1. Where did the first cell come from?

Life almost certainly began in water. But not necessarily in the deep ocean — many researchers now think the real cradle was a small pool whose conditions kept changing: heating, cooling, drying, filling again. That constant churn is what pushes simple chemicals into becoming something complicated.

Hot springs are living examples of such places. In India, the Puga Valley hot springs in Ladakh stay close to the boiling point of water even though the surrounding climate is freezing. Conditions there resemble early Earth roughly 3.5 billion years ago.

Scientists at the Birbal Sahni Institute of Palaeosciences, Lucknow studied these springs and found calcium carbonate forming rapidly around them. Those crusty deposits may have done two important jobs:

• Shielded fragile early organic molecules from harmful radiation and extreme conditions.
• Helped form the first protective membrane — the boundary that turns a soup of chemicals into a cell.

One cell or many?

The ladder of organisation

2. How do we study cells?

Hold up two dots drawn close together on paper and slowly bring them nearer. At some point your eye gives up and sees one dot instead of two. That breaking point has a name.

Most cells are far smaller than 0.1 mm. So how did biologists ever see them? Answer: lenses. A convex lens — or better, a combination of an objective lens and an eyepiece — magnifies the object until it crosses our resolution limit.

Robert Hooke, 1665

Hooke built his own microscope (roughly 200–300X) and looked at a thin slice of cork. He saw tiny box-like compartments and named them ‘cells’ — from the Latin for small rooms. He was actually looking at dead cell walls, but the name stuck forever.

Fig. 2.2 — Structure of a light microscope

Fig. 2.4 — SEM image: stomata on a Colocasia leaf

Who can see what?

Total magnification

Total magnification = eyepiece power × objective power. If both are 10X, total = 100X. So a 200 µm onion cell will appear 100 times larger — but its real size is still 200 µm. Magnification changes appearance, not reality.

Electron microscope

Instead of light, it fires a beam of electrons. This gives magnified images sharp enough to see structure at the nanometre scale (1 nm = one-billionth of a metre). A Scanning Electron Microscope image of a Colocasia leaf’s lower surface, for example, shows the stomata beautifully.

3. The cell membrane — the one thing every cell has

A cell has to talk to the world: take in what it needs, throw out what it doesn’t, and still stay itself. All of that happens at its boundary — the cell membrane, also called the plasma membrane.

The fluid-mosaic model

Structurally the membrane is astonishingly thin: only 7 to 10 nanometres (1 nm = 0.000001 mm). It is built from lipids (fats) and proteins, arranged as described by the fluid-mosaic model.

Why “fluid”? Why “mosaic”?

• Fluid — the molecules are not nailed down. They can move sideways, flip and rotate within the membrane.
• Mosaic — the proteins sit scattered among the lipids like tiles in a mosaic pattern.
• Proteins act as gatekeepers, helping selected substances pass through.

4. Osmosis, diffusion & tonicity

The experimental set-up: Beaker A and Beaker B

Initial state of both potato pieces

Final state: (a) swollen in water, (b) shrunken in salt

The difference in concentration that drives all this is called a concentration gradient. In plants, this is exactly how water from the soil enters root cells.

What if a cell is put in different solutions?

5. The cell wall — armour for cells that can’t run away

Animals escape trouble by moving. Plants can’t. Fixed in one place, a plant must simply endure wind, rain and heat. That’s why plant cells carry an extra rigid covering outside the cell membrane: the cell wall.

Fig. 2.8(a) — Onion peel cells: box-shaped, regular

Fig. 2.8(b) — Human cheek cells: irregular

Fig. 2.9(a) — Cradle lily peel in water

Fig. 2.9(b) — Same cells in 20% sugar solution

This is the whole point of the cell wall: even when a plant cell loses water by osmosis, it does not lose its shape. The cell membrane pulls inward, the wall stays where it is. Without a wall, animal cells simply shrink — but that flexibility is also useful, since it lets animal tissues move and change shape.

6. Prokaryotic vs Eukaryotic cells

Most cells have three basic parts: a plasma membrane, a semi-fluid jelly-like cytoplasm, and a nucleus. Inside the cytoplasm sit tiny sub-cellular structures called organelles, most of which need an electron microscope to be seen.

But not every cell has a proper nucleus — and that single difference splits the entire living world in two.

7. Inside the cell — a tiny living factory

Think of a cell as a factory. Nothing in a factory does everything; each department has one job, and they run at the same time, independently. Cell organelles work the same way — building new materials, removing waste, supplying energy.

🧠 Nucleus — house of coded instructions

Fig. 2.11 — Structure of a nucleus

Fig. 2.12 — From cell to DNA

• Nuclear membrane — double-layered, with pores that allow material to transfer between nucleus and cytoplasm.
• Nucleolus — the dense round body inside the nucleus, where ribosomal subunits are made. They exit to the cytoplasm where one large + one small subunit assemble into a ribosome.
• Chromosomes — rod-shaped, visible only when the cell is about to divide. Made of DNA + specific proteins. They carry information for inheritance from parents to offspring.
• Genes — the functional segments of DNA.
• Chromatin — in a non-dividing cell, DNA appears as an entangled mass of thread-like structures.

⚙️ Ribosomes — the protein factories

Tiny structures found either free in the cytoplasm or attached to the endoplasmic reticulum. They are the sites of protein synthesis. Note: ribosomes are not membrane-bound, which is why even prokaryotes have them.

🏭 Endoplasmic Reticulum (ER) — the manufacturing floor

A large organelle spreading like a network through the cytoplasm, continuous with the outer membrane of the nuclear envelope. It handles the synthesis and transport of proteins, fats (lipids) and some hormones.

📮 Golgi apparatus — packaging & shipping

Stacks of flattened, sac-like structures. Functionally linked to the ER, the cell membrane and other organelles. Think of it as the cell’s post office: it modifies, sorts and packages proteins and lipids into vesicles for transport, secretion, or lysosome formation.

⭐ Meet a scientist

First observed in 1898 by the Italian scientist Camillo Golgi, in the nerve cells of a barn owl. Early microscopes couldn’t resolve it clearly, so many doubted it even existed — electron microscopes confirmed it decades later, and it was named in his honour.

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🧹 Lysosomes — the clean-up crew

Single membrane-bound sacs filled with digestive enzymes. They break down unwanted proteins, carbohydrates, fats and even damaged parts of the cell, keeping it clean and healthy. The breakdown products are released into the cytoplasm to be reused.

🔋 Mitochondria — the powerhouse

Fig. 2.14 — Structure of a mitochondrion

Fig. 2.15 — Structure of a chloroplast

• Outer membrane — smooth and porous.
• Inner membrane — folded into finger-like projections called cristae, which increase surface area for chemical reactions and boost energy production.
• Glucose and other molecules are broken down here to release energy — the process of cellular respiration.
• The energy is stored as ATP (Adenosine Triphosphate), the cell’s energy currency.

🌿 Plastids — food factories of the plant cell

Animals fetch food from outside; plants make their own. Plastids are the plant organelles for food synthesis and storage. There are three kinds:

Inside a chloroplast is a semi-fluid substance called the stroma. Within it sit disc-shaped membrane structures containing chlorophyll, which absorb light energy. The sugars made during photosynthesis are stored in the stroma along with starch granules.

💧 Vacuoles — storage and support

• A mature plant cell usually has one large central vacuole surrounded by a single selectively permeable membrane.
• It is filled with a watery fluid called cell sap and stores water, minerals, sugars and waste material.
• By storing large amounts of water it maintains pressure inside the cell, keeping the plant cell firm.
• When water runs short, the vacuole loses water → cells become less firm → the plant wilts.
• Animal cells sometimes have vacuoles too, but small ones, used for temporary storage.

8. Plant cell vs Animal cell

(a) A typical bacterial cell

(b) A typical plant cell

(c) A typical animal cell

9. Cell division — mitosis & meiosis

A cut on your skin heals in a few days. Fallen hair grows back. Both happen because cells can grow and divide to replace old, dead or damaged cells.

Every day, hundreds of billions of cells in your body are replaced — roughly 1% of your total cells. Both prokaryotic and eukaryotic cells divide, but eukaryotic division is far more controlled and orderly, following a process called the cell cycle.

Fig. 2.16 — Growing onion roots in a jar of water

Fig. 2.17 — Stages of cell division in onion root tip cells

Fig. 2.18 — Mitosis: two identical daughter cells

Fig. 2.19 — Meiosis: four gametes, half the chromosomes

Mitosis

• The most common type of cell division.
• Produces two genetically identical daughter cells from one parent cell.
• Each new cell gets the same DNA and the same number of chromosomes as the parent.
• Important for growth, repair, maintenance and asexual reproduction.
• Every human begins as one fertilised egg, which divides by mitosis into trillions of cells.

Meiosis

• Produces gametes and occurs only in the cells of reproductive organs.
• In animals (including humans): in the testes (→ sperm) and ovaries (→ eggs).
• In plants: in the anthers (→ pollen grains, which later produce sperm cells) and ovaries (→ egg cells).
• The parent cell divides twice, one after the other, forming four daughter cells.
  • First division → two cells, chromosome number reduced to half.
  • Second division → similar to mitosis; each cell divides into two, giving four cells with half the chromosomes.
• During fertilisation, gametes from two individuals combine and the original chromosome number is restored.
• Creates variation and diversity — which is why children resemble their parents but are never exactly the same.

10. Cell theory — biology’s unifying principle

⭐ Meet a scientist — Gottlieb Haberlandt

In 1902, this Austrian botanist proposed that any living plant cell — even a fully mature one from permanent tissue — can develop into a complete plant if given suitable nutrients and favourable conditions. This ability is called totipotency, and his idea laid the foundation for Plant Tissue Culture Technology.

11. Do cells live forever? Contact inhibition & cancer

Healthy cells follow rules. They grow and divide in a controlled way, stay in the right place, do their job, and die when they are no longer needed. Dead cells are replaced by new ones performing the same function. So every cell has a definite life span. Problems appear when cells die too early — or refuse to die at all.

• Cancer cells lose this control and keep dividing uncontrollably, forming tumours.
• Tumours may be benign (stay in one place) or malignant (cancerous — they invade nearby tissues and can spread to other parts of the body to form new tumours).
• Plant cells do not show contact inhibition because of their rigid cell walls, and follow a different pattern of growth.

12. At a glance — night-before revision

Organelle → function cheat sheet

13. Practice questions — tap to reveal the answer

Identify the parts (a)–(g) in the cell diagram below and match them to their functions.

Work from the outside in. (a) and (c) sit on the left, (d)–(g) on the right.

Cell wall → provides structural rigidity to the cell. Cell membrane → separates cell contents from the surroundings. Nucleus → controls all the activities of the cell. Mitochondrion → site of cellular respiration. Golgi apparatus → packs and stores materials received from the ER. Vacuole → storage organelle that also provides rigidity. Chloroplast → helps in manufacturing food.

A carrot in plain water stays stiff and crunchy, but in concentrated salt solution becomes rubbery and limp. Why?

In plain water (hypotonic) water enters the carrot cells by osmosis. The vacuoles fill up, pressure builds against the cell walls, and the cells become turgid — so the carrot stays stiff. In salt solution (hypertonic) water moves out of the cells. The vacuoles shrink, cells lose turgidity, and the carrot becomes limp and rubbery.

Four potato cups are placed in beakers of water — A empty, B with sugar, C with salt, D is a boiled potato with sugar. Explain what happens.

Cups B and C — water gathers in the hollow. The sugar/salt makes the inside of the cup hypertonic, so water moves from the beaker, through the living potato cells, into the cavity by osmosis.

Cup A — necessary as the control. It has no solute, so no concentration gradient exists and no water collects. It proves the water movement in B and C is caused by the solute, not by the potato itself.

Cup D — no water gathers. Boiling kills the cells and destroys their selectively permeable membranes, so osmosis cannot occur. This shows osmosis needs a living, intact membrane.