Cell: The Structural and Functional Unit of Life — ICSE Class 10 Biology Important Questions

Answer: (b) Mitochondrion.

Mitochondria carry out aerobic respiration and release energy stored as ATP, so they are called the powerhouse of the cell.

Answer: (a) Lysosome.

Lysosomes contain digestive (hydrolytic) enzymes; on rupture they can digest the cell's own contents, hence the name 'suicide bags'.

Answer: (c) Cell wall.

The rigid cellulose cell wall (and plastids) occur only in plant cells; mitochondria, ribosomes and the nucleus are common to both.

Answer: (b) Green leaf of a plant.

Presence of plastids and a large central vacuole with no centrioles is characteristic of a eukaryotic plant cell such as a leaf mesophyll cell.

Answer: (c) A is true, R is false. Mature mammalian RBCs are enucleate but are still cells because they are bounded by a membrane and were formed from nucleated cells; however the nucleus is the control centre of the whole cell, not merely for division, so R is incorrect.

(i) Protoplasm: The living, jelly-like substance of a cell comprising the cytoplasm and the nucleus, in which all life activities take place.

(ii) Selectively permeable membrane: A membrane that allows only certain selected substances to pass through it while restricting others, thus controlling exchange of materials between the cell and its surroundings.

(i) Ribosomes: They are the sites of protein synthesis, joining amino acids into polypeptide chains.

(ii) Golgi apparatus: It packages, modifies and secretes materials such as enzymes and hormones, forms lysosomes, and helps synthesise the cell wall and secretory vesicles.

The animal cell is bounded by a plasma (cell) membrane, has a central nucleus, and its cytoplasm contains organelles such as mitochondria, ribosomes and the Golgi body.

(Any three differences are acceptable.)

Structure: The nucleus is a spherical, membrane-bound organelle usually located near the centre of the cell. It is enclosed by a double nuclear membrane bearing nuclear pores, and encloses the nucleoplasm containing one or more nucleoli and a network of chromatin (DNA + protein) that condenses into chromosomes during division.

Functions: (i) It controls all the metabolic activities of the cell and is the control centre of the cell. (ii) It carries the hereditary information in genes on the chromosomes and passes it to daughter cells during cell division. (iii) The nucleolus synthesises ribosomes.

A plant cell is a rectangular eukaryotic cell bounded by a rigid cell wall, within which lie the cell membrane, cytoplasm, a large central vacuole, a nucleus and green chloroplasts.

Functions: (i) Cell wall — a rigid cellulose layer that gives shape, rigidity and protection to the cell. (ii) Cell membrane — a selectively permeable layer controlling entry and exit of substances. (iii) Nucleus — controls cell activities and carries hereditary information. (iv) Chloroplast — contains chlorophyll and carries out photosynthesis. (v) Vacuole — a sap-filled cavity that maintains turgidity and stores food, water and wastes.

(i) Mitochondrion: Its inner membrane is folded into cristae, greatly increasing surface area for the enzymes of aerobic respiration; this maximises ATP production, suiting its role as the powerhouse of the cell.

(ii) Chloroplast: It contains stacked membranes (grana) packed with chlorophyll to trap sunlight, and a stroma with enzymes, making it ideal for capturing light energy and synthesising food during photosynthesis.

(iii) Endoplasmic reticulum: Its extensive network of tubules and channels provides a large internal surface and a transport system; rough ER (with ribosomes) helps in protein synthesis and smooth ER in lipid synthesis and transport.

(iv) Lysosome: A single membrane encloses powerful hydrolytic enzymes, keeping them safely separated so the sac can digest worn-out organelles, food particles and, on rupture, the cell itself.

(v) Plasma membrane: Its fluid lipid bilayer with embedded proteins is selectively permeable, allowing controlled exchange of materials and giving flexibility for processes such as endocytosis.

(i) Cell X is a plant cell and cell Y is an animal cell.

(ii) The green bodies are chloroplasts; they contain chlorophyll and carry out photosynthesis, synthesising food (glucose) using sunlight, water and carbon dioxide.

(iii) The rigid boundary is the cell wall; it is chemically made of cellulose and gives shape, rigidity and protection to the cell.

(iv) Centrioles form the spindle fibres during cell division, which help in the movement and separation of chromosomes.