The Cell and Its Structure
NCERT Class 9 • Science • Chapter "The Fundamental Unit of Life" | NCERT Class 8 • Science • Chapter "Cell Structure and Functions"
The Cell and Its Structure
The cell is the basic structural and functional unit of every living organism. Some organisms are made of a single cell while others contain trillions, but in both cases every activity of life takes place inside cells, which is why the cell is called the fundamental unit of life.
Discovery and Cell Theory
- Robert Hooke first saw and named cells while examining a thin slice of cork under a microscope.
- Leeuwenhoek was the first to observe free living cells in pond water.
- Robert Brown discovered the nucleus, and Purkinje named the fluid inside the cell protoplasm.
- The cell theory, given by Schleiden and Schwann, states that all plants and animals are made of cells and that the cell is the basic unit of life.
- Virchow added that all new cells arise from pre existing cells.
- Unicellular organisms such as amoeba and paramecium carry out all functions within one cell; multicellular organisms divide the work among specialised cells.
- Prokaryotic cells such as bacteria lack a true nucleus and membrane bound organelles; eukaryotic cells have both.
Plant and Animal Cells
- Every cell has a plasma membrane, which is selectively permeable and controls what enters and leaves.
- Movement of substances across it happens by diffusion for gases and by osmosis for water.
- Osmosis is the movement of water from a region of higher water concentration to lower across a semipermeable membrane.
- A plant cell has a rigid cell wall made of cellulose outside the membrane, which an animal cell lacks.
- A plant cell has a large central vacuole and plastids including chloroplasts; an animal cell has small vacuoles and no plastids.
- An animal cell has a centrosome, which most plant cells do not.
- The nucleus contains chromosomes carrying the hereditary information in the form of DNA.
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present, made of cellulose | Absent |
| Plastids | Present | Absent |
| Vacuole | One large central vacuole | Small or absent |
| Centrosome | Usually absent | Present |
Osmosis — the movement of water across a semipermeable membrane from a region of higher water concentration to one of lower concentration.
Hindi me samjhein
पादप कोशिका में कोशिका भित्ति, बड़ी रिक्तिका और लवक होते हैं जो जंतु कोशिका में नहीं मिलते। जंतु कोशिका में तारककाय होता है जो अधिकांश पादप कोशिकाओं में नहीं होता।
Exam me kaise aata hai
- Who discovered the cell — Robert Hooke
- Who discovered the nucleus — Robert Brown
- Cells lacking a true nucleus are called — prokaryotic
- The plant cell wall is made of — cellulose
UPSC/State PSC ke liye
- The plasma membrane is selectively permeable, which is what lets a cell maintain an internal composition different from its surroundings.
- The cell wall gives a plant cell turgidity, so a plant can stand upright without a skeleton.
Yahan confuse hote hain
✗ All cells have a nucleus | Prokaryotic cells lack a true nucleus | ✓
✗ Animal cells have a cell wall | Only plant cells have a cell wall | ✓
✗ Diffusion and osmosis are the same | Osmosis is the movement of water across a semipermeable membrane | ✓
Ek nazar me
- Hooke discovered cells; Schleiden, Schwann and Virchow gave the cell theory.
- Prokaryotic cells lack a true nucleus; eukaryotic cells have one.
- Plant cells have a cell wall, plastids and a large vacuole.
- Plasma membrane is selectively permeable; osmosis moves water across it.