Every living thing is made of cells, and almost every biology course starts here because nearly everything later (genetics, metabolism, disease) happens inside them. The good news: cells follow a small set of patterns. Once you know what each part does and why it is there, the diagrams stop being a list to memorize and start making sense.
Two big kinds of cells
Prokaryotic cells (bacteria and archaea) are small, usually 1 to 10 micrometers across, and have no nucleus. Their DNA sits in a region called the nucleoid, often as a single circular chromosome, sometimes with extra small rings called plasmids. They have ribosomes, a cell membrane, cytoplasm and usually a cell wall, but no membrane-bound organelles.
Eukaryotic cells (animals, plants, fungi and protists) are typically 10 to 100 micrometers and keep their DNA inside a nucleus. They are divided into compartments by internal membranes, which lets different chemical jobs happen side by side without interfering, a bit like rooms in a house.
The organelles and their jobs
- Nucleus: stores DNA and controls the cell by deciding which genes are read. It is surrounded by a double membrane (the nuclear envelope) with pores that let RNA out. The nucleolus inside it builds ribosome parts.
- Ribosomes: build proteins by reading messenger RNA. Free ribosomes float in the cytoplasm and make proteins used inside the cell; ribosomes attached to the rough ER make proteins that will be exported or put into membranes.
- Rough endoplasmic reticulum (rough ER): a folded membrane network studded with ribosomes. Proteins are made into it, folded and sent on.
- Smooth ER: no ribosomes. Makes lipids (including steroid hormones), detoxifies drugs and alcohol (very common in liver cells) and stores calcium in muscle cells.
- Golgi apparatus: the cell's post office. It receives proteins from the ER, modifies them (for example by adding sugar chains), sorts them and ships them in vesicles to the membrane, to lysosomes or out of the cell.
- Mitochondria: release usable energy from food through cellular respiration, producing ATP. They have their own DNA and a folded inner membrane (cristae) that increases surface area. Muscle cells have many.
- Chloroplasts (plants and algae only): carry out photosynthesis, turning light energy into chemical energy in glucose. Like mitochondria, they have their own DNA, which supports the endosymbiotic theory that both were once free-living bacteria.
- Lysosomes (mainly animal cells): sacs of digestive enzymes that break down worn-out parts, food particles and invading bacteria.
- Vacuoles: storage sacs. Plant cells usually have one large central vacuole that stores water and keeps the cell firm (turgor pressure). When a plant wilts, those vacuoles have lost water.
- Cell membrane: a phospholipid bilayer with embedded proteins that controls what enters and leaves. It is selectively permeable.
- Cell wall (plants, fungi, bacteria): a rigid layer outside the membrane. Plant walls are made of cellulose, fungal walls of chitin, and bacterial walls of peptidoglycan.
- Cytoskeleton: protein fibers (microtubules, microfilaments and intermediate filaments) that give shape, move organelles and help the cell divide.
Plant cells vs. animal cells
Both are eukaryotic and share the nucleus, ER, Golgi, mitochondria, ribosomes and membrane. Plant cells add three things: a cellulose cell wall, chloroplasts and a large central vacuole. Animal cells usually have small vacuoles, lysosomes and centrioles, and their shape is more flexible because there is no wall. A quick way to remember the plant extras: plants need to stand up (wall and vacuole) and make their own food (chloroplasts).
Worked example: follow a protein out of the cell
Suppose a pancreas cell makes insulin, a hormone that must leave the cell. Trace the path:
- The gene for insulin is copied into mRNA in the nucleus.
- The mRNA leaves through a nuclear pore and attaches to a ribosome on the rough ER.
- The growing protein is fed into the rough ER, where it folds.
- A vesicle carries it to the Golgi apparatus, which modifies and packages it.
- A secretory vesicle travels to the cell membrane, fuses with it and releases insulin outside (exocytosis).
This pathway is one of the most common exam questions, so it is worth being able to say it from memory.
Common mistakes
- Saying plant cells have no mitochondria. They do. Plants photosynthesize and also respire, so they need both chloroplasts and mitochondria.
- Calling the cell wall a membrane. The wall is outside the membrane and is rigid; the membrane controls transport.
- Saying mitochondria make energy. Energy cannot be created. Mitochondria convert the chemical energy in glucose into ATP.
- Forgetting bacteria have ribosomes. Prokaryotes lack membrane-bound organelles, but ribosomes are not membrane-bound, so bacteria have them.
Memory tricks
Think of the cell as a factory: the nucleus is the head office, ribosomes are workers, the ER is the assembly line, the Golgi is shipping, mitochondria are the power plant, lysosomes are the cleaning crew and the membrane is security at the gate. Linking each organelle to a job makes questions like "which organelle would a cell that secretes lots of enzymes have more of?" easy: more rough ER and Golgi.
Practice questions
- Which organelle would you expect in large numbers in a muscle cell, and why?
- Name three structures found in plant cells but not animal cells.
- A cell has ribosomes, a cell wall and DNA but no nucleus. What kind of cell is it?
- Why does a plant wilt when it is not watered?
Answers: (1) Mitochondria, because muscles need lots of ATP to contract. (2) Cell wall, chloroplasts and a large central vacuole. (3) A prokaryotic cell, such as a bacterium. (4) The central vacuoles lose water, turgor pressure drops and the cells go limp.
Quick FAQ
Do all cells have a nucleus? No. Prokaryotes never do, and mature human red blood cells lose theirs to make room for hemoglobin.
Why are cells so small? As a cell grows, its volume increases faster than its surface area, so a large cell could not move nutrients in and waste out fast enough through its membrane.
Last reviewed 2026-10-23 by the BrainBoost team.