Cell: The Building Block of Life shows that every organism, from a bacterium to a tree, is made of cells. It explains how microscopes reveal cells, how the selectively permeable cell membrane and the rigid cell wall control water movement by osmosis, how prokaryotic and eukaryotic cells differ, and what each organelle does. It ends with mitosis, meiosis, the Cell Theory and why cancer cells escape normal control. School exams lean on comparisons, labelled cell diagrams and the potato and Rhoeo osmosis activities, and the chapter underpins Class 10 life processes and heredity.
Key Concepts
1. The Cell as the Unit of Life, and How We Study It
The chapter opens with the origin of life. Life probably began in water, possibly in small pools such as hot springs. The Puga Valley hot springs in Ladakh, nearly at the boiling point of water, host heat-loving unicellular bacteria called thermophiles, and calcium carbonate deposits around such springs may have helped form the first protective membrane, the barrier that defines a cell.
All living organisms are made up of cells. The cell is the basic level at which life exists.
- Unicellular organisms consist of one cell, for example bacteria and yeast.
- Multicellular organisms, such as plants, fish, birds and humans, are made of millions of cells that work together.
Levels of organisation: a group of similar cells performing similar functions forms a tissue. Different tissues are organised into an organ, and several organs work together as an organ system. For example, the nasal pores, nasal cavity, trachea and lungs together form the respiratory system.
Why we need a microscope
Draw two tiny dots on paper and move them closer. At some point they merge into one. Viewed from about 25 cm (the near point of the human eye), two points about 0.1 mm apart can be seen as separate; closer than that, they look like a single point. This distance is the limit of resolution of the human eye, 0.1 mm. A cell is usually too small to be seen by the unaided eye, so cell biologists rely on lenses.
A convex lens, or a combination of lenses (an objective lens and an eyepiece), magnifies an object so that it appears larger. This is the principle of the microscope.
- Robert Hooke was the first person to observe a cell, in 1665, using a self-designed microscope capable of about 200 to 300X magnification. In a thin slice of cork he saw small box-like compartments and named them cells.
- Light microscopes in school laboratories use visible light and different objective lenses (for example 10X and 40X) for better magnification and resolution. The parts labelled in the textbook are the eyepiece, body tube, coarse adjustment knob, fine adjustment knob, objective lens, handle, stage, mirror and base.
- Electron microscopes use a beam of electrons instead of light. They produce highly magnified images and show cell structure at the nanometre scale. A nanometre is one-billionth of a metre. A picture of the lower surface of a Colocasia leaf taken with a Scanning Electron Microscope shows its stomata clearly.
Estimating the size of a cell (Activity 2.1)
Measure the diameter of the field of view with a transparent millimetre ruler on the stage, convert it to micrometres (1 mm = 1000 µm), then count the onion peel cells lying in a straight line across the same diameter. Estimated size of one cell = diameter of the field in µm / number of cells along the diameter.
With 25 cells across a 5000 µm field, one onion cell is 5000/25 = 200 µm.
Total magnification depends on the magnifying power of both the eyepiece and the objective lens. With a 10X eyepiece and a 10X objective, the total magnification is 10 × 10 = 100X, so a 200 µm cell appears 100 times larger.
Microscopes have been improved through three features: resolution (a measure of clarity), contrast (the difference in brightness between parts of an object) and magnification.
2. The Cell Membrane and Osmosis
Cells in tissues and organs must interact with one another and with their surroundings. These interactions happen at the cell boundary. Even single-celled organisms exchange materials and respond to their environment through the cell membrane.
The cell membrane, also called the plasma membrane, is a thin boundary that surrounds a cell and protects its contents. It defines the individuality of a cell and is the universal feature of all cells. It is selectively permeable: it allows some substances to pass through and blocks others.
Activity 2.2: Potato pieces in water and in salt solution
A potato is cut into two pieces of roughly equal size and both are weighed. One piece goes into Beaker A with plain water; the other into Beaker B with 20 per cent salt or sugar solution. After about an hour, both are weighed again.
| Beaker | Liquid | Observation | Weight |
|---|---|---|---|
| A | Plain water | Potato piece swells | Increases |
| B | 20 per cent salt or sugar solution | Potato piece shrinks | Decreases |
Reason: the cell membrane lets water move in and out of the cell but does not let the salt or sugar molecules through. Water moves from an area with more water and less solute (dilute solution) to an area with less water and more solute (concentrated solution) until the concentrations become equal. In Beaker A water enters the potato cells; in Beaker B water leaves them.
Diffusion and osmosis
- Diffusion is the net movement of particles from a higher to a lower concentration. It happens because of a concentration difference, called a concentration gradient, and it occurs even without a membrane (dye spreading in water, fragrance spreading in air).
- Osmosis is the diffusion of water across a selectively permeable membrane. In plants, water from the soil enters root cells by osmosis.
Cells in solutions of different strengths
| Type of outside solution | Solute concentration compared with inside the cell | Net movement of water | Effect on an animal cell |
|---|---|---|---|
| Isotonic | Outside = inside | No net movement | Stays the same size |
| Hypotonic | Outside < inside | Into the cell | Swells |
| Hypertonic | Outside > inside | Out of the cell | Shrinks |
Structure of the membrane: the fluid-mosaic model
The cell membrane is extremely thin, about 7 to 10 nanometres thick (1 nanometre = 0.000001 mm). It is made of lipids (fats) and proteins. The fluid-mosaic model explains its structure:
- It has a lipid bilayer: two layers of special fat molecules, with water-attracting heads facing outwards and water-repelling tails facing inwards. Proteins are embedded in this bilayer.
- The molecules can move sideways, flip and rotate within the membrane. Hence it is fluid.
- Proteins in the membrane act like gatekeepers that help substances pass through.
- The molecules are arranged like tiles in a mosaic, so it is called the mosaic model.
3. The Cell Wall: Plant Cells versus Animal Cells
All living cells communicate with their surroundings through the cell membrane. However, cells of plants, fungi and bacteria have an additional layer outside the cell membrane, called the cell wall.
Activity 2.3: Onion or Rhoeo peel and cheek cells
- A thin peel of an onion leaf or a Rhoeo (Cradle lily) leaf is mounted with safranin stain. Cheek cells, scraped gently from the inside of the cheek with a cotton swab or the blunt end of a toothpick, are spread on a slide with a drop of water and a few drops of methylene blue stain.
- Onion and Rhoeo peel cells are box-shaped and regularly arranged. Cheek cells are irregularly arranged.
Why plants need a cell wall
Plants generally cannot move from place to place, so they need a rigid structure to withstand stresses like wind and rain. The cell wall:
- gives strength and helps leaves and flowers stay firm and keep their shape,
- helps plants stay upright,
- is rigid but permeable: water and some dissolved minerals can pass through it. Together with the selective permeability of the cell membrane, this helps roots absorb water and nutrients from the soil.
In concentrated sugar solution a plant cell loses water by osmosis, but the rigid cell wall keeps the cell’s shape. Only the inner content shrinks as the cell membrane pulls away from the cell wall. The chapter describes this without naming it; the standard name is plasmolysis. Cheek cells, with no wall, shrink as a whole.
Without a rigid wall, animal cells can change shape easily. This flexibility supports the movement and working of animal tissues.
Composition: the plant cell wall is mainly made of cellulose, a carbohydrate formed from many glucose units linked together. Cellulose in our diet acts as roughage and helps digestion. Fungi and bacteria also have cell walls for protection and structural support.
| Basis | Cell membrane | Cell wall |
|---|---|---|
| Present in | All cells | Plants, fungi and bacteria only |
| Permeability | Selectively permeable | Permeable |
| Nature | Thin, flexible, fluid | Rigid |
| Made of | Lipids and proteins | Mainly cellulose (in plants) |
4. Prokaryotic and Eukaryotic Cells
Most cells have three basic parts:
- a selectively permeable membrane called the plasma membrane,
- a semi-fluid, jelly-like substance called the cytoplasm, and
- a prominent nucleus.
Besides the nucleus, the cytoplasm contains sub-cellular components called organelles, most of which are visible only with an electron microscope.
A bacterial cell lacks a well-defined nucleus and membrane-bound organelles (organelles surrounded by their own membranes). Such cells are prokaryotic (pro = primitive, karyon = nucleus). Most cellular activities in them take place directly in the cytoplasm. Their DNA is a single circular molecule associated with specific proteins, and the region holding it is the nucleoid.
Plant and animal cells have a well-defined nucleus and several membrane-bound organelles. They are eukaryotic (eu = true, karyon = nucleus).
Table 2.1 filled in: bacterial, plant and animal cells
| Cell structure | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|
| Cell membrane | Present | Present | Present |
| Cell wall | Present | Present | Absent |
| Cytoplasm | Present | Present | Present |
| Well-defined nucleus (genetic material enclosed by a membrane) | Absent | Present | Present |
| Primitive nucleus or nucleoid (genetic material without a membrane) | Present | Absent | Absent |
| Membrane-bound organelles | Absent | Present | Present |
Table 2.2: Prokaryotic versus eukaryotic cells
| Characteristic | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Primitive nucleus (nucleoid) | Present | Absent |
| Diameter of a typical cell | 1 to 10 µm | 10 to 100 µm |
| Number of cells in an organism | Usually unicellular | Can be unicellular or multicellular |
| Membrane-bound organelles | Absent | Present |
| Membrane-bound nucleus | Absent | Present |
Other parts of the cytoplasm
- Cytoskeleton: in eukaryotic cells, a network of fine fibres that gives structural support, maintains cell shape, and enables cell movement and internal transport. It is seen as a separate entity only under an electron microscope.
- Cell inclusions: stored materials in the cytoplasm, such as starch (in plant cells) or crystals of calcium oxalate or silica (in some plant cells).
Acellular agents
Viruses, viroids and prions are acellular (they are not cells) infectious agents, too small to be seen under a light microscope.
- Viruses: some genetic material with a protein coat.
- Viroids: genetic material with no protein coat around it.
- Prions: misfolded proteins that lack genetic material.
5. Cell Organelles: A Living Factory
Eukaryotic cells carry out different life processes in different organelles, independently and at the same time. Organelles build new materials, remove waste and provide energy, and they work together. A cell is like a tiny living factory in which each part does a specific job.
Nucleus: house of coded instructions
- Covered by a double-layered nuclear membrane with pores that allow transfer of material between the nucleus and the cytoplasm.
- The nucleolus is a dense round body inside the nucleus where ribosomal subunits are made. These subunits move out to the cytoplasm, where one large and one small subunit assemble to form a ribosome.
- The nucleus contains chromosomes, visible as rod-shaped structures only when the cell is about to divide. Chromosomes are made of DNA (deoxyribonucleic acid) and specific proteins, and they carry information for the inheritance of characters from parents to the next generation.
- Functional segments of DNA are called genes.
- In a non-dividing cell, DNA is part of the chromatin material, an entangled mass of thread-like structures. When the cell is about to divide, chromatin gets organised into chromosomes.
Cells without a nucleus: mature human red blood cells (RBCs) have no nucleus (they are enucleate). This leaves more space for haemoglobin, so they carry more oxygen. Without a nucleus they cannot repair or divide themselves, so they live only about 120 days.
Evidence that DNA controls the cell: in 2010, J. Craig Venter and his team put a chemically made copy of the DNA of the bacterium Mycoplasma mycoides into a related bacterium whose own DNA had been removed. The cell grew and divided following the synthetic DNA. Only the DNA was synthetic; the rest of the cell came from an existing living cell.
Ribosomes: the protein factories
Tiny structures present either freely in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are the sites of protein synthesis.
Endoplasmic reticulum (ER): manufacturing factory
A large organelle that spreads like a network in the cytoplasm and is continuous with the outer membrane of the nuclear envelope. It plays a key role in making and transporting proteins, fats (lipids) and some hormones. Its structure varies with the cell’s function.
| Basis | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Structure | Ribosomes attached to its surface, so it looks rough under an electron microscope | No ribosomes on its surface, so it looks smooth |
| Main function | Protein synthesis and protein secretion (e.g. pancreatic gland cells) | Synthesis and storage of fats and hormones |
Golgi apparatus: packaging and shipping centre
Stacks of flattened, sac-like structures. It is linked in function to the ER, the cell membrane and other organelles. Like the cell’s post office, it modifies, sorts and packages proteins and lipids into vesicles for transport, secretion or lysosome formation. It was first observed in 1898 by the Italian scientist Camillo Golgi in the nerve cells of a barn owl.
The secretion pathway: RER (proteins made) → vesicles → Golgi apparatus (modified, sorted, packaged) → vesicles → cell membrane → secreted outside.
Lysosomes: the clean-up system
Single membrane-bound sacs filled with enzymes that break down unwanted proteins, carbohydrates, fats and even damaged parts of the cell. The products are released into the cytoplasm and may be reused. Human sperm cells contain lysosomal enzymes that help break down the outer layer of the egg during fertilisation.
Mitochondria: the powerhouse of the cell
- Each mitochondrion has two membranes. The outer membrane is smooth and porous. The inner membrane is folded into finger-like projections called cristae, which increase the surface area for chemical reactions and help energy production.
- Glucose and other molecules are broken down here to release energy by cellular respiration.
- The energy is stored as ATP (adenosine triphosphate), the energy currency of the cell, used for most cellular activities.
Plastids: food synthesis and storage in plant cells
Plants make food by photosynthesis. The green pigment chlorophyll, present in chloroplasts, absorbs sunlight. Chloroplasts are double-membrane-bound, like mitochondria. Inside is a semi-fluid substance, the stroma, which contains disc-shaped membrane structures holding chlorophyll. Sugars made in photosynthesis are stored in the stroma along with starch granules.
| Plastid | Pigment | Function or example |
|---|---|---|
| Chloroplast | Chlorophyll (green) | Photosynthesis; temporary food storage |
| Chromoplast (chroma = colour) | Yellow, orange or red pigments | Colour of flower petals and fruits; attracts pollinators and fruit-eating animals that disperse seeds |
| Leucoplast (leukos = white) | None (colourless) | Stores starch, oils or proteins; potato and taro (Colocasia) cells store starch |
Mitochondria and plastids resemble certain bacteria: both have their own DNA and ribosomes, so they can make some of their own proteins. This suggests they share an evolutionary history with such single-celled organisms.
Vacuoles: storage and support
- A mature plant cell usually has one large central vacuole, surrounded by a single selectively permeable membrane and filled with a watery fluid called cell sap.
- It stores water, minerals, sugars and waste material.
- By holding a lot of water it keeps pressure inside the cell, which keeps the cell firm. When a plant gets too little water, vacuoles lose water, cells become less firm and the plant wilts.
- Animal cells sometimes have vacuoles, smaller than plant vacuoles, for temporary storage.
6. Cell Division: Mitosis and Meiosis
A cut on the skin heals and fallen hair grows back because body cells grow and divide to replace old, dead or damaged cells. Cells can grow only up to a certain size, so the body grows mainly because cells divide to form new cells. Every day an estimated hundreds of billions of cells in our body are replaced, almost 1 per cent of the total.
Activity 2.5: Onion root tip
Roots grown from an onion bulb over water for 5 to 6 days are cut (2 to 3 cm), fixed in aceto-alcohol (glacial acetic acid : ethanol :: 1 : 3) for 24 hours and kept in 70 per cent ethanol. A root tip is softened with dilute hydrochloric acid, stained with aceto-carmine, warmed gently, squashed under a coverslip and observed.
Observation: root tip cells look different from one another because the growing tip divides continuously and its cells are caught at different stages of division.
Cell division is the process by which new cells form from pre-existing cells. It lets organisms grow, repair damaged tissues and reproduce. Some cells, such as skin cells, divide continuously to replace cells lost regularly. Both prokaryotic and eukaryotic cells divide, but eukaryotic cells divide in a more controlled, orderly way through the cell cycle.
Mitosis
Every human begins as a single fertilised egg that divides again and again to form trillions of cells. Mitosis is the most common type of cell division. One parent cell gives two genetically identical daughter cells, each with the same DNA and the same number of chromosomes as the parent. It is important for normal growth, repair, maintenance and asexual reproduction.
Meiosis
Meiosis produces gametes and occurs only in cells of reproductive organs: the testes (sperm) and ovaries (eggs) in animals, and the anthers (pollen grains, which later produce sperm cells) and ovaries (egg cells) in plants.
- The parent cell divides twice, one division after the other, forming four daughter cells.
- In the first division the cell divides into two and the chromosome number in each daughter cell is reduced to half.
- The second division is similar to mitosis: each daughter cell divides into two.
- Each gamete has half the DNA of the parent cell. At fertilisation, two gametes combine and the original chromosome number is restored.
- Meiosis creates variation, which is why children resemble their parents but are not exactly the same.
| Basis | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two, one after the other |
| Daughter cells | Two | Four |
| Chromosome number | Same as parent | Half of parent |
| Genetic make-up | Identical to parent | Creates variation |
| Purpose | Growth, repair, maintenance, asexual reproduction | Gametes for sexual reproduction; genetic diversity |
When division goes wrong: errors in mitosis can lead to uncontrolled cell division, forming tumours, and to an abnormal number of chromosomes in body cells. Errors in meiosis may cause genetic disorders, which may come with developmental problems or distinctive physical features, and faulty meiosis may also cause early pregnancy loss or reduced fertility.
7. The Cell Theory, Cell Life Span and Cancer
In 1838 the German botanist Matthias Schleiden reported that all plants are made of cells. In 1839 the German zoologist Theodor Schwann found the same for animals. In 1855 Rudolf Virchow added that new cells form only from pre-existing cells.
According to the classical Cell Theory:
- All living organisms are made up of one or more cells.
- The cell is the basic unit of structure and function in living beings.
- All cells arise from pre-existing cells.
The theory unifies all biology, from bacteria to humans, and explains the continuity of life through cell division.
Totipotency: Haberlandt proposed that any living plant cell, even a fully mature cell from a permanent tissue, can develop into a complete plant if given suitable nutrients and favourable conditions. This laid the foundation of Plant Tissue Culture Technology.
Do cells live forever?
Cells grow and divide in a controlled way, stay in the right place, do their work and eventually die when no longer needed. New cells replace them. Every cell has a definite life span. Problems arise if cells do not die when they should, or die too early.
- Contact inhibition: in many animal cells, division usually stops when the cells come in contact with neighbouring cells.
- Cancer cells lose this control and keep dividing uncontrollably, forming tumours. Tumours may be benign or malignant. Cancerous tumours can invade nearby tissues and 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 they follow a different pattern of growth.
- Programmed Cell Death (PCD) is a genetically regulated, organised process of selective cell destruction, needed for normal development, cellular quality control and immune function. In an embryo, PCD removes the cells between the digits to form separate fingers; without it we would have webbed hands.
Formula and Reaction Sheet
| Quantity | Relation | Example from the chapter |
|---|---|---|
| Unit conversion | 1 mm = 1000 µm | 5 mm = 5 × 1000 = 5000 µm |
| Nanometre | 1 nm = one-billionth of a metre = 0.000001 mm | Membrane thickness 7 to 10 nm |
| Estimated cell size | Diameter of field of view (µm) / number of cells along the diameter | 5000 µm / 25 = 200 µm |
| Total magnification | Magnifying power of eyepiece × magnifying power of objective | 10X × 10X = 100X |
| Typical cell diameter | Prokaryotic 1 to 10 µm; eukaryotic 10 to 100 µm | Table 2.2 |
| Meiosis count | 1 parent cell → 4 gametes, each with half the chromosomes | Fertilisation restores the full number |
| Cellular respiration | Glucose broken down in mitochondria → energy stored as ATP | ATP is the energy currency |
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Important Definitions
- Limit of resolution of the human eye: the smallest distance (about 0.1 mm, seen from about 25 cm) at which two points can be seen as separate and distinct.
- Cell membrane (plasma membrane): the thin, selectively permeable boundary that surrounds a cell, protects its contents and defines its individuality.
- Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration.
- Osmosis: the diffusion of water across a selectively permeable membrane, from a dilute solution to a concentrated solution.
- Cell wall: the rigid, permeable outer covering outside the cell membrane in plant, fungal and bacterial cells; mainly cellulose in plants.
- Prokaryotic cell: a cell without a well-defined nucleus and without membrane-bound organelles.
- Eukaryotic cell: a cell with a well-defined nucleus and membrane-bound organelles.
- Nucleoid: the region of a prokaryotic cell containing its genetic material, with no membrane around it.
- Chromosome: a rod-shaped structure of DNA and specific proteins, visible when the cell is about to divide, that carries hereditary information.
- ATP: adenosine triphosphate, the molecule in which energy from cellular respiration is stored; the energy currency of the cell.
- Mitosis: cell division that forms two genetically identical daughter cells from one parent cell.
- Meiosis: a two-step cell division in reproductive organs that forms four gametes, each with half the number of chromosomes.
- Contact inhibition: the stopping of cell division when animal cells come in contact with neighbouring cells.
- Totipotency: the ability of a living plant cell to develop into a complete plant under suitable conditions.
- Programmed Cell Death (PCD): a genetically regulated, organised process of selective cell destruction.
Solved Examples (NCERT-Based)
Example 1: Estimating the size of an onion cell (Activity 2.1)
Question: The field of view of a microscope measures 5 mm across. 25 onion peel cells lie along its diameter. The eyepiece and the objective are both 10X. Find the estimated size of one cell and how large it appears.
Solution:
- Convert the field to micrometres: 5 mm × 1000 = 5000 µm.
- Size of one cell = 5000 µm / 25 = 200 µm.
- Total magnification = 10 × 10 = 100X, so the 200 µm cell appears 100 times larger.
Example 2: Potato pieces in water and salt solution (Activity 2.2)
Question: Why does the potato piece in plain water gain weight while the piece in 20 per cent salt solution loses weight? Why must both pieces be of roughly equal size and weighed first (Pause and Ponder 3)?
Solution: Water moves by osmosis towards the side with more solute: into the cells from plain water, out of the cells into the 20 per cent salt solution, while the membrane blocks the salt. Equal sizes make the two pieces comparable, and the initial weights are needed to calculate the change in weight, which is the measured result.
Example 3: Revise, Reflect, Refine Question 2
Question: Two similar animal cells are placed in two different solutions: Cell X is placed in pure water. Cell Y is placed in a concentrated salt solution. Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations? (i) Salt molecules moved into Cell Y, causing it to shrink. (ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it. (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. (iv) Solute movement caused osmosis in both cells.
Solution: (iii). Pure water is hypotonic, so water enters X; the concentrated salt solution is hypertonic, so water leaves Y. Options (i), (ii) and (iv) involve salt or solute moving, but the membrane blocks the salt and osmosis is the movement of water.
Example 4: Revise, Reflect, Refine Question 9
Question: Indicate the presence or absence of following structures in bacterial and animal cells: Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts.
| Structure | Bacterial cell | Animal cell | Reason |
|---|---|---|---|
| Chromosome | Present as one circular DNA molecule with proteins, lying in the nucleoid | Present | Bacterial DNA has no nucleus around it; animal chromosomes lie inside the nucleus |
| Nucleus | Absent | Present | Prokaryotes have only a nucleoid |
| Mitochondria | Absent | Present | Membrane-bound organelle |
| Golgi complex | Absent | Present | Membrane-bound organelle |
| Chromoplasts | Absent | Absent | Plastids occur in plant cells |
Example 5: Revise, Reflect, Refine Question 10 (potato cups)
Question: Four scooped potato cups stand in beakers of water. Cup A is empty, Cup B has one teaspoon sugar, Cup C has one teaspoon salt, and Cup D is made from a boiled potato with one teaspoon sugar. (i) Explain why water gathers in the hollowed portion of Cup B and Cup C. (ii) Why is Cup A necessary for this experiment? (iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Solution:
- (i) In B and C the sugar or salt dissolves in moisture in the cup and makes a concentrated solution. The beaker water is dilute. The living potato cells act as a selectively permeable layer, so water moves by osmosis from the beaker, through the cells, into the cup.
- (ii) Cup A is the control. It is the same potato and set-up without any solute, so it shows that water gathers in B and C because of the added sugar or salt, and not merely because a potato cup stands in water.
- (iii) Cup A has no solute inside, so there is no concentration difference to pull water in. Cup D has solute, but boiling kills the potato cells, so their membranes no longer work as selectively permeable membranes and osmosis does not take place.
Example 6: Revise, Reflect, Refine Question 8 (carrots)
Question: A researcher placed one carrot in plain water and another of similar size in concentrated salt solution for 24 hours. (i) What hypothesis does she want to test through this experiment? (ii) What would you suggest for the improvement of this experiment? (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
Solution:
- (i) Hypothesis: water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution.
- (ii) Weigh each carrot before and after, keep all other conditions the same, and repeat with several carrots or solutions of different strengths.
- (iii) In plain water, water enters the cells; the vacuoles stay full, pressure inside the cells keeps them firm, and the carrot is crunchy. In salt solution, water leaves the cells by osmosis; the vacuoles lose water, the cells become less firm, and the carrot goes limp, just as a plant wilts when short of water.
Example 7: Revise, Reflect, Refine Question 14 (making the cell membrane)
Question: The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane.
Solution:
- Proteins are made by ribosomes on the rough ER.
- Lipids (fats) are made by the smooth ER.
- They travel in vesicles to the Golgi apparatus, which modifies, sorts and packages them.
- Vesicles from the Golgi apparatus carry them to the cell membrane.
Path: RER (proteins) and SER (lipids) → vesicles → Golgi apparatus → vesicles → cell membrane. In the labelled diagram, draw the nucleus, the ER network continuous with it, the Golgi stack, small round vesicles and the membrane, with arrows along this path.
Example 8: Counting cells in mitosis and meiosis
Question: A cell of an organism has 8 chromosomes. (a) How many cells and how many chromosomes per cell after one mitosis? (b) After meiosis? (c) After two such gametes fuse?
Solution:
- (a) Mitosis: 2 daughter cells, each with 8 chromosomes, identical to the parent.
- (b) Meiosis: the first division gives 2 cells with 4 chromosomes each; the second division, like mitosis, splits each into 2. Result: 4 gametes, each with 4 chromosomes.
- (c) Fertilisation: 4 + 4 = 8, the original number is restored.
The same reasoning answers Question 15, “What would happen if gametes are formed by mitotic divisions?”: gametes would carry the full number, so the fused cell would have double the number (16 here), and the number would keep doubling every generation.
Competency-Based Questions (with answers)
1. Case-based: Pickles and sharbat (Revise, Reflect, Refine Question 16)
A farmer, Deepa, sold only one-fourth of her amla and lemon harvest. She turned the rest into pickles, murabbas and sharbat by adding appropriate amounts of salt, sugar or jaggery to small pieces of fruit and their juices, and stored them in small glass bottles.
(i) Which scientific concept has the farmer applied in the preservation of the farm produce? Osmosis. High salt or sugar makes a strongly hypertonic medium.
(ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi? The solution outside the microbes has far more solute than their cells. Water leaves the microbial cells by osmosis, they shrink and cannot grow or multiply, so the food does not spoil.
(iii) Suggest a healthy recipe of this kind for food preservation. Amla pieces in jaggery syrup, stored in clean, dry glass bottles.
(iv) What are the scientific values addressed in this case? Less post-harvest waste, food security and income from agro-processing.
2. Assertion-Reason
Assertion (A): A Rhoeo leaf cell placed in concentrated sugar solution keeps its outer shape. Reason (R): The rigid cell wall maintains the shape of a plant cell even when it loses water.
Answer: (a) Both A and R are true, and R correctly explains A. Only the membrane and contents shrink away from the wall.
3. Assertion-Reason
Assertion (A): Mature human red blood cells live only about 120 days. Reason (R): They have no nucleus, so they cannot repair or divide themselves.
Answer: (a) Both A and R are true, and R correctly explains A.
4. Assertion-Reason
Assertion (A): Bacteria are eukaryotic organisms. Reason (R): The genetic material of bacteria lies in a nucleoid without a membrane around it.
Answer: (d) A is false, R is true. Having a nucleoid and no membrane-bound organelles makes bacteria prokaryotic.
5. Source-based: Contact inhibition and tumours (Revise, Reflect, Refine Question 13, first part)
“In many animal cells, cell division usually stops when cells come in contact with neighbouring cells. However, cancer cells lose this control and keep dividing uncontrollably, leading to the formation of tumours.”
(a) Which phenomenon inhibits the formation of tumors in the human body? Contact inhibition.
(b) Why do plant cells not show this phenomenon? Their rigid cell walls give them a different pattern of growth, and the chapter states that plant cells do not show contact inhibition.
(c) Name the two kinds of tumour. Benign and malignant. Cancerous tumours can invade nearby tissues and spread to other parts of the body.
6. Reasoning: One giant mitochondrion (Pause and Ponder 6)
Question: Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?
Answer: Energy production depends on the membrane surface where reactions happen, which is why the inner membrane is folded into cristae. Many small mitochondria have a much larger total surface area than one giant mitochondrion of the same volume, and they can be spread through the cytoplasm close to where energy is needed.
7. Competency MCQ: Revise, Reflect, Refine Question 11
Question: Identify the pair that incorrectly matches the cell organelle with its function. (i) Ribosome: Protein synthesis (ii) SER: Lipid and cellulose synthesis (iii) Lysosome: Digestion of foreign agents
Answer: (ii). The chapter gives the SER’s role as the synthesis and storage of fats and hormones; cellulose is not an SER product. Ribosomes make proteins, and lysosome enzymes break down unwanted material.
8. Competency MCQ: Revise, Reflect, Refine Question 4
Question: Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories (present in the plant cells, absent in the animal cells)? (i) Leucoplast, Cell wall (ii) Mitochondria, Ribosome (iii) Cell wall, Golgi apparatus (iv) Lysosome, Endoplasmic reticulum
Answer: (i). Leucoplasts are present in plant cells and a cell wall is absent in animal cells. Ribosomes, the Golgi apparatus and the ER are all present in animal cells, so (ii), (iii) and (iv) fail.
Important Questions for Exams
1-Mark Questions
- Who first observed cells, and in what? Robert Hooke, in 1665, in a thin slice of cork.
- What is the limit of resolution of the human eye? About 0.1 mm, viewed from about 25 cm.
- Which organelle is called the powerhouse of the cell? Mitochondrion.
- Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes (ii) Mitochondria, Nucleus.
- What are functional segments of DNA called? Genes.
2-Mark Questions
- Differentiate between cell membrane and cell wall (permeability). The cell membrane is selectively permeable: it lets some substances pass and blocks others. The cell wall is permeable: water and some dissolved minerals can pass through it.
- Differentiate between RER and SER (structure). RER has ribosomes attached to its surface and looks rough under an electron microscope. SER has no ribosomes on its surface and looks smooth.
- Differentiate between chloroplasts and chromoplasts (pigments). Chloroplasts contain the green pigment chlorophyll, which absorbs sunlight for photosynthesis. Chromoplasts contain yellow, orange or red pigments that colour flowers and fruits.
- What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell? The cell cannot carry out cellular respiration, so it cannot make ATP. Without energy most cellular activities stop, and the cell would eventually die.
3-Mark Questions
- Give three differences between prokaryotic and eukaryotic cells. (1) Prokaryotic: genetic material in a nucleoid with no membrane; eukaryotic: membrane-bound nucleus. (2) Prokaryotic: no membrane-bound organelles; eukaryotic: several membrane-bound organelles. (3) Prokaryotic: typical diameter 1 to 10 µm, usually unicellular; eukaryotic: 10 to 100 µm, unicellular or multicellular.
- Explain the fluid-mosaic model of the cell membrane. The membrane is a lipid bilayer, with water-attracting heads outwards and water-repelling tails inwards, and proteins embedded in it. The molecules can move sideways, flip and rotate, so it is fluid. They are arranged like tiles, so it is a mosaic. Proteins act as gatekeepers for substances passing through.
- Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer. Renu. Rohit assumes that plastids are only chloroplasts. Plastids also include colourless leucoplasts, which store starch, oils or proteins, and coloured chromoplasts. Non-green parts such as roots still contain leucoplasts. The chapter’s own example is starch-storing leucoplasts in potato and taro, underground parts that never photosynthesise.
5-Mark Questions
- Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.
Similarities: both are bounded by two membranes; both have their own DNA and ribosomes, so both can make some of their own proteins; both deal with energy in the cell; both resemble certain bacteria, suggesting a shared evolutionary history.
Differences: mitochondria occur in plant and animal cells, chloroplasts only in plant cells. The inner mitochondrial membrane is folded into cristae; a chloroplast contains a semi-fluid stroma with disc-shaped membrane structures holding chlorophyll. Mitochondria break down glucose in cellular respiration and store the energy as ATP; chloroplasts absorb light and make sugars by photosynthesis, storing them in the stroma with starch granules. - Describe mitosis and meiosis and compare them. What happens if either goes wrong?
Mitosis: one division of a body cell gives two genetically identical daughter cells with the same DNA and chromosome number; it serves growth, repair, maintenance and asexual reproduction. Meiosis: in reproductive organs (testes, ovaries, anthers), the cell divides twice; the first division halves the chromosome number, the second is like mitosis; four gametes form, each with half the chromosomes, and fertilisation restores the number. Meiosis creates variation.
Errors in mitosis can cause uncontrolled division, tumours and abnormal chromosome numbers in body cells. Errors in meiosis may cause genetic disorders, early pregnancy loss or reduced fertility.
Common Mistakes and Examiner Tips
- Mistake: writing that salt or sugar moves in osmosis. Fix: osmosis is the movement of water only. The solute stays put because the membrane blocks it.
- Mistake: mixing up hypotonic and hypertonic. Fix: both words describe the outside solution compared with the cell. Hypertonic outside means the cell shrinks.
- Mistake: calling the cell wall selectively permeable. Fix: the cell wall is permeable; the cell membrane is selectively permeable.
- Mistake: writing that only plant cells have a cell wall. Fix: plants, fungi and bacteria have cell walls; animal cells do not.
- Mistake: saying bacteria have no genetic material or no DNA. Fix: they have DNA as a single circular molecule in the nucleoid; they lack a membrane around it.
- Mistake: saying plastids occur only in green parts. Fix: leucoplasts and chromoplasts occur in non-green parts; white or underground storage tissues still contain plastids.
- Mistake: writing “meiosis gives two cells” or “mitosis halves the chromosomes”. Fix: mitosis: 2 identical cells, same number. Meiosis: 2 divisions, 4 gametes, half the number.
- Mistake: saying chromosomes are always visible in the nucleus. Fix: in a non-dividing cell the DNA is in thread-like chromatin; chromosomes become visible only when the cell is about to divide.
- Mistake: adding total magnification (10X + 10X = 20X). Fix: multiply the powers of the eyepiece and the objective: 10X × 10X = 100X.
- Tip: in “differentiate” questions, answer only on the basis given in brackets (permeability, structure, pigments) and write the point for both items, ideally in a two-column table.
Quick Revision Points
- The cell is the basic structural and functional unit of all living organisms.
- Cells → tissues → organs → organ systems (e.g. nasal pores, nasal cavity, trachea and lungs form the respiratory system).
- Robert Hooke saw cells in cork in 1665 with a microscope of about 200 to 300X.
- Cell size = field diameter in µm / cells along the diameter; total magnification = eyepiece × objective.
- The cell membrane (plasma membrane) is selectively permeable, 7 to 10 nm thick, made of lipids and proteins.
- Diffusion: net movement from higher to lower concentration. Osmosis: diffusion of water across a selectively permeable membrane.
- Hypotonic outside: cell swells. Hypertonic outside: cell shrinks. Isotonic: no change.
- Plants, fungi and bacteria have a cell wall; in plants it is mainly cellulose, rigid but permeable.
- In concentrated sugar solution plant cells keep their shape while the contents pull away from the wall; cheek cells shrink.
- Prokaryotic: nucleoid, no membrane-bound organelles, 1 to 10 µm. Eukaryotic: true nucleus, organelles, 10 to 100 µm.
- Nucleus: double membrane with pores; nucleolus makes ribosomal subunits; chromatin becomes chromosomes before division.
- Ribosomes make proteins; RER makes and secretes proteins; SER makes and stores fats and hormones.
- Golgi apparatus modifies, sorts and packages into vesicles; first seen by Camillo Golgi in 1898.
- Mitochondria: two membranes, cristae, cellular respiration, ATP.
- Plastids: chloroplast (chlorophyll, stroma), chromoplast (colour), leucoplast (storage). Mitochondria and plastids have their own DNA and ribosomes.
- Mature plant cells have one large central vacuole full of cell sap; loss of water causes wilting.
- Mitosis: 2 identical daughter cells. Meiosis: 2 divisions, 4 gametes with half the chromosomes; fertilisation restores the number.
- Cell Theory: Schleiden 1838, Schwann 1839, Virchow 1855. Totipotency: Haberlandt 1902. Contact inhibition stops division in many animal cells; cancer cells lose it.
Weightage and Exam Pattern
Class 9 has no board exam. This chapter is assessed in school exams (periodic tests, half-yearly and annual papers), and the NCERT source gives no mark allocation for it. It is a descriptive chapter with a strong practical side, so questions come from definitions, comparisons, diagrams and the activities.
| Question type | What this chapter typically supplies |
|---|---|
| MCQ and one-word | Organelle and function pairs, organelles with DNA, stains used, Hooke and cork, limit of resolution |
| Assertion-Reason | Cell wall and shape, RBCs without a nucleus, prokaryote or eukaryote, own DNA in mitochondria and plastids |
| Short answer | Differentiate pairs (membrane and wall, RER and SER, chloroplast and chromoplast), osmosis, wilting |
| Long answer | Mitochondria versus chloroplasts, mitosis versus meiosis, the potato osmosis experiment, a labelled plant or animal cell |
| Case-based and experiment-based | Potato pieces and potato cups, carrots in salt solution, pickles and sharbat, onion root tip |
Practise a labelled plant cell and animal cell diagram. This chapter is the foundation for Class 10 boards, where life processes and heredity build on cells, DNA and gametes.
Class 9 Science · Chapter 2 – swipe through all 10 cards to understand the whole chapter.
Microscopes and Cell Size
The eye cannot separate two points closer than about 0.1 mm, so cells need a microscope.
Total magnification = eyepiece × objective, so 10X × 10X = 100X.
- Robert Hooke saw cells in cork in 1665
- Electron microscopes use a beam of electrons
- 5000 µm field ÷ 25 cells = 200 µm per cell
Cell Membrane: Fluid-Mosaic Model
Every cell has a selectively permeable plasma membrane about 7 to 10 nm thick.
Lipid bilayer with water-attracting heads outward and water-repelling tails inward.
- Made of lipids and proteins
- Molecules move sideways, flip and rotate: fluid
- Proteins act as gatekeepers for substances
Osmosis and Solutions
Osmosis is the diffusion of water across a selectively permeable membrane.
In the potato activity the piece in water swells and the piece in salt solution shrinks.
- Hypotonic outside: water enters, cell swells
- Hypertonic outside: water leaves, cell shrinks
- Isotonic: no net movement of water
The Cell Wall
Plants, fungi and bacteria have a rigid but permeable wall outside the cell membrane.
In sugar solution a Rhoeo cell keeps its shape while its contents pull away from the wall.
- Plant cell wall is mainly cellulose
- Keeps leaves firm and plants upright
- Animal cells lack it and can change shape
Prokaryotic vs Eukaryotic
Bacteria are prokaryotic; plant and animal cells are eukaryotic.
Viruses, viroids and prions are acellular infectious agents.
- Prokaryote DNA is a single circular molecule in the nucleoid
- Eukaryotes have a membrane-bound nucleus
- Only eukaryotes have membrane-bound organelles
Nucleus, DNA and Chromosomes
The nucleus is the house of coded instructions, covered by a double membrane with pores.
Mature human RBCs have no nucleus and live about 120 days.
- Nucleolus makes ribosomal subunits
- Chromosomes are DNA plus specific proteins
- Genes are functional segments of DNA
Ribosomes, ER, Golgi, Lysosomes
Organelles make, pack, ship and clean up like departments of a factory.
Camillo Golgi first observed the Golgi apparatus in 1898.
- Ribosomes: sites of protein synthesis
- RER makes proteins; SER makes fats and hormones
- Lysosomes: enzyme sacs that break down waste
Mitochondria and Plastids
Mitochondria release energy; chloroplasts make food using sunlight.
Both have two membranes and their own DNA and ribosomes.
- Cristae increase surface area for reactions
- Chromoplasts colour flowers and fruits
- Leucoplasts store starch, oils or proteins
Mitosis and Meiosis
New cells come only from pre-existing cells, by two kinds of division.
Fertilisation restores the original chromosome number.
- Mitosis: growth, repair, asexual reproduction
- Meiosis: only in testes, ovaries and anthers
- Errors can cause tumours or genetic disorders
Cell Theory and Cell Life Span
All organisms are made of cells, and every cell has a definite life span.
Contact inhibition stops division in many animal cells; cancer cells lose this control.
- All cells arise from pre-existing cells
- Haberlandt proposed totipotency of plant cells
- Programmed Cell Death separates fingers in an embryo
📝 Practice Cell: The Building Block of Life - 10 questions
Concept-check questions · with answers & explanations
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Why C: Resolution is the measure of clarity, the ability to show two close points as separate. Microscope P enlarges the image a lot but cannot separate the two structures, so its resolution is poor. Microscope Q resolves them despite lower magnification. A big blurry image is not a clear image.
Why not A: P’s magnification is 1000X, higher than Q’s 400X, so magnification is not what it lacks.
Why not B: Contrast is the difference in brightness between parts of the object; the problem described is separating two close points, not brightness.
Why not D: Focal length is not one of the three features (resolution, contrast, magnification) that decide image quality here.
Remember: Magnification makes it bigger; resolution makes it clear.
Why C: A high concentration of salt or sugar makes the medium hypertonic compared with the contents of any bacterial or fungal cell that lands on the food. Water moves out of these cells by osmosis across their selectively permeable membranes, dehydrating them, so they cannot grow and multiply and the food does not spoil.
Why not A: Salt and sugar molecules cannot cross the cell membrane; only water moves, and it moves out, not in.
Why not B: A medium loaded with solute is hypertonic, not hypotonic; cells lose water rather than swelling.
Why not D: Spoilage bacteria and fungi do not make food from sunlight; they feed on the food itself, so blocking light would not stop them.
Remember: Strong salt or sugar pulls water out of microbes, so pickles keep.
Why A: Plants are fixed in one place and need a rigid structure to withstand wind and rain. The cell wall keeps leaves and flowers firm, maintains cell shape and helps the plant stay upright. A wall as flexible as a membrane could not do this, so the plant would droop and collapse.
Why not B: The cell wall is not what limits light entry; photosynthesis depends on chlorophyll in chloroplasts, not wall rigidity.
Why not C: Water crosses the membrane as well, by osmosis; a flexible wall would still be permeable.
Why not D: The nucleus is inside the cell and is unaffected by the wall; a plant cell stays eukaryotic.
Remember: The wall is the plant’s skeleton; make it soft and the plant wilts.
Why C: Prions are misfolded proteins which lack genetic material. Viruses are composed of some genetic material with a protein coat, and viroids have genetic material without a protein coat. All three are acellular and too small to be seen under a light microscope.
Why not A: A virus has genetic material inside its protein coat.
Why not B: A viroid is genetic material without a protein coat, the opposite of a prion.
Why not D: Prions are the exception; they have no genetic material.
Remember: Virus = gene plus coat; viroid = gene, no coat; prion = protein, no gene.
Why A: The inner membrane of a mitochondrion is folded into finger-like projections called cristae. These folds increase the surface area available for the chemical reactions of cellular respiration and so facilitate energy production. More folds mean more surface and more ATP, which suits a high-energy muscle cell.
Why not B: The outer membrane is smooth and porous on its own; the folds belong to the inner membrane and do not change the outer one.
Why not C: Starch granules are stored in the stroma of chloroplasts or in leucoplasts, not in mitochondrial folds.
Why not D: Chlorophyll is found in chloroplasts, not mitochondria; mitochondria release energy from food, not from light.
Remember: More cristae, more surface, more ATP.
Why B: Mitosis produces daughter cells with the same chromosome number as the parent, so mitotic gametes would each carry the full set. At fertilisation two gametes combine, so the offspring would have double the parental chromosome number, and this would double again in every generation. Meiosis halves the number in gametes precisely so that fertilisation restores, rather than doubles, the original number.
Why not A: Mitosis keeps each gamete’s number constant, but fertilisation adds two gametes together, so the total doubles.
Why not C: Fertilisation combines two sets; it never halves anything. Halving is the job of meiosis.
Why not D: Fertilisation is exactly the joining of two gametes’ chromosomes; that is how the original number is restored.
Remember: Meiosis halves so that fertilisation can restore.
Why C: Step 1 (unit conversion, 1 mm = 1000 µm): 4 mm = 4 × 1000 = 4000 µm. Step 2 (cell size = field diameter ÷ number of cells along it): 4000 µm ÷ 20 = 200 µm.
Why not A: 0.2 comes from dividing 4 by 20 without converting mm to µm.
Why not B: 80 comes from multiplying 4 by 20 instead of dividing the field by the number of cells.
Why not D: 4000 µm is the whole field diameter, not one cell; it must be shared among 20 cells.
Remember: Convert to µm first, then divide by the cell count.
Why D: When the solute concentration of the outside medium is lower than that inside the cell, the solution is hypotonic. The outside has more water, so water moves into the cell by osmosis and the cell swells.
Why not A: A hypertonic solution has more solute outside than inside; that is the opposite of the case given.
Why not B: Isotonic means equal solute concentrations inside and out, with no net water movement; the concentrations here are unequal.
Why not C: The name is right but the effect is wrong; a hypotonic solution pushes water in, so the cell swells, not shrinks.
Remember: Hypo means less solute outside, so water flows in and the cell swells.
Why B: The 20 per cent sugar solution is more concentrated than the cell contents, so the cell loses water by osmosis. The rigid cell wall keeps the outer boundary unchanged, but the inner content shrinks and the cell membrane pulls away from the wall, so the gap between the inner and outer boundaries increases.
Why not A: A plant cell does not shrink as a whole because the wall holds its shape; only wall-less animal cells shrink entirely.
Why not C: Water leaves the cell, since the outside is more concentrated; the cell does not swell.
Why not D: Cellulose does not dissolve in sugar solution; the wall stays intact and rigid.
Remember: Wall stays put, contents pull away.
Why B: A typical prokaryotic cell is 1 to 10 µm in diameter and the organism is usually unicellular. A typical eukaryotic cell is 10 to 100 µm and the organism can be unicellular (yeast) or multicellular (plants, animals). Eukaryotes also have a membrane-bound nucleus and organelles, which prokaryotes lack.
Why not A: The sizes are swapped, and neither group is ‘always’ one form; bacteria are usually unicellular and eukaryotes can be either.
Why not C: Membrane-bound organelles are absent in prokaryotes and present in eukaryotes, the reverse of this row.
Why not D: The membrane-bound nucleus is present in eukaryotes and absent in prokaryotes, again reversed.
Remember: Prokaryotes are small and simple; eukaryotes are ten times larger with a true nucleus.
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Frequently Asked Questions
Diffusion is the net movement of particles from a region of higher concentration to lower concentration, and it happens even without a membrane, like a dye spreading in water. Osmosis is a special case: the diffusion of water across a selectively permeable membrane, from a dilute solution to a concentrated one. Water entering root cells from the soil is osmosis.
Both cells lose water by osmosis in a concentrated sugar solution. The plant cell has a rigid cellulose cell wall that holds its shape, so only the membrane and contents shrink away from the wall. The cheek cell is an animal cell with no cell wall, so the whole cell shrinks considerably.
Prokaryotic cells, such as bacteria, have their DNA in a nucleoid with no membrane around it, have no membrane-bound organelles, are about 1 to 10 µm across and are usually unicellular. Eukaryotic cells, such as plant and animal cells, have a membrane-bound nucleus and organelles, are about 10 to 100 µm across and can be unicellular or multicellular.
The nucleus holds DNA in its chromosomes. Mitochondria and plastids such as chloroplasts also have their own DNA and ribosomes, so they can make some of their own proteins. This similarity to certain bacteria suggests they share an evolutionary history with single-celled organisms. Ribosomes, lysosomes and the Golgi apparatus have no DNA.
In mitochondria, glucose and other molecules are broken down during cellular respiration to release energy. This energy is stored as ATP, the energy currency used for most cellular activities. The inner membrane is folded into cristae, which increase the surface area for these reactions.
Mitosis is one division of a body cell that gives two genetically identical daughter cells with the same chromosome number, used for growth, repair and asexual reproduction. Meiosis happens only in reproductive organs: the cell divides twice to give four gametes, each with half the chromosomes, and it creates variation. Fertilisation restores the full number.
The classical Cell Theory states that all living organisms are made up of one or more cells, that the cell is the basic unit of structure and function, and that all cells arise from pre-existing cells. Schleiden (1838) showed plants are made of cells, Schwann (1839) showed the same for animals, and Virchow (1855) added that new cells form only from pre-existing cells.
A mature plant cell has a large central vacuole filled with cell sap. By holding water, the vacuole keeps pressure inside the cell, which keeps the cell firm. When the plant is short of water, the vacuoles lose water, the cells become less firm, and the leaves and stems droop.
