Class 9 Science Chapter 2: Cell: The Building Block of Life — Notes, Examples & Practice

In 1665 Robert Hooke looked at a thin slice of cork through a simple microscope and saw tiny box-like compartments, which he called cells. He was actually seeing the empty walls of dead cells. A few years later Antonie van Leeuwenhoek, with better lenses, saw living single-celled organisms in pond water.

Later scientists built the cell theory: (1) all living organisms are made of one or more cells; (2) the cell is the basic unit of structure and function of life; (3) new cells come only from existing cells (Rudolf Virchow, 1855).

Better microscopes — and later the electron microscope — revealed structures inside cells, showing that a cell is not just a bag of jelly but a highly organised system.

Unicellular organisms (Amoeba, Paramecium, bacteria, yeast) are a single cell that does everything — feeding, moving, reproducing. Multicellular organisms have many cells that divide the work: nerve cells carry messages, red blood cells carry oxygen, muscle cells contract.

Prokaryotic cells (bacteria) have no nuclear membrane — their genetic material lies in the cytoplasm in a region called the nucleoid — and they lack membrane-bound organelles such as mitochondria. Eukaryotic cells (plants, animals, fungi, protists) have a true nucleus enclosed by a membrane and many membrane-bound organelles.

Shape and size suit function: nerve cells are long to carry signals over distances; red blood cells are disc-shaped and flexible to squeeze through capillaries. 'Structure is connected with function' is a theme of the whole biology unit.

Every cell is surrounded by a thin, flexible plasma membrane (cell membrane) made of lipids and proteins. It is selectively permeable: it lets some substances (water, oxygen, carbon dioxide) pass easily and controls the movement of others.

Plant cells (and fungi and bacteria) also have a cell wall outside the membrane. In plants it is made mainly of cellulose, which gives strength and shape and stops the cell from bursting when it takes in a lot of water. The cell wall is fully permeable — it is a support, not a filter.

The flexibility of the animal cell membrane lets cells change shape — an Amoeba engulfs food by endocytosis, wrapping its membrane around the particle.

The nucleus is the control centre. It is enclosed by a double nuclear membrane with pores. Inside are chromosomes, made of DNA and proteins. DNA carries the instructions (genes) for making proteins and for passing characteristics to the next generation. When the cell is not dividing, the chromosomes are spread out as a network called chromatin. A darker region, the nucleolus, makes ribosomes.

The cytoplasm is the jelly-like fluid between the membrane and the nucleus. The organelles float in it, and many chemical reactions of the cell happen there.

Protoplasm is the cytoplasm plus the nucleus — the whole living content of the cell.

Organelles work together like departments of a factory. Mitochondria release energy from food by respiration and store it as ATP — they are the 'powerhouses'. Ribosomes make proteins. The endoplasmic reticulum (ER) is a network of membranes: rough ER (with ribosomes) helps make and transport proteins; smooth ER makes lipids and helps detoxify poisons.

The Golgi apparatus packages and dispatches materials made in the ER, and helps form lysosomes. Lysosomes contain digestive enzymes that break down worn-out parts and foreign material; if a cell is damaged they may burst and digest it, so they are called 'suicide bags'.

Plastids are found only in plant cells: chloroplasts contain chlorophyll for photosynthesis; leucoplasts store starch, oil or protein. Vacuoles store water, food and wastes; plant cells usually have one large central vacuole that keeps the cell firm (turgid). Mitochondria and plastids have their own DNA and ribosomes.

Diffusion is the spreading of particles from a region of higher concentration to lower concentration because of their random motion. Oxygen diffuses into cells and carbon dioxide diffuses out this way; no energy from the cell is needed.

Osmosis is the movement of water across a selectively permeable membrane from a region of higher water concentration (a dilute solution) to lower water concentration (a concentrated solution).

Place a cell in a hypotonic solution (more dilute than the cell) and water enters — the cell swells; an animal cell may burst, but a plant cell becomes firm (turgid) as its wall resists. In a hypertonic solution (more concentrated) water leaves — the cell shrinks; in plant cells the membrane pulls away from the wall (plasmolysis). In an isotonic solution there is no net movement.