Learning Objectives
By the end of this chapter, you will be able to:
- Explain how life is believed to have originated in water environments
- Describe the role of hot springs in understanding early Earth conditions
- Identify the cell as the fundamental unit of life in all organisms
- Distinguish between unicellular and multicellular organisms with examples
- Explain the principles of microscopy and estimate the size of a cell
- Describe the structure and function of the cell membrane
- Define osmosis, diffusion, and selective permeability with examples
- Explain the fluid-mosaic model of the cell membrane
- Describe the structure and significance of the cell wall
- Compare plant cells and animal cells using experimental evidence
- Interpret the effects of hypertonic, hypotonic, and isotonic solutions on a cell
The Origin of Life — Where Did It All Begin?
The question of life's origin has fascinated scientists and philosophers across centuries. Today, it is widely accepted within the scientific community that life originated in water. Water provides the essential medium for biochemical reactions — it dissolves molecules, enables transport of nutrients, and stabilises temperature. Without water, the intricate chemistry of life cannot occur.
While the vast oceans were once assumed to be the cradle of life, a newer and increasingly supported hypothesis suggests something more intriguing: life may have originated in small, isolated water pools where environmental conditions fluctuated dramatically. These changing conditions — cycles of wetting and drying, heating and cooling — could have driven the concentration and polymerisation of organic molecules, leading to the first self-replicating structures. Hot springs are among the most compelling candidates for such environments.
2.0 Hot Springs — Windows to Early Earth
In India, the hot springs of Puga Valley in Ladakh offer a remarkable glimpse into what conditions on early Earth may have resembled. Situated at high altitude in a cold desert, these springs maintain temperatures close to the boiling point of water — a dramatic contrast to the freezing surroundings. This thermal energy is driven by geothermal activity beneath Earth's crust.
Scientists estimate that approximately 3.5 billion years ago, Earth's surface looked very similar to these hot springs today — high temperatures, mineral-rich waters, fluctuating chemical conditions, and intense ultraviolet radiation. It is in such environments that the first molecules of life may have assembled.
Scientists from the Birbal Sahni Institute of Palaeosciences, Lucknow discovered that calcium carbonate (CaCO₃) forms rapidly around these Ladakhi hot springs. These mineral deposits may have acted as a natural shield, protecting early organic molecules from harmful ultraviolet radiation and extreme chemical conditions. Moreover, these deposits may have provided a scaffold for the formation of the very first protective membranes — a critical step toward the emergence of the first true cells.
The Organisms of Puga Valley: Thermophiles
The organisms found thriving in these near-boiling springs are known as thermophiles (from Greek: thermos = heat, philos = loving). These are heat-loving bacteria that are unicellular — consisting of a single cell. Their ability to survive in such extreme conditions makes them members of a broader group called extremophiles. Studying thermophiles helps us understand not only the biochemical limits of life, but also what the earliest life forms on Earth may have been like.
The DNA-copying enzyme Taq polymerase — derived from the thermophilic bacterium Thermus aquaticus found in Yellowstone hot springs — is used in the Polymerase Chain Reaction (PCR) technique, which forms the basis of COVID-19 testing, forensics, and genetic research. Studying extremophiles from Indian hot springs may similarly yield biotechnological breakthroughs.
The Cell — Life's Fundamental Unit
All living organisms are made up of cells. This single statement — one of the most powerful in all of biology — is known as Cell Theory, first articulated in the 19th century. The cell is not merely a structural unit; it is the basic functional unit of life. Everything that makes a living organism alive — metabolism, growth, response to stimuli, reproduction — occurs at the level of the cell.
From Cells to Organ Systems: The Hierarchy of Life
In multicellular organisms, specialisation and cooperation are key. A group of similar cells performing a similar function forms a tissue. Multiple tissues are organised to form an organ, and several organs working in coordination form an organ system. For example:
Even when cells are organised into tissues, organs, and organ systems, the cell remains the fundamental unit of structure and function in all living organisms. This raises a profound question: how do such tiny, microscopic units manage to perform all the complex activities necessary for life?
Cell Theory has three core statements: (1) All living organisms are made of cells. (2) The cell is the basic structural and functional unit of life. (3) All cells arise from pre-existing cells (Virchow's addition). This is a frequent source of exam questions — know which scientist contributed which postulate.
2.1 How to Study Cells?
The Limit of Resolution — Why We Cannot See Cells with the Naked Eye
The human eye is a remarkable optical instrument, but it has a fundamental physical constraint: its limit of resolution. This refers to the minimum distance between two points at which they can be perceived as separate and distinct objects. When viewed from the standard reading distance of about 25 cm (the near point of the human eye), two points must be at least 0.1 mm (100 µm) apart to appear as separate dots. Any closer, and they blur into a single point.
Most cells measure between 10 µm and 100 µm — well below the threshold of human vision. This is why, despite being surrounded by billions of cells at all times, we cannot see them without assistance. It is one of the most profound limitations that drove the development of microscopy.
The Light Microscope — Our Window into the Cellular World
The pivotal moment in the history of biology came in 1665, when Robert Hooke designed a microscope capable of 200–300× magnification and examined a thin slice of cork. He observed small box-like compartments and named them 'cells' — derived from the Latin word cellula, meaning "small room." This observation marked the beginning of cell biology as a scientific discipline.
In school laboratories, light microscopes use visible light passing through a series of lenses to magnify objects. They typically include multiple objective lenses (e.g., 10×, 40×, 100×) that can be rotated to achieve different magnifications. The total magnification of a light microscope is the product of the magnifying power of the eyepiece and the objective lens:
Example: 10× eyepiece × 10× objective = 100× total magnification
The three critical properties that define microscope quality are:
- Resolution: The ability to distinguish two closely-spaced points as separate. A higher-resolution microscope reveals finer details.
- Contrast: The difference in brightness between the specimen and its background. Staining with dyes (safranin, methylene blue) greatly improves contrast.
- Magnification: The degree to which the specimen appears larger than its real size.
Improvements in all three of these properties over centuries have transformed the microscope from a simple curiosity into a powerful investigative instrument.
The Electron Microscope — Seeing at the Nanometre Scale
For structures smaller than the wavelength of visible light (about 400–700 nm), even the best light microscopes become inadequate. Electron microscopes overcome this limitation by using a beam of electrons — which have a far shorter wavelength than visible light — to produce highly magnified images with extraordinary resolution, down to the nanometre scale (1 nm = 0.000001 mm).
Two major types of electron microscopes are used in cell biology:
- Transmission Electron Microscope (TEM): Passes electrons through a thin specimen, revealing internal structures like organelles, membranes, and ribosomes at extremely high resolution.
- Scanning Electron Microscope (SEM): Scans the surface of a specimen, producing three-dimensional images of surface features — as seen in the micrograph of the Colocasia leaf (Fig. 2.4).
Objective: To estimate the real size of an onion peel cell using a light microscope and a transparent ruler, applying the principle of field-of-view measurement.
- Take a transparent ruler with millimetre (mm) markings.
- Place the ruler on the stage of the microscope, focus on it using the adjustment knob, and observe the diameter of the circular field of view through the eyepiece. Measure this diameter in mm.
- Convert the diameter from mm to micrometres (µm). Recall: 1 mm = 1000 µm. For example, if the diameter is 5 mm → 5 × 1000 = 5000 µm.
- Remove the ruler. Place a prepared onion peel slide on the stage and focus carefully.
- Count the number of cells present along the diameter of the field of view, in one straight line.
- Apply the formula below to calculate the estimated size of one cell.
Example: 5000 µm ÷ 25 cells = 200 µm per cell
Students often confuse magnification with resolution. A microscope can magnify an image many times, but if the resolution is poor, the magnified image will be blurry and indistinct. Resolution determines clarity; magnification determines apparent size. An electron microscope is valued for its superior resolution, not just its magnification.
- Who was the first person to observe a cell?
- The minimum distance at which two points can be seen as distinct by the human eye is:
- The total magnification of a microscope with a 10× eyepiece and 40× objective is:
- Electron microscopes use ______ instead of light to produce magnified images.
- Robert Hooke observed cells in a slice of:
- The unit 1 mm = _____________ µm.
- The _____________ microscope uses a beam of electrons to form highly magnified images.
- The three key properties of a microscope are resolution, contrast, and _____________.
- Robert Hooke coined the term _____________ in 1665.
- The limit of resolution of the human eye is _____________ mm.
- The coarse adjustment knob is used for precise, sharp focusing at high magnification. (False)
- Electron microscopes can resolve structures at the nanometre scale. (True)
- A light microscope uses visible light to form magnified images. (True)
- Magnification and resolution are the same property of a microscope. (False)
- Most plant and animal cells can be seen with the unaided eye. (False)
- Define "limit of resolution" of the human eye. Why does this make it impossible to see most cells without a microscope?
- Calculate: If the field of view diameter is 3 mm and 15 onion cells are seen along the diameter, what is the estimated size of one cell?
- State two differences between a light microscope and an electron microscope.
- Why is staining (e.g., with safranin or methylene blue) important when observing cells under a light microscope?
2.2 Structure of a Cell
We have established that cells are the fundamental units of life, and that they are organised into tissues, organs, and organ systems. But for this organisation to work, cells must communicate with one another and with their external environment. These interactions occur primarily at the cell boundary — the interface between the inside of the cell and the outside world. Substances must enter and leave the cell in a precisely regulated manner, and this regulation is the responsibility of the cell membrane.
2.2.1 Cell Membrane — The Universal Feature of All Cells
The cell membrane (also called the plasma membrane) is a thin, flexible boundary that encloses the cell and separates its contents from the external environment. It is present in all living cells — whether bacterial, plant, or animal — making it the most universal feature of cellular life.
The cell membrane is described as selectively permeable: it allows certain substances to pass through while blocking others. This selective control over what enters and exits the cell is fundamental to maintaining the cell's internal environment — a concept known as homeostasis.
Consider the cells lining the alveoli (air sacs) in the lungs. Oxygen must move from the air inside the alveoli into the blood, and carbon dioxide must move in the opposite direction. The selectively permeable cell membranes of the alveolar cells make this exchange possible — allowing gases to diffuse through while maintaining the cell's internal chemistry. This is a direct, real-world application of cell membrane function you have studied in Grade 7.
Objective: To demonstrate that the cell membrane is selectively permeable, allowing water to move in and out by osmosis.
- With the help of a kitchen knife, carefully cut a potato into two pieces of roughly equal size.
- Measure and record the initial weight of both pieces using a weighing balance.
- Place one piece (a) in Beaker A containing plain water.
- Place the other piece (b) in Beaker B containing a 20% salt or sugar solution.
- Leave both beakers undisturbed for about one hour or until a visible change in size is observed.
- Measure and record the final weight of each piece. Calculate the difference from initial weight.
Beaker B (20% Salt Solution): The potato piece shrinks — its weight decreases.
This occurs because the cell membrane allows water molecules to move across it but NOT large solute molecules (salt/sugar). Water moves from the region of higher water concentration (dilute solution, Beaker A) into the cell → cell swells. In Beaker B, the cell's water is less concentrated than outside, so water exits → cell shrinks.
Osmosis — The Directed Movement of Water
Osmosis is defined as the movement of water molecules from a region of higher water concentration (dilute solution, lower solute concentration) to a region of lower water concentration (concentrated solution, higher solute concentration), through a selectively permeable membrane, until equilibrium is reached.
It is important to distinguish osmosis from diffusion:
| Property | Diffusion | Osmosis |
|---|---|---|
| Particles involved | Any particles (solute, gas, etc.) | Water molecules only |
| Membrane required? | No — occurs without a membrane | Yes — requires a selectively permeable membrane |
| Direction | High concentration → Low concentration | High water conc. → Low water conc. |
| Example | Perfume spreading in a room; ink in water | Water entering root cells from soil |
In plants, water from the soil enters root cells through osmosis — because the root cell cytoplasm has a higher concentration of dissolved substances (lower water concentration) than the surrounding soil water. This is the primary mechanism by which plants absorb water.
Types of Solutions Based on Concentration
When a cell is placed in a solution, the relative concentration of the solution compared to the cell's internal fluids determines whether water enters or leaves the cell:
| Solution Type | Definition | Effect on Cell | Example |
|---|---|---|---|
| Isotonic | Solute conc. outside = Solute conc. inside cell | No net water movement; cell shape maintained | Normal saline (0.9% NaCl) for blood cells |
| Hypotonic | Solute conc. outside < Solute conc. inside cell (external solution more dilute) | Water enters the cell; cell swells (may burst in animal cells — lysis) | Plain water for potato piece |
| Hypertonic | Solute conc. outside > Solute conc. inside cell (external solution more concentrated) | Water exits the cell; cell shrinks (crenation in animal cells; plasmolysis in plant cells) | 20% salt solution for potato piece |
Consider mung beans soaked in water for 12 hours — they absorb water by osmosis and swell. If these swollen seeds are then placed in a concentrated solution, the external solution is hypertonic to the seed's cells. Water will now exit the cells by osmosis, and the seeds will shrink and become wrinkled. This is the same principle as the potato experiment, applied to seeds.
2.2.2 Structure of the Cell Membrane — The Fluid-Mosaic Model
The cell membrane is extraordinarily thin — measuring only 7 to 10 nanometres (nm) in thickness. To put this in perspective, 10 nm is about 10,000 times thinner than a single human hair. Despite this minuscule thickness, the membrane is a sophisticated, dynamic structure responsible for regulating all traffic in and out of the cell.
The most widely accepted explanation of membrane structure is the Fluid-Mosaic Model (Singer and Nicolson, 1972). The name itself encodes its key properties:
A powerful analogy for the fluid-mosaic model: imagine protein molecules (large islands) floating freely in a sea of lipid molecules (the fluid ocean). The lipid bilayer is the sea; the proteins are the scattered islands. Both components move, interact, and contribute to the membrane's functions. This analogy makes the model intuitive and memorable for exams.
Key Terminology — Cell Membrane
Objective: To observe and compare the shape, arrangement, and response to osmosis in plant cells (onion/Rhoeo leaf) and animal cells (human cheek cells).
- Prepare a temporary slide of a thin peel of an onion leaf or a Rhoeo (Cradle lily) leaf. Mount it with safranin stain and cover with a coverslip.
- Prepare a cheek cell slide: gently scrape the inner side of your cheek with a clean cotton swab. Spread on a glass slide, add a drop of water, then a few drops of methylene blue stain. Carefully place a coverslip.
- Observe both slides under a microscope. Note the shape and arrangement of the cells.
- Prepare two new slides of Rhoeo leaf peel and cheek cells. Apply a drop of 20% sugar solution onto each slide. Wait 30 minutes and re-observe.
After Sugar Solution: Plant cells — the outer boundary (cell wall) stays intact, but the inner contents (protoplast) shrink and pull away from the cell wall, creating a gap. This is called plasmolysis. Cheek cells shrink considerably in all dimensions because there is no cell wall to maintain their outer shape.
2.2.3 Cell Wall — The Outer Armour of Plant, Fungal, and Bacterial Cells
While the cell membrane is universal, certain groups of organisms — plants, fungi, and bacteria — possess an additional protective layer outside the cell membrane called the cell wall. This outer covering is rigid, tough, and provides significant mechanical support.
Why Do Plants Need a Cell Wall?
Plants, unlike most animals, are stationary organisms. They cannot relocate to escape wind, rain, or physical stress. They must withstand mechanical forces imposed by gravity, wind, and the weight of their own structures. The rigid cell wall provides this mechanical strength, ensuring that leaves, stems, and flowers maintain their shape and remain upright. It also helps leaves and petals remain firm and turgid.
The plant cell wall is primarily composed of cellulose — a complex carbohydrate made of many glucose units linked together by β-1,4-glycosidic bonds. This arrangement makes cellulose fibres exceptionally strong and resistant to stretching. Interestingly, the same cellulose that gives plants their structural rigidity acts as dietary fibre (roughage) in our digestive system, aiding the peristaltic movement of food through the intestines.
Cell Wall Properties
| Property | Cell Membrane | Cell Wall |
|---|---|---|
| Presence | All living cells | Plants, fungi, bacteria (NOT animal cells) |
| Rigidity | Flexible, fluid | Rigid and tough |
| Permeability | Selectively permeable | Freely permeable (porous) |
| Primary Composition | Phospholipids and proteins | Cellulose (plants); Chitin (fungi); Peptidoglycan (bacteria) |
| Main Function | Regulates entry/exit of substances | Structural support, shape maintenance, protection |
| Thickness | 7–10 nm | 0.1 µm to several µm |
Although rigid, the cell wall is freely permeable — water and small dissolved minerals pass through it without restriction. The selective regulation of entry into the cell is handled entirely by the cell membrane within. Together, the permeable cell wall and the selectively permeable cell membrane form a two-stage system for controlling what enters the cell — a critical mechanism for plant root cells absorbing water and nutrients from the soil.
Evidence from Osmosis Experiments
The potato experiment and the Rhoeo leaf peel experiment both elegantly demonstrate the difference between cells with and without a cell wall:
- In a hypertonic solution, plant cells lose water but maintain their external dimensions — the rigid cell wall prevents the cell from shrinking. Only the internal contents (protoplast) pull away from the wall — this is plasmolysis.
- Animal cells (cheek cells) in the same hypertonic solution shrink entirely, as there is no cell wall to constrain their outer boundary.
- The flexibility of animal cells — a direct consequence of lacking a rigid cell wall — enables animal tissues to move, change shape, and function dynamically (e.g., muscle contraction, blood cell flexibility through capillaries).
Fungi have cell walls made of chitin — the same tough material found in insect exoskeletons — rather than cellulose. Bacteria have cell walls made of a unique material called peptidoglycan. The differences in cell wall composition between bacteria, fungi, and plants are exploited in medicine: antibiotics like penicillin target bacterial cell wall synthesis without affecting human cells (which have no cell wall), making them effective and relatively safe antimicrobial agents.
1. What argument would you give for the necessity of a cell wall in plants (usually fixed in one place) versus in animals (usually moving from place to place)?
2. What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
3. Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Students often believe the cell wall controls which substances enter the cell. This is incorrect. The cell wall is freely permeable — almost anything can pass through it. It is the cell membrane just inside the cell wall that performs the critical role of selective regulation. The cell wall's primary job is mechanical support and protection.
- Which model best explains the structure of the cell membrane?
- When a plant cell is placed in a hypertonic solution, what is observed?
- Osmosis is specifically defined as the diffusion of:
- The thickness of the cell membrane is approximately:
- The cell wall of plants is primarily made of:
- Which of the following does NOT have a cell wall?
- In the fluid-mosaic model, proteins in the membrane act as:
- What happens to a red blood cell placed in pure water?
- The movement of water through a selectively permeable membrane from a dilute to a concentrated solution is called _____________.
- A solution in which the external solute concentration equals the internal solute concentration of the cell is called a(n) _____________ solution.
- The shrinkage of the protoplast away from the cell wall in a hypertonic solution is called _____________.
- The cell membrane is described as _____________ permeable because it allows only certain substances to pass through it.
- The lipid bilayer has hydrophilic _____________ facing outward and hydrophobic _____________ facing inward.
- Cellulose in our diet acts as _____________, helping in digestion.
- The cell wall is selectively permeable, regulating the entry of substances. (False — It is the cell membrane that is selectively permeable; the cell wall is freely permeable.)
- Animal cells lack a cell wall, which allows them to change shape easily. (True)
- In a hypotonic solution, water exits the cell, causing it to shrink. (False — Water enters the cell in a hypotonic solution, causing it to swell.)
- The fluid-mosaic model was proposed by Singer and Nicolson in 1972. (True)
- Fungi have cell walls made of cellulose, like plant cells. (False — Fungal cell walls are made of chitin.)
Answers: 1-D, 2-C, 3-A, 4-B, 5-E, 6-F
- Define osmosis. How is it different from diffusion?
- Why does the potato piece in the salt solution lose weight, while the one in plain water gains weight?
- Describe the fluid-mosaic model of the cell membrane in your own words, explaining both the "fluid" and "mosaic" aspects.
- What is plasmolysis? In which type of cells is it observed, and why not in animal cells?
- State two functions of the cell wall in plant cells.
- Why would a plant wilt if kept without water for a long time? Relate your answer to osmosis and turgor pressure.
- Animal cells can change shape easily, but plant cells cannot — give a reason.
- Antibiotics like penicillin can kill bacteria without harming human cells — give a reason based on cell wall composition.
- Water from the soil enters root cells even though the plant is not actively "drinking" — give a reason.
- A person drinking very salty seawater becomes more dehydrated — give a reason using osmosis.
- Doctors use 0.9% normal saline (isotonic) rather than plain water to rehydrate patients intravenously. Explain why using plain water intravenously could be dangerous for red blood cells.
- When you sprinkle salt on a cut tomato or cucumber, water appears on the surface within minutes. Explain this observation using the concept of osmosis.
- A student notices that freshly cut flowers placed in a vase of water stay fresh longer than those kept without water. Explain this in terms of cell turgor and osmosis.
- Design an experiment (similar to Activity 2.2) to test whether osmosis occurs across an egg membrane. What would you observe if you removed the shell of a boiled egg and placed it in a concentrated sugar solution?
- Describe the fluid-mosaic model of the cell membrane in detail. Include in your answer: (a) the components of the lipid bilayer, (b) the role of proteins, (c) the property of fluidity, and (d) why the membrane is described as "mosaic".
- With the help of a diagram or table, compare plant cells and animal cells with reference to: cell membrane, cell wall, osmotic behaviour in hypertonic and hypotonic solutions, and shape flexibility.
- Explain the process of osmosis. Describe Activity 2.2 (the potato experiment) in full, including: purpose, procedure, observations, inference, and the scientific principle demonstrated.
- Life is widely believed to have originated in water. Small, fluctuating water pools like hot springs (e.g., Puga Valley, Ladakh) may resemble early Earth conditions from ~3.5 billion years ago.
- Thermophiles — heat-loving unicellular bacteria — inhabit hot springs and are valuable for understanding primitive life and biotechnology.
- Calcium carbonate deposits around hot springs may have protected early organic molecules and assisted in forming the first cell-like membranes.
- The cell is the basic structural and functional unit of life. Organisms may be unicellular (bacteria, yeast, amoeba) or multicellular (plants, animals).
- Organisation: Cells → Tissues → Organs → Organ Systems → Organism.
- The limit of resolution of the human eye is 0.1 mm; most cells are smaller and require a microscope to be seen.
- Robert Hooke first observed cells in cork in 1665. Light microscopes use visible light; electron microscopes use electron beams for nanometre-scale resolution.
- Total magnification = Eyepiece power × Objective lens power. Cell size can be estimated using the field-of-view method.
- The cell membrane (plasma membrane) is a universal feature of all living cells — selectively permeable, it regulates entry and exit of substances.
- Osmosis is the movement of water through a selectively permeable membrane from high water concentration (dilute solution) to low water concentration (concentrated solution).
- Isotonic solution: no net water movement. Hypotonic solution: water enters cell (swells). Hypertonic solution: water exits cell (shrinks/plasmolysis).
- The fluid-mosaic model describes the cell membrane as a fluid lipid bilayer with proteins embedded throughout. Hydrophilic heads face outward; hydrophobic tails face inward. Proteins act as gatekeepers.
- Plant, fungal, and bacterial cells have a cell wall outside the cell membrane. Plant cell walls are made of cellulose. The cell wall is rigid (providing structural support) but freely permeable.
- Animal cells lack a cell wall — this makes them flexible and able to change shape, but also means they shrink and swell more dramatically in osmotic conditions compared to plant cells.
- Which of the following organisms is an example of a thermophile?
- The term "cell" was first coined after observing:
- Which of the following is NOT a function of membrane proteins?
- In a hypotonic solution, water moves:
- Which of the following correctly describes the lipid bilayer?
- What is the primary composition of the cell wall of fungi?
- The Scanning Electron Microscope (SEM) is used to observe:
- Which of the following is a real-world application of research on thermophilic bacteria?
- The hot springs of _____________ valley in Ladakh are inhabited by thermophilic bacteria.
- The scientist who first observed and named cells was _____________.
- The three properties of a good microscope are resolution, contrast, and _____________.
- Osmosis is a special case of _____________, specifically involving water molecules.
- In the fluid-mosaic model, lipid molecules have a _____________ head and a _____________ tail.
- When a plant cell is placed in a hypertonic solution, its inner contents shrink away from the cell wall — a phenomenon called _____________.
- The process by which water enters root cells from the soil is called _____________.
- The cell wall of bacteria is made of _____________, which is targeted by antibiotics such as penicillin.
- Life is believed to have originated in small fluctuating water pools rather than necessarily the open oceans. (True)
- All living organisms are made of cells — this principle is part of Cell Theory. (True)
- The electron microscope uses visible light at extremely high intensity. (False — It uses a beam of electrons.)
- The cell membrane of animal cells is more rigid than the cell wall of plant cells. (False — The cell membrane is flexible; the cell wall is rigid.)
- A cell in an isotonic solution neither gains nor loses water by osmosis. (True)
- Robert Hooke first observed cells in 1665 using a self-designed microscope. (True)
Answers: 1-C, 2-F, 3-E, 4-G, 5-A, 6-B, 7-H, 8-D
- State two similarities and two differences between unicellular and multicellular organisms.
- What is the biological hierarchy from the cell to the organism? Give one example at each level.
- Explain why hot springs in Puga Valley, Ladakh are significant for scientific research on the origin of life.
- Define resolution in the context of microscopy. Why does an electron microscope have better resolution than a light microscope?
- Describe the arrangement of phospholipids in the cell membrane bilayer. Why is this arrangement stable in an aqueous environment?
- What is the role of membrane proteins in the fluid-mosaic model?
- Differentiate between a hypotonic and hypertonic solution. State the effect of each on a plant cell and an animal cell.
- Why is the cell wall said to be rigid yet permeable? How does this help plant cells absorb water from the soil?
- The cell wall maintains the shape of a plant cell even when it is placed in a hypertonic solution.
- The fluid-mosaic model describes the cell membrane as "fluid."
- Early organic molecules on Earth may have been protected by calcium carbonate deposits around hot springs.
- Osmosis requires a selectively permeable membrane, but diffusion does not.
- Cellulose from vegetables acts as roughage in our diet.
- Write a detailed account of how scientists study cells, covering: (a) the limit of resolution of the human eye, (b) the light microscope — its key components and how magnification is calculated, (c) the electron microscope and its advantages, and (d) Activity 2.1 for estimating cell size. Include relevant examples and calculations.
- Describe osmosis in detail. Include: (a) definition and comparison with diffusion, (b) demonstration through Activity 2.2 (potato experiment) — full procedure, observations, and inference, (c) the three types of solutions (isotonic, hypotonic, hypertonic) and their effects on plant and animal cells, and (d) one real-life application of osmosis in plants.
- Explain the structure and functions of the cell wall in plants. Compare the cell wall of plants, fungi, and bacteria. Why do animal cells not have a cell wall, and what is the biological advantage of this?