The Fundamental Unit of Life — NCERT Solutions

Given: The cell membrane is made up of lipids and proteins.

Answer:

• Lipids (fats) that form the cell membrane are synthesised in the Smooth Endoplasmic Reticulum (SER). SER lacks ribosomes and is specialised for lipid synthesis.

• Proteins that are part of the cell membrane are synthesised by ribosomes attached to the Rough Endoplasmic Reticulum (RER). The RER has ribosomes on its surface which manufacture proteins.

• After synthesis, both lipids and proteins are transported to the Golgi apparatus, where they are processed and packaged before being sent to the cell membrane.

Conclusion: Lipids are synthesised in the SER and proteins are synthesised on the RER; both are then assembled to form the cell membrane.

Given: Amoeba is a unicellular organism.

Process — Endocytosis (Phagocytosis):

Amoeba obtains its food by a process called endocytosis, specifically phagocytosis (cell eating). The steps are:

1. When Amoeba senses food (like a bacterium or algal cell), it extends finger-like projections of its cell membrane called pseudopodia (false feet).
2. The pseudopodia surround the food particle from all sides.
3. The tips of the pseudopodia fuse together, enclosing the food particle within a membrane-bound sac called a food vacuole.
4. Digestive enzymes are secreted into the food vacuole, breaking down the food into simpler molecules.
5. The digested nutrients are absorbed into the cytoplasm.
6. Undigested waste is expelled outside the cell.

Conclusion: Amoeba obtains food through endocytosis (phagocytosis) using pseudopodia to engulf food particles, which is possible due to the flexibility of its cell membrane.

Definition:

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration (lower solute concentration / hypotonic solution) to a region of lower water concentration (higher solute concentration / hypertonic solution).

Key points:

• Osmosis is a special type of diffusion involving only water molecules.
• It occurs across a semi-permeable (selectively permeable) membrane.
• It continues until equilibrium is reached (both sides have equal concentration).

Types:

• Endosmosis: Movement of water into the cell when placed in a hypotonic solution — cell swells.
• Exosmosis: Movement of water out of the cell when placed in a hypertonic solution — cell shrinks (plasmolysis in plant cells).

Example: When a raisin is placed in water, it swells up due to osmosis (water moves into the raisin).

Conclusion: Osmosis is the passive movement of water across a selectively permeable membrane from a dilute solution to a concentrated solution.

Setup:

• Cup A: Empty (raw potato) — control
• Cup B: Sugar in raw potato cup
• Cup C: Salt in raw potato cup
• Cup D: Sugar in boiled potato cup
• All cups placed in a trough of water for 2 hours.

(i) Explain why water gathers in the hollowed portion of B and C.

Answer:
The sugar in cup B and salt in cup C dissolve in the small amount of water present and create a concentrated (hypertonic) solution inside the hollow portion. The potato cells surrounding the hollow act as a selectively permeable membrane. The water in the trough (and inside the potato cells) is at a higher concentration than the solution inside the hollow.

Therefore, water moves by osmosis from the potato cells (and from the trough through the potato) into the hollow portion — from the region of higher water concentration to lower water concentration.

This is why water accumulates in the hollowed portions of B and C.

(ii) Why is potato A necessary for this experiment?

Answer:
Potato cup A (empty, raw potato) serves as the control for the experiment.

• It shows that in the absence of any solute (sugar or salt), no water accumulates in the hollow.
• It helps us confirm that the water gathering in B and C is specifically due to the osmotic effect of the dissolved sugar/salt, and not due to any other factor such as the shape of the cup or the water in the trough.
• Without a control, we cannot draw a valid scientific conclusion.

(iii) Explain why water does not gather in the hollowed out portions of A and D.

For Cup A (empty raw potato):
There is no solute (sugar or salt) in the hollow. The concentration of water inside the hollow is the same as (or not significantly different from) the concentration in the potato cells and the trough water. Therefore, there is no osmotic gradient and no net movement of water into the hollow. No water accumulates.

For Cup D (sugar in boiled potato):
Boiling the potato kills the cells and destroys the selectively permeable membrane of the potato cells. Since osmosis requires a functional selectively permeable membrane, the dead cells of the boiled potato cannot act as such a membrane. Even though sugar is present and creates a concentration gradient, water cannot move by osmosis through the dead, ruptured membranes. Therefore, no water gathers in cup D.

Conclusion: This experiment demonstrates that osmosis requires both a concentration gradient AND an intact selectively permeable membrane.

Answer:

1. For growth and repair of the body:

Mitosis (mitotic cell division) is required.

• In mitosis, a parent cell divides to produce two daughter cells, each with the same number of chromosomes as the parent cell (the chromosome number is maintained).
• This ensures that new cells formed during growth or repair are identical to the original cells.
• Example: Healing of a wound, replacement of old blood cells, growth of tissues.

2. For formation of gametes (reproductive cells):

Meiosis (meiotic cell division) is involved.

• In meiosis, a parent cell divides to produce four daughter cells, each with half the number of chromosomes of the parent cell (haploid cells).
• This reduction in chromosome number is essential so that when two gametes (sperm and egg) fuse during fertilisation, the resulting zygote has the correct (diploid) number of chromosomes.
• Example: Formation of sperm cells in testes and egg cells in ovaries.

Conclusion:
$$\text{Growth and Repair} \rightarrow \textbf{Mitosis}$$
$$\text{Formation of Gametes} \rightarrow \textbf{Meiosis}$$