Biology · Introductory biology · Concept
Cell structure: organelles and their jobs
Every cell has a plasma membrane, cytoplasm, ribosomes and DNA. Prokaryotic cells, the bacteria and archaea, are small and have no nucleus or other membrane-bound organelles; their DNA lies in a region called the nucleoid. Eukaryotic cells are larger and divide their work among organelles: the nucleus holds the DNA, ribosomes and the rough ER make proteins, the Golgi apparatus modifies and ships them, and mitochondria make ATP. Plant cells add chloroplasts, a cell wall and a large central vacuole.
What every cell has
A plasma membrane separates the cell from its surroundings and controls what enters and leaves. Inside are the cytoplasm, ribosomes that build proteins, and DNA that carries the cell’s instructions.
Prokaryotic and eukaryotic cells
Prokaryotic cells, typically 0.1–5 µm across, keep their DNA in a nucleoid with no membrane around it, and many carry small rings of extra DNA called plasmids. Most have a cell wall, and some have a capsule or flagella. Eukaryotic cells, typically 10–100 µm across, keep their DNA in a nucleus and have membrane-bound organelles.
The endomembrane system
The nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles and lysosomes work as one system. Ribosomes on the rough ER make proteins for membranes and for export; the smooth ER makes lipids and breaks down toxins. Vesicles carry these products to the Golgi apparatus, which modifies and sorts them and sends them on to the plasma membrane, to lysosomes or out of the cell.
Plant and animal cells
Both have a nucleus, ER, a Golgi apparatus, mitochondria and ribosomes. Plant cells add a cellulose cell wall, chloroplasts for photosynthesis, and a large central vacuole that stores water and keeps the cell firm. Animal cells have no wall, and they have centrosomes with centrioles, and lysosomes, which plant cells lack.
| Structure | Job |
|---|---|
| Cell wall | Supports and protects the cell; made of cellulose |
| Plasma membrane | Controls what enters and leaves |
| Nucleus | Holds the DNA; its nucleolus makes the parts of ribosomes |
| Ribosomes | Build proteins |
| Rough ER | Makes proteins for membranes and export |
| Smooth ER | Makes lipids and breaks down toxins |
| Golgi apparatus | Modifies, sorts and ships proteins |
| Vesicles | Carry materials between organelles |
| Mitochondria | Make ATP by cellular respiration |
| Chloroplasts | Capture light energy by photosynthesis |
| Central vacuole | Stores water and keeps the cell firm |
Structure fits function
A cell’s organelles reflect its work. Pancreatic cells that secrete digestive enzymes are packed with rough ER and Golgi; muscle cells, which use a great deal of energy, are packed with mitochondria; and liver cells that break down drugs have plenty of smooth ER.
Common mistakes
- Thinking plant cells have no mitochondria because they have chloroplasts: plants respire too, and their cells have both.
- Saying prokaryotes have no ribosomes or DNA: they have both; what they lack is a nucleus and other membrane-bound organelles.
- Confusing the cell wall with the plasma membrane: the wall lies outside the membrane, and the membrane controls what enters and leaves.
- Counting ribosomes among the membrane-bound organelles: they are made of RNA and protein and have no membrane.
Key terms
- Prokaryotic cell
- A cell lacking a membrane-bound nucleus. Bacteria and archaea are distinct groups of prokaryotes, with ribosomes and organized cellular processes.
- Eukaryotic cell
- A cell with a membrane-bound nucleus and other compartmentalized structures. Animals, plants, fungi and many unicellular organisms are eukaryotes.
- Organelle
- A specialized cellular structure carrying out particular functions. Some organelles are membrane enclosed; the term’s scope can vary with the teaching convention.
- Plasma membrane
- The phospholipid bilayer, studded with proteins, that surrounds every cell and controls what enters and leaves it.
- Cell wall
- A rigid layer outside the plasma membrane that supports and protects the cell. Plant cell walls are mostly cellulose, fungal walls chitin and bacterial walls peptidoglycan; animal cells have none.
- Cell nucleus
- The membrane-enclosed compartment containing most eukaryotic chromosomal DNA. Its nuclear envelope separates this compartment from the surrounding cytoplasm.
- Nucleoid
- The region of a prokaryotic cell where its chromosome lies. Unlike a nucleus, it has no membrane around it.
- Plasmid
- A small ring of DNA that is copied separately from a cell’s chromosome, common in bacteria. Plasmids often carry genes such as antibiotic resistance, and biotechnology uses them to carry genes into cells.
- Ribosome
- A complex of RNA and proteins that translates messenger RNA into a polypeptide. Ribosomes occur in both prokaryotic and eukaryotic cells.
- Endoplasmic reticulum
- A eukaryotic membrane network. Ribosome-bearing rough ER supports synthesis of many secreted or membrane proteins; smooth ER includes lipid-related functions.
- Golgi apparatus
- Stacks of membrane compartments that modify, sort and package cellular material. Transport vesicles connect its processing functions with other parts of the endomembrane system.
- Lysosome
- A membrane-bound compartment containing enzymes that digest and recycle cellular material under acidic conditions. It participates in the endomembrane system.
- Endomembrane system
- Connected or interacting cellular membranes involved in making, processing, transporting and degrading materials. The ER, Golgi apparatus and lysosomes have different roles.
- Mitochondrion
- An organelle central to aerobic energy metabolism in many eukaryotes. It supports oxidative phosphorylation and also performs other cellular functions.
- Chloroplast
- A photosynthetic organelle in plants and algae. Its internal membranes and surrounding compartments support distinct light-dependent and carbon-fixation processes.
- Central vacuole
- A large membrane-bound sac in a plant cell that stores water, ions and wastes. Water pressing outward from it against the cell wall, called turgor pressure, keeps the cell firm.
Work through an example
A student observes three cells. Cell A is 2 µm long, with a cell wall, ribosomes and a ring of DNA but no nucleus. Cell B is 40 µm across, with a nucleus, a cell wall, chloroplasts and a large central vacuole. Cell C is 20 µm across, with a nucleus, mitochondria and centrioles but no cell wall. Identify each cell.
Identify a cell from its structures →Sources and scope
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