What an organelle is
An organelle is a small structure inside a cell that does a particular job, much as organs do in a body. The word comes from “little organ.” Most organelles in plant and animal cells are wrapped in their own membrane, which keeps their chemistry separate from the rest of the cell. Ribosomes are the main exception: they have no membrane, and some biologists do not call them organelles for that reason.
Control and instructions
The nucleus holds the cell’s DNA, the instructions for making proteins. A double membrane called the nuclear envelope surrounds it, and nuclear pores let messages out to the cytoplasm. Inside, the nucleolus is a dense area that builds the parts of ribosomes.
- Nucleus: stores DNA and controls which genes are used.
- Nuclear envelope: a double membrane with pores.
- Nucleolus: makes ribosome parts.
Making and shipping proteins
Ribosomes read the messages from the nucleus and build proteins. Ribosomes on the rough endoplasmic reticulum make proteins that will go to membranes or leave the cell. The smooth ER, which has no ribosomes, makes lipids and stores calcium. The Golgi apparatus then modifies and sorts these products and packs them into vesicles, small bubbles that carry them where they need to go.
- Ribosomes: build proteins.
- Rough ER: makes and folds proteins.
- Smooth ER: makes lipids, stores calcium.
- Golgi apparatus: sorts and ships.
- Vesicles: carry materials.
Energy, cleanup and support
Mitochondria release energy from food molecules through cellular respiration, storing it in a molecule called ATP. Lysosomes in animal cells hold enzymes that break down worn-out parts and food particles, and peroxisomes break down fatty acids and harmful hydrogen peroxide. The cell membrane controls what enters and leaves, and the cytoplasm is the watery gel in which all of this happens.
Parts only plant cells have
Plant cells add a cellulose cell wall for support, chloroplasts that make sugar by photosynthesis, a large central vacuole that stores water and keeps the cell firm, and plasmodesmata, channels through the wall that connect neighboring cells.
Sources and further reading
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