Cell: The Unit of Life Class 11 Biology: Cell Theory, Organelles and Nucleus

What Cell: The Unit of Life is about

Weightage: This chapter is part of Unit III, Cell: Structure and Function, which carries 15 of the 70 theory marks in the school annual exam together with Biomolecules and Cell Cycle and Cell Division.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit III Cell: Structure and Function

Key concepts

What a cell is, and the cell theory

NCERT section 8.1 What Is a Cell?; 8.2 Cell Theory; 8.3 An Overview of Cell

Unicellular organisms (Amoeba, bacteria) are able to live independently and do every life activity inside a single cell. Multicellular organisms divide this work among many cells. Either way, nothing smaller than a complete cell can live on its own, so the cell is the fundamental unit of life.

History in order:

• Anton von Leeuwenhoek was the first to observe and describe a living cell.
• The nucleus was found later by Robert Brown.
• Matthias Schleiden (1838), a German botanist, studied many plants and found that all plants are made of different kinds of cells that form plant tissues.
• Theodore Schwann (1839), a British zoologist, studied animal cells and found a thin outer layer, now called the plasma membrane. From plant tissues he concluded that the cell wall is a unique feature of plant cells. He proposed that animal and plant bodies are made of cells and their products.
• Schleiden and Schwann together gave the cell theory, but it did not explain how new cells form.
• Rudolf Virchow (1855) explained that cells divide and new cells come from pre-existing cells: *Omnis cellula-e cellula*.

Cell theory as understood today:
1. All living organisms are made of cells and the products of cells.
2. All cells arise from pre-existing cells.

Variety of cells: the smallest cells are mycoplasmas (about 0.3 µm long); bacteria are about 3 to 5 µm; human red blood cells are about 7 µm across; nerve cells are among the longest cells; the largest isolated single cell is the egg of an ostrich. Shapes vary with function: disc-like (RBC), amoeboid (WBC), columnar (epithelium), long and branched (nerve cell), elongated (tracheid).

Worked example

State the two points of the modern cell theory. What was missing from the theory as first proposed by Schleiden and Schwann, and who supplied it? (3 marks)

Try it first, then show the solutionSolution

1. All living organisms are composed of cells and the products of cells. (1)
2. All cells arise from pre-existing cells. (1)
3. The original theory did not explain how new cells are formed; Rudolf Virchow (1855) showed that cells divide and new cells come from existing cells, Omnis cellula-e cellula. (1)

Answer: Organisms are made of cells and cell products; cells come only from pre-existing cells (Virchow's addition).

Prokaryotic cells: organisation, cell envelope and appendages

NCERT section 8.4 Prokaryotic Cells (8.4.1 Cell Envelope and its Modifications; 8.4.2 Ribosomes and Inclusion Bodies)

Who: bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (pleuro-pneumonia-like organisms). They are usually smaller than eukaryotic cells and multiply faster.

Bacterial shapes: bacillus (rod), coccus (sphere), vibrio (comma) and spirillum (spiral).

Basic plan:

• A cell wall around the cell membrane in all prokaryotes except mycoplasma.
• No well-defined nucleus; the genetic material is naked (no nuclear membrane). The main DNA is a single circular chromosome.
• Many bacteria also carry small circular DNA outside the main chromosome, called plasmids. These give extra traits such as antibiotic resistance.
• No membrane-bound organelles; only ribosomes are present.
• Mesosome: an infolding of the plasma membrane in the form of vesicles, tubules or lamellae. It helps in cell wall formation, DNA replication and sharing DNA between daughter cells, respiration and secretion, and it increases the membrane's surface area and enzyme content.
• Chromatophores: membranous extensions in cyanobacteria that hold pigments for photosynthesis.

Cell envelope: three tightly bound layers acting as one protective unit, from outside in:
1. Glycocalyx: a loose slime layer in some bacteria, a thick tough capsule in others.
2. Cell wall: gives shape and strong support and stops the cell from bursting or collapsing.
3. Plasma membrane: selectively permeable, similar in structure to the eukaryotic membrane.

Based on the envelope and the Gram stain, bacteria are Gram positive (take up the stain) or Gram negative (do not).

Surface structures:

• Flagellum (for movement): three parts, filament (longest, extends outside), hook and basal body.
• Pili: elongated tubular structures made of a special protein.
• Fimbriae: small bristle-like fibres that help some bacteria attach to rocks in streams or to host tissues.

Ribosomes and inclusions: prokaryotic ribosomes are attached to the plasma membrane, about 15 nm by 20 nm, 70S (subunits 50S and 30S). Several ribosomes on one mRNA form a polysome, which translates the mRNA into protein. Inclusion bodies store reserve material and lie free in the cytoplasm without a membrane: phosphate granules, cyanophycean granules, glycogen granules; gas vacuoles occur in blue-green, purple and green photosynthetic bacteria.

Worked example

Name the three layers of a bacterial cell envelope in order from outside, and give one role each of the mesosome and the fimbriae. (3 marks)

Try it first, then show the solutionSolution

1. Glycocalyx (slime layer or capsule) → cell wall → plasma membrane. (1½)
2. Mesosome: any one of cell wall formation, DNA replication and distribution to daughter cells, respiration, secretion, or increasing surface area. (1)
3. Fimbriae: help the bacterium attach to surfaces such as rocks or host tissue. (½)

Answer: Glycocalyx, cell wall, plasma membrane; mesosome helps in wall formation, DNA replication, respiration; fimbriae help attachment.

Eukaryotic cells, the plasma membrane and the cell wall

NCERT section 8.5 Eukaryotic Cells (8.5.1 Cell Membrane; 8.5.2 Cell Wall)

Eukaryotes: all protists, plants, animals and fungi. Their cytoplasm is divided into compartments by membrane-bound organelles, and they have a true nucleus with a nuclear envelope.

Plant cell vs animal cell:

• Plant cells have a cell wall, plastids and a large central vacuole; these are absent in animal cells.
• Animal cells have centrioles, which are absent in almost all plant cells.

Plasma membrane (cell membrane):

• Studies on human RBC membranes showed it is made mainly of lipids and proteins: about 52% protein and 40% lipid in the RBC membrane (the ratio differs between cell types).
• The main lipids are phospholipids in a bilayer: the polar heads face outwards (towards water) and the non-polar hydrocarbon tails face inwards, away from water. Cholesterol is also present.
• Proteins are peripheral (on the surface) or integral (partly or fully buried in the bilayer).
• Fluid mosaic model (Singer and Nicolson, 1972): the lipid layer is quasi-fluid, so proteins can move sideways (lateral movement) within it. This fluidity matters for cell growth, intercellular junctions, secretion, endocytosis and cell division.

Transport across the membrane (the membrane is selectively permeable):

• Passive transport: no energy used; molecules move down a concentration gradient (high to low). Neutral solutes cross by simple diffusion. Water moves by osmosis.
• Polar molecules cannot cross the non-polar lipid bilayer by themselves; they need carrier proteins.
• Active transport: molecules move against the concentration gradient using ATP, for example the Na⁺/K⁺ pump.

Cell wall (non-living rigid layer outside the plasma membrane of fungi and plants): gives shape, protects from mechanical damage and infection, helps cell-to-cell contact and blocks unwanted large molecules.

• Algae: cellulose, galactans, mannans and minerals such as calcium carbonate.
• Plants: cellulose, hemicellulose, pectins and proteins.
• Young cells have a primary wall, which can grow; it shrinks in importance as the cell matures and a secondary wall forms on the inner side (towards the membrane).
• The middle lamella, mainly calcium pectate, cements neighbouring cells together.
• Plasmodesmata: cytoplasmic strands crossing the wall and middle lamella, linking the cytoplasm of neighbouring cells.

Worked example

Oxygen enters a cell freely, but the cell keeps K⁺ ions at a much higher concentration inside than outside. Explain how each crosses the plasma membrane and why the two methods differ. Name the model of membrane structure involved and its proposers. (3 marks)

Try it first, then show the solutionSolution

1. O₂ is a small neutral molecule: it passes through the non-polar lipid core by simple diffusion, down its concentration gradient, with no energy used (passive transport). (1)
2. K⁺ is charged, so it cannot cross the non-polar core on its own and needs a carrier protein; because it is moved against its concentration gradient, ATP is spent (active transport, as in the Na⁺/K⁺ pump). (1)
3. Fluid mosaic model, proposed by Singer and Nicolson (1972). (1)

Answer: O₂: simple diffusion (passive); K⁺: carrier protein using ATP against the gradient (active); fluid mosaic model of Singer and Nicolson.

Endomembrane system: ER, Golgi apparatus, lysosomes and vacuoles

NCERT section 8.5.3 Endomembrane System

Organelles whose functions are coordinated and work like one system form the endomembrane system: ER, Golgi complex, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes are not included, because their functions are not coordinated with these.

Endoplasmic reticulum (ER): a network of tiny tubules spread through the cytoplasm, dividing it into a luminal compartment (inside the ER) and an extra-luminal compartment (cytoplasm).

• Rough ER (RER): ribosomes attached to its outer surface; common in cells active in protein synthesis and secretion; extensive and continuous with the outer membrane of the nucleus.
• Smooth ER (SER): no ribosomes; the main site of lipid synthesis. In animal cells, lipid-like steroid hormones are made in the SER.

Golgi apparatus: first seen by Camillo Golgi (1898) as densely stained reticular structures near the nucleus.

• Made of flat, disc-shaped sacs called cisternae (0.5 µm to 1.0 µm across), stacked parallel to each other.
• Concentrically arranged near the nucleus with a convex cis (forming) face and a concave trans (maturing) face; the two faces are different but linked.
• Main job: packaging materials for delivery inside the cell or for secretion. Materials packed in vesicles from the ER fuse with the cis face, move towards the trans face, and leave from there. This is why the Golgi is closely linked with the ER.
• Proteins made by ribosomes on the ER are modified in the Golgi cisternae; it is an important site for forming glycoproteins and glycolipids.

Lysosomes: membrane-bound vesicles formed by packaging in the Golgi. They are full of hydrolytic enzymes (lipases, proteases, carbohydrases) that work best at acidic pH and can digest carbohydrates, proteins, lipids and nucleic acids.

Vacuoles: membrane-bound spaces holding water, sap, excretory products and unwanted materials. The membrane is the tonoplast.

• In plant cells the vacuole can occupy up to 90% of the cell volume.
• The tonoplast pumps many ions and other materials into the vacuole against their concentration gradient, so their concentration is much higher in the vacuole than in the cytoplasm.
• Contractile vacuole in Amoeba: osmoregulation and excretion. Food vacuoles in many protists form by engulfing food particles.

Worked example

Distinguish between RER and SER (two points), and explain why mitochondria are not part of the endomembrane system. (3 marks)

Try it first, then show the solutionSolution

1. RER bears ribosomes; SER has none. (1)
2. RER makes proteins for secretion; SER makes lipids (steroid hormones in animal cells). (1)
3. The endomembrane system is a set of organelles whose functions are coordinated (ER → Golgi → lysosomes/vacuoles); mitochondria work independently of this flow, so they are excluded. (1)

Answer: RER: ribosomes, proteins; SER: no ribosomes, lipids. Mitochondria are excluded because their function is not coordinated with the ER-Golgi system.

Mitochondria and plastids: the energy organelles

NCERT section 8.5.4 Mitochondria; 8.5.5 Plastids

Mitochondria

• Hard to see under a light microscope unless specially stained. Their number per cell varies with the cell's physiological activity.
• Shape: sausage-shaped or cylindrical; about 0.2 to 1.0 µm in diameter (average 0.5 µm) and 1.0 to 4.1 µm long.
• Double membrane: the outer membrane is smooth and forms the outer boundary; the inner membrane folds inwards into cristae, which increase the surface area.
• Two compartments: the outer compartment (between the membranes) and the inner compartment, filled with a dense, uniform matrix.
• Each membrane has its own specific enzymes for mitochondrial function.
• Site of aerobic respiration; they produce energy as ATP, hence called the power houses of the cell.
• The matrix has a single circular DNA, a few RNA molecules, 70S ribosomes and the components needed for protein synthesis.
• They divide by fission.

Plastids: found in all plant cells and in euglenoids; easily seen because they are large. Classified by pigment:

• Chloroplasts: chlorophyll and carotenoid pigments that trap light for photosynthesis.
• Chromoplasts: fat-soluble carotenoids such as carotene and xanthophylls that give yellow, orange or red colours (petals, ripe fruits).
• Leucoplasts: colourless storage plastids: amyloplasts store starch (potato), elaioplasts store oils and fats, aleuroplasts store proteins.

Chloroplast structure:

• Mostly in the mesophyll cells of leaves; lens-shaped, oval, spherical, discoid or ribbon-like; about 5 to 10 µm long and 2 to 4 µm wide.
• Number varies from one per cell in *Chlamydomonas* (a green alga) to 20 to 40 per cell in mesophyll.
• Double membrane; the inner membrane is relatively less permeable.
• Inside is the stroma. Flat membranous sacs, thylakoids, are stacked like coins into grana (singular granum); flat tubules called stroma lamellae connect thylakoids of different grana. The space inside a thylakoid is the lumen.
• The stroma has enzymes for synthesis of carbohydrates and proteins, plus small, double-stranded circular DNA and 70S ribosomes (smaller than the 80S cytoplasmic ribosomes).
• Chlorophyll pigments are in the thylakoids.

Worked example

Give three features that mitochondria and chloroplasts share, and one way in which their internal membranes differ. (3 marks)

Try it first, then show the solutionSolution

1. Both are bounded by a double membrane. (½)
2. Both contain their own circular DNA and 70S ribosomes, so they can make some of their own proteins. (1)
3. Both make ATP / are involved in energy conversion. (½)
4. Mitochondrial inner membrane folds into cristae; in chloroplasts a separate system of thylakoids stacked into grana lies inside the stroma. (1)

Answer: Shared: double membrane, own DNA and 70S ribosomes, energy role. Different: cristae vs thylakoids/grana.

Ribosomes, cytoskeleton, cilia, flagella and centrioles

NCERT section 8.5.6 Ribosomes; 8.5.7 Cytoskeleton; 8.5.8 Cilia and Flagella; 8.5.9 Centrosome and Centrioles

Ribosomes: first seen as dense particles by George Palade (1953). Made of RNA and protein; not surrounded by any membrane.

• Eukaryotic ribosomes are 80S (subunits 60S + 40S); prokaryotic ribosomes are 70S (subunits 50S + 30S).
• S stands for the sedimentation coefficient (Svedberg unit), which indirectly measures density and size. S values are not added like masses: 60S + 40S gives 80S, not 100S.
• Ribosomes are found free in the cytoplasm, on RER, and inside mitochondria and chloroplasts.

Cytoskeleton: a network of filamentous protein structures in the cytoplasm, made of microtubules, microfilaments and intermediate filaments. It gives mechanical support, helps motility and maintains the shape of the cell.

Cilia and flagella: hair-like outgrowths of the cell membrane.

• Cilia: small, work like oars, moving the cell or the fluid around it.
• Flagella: comparatively longer; responsible for cell movement. (Prokaryotic flagella are structurally different from eukaryotic ones.)
• Electron microscope view: covered with plasma membrane; the core, axoneme, has many microtubules running parallel to the long axis.
• 9+2 arrangement: nine pairs (doublets) of microtubules around the edge plus two single microtubules in the centre.
• The central tubules are joined by bridges and enclosed by a central sheath, which is connected to one tubule of each peripheral doublet by a radial spoke (so there are nine radial spokes). Neighbouring peripheral doublets are joined by linkers.
• Both cilia and flagella arise from a centriole-like structure called the basal body.

Centrosome and centrioles:

• The centrosome usually has two cylindrical centrioles, lying perpendicular to each other, surrounded by amorphous pericentriolar material.
• Each centriole looks like a cartwheel: nine evenly spaced peripheral fibrils of the protein tubulin, each fibril a triplet; adjacent triplets are linked.
• The central part is the proteinaceous hub, joined to the peripheral triplets by radial spokes.
• Centrioles form the basal bodies of cilia and flagella and give rise to spindle fibres (spindle apparatus) during cell division in animal cells.

Worked example

Compare the arrangement of microtubules in the axoneme of a cilium with that of a centriole. (3 marks)

Try it first, then show the solutionSolution

1. Axoneme: nine peripheral doublets plus two central single microtubules (9+2), with a central sheath, radial spokes and linkers. (1½)
2. Centriole: nine peripheral triplets of tubulin, no central microtubules (9+0), with a central proteinaceous hub joined by radial spokes (cartwheel). (1)
3. The basal body at the base of a cilium is centriole-like. (½)

Answer: Cilium: 9 doublets + 2 central (9+2); centriole: 9 triplets, no central tubules, central hub (9+0).

Nucleus, chromosomes and microbodies

NCERT section 8.5.10 Nucleus; 8.5.11 Microbodies

Nucleus: first described by Robert Brown (1831). The material stained by basic dyes was later named chromatin by Flemming.

Interphase nucleus (not dividing) has:

• Chromatin: highly extended, elaborate nucleoprotein fibres.
• Nuclear matrix (nucleoplasm) with one or more spherical nucleoli (singular nucleolus).
• Nuclear envelope: two parallel membranes with a perinuclear space of 10 to 50 nm between them. The outer membrane is usually continuous with the ER and bears ribosomes.
• Nuclear pores: the envelope is broken at several places where the two membranes fuse. These pores are the passages for RNA and protein to move in both directions between the nucleus and the cytoplasm.
• Nucleolus: not membrane-bound, so its contents are continuous with the nucleoplasm. It is the site of active ribosomal RNA (rRNA) synthesis; cells actively making proteins have larger and more numerous nucleoli.

Number: normally one nucleus per cell, but some cells have more than one. Mature mammalian RBCs and the sieve tube cells of vascular plants lack a nucleus. (Do not call them prokaryotic; they are eukaryotic cells that lose the nucleus.)

Chromatin and chromosomes: chromatin contains DNA, basic proteins called histones, some non-histone proteins and RNA. A single human cell has about two metre long thread of DNA spread among its 46 chromosomes (23 pairs). During cell division, chromatin condenses into structured chromosomes.

Chromosome structure: every chromosome has a primary constriction, the centromere, holding two chromatids together. Disc-shaped kinetochores lie on the sides of the centromere.

Types by centromere position:

• Metacentric: centromere in the middle; two equal arms.
• Sub-metacentric: centromere slightly away from the middle; one shorter and one longer arm.
• Acrocentric: centromere close to one end; one extremely short and one very long arm.
• Telocentric: centromere at the end (terminal).

Some chromosomes have a secondary constriction at a constant location, giving a small fragment called the satellite.

Microbodies: many membrane-bound minute vesicles containing various enzymes, found in both plant and animal cells.

Worked example

A chromosome has one arm about twice as long as the other. Classify it and explain your reasoning. Where are its kinetochores found? (2 marks)

Try it first, then show the solutionSolution

1. Unequal arms but not extremely short: the centromere is slightly away from the middle, so it is sub-metacentric. (1)
2. Kinetochores are disc-shaped structures on the sides of the centromere (where spindle fibres attach during division). (1)

Answer: Sub-metacentric; kinetochores sit on either side of the centromere.

Cell: The Unit of Life in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Cell Theory and Prokaryotic Cells
• Mitochondria, Plastids and Ribosomes

Cell: The Unit of Life: key facts

Cell theory (Schleiden, Schwann, Virchow)

$\text{organisms}=\text{cells}+\text{cell products};\text{new cell}\leftarrow \text{pre-existing cell}$

Omnis cellula-e cellula (Virchow, 1855).

Bacterial cell envelope (outside to inside)

$\text{glycocalyx}\to \text{cell wall}\to \text{plasma membrane}$

Glycocalyx: slime layer (loose) or capsule (thick).

Ribosome sizes

$\text{70S}=\text{50S}+\text{30S (prokaryote)};\text{80S}=\text{60S}+\text{40S (eukaryote)}$

S values are sedimentation coefficients and are not added arithmetically.

Secretory pathway

$\text{RER}\to \text{vesicle}\to \text{cis face of Golgi}\to \text{trans face}\to \text{secretory vesicle / lysosome}$

Golgi forms glycoproteins and glycolipids.

Microtubule patterns

$\text{cilium/flagellum axoneme}=9\text{doublets}+2\text{central};\text{centriole}=9\text{triplets}+\text{hub}$

Basal body is centriole-like.

Chloroplast compartments

$\text{outer membrane}\to \text{inner membrane}\to \text{stroma}\supset \text{grana (thylakoid stacks)}+\text{stroma lamellae}$

Pigments in thylakoids; enzymes, DNA, 70S ribosomes in stroma.

Cell: The Unit of Life: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Cell: The Unit of Life

The question patterns to expect, and the method that works for each.

Tricks to remember Cell: The Unit of Life

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Bacteria shapes: Big Cats Visit Spain

  Bacillus (rod), Coccus (sphere), Vibrio (comma), Spirillum (spiral).

  Use it when: Four basic bacterial shapes.

• Envelope GCP

  Glycocalyx, Cell wall, Plasma membrane: outside to inside, like a Gift wrap, Carton, Plastic bag.

  Use it when: Order of the bacterial cell envelope.

• ALE of leucoplasts

  Amyloplast = Amylum (starch); Elaioplast = Elaion (oil); Aleuroplast = proteins (aleurone grains in cereals).

  Use it when: Matching storage plastids to their stored food.

• Cilia 9+2, centriole 9+0 triplets

  Cilia have 'two in the middle, doubles outside'; centrioles have 'nothing in the middle, triples outside'.

  Use it when: Distinguishing axoneme from centriole.

• M-S-A-T: centromere walks to the end

  Metacentric (middle) → Sub-metacentric (slightly off) → Acrocentric (near end) → Telocentric (at the end, T for terminal).

  Use it when: Classifying chromosomes.

• Cis comes in, trans takes out

  Cis = forming face (receives from ER); trans = maturing face (sends out vesicles).

  Use it when: Golgi polarity.

Cell: The Unit of Life: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Cell: The Unit of Life

The slips examiners see most often, and what to write instead.

• ✗Mistake: Writing 50S + 30S = 80S or 60S + 40S = 100S

  Students add S values like masses.

  ✓Do this: 70S = 50S + 30S; 80S = 60S + 40S. S is a sedimentation coefficient and does not add up arithmetically.

  Usually costs: 1 mark.

• ✗Mistake: Saying Robert Brown discovered the cell

  Two early names are mixed up.

  ✓Do this: Leeuwenhoek first saw a live cell; Robert Brown discovered the nucleus (1831).

  Usually costs: ½-1 mark.

• ✗Mistake: Including mitochondria or chloroplasts in the endomembrane system

  They are membrane-bound, so students assume they belong.

  ✓Do this: Endomembrane system = ER, Golgi, lysosomes, vacuoles only; mitochondria, chloroplasts and peroxisomes are excluded.

  Usually costs: 1 mark.

• ✗Mistake: Calling mature RBCs prokaryotic because they lack a nucleus

  'No nucleus' is linked to prokaryotes.

  ✓Do this: Mature mammalian RBCs and sieve tube cells are eukaryotic cells that lose their nucleus.

  Usually costs: 1 mark.

• ✗Mistake: Drawing the cilium with nine triplets or the centriole with a central pair

  The two patterns are swapped.

  ✓Do this: Cilium/flagellum: 9 doublets + 2 central. Centriole: 9 triplets + central hub, no central microtubules.

  Usually costs: 1-2 marks in a diagram.

• ✗Mistake: Saying the secondary wall forms outside the primary wall

  'Secondary' is taken to mean 'outer'.

  ✓Do this: The secondary wall is laid down on the inner side, towards the plasma membrane.

  Usually costs: ½ mark.

NCERT exercise guide: Cell: The Unit of Life

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Cell: The Unit of Life

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The cell wall of algae may contain all of the following except:
   (a) galactans (b) mannans (c) calcium carbonate (d) chitin

   Hint

   Chitin is a fungal and arthropod material.

   Answer

   (d) chitin

2. Q2 · 1 mark · MCQExam level

   Which organelle stores oils and fats in plant cells?
   (a) Amyloplast (b) Elaioplast (c) Aleuroplast (d) Chromoplast

   Hint

   Elaion = oil.

   Answer

   (b) Elaioplast

3. Q3 · 2 marks · Very short answerExam level

   Name the three parts of a bacterial flagellum, and state the function of fimbriae. (2 marks)

   Hint

   Longest part extends outside.

   Answer

   Filament, hook and basal body (1); fimbriae help bacteria attach to rocks in streams or to host tissues (1).

4. Q4 · 2 marks · Very short answerHarder exam level

   A student says: 'A mitochondrion has its own DNA and ribosomes, so it is a living unit just like a cell.' Give two reasons why this claim is wrong. (2 marks)

   Hint

   What can a whole cell do that a part of it cannot?

   Answer

   Nothing less than a complete cell can live independently; a mitochondrion removed from the cell cannot take in nutrients, grow and multiply on its own (1). It performs only part of the cell's work (aerobic respiration and ATP production), whereas a cell carries out all the activities of life (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   Explain the difference between active and passive transport across the plasma membrane with one example of each. (2 marks)

   Hint

   Gradient and ATP.

   Answer

   Passive: along the concentration gradient without energy, e.g. diffusion of neutral solutes or osmosis of water (1). Active: against the gradient using ATP, e.g. the Na⁺/K⁺ pump (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   Why is the inner mitochondrial membrane folded? How does the number of mitochondria vary between cells? (2 marks)

   Hint

   Surface area; activity.

   Answer

   Folding into cristae increases surface area for the enzymes of respiration (1). The number per cell depends on its physiological activity; active cells such as muscle have more (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Compare the primary and secondary cell wall and state the role of plasmodesmata. (3 marks)

   Hint

   Young vs mature cells; position.

   Answer

   Primary wall: in young cells, capable of growth, gradually diminishes as the cell matures (1). Secondary wall: formed later on the inner side of the primary wall, towards the membrane (1). Plasmodesmata are cytoplasmic strands through walls and middle lamella connecting the cytoplasm of neighbouring cells (1).

8. Q8 · 3 marks · Short answerHarder exam level

   List three differences between plant and animal cells. Which one of these features would you look for first under a light microscope, and why? (3 marks)

   Hint

   Wall, plastids, vacuole, centrioles.

   Answer

   Plant cells have a cell wall, plastids and a large central vacuole; animal cells lack these but have centrioles (any three, 2). The cell wall (or chloroplasts) is the easiest to see because it is a thick, rigid outer boundary visible at low power (1).

9. Q9 · 3 marks · Short answerHarder exam level

   A drug makes the nucleoli of a pancreatic cell disappear while the nuclear envelope and chromatin stay intact. (a) Which molecule's synthesis is directly stopped? (b) Which cytoplasmic structures will become scarce over the next few hours? (c) How will enzyme secretion by this cell change, and why? (3 marks)

   Hint

   Nucleolus → rRNA → ribosomes → protein on RER.

   Answer

   (a) Ribosomal RNA (rRNA), which is made actively in the nucleolus (1). (b) Ribosomes, which are built of rRNA and protein, so new ones cannot be assembled (1). (c) Secretion falls: fewer ribosomes on the RER means less enzyme protein is made for the Golgi to package; this is why such protein-making cells normally have large and numerous nucleoli (1).

10. Q10 · 5 marks · Long answerHarder exam level

    (a) Describe the ultrastructure of a cilium. (b) How is a centriole different? (c) State two functions of centrioles. (5 marks)

    Hint

    9+2 vs 9 triplets.

    Answer

    (a) Covered by plasma membrane; core axoneme with nine peripheral doublet microtubules and two central single microtubules; central sheath joined to each doublet by a radial spoke (nine spokes); linkers join the doublets; arises from a basal body (2). (b) Centriole: nine peripheral triplets of tubulin, no central microtubules, a central proteinaceous hub joined by radial spokes giving a cartwheel; two centrioles lie perpendicular in the centrosome with pericentriolar material (2). (c) Form basal bodies of cilia and flagella; form spindle fibres in animal cell division (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    Suppose a drug blocks the transport vesicles that carry material from the ER to the Golgi apparatus. Predict two effects on a cell that secretes digestive enzymes and one effect on its lysosomes. (3 marks)

    Hint

    Follow the secretory pathway.

    Answer

    Proteins made on the RER would pile up in the ER lumen instead of reaching the Golgi (1). Enzymes would not be modified (e.g. glycoproteins not formed) or packed into secretory vesicles, so secretion stops (1). Lysosomes are formed by Golgi packaging of hydrolases, so new lysosomes would not form and intracellular digestion would fall (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): Sieve tube cells of plants are eukaryotic even though mature ones lack a nucleus.
    Reason (R): Eukaryotic cells may lose the nucleus on maturity while keeping other membrane-bound organelles of eukaryotes.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Being eukaryotic depends on cell organisation, not just a nucleus at one stage.

    Answer

    (a)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who explained that new cells come from pre-existing cells?Show answer

   Rudolf Virchow (1855): Omnis cellula-e cellula.

2. 2.Which prokaryote lacks a cell wall?Show answer

   Mycoplasma.

3. 3.What are plasmids?Show answer

   Small circular DNA outside the main bacterial chromosome, giving traits such as antibiotic resistance.

4. 4.What is a polysome?Show answer

   Several ribosomes attached to one mRNA, translating it into protein.

5. 5.What did Singer and Nicolson propose?Show answer

   The fluid mosaic model of the plasma membrane (1972).

6. 6.What is the middle lamella mainly made of?Show answer

   Calcium pectate.

7. 7.Name the membrane around the plant vacuole.Show answer

   Tonoplast.

8. 8.Which plastid stores starch in potato?Show answer

   Amyloplast.

9. 9.What does S stand for in 80S?Show answer

   Svedberg unit (sedimentation coefficient).

10. 10.What is the main function of the nucleolus?Show answer

    Synthesis of ribosomal RNA.

11. 11.What is the gap between the two nuclear membranes called, and how wide is it?Show answer

    Perinuclear space, 10-50 nm.

12. 12.Name the chromosome with the centromere at its tip.Show answer

    Telocentric.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you state the cell theory and name the four scientists linked to it?
• ☐Can you list five differences between prokaryotic and eukaryotic cells?
• ☐Can you name the three layers of the bacterial envelope and four functions of the mesosome?
• ☐Can you explain the fluid mosaic model and the difference between passive and active transport?
• ☐Can you name the four members of the endomembrane system and say why mitochondria are excluded?
• ☐Can you draw and label a mitochondrion and a chloroplast?
• ☐Can you draw the 9+2 axoneme and the cartwheel centriole?
• ☐Can you classify chromosomes by centromere position with a sketch of each?

Cell: The Unit of Life: frequently asked questions