Cells as the unit of life and how we study them
A sugar cube looks like one piece, yet a magnifier shows it is made of tiny crystals. In the same way, a leaf or your skin looks smooth, but under a microscope it is made of tiny units called cells.
A cell is the basic structural and functional unit of life. Cells join to form tissues, tissues form organs, and organs work together as an organ system. Most cells are too small for the unaided eye, so we use microscopes.
In the figure : Each level is built from the level before it.
Rule 1: Cells build the body step by step — Similar cells doing one job form a tissue; tissues form an organ; organs form an organ system.
- Easy : Muscle cells joined together form muscle tissue, and muscle tissue is a part of the heart (an organ).
- Medium : A bacterium or yeast is a whole living thing made of just one cell, so it is unicellular.
- Exam-type : Why do we say the cell is the unit of life? Because everything that makes an organism alive starts at the level of the cell.
Rule 2: Use a microscope and simple formulas to study cells — Total magnification = eyepiece × objective. Size of one cell = field of view ÷ number of cells across it.
- Easy : Eyepiece 10X and objective 40X give 10 × 40 = 400X.
- Medium : Field of view 2 mm (2000 µm) with 20 cells in a line: one cell = 2000 ÷ 20 = 100 µm.
- Exam-type : The eye cannot separate points closer than about 0.1 mm, so cells of 10–100 µm need a microscope. Electron microscopes, which use electrons, show details of a few nanometres.
1 mm = 1000 µm and 1 µm = 1000 nm. Change the unit first, then divide.
Mixing mm and µm while finding cell size, or adding the two lens powers instead of multiplying them.
Memory clue: C-T-O-S — Cell, Tissue, Organ, System (small to big). For magnification, remember: multiply the lenses.
Spot the difference
| Point | Unaided eye | Microscope |
|---|---|---|
| Smallest gap seen | About 0.1 mm | Far smaller; depends on the type |
| Cells of 10–100 µm | Cannot be seen | Can be seen |
| Details at nm scale | No | Only electron microscope |
Practice questions
- 1Who first saw cells and in what material?
- 2Write the order of organisation from cell to organ system.
- 3Convert 1 mm into µm.
- 4The eyepiece is 12X and the objective is 40X. Find the total magnification.
- 5Why can't we see most cells with the naked eye?
- 6Which microscope shows structures at the nanometre scale?
- 7The field of view is 1.5 mm and 25 cells fit in a line across it. Find the size of one cell.
- 8A cell of 25 µm is seen at 200X. How large (in mm) does its image appear?
Show answers (try them yourself first)
- 1Robert Hooke, in a thin slice of cork, in 1665.
- 2Cell → tissue → organ → organ system.
- 31000 µm.
- 412 × 40 = 480X.
- 5They are smaller than the eye's limit of resolution, about 0.1 mm.
- 6The electron microscope.
- 71500 µm ÷ 25 = 60 µm.
- 825 × 200 = 5000 µm = 5 mm.
Cell membrane, diffusion and osmosis
A raisin dropped in water slowly swells, and a slice of tomato sprinkled with salt turns wet. Water is moving across the cell boundary in both cases.
The cell membrane (plasma membrane) is a thin boundary of lipids and proteins around every cell. It is selectively permeable: it lets some substances pass and blocks others. Diffusion is movement from higher to lower concentration, and osmosis is the diffusion of water across such a membrane.
In the figure : Water moves toward the side that has more solute.
Rule 1: Water moves from dilute to concentrated solution — Dilute means more water, concentrated means less water. Movement goes on until the two sides become equal.
- Easy : A raw potato strip in plain water swells, because water enters its cells.
- Medium : A raw potato strip in 20% sugar solution shrinks, because water leaves its cells.
- Exam-type : Salt sprinkled on tomato slices makes the outside concentrated, so water comes out of the cells and the slices turn wet.
Rule 2: Name the solution by comparing it with the cell — Hypotonic means less solute outside, so water goes in. Isotonic means equal, so there is no net movement. Hypertonic means more solute outside, so water goes out.
- Easy : Animal cell in pure water: the water is hypotonic, so the cell swells.
- Medium : Animal cell in strong salt solution: the solution is hypertonic, so the cell shrinks.
- Exam-type : Pickles keep for long because the strong salt solution pulls water out of microbes and stops their growth.
Osmosis needs a selectively permeable membrane; diffusion does not. Water follows the solute, moving toward the more concentrated side.
Saying that the solute moves in osmosis. In osmosis it is the water that moves.
Memory clue: Hypo = Help water in, Hyper = Help water out. Iso = same, so no change.
Spot the difference
| Solution outside | Solute outside | Effect on animal cell |
|---|---|---|
| Hypotonic | Less than inside | Cell swells |
| Isotonic | Equal to inside | No change in size |
| Hypertonic | More than inside | Cell shrinks |
Practice questions
- 1What does 'selectively permeable' mean?
- 2What is osmosis?
- 3In osmosis, does water move from dilute to concentrated solution or the other way?
- 4A raisin is kept in plain water. What happens and why?
- 5Why does water collect on tomato slices after sprinkling salt?
- 6What kind of solution is plain water for an animal cell?
- 7A cell has 0.9% salt inside and is placed in a 0.9% salt solution. Name the solution and describe what happens.
- 8Explain why a drooping flower stem becomes firm when kept in water.
Show answers (try them yourself first)
- 1It lets some substances pass and blocks others.
- 2The diffusion of water across a selectively permeable membrane.
- 3From dilute (more water) to concentrated (less water) solution.
- 4It swells, because water enters by osmosis.
- 5The salt makes the outside concentrated, so water leaves the cells by osmosis.
- 6Hypotonic.
- 7Isotonic; there is no net movement of water and the cell stays the same size.
- 8Its cells take in water by osmosis and become firm again.
Cell wall in plant cells
A bamboo stem stands tall in the wind, but a jelly would collapse. Plant cells have an extra firm coat that gives them this strength.
The cell wall is a rigid, permeable layer outside the cell membrane in plant, fungal and bacterial cells. In plants it is mainly made of cellulose, a chain of glucose units. Animal cells have no cell wall.
In the figure : The wall is the outermost coat, then the membrane, then the cytoplasm.
Cell wall in plant cells
In the app you can move this figure with your finger.
Open in the appCell wall in plant cells
In the app you can move this figure with your finger.
Open in the appRule 1: The wall gives shape and strength — It protects the cell, keeps it firm and helps the plant stand upright.
- Easy : A plant stem stays upright after rain because its cells have firm walls.
- Medium : Onion peel cells look like neat boxes because each is held by a wall.
- Exam-type : Plants cannot move away from wind or rain, so they depend on firm walls more than animals do.
Rule 2: The wall is permeable, the membrane chooses — Water and minerals can pass through the wall. The cell membrane decides what finally enters the cell.
- Easy : A Rhoeo peel in strong sugar solution: the inner content shrinks away but the wall keeps its shape.
- Medium : Cheek cells in the same solution shrink completely, because they have no wall.
- Exam-type : We cannot digest cellulose, but it adds roughage to our food and helps digestion.
Wall outside, membrane inside. The wall is rigid and permeable; the membrane is flexible and selectively permeable.
Giving the property 'selectively permeable' to the cell wall. That belongs to the cell membrane.
Memory clue: W-M-C — Wall, Membrane, Cytoplasm, from outside to inside.
Spot the difference
| Feature | Cell wall | Cell membrane |
|---|---|---|
| Present in | Plants, fungi, bacteria | All cells |
| Nature | Rigid, permeable | Flexible, selectively permeable |
| Main material | Cellulose (in plants) | Lipids and proteins |
Practice questions
- 1Name the main substance in a plant cell wall.
- 2Name two groups of organisms, other than plants, that have a cell wall.
- 3Which layer lies outside: wall or membrane?
- 4State one function of the cell wall.
- 5Why does a cheek cell shrink in strong sugar solution while the boundary of a Rhoeo cell stays in shape?
- 6Why is cellulose useful in our diet even though we cannot digest it?
- 7Compare cell wall and cell membrane in two points.
- 8A plant cell lost its wall. Predict two changes in the cell and the plant.
Show answers (try them yourself first)
- 1Cellulose.
- 2Fungi and bacteria.
- 3The cell wall.
- 4It gives shape and strength and protects the cell.
- 5The cheek cell has no wall, but the Rhoeo cell has a rigid wall that holds its boundary.
- 6It works as roughage and helps digestion.
- 7The wall is rigid and permeable, the membrane is flexible and selectively permeable; the wall is outside, the membrane is inside it.
- 8The cell would lose its firm shape, and the plant would droop and may not stand upright.
Prokaryotic and eukaryotic cells; nucleus and DNA
A library has a catalogue room and a store room. Some cells keep their DNA in a separate room, a nucleus; others keep it in an open area.
Prokaryotic cells (bacteria) have no membrane-bound nucleus; their DNA lies in a region called the nucleoid. Eukaryotic cells (plants, animals, fungi) have a true nucleus and membrane-bound organelles. The nucleus holds chromosomes of DNA and proteins, and DNA controls the cell.
In the figure : The bacterium has DNA in the open nucleoid; the other cell has a nucleus.
Prokaryotic and eukaryotic cells; nucleus and DNA
In the app you can move this figure with your finger.
Open in the appRule 1: Prokaryote or eukaryote depends on the nucleus — No nuclear membrane means prokaryote. A nucleus with a double membrane and pores means eukaryote.
- Easy : A bacterium has a membrane, cytoplasm, ribosomes and a nucleoid, but no nucleus.
- Medium : A human cheek cell has a nucleus and organelles, so it is eukaryotic.
- Exam-type : Prokaryotic cells are usually 1–10 µm and eukaryotic cells 10–100 µm.
Rule 2: DNA carries the instructions — Genes are working pieces of DNA. In a resting cell DNA appears as chromatin; before division it forms rod-like chromosomes.
- Easy : Mature human red blood cells have no nucleus, so they cannot repair or divide and last only about 120 days.
- Medium : A virus is acellular: genetic material inside a protein coat, with no cell at all.
- Exam-type : In 2010 scientists put synthetic DNA in a cell whose own DNA was removed and the cell divided, showing that DNA directs the cell.
Every cell has a membrane and cytoplasm; the nucleus separates eukaryotes from prokaryotes.
Saying that bacteria have no genetic material. They do, but it lies in the nucleoid.
Memory clue: Pro = Primitive, Eu = True (nucleus).
Spot the difference
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No true nucleus (nucleoid) | True nucleus with membrane |
| Organelles | No membrane-bound ones | Many membrane-bound ones |
| Usual size | 1–10 µm | 10–100 µm |
Practice questions
- 1What is the nucleoid?
- 2Name two features of eukaryotic cells.
- 3Chromosomes are made of which two substances?
- 4Give two differences between prokaryotic and eukaryotic cells.
- 5Why do mature red blood cells live only about 120 days?
- 6What is chromatin?
- 7Why is a virus not called a cell?
- 8What did the 2010 experiment with synthetic DNA show?
Show answers (try them yourself first)
- 1The region of a prokaryotic cell that holds its DNA, with no membrane around it.
- 2A true nucleus and many membrane-bound organelles.
- 3DNA and proteins.
- 4Prokaryotes have no true nucleus (a nucleoid) and no membrane-bound organelles; eukaryotes have both.
- 5They have no nucleus, so they cannot repair themselves or divide.
- 6The loose, thread-like form of DNA in a cell that is not dividing.
- 7It has no cell structure: only genetic material in a protein coat.
- 8That DNA directs the structure and activities of a cell.
Ribosomes, ER, Golgi apparatus and lysosomes
A school office receives forms, files them, packs the replies and sends them out. A cell handles its proteins in a similar chain.
Ribosomes make proteins. The endoplasmic reticulum (ER) is a network of membranes: rough ER (with ribosomes) handles proteins, smooth ER handles fats and hormones. The Golgi apparatus modifies, sorts and packs materials, and lysosomes digest waste.
In the figure : A protein travels from nucleus instructions to the cell membrane.
Rule 1: Follow the protein path — DNA gives the instructions, ribosomes build the protein, the ER carries it, the Golgi packs it, and a vesicle delivers it.
- Easy : A hormone made of protein: nucleus → ribosome on rough ER → Golgi → cell membrane → outside the cell.
- Medium : A gland cell that secretes protein has plenty of rough ER.
- Exam-type : Why is the Golgi called the post office? It sorts and packs materials into vesicles for delivery.
Rule 2: Lysosomes keep the cell clean — They are single-membrane sacs full of enzymes that break down unwanted material and worn-out organelles.
- Easy : Without lysosomes, waste and old organelles would pile up in the cell.
- Medium : A sperm carries lysosomal enzymes that help it break the outer layer of the egg.
- Exam-type : Smooth ER, not rough ER, is abundant in cells that make fat-based hormones.
Ribosome = builds, ER = carries, Golgi = packs, Lysosome = cleans.
Thinking that lysosomes make proteins or that ribosomes digest waste.
Memory clue: R-E-G-L — Ribosome builds, ER carries, Golgi packs, Lysosome cleans.
Spot the difference
| Organelle | Job | Short name |
|---|---|---|
| Ribosome | Makes proteins | Protein factory |
| Golgi apparatus | Modifies, sorts, packs | Post office |
| Lysosome | Digests waste | Clean-up crew |
Practice questions
- 1Which organelle is called the protein factory?
- 2What makes rough ER look 'rough'?
- 3What do lysosomes do?
- 4Write the path of a protein that is secreted from a cell.
- 5Why is the Golgi apparatus called the cell's post office?
- 6Which kind of ER is common in cells that make fat-based hormones?
- 7What would happen if a cell had no lysosomes?
- 8How does lysosomal enzyme in the sperm help in fertilisation?
Show answers (try them yourself first)
- 1The ribosome.
- 2Ribosomes attached to its surface.
- 3They digest unwanted material and worn-out organelles.
- 4Nucleus → ribosome on rough ER → Golgi apparatus → vesicle → cell membrane.
- 5It modifies, sorts and packs materials and sends them out in vesicles.
- 6Smooth ER.
- 7Waste and old organelles would collect and the cell would suffer.
- 8It helps break down the outer layer of the egg.
Mitochondria, plastids and vacuoles
A phone needs a battery and a charger. Cells have organelles that supply energy and make food, plus a tank that keeps them firm.
Mitochondria release energy from food as ATP (cellular respiration). Plastids occur in plant cells: chloroplasts make food by photosynthesis, chromoplasts hold bright pigments, leucoplasts store starch, oil or protein. The vacuole stores cell sap and keeps a plant cell firm.
In the figure : Energy from food, food from light, and water storage.
Rule 1: Mitochondria release energy; chloroplasts capture it — Mitochondria have two membranes with folds called cristae. Chloroplasts have chlorophyll and a stroma.
- Easy : Glucose is broken down in mitochondria and the energy is stored in ATP.
- Medium : Mitochondria and chloroplasts have their own DNA and ribosomes, like some bacteria.
- Exam-type : Chloroplasts are found only in plant cells, so a green leaf cell can make food but a muscle cell cannot.
Rule 2: Plastids and vacuoles do special jobs — Chromoplasts colour fruits and flowers; leucoplasts store food; vacuoles hold sap and pressure.
- Easy : Bright fruit and flower colours attract animals that help in pollination and seed dispersal.
- Medium : Potato tubers have leucoplasts that store starch.
- Exam-type : A plant wilts when its vacuoles lose water and the cells become less firm.
Mitochondria: energy from food. Chloroplast: food from light. Vacuole: water store and firmness.
Saying that plants have chloroplasts instead of mitochondria. Plants have both.
Memory clue: M = Make energy, C = Capture sunlight, V = Vessel of water.
Spot the difference
| Organelle | Found in | Main job |
|---|---|---|
| Mitochondrion | Plant and animal cells | Releases energy as ATP |
| Chloroplast | Plant cells | Photosynthesis |
| Vacuole | Large in plants | Stores cell sap, keeps cell firm |
Practice questions
- 1Which organelle is called the powerhouse of the cell?
- 2What is ATP?
- 3Name the green pigment present in chloroplasts.
- 4Why do mitochondria have folded inner membranes?
- 5What is the difference between chromoplasts and leucoplasts?
- 6Why does a plant wilt if it is not watered?
- 7Compare mitochondria and chloroplasts in one line.
- 8Why can roots store food even though they do not photosynthesise?
Show answers (try them yourself first)
- 1Mitochondrion.
- 2The molecule that stores the energy released in the cell and works as its energy currency.
- 3Chlorophyll.
- 4The folds (cristae) give a larger surface for reactions that release energy.
- 5Chromoplasts hold bright pigments; leucoplasts are colourless and store food.
- 6Its vacuoles lose water, so the cells lose firmness.
- 7Mitochondria release energy from food; chloroplasts use light to make food.
- 8Roots have colourless leucoplasts that store starch and other food.
Cell division: mitosis and meiosis
A cut on your finger heals in days. The body does this by making new cells to replace the lost ones.
Cell division forms new cells from existing cells, for growth, repair and reproduction. In mitosis one cell gives two identical cells with the same number of chromosomes. In meiosis, which happens in reproductive organs, one cell gives four cells with half the number, the gametes.
In the figure : Mitosis keeps the number; meiosis halves it.
Rule 1: Mitosis copies the cell — Each daughter cell receives the same DNA as the parent, so the chromosome number is unchanged.
- Easy : A body cell with 46 chromosomes divides by mitosis: both new cells have 46.
- Medium : Skin heals after a cut because cells divide and replace the damaged ones.
- Exam-type : Errors in mitosis can lead to uncontrolled division and tumours.
Rule 2: Meiosis halves the number for gametes — Fusion of two gametes brings the number back to normal in the new individual.
- Easy : 46 chromosomes in a human body cell: sperm and egg each get 23, and the fertilised egg again has 46.
- Medium : Meiosis happens in the testes and ovaries of animals and in the anthers and ovaries of flowers.
- Exam-type : Different gametes carry different combinations, so children are similar to, but not exactly like, their parents.
Mitosis: 1 → 2, same number. Meiosis: 1 → 4, half number.
Believing meiosis makes body cells for growth. It only makes gametes.
Memory clue: Mi-TWO-sis gives TWO cells; Mei-FOUR-sis gives FOUR cells.
Spot the difference
| Point | Mitosis | Meiosis |
|---|---|---|
| Cells formed | 2, identical | 4, with half the chromosomes |
| Where | Body cells | Reproductive organs |
| Use | Growth, repair | Gametes, variation |
Practice questions
- 1Name the two types of cell division.
- 2How many cells does mitosis produce from one cell?
- 3Where does meiosis occur in animals?
- 4A human body cell has 46 chromosomes. How many are there in a cell made by mitosis? In a gamete?
- 5Why does a cut on the skin heal?
- 6Why are children not exactly like their parents?
- 7Why must gametes have half the number of chromosomes?
- 8What might happen if mitosis goes wrong?
Show answers (try them yourself first)
- 1Mitosis and meiosis.
- 2Two identical cells.
- 3In the testes and ovaries.
- 446 and 23.
- 5Cells divide by mitosis and replace the damaged cells.
- 6Gametes made by meiosis carry different combinations of genes.
- 7So that after fertilisation the chromosome number returns to normal instead of doubling.
- 8Cells may divide without control and form a tumour.
Cell theory, life span of cells and cancer
A brick wall is built brick by brick, yet the wall as a whole stays old and new at once. The living body is like this: every cell is born from an earlier cell, works for some time and then dies.
The cell theory says all living things are made of one or more cells, the cell is the basic unit of structure and function, and new cells arise only from existing cells. Cells have a limited life span; normal growth and death are controlled. In cancer, control is lost and cells divide without stopping.
In the figure : Three steps built the cell theory.
Rule 1: Three ideas make the cell theory — Schleiden (plants, 1838) and Schwann (animals, 1839) showed all organisms are made of cells; Virchow (1855) showed new cells come from old cells.
- Easy : A skin cell dividing to replace lost cells supports the idea that cells come from cells.
- Medium : Cells do not appear from air, sunlight or stones.
- Exam-type : A virus is acellular, so it is not counted as a living thing made of cells.
Rule 2: Growth and death are controlled — Contact inhibition stops many animal cells from dividing when they touch neighbours; cancer cells ignore this. Programmed cell death (PCD) removes cells in an organised way.
- Easy : In an embryo, PCD removes the cells between the fingers so that they become separate.
- Medium : A malignant tumour can invade nearby tissue and spread to other parts of the body.
- Exam-type : Haberlandt suggested that a living plant cell can grow into a whole plant; this ability, totipotency, is the base of plant tissue culture.
Normal cells grow, work, and die when needed; cancer cells do not obey these controls.
Thinking that all cell death is bad. PCD is a planned and healthy process.
Memory clue: H-S-V — Hooke 1665, Schleiden-Schwann 1838–39, Virchow 1855.
Spot the difference
| Term | Meaning | Example |
|---|---|---|
| Contact inhibition | Cells stop dividing on touching others | Many animal cells |
| Programmed cell death | Planned removal of cells | Separating fingers in an embryo |
| Tumour | Mass of cells dividing without control | Cancer |
Practice questions
- 1Who saw cells in 1665, who reported plant cells in 1838 and who stated in 1855 that cells come from cells?
- 2State the three points of the cell theory.
- 3What is contact inhibition?
- 4What is programmed cell death?
- 5How do cancer cells differ from normal cells?
- 6What is totipotency?
- 7Explain how PCD helps in forming separate fingers.
- 8Why is the cell theory not applied to viruses?
Show answers (try them yourself first)
- 1Hooke; Schleiden; Virchow.
- 2All living things are made of cells; the cell is the basic unit of structure and function; all cells arise from existing cells.
- 3In many animal cells, division stops when cells touch neighbouring cells.
- 4A genetically controlled, organised removal of cells.
- 5They lose control and keep dividing, forming tumours that may spread.
- 6The ability of a living plant cell to develop into a whole plant.
- 7Cells between the digits are removed, so the fingers separate.
- 8Viruses are acellular: they are not made of cells.