Chapter 5: The Fundamental Unit of Life - NCERT Solutions
In-Text Questions (Page No- 51)
Q1. Who discovered cells, and how?
Answer - Cells were discovered by Robert Hooke in 1665. He observed thin slices of cork under a microscope and saw small box-like structures, which he named 'cells'.
Q2. Why is the cell called the structural and functional unit of life?
Answer - The cell is called the structural and functional unit of life because all living organisms are made up of cells. They perform all the basic functions necessary for life, such as nutrition, respiration, excretion, and reproduction.
In-Text Questions (Page No- 53)
Q1. How do substances like CO2 and water move in and out of the cell? Discuss.
Answer - Substances like carbon dioxide (CO2) and water move across the cell membrane primarily by diffusion and osmosis, respectively. CO2, being a small nonpolar molecule, diffuses directly through the lipid bilayer from an area of higher concentration to lower concentration. Water moves in and out of the cell by osmosis, which is the diffusion of water molecules through a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.
Q2. Why is the plasma membrane called a selectively permeable membrane?
Answer - The plasma membrane is called selectively permeable because it allows certain substances to pass through while restricting others. This selective permeability ensures that essential nutrients enter the cell, waste products exit, and harmful substances are kept out, maintaining the cell's internal environment.
In-Text Questions (Page No- 55)
Q1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Size | Usually larger (10-100 µm) | |
| Nucleus | Absent | |
| Membrane-bound organelles | Present | |
| Cell division | Binary fission | |
| Genetic material | Multiple linear chromosomes | |
| Examples | Bacteria and Archaea |
Answer -
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Size | Usually small (1-10 µm) | Usually larger (10-100 µm) |
| Nucleus | Absent, nucleoid region present | Present, well-defined nucleus |
| Membrane-bound organelles | Absent | Present (e.g., mitochondria, ER) |
| Cell division | Binary fission | Mitosis and meiosis |
| Genetic material | Single circular DNA molecule | Multiple linear chromosomes |
| Examples | Bacteria and Archaea | Plants, animals, fungi, protists |
In-Text Questions (Page No- 57)
Q1. Can you name the two organelles we have studied that contain their own genetic material?
Answer - The two organelles that contain their own genetic material are mitochondria and chloroplasts.
Q2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer - If the organisation of a cell is destroyed by physical or chemical influences, the cell will lose its ability to function properly and eventually die.
Q3. Why are lysosomes known as suicide bags?
Answer - Lysosomes are called suicide bags because they contain digestive enzymes that can break down cellular components, and if they rupture, they can digest the cell itself.
Q4. Where are proteins synthesised inside the cell?
Answer - Proteins are synthesised on ribosomes, which may be free in the cytoplasm or attached to the endoplasmic reticulum.
Exercises
Q1. Make a comparison and write down ways in which plant cells are different from animal cells.
Answer -
| Plant Cell | Animal Cell | |
|---|---|---|
| Cell Wall | Present (made of cellulose) | Absent |
| Chloroplasts | Present (for photosynthesis) | Absent |
| Vacuole | Large central vacuole present | Small or absent vacuoles |
| Centrioles | Absent (except in some lower plants) | Present |
| Lysosomes | Rarely present | Commonly present |
| Shape | Usually fixed and rectangular | Usually irregular and round |
Q2. How is a prokaryotic cell different from a eukaryotic cell?
Answer -
| Prokaryotic Cell | Eukaryotic Cell | |
|---|---|---|
| Nucleus | Absent (no true nucleus) | Present (true nucleus with nuclear membrane) |
| Size | Generally smaller (1-10 µm) | Generally larger (10-100 µm) |
| Membrane-bound Organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Genetic Material | Single circular DNA molecule | Multiple linear chromosomes |
| Cell Division | Binary fission | Mitosis and meiosis |
| Examples | Bacteria and Archaea | Plants, animals, fungi, protists |
Q3. What would happen if the plasma membrane ruptures or breaks down?
Answer - If the plasma membrane ruptures, the cell loses its selective permeability, leading to uncontrolled entry or exit of substances and eventually cell death.
Q4. What would happen to the life of a cell if there was no Golgi apparatus?
Answer - Without the Golgi apparatus, proteins and lipids cannot be modified, sorted, or transported properly, affecting cell function and secretion.
Q5. Which organelle is known as the powerhouse of the cell? Why?
Answer - The mitochondrion is called the powerhouse of the cell because it produces energy in the form of ATP through cellular respiration.
Q6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer - Lipids are synthesised in the smooth endoplasmic reticulum, and proteins are synthesised on ribosomes, often attached to the rough endoplasmic reticulum.
Q7. How does an Amoeba obtain its food?
Answer - An Amoeba obtains food by engulfing it through a process called phagocytosis, where it surrounds and encloses food particles using pseudopodia.
Q8. What is osmosis?
Answer - Osmosis is the diffusion of water molecules from a region of lower solute concentration to a region of higher solute concentration through a selectively permeable membrane.
Q9. Carry out the following osmosis experiment:
• Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed out portions of A and D.
Answer - (i) Water moves into cups B and C by osmosis because sugar and salt solutions inside create higher solute concentration compared to the surrounding water.
(ii) Potato A acts as a control to show water movement without solutes.
(iii) Water does not gather in A because it is empty, and in D because boiling destroys the cell membranes, preventing osmosis.
Q10. 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 responsible for growth and repair, while meiosis produces gametes for sexual reproduction.