CLASS 9 SCIENCE · CHAPTER 2 · STUDY NOTES
Cell: The Building Block of Life
Explore a cell, follow its materials and see how it divides. Short explanations, NCERT pictures and original visual models help you understand each idea before you check yourself.
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One cell can be a whole living thing
A cell is the smallest unit that carries out the basic activities of life.One cell = one organism
A bacterium does all its life processes in a single cell.
- CellOne working unit
- TissueCells with related jobs
- OrganDifferent tissues together, such as a lung
- Organ systemOrgans working together, such as the respiratory system
- OrganismThe whole living thing
This second path describes a multicellular body.
- Cell
- The basic structural and functional unit of a living organism.
- Unicellular / multicellular
- Made of one cell / made of many cells.
A bacterium lives as one cell. A plant or a person has many cells that work together. Both need to take in materials, release usable energy, remove wastes and respond to their surroundings.
In a multicellular body, cells with related jobs form tissues. Tissues work together in an organ, such as a leaf or a lung. Organs work together in an organ system. All these systems belong to one organism.
Let’s look a little closer.
Go deeper: Where did the first cells come from?
Evidence points to very ancient life in water. Scientists investigate how non-living molecules could have formed organised, membrane-bound systems. This question is still being studied.
The chapter describes hot springs in Puga Valley, Ladakh, as one setting that may help scientists study early-life conditions. Mineral deposits might protect molecules. This is a possible explanation to investigate, not proof that life began at that place.
Take a moment. What do you think?
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Question
Is yeast unicellular or multicellular? Does one yeast cell still carry out life processes?
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Make a tiny cell visible
A microscope makes an image larger. Good resolution lets you tell nearby details apart.Estimate one cell’s width
How many cells fit in a row across the field?
- Magnification
- How many times larger the image appears than the object.
- Resolution
- The ability to show two close points as separate points.
- Contrast
- A visible difference between an object and its background.
Robert Hooke saw small compartments in thin cork in 1665 and called them cells. Cork was dead plant tissue: he mainly saw the remaining walls. Better microscopes later revealed living cells and their contents.
A light microscope uses lenses. Multiply the eyepiece magnification by the objective magnification. Staining can make faint details easier to see. An electron microscope can show much finer details than a light microscope.
Estimate the width of a cell
Original example: 20 cells fit across a 2 mm field of view.
Cell width ≈ 2 ÷ 20 = 0.1 mm = 100 µmFirst divide the field width by the number of cells. Then use 1 mm = 1000 µm. This is an estimate: the cells must be lined up with little gap.
Go deeper: Size is not the same as clarity
The unaided eye cannot usually separate points closer than about 0.1 mm at a viewing distance of 25 cm. Magnification enlarges an image; resolution determines which details are distinguishable. Contrast makes those details stand out.
Go deeper: Why a microscope slide needs a thin sample
Light must pass through a thin sample to reveal individual cells clearly. A cover slip keeps it flat; a suitable stain gives faint structures contrast. Onion epidermis shows a regular pattern of cell walls. Cheek cells have membranes but no cell walls. Prepare and handle slides only as your teacher directs.
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Question
An eyepiece is 10× and the objective is 40×. What is the total magnification? Does making a blurry image larger add new detail?
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A thin boundary with a selective gate
Every cell has a plasma membrane. It controls exchange with the surroundings.- Plasma membrane
- A thin, flexible boundary that separates the cell contents from the surroundings.
- Selectively permeable
- Lets some substances cross more easily than others.
The membrane has a double layer of lipids with proteins in it. The water-loving heads face the watery surroundings and cytoplasm. The water-avoiding tails face inward, towards each other.
The membrane is not an open sieve. Small molecules may pass through the lipid layer, while other substances need particular proteins. The membrane helps the cell keep a suitable internal environment.
Go deeper: A flexible boundary
Many lipids and proteins can move sideways within the membrane. This makes it fluid and flexible. “Selectively permeable” does not mean the cell can choose by thinking; passage depends on molecular properties and transport mechanisms.
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Question
Why do the lipid tails face inward rather than towards the water around the cell?
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Particles spread out by diffusion
Particles keep moving. When one region has more, there is usually a net movement towards the region with fewer.More particles start at the left. Random movement produces an overall spread towards less concentrated regions.
- Diffusion
- Net movement of particles from a region of higher concentration to a region of lower concentration.
- Concentration
- How much of a substance is present in a given space.
Imagine a drop of dye spreading through still water. Individual particles move in many directions. Because more particles start near the drop, the overall movement spreads the dye out.
Diffusion can happen without a membrane. In cells, gases such as oxygen and carbon dioxide can cross membranes by diffusion when a concentration difference exists.
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Question
When the dye is evenly spread, have its particles stopped moving?
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Water moves across a selective membrane
Compare the solutions on the two sides, then predict the net movement of water.Water crosses both ways at equal overall rates. The cell keeps roughly the same size.
- Osmosis
- Net movement of water through a selectively permeable membrane towards the side with a higher concentration of dissolved solute.
- Isotonic
- The outside solution has the same effective solute concentration as the cell contents.
For these simple examples, the membrane lets water cross but keeps the dissolved solute on its own side. A dilute outside solution gives a net movement of water into the cell. A more concentrated outside solution gives a net movement out.
“Hypotonic” and “hypertonic” compare the outside solution with the cell contents. Hypotonic outside: water enters. Hypertonic outside: water leaves. Isotonic outside: water still crosses both ways, but there is no net movement.
Go deeper: Read the potato investigation fairly
Use pieces with the same initial size, from comparable potato tissue, for the same time and temperature. Compare their initial and final mass or firmness. Plain water and a concentrated salt or sugar solution produce different net water movements.
Water entering cells can make a piece firm; water leaving can make it limp. To support a conclusion, measure the change rather than judging only by appearance. Follow the teacher’s instructions when cutting or preparing samples.
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Question
Why does an animal cell shrink in a solution that is more concentrated than its contents?
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A plant cell has a wall and a membrane
The firm outer wall supports the cell. The thinner inner membrane controls exchange.- Cell wall
- A firm layer outside the plasma membrane; in plants it is mainly made of cellulose.
- Plasmolysis
- The cell contents and membrane pull away from the wall after a plant cell loses water.
Water entering a plant cell fills its vacuole and pushes the contents against the wall. The wall resists expansion, so the cell becomes firm, or turgid. An animal cell has no cell wall and may burst if too much water enters.
In a concentrated outside solution, water leaves a plant cell. Its contents shrink and the membrane may pull away from the wall. The wall itself stays in place. This separation is plasmolysis.
Go deeper: Not all walls have the same material
Many bacteria and fungi also have cell walls, but their wall material differs from a plant’s cellulose wall. The wall is generally permeable to many substances; the plasma membrane is the main selective boundary.
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Question
During plasmolysis, does the cell wall shrink along with the membrane?
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Look for a membrane-bound nucleus
Plant and animal cells are eukaryotic. A bacterium is prokaryotic.Bacterium
DNA in nucleoid · ribosomes · no membrane-bound nucleus
Green plant cell
Wall + membrane · large vacuole · chloroplasts
Animal cell
Membrane · nucleus · no cell wall
Nucleus: Keeps the DNA instructions inside a membrane-bound compartment. The selected structure glows in the animal-cell model.
- Prokaryotic cell
- A cell without a membrane-bound nucleus or other membrane-bound organelles.
- Eukaryotic cell
- A cell with a membrane-bound nucleus and membrane-bound organelles.
- Nucleoid
- The region containing the main DNA of a prokaryotic cell.
- Cytoplasm
- The material inside the membrane, excluding the nucleus in eukaryotic cells. It contains organelles, and many reactions happen here.
A bacterium still has a membrane, cytoplasm, DNA and ribosomes. Its DNA lies in a nucleoid rather than inside a nucleus. Most bacterial cells are smaller than most plant and animal cells.
Plant and animal cells both have a nucleus, mitochondria, ER and other organelles. A typical plant cell also has a cell wall, a large central vacuole and plastids. Chloroplasts occur in green, photosynthesising cells; do not add them to every plant cell.
Go deeper: Useful size ranges, not definitions
Typical prokaryotic cells are about 1–10 µm and typical eukaryotic cells about 10–100 µm. There are exceptions, so size alone is not a reliable classification.
Viruses, viroids and prions are not cells. A virus depends on a host cell to reproduce; it is not a very small bacterium.
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Question
A yeast cell lives alone but has a membrane-bound nucleus. Is it prokaryotic?
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Keep the cell’s instructions organised
The nucleus contains DNA. Genes are stretches of DNA that carry information for particular products and functions.- Chromatin
- DNA and associated proteins in the nucleus.
- Chromosome
- An organised DNA–protein structure; it becomes compact and easy to see during cell division.
- Gene
- A segment of DNA that carries information for a functional product.
The nucleus has a double membrane with pores for exchange. Its DNA provides instructions that help control cell activities. The nucleolus helps make ribosomal components.
DNA is not separate from chromosomes: it is part of them. Before a cell divides, it copies its DNA. Chromatin then becomes compact chromosomes so the copied material can be separated.
Go deeper: A useful exception
A mature human red blood cell has no nucleus and normally survives for about 120 days. This specialised exception does not mean that animal cells in general lack nuclei.
Go deeper: An Indian contribution
Arun Kumar Sharma made important contributions to the study of chromosomes and plant cell biology. His work reminds us that scientific knowledge is built by researchers in many places.
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Question
Are DNA, genes and chromosomes three unrelated substances?
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Follow a protein from making to delivery
Ribosomes make proteins. ER and Golgi help process and move many of them.Ribosomes on rough ER: A protein for secretion is made and enters the rough ER.
- Ribosome
- A structure that joins amino acids to make proteins.
- Rough / smooth ER
- Rough ER has ribosomes on its surface; smooth ER does not.
- Golgi apparatus
- Membrane sacs that modify, sort and package materials.
Many proteins for secretion or membranes are made by ribosomes on rough ER. They enter the ER, travel in small membrane vesicles to the Golgi, then move in new vesicles towards their destination.
Smooth ER helps make lipids. In some cells it also helps make steroid hormones or remove harmful substances. Ribosomes in the cytoplasm make other proteins; not every protein follows the secretion route.
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Question
Put these in order for a protein that is secreted: Golgi, rough ER, vesicle at the cell membrane.
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