1. The Cell: Life's Basic Unit

Every living thing — a bacterium, an oak tree, you — is built from cells. The cell theory sums up three big ideas:

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in life.
• All cells come from existing cells.

There are two broad types. Prokaryotic cells (bacteria) are small and have no nucleus — their DNA floats freely. Eukaryotic cells (plants, animals, fungi) are larger and keep their DNA inside a nucleus, along with other specialized parts called organelles.

Key organelles and what they do:

• Nucleus — the control center; holds DNA, the cell's instructions.
• Mitochondria — the 'powerhouse'; release energy (ATP) from food.
• Cell membrane — the gatekeeper; controls what enters and leaves.
• Cytoplasm — the jelly-like fluid where everything sits and reactions happen.
• Ribosomes — build proteins.

Plant cells add two parts animal cells don't have: a rigid cell wall for support, and chloroplasts that capture sunlight to make food.

⚠️ Common misconception: bacteria are not 'simple animals'. They are prokaryotes — a completely separate, nucleus-free kind of cell.

💡 Tip: remember 'mitochondria = powerhouse' and 'nucleus = control center'. Those two come up constantly.