"तब याद तुम्हारी आती है" कविता के कवि कौन हैं?

रामनरेश त्रिपाठी

कविता में कवि को किसकी याद आती है?

सिरजनहार (ईश्वर) की

"सिरजनहार" शब्द का क्या अर्थ है?

सृष्टि की रचना करने वाला ईश्वर

सवेरे किन दृश्यों से कवि को प्रभु की याद आती है?

चिड़ियों के गीत, खिलती कलियाँ और ठंडी हवा से

रात के समय कौन-से दृश्य कवि को प्रभावित करते हैं?

चमकते तारे और शान से उठता चाँद

What is the limit of resolution of the human eye, and why does it matter for studying cells?

It is about 0.1 mm at the near point (25 cm) — two points closer than this look like one. Most cells are smaller than this, so microscopes are needed to see them.

Who first observed cells, and how?

Robert Hooke in 1665, using a self-made microscope, examined a thin slice of cork and saw tiny box-like compartments which he named "cells".

Differentiate between diffusion and osmosis.

Diffusion is the net movement of particles from higher to lower concentration and can occur without a membrane; osmosis is specifically the diffusion of water across a selectively permeable membrane.

What does the fluid-mosaic model say about the cell membrane?

The membrane is a lipid bilayer (water-attracting heads outward, water-repelling tails inward) with proteins embedded like tiles in a mosaic; the molecules can move, so the membrane is fluid, and proteins act as gatekeepers for what enters or leaves.

Why do plant cells have a cell wall while animal cells do not?

Plants are usually fixed in one place and need a rigid cellulose wall to withstand wind, rain and pressure changes and to stay upright; animal cells move and change shape, so a rigid wall would restrict them.

Distinguish prokaryotic from eukaryotic cells.

Prokaryotic cells (e.g. bacteria) lack a membrane-bound nucleus and organelles, keeping their DNA in a nucleoid; eukaryotic cells (plants, animals) have a true nucleus and membrane-bound organelles and are generally larger.

Why are mitochondria called the powerhouse of the cell?

They break down glucose and other molecules during cellular respiration and store the released energy as ATP, the energy currency used for most cellular activities.

Name the three types of plastids and their roles.

Chloroplasts (green, contain chlorophyll, carry out photosynthesis), chromoplasts (coloured pigments giving flowers and fruits their colours) and leucoplasts (colourless, store starch, oils or proteins).

Compare mitosis and meiosis.

Mitosis produces two genetically identical daughter cells with the full chromosome number, used for growth and repair; meiosis is a two-step division producing four cells with half the chromosomes, used to make gametes for reproduction.

State the cell theory and name the scientists who developed it.

All living organisms are made of one or more cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells; it was built up by Schleiden (plants), Schwann (animals) and Virchow (new cells from existing cells).

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Chapter 2: Cell: The Building Block of LifeClass 9 Science — summary, notes, extra questions & NCERT solutions

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Who first observed and named "cells" while looking at a slice of cork in 1665?

Summary

This chapter establishes the cell as the basic structural and functional unit of life. Some organisms are unicellular (a single cell, like bacteria or yeast) while others are multicellular; in the latter, similar cells form tissues, tissues build organs, and organs form organ systems, yet the cell stays the fundamental unit throughout. Because cells are smaller than the eye’s limit of resolution (about 0.1 mm at the near point of 25 cm), we study them with microscopes — Robert Hooke first saw cork "cells" in 1665, and modern light and electron microscopes reveal ever finer detail. Every cell is bounded by a selectively permeable cell (plasma) membrane, described by the fluid-mosaic model as a lipid bilayer with embedded proteins; substances cross it by diffusion and water by osmosis. Plant, fungal and bacterial cells add a rigid cellulose cell wall. Cells are classed as prokaryotic (no membrane-bound nucleus or organelles; genetic material in a nucleoid) or eukaryotic (true nucleus plus organelles). Key organelles include the nucleus (holding DNA-bearing chromosomes), ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria (the ATP-producing "powerhouse"), plastids in plants (chloroplast, chromoplast, leucoplast) and vacuoles. Cells multiply by division — mitosis (two identical daughter cells for growth and repair) and meiosis (four cells with half the chromosomes, for reproduction). The cell theory of Schleiden, Schwann and Virchow unifies all of this: all organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Studying cells: resolution and microscopesCell membrane, diffusion and osmosisCell wall; prokaryotic vs eukaryotic cellsCell organelles and their functionsCell division and cell theory

Key terms

Cell membrane (plasma membrane)

The selectively permeable boundary of every cell, described by the fluid-mosaic model as a lipid bilayer with embedded proteins.

Osmosis

The movement of water across a selectively permeable membrane from a dilute (more water) to a concentrated (less water) region.

Prokaryotic cell

A cell without a membrane-bound nucleus or organelles; its genetic material lies in a region called the nucleoid (e.g. bacteria).

Eukaryotic cell

A cell with a true, membrane-bound nucleus and several membrane-bound organelles (e.g. plant and animal cells).

Mitochondria

Double-membraned organelles, the "powerhouse" of the cell, that release energy as ATP during cellular respiration.

Cell theory

The unifying idea that all organisms are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells.

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The selectively permeable boundary of every cell, described by the fluid-mosaic model as a lipid bilayer with embedded proteins.

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Chapter 2: Cell: The Building Block of LifeClass 9 Science — summary, notes, extra questions & NCERT solutions

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