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.
A Class 11 student meets this topic now because understanding cell structure is essential before studying biochemistry, cell division, genetics and physiology in later chapters. The cell is where molecules become life; grasping its organisation helps you see how organs, tissues and whole organisms work.
After studying this chapter, you should be able to describe the structure and function of each cell organelle, compare plant and animal cells, differentiate prokaryotes from eukaryotes, and explain how the plasma membrane regulates what enters or leaves a cell.
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
1. Resolution of a light microscope: approximately 0.2 µm (200 nm); electron microscopes resolve down to about 0.2 nm.
2. Size range of cells: most animal cells 10–30 µm; most plant cells 10–100 µm; bacteria typically 1–5 µm.
3. Plasma membrane composition: phospholipid bilayer (hydrophilic heads outward, hydrophobic tails inward) plus cholesterol, proteins and carbohydrates.
4. 70S ribosomes occur in prokaryotes (made of 50S + 30S subunits); 80S ribosomes occur in eukaryotic cytoplasm (60S + 40S subunits).
5. Cell wall: present in plants (cellulose), fungi (chitin) and bacteria (peptidoglycan); absent in animal cells.
6. 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.
7. Vacuole: large central vacuole in plant cells maintains turgor pressure; animal cells have smaller, temporary vacuoles.
8. Fluid mosaic model (Singer and Nicolson, 1972): describes the plasma membrane as a fluid phospholipid sea with proteins floating like a mosaic.
Worked examples
### Example 1: Calculating magnification
A student observes an onion epidermal cell under a compound microscope. The eyepiece lens is 10× and the objective lens is 40×. What is the total magnification?
Solution
Total magnification = eyepiece magnification × objective magnification = 10 × 40 = 400×
The image appears 400 times larger than the actual cell size.
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### Example 2: Identifying cell type from features
A biologist examines a cell and notes the following: cell wall present, large central vacuole, chloroplasts visible, membrane-bound nucleus present. Is this cell prokaryotic or eukaryotic, and is it from a plant or an animal?
Solution
Step 1: Membrane-bound nucleus indicates a eukaryotic cell (prokaryotes lack this).
Step 2: Chloroplasts and a large central vacuole are characteristic of plant cells; animal cells lack both.
Step 3: Cell wall composed of cellulose confirms it is a plant cell.
Answer: Eukaryotic plant cell.
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### Example 3: Ribosome identification
A researcher isolates ribosomes from two sources: bacterium Bacillus and human liver tissue. Sedimentation analysis shows one sample has 70S ribosomes and the other has 80S ribosomes. Assign each ribosome type to its source.
Solution
Prokaryotes (like Bacillus) contain 70S ribosomes. Eukaryotic cytoplasm (human liver cells) contains 80S ribosomes.
Answer: 70S — Bacillus; 80S — human liver cells.
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.