Cells and Organelles
Every living thing is made of cells. This guide explains what the main organelles do, how they differ between animal and plant cells, and why their structure fits their function.
Primary sources: Molecular Biology of the Cell (Alberts et al., NCBI) · Nature Scitable — Cell Organelles
01 · What is a cell
The basic unit of life.
All living organisms are made from one or more cells. The cell theory, established by Schleiden, Schwann and Virchow in the 19th century, states three things: all organisms consist of cells; the cell is the basic unit of life; all cells come from pre-existing cells.
Cells fall into two broad groups. Prokaryotic cells — bacteria and archaea — have no membrane-bound nucleus. Their DNA floats in the cytoplasm in a region called the nucleoid. Eukaryotic cells — animals, plants, fungi and protists — have a true nucleus enclosed by a double membrane, and a range of specialised membrane-bound compartments called organelles.
This guide focuses on eukaryotic cells, because they are the ones covered in most secondary-school and first-year university biology courses. For the full comparison, see the NCBI Molecular Biology of the Cell.
02 · The organelles
Ten structures, one cell.
Each organelle has a specific job. The list below covers both animal and plant cells — those marked "Plant only" are not found in animal cells.
Nucleus
Stores DNA and controls gene expression.
Mitochondrion
Produces ATP by aerobic respiration.
Ribosome
Assembles amino acids into proteins.
Endoplasmic reticulum
Rough ER folds proteins; smooth ER makes lipids.
Golgi apparatus
Modifies, packages and ships proteins.
Lysosome
Contains digestive enzymes; breaks down waste.
Cell membrane
Phospholipid bilayer; controls what enters and leaves.
Chloroplast
Plant only — captures light for photosynthesis.
Cell wall
Plant/fungi only — rigid outer layer for support.
Vacuole
Storage; central vacuole in plants maintains turgor pressure.
03 · Animal vs plant cells
Same toolkit, different extras.
Both animal and plant cells share a nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus and a cell membrane. Plant cells have three additional structures:
- Cell wall — made of cellulose. Provides rigid support and prevents over-expansion.
- Chloroplasts — contain chlorophyll and carry out photosynthesis. See the photosynthesis guide for details.
- Large central vacuole — stores water and maintains turgor pressure, keeping the plant upright.
Animal cells have centrioles (used in cell division) and no cell wall; their vacuoles, if present, are small. For cell division details, see the mitosis and meiosis guide.
04 · The nucleus
The cell's control centre.
The nucleus is enclosed by a nuclear envelope — two membranes with nuclear pores that regulate the passage of molecules. Inside, DNA is wrapped around histone proteins to form chromatin. When a cell divides, chromatin condenses into visible chromosomes. Human cells contain 46 chromosomes arranged in 23 pairs. For how DNA is copied and expressed, see the DNA, genes and heredity guide and the protein synthesis guide.
The nucleolus is a dense region inside the nucleus where ribosomal RNA is made. Ribosomes assembled here move to the rough ER where they translate mRNA into protein.
Questions about cells and organelles
- Prokaryotic cells (bacteria, archaea) have no membrane-bound nucleus and are generally smaller. Eukaryotic cells (animals, plants, fungi, protists) have a nucleus and membrane-bound organelles. See the Molecular Biology of the Cell overview (NCBI) for a full comparison.
- Mitochondria produce ATP — the cell's energy currency — through aerobic respiration. The inner membrane folds (cristae) increase surface area for this process. Read more in our cellular respiration guide and the Khan Academy cell structure module.
- Yes. Plant cells have both mitochondria and chloroplasts. Mitochondria handle respiration; chloroplasts handle photosynthesis. See our photosynthesis guide for how chloroplasts capture light energy.
- A phospholipid bilayer with embedded proteins. The hydrophobic tails face inward; hydrophilic heads face the water on each side — the fluid-mosaic model described by Singer and Nicolson (1972). The Nature Scitable cell membrane page has an illustrated explanation.
Ready for the next step?
Follow the biology foundations path: DNA → mitosis → photosynthesis → respiration.