Cell Biology

The cell is the basic structural and functional unit of life. All living organisms are made of cells. Unicellular organisms (like amoeba, bacteria) perform all life functions in a single cell, while multicellular organisms (like humans, plants) have billions of specialised cells working together.

Cell theory, proposed by Schleiden and Schwann, states: (1) All living things are made of cells. (2) The cell is the basic unit of life. (3) All cells arise from pre-existing cells (added by Rudolf Virchow). This is one of the foundational principles of biology.

The nucleus is the control centre of the cell. It contains DNA (genetic material) that carries instructions for all cellular activities. The nucleus is bounded by a double membrane called the nuclear envelope, and contains the nucleolus (site of ribosome synthesis).

Mitochondria are the sites of aerobic respiration. They convert glucose into ATP (energy currency) through oxidative phosphorylation. They have a double membrane — the inner membrane is folded into cristae. Mitochondria contain their own DNA, suggesting an evolutionary origin as free-living bacteria.

Chloroplasts are found only in plant cells and are the site of photosynthesis. They contain chlorophyll (green pigment) that captures sunlight. Other key organelles: Ribosomes (protein synthesis), Endoplasmic Reticulum (transport network), Golgi apparatus (packing and secretion), Lysosomes (digestion, found in animal cells).