Correct option: (B) Cell.

The cell is the lowest level of biological organisation at which the essential characteristics of life are expressed. Tissues, organs and organ systems are formed through organisation of cells.

Correct option: (B) 1665.

Hooke examined a thin slice of cork using his self-designed microscope and observed small box-like compartments. He called these compartments “cells”.

Correct option: (C) 0.1 mm.

Two points separated by about 0.1 mm can normally be distinguished by the eye at its near point. At smaller separations they appear as one point.

Correct option: (B) 400X.

Correct option: (C) 200 µm.

Correct option: (B).

An electron microscope uses a beam of electrons rather than visible light and can reveal structures at the nanometre scale with much greater detail.

Correct option: (C).

The cell membrane controls exchange between the cell and its surroundings by permitting certain substances to pass while restricting others.

Correct option: (B).

Osmosis specifically involves movement of water through a selectively permeable membrane from a region with more water and less solute towards one with less water and more solute.

Correct option: (B).

Plain water is relatively dilute compared with the cell contents. Water therefore moves into the potato cells across their selectively permeable membranes.

Correct option: (C).

The surrounding solution is hypertonic. Water moves from the cell into the more concentrated external solution, causing the animal cell to shrink.

Correct option: (B).

The fluid-mosaic model describes the plasma membrane as a lipid bilayer containing embedded proteins. Its components can move within the membrane, making it fluid.

Correct option: (B) Cellulose.

Cellulose is a carbohydrate made of many linked glucose units. It contributes rigidity and mechanical support to plant cell walls.

Correct option: (C) Cell membrane.

The plasma membrane is a universal cellular feature. Bacteria lack a membrane-bound nucleus and many membrane-bound organelles, while chloroplasts and large central vacuoles are characteristic of plant cells.

Correct option: (B) Nucleoid.

Prokaryotic cells do not have a membrane-bound nucleus. Their genetic material lies in a cytoplasmic region termed the nucleoid.

Correct option: (C).

Eukaryotic cells possess a well-defined membrane-bound nucleus and several membrane-bound organelles. These are absent from prokaryotic cells.

Correct option: (B) Nucleolus.

Ribosomal subunits are produced in the nucleolus and later move into the cytoplasm, where they assemble to form functional ribosomes.

Correct option: (A).

In non-dividing cells, DNA and associated proteins are present as thread-like chromatin. Before cell division, this material becomes organised into visible chromosomes.

Correct option: (A).

Ribosomes are the sites at which proteins are synthesised. They may occur freely in the cytoplasm or attached to rough ER.

Correct option: (B).

Numerous ribosomes are attached to the surface of RER, giving it a rough appearance under an electron microscope and linking it strongly with protein synthesis and secretion.

Correct option: (A) Smooth ER.

SER lacks ribosomes and is involved in the synthesis and storage of fats and certain hormones.

Correct option: (A).

The Golgi apparatus receives cellular products, modifies and sorts them, and packages them into vesicles for transport, secretion or formation of structures such as lysosomes.

Correct option: (A).

Lysosomal enzymes break down unwanted materials and damaged cellular components. Breakdown products can subsequently be released into the cytoplasm and reused.

Correct option: (B) Cristae.

Cristae greatly increase the surface area of the mitochondrial inner membrane for chemical reactions associated with cellular energy production.

Correct option: (B).

Energy released during cellular respiration is stored in ATP and subsequently used to support many cellular activities.

Correct option: (C) Leucoplast.

Leucoplasts are colourless plastids specialised for food storage. Some leucoplasts in potato and Colocasia cells store starch.

Correct option: (B).

The large central vacuole contains cell sap. Accumulated water helps maintain internal pressure, contributing to firmness of the plant cell.

Correct option: (B).

Mitosis produces two daughter cells that receive essentially the same DNA and chromosome number as the parent cell. It is important for growth, repair and maintenance.

Correct option: (B).

Meiosis involves two divisions and produces four daughter cells. Each receives half the chromosome number of the parent cell, making meiosis important in gamete formation.

Correct option: (C) Rudolf Virchow.

Virchow expanded Cell Theory in 1855 by stating that new cells originate only from pre-existing cells.

Correct option: (B).

Normal animal cells generally stop dividing when they contact neighbouring cells. Cancer cells may lose this growth control and continue dividing, contributing to tumour formation.

Cells provide the basic physical structure of all living organisms. At the same time, essential life activities occur through cellular processes. Even in multicellular organisms, tissues and organs function because their constituent cells perform coordinated activities.

Magnification is the extent to which an object appears larger than its actual size. Resolution refers to clarity and the ability to distinguish two closely spaced points as separate. A large image is not necessarily useful if it has poor resolution.

Therefore, the object appears approximately 600 times larger.

Diffusion: net movement of particles from higher to lower concentration and does not necessarily require a membrane.

Osmosis: diffusion of water across a selectively permeable membrane.

In plain water, water enters potato cells by osmosis, increasing their mass and size. In concentrated salt solution, water moves out of the cells toward the more concentrated external solution, causing loss of mass and shrinkage.

The rigid cell wall allows water and some dissolved substances to pass through it relatively freely. The underlying plasma membrane regulates which substances actually enter or leave the living cell, making it selectively permeable.

• Prokaryotes lack a membrane-bound nucleus; eukaryotes possess one.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain several such organelles.

Absence of a nucleus leaves additional space for haemoglobin, helping the RBC transport more oxygen. However, enucleated mature RBCs cannot divide or effectively repair themselves, contributing to their limited lifespan of about 120 days.

RER has ribosomes on its surface and is strongly associated with protein synthesis and secretion. SER lacks surface ribosomes and participates in synthesis and storage of fats and certain hormones.

Lysosomes contain digestive enzymes capable of breaking down unwanted materials and damaged or worn-out cellular parts. The useful breakdown products may then return to the cytoplasm and be reused.

Cristae are folds of the inner mitochondrial membrane. They increase its surface area, providing more area for chemical reactions associated with energy production during cellular respiration.

• Chloroplasts: contain chlorophyll and participate in photosynthesis.
• Chromoplasts: contain coloured pigments, often yellow, orange or red.
• Leucoplasts: lack pigments and mainly store materials such as starch, oils or proteins.

Mitosis produces two genetically similar daughter cells with the same chromosome number and is important in growth and repair. Meiosis involves two divisions, produces four daughter cells with half the chromosome number and is associated with formation of gametes.

Any two:

• All living organisms consist of one or more cells.
• The cell is the basic structural and functional unit of living organisms.
• All cells arise from pre-existing cells.

Programmed Cell Death is a genetically regulated and organised process of selective cell destruction. It contributes to normal development and cellular quality control. The chapter illustrates this with removal of cells between developing fingers.

Hence, the approximate size of one cell is 200 µm.

• Magnification makes a small specimen appear larger.
• Resolution determines whether nearby details can be seen distinctly.
• Contrast makes different parts easier to distinguish by increasing differences in brightness or appearance.

Effective microscopy depends on improvement of all three rather than magnification alone.

1. Take roughly equal potato pieces and record their initial masses.
2. Place one in plain water and the other in concentrated salt or sugar solution.
3. The first gains water and swells, whereas the second loses water and shrinks.

The results show that water crosses selectively permeable cell membranes according to differences in water/solute concentration.

• Isotonic: external and internal solute concentrations are similar; there is no major net change in cell size.
• Hypotonic: external solution is more dilute; water enters and the cell swells.
• Hypertonic: external solution is more concentrated; water leaves and the cell shrinks.

• The membrane contains a lipid bilayer with water-attracting heads facing outward and water-repelling tails inward.
• Different proteins are embedded in the bilayer and may function as gatekeepers.
• Membrane components can move sideways, rotate and otherwise shift within the membrane, producing its fluid character.

Both cells lose water by osmosis. The inner living content of a Rhoeo cell shrinks and its membrane moves away from the wall, but the rigid cell wall preserves the overall outer shape. Cheek cells lack a cell wall, so the entire cells shrink considerably.

• It provides rigidity and mechanical support.
• It helps cells retain shape and assists leaves and flowers in remaining firm.
• It helps plants withstand environmental stresses such as wind and rain while still remaining permeable to water and some dissolved minerals.

DNA carries genetic information. Functional segments of DNA are called genes. In a non-dividing cell, DNA and associated proteins occur as thread-like chromatin. As the cell prepares to divide, chromatin becomes organised into distinct chromosomes.

A simplified pathway is:

Ribosomes synthesise the protein, ER participates in its processing/transport, and Golgi modifies, sorts and packages it for secretion.

• RER: ribosome-bearing; closely associated with protein synthesis and secretion.
• SER: lacks ribosomes; involved in synthesis/storage of fats and some hormones.
• Golgi: modifies, sorts and packages proteins and lipids into vesicles.

• A mitochondrion possesses two surrounding membranes.
• The inner membrane forms numerous folds called cristae.
• Cristae increase the available surface area for reactions involved in cellular energy production. Energy released is stored in ATP.

• Both are double-membrane-bound organelles.
• Both contain their own DNA.
• Both contain ribosomes and can make some of their own proteins.

The chapter notes that these characteristics suggest an evolutionary relationship with certain single-celled organisms.

Not all plastids are chloroplasts. Roots may contain colourless leucoplasts, which store food such as starch, oils or proteins. Therefore the absence of photosynthesis does not imply the absence of plastids.

The large central vacuole stores cell sap containing water and other dissolved substances. Water in the vacuole contributes to internal pressure and cellular firmness. During water shortage, vacuoles lose water, the cells become less firm and the plant may wilt.

The experiment showed that laboratory-synthesised DNA could direct the growth and division of a recipient bacterial cell, supporting the role of DNA in controlling cellular structure and activities.

It did not represent creation of an entirely new cell from non-living chemicals because the cytoplasm, membrane and other cell components came from an existing living cell.

• Growth by increasing cell number.
• Repair and replacement of damaged or dead cells.
• Reproduction: mitosis can support asexual reproduction, while meiosis produces gametes for sexual reproduction.

Meiosis produces gametes with half the chromosome number of the parent cell. During fertilisation, two gametes combine. Their chromosome sets together restore the original chromosome number, preventing chromosome number from doubling in every generation.

• Matthias Schleiden, 1838: reported that plants are made of cells.
• Theodor Schwann, 1839: concluded that animals are also made of cells.
• Rudolf Virchow, 1855: stated that new cells arise only from pre-existing cells.

A high concentration of salt or sugar creates a strongly concentrated external environment. Water tends to move out of cells of many spoilage-causing microorganisms by osmosis. Loss of available water makes conditions unfavourable for their normal growth and multiplication, thereby helping preserve the food.

1. Measure the diameter of the microscope’s circular field using a transparent ruler.
2. Convert the measured field diameter from millimetres to micrometres using 1 mm = 1000 µm.
3. Observe an onion peel and count how many cells lie in one straight line across the field diameter.
4. Divide field diameter in micrometres by number of cells.

For example, 5000 µm ÷ 25 = 200 µm.

• The cell membrane is about 7–10 nm thick and consists mainly of lipids and proteins.
• Lipids form a bilayer, with water-attracting heads outward and water-repelling tails inward.
• Various proteins are embedded in this bilayer.
• Membrane molecules can move, so the membrane is fluid, while the mixture of molecular components gives the mosaic description.

The embedded proteins can act as gatekeepers, helping regulate movement of particular substances and therefore contributing to selective permeability.

Water moves out of both plant and animal cells by osmosis because the external solution is concentrated.

• In a plant cell, the inner content contracts and the plasma membrane moves away from the wall, but the rigid cell wall preserves the cell’s outer form.
• An animal cell has no cell wall, so the entire cell can shrink considerably.

The comparison demonstrates the supporting and shape-maintaining function of the plant cell wall.

Both nevertheless contain fundamental cellular components such as plasma membrane, cytoplasm, genetic material and ribosomes.

• The nucleus has a double-layered nuclear membrane containing pores.
• The nucleolus produces ribosomal subunits that later assemble in the cytoplasm.
• The nucleus contains DNA, which carries genetic information.
• Functional DNA segments are genes. In non-dividing cells DNA forms chromatin, which condenses into chromosomes before division.

Thus the nucleus combines genetic control with an important role in production of cellular protein-making machinery.

• Nucleolus: supplies ribosomal subunits.
• Ribosomes/RER: synthesize proteins intended for secretion.
• Golgi apparatus: modifies, sorts and packages these proteins into vesicles.
• Mitochondria: provide ATP needed for active cellular work.

The organelles therefore form a coordinated manufacturing, processing and energy-supply system.

Both can produce some of their own proteins because they contain DNA and ribosomes.

• Chloroplasts: capture light and participate in food synthesis.
• Chromoplasts: supply bright colours to many flowers and fruits, helping attract pollinators or seed-dispersing animals.
• Leucoplasts: store food such as starch, oils or proteins.
• Vacuoles: store water, minerals, sugars and waste materials and help maintain cellular firmness.

• It increases cell number during growth.
• It replaces lost or dead cells.
• It repairs damaged tissues, such as after a cut.
• It produces daughter cells with essentially the same DNA and chromosome number, helping maintain genetic continuity among body cells.

Meiosis occurs in cells of reproductive organs and consists of two successive divisions.

• After the first division, chromosome number is reduced to half.
• A second division produces four daughter cells.
• Each gamete therefore contains half the parental chromosome number.
• When male and female gametes unite during fertilisation, the normal chromosome number is restored.

This mechanism also contributes to variation among offspring.

Errors in mitosis:

• May produce abnormal chromosome numbers in body cells.
• Loss of normal division control can contribute to abnormal growth and tumours.

Errors in meiosis:

• May cause genetic disorders or developmental problems.
• May contribute to reduced fertility or early pregnancy loss.

Fresh onion roots are grown, fixed/preserved, softened using dilute HCl, stained with aceto-carmine and gently squashed beneath a coverslip before microscopic observation.

Root tips are suitable because they contain actively growing cells that divide continuously. Consequently, a single prepared root-tip sample may contain cells displaying different structural stages of division.

Classical Cell Theory states:

1. All living organisms consist of one or more cells.
2. The cell is the basic structural and functional unit of living beings.
3. All cells arise from pre-existing cells.

The third principle links growth, repair and reproduction with cell division and explains how cellular life continues from existing cells.

• Contact inhibition: many normal animal cells stop dividing when they contact neighbouring cells.
• Cancer: abnormal cells may lose growth control and continue to divide, potentially forming tumours.
• Programmed Cell Death: genetically controlled removal of selected cells helps normal development and quality control.

Together these show that healthy tissues depend on a balance between cell production and cell removal.

Cell culture is the growth of plant or animal cells outside the organism in controlled conditions using a nutrient-rich medium.

Important requirements include suitable temperature, appropriate acidic/alkaline conditions, moisture and sterile conditions.

Applications include studying cellular functions and production of biochemicals, foods, medicines and vaccines.

The original cell size is:

If only six cells fit across the second field:

Thus the second visible field has one-third the diameter of the first field. Greater magnification generally shows a smaller field of view.

Student B has the better argument. Magnification only describes enlargement. If resolution or contrast is poor, increasing magnification may simply produce a larger blurred image. Useful microscopy therefore requires adequate magnification, resolution and contrast together.

Solution P is approximately isotonic with the cell, so external and internal solute concentrations are similar and there is no substantial net osmotic change.

Solution Q is hypertonic relative to the cell. It contains a higher solute concentration, causing water to leave the cell.

Both lose water. The animal cell shrinks as a whole. In the plant cell, the inner cellular content contracts but the rigid outer cell wall largely maintains the external shape. This contrast demonstrates the structural-support role of the cell wall.

The seed is expected to lose water and become less swollen because the external solution has a higher solute concentration. Water therefore moves outward through selectively permeable cell membranes by osmosis.

Selective transport would be strongly affected because membrane proteins act as gatekeepers for passage of many substances. The membrane boundary may still physically exist because the lipid bilayer remains, but its regulated exchange functions would be impaired.

No. Its size and lack of a true nucleus suggest a prokaryotic cell. The chapter states that prokaryotic cells carry out many cellular activities directly in the cytoplasm even though they lack membrane-bound organelles. Absence of mitochondria therefore does not mean the cell is non-living or incapable of energy-releasing processes.

It is a eukaryotic cell because it possesses a membrane-bound nucleus and membrane-bound organelles. The absence of a cell wall makes it more consistent with an animal-type cell than a typical plant cell.

A nucleus could potentially restore greater capacity for cellular control, repair or division, but it would occupy space that is normally available for haemoglobin. The specialised advantage of carrying more haemoglobin—and therefore more oxygen—would be reduced.

Accurate cell division would be threatened. Before division, chromatin normally becomes organised into chromosomes. Failure to organise genetic material properly would interfere with orderly distribution of DNA to daughter cells.

The nucleolus normally synthesises ribosomal subunits. Damage to it would therefore reduce formation of new ribosomes. Since ribosomes are required for protein synthesis, protein production would eventually be impaired even if the DNA itself remained intact.

• The protein may not be properly modified, sorted or packaged for its correct destination.
• Formation and transport of secretory vesicles would be impaired, so effective secretion through the plasma membrane would decrease.

Waste materials and damaged or worn-out cellular components would accumulate because lysosomes would no longer efficiently break them down. Recycling of useful breakdown products would also decrease, eventually disturbing overall cellular health.

Not necessarily. Numerous smaller mitochondria can collectively provide a large membrane surface area and distribute energy-producing capacity throughout the cell. Within each mitochondrion, folding of the inner membrane into cristae further increases reaction surface area. A single giant organelle could reduce this distributed surface advantage.

No. Chloroplasts help synthesise food, whereas mitochondria release usable energy from food during cellular respiration and store it in ATP. Sugar production alone cannot replace the mitochondrial role in supplying ATP for most cellular activities.

Bright colours produced by chromoplast pigments can help attract pollinators. Loss of such pigments in flowers could therefore reduce visual attraction of pollinating organisms, even if photosynthesis elsewhere in the plant remained normal.

The student incorrectly treats “plastid” as synonymous with “chloroplast”. Underground storage tissues may contain colourless leucoplasts. The chapter specifically notes leucoplasts in potato cells that store starch.

• Internal pressure supporting cellular firmness would fall.
• Plant tissues could become less firm and the plant could wilt despite retaining the rigid cell-wall framework.

Storage of dissolved substances in cell sap would also be disturbed.

Only the DNA was synthesised. The recipient cell membrane, cytoplasm, ribosomes and other necessary cellular machinery were obtained from a pre-existing living cell. The experiment therefore demonstrates the controlling role of DNA but not creation of an entire living cell from non-living components.

After normal mitosis, each daughter cell retains the same chromosome number as the parent:

Meiosis reduces chromosome number to half:

If each gamete retained the full parental chromosome number, fusion of two gametes would produce a fertilised cell with approximately twice the normal chromosome number. Repetition in later generations could cause chromosome number to increase repeatedly. Meiosis avoids this by halving the chromosome number before fertilisation.

Meiosis produces four cells with half the chromosome number and is specialised for gamete formation. Tissue repair requires daughter cells genetically and chromosomally comparable to normal body cells. Therefore meiotic products would not properly replace ordinary skin cells.

Culture B may have lost normal contact inhibition. Normal animal cells often stop dividing when they contact neighbouring cells. Loss of this control allows continued division and accumulation of abnormal cell masses, a behaviour associated with tumour formation.

Cells between developing digits would remain rather than being selectively removed. Consequently the fingers could remain connected by tissue, producing a webbed appearance. The example shows why controlled cell death is as important as controlled cell division.

The observation supports totipotency, the ability of a living plant cell to give rise to a complete plant when provided with suitable conditions.

The idea was associated in the chapter with Gottlieb Haberlandt and became foundational to plant tissue culture technology. It demonstrates that even a specialised plant cell can retain the genetic information required to generate an entire organism under appropriate conditions.