the vocabulary first so nothing trips you up later.

• Organelle: a small working part inside a cell, with its own structure and its own job.
• Cytoplasm: the jelly-like fluid between the cell membrane and the nucleus, in which the organelles float.
• Eukaryote: an organism whose cells have a true nucleus wrapped in a membrane. Plants, animals, fungi and protozoa.
• Prokaryote: an organism whose cells have no true nucleus, such as bacteria.
• Membrane-bound organelle: a part wrapped in its own membrane, such as the nucleus, mitochondria and chloroplasts.
• Tissue: a group of similar cells doing one job together.
• Magnification: how many times bigger than real life a microscope makes something look.

The organelle tour, one by one

Picture the cell as a small town: an administration office, a power station, factories, a post office, a water tank, a clean-up crew and a boundary wall around it all. The comparison is not perfect, but it works well for learning. Click each part in the simulation as you read the matching paragraph below.

Plant cell vs animal cell: where they differ

Both cells have a nucleus, cytoplasm, cell membrane, mitochondria, ER, ribosomes and Golgi bodies. The differences sit in a few clear places, and they come from how the two organisms live. A plant stands in one place and makes its own food, so it needs a light-catching machine and a rigid frame. An animal moves to find food, so it needs soft, flexible cells and a bag of digestive enzymes.

Prokaryote and eukaryote

Both cells you have been exploring are eukaryotic ("eu" means true, "karyon" means kernel or nucleus), with DNA inside a membrane-wrapped nucleus. Prokaryotes such as bacteria have no such nucleus. Their DNA lies loose in a region called the nucleoid, with no membrane wall around it.

Try it in the simulation

Understanding by doing beats understanding by reading. Run these experiments one at a time.

• Experiment 1, match the address: with "All parts" selected, click an unknown part, guess its name, then check the caption under the picture. After ten tries you will have every name.
• Experiment 2, plant-only parts: choose "Plant cell" and click the cell wall, a chloroplast and the vacuole. Then switch to "Animal cell" and try to pick the same parts from the list; nothing lights up and the caption says the part is not in this cell.
• Experiment 3, watch the streaming: choose "Cell", pick cytoplasm and the blue vesicles grow and become easy to follow. Compare how they circle the vacuole in the plant cell with how they wander in the animal cell.
• Experiment 4, hide the names: turn off "Show part names", look at the diagram and quiz yourself, then click to check. Exams give unlabelled diagrams.
• Experiment 5, magnification: slide the magnification between 40 and 1000 and watch how the width of the cell image changes. Work out why the real width does not.
• Experiment 6, side by side: pick "Side by side" and select a shared part such as mitochondria; it lights up in both cells. Now select chloroplast and only the right-hand cell responds.

Size maths: from the picture to the real thing

Cell sizes are quoted in micrometres (µm). 1 mm = 1000 µm, and 1 µm = 1000 nanometres (nm). Keep this conversion in your head and the rest of the maths is easy.

Mistakes everyone makes

Marks on this topic are rarely lost for lack of knowledge. They go to small slips. Here are the usual traps.

• "Every cell has a nucleus": wrong. Mature red blood cells and the sieve tube cells of plants have none, while a skeletal muscle fibre has many.
• Mixing up the cell wall and the cell membrane: the wall is rigid, fully permeable and only in plants; the membrane is thin, selective and in every cell.
• "All plant cells have chloroplasts": wrong. Root cells never see light, so they have none.
• "Mitochondria make energy": energy is not created. The chemical energy in glucose is converted into ATP. Write "releases energy" or "makes ATP".
• Confusing lysosomes with vacuoles: lysosomes hold digestive enzymes, while a vacuole is mainly a storage and pressure sac.
• Leaving out the membrane in a drawing: when sketching a plant cell, many people forget the separate membrane line just inside the wall.
• Mixing up RER and SER: if the surface is dotted with ribosomes, it is rough ER.

Where this shows up in real life

Cell structure is not an isolated lesson. A lot of everyday events are explained by it.

• Why plants wilt: with too little water the vacuole loses pressure and the cells go soft. Water them and the cells firm up again.
• Muscle fatigue: muscle cells are packed with mitochondria, so they are the first to suffer when oxygen runs short.
• Why antibiotics kill bacteria but not you: many attack the bacterial cell wall or the smaller bacterial ribosomes, which human cells do not have.
• Some inherited diseases: if a lysosome enzyme is missing, waste builds up inside the cell and does damage.
• Dietary fibre: the fibre in vegetables is the cellulose of plant cell walls, which we cannot digest.
• Why salt draws water out of a cucumber: a strong solution outside the cell pulls water out of the vacuole and cytoplasm by osmosis.

Exam corner: CBSE and school-level questions

Cell structure sits at the heart of the chapter "The Fundamental Unit of Life" in the NCERT Class 9 science book, and it returns in biology at every later level. Questions come in four typical shapes.

• Definition: What is a cell wall? What are mitochondria? What is the Golgi apparatus? Give one line plus the job.
• Reason: Why are mitochondria called the powerhouses of the cell? Why are lysosomes called suicide bags? Answer with the reason in two or three sentences.
• Application: a description or diagram of a cell is given and you must say what kind of cell it is, why, and what would happen if a named organelle were missing.
• Difference: plant cell and animal cell, cell wall and cell membrane, rough ER and smooth ER, prokaryote and eukaryote. A table with four points is best.

Quick revision list

Skim this the night before the test.

• The cell is the basic unit of structure and function; the cell theory is credited to Schleiden, Schwann and Virchow; Hooke named the cell.
• Cell membrane: in all cells, selective, made of phospholipid and protein.
• Cell wall: plants only (cellulose), rigid, fully permeable.
• Nucleus: DNA and control; nucleolus: makes ribosome parts.
• Mitochondria: make ATP, two membranes, cristae, own DNA.
• Chloroplast: photosynthesis, chlorophyll, grana, plants only.
• Vacuole: one huge sac in plants, stores and keeps the cell turgid.
• Ribosome: builds proteins; rough ER: with ribosomes; smooth ER: lipids and detox.
• Golgi body: sorts, modifies, packs; lysosome: digestion and clean-up.
• Centrosome: builds the spindle in animal cell division.
• Real size = image size ÷ magnification; 1 mm = 1,000 µm.

Frequently asked questions