Class 9 - Ch5 - Cell:Fundamental Unit of Life - NCERT Q & A
Q1: Who observed the cells first time?
Answer: Robert Hooke. He observed thin slice of cork cells under a simple microscope.
Q2:Define Cell
Answer: Cell is the structural and functional unit of life.
Q3: Who proposed the cell theory?
Answer: Schleiden (1838) and Schwann (1839).
Q4: Who expanded cell theory by suggesting that all cells arise from pre-existing cells?
Answer: Virchow (1855.
Q5: In which year electron microscope was invented?
Answer: 1940
Q6: Name the book in which Robert Hooke published his observations about cork cells.
Answer: Micrographia.
Q7: Who discovered nucleus in the cell?
Answer: Robert Brown (1831)
Q8: Name the two postulates of the cell theory.
Answer:
Answer: Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell.
Q10: Name the largest cell?
Answer: The egg of the ostrich is the largest cell (170 x 135 mm) which can be seen by an unaided eye.
Q11: Name the world's smallest cell.
Answer: It is a type of bacteria called Mycoplasma about a diameter of 10µm (micrometre).
Q12: Name the smallest cell in human body.
Answer: Human sperm cell about 5x3 µm.
Q13: Name the biggest cell in human body.
Answer: Human ovam about 1mm in diameter.
Q14: Name the longest cell in human body.
Answer: Human nerve cell about 1 meter long.
Q15: Name the cell in human body which cannot reproduce.
Answer: RBCs or Red Blood Corpuscles/cells.
Q16: Give an example of anucleate cell i.e. cell without nucleus.
Answer: RBCs or Red Blood Corpuscles/cells in mammals.
Q17: Give an example of cells containing two nuclei (Binucleate).
Answer: Paramecium
Q18: Give examples of cells which are muti-nucleate (i.e. having many nuclei).
Answer: Striated muscles, few types algae and fungi.
Q19: What is the plasma membrane composed of?Answer: Lipids and Proteins. It has viscous bilayer of lipid with protein molecules occurring on inner and outer sides of lipid bilayer. (Phospholipid bilayer)
Q20: Who proposed fluid-mosaic model of cell or plasma membrane?
Answer: Singer and Nicholsan (1972)
Q21: Is plasma membrane permeable or selectively permeable?
Answer: Selectively permeable i.e. it allows certain molecules or ions to pass through it.
Q22: What are different types transport of components across cell membrane?
Answer: Two types:
Answer: Passive transport is the net movement of materials across membranes that does not require any special source of energy.
Q24: What is diffusion?
Answer: Diffusion is the net movement of molecules from high to regions of lower concentration as a result of random molecular motion. Diffusion tends to distribute molecules uniformly.
Exchange of gases like CO2 and O2 in lungs, plants, blood cells, O2 entering stomata of leaves are examples of diffusion.
Diffusion is passive transport i.e. no external energy is provided for the movement of molecules. The natural kinetic energy of the particle supplies the energy.
Q25:Define Osmosis. What are different types of osmosis? Give examples of osmosis.
Answer: Osmosis is a special type of diffusion i.e. passage of water (solvent) across a selectively permeable membrane, from a region of high water concentration (dilute solution) to the region of low water concentration (concentrated solution).
Osmosis is also a passive transport i.e. no external energy is required for the passage.
There are two type of osmosis:
(a) Hypotonic solution
(b) Isotonic solution
(c) Hypertonic solution
Answer:
(a) Hypotonic Solution: external solution is dilute i.e. high water concentration (low solute concentration). In this case water will rush into the cell and it will gain water.
(b) Isotonic Solution: external solution concentration is same as internal solution concentration of the cell. In this case, there is no net movement of water.
(c) Hypertonic Solution: external solution is of high solute concentration i.e. low water concentration. Here water will rush out from cell and it will shrivel/shrink.
The figure below shows red blood cell and plant cell osmotic behaviour in various types of solutions.
Q 27: What is plasmolysis?
Answer: When a living plant cell loses water placed in hypertonic solution, the plant cell shrinks away from the cell wall. This process is called plasmolysis.Plants with plasmolysed cells wilt.
Q28: Place a de-shelled egg in water for five minutes. What do you observe?
Q31: What would happen if the plasma membrane ruptures or breaks down?
Answer: Plasma or Cell membrane is a selectively permeable membrane i.e. it allows movement of certain materials in and out to the cell. In case plasma membrane ruptures (like in hypertonic solution), the cell will shrivel and eventually die. At this time, lysosomes of the cell will come into action. Lysosomes will release digestive enzymes and eat up worn out cell organelles and foreign material.
Q32: What do you mean by Endocytosis? How does an Amoeba obtain its food?
Answer: The process by which a cell engulfs material to bring it into the cell is called endocytosis. It is a type of active transport. Endocytosis is of two types:
Q33: Define Exocytosis.
Answer: Exocytosis is process to move material to the outside of the cell. A vesicle fuses with the plasma membrane and discharges its contents outside. This allows cells to secrete molecules. It is a type of active transport.
Q34: Why are lysosomes known as suicide bags?
Answer: Lysosomes are organelles that contain digestive enzymes. They digest excess or worn out organelles, food particles, and engulfed viruses or bacteria. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuole, digesting its contents. Sometimes, lysosomes digest the cells that contain them. It happens when cells get damaged or destroyed. That's why lysosomes are called suicidal bags.
E.g. When a tadpole develops into a frog, lysosomes within the cells of the tadpole’s tail cause its digestion. Or Lysosomes help in defense by digesting germs, as in white blood cells.
Answer: Chromosomes are compact rod like bodies, which are formed at the time of cell division from chromatin material. They are made up of DNA and carry genes (unit of inheritance).
Answer: DNA - Deoxyribo Nucleic Acid
Q62: Who discovered Virus?
Answer: Virus was discovered by Ivanowsky, a Russian Botanist in 1892.
Q63: What are the function of nucleus?
Answer:
Volume of cube (V) = length x length length
Surface Area to Volume Ratio = SA/V
SA/V ratio of cube of 1 cm length = 6:1
SA/V ratio of cube of 2 cm length = 24/8 = 3:1
SA/V ratio of cube of 4 cm length = 96/64 = 1.5: 1
⇒ SA/V ratio decreases as the sizes of the cube increases
Q64: Why can't single cells grow very large? Or Big organisms like human beings are multi-cellular? Why can't such big organisms be a single large cell?
Answer: Cell is the fundamental unit of life. All metabolic activities (necessary for life) takes place inside the cell. All raw materials required for these metabolic activities enter through the cell surface via cell membrane. Greater the surface are, large amount of raw material can enter (this is the case of uni-cellular micro-organisms). As the size of cell grows larger, the Surface Area to Volume (SA/V) ratio decreases, it means raw material required for the cell to survive will not be sufficient. Thus, SA/V ratio limits the size of the cell. Therefore big organisms like human beings are mulch-cellular.
Q65(NTSE Stage2): Which of the following is an example of a single cell that does not function as a full fledged organism?
(a) White blood cells (WBC)
(b) Amoeba
(c) WBC and Amoeba
(d) Paramecium
Answer: (a) WBC
Q66: Why do vegetable vendors (subzi-walla) regularly sprinkle water on the vegetables in their baskets?
Answer: Water makes a hypotonic environment for the plant cells and they take up water to avoid plasmolysed condition. Therefore vegetables look crisp and fresh.
Q67: Why do we stain cells while observing under microscope? List commonly used stains.
Answer: Stains help us visualize specific organelles of cells in better way. Otherwise these organelles are not easily recognizable via microscope. Stains or dyes are basic or acidic in nature have affinity to certain organelles. For example, methylene blue is a basic dye which has affinity to nucleic acid and hence it can stain nucleus. Methylene blue is commonly used to stain animal cells. Similarly, iodine dye is used to detect starch and used to stain plant cells. Eosin is used to identify RBCs, cell membrane and cytoplasm. Safarnin is another nuclear stain, and also used to stain Rheo leaves to visualize stomata.
(Read a nice article on Cell Stain)
Q68: Are there any exceptions to cell theory proposed by Schleiden & Schwann and Virchow? If yes, what are those?
Answer: Yes there are exceptions to cell theory. These are:
Answer: 7-10 nano-meters.
Q70: Why is mitochondria absent in red blood cells?
Answer: RBCs are the oxygen and food transporters to different body cells. If they have mitcochondira, they will consume the carrying oxygen for themselves.
Q71: Name the cell organelles which their own DNA and Ribosomes.
Answer: Mitochondria and Plastids.
Q72: What is cytoskeleton?
Answer: It is a dynamic network of protein fibres which provide structural support and shape to the cell.
Q73: Name the cell organelles involved in synthesis, packaging and movement of protein (or other macromolecules) inside a cell.
Answer: Ribosomes, Golgi Bodies and Endoplasmic Reticulum (Smooth and Rough ER)
Answer: Robert Hooke. He observed thin slice of cork cells under a simple microscope.
 | ||||
| Simple Microscope made by Robert Hooke (credits:Wikipedia) |
Q2:Define Cell
Answer: Cell is the structural and functional unit of life.
Q3: Who proposed the cell theory?
Answer: Schleiden (1838) and Schwann (1839).
Q4: Who expanded cell theory by suggesting that all cells arise from pre-existing cells?
Answer: Virchow (1855.
Q5: In which year electron microscope was invented?
Answer: 1940
Q6: Name the book in which Robert Hooke published his observations about cork cells.
Answer: Micrographia.
Q7: Who discovered nucleus in the cell?
Answer: Robert Brown (1831)
Q8: Name the two postulates of the cell theory.
Answer:
- Cell is the basic structural and functional unit of life for all living beings.
- All cells arise from pre-existing cells.
Answer: Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell.
Q10: Name the largest cell?
Answer: The egg of the ostrich is the largest cell (170 x 135 mm) which can be seen by an unaided eye.
 |
| An ostrich egg (Source:wikipedia) |
Answer: It is a type of bacteria called Mycoplasma about a diameter of 10µm (micrometre).
Q12: Name the smallest cell in human body.
Answer: Human sperm cell about 5x3 µm.
Q13: Name the biggest cell in human body.
Answer: Human ovam about 1mm in diameter.
Q14: Name the longest cell in human body.
Answer: Human nerve cell about 1 meter long.
Q15: Name the cell in human body which cannot reproduce.
Answer: RBCs or Red Blood Corpuscles/cells.
Q16: Give an example of anucleate cell i.e. cell without nucleus.
Answer: RBCs or Red Blood Corpuscles/cells in mammals.
Q17: Give an example of cells containing two nuclei (Binucleate).
Answer: Paramecium
Q18: Give examples of cells which are muti-nucleate (i.e. having many nuclei).
Answer: Striated muscles, few types algae and fungi.
Q19: What is the plasma membrane composed of?Answer: Lipids and Proteins. It has viscous bilayer of lipid with protein molecules occurring on inner and outer sides of lipid bilayer. (Phospholipid bilayer)
Q20: Who proposed fluid-mosaic model of cell or plasma membrane?
Answer: Singer and Nicholsan (1972)
Note: The fluid mosaic model describes the plasma membrane as a phospholipid bilayer with embedded proteins.
Q21: Is plasma membrane permeable or selectively permeable?
Answer: Selectively permeable i.e. it allows certain molecules or ions to pass through it.
Q22: What are different types transport of components across cell membrane?
Answer: Two types:
- Passive Transport: e.g. Diffusion, Osmosis
- Active Transport: e.g. endocytosis, exocytosis
Answer: Passive transport is the net movement of materials across membranes that does not require any special source of energy.
Q24: What is diffusion?
Answer: Diffusion is the net movement of molecules from high to regions of lower concentration as a result of random molecular motion. Diffusion tends to distribute molecules uniformly.
Exchange of gases like CO2 and O2 in lungs, plants, blood cells, O2 entering stomata of leaves are examples of diffusion.
Diffusion is passive transport i.e. no external energy is provided for the movement of molecules. The natural kinetic energy of the particle supplies the energy.
Q25:Define Osmosis. What are different types of osmosis? Give examples of osmosis.
Answer: Osmosis is a special type of diffusion i.e. passage of water (solvent) across a selectively permeable membrane, from a region of high water concentration (dilute solution) to the region of low water concentration (concentrated solution).
Osmosis is also a passive transport i.e. no external energy is required for the passage.
There are two type of osmosis:
- Endosmosis: The process in which the water molecules enter into the cell is known as endosmosis.
- Exosmosis: The process in which the water molecules move out of the cell is known as exosmosis.
- Absorption of water by plant roots.
- Absorption of water by alimentary canal.
- Re-absorption of water by tubules of the nephron in kidneys.
(a) Hypotonic solution
(b) Isotonic solution
(c) Hypertonic solution
Answer:
(a) Hypotonic Solution: external solution is dilute i.e. high water concentration (low solute concentration). In this case water will rush into the cell and it will gain water.
(b) Isotonic Solution: external solution concentration is same as internal solution concentration of the cell. In this case, there is no net movement of water.
(c) Hypertonic Solution: external solution is of high solute concentration i.e. low water concentration. Here water will rush out from cell and it will shrivel/shrink.
The figure below shows red blood cell and plant cell osmotic behaviour in various types of solutions.
Answer: When a living plant cell loses water placed in hypertonic solution, the plant cell shrinks away from the cell wall. This process is called plasmolysis.Plants with plasmolysed cells wilt.
Q28: Place a de-shelled egg in water for five minutes. What do you observe?
(Note: De-shelled egg means,the shell of an egg is removed by dissolving it in dilute hydrochloric acid. The shell is mostly calcium carbonate. A thin outer skin now encloses the egg.)
Answer: The egg swells because water passes into it by osmosis. (hypotonic solution).
Q29: What will happen if a de-shelled egg in a concentrated salt solution for 5 minutes?
Answer: The egg shrinks because water passes out of the egg into saLt solution. The salt solution is more
concentrated than the inside of the egg. (Hypertonic)
Q30 (CBSE): Put dried raisins in plain water and leave them for some time. Then place them in concentrated solution of sugar or salt. What do you observe in both cases?
Answer: Case I: Raisins gain water and swells when placed in pure water.
Case II: When placed in the concentrated solution, raisins lose water and hence shrink.
Q31: What would happen if the plasma membrane ruptures or breaks down?
Answer: Plasma or Cell membrane is a selectively permeable membrane i.e. it allows movement of certain materials in and out to the cell. In case plasma membrane ruptures (like in hypertonic solution), the cell will shrivel and eventually die. At this time, lysosomes of the cell will come into action. Lysosomes will release digestive enzymes and eat up worn out cell organelles and foreign material.
Q32: What do you mean by Endocytosis? How does an Amoeba obtain its food?
Answer: The process by which a cell engulfs material to bring it into the cell is called endocytosis. It is a type of active transport. Endocytosis is of two types:
- Phagocytosis: It refers to the process of engulfing large particles (or cell eating).
- Pinocytosis: it refers to engulfing macromolecules. (or cell drinking)
Q33: Define Exocytosis.
Answer: Exocytosis is process to move material to the outside of the cell. A vesicle fuses with the plasma membrane and discharges its contents outside. This allows cells to secrete molecules. It is a type of active transport.
Q34: Why are lysosomes known as suicide bags?
Answer: Lysosomes are organelles that contain digestive enzymes. They digest excess or worn out organelles, food particles, and engulfed viruses or bacteria. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuole, digesting its contents. Sometimes, lysosomes digest the cells that contain them. It happens when cells get damaged or destroyed. That's why lysosomes are called suicidal bags.
E.g. When a tadpole develops into a frog, lysosomes within the cells of the tadpole’s tail cause its digestion. Or Lysosomes help in defense by digesting germs, as in white blood cells.
Concept Map on Cell Structure
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| Cell Structure (concept map) |
Q35: Viruses are
(a) Uni cellular micro-organisms
(b) Bi-Cellular micro-organisms
(c) Multi-cellular micro-organisms
(d) Non-cellular micro-organisms
Answer: (d). Non-Cellular. Viruses lie on the line of division between living and non-living. They are non-living carrying RNA material. They act as living when find a suitable host and multiply.
Q36: Who is known as Father of Biology?
Answer: Aristotle.
Q37: Who discovered Golgi apparatus?
Answer: Camillo Golgi
Q38: Which cell organelle is known as "protein factory"?
Answer: Ribosomes
Q39: What is the energy currency of the cell called?
Answer: ATP (Adenosine triphosphate)
Q40: When chromosomes are visible in the nucleus?
Answer: During cell division.
Q41: Which of the following is NOT involved in the synthesis of proteins?
(a) rough ER
(b) smooth ER
(c) Golgi body
(d) ribosomes
Answer: (b) smooth ER.
Q37: Who discovered Golgi apparatus?
Answer: Camillo Golgi
Q38: Which cell organelle is known as "protein factory"?
Answer: Ribosomes
Q39: What is the energy currency of the cell called?
Answer: ATP (Adenosine triphosphate)
Q40: When chromosomes are visible in the nucleus?
Answer: During cell division.
Q41: Which of the following is NOT involved in the synthesis of proteins?
(a) rough ER
(b) smooth ER
(c) Golgi body
(d) ribosomes
Answer: (b) smooth ER.
Rough ER is the site for protein synthesis and has ribosomes embedded. Ribosomes on the rough ER produce
proteins that are passed from the cell; the Golgi body then packages them for this purpose. Smooth ER is used mainly in the making of lipids and detoxifying enzymes.
Q42: Which of the following often distinguishes plant cells from animal cells?
(a) centrioles
(b) nucleus
(c) chromatin
(d) rough ER
Answer: (a) Centrioles
Q43: Are plastids present in all cells? What are its types?
Answers: Plastids are present only in plant cells. Based on colour pigments, there are three types of plastids:
Answer: Sac like structures in chloroplasts are called thylakoids.
Q45: What are the conditions for osmosis?
Answer: Conditions for osmosis are:
Q46: Will the temperature have any effect on the process of the osmosis?
Answer: Yes the higher temperature will increase the rate of water entering the cell as it increases the kinetic energy of the solution.
Q47: What is osmoregulation?
Answer: The regulation of the water content of the cell is called osmoregulation.
Q48: Which organ of the plant body helps in osmoregulation?
Answer: Leaves
Q49: Which organelle of the cell in animals helps in osmregulation?
Answer: Contractile Vacuole.
Q50 What are centrosomes? What functions do they perform?
Answer: Centrosome is a small, star shaped protoplamic organelle and is present near to nucleus. In animal cells, it consists of two small granules called centrioles. These centrioles are orthogonally arranged i.e. lie at right angles to each other. In plant cells, centrioles are absent.
Q51: Who is known as 'Father of Microscopy'?
Answer: Antony van Leeuwenhoek
Q52: Are Viruses Prokaryotic or Eukaryotic?
Answer: Neither of the two. Viruses are smaller than prokaryotic cells. A virus is considered as a simple nucleic acid surround by a protein coat. They act as non-living and become living on finding a suitable host.
Q53: Which cell organelle is called "kitchen of plant"?
Answer: Plastids
Q54: Which cell organelle is called 'control center'?
Answer: Nucleus.
Q55: Which cell organelle is called 'transport system'?
Answer: Endoplasmic Reticulum
Q55: What is Endoplasmic Reticulum(ER)? Name its types?
Answer: The cell organelle, Endoplasmic reticulum (ER) is an extensive, interconnected, membrane-bound network of tubes and sheets. It consists of various channels like cisternae, vesicles and tubules. ER carries materials throughout the cell. There are two type of ERs:
Q56: What are the functions of Endoplasmic Reticulum (ER)?
Answer:
Q57: What are the components nucleus?
Answer:
Q58: What is the function of nucleoplasm?
Answer: Nucleoplasm is a semi-fluid which contains a number of enzymes which help in synthesis and functioning of DNA and RNA (genetic material).
Q59: How chromatic network is related to chromosomes?
Answer: Chromatin network is a tangled fibrous mass inside nucleus. The chromatin threads condense and turn into chromosomes during cell division.
proteins that are passed from the cell; the Golgi body then packages them for this purpose. Smooth ER is used mainly in the making of lipids and detoxifying enzymes.
Q42: Which of the following often distinguishes plant cells from animal cells?
(a) centrioles
(b) nucleus
(c) chromatin
(d) rough ER
Answer: (a) Centrioles
Q43: Are plastids present in all cells? What are its types?
Answers: Plastids are present only in plant cells. Based on colour pigments, there are three types of plastids:
- Leucoplasts: White or colourless.
- Chromoplast: Coloured like blue, red, yellow.
- Chloroplast: Green in colour.
Answer: Sac like structures in chloroplasts are called thylakoids.
Q45: What are the conditions for osmosis?
Answer: Conditions for osmosis are:
- There should be two solutions of different concentrations.
- The membrane separating these two solutions should be semipermeable.
Q46: Will the temperature have any effect on the process of the osmosis?
Answer: Yes the higher temperature will increase the rate of water entering the cell as it increases the kinetic energy of the solution.
Q47: What is osmoregulation?
Answer: The regulation of the water content of the cell is called osmoregulation.
Q48: Which organ of the plant body helps in osmoregulation?
Answer: Leaves
Q49: Which organelle of the cell in animals helps in osmregulation?
Answer: Contractile Vacuole.
Q50 What are centrosomes? What functions do they perform?
 |
| Cetrosomes during cell division |
Answer: Centrosome is a small, star shaped protoplamic organelle and is present near to nucleus. In animal cells, it consists of two small granules called centrioles. These centrioles are orthogonally arranged i.e. lie at right angles to each other. In plant cells, centrioles are absent.
Functions of Centrosomes:
- Initiate cell division
- During cell division, centrioles move to opposite poles of the cell and help in spindle formation.
- They form basal bodies (microtubules like sturtures) which form cillia or flagella.
Click on the image below to see video showing cell division in pig kidney cells. Hamamatsu.com, a Japanese company, sells microscopy related cameras has a nice collection of cell images.
 | |
| Cell Division (source: http://learn.hamamatsu.com) |
Q51: Who is known as 'Father of Microscopy'?
Answer: Antony van Leeuwenhoek
Q52: Are Viruses Prokaryotic or Eukaryotic?
Answer: Neither of the two. Viruses are smaller than prokaryotic cells. A virus is considered as a simple nucleic acid surround by a protein coat. They act as non-living and become living on finding a suitable host.
Q53: Which cell organelle is called "kitchen of plant"?
Answer: Plastids
Q54: Which cell organelle is called 'control center'?
Answer: Nucleus.
Q55: Which cell organelle is called 'transport system'?
Answer: Endoplasmic Reticulum
Q55: What is Endoplasmic Reticulum(ER)? Name its types?
Answer: The cell organelle, Endoplasmic reticulum (ER) is an extensive, interconnected, membrane-bound network of tubes and sheets. It consists of various channels like cisternae, vesicles and tubules. ER carries materials throughout the cell. There are two type of ERs:
- Rough Endoplasmic Reticulum (RER) - bears ribosomes, involved in protein synthesis.
- Smooth Endoplasmic Reticulum (SER) - Does not bear ribosomes, involved in lipid synthesis.
Q56: What are the functions of Endoplasmic Reticulum (ER)?
Answer:
- Helps in transporting materials from one part to other inside cell.
- Acts as skeletal framework and provides mechanical strength to cytoplasmic matrix.
- Provides larger surface area for synthesis of various metabolic activities.
- It contains various enzymes which act as catalyst in synthesis of lipids and proteins.
- SER is also involved in the process of detoxification of drugs and poison.
Q57: What are the components nucleus?
Answer:
- nucleolus
- nuclear membrane
- chromatin network (forms in chromosomes and carry genes).
- nucleoplasm
Q58: What is the function of nucleoplasm?
Answer: Nucleoplasm is a semi-fluid which contains a number of enzymes which help in synthesis and functioning of DNA and RNA (genetic material).
Q59: How chromatic network is related to chromosomes?
Answer: Chromatin network is a tangled fibrous mass inside nucleus. The chromatin threads condense and turn into chromosomes during cell division.
Q60: What are chromosomes?
Answer: Chromosomes are compact rod like bodies, which are formed at the time of cell division from chromatin material. They are made up of DNA and carry genes (unit of inheritance).
Q61: What is the full form of DNA and RNA?
Answer: DNA - Deoxyribo Nucleic Acid
RNA - Ribo nucleic acid
Q62: Who discovered Virus?
Answer: Virus was discovered by Ivanowsky, a Russian Botanist in 1892.
Q63: What are the function of nucleus?
Answer:
- Nucleus controls all the cellular activities of the cell.
- Nucleolus is involved in synthesis of ribosomes.
- Nucleus contains hereditary information of cell.
- It is responsible for cell division (reproduction) and transmission of genetic information from parents to offspring.
Surface Area to Volume Ratio
Surface Area of a cube (SA) = (length x Width) x 6 FacesVolume of cube (V) = length x length length
Surface Area to Volume Ratio = SA/V
SA/V ratio of cube of 1 cm length = 6:1
SA/V ratio of cube of 2 cm length = 24/8 = 3:1
SA/V ratio of cube of 4 cm length = 96/64 = 1.5: 1
⇒ SA/V ratio decreases as the sizes of the cube increases
Q64: Why can't single cells grow very large? Or Big organisms like human beings are multi-cellular? Why can't such big organisms be a single large cell?
Answer: Cell is the fundamental unit of life. All metabolic activities (necessary for life) takes place inside the cell. All raw materials required for these metabolic activities enter through the cell surface via cell membrane. Greater the surface are, large amount of raw material can enter (this is the case of uni-cellular micro-organisms). As the size of cell grows larger, the Surface Area to Volume (SA/V) ratio decreases, it means raw material required for the cell to survive will not be sufficient. Thus, SA/V ratio limits the size of the cell. Therefore big organisms like human beings are mulch-cellular.
Q65(NTSE Stage2): Which of the following is an example of a single cell that does not function as a full fledged organism?
(a) White blood cells (WBC)
(b) Amoeba
(c) WBC and Amoeba
(d) Paramecium
Answer: (a) WBC
Q66: Why do vegetable vendors (subzi-walla) regularly sprinkle water on the vegetables in their baskets?
Answer: Water makes a hypotonic environment for the plant cells and they take up water to avoid plasmolysed condition. Therefore vegetables look crisp and fresh.
Q67: Why do we stain cells while observing under microscope? List commonly used stains.
Answer: Stains help us visualize specific organelles of cells in better way. Otherwise these organelles are not easily recognizable via microscope. Stains or dyes are basic or acidic in nature have affinity to certain organelles. For example, methylene blue is a basic dye which has affinity to nucleic acid and hence it can stain nucleus. Methylene blue is commonly used to stain animal cells. Similarly, iodine dye is used to detect starch and used to stain plant cells. Eosin is used to identify RBCs, cell membrane and cytoplasm. Safarnin is another nuclear stain, and also used to stain Rheo leaves to visualize stomata.
(Read a nice article on Cell Stain)
Q68: Are there any exceptions to cell theory proposed by Schleiden & Schwann and Virchow? If yes, what are those?
Answer: Yes there are exceptions to cell theory. These are:
- Cell theory do not explain nature of Virus. They become live and multiply though they are made up of non-living material.
- How come the first living cell originated or how life began on the earth. (Scientists J.B.S. Haldane, Stanley L. Miller and Harold C. Urey did experiments to on this.).
- Mitochondria and chloroplasts have their own genetic material and can reproduce independently.
Answer: 7-10 nano-meters.
Q70: Why is mitochondria absent in red blood cells?
Answer: RBCs are the oxygen and food transporters to different body cells. If they have mitcochondira, they will consume the carrying oxygen for themselves.
Q71: Name the cell organelles which their own DNA and Ribosomes.
Answer: Mitochondria and Plastids.
Q72: What is cytoskeleton?
Answer: It is a dynamic network of protein fibres which provide structural support and shape to the cell.
Q73: Name the cell organelles involved in synthesis, packaging and movement of protein (or other macromolecules) inside a cell.
Answer: Ribosomes, Golgi Bodies and Endoplasmic Reticulum (Smooth and Rough ER)
Watch youtube video on The Wacky History of Cell Theory by TED.com
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