Cellular Organization
Complete Notes & Free Quiz

Cells are the basic building blocks of all living things. Every living thing, from the smallest bacteria to the largest animal, is made up of cells.

Unicellular organisms — made up of only one single cell. Example: Bacteria.
Multicellular organisms — made up of millions of cells joined together. Examples: most animals and plants.

Animal and plant cells both perform the same basic jobs:

1. Take in food
2. Release energy
3. Get rid of waste
4. Grow
5. Reproduce

The nucleus is the control center of the cell. It contains genetic information — the instructions that tell every part of the cell what to do and how to grow. It controls everything that goes on in the cell. Without the nucleus, the cell cannot carry out its functions.

Cytoplasm — a jelly-like fluid inside the cell (all living matter except the nucleus). Chemical reactions in the cell take place inside the cytoplasm.

Cell Membrane — a thin skin around the outside of the cell. It prevents cell contents from escaping but allows substances like water and small molecules to cross in and out.

Mitochondria are where cellular respiration takes place. During respiration, glucose is broken down to release energy. This energy powers all cell activities. That is why mitochondria are called the powerhouse of the cell.

A cell wall is a rigid (firm) coating around the outside of the cell membrane, found only in plant cells. It is made of cellulose. It helps the plant cell keep its shape and makes stems and branches strong. Animal cells do not need a cell wall because their shape is maintained by the cytoskeleton.

Chloroplasts are found only in plant cells. They contain a green chemical called chlorophyll. Chlorophyll enables green plants to use the Sun's energy to make food during photosynthesis.

Animal cells do not have chloroplasts because animals get their energy from the food they eat, not by making it themselves.

A vacuole is a space inside the cell that stores materials.

Plant cells — one large central vacuole filled with cell sap (helps keep the plant firm)
Animal cells — many small vacuoles that store water, air, and food

Feature	Animal Cell	Plant Cell
Nucleus	✅ Present	✅ Present
Cytoplasm	✅ Present	✅ Present
Cell Membrane	✅ Present	✅ Present
Mitochondria	✅ Present	✅ Present
Cell Wall	❌ Absent	✅ Present (cellulose)
Chloroplasts	❌ Absent	✅ Present
Vacuole	Many small	One large central

Nerve cells (neurons) have long thin fibers which carry messages from one part of the body to another. The long shape allows electrical signals to travel quickly across long distances in the body.

Red Blood Cells — carry oxygen around the body. They have a large surface area (biconcave disc shape) which enables them to pick up lots of oxygen.

Muscle Cells — long and thin with pointed ends. This shape helps the cells slide over each other easily as muscles contract and relax to cause movement.

Pollen Grains — cells in plants. Some have a spiky surface to stick to insects; others have tiny wings to be carried by the wind. Both adaptations help in pollination — the transfer of pollen from one plant to another.

Epithelial Cells — thin and flat, found in layers covering surfaces such as the skin, lining of blood vessels, and the alimentary canal. They help with protection and absorption.

Living organisms are organized in a biological hierarchy:

Cell → the basic unit (e.g. muscle cell)
Tissue → a group of similar cells working together (e.g. muscle tissue)
Organ → a group of different tissues working together for one function (e.g. the heart)
Organ System → organs working together (e.g. circulatory system)
Organism → the complete living thing