Comprehensive Chapter Summary
1. Discovery of the Cell
Robert Hooke discovered cells in 1665 while examining a thin slice of cork under a self-designed microscope, noting that the cork resembled a honeycomb with many little compartments or 'cells'. Cork comes from the bark of a tree, and this chance observation was the first time someone noted that living things consist of separate units. The term 'cell' is Latin for 'a little room' and has been used in biology ever since. This incident is significant in science history as it laid the foundation for understanding that all living organisms are composed of cells. Anton van Leeuwenhoek improved the microscope in 1674 and discovered free-living cells in pond water for the first time. Robert Brown in 1831 discovered the nucleus in the cell. Purkinje in 1839 coined the term 'protoplasm' for the fluid substance of the cell. The cell theory was presented by Schleiden (1838) and Schwann (1839), stating that all plants and animals are composed of cells, which are the basic unit of life. Virchow (1855) expanded it by suggesting that all cells arise from pre-existing cells. The electron microscope in 1940 allowed observation of complex cell structures and organelles.
Historical Note: Robert Hooke
Hooke (1635-1703), an English scientist, observed cork slices resembling honeycombs. His book 'Micrographia' detailed these observations, coining 'cell' from Latin for 'little room'. This was a pivotal moment, leading to further explorations in microscopy and biology.
Historical Note: Cell Theory Development
The cell theory evolved through contributions from multiple scientists. Schleiden focused on plants, Schwann on animals, and Virchow on cell division. This theory revolutionized biology, emphasizing cells as the fundamental building blocks of life.
2. Cells as Building Blocks
Unicellular Organisms
Organisms like Amoeba, Chlamydomonas, Paramoecium, and bacteria consist of a single cell that performs all life functions. These are called unicellular (uni = single). The invention of magnifying lenses led to discovering such microscopic worlds. Activity 5.2 involves preparing temporary mounts of leaf peels, tip of roots of onion, or onion peels of different sizes to observe cell similarities and differences.
Multicellular Organisms
Many cells group together in a single body, assuming different functions to form various body parts in multicellular organisms (multi = many), such as some fungi, plants, and animals. Every multicellular organism comes from a single cell through cell division, producing cells of their own kind. All cells come from pre-existing cells. Human body cells include sperm, bone cell, smooth muscle cell, blood cells, ovum, nerve cell, fat cell (Fig. 5.3).
Cell Shape and Size
Cells vary in shape and size related to their function. Some like Amoeba have changing shapes, while nerve cells have fixed and peculiar shapes. Sizes range from small prokaryotic cells (1-10 µm) to larger eukaryotic cells (5-100 µm). In multicellular organisms, division of labor means different parts perform specific functions, like heart pumping blood or stomach digesting food.
Similarities and Differences in Cells
All cells have plasma membrane, nucleus, and cytoplasm. However, cells from different parts of a plant body differ, e.g., leaf cells vs. root cells. Onion bulbs of different sizes have similar small structures under microscope, showing cells as basic units.
Experiment: Onion Peel (Activity 5.1)
Take a small piece from an onion bulb, peel the epidermis from the concave side, place in water to prevent folding or drying. Transfer to slide with water, add safranin, cover slip avoiding air bubbles. Observe under compound microscope at low then high power (Fig. 5.1). Draw observed structures resembling Fig. 5.2, showing cells with nucleus.
Experiment: Temporary Mounts (Activity 5.2)
Prepare mounts of leaf peels, root tips, or onion peels of different sizes. Observe and answer: (a) Not all cells alike in shape/size; (b) Not alike in structure; (c) Differences among plant parts; (d) Similarities like membrane, nucleus.
3. Cell Structure and Organisation
Cells have special components called organelles that perform specific functions, like making new material or clearing waste. All cells have the same organelles regardless of function or organism. Under microscope, cells show plasma membrane, nucleus, cytoplasm. Plasma membrane separates cell contents from environment, selectively permeable for entry/exit of materials.
Plasma Membrane
Lipid-protein layer, flexible. Allows diffusion of substances like CO2, O2 from high to low concentration. Osmosis is water diffusion through selectively permeable membrane toward higher solute concentration. In hypotonic solution, cell gains water and swells; isotonic no net movement; hypertonic loses water and shrinks. Activity 5.3 with de-shelled egg: swells in water (hypotonic), shrinks in salt (hypertonic). Activity 5.4 with raisins/apricots: swell in water, shrink in concentrated solution.
Diffusion and Osmosis Applications
Diffusion important for gas exchange between cells and environment, water absorption by plant roots. Unicellular freshwater organisms and plant cells gain water by osmosis. The plasma membrane enables endocytosis, where cell engulfs food from external environment, as in Amoeba acquiring food.
Cell Wall
In plants, fungi, bacteria; cellulose in plants provides rigidity. Allows cell to withstand hypotonic solutions without bursting due to turgor pressure. Plasmolysis occurs in hypertonic solutions where cell contents shrink away from wall.
Nucleus
Double membrane with pores; contains chromatin (DNA+protein) that condenses into chromosomes carrying genes. Controls cell activities, reproduction, inheritance. Prokaryotes have undefined nuclear region called nucleoid without membrane.
Cytoplasm and Organelles
Fluid inside membrane containing organelles for specific functions: Endoplasmic reticulum (ER) for transport and synthesis (rough ER with ribosomes for proteins, smooth ER for lipids); Golgi apparatus for packaging and modification; lysosomes for digestion with enzymes, known as suicide bags; mitochondria for ATP production with own DNA and ribosomes; plastids in plants (chloroplasts for photosynthesis with chlorophyll, leucoplasts for storage, own DNA); vacuoles for storage and turgidity, larger in plants.
Prokaryotic vs Eukaryotic Cells
Prokaryotic: Small, no nuclear membrane, single chromosome, no membrane-bound organelles. Eukaryotic: Larger, nuclear membrane, multiple chromosomes, membrane-bound organelles. Bacteria are prokaryotic; plants/animals/fungi are eukaryotic.
4. Cell Division
Cells divide for growth, repair, reproduction. Mitosis: One cell divides into two identical daughter cells for growth and repair (Fig. 5.7). Meiosis: Reduces chromosome number by half for gamete formation, involving two divisions to produce four cells (Fig. 5.8).
Practical Applications
Osmosis in plant water absorption, diffusion in gas exchange. Lysosomes digest wastes; mitochondria power activities. Isotonic solutions in medical IVs prevent cell damage. Cell theory applications in biotechnology, medicine, understanding diseases like cancer from uncontrolled division.
Drawbacks and Theories
Early models had drawbacks; electron microscope revealed more. Endosymbiotic theory explains mitochondria and plastids as former bacteria with own DNA.
Questions and Answers from Chapter
Short Questions
Q1. Who discovered cells, and how?
Answer: Robert Hooke in 1665, by observing cork under a microscope.
Q2. Why is the cell called the structural and functional unit of life?
Answer: It performs all life functions and builds organisms.
Q3. How do substances like CO2 and water move in and out of the cell? Discuss.
Answer: By diffusion across the plasma membrane.
Q4. Why is the plasma membrane called a selectively permeable membrane?
Answer: It allows some substances to pass while blocking others.
Q5. Can you name the two organelles we have studied that contain their own genetic material?
Answer: Mitochondria and plastids.
Q6. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer: The cell will die.
Q7. Why are lysosomes known as suicide bags?
Answer: They can burst and digest their own cell.
Q8. Where are proteins synthesised inside the cell?
Answer: On ribosomes.
Q9. What is osmosis?
Answer: Diffusion of water across a selectively permeable membrane.
Q10. What is plasmolysis?
Answer: Shrinkage of cell contents away from cell wall in hypertonic solution.
Q11. What is a prokaryotic cell?
Answer: Cell lacking nuclear membrane.
Q12. What is a eukaryotic cell?
Answer: Cell with nuclear membrane.
Q13. What is the function of mitochondria?
Answer: Produce ATP energy.
Q14. What are plastids?
Answer: Organelles in plant cells for photosynthesis or storage.
Q15. What is the cell wall made of in plants?
Answer: Cellulose.
Q16. What is the nucleus?
Answer: Control center of the cell.
Q17. What is cytoplasm?
Answer: Fluid content inside plasma membrane.
Q18. What is mitosis?
Answer: Cell division for growth.
Q19. What is meiosis?
Answer: Division for gametes.
Q20. What is endocytosis?
Answer: Engulfing food by cell membrane.
Medium Questions
Q1. Do all cells look alike in terms of shape and size?
Answer: No, cells vary greatly in shape and size depending on their function. For example, Amoeba has a constantly changing shape to aid in movement and feeding, while nerve cells have a fixed, elongated shape for transmitting signals over long distances. Size ranges from 1-10 micrometers in prokaryotes to 5-100 micrometers in eukaryotes, with shape directly related to specific roles in the organism. (3 marks)
Q2. Do all cells look alike in structure?
Answer: No, cell structures differ between types; prokaryotic cells lack a defined nucleus and membrane-bound organelles, while eukaryotic cells have them. Plant cells have additional structures like cell walls and plastids, absent in animal cells. These structural differences enable specialized functions, such as photosynthesis in plants. (3 marks)
Q3. Could we find differences among cells from different parts of a plant body?
Answer: Yes, cells from different plant parts vary; leaf cells contain chloroplasts for photosynthesis, while root cells focus on absorption and lack chloroplasts. Stem cells may be elongated for support. These differences reflect division of labor in multicellular plants. (3 marks)
Q4. What similarities could we find?
Answer: All cells share basic features like plasma membrane for protection, cytoplasm for chemical reactions, and nucleus for control. These common elements ensure fundamental life processes occur similarly. Even in varied organisms, organelles like ribosomes are universal. (3 marks)
Q5. What do we observe as we look through the lens?
Answer: Through the microscope lens, we see onion peel cells as brick-like structures with distinct walls and a central nucleus stained by safranin (Fig. 5.2). The cells appear uniform regardless of onion size. This observation confirms cells as basic units. (3 marks)
Q6. What do we infer from Activity 5.6?
Answer: From plasmolysis in Rhoeo leaves, we infer only living cells respond to hypertonic solutions by shrinking, while boiled (dead) cells do not. This shows osmosis requires living, selectively permeable membranes. It distinguishes living from non-living matter. (3 marks)
Q7. What is the shape of the cells we see in cheek scrape?
Answer: Cheek cells are irregular or polygonal in shape with a central nucleus, stained by methylene blue. They lack cell walls, typical of animal cells. This contrasts with plant cells' rigid shapes. (3 marks)
Q8. What would happen if the plasma membrane ruptures or breaks down?
Answer: If the plasma membrane ruptures, cell contents would leak out, disrupting internal environment and leading to cell death. It loses selective permeability, unable to regulate substances. Lysosomes might release enzymes, causing further damage. (3 marks)
Q9. What would happen to the life of a cell if there was no Golgi apparatus?
Answer: Without Golgi, cells couldn't package or modify proteins/lipids, halting secretion and lysosome formation. Waste accumulation would occur, impairing function. Cell survival would be compromised due to disrupted transport. (3 marks)
Q10. Which organelle is known as the powerhouse of the cell? Why?
Answer: Mitochondria is the powerhouse, producing ATP through cellular respiration. Its cristae increase surface for energy reactions. Own DNA allows semi-autonomous function. (3 marks)
Q11. Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer: Lipids in smooth ER, proteins in rough ER attached to ribosomes. They contribute to membrane biogenesis. Golgi modifies them further. (3 marks)
Q12. How does an Amoeba obtain its food?
Answer: Amoeba uses pseudopodia for endocytosis, engulfing food particles into vacuoles. Lysosomes digest the food. This process is phagocytosis for solids. (3 marks)
Q13. Explain why water gathers in the hollowed portion of B and C in potato experiment.
Answer: Water enters by osmosis from hypotonic outside to hypertonic sugar/salt inside potato cups. Concentration gradient drives movement. Living cells enable this. (3 marks)
Q14. Why is potato A necessary for this experiment?
Answer: Potato A is a control without solute, showing no water gathering. It confirms osmosis due to concentration difference. Validates experimental setup. (3 marks)
Q15. Explain why water does not gather in the hollowed out portions of A and D.
Answer: A lacks solute for gradient; D has dead cells from boiling, no active membrane for osmosis. Demonstrates need for living cells and concentration difference. (3 marks)
Q16. What do we observe after 5 minutes in egg Activity 5.3(a)?
Answer: Egg swells as water enters by osmosis from hypotonic pure water to egg's hypertonic interior. Membrane allows water passage. Shows hypotonic effect. (3 marks)
Q17. The egg shrinks in Activity 5.3(b). Why?
Answer: Water leaves egg to hypertonic salt solution, causing shrinkage. Gradient favors outward movement. Demonstrates hypertonic effect on cells. (3 marks)
Q18. What happens to raisins in Activity 5.4(a)?
Answer: Raisins gain water by osmosis from hypotonic plain water, swelling up. Rehydrates dried cells. Shows endosmosis. (3 marks)
Q19. What happens in Activity 5.4(b)?
Answer: Raisins lose water to hypertonic sugar/salt solution, shrinking further. Exosmosis occurs. Contrasts with swelling in water. (3 marks)
Q20. Did plasmolysis occur in boiled Rhoeo leaf?
Answer: No, boiled leaves have dead cells without functional membranes for osmosis. No shrinkage observed. Highlights living cell requirement. (3 marks)
Long Questions
Q1. Make a comparison and write down ways in which plant cells are different from animal cells.
Answer: Plant cells differ from animal cells in several key ways: plant cells have a rigid cell wall made of cellulose providing structural support, while animal cells lack this wall and are more flexible. Plant cells contain large central vacuoles occupying up to 90% of cell volume for storage and turgidity, whereas animal cells have small, multiple vacuoles. Plastids like chloroplasts for photosynthesis are present in plant cells but absent in animal cells. Animal cells often have centrioles for cell division, not typically found in plant cells. These differences adapt plants for stationary life with photosynthesis and animals for mobility and ingestion (Figs. 5.5, 5.6). Additionally, plant cells can withstand hypotonic solutions without bursting due to the cell wall, unlike animal cells that may lyse.
Q2. How is a prokaryotic cell different from a eukaryotic cell?
Answer: Prokaryotic cells differ from eukaryotic cells primarily in lacking a defined nuclear membrane, with DNA in a nucleoid region instead of a nucleus. Prokaryotes have a single chromosome, while eukaryotes have multiple linear chromosomes. Membrane-bound organelles like mitochondria, ER, Golgi are absent in prokaryotes but present in eukaryotes. Prokaryotic ribosomes are smaller (70S) compared to eukaryotic (80S). Prokaryotes are generally smaller (1-10 µm) and include bacteria, while eukaryotes are larger (5-100 µm) and include plants, animals, fungi (Fig. 5.4). Eukaryotes have complex cytoskeletons; prokaryotes simpler. These differences reflect evolutionary divergence, with eukaryotes having compartmentalized functions.
Q3. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
Answer: Prokaryotic cells: Size mostly 1-10 µm; Nuclear region poorly defined due to absence of nuclear membrane; Chromosome single; No membrane-bound organelles. Eukaryotic cells: Size mostly 5-100 µm; Nuclear region well defined and surrounded by nuclear membrane; More than one chromosome; Membrane-bound organelles present. This table highlights fundamental structural differences enabling varied complexity in life forms.
Q4. Carry out the following osmosis experiment: Explain observations.
Answer: In the potato osmosis experiment, cut potato cups, place in water dish. Fill A empty (control), B sugar, C salt, D boiled with sugar. Water gathers in B and C by osmosis from hypotonic water to hypertonic interiors, rising levels. No change in A due to no gradient. D shows no osmosis as boiling kills cells, destroying membrane selectivity. This demonstrates osmosis requires living cells and concentration differences, with applications in understanding plant water uptake and cell turgor.
Q5. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
Answer: Mitosis is required for growth and repair, where one cell divides into two identical daughter cells with the same chromosome number, ensuring tissue maintenance (Fig. 5.7). Meiosis forms gametes, reducing chromosome number by half through two divisions producing four haploid cells for sexual reproduction (Fig. 5.8). Mitosis maintains genetic stability; meiosis introduces variation through crossing over and independent assortment. These processes are crucial for multicellular organism development and species continuity.
Q6. Describe the structure and function of plasma membrane.
Answer: The plasma membrane is the outermost covering, composed of lipids and proteins, forming a flexible, selectively permeable barrier separating cell contents from the environment. It regulates entry and exit of materials via diffusion for gases like CO2 and O2, and osmosis for water. In hypotonic solutions, it allows water influx causing swelling; in hypertonic, efflux causing shrinkage. Its flexibility enables endocytosis for food intake in Amoeba. The membrane maintains cell integrity, facilitates communication, and is vital for homeostasis.
Q7. Explain the cell theory and its expansion.
Answer: The cell theory states all organisms are composed of cells, the basic unit of life (Schleiden 1838 for plants, Schwann 1839 for animals). Virchow (1855) expanded it: all cells arise from pre-existing cells, emphasizing continuity. This theory unified biology, explaining growth via division and heredity through cells. Historical context includes Hooke's discovery, Leeuwenhoek's microbes, Brown's nucleus. It underpins modern fields like genetics and biotechnology.
Q8. Describe endoplasmic reticulum types and functions.
Answer: Endoplasmic reticulum (ER) is a network of tubules and sacs. Rough ER (RER) has ribosomes for protein synthesis and folding, aiding secretion. Smooth ER (SER) lacks ribosomes, synthesizes lipids, steroids, detoxifies drugs. Both contribute to membrane biogenesis by producing lipids/proteins for plasma membrane. ER transports materials within cell, connects to nuclear membrane. Dysfunction can lead to diseases like ER stress-related disorders.
Q9. What is the function of Golgi apparatus?
Answer: Golgi apparatus, discovered by Camillo Golgi (1898), consists of stacked cisternae for storage, modification, and packaging of ER products like proteins and lipids. It adds carbohydrates for glycoproteins, forms lysosomes with digestive enzymes. Golgi sorts and directs molecules to destinations, crucial for secretion in glandular cells. In plants, it synthesizes cell wall polysaccharides. Its role in processing is essential for cell function and organism health.
Q10. Explain lysosomes and their role.
Answer: Lysosomes are membrane-bound vesicles containing hydrolytic enzymes for breaking down wastes, organelles, and ingested materials. Formed by Golgi, they digest via autophagy (self-digestion) or heterophagy (foreign matter). Known as suicide bags, they can burst in distressed cells, releasing enzymes to digest the cell. Lysosomes maintain cellular hygiene, recycle components, and are involved in defense against pathogens. Deficiencies cause storage diseases like Tay-Sachs.
Q11. Describe mitochondria structure and function.
Answer: Mitochondria have double membranes: outer permeable, inner folded into cristae increasing surface area for respiration. Matrix contains enzymes, DNA, ribosomes for semi-autonomy. They produce ATP via oxidative phosphorylation, powering cellular activities. Involved in apoptosis, calcium signaling. Endosymbiotic origin explains own genome. Number varies by cell energy needs, e.g., more in muscle cells.
Q12. What are plastids? Types and functions.
Answer: Plastids are plant organelles with double membranes and own DNA/ribosomes. Chloroplasts contain chlorophyll for photosynthesis, converting light to chemical energy. Leucoplasts store starch, oils, proteins; chromoplasts give color to fruits/flowers. Amyloplasts for starch in roots. Plastids differentiate from proplastids, essential for plant nutrition, attraction of pollinators. Endosymbiotic origin similar to mitochondria.
Q13. Explain vacuoles in plant and animal cells.
Answer: Vacuoles are membrane-bound sacs for storage. In plants, large central vacuole (up to 90% volume) stores water, ions, wastes, maintains turgor pressure for rigidity. Contains sap with pigments, enzymes. Animal cells have small, multiple vacuoles for temporary storage/digestion. Contractile vacuoles in protists expel excess water. Vacuoles aid detoxification, pigmentation, and in plants, defense via toxins.
Q14. Describe Activity 5.1 in detail.
Answer: For onion peel: Peel epidermis from concave side using forceps, place in water watch-glass to prevent drying. Transfer flat piece to slide with water drop, add safranin for staining, cover with slip avoiding bubbles using needle. Observe under low then high power microscope (Fig. 5.1). See brick-like cells with walls, cytoplasm, nucleus (Fig. 5.2). Demonstrates basic cell structure, importance of staining for visibility.
Q15. Explain differences in hypotonic, isotonic, hypertonic solutions.
Answer: Hypotonic: Lower solute than cell, water enters causing swelling or bursting in animal cells, turgidity in plants. Isotonic: Equal concentration, no net water movement, cell stable. Hypertonic: Higher solute, water exits causing shrinkage (plasmolysis in plants). Examples: Pure water hypotonic, saline isotonic, sugar solution hypertonic. Crucial for IV fluids, food preservation.
Q16. Describe cell wall and plasmolysis.
Answer: Cell wall is rigid outer layer of cellulose in plants, providing shape, protection, preventing bursting in hypotonic solutions via turgor. Plasmolysis: In hypertonic solutions, water loss causes cytoplasm shrinkage away from wall, observable in Rhoeo leaves. Reversible if returned to hypotonic. Demonstrates membrane permeability, used in studying cell viability.
Q17. Explain nucleus structure.
Answer: Nucleus has double nuclear membrane with pores for material exchange. Inside, chromatin (DNA+histones) condenses into chromosomes during division, carrying genes for traits. Nucleolus synthesizes ribosomes. Controls gene expression, replication, cell activities. Absence in prokaryotes as nucleoid. Vital for eukaryotes' complexity.
Q18. Describe Activity 5.7 (cheek cells).
Answer: Gently scrape inside cheek with toothpick, transfer to slide with water drop. Add methylene blue for staining, cover slip. Observe under microscope: irregular cells, central nucleus, no cell wall. Demonstrates animal cell structure, contrasts with plant cells. Safety: Use clean tools, avoid injury.
Q19. Explain prokaryotic cell (Fig. 5.4).
Answer: Prokaryotic cells like bacteria have plasma membrane, cytoplasm with ribosomes, nucleoid with single circular DNA. Lack nucleus, organelles. Cell wall (peptidoglycan) provides shape. Flagella for movement in some. Simple, rapid division enables quick adaptation. Basis for many biotechnologies.
Q20. Describe animal cell (Fig. 5.5) vs plant cell (Fig. 5.6).
Answer: Animal cell: Irregular shape, plasma membrane, small vacuoles, centrioles, no wall/plastids. Plant cell: Rectangular, cell wall, large vacuole, chloroplasts, no centrioles. Animal flexible for movement; plant rigid for support. Both have nucleus, ER, Golgi, mitochondria, ribosomes. Differences adapt to lifestyles: ingestion vs. photosynthesis.
