Inside the cell, the basic unit of life
Robert Hooke first saw cells in 1665 when he looked at a thin slice of cork under a microscope. The cell theory states that all living things are made of cells, the cell is the basic unit of life, and new cells come from existing cells. Some organisms, like Amoeba, are single-celled, while humans have trillions of cells organised into tissues and organs.
Every cell is surrounded by a plasma membrane that controls what enters and leaves; water moves across it by osmosis. Plant cells also have a rigid cell wall made of cellulose. The nucleus contains chromosomes made of DNA and controls cell activities. Mitochondria are the powerhouses that release energy through respiration. Ribosomes make proteins, the endoplasmic reticulum transports materials, the Golgi apparatus packages them, and lysosomes digest waste, which is why they are called suicide bags. Plant cells have chloroplasts for photosynthesis and large vacuoles, while animal cells lack chloroplasts. Prokaryotic cells, such as bacteria, have no true nucleus.
Why it matters
Understanding cells explains how wounds heal, how diseases like cancer start when cells divide uncontrollably, and why salting pickles kills microbes through osmosis. Doctors examine blood cells to diagnose illnesses. This chapter is the foundation for tissues, life processes, heredity and all of biology in later classes.
Key terms
- Cell theory
- The principle that all living things are made of cells and new cells arise from existing cells.
- Plasma membrane
- The thin, selectively permeable layer that surrounds every cell.
- Mitochondria
- Cell organelles that release energy from food through cellular respiration.
- Osmosis
- The movement of water through a selectively permeable membrane from a dilute to a concentrated solution.