The Fundamental Unit of Life Notes

Robert Hooke, in 1665, examined a thin slice of cork under his self-made microscope and saw tiny box-like compartments. He called them 'cells', from the Latin word for a little room. He was actually looking at the dead cell walls of plant cells.

Later, Leeuwenhoek saw free-living cells in pond water, Robert Brown discovered the nucleus, and Purkinje coined the term 'protoplasm' for the living substance of the cell.

Schleiden and Schwann proposed the cell theory: all plants and animals are made of cells, and the cell is the basic unit of life. Virchow added that all cells arise from pre-existing cells. Organisms may be unicellular (a single cell does everything, e.g. Amoeba, Paramecium) or multicellular (many cells, each group specialised for a job).

A prokaryotic cell has no true, membrane-bound nucleus — its genetic material lies free in the cytoplasm in a region called the nucleoid. It also lacks membrane-bound organelles such as mitochondria and plastids. Bacteria and blue-green algae are prokaryotes.

A eukaryotic cell has a true nucleus enclosed by a nuclear membrane, and it has many membrane-bound organelles. All plants, animals, fungi, and protists are made of eukaryotic cells.

In both types the cell carries out the same basic activities, but the eukaryotic cell divides its work among organelles, which allows it to be larger and more efficient.

The plasma membrane (cell membrane) is the outermost covering of an animal cell and lies just inside the cell wall in a plant cell. It is made of lipids and proteins and is selectively permeable — it allows some substances to pass and holds others back.

Diffusion is the movement of a substance from a region of higher concentration to a region of lower concentration. Gases such as CO₂ and O₂ move in and out of the cell by diffusion.

Osmosis is the movement of water molecules through a selectively permeable membrane from a solution of lower solute concentration to one of higher solute concentration. In a hypotonic solution a cell gains water and swells; in a hypertonic solution it loses water and shrinks; in an isotonic solution there is no net change. Plasmolysis is the shrinking of the cell contents away from the cell wall when a plant cell loses water in a strong solution.

Plant cells have a rigid outer cell wall made mainly of cellulose, lying outside the plasma membrane. It gives the cell shape, strength, and protection, and lets plant cells withstand large changes in the surrounding medium.

When a plant cell is placed in a dilute solution it takes up water and becomes turgid, and the wall stops it from bursting. This turgidity provides support to soft plant parts. Animal cells have no cell wall.

Adjacent plant cells are connected through fine strands of cytoplasm called plasmodesmata, which pass through the walls.

Nucleus: bounded by a nuclear membrane with pores; contains chromatin (DNA + protein) that condenses into chromosomes during division; controls all cell activities and carries hereditary information. The nucleolus inside it makes ribosomes.

Endoplasmic reticulum (ER): a network of membranes; rough ER (with ribosomes) makes proteins, smooth ER makes fats and helps detoxify. Golgi apparatus: stacks of membranes that modify, package, and dispatch materials; also forms lysosomes. Ribosomes: sites of protein synthesis. Mitochondria: 'powerhouse of the cell', release energy as ATP by respiration; have their own DNA. Plastids (in plant cells): chloroplasts contain chlorophyll for photosynthesis; leucoplasts store starch, oils, proteins. Vacuole: fluid-filled sac; small in animal cells, a large central vacuole in plant cells provides turgidity and stores cell sap. Lysosomes: 'suicide bags', contain digestive enzymes that break down worn-out organelles and foreign material.