The cell is the basic unit of life, and cell structure is one of the first topics in BIO 101. This guide covers what you need to know, with simple ways to remember each part.
Cell theory
Cell theory has three main statements:
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells come from pre-existing cells.
Prokaryotic and eukaryotic cells
Prokaryotic cells, such as bacteria, are small and simple. They have no true nucleus: their DNA lies in a region called the nucleoid. They also lack membrane-bound organelles like mitochondria.
Eukaryotic cells, found in plants, animals, fungi and protists, are larger and have a true nucleus enclosed by a nuclear membrane, as well as many membrane-bound organelles.
The main organelles and what they do
- Cell (plasma) membrane: a thin, flexible barrier made mainly of a phospholipid bilayer with proteins. It is selectively permeable, meaning it controls what enters and leaves the cell. It is often described by the fluid mosaic model.
- Nucleus: contains the cell's genetic material (DNA) and controls the cell's activities. The nucleolus inside it makes ribosomes.
- Mitochondria: the site of aerobic respiration, where energy from food is released and stored as ATP. Often called the "powerhouse of the cell". Active cells such as muscle cells have many mitochondria.
- Ribosomes: tiny structures where proteins are made (protein synthesis). Found free in the cytoplasm or attached to the rough endoplasmic reticulum.
- Endoplasmic reticulum (ER): a network of membranes. Rough ER has ribosomes and helps make and transport proteins. Smooth ER has no ribosomes and makes lipids.
- Golgi apparatus (Golgi body): modifies, packages and sends out proteins and lipids, like a cell's post office.
- Lysosomes: contain digestive enzymes that break down worn-out parts and foreign materials. Mainly in animal cells.
- Cytoplasm: the jelly-like fluid where organelles sit and many chemical reactions happen.
Structures found in plant cells
- Cell wall: a rigid layer made of cellulose outside the membrane. It gives shape and support.
- Chloroplasts: contain chlorophyll and are the site of photosynthesis, where light energy is used to make food.
- Large central vacuole: stores cell sap and keeps the cell firm (turgid) by holding water.
Plant cell versus animal cell
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Vacuole | One large central vacuole | Small, temporary vacuoles if any |
| Shape | Usually regular | Usually irregular |
| Centrioles | Usually absent in higher plants | Present |
Memory tip
Think of the cell as a factory. The nucleus is the manager's office, ribosomes are the workers making products (proteins), the ER is the assembly line, the Golgi is the packaging and shipping department, mitochondria are the generators, and the membrane is the security gate.
Exam tips
- When drawing a cell, use a sharp pencil, draw clean lines and label with straight ruled lines that do not cross.
- Always state both the structure and its function. Many questions award one mark for each.