Cell: The Building Block of Life Class 9 Notes and Questions

These notes on Cell: The Building Block of Life for Class 9 take you through the chapter in order: what a cell is, how we see it, how things move in and out of it, what each organelle does, and how new cells are made. Each idea is kept short, and the important questions at the end come with model answers you can use for revision.

The cell: the fundamental unit of life

All living organisms are made of cells. The cell is the basic level at which life exists and works, so it is called the fundamental unit of structure and function.

Scientific evidence suggests that life began in water about 3.5 billion years ago. Early organic molecules may have been protected by calcium carbonate deposits around hot springs, such as those in Puga Valley, Ladakh. These deposits may have helped form the first protective membranes, the barrier that defines a cell.

• Unicellular organisms have a single cell, like bacteria or yeast.
• Multicellular organisms are made of millions of cells, like plants and humans.

In multicellular organisms there is a clear hierarchy: similar cells form tissues, different tissues form an organ, and several organs work together as an organ system. The nasal pores, trachea and lungs together form the respiratory system.

A handy picture: a cell is like a tiny living factory, where different parts do specific jobs such as making energy, removing waste and building new materials.

Seeing cells: microscopes and measurement

The human eye has a limit of resolution of about 0.1 mm. Two points closer than this look like one point. Most plant and animal cells are 10 to 100 micrometres (µm) across, and bacteria are even smaller, around 1 µm, so we need instruments to see them.

• Robert Hooke first observed cells in 1665, looking at cork through a microscope he designed himself.
• Light microscopes use visible light and lenses to magnify.
• Electron microscopes use beams of electrons and show details at the nanometre scale (one-billionth of a metre).

Total magnification = power of eyepiece × power of objective lens. A 10X eyepiece with a 40X objective gives 10 × 40 = 400X.

Estimating cell size: measure the diameter of the visible field in µm, then divide by the number of cells seen in a straight line across it. If the field is 5 mm (5000 µm) and 25 cells fit across it, one cell is about 5000 ÷ 25 = 200 µm.

The cell membrane and transport

The cell membrane (plasma membrane) is very thin, 7 to 10 nm. It is selectively permeable: it lets only certain substances through and blocks others.

It follows the fluid-mosaic model. It is a lipid bilayer (two layers of fat molecules) with proteins embedded in it. It is "fluid" because molecules can move sideways, and "mosaic" because the proteins sit like tiles. These proteins act as gatekeepers.

• Diffusion: net movement of particles from higher to lower concentration.
• Osmosis: diffusion of water across a selectively permeable membrane. Plant roots take in water from the soil by osmosis.

Potato osmosis experiment: Weight change = final weight − initial weight. A potato piece in plain water (hypotonic) gains water and its weight increases. A piece in 20% salt solution (hypertonic) loses water and its weight decreases.

The cell wall

Plants, fungi and bacteria have a rigid cell wall outside the cell membrane. In plants it is mainly made of cellulose, a complex carbohydrate. Unlike the membrane, the cell wall is permeable: water and dissolved minerals pass through easily.

The wall gives mechanical strength, helps plants stand up to wind and rain (they cannot move away), keeps the cell's shape, and stops the cell from bursting when it takes in a lot of water.

In a concentrated solution, a plant cell's contents shrink away from the wall. This is plasmolysis. The wall keeps the outer shape. Animal cells have no cell wall, so they are flexible, which helps animal tissues move. But it also means an animal cell, like a human cheek cell, shrinks completely when it loses water in a hypertonic solution.

Prokaryotic and eukaryotic cells

"Pro" means primitive and "karyon" means nucleus; "eu" means true. Bacteria are typical prokaryotes; their genetic material lies in an undefined region called the nucleoid. Plants and animals are eukaryotes, with a nucleus inside a membrane and a cytoskeleton (a network of fibres for support and transport). Prokaryotes carry out activities directly in the cytoplasm, while eukaryotes use organelles that work independently and at the same time.

Organelles and their jobs

• Nucleus: the "house of coded instructions". It has a double nuclear membrane with pores for exchanging materials. It holds the nucleolus (where ribosomes are made) and chromatin, which forms rod-shaped chromosomes during division. Genes are functional segments of DNA that carry hereditary information.
• Rough ER: has ribosomes; makes and secretes proteins.
• Smooth ER: no ribosomes; makes and stores lipids and some hormones. It does not make cellulose.
• Golgi apparatus: stacks of flat sacs; the cell's "post office" that packages materials into vesicles.
• Lysosomes: the "clean-up system"; their enzymes break down unwanted proteins, carbohydrates, fats and damaged parts.
• Mitochondria: the "powerhouses". The folded inner membrane (cristae) increases surface area for cellular respiration, which releases energy as ATP (adenosine triphosphate), the cell's energy currency.
• Plastids (plants only): chloroplasts (chlorophyll, photosynthesis), chromoplasts (yellow, orange or red pigments in flowers and fruits) and leucoplasts (colourless, store starch, oils or proteins, as in potato and taro).
• Vacuoles: mature plant cells have a large central vacuole of cell sap that gives turgidity; animal cells have small, temporary ones.

Mitochondria and plastids are special because they have their own DNA and ribosomes and can make some of their own proteins. Mature human red blood cells have no nucleus, leaving more room for haemoglobin to carry oxygen, but they live only about 120 days.

Cell division, cell theory and growth control

Mitosis is used for growth and repair. It gives two daughter cells that are genetically identical to the parent, with the same chromosome number.

Meiosis is a two-step division for sexual reproduction. The parent cell divides twice to give four gametes with half the chromosome number. In plants it happens in the anthers (forming pollen grains, which later produce sperm cells) and in the ovaries (producing egg cells).

The Cell Theory came from Schleiden (1838), Schwann (1839) and Virchow (1855): all organisms are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells. Gottlieb Haberlandt proposed totipotency, the ability of any living plant cell to grow into a complete plant under suitable conditions. This idea laid the foundation for plant tissue culture.

Normal animal cells show contact inhibition: they stop dividing when they touch their neighbours. Cancer cells lose this control and form tumours. Programmed Cell Death (PCD) removes chosen cells, for example the cells between fingers and toes, so hands are not webbed.

Remember this

• Cell → tissue → organ → organ system.
• Eye's limit of resolution: about 0.1 mm.
• Total magnification = eyepiece power × objective power.
• Membrane: selectively permeable. Cell wall: permeable.
• Hypotonic: cell swells. Hypertonic: cell shrinks.
• Mitosis: 2 identical cells. Meiosis: 4 gametes with half the chromosomes.

Important questions with answers

1. What is the hierarchy from a cell to an organ system? Similar cells form tissues, different tissues form an organ, and several organs work together as an organ system.

2. A microscope has a 15X eyepiece and a 10X objective. What is the total magnification? 15 × 10 = 150X.

3. A field of view is 4 mm wide and 20 cells fit across it. Estimate the size of one cell. 4 mm = 4000 µm. Size = 4000 ÷ 20 = 200 µm.

4. Why is the cell membrane called fluid-mosaic? Its molecules can move sideways (fluid), and its proteins are set in the lipid bilayer like tiles (mosaic).

5. Why does a plant cell keep its shape in a concentrated sugar solution? Its contents shrink away from the wall as water leaves (plasmolysis), but the rigid cell wall keeps the outer shape.

6. What is a nucleoid? The region in a prokaryotic cell where the genetic material lies, not enclosed by a nuclear membrane.

7. Give the functions of smooth ER. It makes and stores lipids and some hormones. It does not make cellulose.

8. Why is meiosis needed for sexual reproduction? It halves the chromosome number in gametes, so the original number is restored when two gametes combine at fertilisation.

9. How does PCD help an embryo develop? It removes specific cells, such as those between the digits, so fingers and toes are shaped properly.

Common mistakes to avoid

• Calling the cell wall selectively permeable. It is the membrane that is selectively permeable; the wall is permeable.
• Mixing up hypotonic and hypertonic. "Hypo" outside means less solute outside, so water enters and the cell swells.
• Adding the eyepiece and objective powers instead of multiplying them.
• Forgetting to convert mm to µm before dividing in cell-size questions.
• Writing that smooth ER makes cellulose.

When you are ready to test yourself on each organelle, you can study this chapter with Joy.