What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

State the cell theory.

All living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells.

Who modified the cell theory and how?

Rudolf Virchow modified it by explaining that new cells are formed from pre-existing cells (Omnis cellula-e cellula).

What is a mesosome and what does it do?

A mesosome is an infolding of the prokaryotic plasma membrane that helps in cell wall formation, DNA replication and distribution, respiration and secretion.

Name two double-membrane-bound organelles and their main function.

Mitochondria, which produce ATP by aerobic respiration, and chloroplasts, which carry out photosynthesis.

How do neutral solutes move across the plasma membrane?

Neutral solutes move by simple diffusion along the concentration gradient without energy; polar molecules need carrier proteins and some ions need active transport using ATP.

What are nuclear pores and their function?

Nuclear pores are openings in the nuclear envelope formed by fusion of its two membranes, allowing movement of RNA and protein molecules between nucleus and cytoplasm.

Differentiate between rough and smooth endoplasmic reticulum.

Rough ER bears ribosomes and is the site of protein synthesis and secretion, while smooth ER lacks ribosomes and is the site of lipid synthesis.

What is the 9+2 array?

It is the arrangement of microtubules in the axoneme of cilia and flagella: nine peripheral doublets surrounding a central pair.

How is the position of the centromere used to classify chromosomes?

Chromosomes are metacentric (centromere in the middle), sub-metacentric (slightly off-centre), acrocentric (near one end) or telocentric (terminal).

Why are lysosomes and vacuoles both endomembrane structures yet different?

Lysosomes are vesicles rich in hydrolytic enzymes for intracellular digestion, while vacuoles are membrane-bound spaces storing water, sap and wastes, sometimes aiding osmoregulation.

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Chapter 8: Cell: The Unit of Life — Class 11 Biology

Biology · 19 chapters

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Chapter 8: Cell: The Unit of Life

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The cell theory was formulated by:

Summary

This chapter establishes the cell as the fundamental structural and functional unit of life and compares prokaryotic and eukaryotic cells. The cell theory, formulated by Schleiden and Schwann and completed by Virchow (who showed cells arise from pre-existing cells), states that all organisms are made of cells and products of cells. Prokaryotic cells, found in bacteria, cyanobacteria, mycoplasma and PPLO, lack a membrane-bound nucleus and organelles; they have a cell envelope (glycocalyx, cell wall, plasma membrane), genomic DNA with plasmids, mesosomes, ribosomes (70S), inclusion bodies, and surface structures such as flagella, pili and fimbriae. Eukaryotic cells have a true nucleus and membrane-bound organelles. The chapter describes the fluid mosaic model of the plasma membrane, the cell wall, and the endomembrane system (endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles). It then covers the double-membraned mitochondria (power houses producing ATP) and plastids (chloroplasts, chromoplasts, leucoplasts), ribosomes (80S in eukaryotes), the cytoskeleton, cilia and flagella with their 9+2 axoneme, centrosome and centrioles, and the nucleus with its envelope, nucleolus, chromatin and chromosomes classified by centromere position.

Cell theory and overview of the cellProkaryotic cell: envelope, ribosomes, inclusionsPlasma membrane and cell wallEndomembrane system: ER, Golgi, lysosomes, vacuolesMitochondria, plastids and ribosomesCytoskeleton, cilia, centrosome and nucleus

Key terms

Cell theory

The principle that all living organisms are composed of cells and that all cells arise from pre-existing cells.

Prokaryotic cell

A cell lacking a membrane-bound nucleus and membrane-bound organelles, as in bacteria.

Fluid mosaic model

The model of the plasma membrane (Singer and Nicolson) in which proteins move laterally within a fluid lipid bilayer.

Endomembrane system

The coordinated organelles ER, Golgi apparatus, lysosomes and vacuoles.

Mesosome

An infolding of the plasma membrane in prokaryotes aiding cell wall formation, DNA replication and respiration.

Centromere

The primary constriction of a chromosome bearing the kinetochores, used to classify chromosome types.

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The principle that all living organisms are composed of cells and that all cells arise from pre-existing cells.

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Practice quiz · Cell: The Unit of Life

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