Discovery and the cell theory
Robert Hooke (1665) saw compartments in cork and named them cells.
Leeuwenhoek (1674) first saw free-living cells in pond water with an improved microscope.
Robert Brown (1831) discovered the nucleus.
Purkinje (1839) coined the term protoplasm.
Schleiden (1838) and Schwann (1839) proposed the cell theory: all plants and animals are made of cells, and the cell is the basic unit of life. Virchow (1855) added that all cells arise from pre-existing cells.
Prokaryotes and eukaryotes
Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
Nucleus | not enclosed by a membrane; genetic material lies in a region called the nucleoid | well-defined, enclosed by a nuclear membrane |
Membrane-bound organelles | absent | present (mitochondria, plastids, etc.) |
Size | generally small (1–10 μm) | generally larger (5–100 μm) |
Examples | bacteria, blue-green algae | plants, animals, fungi |
The plasma membrane
The plasma membrane (cell membrane) is the outermost covering of an animal cell. It is selectively permeable: it lets some substances in and out but not others. It is made of lipids and proteins and is flexible, which lets cells such as amoeba engulf food (endocytosis).
Diffusion is the movement of a substance from a region of higher concentration to one of lower concentration. Carbon dioxide made in the cell diffuses out; oxygen diffuses in.
Osmosis is the movement of water through a selectively permeable membrane from a region of higher water concentration (dilute solution) to one of lower water concentration (concentrated solution).
Surrounding solution | Effect on an animal cell |
|---|---|
Hypotonic (more dilute than the cell) | water enters; the cell swells and may burst |
Isotonic (same concentration) | no net movement; the cell keeps its size |
Hypertonic (more concentrated) | water leaves; the cell shrinks |
Raisins swell in water and dry fruit pickled in sugar shrinks: both are osmosis. Unicellular freshwater organisms and plant roots take in water by osmosis.
The cell wall
Plant cells have a rigid cell wall outside the membrane, made mainly of cellulose. It lets plant cells survive in dilute surroundings without bursting: water entering by osmosis pushes against the wall, keeping the cell firm (turgid).
When a living plant cell loses water in a hypertonic solution, its contents shrink away from the wall. This is plasmolysis.
The nucleus
The nucleus has a double-layered nuclear membrane with pores. Inside are chromosomes, visible only when the cell divides, made of DNA and protein. DNA carries the information for making and organising cells. Functional segments of DNA are genes. In a non-dividing cell, the DNA appears as a tangled mass called chromatin.
The nucleus controls the cell's activities and plays the central role in cell division and inheritance.
Organelles in the cytoplasm
Organelle | Structure | Function |
|---|---|---|
Endoplasmic reticulum (ER) | network of membrane tubes and sheets | Rough ER has ribosomes and makes proteins; smooth ER makes fats (lipids) and detoxifies poisons in liver cells; both transport materials |
Ribosomes | tiny particles, on the rough ER or free | site of protein synthesis |
Golgi apparatus | stacks of flattened sacs (cisterns) | stores, modifies and packages products and sends them where needed; forms lysosomes |
Lysosomes | membrane bags of digestive enzymes | break down worn-out parts and foreign material; called suicide bags because they can digest the cell itself if it is damaged |
Mitochondria | double membrane; inner one deeply folded | release energy as ATP (adenosine triphosphate) during respiration; the powerhouse of the cell; have their own DNA and ribosomes |
Plastids (plants only) | double membrane | chloroplasts contain chlorophyll for photosynthesis; leucoplasts store starch, oils and proteins; also have their own DNA and ribosomes |
Vacuoles | membrane-bound sacs | store water, wastes and other materials; in plant cells, a large central vacuole (up to 50–90% of the cell volume) keeps the cell turgid |
The cell membrane and the membranes of the ER are made together by the ER, a process called membrane biogenesis.
Plant cell and animal cell
Plant cell | Animal cell | |
|---|---|---|
Cell wall | present | absent |
Plastids | present | absent |
Vacuole | large, central | small, many or absent |
Centrosome | absent in most | present |
Cell division
New cells are formed for growth, to replace old and injured cells, and to make gametes.
Mitosis: one cell divides into two identical daughter cells, each with the same number of chromosomes as the mother cell. It is used for growth and repair.
Meiosis: occurs in reproductive organs to form gametes. It gives four cells, each with half the number of chromosomes.
Summary
All living things are made of cells, and all cells arise from pre-existing cells.
The plasma membrane is selectively permeable; water moves by osmosis.
Mitochondria release energy, ribosomes make proteins, the Golgi packages, lysosomes digest.
Plant cells have a cell wall, plastids and a large vacuole.