Cell: Notes

Cells were seen only after microscopes were made. Robert Hooke (1665) looked at thin cork and named the box-like compartments cells. Antonie van Leeuwenhoek (1670s) saw living single cells in pond water. Schleiden (1838, plants) and Schwann (1839, animals) said that organisms are made of cells, and Virchow (1855) said that new cells come only from existing cells.

The cell theory: (1) all organisms are made of one or more cells; (2) the cell is the basic unit of structure and function; (3) all cells arise from pre-existing cells.

Cells are measured in micrometres: $1\ \text{mm} = 1000\ \mu\text{m}$. Magnification $= \dfrac{\text{size of drawing}}{\text{actual size}}$.

Both kinds have a cell membrane, cytoplasm, a nucleus, mitochondria, ER and Golgi apparatus. A plant cell also has a cellulose cell wall, a large central vacuole and plastids. An animal cell has none of these three, and its vacuoles are small.

Plants store food as starch and animals as glycogen. The cell wall gives the plant cell a fixed shape and lets it become firm (turgid) when it takes in water.

In a prokaryotic cell the DNA lies in the nucleoid, not enclosed by a nuclear membrane, and there are no membrane-bound organelles. Bacteria and cyanobacteria are prokaryotes. They are usually about 1 to 5 micrometres long.

A eukaryotic cell has a true nucleus with a nuclear membrane and organelles with membranes. Amoeba, yeast, plants, animals and fungi are eukaryotes, and their cells are usually 10 to 100 micrometres across.

• Nucleus: double membrane with pores; holds DNA in chromosomes; controls the cell; has a nucleolus.
• Mitochondria: double membrane, inner folds; release energy from food in respiration (powerhouses).
• Endoplasmic reticulum: network of membranes. Rough ER has ribosomes and makes proteins; smooth ER makes lipids and helps remove harmful substances.
• Golgi apparatus: stacked sacs that modify, pack and dispatch products in vesicles.
• Lysosomes: sacs of digestive enzymes; break down worn-out parts and germs.
• Vacuoles: store water, food and wastes; the large plant vacuole keeps the cell firm.
• Plastids: chloroplasts (green, photosynthesis), chromoplasts (colours), leucoplasts (store starch, oil, protein).
• Cell wall: cellulose, in plants; shape and protection, fully permeable.

The plasma membrane is selectively permeable. Diffusion is the net movement of particles from higher to lower concentration. Osmosis is the net movement of water through a selectively permeable membrane towards the region of more dissolved substance.

Activities: a shell-less egg swells in pure water and shrinks in strong sugar solution; dry raisins swell in water; in a potato osmometer the level rises as water enters the sugar solution in the cavity.

In a hypotonic solution a cell gains water (endosmosis) and a plant cell becomes turgid. In a hypertonic solution it loses water (exosmosis), and a plant cell shows plasmolysis. In an isotonic solution there is no net movement.

Mitosis gives two identical cells with the same chromosome number: growth, repair and replacement. Stages: prophase, metaphase, anaphase, telophase, then the cytoplasm divides. Meiosis happens in reproductive organs: two divisions give four gametes with half the chromosome number; fertilisation restores it. A cell with $2n$ chromosomes gives gametes with $n = 2n \div 2$.

Recent advances (textbook level): stem cells can divide and become specialised cells, and bone marrow transplants are an established use, while many other uses are still being researched. Fluorescence microscopes light up chosen parts of a cell with dyes. Electron microscopes show structures too small for light microscopes. Cell culture grows cells in a dish for testing medicines.