Schooling · Class 11 · Biology · Cell : The Unit of Life · Chapter on a page
CBSE Class 11 · Biology · NCERT Biology
Cell : The Unit of Life: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Cell theory
- Prokaryotic cells
- Eukaryotic cells
- plasma membrane
- Nucleus
- Mitochondria
- Chloroplasts
- Formulas and facts
- Resolution of a light microscope
- Size range of cells
- Plasma membrane composition
- 70S ribosomes
- Cell wall
- Centrioles
- Vacuole
- Worked examples
- Calculating magnification
- Identifying cell type from features
- Ribosome identification
- Watch out for
- Thinking all cells have a cell wall
- Confusing 70S and 80S ribosomes
- Believing mitochondria are found…
- Assuming the nucleolus is a separate…
- Forgetting that prokaryotes have no…
What this chapter is about
This chapter introduces the cell as the fundamental structural and functional unit of all living organisms. You will learn that every life process — from nutrition to reproduction — ultimately happens within or between cells. The chapter traces the discovery of cells, explains the cell theory, and distinguishes between prokaryotic and eukaryotic cell types.
Key ideas
- Cell theory states that all living organisms are made of cells, the cell is the basic unit of life, and new cells arise only from pre-existing cells (proposed by Schleiden and Schwann; later extended by Virchow).
- Prokaryotic cells (bacteria, cyanobacteria) lack a membrane-bound nucleus and most organelles; their genetic material lies in a nucleoid region.
- Eukaryotic cells (plants, animals, fungi, protists) possess a true nucleus enclosed by a double membrane and contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
- The plasma membrane is a selectively permeable phospholipid bilayer with embedded proteins; it controls the passage of ions, nutrients and wastes.
- Nucleus contains chromatin (DNA + histone proteins), nucleolus (ribosome assembly) and is surrounded by a nuclear envelope with pores.
- Mitochondria are double-membraned organelles that carry out aerobic respiration and produce ATP; they have their own circular DNA.
- Chloroplasts (in plant cells) have thylakoid membranes containing chlorophyll for photosynthesis; they also possess their own DNA.
- Endoplasmic reticulum (ER) is a network of membranes: rough ER (with ribosomes) synthesises proteins; smooth ER synthesises lipids and detoxifies substances.
Formulas and facts to remember
- Resolution of a light microscope: approximately 0.2 µm (200 nm); electron microscopes resolve down to about 0.2 nm.
- Size range of cells: most animal cells 10–30 µm; most plant cells 10–100 µm; bacteria typically 1–5 µm.
- Plasma membrane composition: phospholipid bilayer (hydrophilic heads outward, hydrophobic tails inward) plus cholesterol, proteins and carbohydrates.
- 70S ribosomes occur in prokaryotes (made of 50S + 30S subunits); 80S ribosomes occur in eukaryotic cytoplasm (60S + 40S subunits).
- Cell wall: present in plants (cellulose), fungi (chitin) and bacteria (peptidoglycan); absent in animal cells.
- Centrioles: paired cylindrical structures (9 + 0 arrangement of microtubule triplets) involved in spindle formation during cell division; present in animal cells, absent in most plant cells.
- Vacuole: large central vacuole in plant cells maintains turgor pressure; animal cells have smaller, temporary vacuoles.
- Fluid mosaic model (Singer and Nicolson, 1972): describes the plasma membrane as a fluid phospholipid sea with proteins floating like a mosaic.
Common mistakes
- Thinking all cells have a cell wall → only plant, fungal and bacterial cells have a cell wall; animal cells have only a plasma membrane.
- Confusing 70S and 80S ribosomes → remember prokaryotes have 70S (smaller number for simpler organisms) and eukaryotic cytoplasm has 80S.
- Believing mitochondria are found only in animal cells → mitochondria are present in nearly all eukaryotic cells, including plant cells.
- Assuming the nucleolus is a separate organelle with its own membrane → the nucleolus is a non-membrane-bound region inside the nucleus where ribosomal RNA is synthesised.
- Forgetting that prokaryotes have no membrane-bound organelles → they still have ribosomes (non-membrane structures), a cell membrane and often a cell wall.
Quick revision
- Cell theory: all life is cellular; cells arise from pre-existing cells.
- Prokaryotes = no true nucleus, 70S ribosomes; Eukaryotes = membrane-bound nucleus, 80S ribosomes.
- Plasma membrane = phospholipid bilayer + proteins; selectively permeable.
- Mitochondria and chloroplasts have their own DNA and double membranes.
- Plant cells have a cell wall (cellulose), large vacuole and plastids; animal cells lack these.
- Fluid mosaic model describes the dynamic structure of the cell membrane.