The Fundamental Unit of Life — Science Class 9 Notes (CBSE & HBSE)
Free NCERT Science notes for The Fundamental Unit of Life (Class 9) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — The Fundamental Unit of Life (CBSE & HBSE)
CBSE focuses on detailed structure-function of organelles, osmosis experiments and well-labelled diagram-based reasoning (plant vs animal cell, prokaryote vs eukaryote). HBSE emphasises definitions, differences (cell wall vs cell membrane, plant vs animal cell) and the discovery of the cell as short-answer recall.
Discovery of the Cell, Prokaryotic vs Eukaryotic, Plant vs Animal Cell
Discovery of the Cell
- Robert Hooke (1665) first observed dead cork cells under a self-made microscope and coined the term cell (Latin cellula = little room).
- Leeuwenhoek discovered free-living cells (living cells) in pond water.
- Robert Brown discovered the nucleus.
- Purkinje coined the term protoplasm for the fluid substance of the cell.
- Schleiden and Schwann proposed the Cell Theory: all plants and animals are composed of cells, and the cell is the basic unit of life.
- Virchow extended it: all cells arise from pre-existing cells (Omnis cellula-e cellula).
The cell is the structural and functional unit of life. Organisms made of a single cell are unicellular (e.g. Amoeba, Paramoecium); those made of many cells are multicellular.
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | No well-defined nucleus (no nuclear membrane) | Well-defined nucleus with nuclear membrane |
| Size | Generally small (1-10 \u00b5m) | Generally large (5-100 \u00b5m) |
| Nuclear material | Nucleoid (lies free in cytoplasm) | Enclosed nucleus |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, etc.) |
| Examples | Bacteria, blue-green algae | Plant and animal cells, fungi |
Trap: Prokaryotes lack a nuclear membrane and membrane-bound organelles, but they DO have ribosomes and genetic material.
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (made of cellulose) | Absent |
| Plastids | Present (e.g. chloroplast) | Absent (except Euglena) |
| Vacuole | Large central vacuole | Small or absent |
| Centrosome | Usually absent | Present |
| Shape | Usually fixed and rectangular | Usually round/irregular |
Cell Membrane, Osmosis, Cell Wall and Nucleus
Plasma (Cell) Membrane
The plasma membrane is the outermost covering of an animal cell. It is selectively permeable \u2014 it allows the entry and exit of only some substances and prevents others. It is made of lipids and proteins and is flexible.
Movement across the membrane:
- Diffusion: movement of substances (e.g. CO\u2082, O\u2082) from a region of higher concentration to lower concentration.
- Osmosis: movement of water molecules through a selectively permeable membrane from a region of higher water concentration (dilute solution) to lower water concentration (concentrated solution).
Types of Solutions and Osmosis
| Solution | Concentration outside cell | Net water movement | Effect on cell |
|---|---|---|---|
| Hypotonic | More dilute (more water outside) | Water enters cell (endosmosis) | Cell swells / may burst |
| Isotonic | Equal concentration | No net movement | No change |
| Hypertonic | More concentrated (less water outside) | Water leaves cell (exosmosis) | Cell shrinks (plasmolysis in plant cell) |
Trap: In a hypotonic solution an animal cell may burst (no cell wall to protect it), whereas a plant cell only swells (the rigid cell wall prevents bursting). Plasmolysis = shrinking of the cell contents away from the cell wall in a hypertonic solution.
Cell Wall
The cell wall is a rigid outer covering present only in plant cells (outside the plasma membrane), made mainly of cellulose. It provides structural strength and shape and allows plant cells to withstand changes in the surrounding medium.
Nucleus
The nucleus is the control centre of the cell. It has:
- A double-layered nuclear membrane with pores that allow transfer of material.
- Chromatin material that condenses into chromosomes during cell division; chromosomes carry genes (units of heredity) made of DNA and protein.
- A nucleolus.
Functions: controls all cellular activities and plays a central role in cellular reproduction and inheritance.
Cell Organelles: ER, Golgi, Mitochondria, Plastids, Lysosomes and Vacuoles
Endoplasmic Reticulum (ER)
A network of membranes forming tubes and sheets in the cytoplasm. Two types:
- Rough ER (RER): has ribosomes on its surface; helps in protein synthesis and transport.
- Smooth ER (SER): lacks ribosomes; helps in lipid (fat) synthesis and detoxification in liver cells.
The ER also serves as a channel for transport of materials and acts as a cytoplasmic framework.
Golgi Apparatus
Discovered by Camillo Golgi. It consists of membrane-bound sacs called cisternae. Functions:
- Storage, modification and packaging of substances made in the ER.
- Forms lysosomes.
- Involved in the formation of complex sugars and secretory vesicles.
Mitochondria
The mitochondrion is the powerhouse of the cell. It has a double membrane; the inner membrane is folded into cristae to increase surface area. It releases energy in the form of ATP (Adenosine Triphosphate) during respiration. Mitochondria have their own DNA and ribosomes.
Plastids (only in plant cells)
| Plastid | Colour/Pigment | Function |
|---|---|---|
| Chloroplast | Green (chlorophyll) | Photosynthesis |
| Chromoplast | Coloured (red/orange/yellow) | Colour to flowers and fruits |
| Leucoplast | Colourless | Storage of starch, oils, proteins |
Like mitochondria, plastids also have their own DNA and ribosomes.
Lysosomes
Lysosomes are membrane-bound sacs containing powerful digestive (hydrolytic) enzymes. They digest worn-out organelles and foreign material. When a cell is damaged, lysosomes may burst and digest their own cell, so they are called the suicidal bags of the cell.
Vacuoles
Vacuoles are storage sacs filled with cell sap. In plant cells the central vacuole is large (up to 50-90% of cell volume) and provides turgidity and rigidity; it stores water, sugars, amino acids and waste. In animal cells vacuoles are small.
Trap: Mitochondria and plastids are described as semi-autonomous because they have their own DNA and ribosomes and can make some of their own proteins.
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