Absorption by Roots: The Processes Involved — ICSE Class 10 Biology Important Questions

Answer: (b) Osmosis.

Osmosis specifically involves water moving through a selectively permeable membrane down its water-potential gradient.

Answer: (c) Root hair zone.

Root hairs greatly increase the surface area of the root and are the main site of water and mineral absorption.

Answer: (c) Active transport.

Moving ions from a lower concentration in the soil to a higher concentration in the root needs energy (ATP), i.e. active transport.

Answer: (c) Undergo plasmolysis.

The external solution is hypertonic, so water leaves the cell by exosmosis; the protoplasm shrinks away from the cell wall, i.e. plasmolysis occurs.

Answer: (a) Both statements are true and R explains A: the thin wall, large surface area and concentrated cell sap of the root hair allow water to enter rapidly by osmosis.

(i) Osmosis: The movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively permeable membrane.

(ii) Plasmolysis: The shrinkage of the protoplasm away from the cell wall when a cell is placed in a hypertonic (strong) solution, due to loss of water by exosmosis.

Endosmosis: The movement of water into a cell when it is placed in a hypotonic (dilute) solution; the cell swells and becomes turgid.

Exosmosis: The movement of water out of a cell when it is placed in a hypertonic (concentrated) solution; the cell shrinks and may become flaccid or plasmolysed.

The root hair is a tubular outgrowth of an epidermal cell; its thin cellulose wall and cell membrane are in contact with the soil water, and its vacuole contains concentrated cell sap that draws water in by osmosis.

Water from the soil (higher water potential) enters the root hair by osmosis because the cell sap is more concentrated (lower water potential).

(Any three differences are acceptable.)

Water present in the soil is absorbed by the root hairs by osmosis, since the cell sap of the root hair is more concentrated than the soil water.

From the root hair, water passes from cell to cell across the cortex by osmosis, moving down a gradient of increasing concentration (each inner cell has more concentrated sap than the one outside it).

The water then crosses the endodermis and reaches the xylem vessels of the root. From the xylem it is carried upward to the stem and leaves.

Path: soil water $\rightarrow$→ root hair $\rightarrow$→ cortex cells $\rightarrow$→ endodermis $\rightarrow$→ xylem.

Aim: To demonstrate osmosis.

Materials: A large potato, sugar (or strong sugar solution), water, a trough, pins.

Method:
(i) Peel and cut the base of a potato flat so it can stand. Scoop out the centre to form a hollow cup.
(ii) Place some concentrated sugar solution in the cavity and mark its level with a pin.
(iii) Stand the potato cup in a trough of water so that water reaches about half the height of the potato.
(iv) Leave the set-up for a few hours.

Control: A similar potato cup boiled first (to kill the cells) and filled with sugar solution shows no rise.

Observation: In the living potato, the level of the sugar solution in the cavity rises, while no rise occurs in the boiled potato.

Conclusion: Water passes from the trough (higher water potential) through the living cells of the potato (which act as a selectively permeable membrane) into the concentrated sugar solution (lower water potential). This inward movement of water is osmosis. The boiled potato shows no rise because its cell membranes are killed and are no longer selectively permeable.

(a) Root pressure: It is the positive hydrostatic pressure developed in the xylem of the roots as a result of the active absorption of water by the root. As water is continuously drawn into the root cells and forced into the xylem, a pressure builds up that helps push water a short distance up the stem. It can be seen as sap exuding from the cut stump of a plant (bleeding).

(b) Effect of solutions of different concentration:

• Turgid: When a cell is placed in a hypotonic (dilute) solution or pure water, water enters by endosmosis; the vacuole swells and pushes the protoplasm against the cell wall, making the cell firm or turgid.
• Flaccid: When a cell is in a solution of the same concentration (isotonic) or loses some water, it becomes limp or flaccid as turgor pressure falls.
• Plasmolysed: When a cell is placed in a hypertonic (concentrated) solution, water leaves by exosmosis, the protoplasm shrinks and pulls away from the cell wall; the cell is plasmolysed.

(c) Wilted vegetable becoming firm: A wilted vegetable has lost water and its cells are flaccid. When kept in water, water enters the cells by endosmosis, the vacuoles fill up and the cells become turgid again, so the vegetable regains its firmness and freshness.

(i) The strip in solution P (distilled water) gains weight, because water enters the potato cells by endosmosis (the cell sap is more concentrated than distilled water), making the cells turgid.

(ii) The strip in solution Q (5% salt solution) loses weight, because water leaves the cells by exosmosis (the salt solution is hypertonic). The process responsible is osmosis (exosmosis).

(iii) In solution R there is no change (or negligible change) in weight, since it is of the same concentration as the cell sap; such a solution is called an isotonic solution.

(iv) The selective (semi-) permeability of the cell membrane makes these osmotic changes possible.