🧬 Biology · Topic 7 of 10
Start with the study notes on Cell Structure & Functions (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
The cell is the basic unit of life. Every living thing is made of one or more cells, just as a wall is built from bricks.
Schleiden and Schwann said that all living things are made of cells. Virchow added that new cells come only from existing cells.
Cells come in many sizes and shapes, each suited to its job. Most are so small that we need a microscope to see them.
Examples:
The thin outer covering of every cell. It is selectively permeable: it lets only certain substances in and out.
The jelly-like substance between the cell membrane and the nucleus. It holds the organelles, the cell's tiny working parts.
The control centre of the cell. It contains chromosomes, which carry genes, the units that pass features from parents to children.
Example: Human body cells have 46 chromosomes (23 pairs).
Remember: Bacteria have no proper nucleus; their genetic material lies loose in the cytoplasm. They are prokaryotes. Cells with a proper nucleus are eukaryotes.
Both have a cell membrane, cytoplasm, nucleus and mitochondria. Only plant cells have:
Examples:
Remember: Put in water, a plant cell swells but does not burst, because its cell wall holds firm. An animal cell may burst.
In multicellular organisms, cells are specialised for different jobs. This is called division of labour.
Cell → Tissue → Organ → Organ system → Organism
Examples:
Cells are seen with a microscope. Stains such as iodine or safranin colour the cell parts so they show up clearly.
Cells divide for growth, to repair damage, and for reproduction.
| Misconception | Correct understanding |
|---|---|
| Hooke saw living cells in cork. | He saw the empty walls of dead cork cells. |
| Animal cells have a cell wall. | Only plant cells have a cell wall; all cells have a cell membrane. |
| All cells are the same shape. | Cell shape depends on its job: long nerve cells, disc-shaped red blood cells. |
| All cells can be seen without a microscope. | Almost all cells need a microscope; an egg is a rare exception. |
| Every cell has a proper nucleus. | Bacteria (prokaryotes) do not; mature red blood cells lose theirs. |
Who discovered the cell, and in what?
Robert Hooke, in a thin slice of cork.
Which is the longest cell in the human body?
The nerve cell (neuron).
Why is the cell membrane called selectively permeable?
It lets only certain substances pass in and out.
Which organelle is the powerhouse of the cell?
The mitochondrion.
Name three things found in plant cells but not animal cells.
Cell wall, chloroplasts and a large central vacuole.
How many chromosomes are in a human body cell?
46 (23 pairs).
Put these in order: organ, cell, organism, tissue.
Cell → tissue → organ → organism.
Why doesn't a plant cell burst in water?
Its rigid cell wall holds it in.
Key takeaway: Every living thing is made of cells. Each cell has a membrane, cytoplasm and a nucleus, and plant cells also have a wall, chloroplasts and a big vacuole.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The basic structural and functional unit of all living organisms is the:
Answer: A — Cell
Every living thing, from a bacterium to a blue whale, is made of cells. The cell is the smallest unit that shows all the characteristics of life.
Q2. easy
Who discovered the cell in 1665?
Answer: A — Robert Hooke
Robert Hooke examined a thin slice of cork under his microscope and saw tiny box-like compartments. He named them 'cells' because they reminded him of the small rooms in a monastery.
Q3. medium
Robert Hooke observed cells in a thin slice of:
Answer: A — Cork
Cork is dead plant tissue from tree bark. What Hooke actually saw were the empty walls of dead cells - he never saw a living cell.
Q4. medium
Who first observed LIVING cells (tiny moving organisms) under a microscope?
Answer: A — Anton van Leeuwenhoek
Leeuwenhoek made powerful simple microscopes and in 1674 observed living single-celled organisms in pond water, which he called 'animalcules'.
Q6. medium
Organisms made of only ONE cell are called:
Answer: A — Unicellular
Amoeba, Paramecium, bacteria and yeast are unicellular. That single cell performs every life process by itself.
Q7. medium
Organisms made of MANY cells are called:
Answer: A — Multicellular
Plants, animals and humans are multicellular. Their cells are specialised for different jobs and work together as tissues, organs and organ systems.
Q9. medium
Which is generally the LONGEST cell in the human body?
Answer: A — The nerve cell (neuron)
A single neuron can be a metre long, running from the spinal cord to the toe. Its length lets it carry signals over long distances quickly.
Q11. medium
The three basic parts found in ALL cells are:
Answer: A — Cell membrane, cytoplasm and nucleus (or nuclear material)
Every cell is bounded by a cell membrane, filled with cytoplasm, and contains genetic material. Cell walls and chloroplasts are found only in plant cells.
Q13. medium
The cell membrane is described as SEMI-PERMEABLE because it:
Answer: A — Allows only certain substances to pass in and out
This selective control keeps useful substances in and harmful ones out, and lets the cell regulate its internal environment.
Q15. medium
The jelly-like substance between the cell membrane and the nucleus is:
Answer: A — Cytoplasm
Cytoplasm holds all the cell organelles and is the site of many chemical reactions that keep the cell alive.
Q17. medium
The nucleus is often called the 'brain' or control centre of the cell because it:
Answer: A — Controls all the activities of the cell and carries the hereditary material
The nucleus directs growth, repair and reproduction, and contains chromosomes carrying the genetic information passed to the next generation.
Q19. medium
The units of inheritance that pass characters from parents to offspring are called:
Answer: A — Genes
Genes are segments of DNA on chromosomes. They determine features such as eye colour, height and blood group, and are passed on during reproduction.
Q21. medium
The dense round body inside the nucleus is the:
Answer: A — Nucleolus
The nucleolus is a small spherical structure within the nucleus. It is involved in making ribosomes, which in turn make proteins.
Q23. medium
Cells WITHOUT a well-organised nucleus, such as bacteria, are called:
Answer: A — Prokaryotic cells
In a prokaryotic cell, the genetic material is not enclosed in a nuclear membrane and floats in the cytoplasm. Such cells also lack most membrane-bound organelles.
Q25. medium
Structures in the cytoplasm that carry out specific functions are called:
Answer: A — Organelles
Just as organs perform tasks in the body, organelles such as mitochondria, ribosomes, plastids and vacuoles perform particular jobs inside the cell.
Q27. medium
Green plastids that contain chlorophyll and carry out photosynthesis are called:
Answer: A — Chloroplasts
Chloroplasts are found in plant cells exposed to light, mainly in leaves. They trap sunlight and make glucose - the process on which all food chains depend.
Q29. medium
The large fluid-filled space in a plant cell is called the:
Answer: A — Vacuole
A mature plant cell usually has one big central vacuole filled with cell sap. It stores water and dissolved substances and keeps the cell firm.
Q31. medium
Animal cells have vacuoles that are:
Answer: A — Small and numerous, if present at all
Animal cells have several tiny vacuoles or none. The huge single vacuole is a distinguishing feature of mature plant cells.
Q33. medium
Plant cells usually have a fixed, rectangular shape because of the:
Answer: A — Rigid cell wall
The tough cellulose wall holds the cell in a definite shape. Animal cells, lacking a wall, are usually rounded and irregular.
Q35. medium
Amoeba has no fixed shape because it:
Answer: A — Constantly changes shape by extending pseudopodia
Amoeba pushes out finger-like pseudopodia to move and to capture food, so its outline is always changing. It has a flexible membrane but no cell wall.
Q37. medium
The correct order of organisation in a multicellular organism is:
Answer: A — Cell → Tissue → Organ → Organ system → Organism
Each level is built from the one before. This hierarchy allows division of labour, with different parts specialising in different jobs.
Q39. medium
While preparing an onion peel slide, a stain such as iodine or safranin is used to:
Answer: A — Make the cell parts clearly visible under the microscope
Cell structures are transparent and hard to see. Stains are absorbed differently by different parts, so the nucleus and cell wall stand out clearly.
Q41. medium
Human cheek cells observed under a microscope appear:
Answer: A — Rounded or irregular, with no cell wall and a clear nucleus
Cheek cells are typical animal cells: a flexible membrane rather than a wall, an irregular flattened shape, and a distinct nucleus when stained.
Q43. medium
Which scientists are credited with proposing the Cell Theory?
Answer: A — Schleiden and Schwann
The botanist Matthias Schleiden and the zoologist Theodor Schwann proposed in 1838-39 that plants and animals alike are composed of cells.
Q45. medium
Red blood cells are unusual because a mature human red blood cell:
Answer: A — Has no nucleus, leaving more room to carry oxygen
Losing the nucleus lets the cell pack in more haemoglobin and take on a flexible biconcave shape, but it also means the cell lives only about 120 days.
Q47. medium
The endoplasmic reticulum is a network of tubes that:
Answer: A — Transports materials within the cell
The endoplasmic reticulum acts as an internal transport and manufacturing system. Rough ER studded with ribosomes makes proteins; smooth ER makes fats.
Q49. medium
Which organelle packages and dispatches materials made by the cell?
Answer: A — The Golgi apparatus
The Golgi apparatus modifies, sorts and packs proteins and other substances into vesicles for storage inside the cell or export outside it.
Q51. medium
The tail of a sperm cell is an adaptation for:
Answer: A — Swimming towards the egg
The whip-like tail propels the sperm through fluid to reach the egg. The head carries the genetic material and enzymes to penetrate the egg's surface.
Q53. medium
Which of these correctly compares a plant cell and an animal cell?
Answer: A — Plant cells have a cell wall, chloroplasts and a large vacuole; animal cells do not
Both have a membrane, cytoplasm, nucleus, mitochondria and ribosomes, but the wall, plastids and large central vacuole are unique to the plant cell.
Q55. medium
A cell placed in pure water swells because water enters by:
Answer: A — Osmosis
Water moves through the semi-permeable membrane from the more dilute outside to the more concentrated cell contents. Plant cells swell only until the cell wall stops them.
Q57. medium
Which overall statement about cells is CORRECT?
Answer: A — All living things are made of cells, each with a membrane, cytoplasm and genetic material, and cells vary in size, shape and function
This sums up the topic: the cell is the fundamental unit of life; organelles divide the work inside it; and specialised cells form tissues, organs and whole organisms.
Q58. medium
Why are most cells microscopic in size rather than very large?
Answer: A — A small size gives a large surface area relative to volume, so materials move in and out efficiently
As a cell grows, its volume increases faster than its surface. A small cell has plenty of membrane relative to its contents, so oxygen, food and wastes cross quickly enough to keep it alive.
Q60. medium
Which structure controls what enters and leaves BOTH plant and animal cells?
Answer: A — The cell membrane
The selectively permeable cell membrane surrounds every cell and regulates the passage of substances. The cell wall, present only in plants, lies outside it and is freely permeable.
Q5. hard
Who discovered the NUCLEUS of the cell in 1831?
Answer: A — Robert Brown
Robert Brown identified the dense round body present in cells and named it the nucleus. It was later found to control all the cell's activities.
Q8. hard
Which is the LARGEST cell known?
Answer: A — The egg of an ostrich
An ostrich egg is a single cell and is the largest known. Among human cells, the female egg cell is the largest and can just be seen without a microscope.
Q10. hard
Which is the SMALLEST cell?
Answer: A — A bacterial cell (Mycoplasma)
Mycoplasma, at about 0.1 micrometre, is the smallest known cell. Bacteria in general are far smaller than plant or animal cells.
Q12. hard
The thin outer covering present in ALL cells is the:
Answer: A — Cell membrane (plasma membrane)
The cell membrane gives the cell its shape and controls which substances enter and leave. Because it is selective, it is called semi-permeable.
Q14. hard
The rigid outer layer found ONLY in plant cells is the:
Answer: A — Cell wall, made of cellulose
The cellulose cell wall lies outside the cell membrane, giving the plant cell a fixed rectangular shape, strength and protection. Animal cells have no cell wall.
Q16. hard
The living substance of the cell, including the cytoplasm and nucleus, is called:
Answer: A — Protoplasm
Protoplasm was described by Purkinje as 'the living substance of the cell'. It comprises the cytoplasm together with the nucleus.
Q18. hard
The thread-like structures inside the nucleus that carry genes are called:
Answer: A — Chromosomes
Chromosomes are made of DNA and protein and carry the genes that determine inherited characteristics. Human body cells contain 46 chromosomes (23 pairs).
Q20. hard
How many chromosomes are present in a normal human body cell?
Answer: A — 46 (23 pairs)
Each body cell has 46 chromosomes - 23 inherited from each parent. Egg and sperm cells contain only 23 each, so the number stays constant across generations.
Q22. hard
Cells that have a well-defined nucleus surrounded by a nuclear membrane are called:
Answer: A — Eukaryotic cells
Plants, animals, fungi and protists have eukaryotic cells. Bacteria and blue-green algae, whose genetic material lies free in the cytoplasm, are prokaryotic.
Q24. hard
The organelle known as the 'powerhouse of the cell' is the:
Answer: A — Mitochondrion
Mitochondria carry out respiration, releasing energy from food and storing it as ATP. Cells that need a lot of energy, such as muscle cells, have many mitochondria.
Q26. hard
The organelles where proteins are made are the:
Answer: A — Ribosomes
Ribosomes are tiny granular structures, free in the cytoplasm or attached to the endoplasmic reticulum, that assemble amino acids into proteins.
Q28. hard
Plastids are found in:
Answer: A — Plant cells only
Plastids - chloroplasts, coloured chromoplasts and colourless food-storing leucoplasts - are unique to plant cells. Animal cells have none.
Q30. hard
The large central vacuole in a plant cell helps by:
Answer: A — Storing water and salts, and keeping the cell firm (turgid)
Water pressure in the vacuole pushes the cytoplasm against the cell wall, giving the cell rigidity. When plants lose water, the vacuoles shrink and the plant wilts.
Q32. hard
Which of these is found in a PLANT cell but NOT in an animal cell?
Answer: A — Cell wall, chloroplasts and a large central vacuole
These three features distinguish plant cells. Both cell types have a membrane, cytoplasm, nucleus, mitochondria and ribosomes.
Q34. hard
Which of these best describes the shape of cells?
Answer: A — Cells vary in shape according to their function - some are round, some long, some spindle-shaped
Nerve cells are long and branched to carry signals, red blood cells are disc-shaped to squeeze through capillaries, and muscle cells are spindle-shaped for contraction.
Q36. hard
A group of similar cells performing a particular function is called a:
Answer: A — Tissue
Muscle tissue, nerve tissue and xylem are examples. Tissues combine to form organs, organs form organ systems, and systems together form the organism.
Q38. hard
Which instrument is used to observe cells?
Answer: A — A microscope
Cells are far too small for the naked eye. A compound microscope magnifies them hundreds of times, and an electron microscope reveals even the organelles inside.
Q40. hard
In an onion peel viewed under a microscope, the cells appear:
Answer: A — Rectangular, arranged closely with distinct walls and a dark nucleus
Onion peel cells form a neat brick-like pattern because of their rigid walls. They contain no chloroplasts because onion bulbs grow underground.
Q42. hard
The Cell Theory states that:
Answer: A — All living things are made of cells, and all cells arise from pre-existing cells
Schleiden and Schwann proposed that all organisms are made of cells, and Virchow added that every cell comes from a pre-existing cell by division.
Q44. hard
Rudolf Virchow added to the cell theory the idea that:
Answer: A — New cells arise only from the division of existing cells
Virchow's phrase 'omnis cellula e cellula' - every cell from a cell - completed the theory and ruled out the idea of life arising spontaneously.
Q46. hard
Cells that carry out specialised functions in a multicellular organism show:
Answer: A — Division of labour - each type of cell does a particular job
Specialisation makes the organism efficient: nerve cells carry signals, muscle cells contract, red blood cells carry oxygen and root hair cells absorb water.
Q48. hard
Lysosomes are called the 'suicide bags' of the cell because they:
Answer: A — Contain digestive enzymes that break down worn-out parts and can destroy the cell itself
Lysosomes digest foreign particles and worn-out organelles. If the cell is damaged, they may burst and digest the whole cell.
Q50. hard
Cells divide mainly for:
Answer: A — Growth, repair of damaged tissue, and reproduction
Cell division adds cells so the organism grows, replaces cells lost through wear and injury, and produces the gametes needed for reproduction.
Q52. hard
Root hair cells are long and narrow because this:
Answer: A — Increases the surface area for absorbing water and minerals
The long extension pushes between soil particles and multiplies the surface in contact with soil water, making absorption far more efficient.
Q54. hard
Why is the cell called the 'building block of life'?
Answer: A — Because every living organism is built from cells, just as a wall is built from bricks
The comparison captures the idea that complex living structures are assembled from vast numbers of small, similar units, each alive in its own right.
Q56. hard
Why does a plant cell NOT burst when placed in water, while an animal cell might?
Answer: A — The rigid cell wall resists the pressure of the incoming water
As water enters, the plant cell becomes turgid and the wall pushes back, stopping further entry. An animal cell has only a flexible membrane and can burst.
Q59. hard
The green plastid, the coloured plastid in flowers and fruits, and the colourless storage plastid are respectively:
Answer: A — Chloroplast, chromoplast and leucoplast
Chloroplasts (green) photosynthesise, chromoplasts give petals and fruits their red, orange and yellow colours, and leucoplasts (colourless) store starch, oil or protein.