Class 9 Science — Chapter 2: Cell: The Building Block of Life

All living things are made of cells. Some organisms consist of just one cell, while others contain many cells that work together. This chapter explores what cells are, how we see them and how their parts help them stay alive.

1. Cells and Living Organisation

1.1 The Cell

Cells take in materials, use energy, respond to their surroundings and make more cells. A cell may live on its own, or it may work as part of a larger organism.

For example, tiny single-celled thermophiles live in the hot springs of Puga Valley in Ladakh. Their survival shows that living cells can adapt to conditions that would be too hot for many other organisms.

1.2 From Cells to Organ Systems

That one cell performs all the tasks needed for life. Many bacteria and the amoeba are unicellular organisms.

In a human, for example, muscle cells help produce movement, while nerve cells carry signals. Similar cells can work together in larger levels of organisation.

Muscle tissue is made of cells that contract and help move body parts.

The stomach is an organ: its tissues work together to store and digest food.

The digestive system includes organs such as the mouth, stomach and intestines, which work together to process food.

In many multicellular organisms, the organisation can be followed from smaller to larger levels: cells → tissues → organs → organ systems → organism.

2. Seeing Cells

2.1 Why We Need Microscopes

Most cells are much smaller than this, so two nearby parts of a cell would blur together if we looked with the unaided eye. A microscope uses lenses to make small objects easier to see and examine.

A light microscope uses visible light and glass lenses. An electron microscope uses a beam of electrons and can reveal much finer details, including structures too small to resolve with an ordinary light microscope.

2.2 Parts of a Light Microscope

• Eyepiece: the lens you look through.
• Body tube: keeps the eyepiece and objective lenses at the correct distance.
• Objective lenses: lenses close to the specimen that provide different magnifications.
• Stage: the platform that holds the slide.
• Mirror or light source: directs light through the specimen so it can be seen.
• Coarse adjustment knob: moves the tube or stage more quickly to bring the image roughly into focus.
• Fine adjustment knob: makes small movements to sharpen the focus.
• Arm or handle: supports the upper part of the microscope and is used to carry it safely.
• Base: supports the microscope on the table.

2.3 Magnification, Resolution and Contrast

For instance, an image magnified 100 times appears 100 times larger in each measured dimension. Greater magnification alone does not always reveal more detail.

When resolution is poor, nearby structures appear as one blurred spot, even if the image is made larger.

Good contrast makes a cell part easier to distinguish from the rest of the image. Stains can increase contrast by colouring particular structures.

3. Cell Boundaries and Transport

3.1 Cell Membrane

Every cell has a cell membrane. It is selectively permeable: some substances pass through it more easily than others. This helps the cell take in useful materials and control the movement of water and dissolved substances.

3.2 Diffusion and Osmosis

Particles move randomly in all directions, but more particles leave the crowded region than return to it. So, overall, they spread out. A smell spreading through a room is an everyday example. Diffusion does not require a membrane.

Water can move into or out of a cell by osmosis. The direction depends on the water concentration on each side of the membrane.

In a simple potato investigation, a hollow in a potato is filled with concentrated sugar solution and the potato is placed in water. Water moves through the potato cells towards the sugar solution, so its level rises. This demonstrates osmosis.

3.3 Isotonic, Hypotonic and Hypertonic Solutions

In a hypotonic solution, an animal cell may swell and can burst if too much water enters. A plant cell becomes firm as water fills its large vacuole; its cell wall helps prevent it from bursting. In a hypertonic solution, a plant cell loses water and its inner contents shrink away from the wall.

3.4 Fluid Mosaic Model

The lipid layer can move, and many of its proteins can move within it. The proteins perform different jobs, including helping particular substances cross the membrane. The mixture of moving parts gives the membrane its mosaic-like pattern.

3.5 Cell Wall

Plant cell walls are mainly made of cellulose. Fungi and bacteria also have cell walls, though their wall materials differ. The wall helps a plant cell keep its shape and resists the pressure created when water enters. Animal cells do not have cell walls.

4. Cell Types and Organelles

4.1 Basic Parts of a Cell

A typical plant or animal cell has a cell membrane, cytoplasm and a nucleus. Bacterial cells have a membrane and cytoplasm too, but their internal organisation is different.

Different organelles perform different tasks, much as different parts of a workshop handle different steps in making a product.

4.2 Prokaryotic and Eukaryotic Cells

Bacteria are prokaryotes. Their DNA is found in a region of the cytoplasm rather than inside a nucleus.

Plants, animals, fungi and many single-celled organisms have eukaryotic cells. Prokaryotic cells are generally smaller, often about 1–10 μm across; eukaryotic cells are often about 10–100 μm across. These are typical ranges, not rules for every cell.

4.3 Nucleus and Nucleoid

A double membrane with pores surrounds the nucleus. The pores allow selected materials to move between the nucleus and cytoplasm. Inside, DNA carries instructions used by the cell. A gene is a section of DNA that contains instructions for a particular product or function. The nucleolus makes parts that are used to build ribosomes. Before cell division, the loose DNA material becomes more tightly packed into visible chromosomes.

The DNA in a typical bacterial nucleoid is arranged mainly as a circular molecule. The nucleoid is not a nucleus or a membrane-bound organelle.

4.4 Ribosomes, Endoplasmic Reticulum and Golgi Apparatus

Proteins do many jobs, including building cell structures and helping chemical reactions happen. Ribosomes are found in both prokaryotic and eukaryotic cells.

Rough ER has ribosomes attached and helps make and process proteins. Smooth ER has no ribosomes attached and helps make lipids and process some substances.

The Golgi apparatus packs materials into small membrane sacs called vesicles. Some vesicles carry materials to other parts of the cell; others release them outside the cell.

4.5 Lysosomes and Mitochondria

The smaller products can be reused by the cell. Lysosomes are especially common in animal cells.

Cells use this energy to carry out their activities. Mitochondria have an outer membrane and a folded inner membrane. The folds provide more surface area for reactions involved in respiration.

Mitochondria also contain a small amount of their own DNA and ribosomes.

4.6 Plastids

Plastids have a double membrane and contain their own DNA and ribosomes. Different kinds of plastids have different roles.

• Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis, using light energy to help make sugars. The fluid inside a chloroplast is called stroma.
• Chromoplasts contain coloured pigments that give many flowers and fruits their yellow, orange or red colour.

For example, starch-storing leucoplasts are common in potato cells. Plastids can change from one type to another as a plant part develops; a green fruit may become coloured as it ripens.

4.7 Vacuoles

Plant cells commonly have one large central vacuole. It stores dissolved materials and helps keep the cell firm. Animal cells may have smaller, temporary vacuoles.

Cell sap may contain water, sugars, salts, pigments and waste substances.

4.8 Cytoskeleton and Cell Inclusions

Cells may also contain stored materials that are not membrane-bound organelles. Examples include starch grains in plant cells and crystals of calcium oxalate or silica in some plants.

5. Cell Division and Cell Theory

5.1 Cell Division

Cell division allows organisms to grow, replace worn-out cells and, in some organisms, reproduce. Before a cell divides, it copies its DNA so the new cells can receive genetic information.

Mitosis is used for growth and for replacing cells. For example, cells near the tip of a growing plant root divide to make new cells.

In animals, meiosis makes reproductive cells such as sperm and eggs. The cells formed are genetically different from one another. The detailed stages of these divisions are studied later; here, remember their different outcomes.

5.2 Cell Culture and Totipotency

Scientists maintain the right temperature, acidity, moisture and nutrients so the cells can survive and multiply. Cell cultures help researchers study cells and produce useful substances, including some medicines and vaccines.

This idea made plant tissue culture possible. For example, a small piece of plant tissue can be grown in a nutrient medium to produce many new plants.

5.3 Cell Theory

Scientists built this idea over time. Matthias Schleiden concluded that plants are made of cells; Theodor Schwann extended the idea to animals; and Rudolf Virchow stated that new cells arise from cells that already exist.

• All living organisms are made of one or more cells.
• The cell is the basic structural and functional unit of life: it forms living bodies and carries out life processes.
• New cells arise from pre-existing cells.

5.4 Contact Inhibition

This helps keep tissues organised. Some cancer cells do not respond normally to these signals, so they may keep dividing and form a mass called a tumour. Plant cells generally do not show contact inhibition in the same way because their rigid walls limit how closely they can pack.

6. Extra Connections

6.1 Viruses, Viroids and Prions

These infectious agents are not made of cells, so they are described as acellular. They cannot carry out all life processes independently and rely on living cells in different ways.

These three examples differ in what they are made of: a virus has genetic material and a protein coat, a viroid is genetic material without a coat, and a prion is protein only.

6.2 Programmed Cell Death

It helps shape developing bodies and remove damaged or unneeded cells.

7. Chapter Summary

• A cell is the smallest unit that carries out the basic processes of life. Organisms may be unicellular or multicellular.
• Cells can form tissues, organs and organ systems. Microscopes reveal cells that the eye cannot resolve.
• Magnification enlarges an image; resolution separates nearby details; contrast makes structures stand out.
• The cell membrane controls exchange with the surroundings. Diffusion moves particles down a concentration difference; osmosis is water movement through a selectively permeable membrane.
• Prokaryotic cells have a nucleoid but no membrane-enclosed nucleus. Eukaryotic cells have a nucleus and membrane-bound organelles.
• Organelles divide cell work: ribosomes make proteins, ER and Golgi process and move materials, lysosomes break down materials, and mitochondria release energy from food.
• Plant cells may have a cell wall, plastids and a large vacuole. Plastids make or store food and pigments.
• Mitosis makes two cells with the same chromosome number; meiosis makes four cells with half the chromosome number.
• Cell theory describes the shared cellular basis of life. Cell culture and totipotency are useful in research and plant propagation.