Cell - the Unit of Life | ISC Class 11 Biology Notes

This note covers cell discovery and cell theory, cell size and shape, types of cells, cell boundaries and transport, internal cell structures and their functions, cellular movement, the nucleus and chromosome organisation.

Why is the cell called the basic unit of life?

A cell is the fundamental structural and functional unit of living organisms. Unicellular organisms consist of one cell; multicellular organisms consist of many cells. A complete unicellular organism can exist independently and perform the essential functions of life.

Anything less than the complete structure of a cell does not ensure independent living. Cell components carry out particular functions, but their organised working together gives the cell its ability to sustain life.

How did observations lead to cell theory?

In 1665, Robert Hooke examined a thin slice of cork with a self-designed microscope. He saw small box-like compartments and named them cells. Antonie Von Leeuwenhoek first saw and described a live cell. Robert Brown described the nucleus in 1831.

In 1838, Matthias Schleiden examined many plants and concluded that different kinds of cells formed plant tissues. A tissue is an organised group of cells performing particular functions.

Theodore Schwann studied animal cells in 1839 and described their thin outer boundary, now called the plasma membrane. His studies of plant tissues also distinguished the cell wall, a rigid covering outside the membrane.

Schleiden and Schwann formulated cell theory, but their original explanation did not establish how new cells formed. In 1855, Rudolf Virchow explained that cells divide and new cells arise from pre-existing cells.

Definition: Cell theory states that living organisms consist of cells and products of cells, and that all cells arise from pre-existing cells.

The microscope made cellular organisation visible. Its improvement, including the development of the electron microscope, revealed finer internal details. Ultrastructure means these fine structural details that can be studied using electron microscopy.

How do cell size, shape and internal organisation vary?

Cells differ greatly in size, shape and activities. Bacteria could be 3 to 5 µm long, where µm means micrometre, one-millionth of a metre. Human red blood cells are about 7.0 µm in diameter. These examples do not define one standard cell size.

The egg of an ostrich is the largest isolated single cell. Nerve cells are some of the longest cells. Cells may be disc-like, polygonal, columnar, cuboid, thread-like or irregular. Their shape may vary with the function they perform.

What occupies the inside of a cell?

The cytoplasm is the semi-fluid matrix that occupies the cell interior. It is a major site of cellular activities and chemical reactions. A nucleus is the membrane-bound structure containing the genetic material in a eukaryotic cell.

DNA, or deoxyribonucleic acid, carries genetic information. Chromosomes are structures containing this genetic material. A cell organelle is a distinct internal structure performing particular cellular functions; some organelles have membranes and others do not.

Eukaryotic cells have a membrane-bound nucleus. Prokaryotic cells lack a membrane-bound nucleus. Both possess cytoplasm and ribosomes, the non-membranous structures that synthesise proteins.

The eukaryotic cytoplasm is extensively divided into compartments by membrane-bound organelles. Such internal compartments allow different structures to perform their particular functions within the same cell. Prokaryotic cells lack these membrane-bound organelles.

A nucleus is not retained in every mature cell. Erythrocytes, meaning red blood cells, of many mammals and the food-conducting sieve tube cells of vascular plants are examples of mature cells that lack a nucleus.

How do prokaryotic and eukaryotic cells differ?

Prokaryotic cells include bacteria, cyanobacteria or blue-green algae, and mycoplasma. Eukaryotes include protists, plants, animals and fungi. Prokaryotic cells are generally smaller and multiply more rapidly than eukaryotic cells; size alone is therefore not the defining distinction.

The clearest distinction is whether a nuclear membrane encloses the genetic material. Bacteria possess genetic material despite lacking a membrane-bound nucleus. Many bacteria also possess plasmids, small circular DNA molecules outside their main genomic DNA.

Plasmids can confer particular observable characteristics, such as resistance to antibiotics. The term genomic DNA refers here to the principal genetic material of the bacterial cell, distinguished from its plasmid DNA.

What forms the bacterial cell envelope?

Most prokaryotic cells, particularly bacterial cells, possess a chemically complex cell envelope. Its three tightly associated layers are the outer glycocalyx, the cell wall and the plasma membrane. Together they act as a protective unit.

The glycocalyx is the outer covering, varying in thickness and composition. It may form a loose slime layer or a thick, tough capsule. The bacterial wall determines shape and provides structural support against bursting or collapse. Mycoplasma lacks a cell wall.

Bacterial shapes include bacillus, meaning rod-like; coccus, spherical; vibrio, comma-shaped; and spirillum, spiral. Inclusion bodies are non-membranous stores of reserve material in the cytoplasm, including granules of phosphate, a phosphorus-containing ion, and glycogen, a storage carbohydrate.

What does the fluid mosaic model explain about the plasma membrane?

The plasma membrane mainly contains lipids, a group including fats and related substances, and proteins. Proteins are molecules built from amino acid units. Its major lipids are phospholipids, molecules with a polar, water-attracting head and nonpolar, water-repelling tails. A bilayer consists of two layers of these molecules.

The polar heads face the aqueous, or water-containing, surroundings on either side. The hydrophobic, meaning water-repelling, tails face inward. This arrangement protects the nonpolar tails from contact with the surrounding water.

The membrane also contains cholesterol, another lipid, and carbohydrates, a group including sugars and starch. The proportions of proteins and lipids vary considerably between cell types. Consequently, one membrane composition should not be applied indiscriminately to every cell.

Where are proteins located?

Peripheral proteins lie on the membrane surface. Integral proteins are partly or wholly buried within the membrane. These categories distinguish their relationship to the lipid bilayer.

Singer and Nicolson proposed the fluid mosaic model in 1972. The quasi-fluid nature of the lipids enables proteins to move laterally within the overall bilayer. Fluidity describes the ability of components to move within the membrane.

This fluid nature is important in cell growth, cell division, secretion, formation of junctions between cells and endocytosis. Endocytosis is the uptake of material through infolding of the cell membrane.

The drawing shows a phospholipid bilayer with proteins at its surface and within it. Labels identify peripheral protein, integral protein, cholesterol and sugar groups. The phospholipid heads face outward on both sides, with tails inside the bilayer.

See Fig. 8.4 in your NCERT textbook

The membrane is selectively permeable: it allows some substances to cross more readily than others. Its structure therefore connects directly with its role in controlling exchanges between the cell and its surroundings.

How do substances cross the plasma membrane?

A concentration gradient is a difference in the concentration of a substance between regions. Movement down that gradient is from higher to lower concentration. Passive transport requires no cellular energy expenditure; active transport moves substances against their gradient using energy.

How does simple diffusion proceed?

Simple diffusion is movement down a concentration gradient without a membrane carrier. Neutral, or electrically uncharged, solutes may cross the membrane in this way. A solute is a substance dissolved in a solvent, such as water.

1. A solute has a higher concentration on one side of the membrane than on the other.
2. If the membrane permits its passage, the solute can move across the membrane.
3. Its net movement follows the concentration gradient, from the higher concentration towards the lower concentration.
4. This movement is passive: the cell does not supply energy to drive it.

Osmosis is the movement of water by diffusion through a selectively permeable membrane. The transported substance is water, whereas diffusion is a broader term applying to movement of substances down their gradients.

What is facilitated diffusion?

Polar molecules cannot cross the nonpolar lipid bilayer in the same way as neutral solutes and require membrane proteins. Facilitated diffusion is passive movement down a concentration gradient with the help of specific membrane proteins.

1. A substance that requires protein assistance is at a higher concentration on one side of the membrane.
2. A suitable membrane transport protein provides a route for that substance to cross.
3. The substance moves down its concentration gradient through the protein-assisted route.
4. The transport remains passive because the cell supplies no energy to drive movement against the gradient.

Carrier-mediated entry of glucose, a simple sugar, down its concentration gradient is an example of facilitated diffusion. A carrier is a membrane protein that binds a transported substance and transfers it across the membrane.

These three terms describe the number and directions of transported substances. They do not, by themselves, establish whether transport is passive or energy-dependent.

What makes active transport different?

Active transport uses energy from ATP, adenosine triphosphate, an energy-carrying molecule. The sodium-potassium pump is an example. The symbols Na⁺ and K⁺ denote positively charged sodium and potassium ions respectively; an ion is an electrically charged atom or group of atoms.

Note: Protein assistance does not automatically mean active transport. Facilitated diffusion uses proteins but remains passive; movement against a concentration gradient requires an energy source.

How do the cell wall and other features distinguish plant and animal cells?

The cell wall is a non-living, rigid covering outside the plasma membrane of plant and fungal cells. It gives shape, protects against mechanical damage and infection, assists cell-to-cell interaction and provides a barrier to undesirable large molecules.

Plant walls contain cellulose, hemicellulose, pectins and proteins. Cellulose, hemicellulose and pectins are structural carbohydrates. Algal walls contain cellulose, galactans, mannans and minerals such as calcium carbonate; galactans and mannans are also wall carbohydrates.

How are neighbouring plant cells connected?

The primary wall of a young plant cell can grow. This capacity gradually diminishes as the cell matures and the secondary wall forms on the inner side, towards the plasma membrane.

The middle lamella, mainly composed of calcium pectate, holds neighbouring cells together. Plasmodesmata are connections that pass through cell walls and the middle lamella, linking the cytoplasm of neighbouring cells.

Plastids are organelles associated with pigments or stored nutrients. Photosynthesis uses light energy to synthesise food; chloroplasts are plastids that trap light for this process. A vacuole is a membrane-bound space in the cytoplasm.

Centrioles are cylindrical structures associated with cell movement structures and division machinery. Mitochondria are double-membrane organelles that produce cellular energy in the form of ATP.

Both plant and animal cells have a plasma membrane, cytoplasm and many shared organelles. A plant cell does not replace its membrane with a wall: the two coverings occur together and perform different functions.

The plant-cell drawing labels a cell wall, middle lamella, plasmodesmata, chloroplast and large vacuole. The animal-cell drawing labels a centriole. Both drawings label the plasma membrane, nucleus, mitochondrion and ribosomes.

See Fig. 8.3 in your NCERT textbook

How does the endomembrane system coordinate synthesis, packaging and digestion?

The endomembrane system groups membrane-bound structures whose functions are coordinated. It includes the endoplasmic reticulum, a membrane network; the Golgi complex, a packaging organelle; lysosomes, digestive vesicles; and vacuoles. Vesicles are small membrane-bound sacs.

Mitochondria, chloroplasts and peroxisomes, a type of enzyme-containing microbody, are not included. Microbodies are minute membrane-bound vesicles containing enzymes. Enzymes are biological catalysts, substances that speed up biological reactions.

What distinguishes rough and smooth endoplasmic reticulum?

The endoplasmic reticulum, abbreviated ER, is a network of small tubular membrane structures scattered through the cytoplasm. It separates the space inside it, called the luminal compartment, from the cytoplasm outside it, called the extra-luminal compartment.

Rough endoplasmic reticulum, or RER, bears ribosomes on its outer surface. It is frequently observed in cells actively synthesising and secreting proteins, and is continuous with the outer nuclear membrane.

Smooth endoplasmic reticulum, or SER, lacks surface ribosomes. It is the major site of lipid synthesis. In animal cells, steroid hormones, which are lipid-like chemical messengers, are synthesised in SER.

How does the Golgi complex handle materials?

The Golgi apparatus, also called Golgi complex, consists of stacks of flattened, disc-shaped sacs called cisternae. Its convex cis face is the forming face; its concave trans face is the maturing face.

The Golgi apparatus packages materials for destinations inside the cell or for secretion outside it. It is closely associated with ER. Vesicles carry materials between these structures.

1. Materials to be packaged reach the Golgi apparatus in vesicles from the endoplasmic reticulum.
2. These vesicles fuse with the cis, or forming, face of the Golgi apparatus.
3. Materials move towards the maturing face; a number of proteins are modified within Golgi cisternae.
4. Packaged materials are released from the trans face for delivery within the cell or secretion outside it.

The Golgi apparatus is an important site for forming glycoproteins and glycolipids, molecules in which carbohydrate groups are associated with proteins and lipids respectively.

What do lysosomes and vacuoles do?

Lysosomes are membrane-bound vesicles formed through Golgi packaging. They contain hydrolytic enzymes, enzymes that digest substances through reactions involving water.

Lysosomal enzymes are optimally active under acidic conditions. They digest carbohydrates, proteins, lipids and nucleic acids, the group of molecules that includes DNA and RNA. RNA means ribonucleic acid.

A vacuole contains water, cell sap or watery fluid, excretory products and other materials not useful to the cell. Its single surrounding membrane is the tonoplast. In plant cells, vacuoles can occupy up to 90 per cent of cell volume.

The plant tonoplast facilitates movement of ions and other substances into the vacuole against concentration gradients. Their concentrations are therefore significantly higher in the vacuole than in the cytoplasm.

In Amoeba, the contractile vacuole helps in osmoregulation, the regulation of water balance, and excretion. In many cells, as in protists, food vacuoles form when food particles are engulfed.

How does mitochondrial structure support its functions?

A mitochondrion, plural mitochondria, is a double-membrane organelle associated with aerobic respiration, the release of energy from food using oxygen. Mitochondria produce cellular energy in the form of ATP, earning the description power houses of the cell.

Their number varies with the physiological activity of the cell. Their shapes and sizes also vary considerably. A mitochondrion is typically sausage-shaped or cylindrical and is not easily visible under the microscope unless specifically stained.

What are the mitochondrial compartments?

The outer membrane forms the continuous limiting boundary. The inner membrane separates the internal matrix, a dense substance, from the compartment between the two membranes. The membranes possess enzymes associated with mitochondrial functions.

The inner membrane folds towards the matrix to form cristae, singular crista. These infoldings increase surface area. Distinguish an inner-membrane fold from the matrix around it: they are different parts of the same organelle.

The cutaway drawing labels the outer membrane, inner membrane, inter-membrane space, matrix and a crista. Repeated folds of the inner membrane project into the matrix.

See Fig. 8.7 in your NCERT textbook

Which internal components are present?

The matrix contains a single circular DNA molecule, a few RNA molecules, ribosomes and components required for protein synthesis. Mitochondria divide by fission, meaning division into separate units.

The presence of DNA and ribosomes distinguishes mitochondria from structures such as lysosomes. Their membranes, matrix and protein-synthesising components should be identified separately when explaining how the organelle is organised.

Mitochondria are excluded from the endomembrane system because their functions are not coordinated with that system's components in the same way. Having a membrane alone does not establish membership of the endomembrane system.

What types of plastids occur, and how is a chloroplast organised?

Plastids occur in plants and euglenoids, a group of protists. They are classified as chloroplasts, chromoplasts and leucoplasts according to pigmentation. Chloroplasts contain chlorophyll and carotenoid pigments that trap light energy for photosynthesis.

Photosynthesis is the process in which light energy supports the synthesis of food. Chlorophyll is a green pigment; carotenoids are pigments including carotene and xanthophylls.

Chromoplasts contain fat-soluble carotenoid pigments that impart yellow, orange or red colours to plant parts. Leucoplasts are colourless plastids containing stored nutrients and have varied shapes and sizes.

What lies inside a chloroplast?

The majority of chloroplasts in green plants occur in leaf mesophyll, the internal photosynthetic tissue. A chloroplast is double-membrane bound, with the inner membrane relatively less permeable. The space enclosed by the inner membrane is the stroma.

Flattened membranous sacs called thylakoids occur within the stroma. They form stacks called grana, singular granum. Stroma lamellae are membrane connections between thylakoids of different grana. Each thylakoid encloses an internal space called its lumen.

Chlorophyll pigments occur in thylakoids. The stroma contains enzymes needed for carbohydrate and protein synthesis, small double-stranded circular DNA molecules and ribosomes. The ribosomes inside chloroplasts are smaller than those in the surrounding eukaryotic cytoplasm.

The drawing labels outer and inner membranes, stroma, a thylakoid, a granum and stroma lamella. Stacks of flattened sacs are shown inside the double outer boundary, with connections extending between stacks.

See Fig. 8.8 in your NCERT textbook

How do ribosomes and microbodies contribute to cell activity?

Ribosomes are granular structures composed of RNA and proteins, without a surrounding membrane. George Palade first observed them as dense particles using the electron microscope in 1953. Their function is protein synthesis.

Ribosomes occur in both prokaryotic and eukaryotic cells. Within eukaryotic cells they may lie in the cytoplasm, attach to rough ER, or occur inside mitochondria and chloroplasts. Their location must be considered when identifying their type.

What do 70S and 80S mean?

S is the Svedberg unit used for a sedimentation coefficient, describing sedimentation behaviour and indirectly reflecting particle size and density. Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S. Mitochondria and chloroplasts contain 70S ribosomes.

A subunit is one of the two component parts of a ribosome. The S labels describe sedimentation coefficients, so the values of the subunits are not added arithmetically to obtain the value for the whole ribosome.

Several ribosomes may attach to one messenger RNA, abbreviated mRNA, the RNA template used in protein synthesis. This chain is called a polysome or polyribosome. Its ribosomes translate the mRNA, meaning they use its information to synthesise proteins.

What are microbodies?

Microbodies are minute membrane-bound vesicles containing various enzymes. They occur in both plant and animal cells. Peroxisomes are a type of microbody and are excluded from the endomembrane system.

The distinction is structural as well as functional: ribosomes lack surrounding membranes and synthesise proteins, whereas microbodies have membranes and contain enzymes. Neither should be confused with non-membranous bacterial inclusion bodies that store reserve materials.

How do the cytoskeleton, cilia, flagella and centrioles support cells?

The cytoskeleton is a network of protein filaments in the cytoplasm. It consists of microtubules, microfilaments and intermediate filaments. Together, these structures contribute to mechanical support, maintenance of cell shape and motility, meaning movement.

Microtubules are hollow protein tubes; microfilaments are thin protein filaments; intermediate filaments are fibres of intermediate thickness. These names describe distinct elements of the supporting network.

What is the structure of cilia and flagella?

Cilia, singular cilium, are small hair-like membrane outgrowths that work like oars, moving the cell or surrounding fluid. Flagella, singular flagellum, are comparatively longer and responsible for cell movement. Bacterial flagella differ structurally from eukaryotic flagella.

A eukaryotic cilium or flagellum is covered by plasma membrane. Its internal core, the axoneme, contains microtubules running parallel to its length. It usually has nine peripheral doublets, each a pair of microtubules, and two centrally placed microtubules.

The notation 9+2 means nine peripheral doublets plus two central microtubules. The central pair is enclosed by a central sheath, connected to the peripheral doublets by radial spokes. Adjacent peripheral doublets are linked to one another.

An electron micrograph is placed beside a diagrammatic cross-section. The diagram labels the plasma membrane, peripheral doublets, central microtubules, central sheath, radial spoke and interdoublet bridge.

See Fig. 8.10 in your NCERT textbook

How are centrioles organised?

A centrosome usually contains two cylindrical centrioles lying perpendicular to one another. They are surrounded by amorphous pericentriolar material, meaning material without a definite shape around the centrioles.

Each centriole has nine evenly spaced peripheral fibrils made of tubulin, a structural protein. Each fibril is a triplet, consisting of three microtubules. Adjacent triplets are linked. A central protein hub connects to peripheral triplets through radial spokes, producing a cartwheel organisation.

Centrioles form basal bodies, the structures from which cilia or flagella emerge. In animal cells they also form spindle fibres that give rise to the spindle apparatus, the fibre system involved in chromosome movement during division.

How are the nucleus and chromosomes organised?

The nuclear envelope consists of two parallel membranes separating nuclear contents from cytoplasm. The space between them is the perinuclear space. The outer membrane usually remains continuous with ER and bears ribosomes.

Nuclear pores occur where the two membranes fuse. They provide passages for RNA and proteins to move in both directions between the nucleus and cytoplasm. The envelope therefore separates compartments while allowing exchange.

What is present in an interphase nucleus?

Interphase refers here to the period when a cell is not dividing. Its nucleus contains extended chromatin, a network of DNA, proteins and RNA, within the nucleoplasm, the internal nuclear matrix. Flemming named the stained nuclear material chromatin.

The nucleus contains one or more nucleoli, singular nucleolus. These spherical structures have no surrounding membrane and are continuous with the nucleoplasm. They are sites of active ribosomal RNA synthesis; ribosomal RNA is the RNA component of ribosomes.

Cells actively synthesising proteins have larger and more numerous nucleoli. Normally, a cell has one nucleus, but variations in nuclear number are frequently observed. The nucleus controls organelle activities and plays a major role in heredity, the transmission of inherited characteristics.

What structures identify a chromosome?

During cell division, chromatin becomes organised into visible chromosomes. Its proteins include histones, DNA-associated proteins, and non-histone proteins. The centromere is the primary constriction holding together the two chromatids, the two component strands of a duplicated chromosome.

Disc-shaped structures called kinetochores occur on the sides of the centromere. The position of the centromere is the basis for classifying chromosomes into four types.

Sometimes a few chromosomes have non-staining secondary constrictions, additional narrow regions at constant locations. A small region beyond such a constriction appears as a fragment called a satellite. This differs from the primary constriction that forms the centromere.

Four drawings show changing centromere positions and corresponding arm lengths. Labels identify centromere, short arm, long arm, secondary constriction and satellite. The satellite is shown beyond the secondary constriction.

See Fig. 8.13 in your NCERT textbook

Glossary

• Cell theory — The explanation that organisms consist of cells and cell products, and new cells arise from pre-existing cells.
• Cytoplasm — The semi-fluid matrix inside a cell where many cellular activities take place.
• Prokaryotic cell — A cell lacking a membrane-bound nucleus and other membrane-bound organelles.
• Fluid mosaic model — A model describing a lipid bilayer that permits lateral movement of membrane proteins.
• Facilitated diffusion — Passive movement down a concentration gradient with assistance from specific membrane transport proteins.
• Plasmodesmata — Connections through plant cell walls that link the cytoplasm of neighbouring cells.
• Endomembrane system — Functionally coordinated organelles comprising endoplasmic reticulum, Golgi complex, lysosomes and vacuoles.
• Cristae — Infoldings of the inner mitochondrial membrane that increase its surface area.
• Stroma — The chloroplast compartment enclosed by the inner membrane and containing thylakoids and enzymes.
• Polysome — A group of ribosomes attached to one messenger RNA during protein synthesis.
• Axoneme — The microtubule-containing internal core of a eukaryotic cilium or flagellum.
• Nucleolus — A non-membrane-bound nuclear structure that actively synthesises the RNA component of ribosomes.
• Centromere — The primary chromosome constriction that holds the two chromatids together.
• Satellite — A small chromosome region appearing beyond a non-staining secondary constriction.

Common errors and misconceptions

• Misconception: A prokaryotic cell has no genetic material. Correct: It has DNA but lacks a membrane-bound nucleus.
• Misconception: A plant cell has a wall instead of a plasma membrane. Correct: The wall lies outside the plasma membrane; both are present.
• Misconception: Any transport involving a protein is active. Correct: Facilitated diffusion uses proteins while moving passively down a concentration gradient.
• Misconception: Every membrane-bound organelle belongs to the endomembrane system. Correct: Mitochondria, chloroplasts and peroxisomes are excluded.
• Misconception: All ribosomes in a eukaryotic cell are 80S. Correct: Mitochondria and chloroplasts contain 70S ribosomes.
• Misconception: A centriole has the same arrangement as an axoneme. Correct: A centriole has nine peripheral triplets; an axoneme usually has nine peripheral doublets and two central microtubules.
• Misconception: Centromere and centrosome mean the same structure. Correct: The centromere is a chromosome constriction; a centrosome usually contains two centrioles.
• Misconception: Every plant cell lacks centrioles without exception. Correct: Centrioles are absent in almost all plant cells.

Exam-style questions with model answers

Q1. State the two propositions of cell theory. [2 marks]

1. All living organisms are composed of cells and products formed by cells.
2. All cells arise from pre-existing cells through division; new cells originate from cells already present.

Q2. Compare prokaryotic and eukaryotic cells with respect to nuclear organisation, membrane-bound organelles and ribosomes. [3 marks]

1. Prokaryotic genetic material is not enclosed by a nuclear membrane, whereas a eukaryotic nucleus is surrounded by a nuclear envelope.
2. Prokaryotic cells lack membrane-bound organelles. Eukaryotic cells possess such organelles, which divide their cytoplasm into distinct compartments.
3. Both have non-membranous ribosomes: prokaryotic ribosomes are 70S, while eukaryotic cytoplasmic ribosomes are 80S. Mitochondria and chloroplasts contain 70S ribosomes.

Q3. A substance crosses a membrane through a specific transport protein from higher to lower concentration, without cellular energy expenditure. Identify the process and explain how each stated feature supports your answer. [4 marks]

1. The process is facilitated diffusion, a form of passive transport across a membrane with protein assistance.
2. The specific transport protein provides assistance for crossing, distinguishing this route from simple diffusion through the lipid bilayer.
3. Movement from higher to lower concentration follows the concentration gradient, rather than proceeding against that gradient.
4. No cellular energy is expended, so the described movement is passive rather than active transport.

Q4. Explain four stages in the movement of materials from the endoplasmic reticulum through the Golgi apparatus for packaging and delivery. [4 marks]

1. Materials that require packaging reach the Golgi apparatus in membrane-bound vesicles derived from the endoplasmic reticulum.
2. The incoming vesicles fuse with the cis face, the forming face of the Golgi apparatus.
3. Materials move towards the maturing face; a number of proteins undergo modification within the Golgi cisternae.
4. Packaged materials leave the trans face for delivery to targets inside the cell or for secretion outside it.

Q5. Describe a mitochondrion under five headings: outer boundary, internal compartments, cristae, matrix contents and principal energy-related function. [5 marks]

1. A mitochondrion is enclosed by two membranes. Its outer membrane forms a continuous limiting boundary around the organelle.
2. The membranes define an outer compartment between them and an inner compartment containing the dense mitochondrial matrix.
3. The inner membrane forms infoldings called cristae projecting towards the matrix. These folds increase the surface area of the inner membrane.
4. The matrix contains a single circular DNA molecule, a few RNA molecules, 70S ribosomes and components needed for protein synthesis.
5. Mitochondria are sites of aerobic respiration and produce cellular energy as ATP. This energy-related role explains their description as the power houses of the cell.

Q6. Give six structural features of a chloroplast: its envelope, stroma, thylakoids, grana, stroma lamellae and internal genetic machinery. [6 marks]

1. The chloroplast is surrounded by an outer and an inner membrane, with the inner membrane relatively less permeable.
2. The stroma is the compartment enclosed by the inner membrane. It contains enzymes required for carbohydrate and protein synthesis.
3. Thylakoids are flattened membrane sacs within the stroma. Their membranes enclose a lumen, and chlorophyll pigments occur in the thylakoids.
4. Grana are stacks of thylakoids arranged like piles of coins; the singular term for one stack is granum.
5. Stroma lamellae are flat membrane connections joining the thylakoids of different grana within the chloroplast.
6. The stroma contains small double-stranded circular DNA molecules and ribosomes. Chloroplast ribosomes are 70S, smaller than the 80S ribosomes in eukaryotic cytoplasm.

Q7. Classify chromosomes into four types by centromere position, explaining the arm lengths associated with each position. [4 marks]

1. A metacentric chromosome has its centromere in the middle, producing two arms of equal length.
2. A sub-metacentric chromosome has its centromere slightly away from the middle, giving one shorter arm and one longer arm.
3. An acrocentric chromosome has its centromere close to one end, producing one extremely short arm and one very long arm.
4. A telocentric chromosome has a terminal centromere, at the end of the chromosome, without a short arm extending beyond it.

Q8. Distinguish a centriole from a eukaryotic ciliary axoneme in terms of peripheral microtubule arrangement and the central organisation. [2 marks]

1. A centriole contains nine peripheral triplets with a protein hub and radial spokes in its proximal region.
2. A ciliary axoneme usually contains nine peripheral doublets surrounding two central microtubules, called the 9+2 arrangement.

Key takeaways

• Cells are structural and functional units of life, and new cells arise from cells already present.
• Prokaryotes lack a membrane-bound nucleus; eukaryotic cells contain membrane-bound compartments with distinct internal functions.
• The plasma membrane has a fluid lipid bilayer with proteins and selectively controls movement across the cell boundary.
• Facilitated diffusion is passive despite using proteins, while active transport requires energy for movement against a gradient.
• The endomembrane system includes ER, Golgi apparatus, lysosomes and vacuoles, whose functions are coordinated.
• Mitochondria and chloroplasts have double membranes, DNA and ribosomes, but differ in internal organisation and functions.
• The cytoskeleton supports cell shape and movement; ciliary axonemes usually contain nine peripheral doublets and two central microtubules.
• Centromere position determines chromosome type, while a satellite is associated with a secondary chromosome constriction.

Test yourself

What did Virchow add to cell theory?

He explained that new cells arise through division of pre-existing cells.

What links the cytoplasm of neighbouring plant cells?

Plasmodesmata pass through their walls and middle lamella, connecting neighbouring cytoplasm.

How does symport differ from antiport?

Symport moves two substances in the same direction; antiport moves them in opposite directions.

Which Golgi face receives vesicles from ER?

The cis face, also called the forming face, receives incoming ER vesicles.

How are cristae and thylakoids different?

Cristae are inner mitochondrial membrane folds; thylakoids are flattened membrane sacs inside chloroplasts.

What does S represent in a ribosome label?

S denotes the Svedberg unit for the sedimentation coefficient, indirectly reflecting size and density.

Why is a nucleolus different from a nucleus?

The nucleolus lacks its own membrane and synthesises ribosomal RNA inside the membrane-bound nucleus.

What distinguishes a sub-metacentric chromosome from an acrocentric chromosome?

A sub-metacentric centromere is slightly off-centre; an acrocentric centromere lies close to an end, making one arm extremely short.