Every living thing — from a tiny bacterium to a blue whale — is built from cells. The cell is the smallest unit that can carry out all the activities of life, which is exactly why it is called the fundamental unit of life.
Cell discovery
Robert Hooke saw dead cork "cells" in 1665 using his own microscope.
Cell theory
All living things are made of cells; the cell is the basic unit of life.
Two cell types
Prokaryotic (no true nucleus) and eukaryotic (true, membrane-bound nucleus).
Organelles
Tiny "organs" inside the cell — nucleus, mitochondria, plastids and more — each with a job.
1. What is a cell & who discovered it?
A cell is the smallest structural and functional unit of a living organism. It is the smallest part of the body able to perform all life processes such as respiration, nutrition, growth and reproduction. The word "cell" was coined by Robert Hooke in 1665 when he examined a thin slice of cork (dead plant tissue) under his self-designed microscope and saw small box-like compartments resembling the rooms (cells) of a monastery. Anton van Leeuwenhoek (1674), with an improved microscope, was the first to see free, living cells in pond water. Robert Brown (1831) discovered the nucleus, and Purkinje (1839) coined the term protoplasm for the living jelly-like substance inside the cell.
2. The Cell Theory
In 1838–39 two scientists, Schleiden (a botanist) and Schwann (a zoologist), proposed the cell theory: all plants and animals are made up of cells, and the cell is the basic unit of life. Later, in 1855, Rudolf Virchow added the crucial idea that new cells arise only from pre-existing cells (omnis cellula e cellula). Together these statements form the modern cell theory.
3. Unicellular vs multicellular & division of labour
Organisms made of a single cell are unicellular (e.g., Amoeba, Paramecium, Chlamydomonas, bacteria); that one cell performs every life function. Organisms made of many cells are multicellular (e.g., humans, plants, fungi). In multicellular organisms there is a division of labour — different cells are specialised for different jobs (nerve cells carry messages, muscle cells contract, red blood cells carry oxygen). Cells vary enormously in shape and size: the smallest cells are bacteria (about 0.1–0.5 micrometre), the largest single cell is the ostrich egg, and the longest are nerve cells.
4. Structure of a cell: three basic parts
Every cell has three fundamental features: a plasma membrane, cytoplasm, and genetic material (usually inside a nucleus).
5. Plasma (cell) membrane
The plasma membrane is the outermost living covering of the cell. It is selectively permeable — it allows some substances to pass while blocking others, controlling movement in and out of the cell. It is flexible and made of lipids and proteins. Substances move across it by diffusion (movement of a substance from a region of high concentration to low concentration, e.g., CO2 and O2 exchange) and by osmosis (the movement of water molecules across a selectively permeable membrane from a region of high water concentration to low water concentration). In a hypotonic solution the cell gains water and swells; in an isotonic solution there is no net change; in a hypertonic solution the cell loses water and shrinks (plasmolysis in plant cells). The flexibility of the membrane also lets some cells engulf food — endocytosis — as Amoeba does.
6. Cell wall
Plant cells (and fungi, bacteria) have an extra rigid layer outside the plasma membrane called the cell wall, made mainly of cellulose in plants. It gives shape, strength and protection, and lets plant cells withstand changes in surrounding medium. Because of the cell wall, plant cells can tolerate much greater variation in the medium than animal cells; when a plant cell loses water in a hypertonic solution, the contents shrink away from the wall — this is called plasmolysis.
7. Nucleus
The nucleus is the control centre of the cell. It is bounded by a double-layered nuclear membrane with pores, and contains chromatin — thread-like structures that condense into chromosomes during cell division. Chromosomes carry the hereditary information in the form of DNA (deoxyribonucleic acid) packed with proteins; a functional segment of DNA is a gene. The nucleus directs all chemical activities and controls reproduction. In organisms whose cells lack a true nucleus (no nuclear membrane), the nuclear region is called a nucleoid; such organisms are prokaryotes (e.g., bacteria, blue-green algae). Cells with a true membrane-bound nucleus are eukaryotes.
8. Cytoplasm and cell organelles
Cytoplasm is the fluid content inside the plasma membrane (excluding the nucleus). Suspended in it are the cell organelles, each performing a special function:
Endoplasmic reticulum (ER): a network of membranous tubes. Rough ER (RER) bears ribosomes and helps make proteins; Smooth ER (SER) makes fats/lipids and detoxifies poisons and drugs in liver cells. The ER also serves as channels for transport of materials — like the "supply highway" of the cell.
Golgi apparatus: stacks of membrane-bound sacs (cisternae) that store, modify, pack and dispatch materials made in the ER. It also forms lysosomes.
Lysosomes: membrane-bound sacs full of digestive enzymes; they clear out worn-out organelles and digest foreign material. When a cell is damaged, lysosomes may burst and digest the cell, so they are called the suicide bags of the cell.
Mitochondria: rod-shaped, double-membraned organelles where respiration releases energy as ATP; hence they are the powerhouse of the cell. They have their own DNA and ribosomes and can make some of their own proteins.
Plastids: found only in plant cells. Chloroplasts (green, contain chlorophyll) carry out photosynthesis; chromoplasts give colour to flowers and fruits; leucoplasts store starch, oils and proteins. Like mitochondria, plastids have their own DNA and ribosomes.
Vacuoles: storage sacs for solid or liquid contents. In plant cells the vacuole is large (up to 50–90% of cell volume) and provides turgidity and stores cell sap; in animal cells vacuoles are small.
Ribosomes: tiny granules (on RER or free in cytoplasm) that are the sites of protein synthesis.
9. Cell division
New cells are formed by cell division. Mitosis produces two identical daughter cells for growth and repair (each with the same number of chromosomes as the parent). Meiosis occurs in reproductive organs to form gametes, producing four cells each with half the number of chromosomes.
- Cell = fundamental structural & functional unit of life.
- Robert Hooke (1665) → coined "cell"; Leeuwenhoek → first living cells.
- Cell theory: Schleiden & Schwann; "cells from cells" → Virchow.
- Diffusion = movement of any substance high → low concentration.
- Osmosis = movement of water across a selectively permeable membrane.
- Prokaryote = no true nucleus (nucleoid); Eukaryote = true nucleus.
- Mitochondria = powerhouse; Lysosomes = suicide bags; Ribosomes = protein factories.
- Plastids & mitochondria have their own DNA.
A few raisins are soaked in water for some hours. They swell up. Explain the process step by step and name it.
- A dry raisin has a high concentration of solutes (sugar) and very little water inside it.
- The skin of the raisin acts as a selectively permeable membrane.
- Pure water outside has a higher water concentration than the inside of the raisin.
- Water molecules move from the region of high water concentration (outside) to low water concentration (inside) across the membrane.
- Water enters the raisin, so it swells and becomes plump.
Trace the journey of a protein made in a cell — from where it is made to where it leaves the cell. Name the organelles involved at each step.
- The protein is synthesised on the ribosomes attached to the rough endoplasmic reticulum (RER).
- The RER transports the newly made protein through its membranous channels.
- The protein reaches the Golgi apparatus, where it is modified, stored and packed into vesicles.
- The Golgi apparatus dispatches the packed protein in vesicles towards the plasma membrane.
- The vesicle fuses with the plasma membrane and the protein is secreted out of the cell.
Remember organelle jobs with: "Mighty Power, Suicidal Bags, Green Kitchens." Mitochondria = Mighty Power (energy/ATP), Lysosomes = Suicidal Bags (digestion), Chloroplasts = Green Kitchens (photosynthesis). For osmosis vs diffusion: Osmosis is for the O in water (H2O) — only water moves.
Do not confuse diffusion and osmosis. Diffusion is the movement of any substance (like gases) from high to low concentration; osmosis is specifically the movement of water across a selectively permeable membrane. Also never say a cell wall is "living and selectively permeable" — the cell wall is non-living and fully permeable; it is the plasma membrane that is living and selectively permeable.
Q1. Why is the plasma membrane called a selectively permeable membrane?
Answer: The plasma membrane is called selectively permeable because it allows only certain substances to move in and out of the cell while preventing the movement of others. It permits the entry and exit of some molecules (such as O2, CO2 and water) but blocks others, thereby regulating the cell’s internal composition. This selective control protects the cell and keeps its internal environment stable.
Q2. What are the differences between a prokaryotic cell and a eukaryotic cell?
Answer: (i) A prokaryotic cell has no true nucleus — its genetic material lies free in the cytoplasm in a region called the nucleoid; a eukaryotic cell has a true, membrane-bound nucleus. (ii) Prokaryotes lack membrane-bound organelles (no mitochondria, ER, Golgi, plastids); eukaryotes have these organelles. (iii) Prokaryotes are usually small and single-celled (e.g., bacteria, blue-green algae); eukaryotes may be unicellular or multicellular (e.g., Amoeba, plants, animals).
Q3. Why are lysosomes known as “suicide bags” of the cell?
Answer: Lysosomes are membrane-bound sacs filled with powerful digestive enzymes. They normally clean up the cell by digesting worn-out organelles and foreign particles. However, when the cell is damaged or during disturbances in cellular metabolism, the lysosomes may burst and their enzymes digest (destroy) the cell’s own contents, causing the cell to die. Because they can cause the cell’s self-destruction, lysosomes are called the suicide bags of the cell.
Q4. What would happen to the life of a cell if there were no Golgi apparatus?
Answer: If there were no Golgi apparatus, the cell could not modify, store, pack and dispatch the materials synthesised by the endoplasmic reticulum. Secretory products such as proteins and enzymes would not be properly processed or transported to their destinations inside or outside the cell. The formation of lysosomes (which arise from the Golgi) would also stop. As a result, secretion and many vital cell functions would be disrupted, and the cell would not survive properly.
- ✅ The cell is the fundamental structural and functional unit of life.
- ✅ Three basic parts: plasma membrane, cytoplasm and nucleus.
- ✅ Plasma membrane is selectively permeable; movement by diffusion and osmosis.
- ✅ Plant cells have an extra non-living cellulose cell wall.
- ✅ Organelles divide the work: mitochondria (energy), ribosomes (protein), Golgi (packing), lysosomes (digestion), plastids (photosynthesis/storage).
- ✅ Prokaryotes lack a true nucleus; eukaryotes have one. New cells come from existing cells by mitosis/meiosis.
Book a free demo class