The Fundamental Unit of Life
CBSE Class 9 · Science · Notes and practice questions
This chapter explains the cell as the fundamental unit of life, covering the differences between prokaryotic and eukaryotic cells, the structure and functions of the plasma membrane, cell wall, nucleus, and all major cell organelles, along with the processes of diffusion and osmosis.
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The chapter begins by establishing that all living organisms are composed of cells, and a single cell is the smallest unit capable of performing life processes. It introduces the two major categories of cells: prokaryotic cells, which lack a nuclear membrane and most organelles, and eukaryotic cells, which have a distinct nucleus and membrane-bound organelles. This distinction forms the foundation for understanding how cell structure relates to function.
The plasma membrane is the outer boundary of every cell, controlling what enters and leaves. Diffusion and osmosis are the two main passive transport processes explained here. Diffusion is the movement of molecules from higher to lower concentration, while osmosis specifically refers to water movement across a semipermeable membrane. The behaviour of cells in hypotonic, isotonic, and hypertonic solutions is a crucial application, explaining phenomena like swelling, shrinkage, and turgidity.
In plant cells and some others, a rigid cell wall surrounds the plasma membrane, providing structural support. Inside the cell, the nucleus houses genetic material and controls cellular activities. The cytoplasm contains several organelles, each with a specific job: the endoplasmic reticulum helps in protein and lipid synthesis, the Golgi apparatus processes and packages materials, lysosomes digest waste, mitochondria release energy, plastids carry out photosynthesis in plant cells, and vacuoles store substances.
A key theme is that each organelle contributes to the overall functioning of the cell, and the loss or dysfunction of any can disrupt life processes. The chapter prepares students to understand how cells are not just simple sacs but highly organised and dynamic units. It also clarifies common misconceptions, such as the idea that all cells are the same or that osmosis is only about the movement of any substance through a membrane.
Key terms
- Cell
- The basic structural and functional unit of all living organisms. Organisms may be unicellular (a single cell) or multicellular (many cells working together).
- Prokaryotic cell
- A cell that lacks a true nucleus and membrane-bound organelles. Genetic material lies directly in the cytoplasm. Examples include bacteria and cyanobacteria.
- Eukaryotic cell
- A cell with a well-defined nucleus enclosed by a nuclear membrane, and membrane-bound organelles like mitochondria and Golgi apparatus. All organisms except bacteria and archaea are made of eukaryotic cells.
- Plasma membrane (cell membrane)
- A selectively permeable thin membrane that surrounds the cell, separating its contents from the outside. It regulates the transport of materials and also provides some protection and shape flexibility.
- Diffusion
- The net movement of molecules from a region of higher concentration to a region of lower concentration, occurring due to the random motion of particles. It does not require energy and continues until concentrations become equal.
- Osmosis
- The movement of water molecules across a selectively permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
- Hypotonic, isotonic and hypertonic solutions
- In a hypotonic solution, the outside has a lower solute concentration than the cell, so water enters and the cell swells. In an isotonic solution, concentrations are equal, so no net water movement occurs. In a hypertonic solution, the outside has a higher solute concentration, so water leaves and the cell shrinks.
- Cell wall
- A rigid and non-living outer layer found in plant cells, fungi, and many bacteria. It is made chiefly of cellulose in plants and provides structural support, while being fully permeable to water and solutes.
- Nucleus
- A double-membrane-bound organelle that contains the cell's genetic material (chromatin/DNA). It controls all cellular activities and is involved in cell division. The porous nuclear membrane allows exchange of materials.
- Cell organelles
- Specialised membrane-bound structures within the cytoplasm, each performing a specific function. They include the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids, and vacuoles.
Practice questions with answers
1. What is the difference between diffusion and osmosis? Give one example of each.
Diffusion is the movement of any substance from a region of higher concentration to lower concentration until uniformly distributed; it does not require a membrane. Osmosis is specifically the movement of water through a selectively permeable membrane from higher water concentration to lower water concentration. Example of diffusion: the smell of perfume spreading in a room. Example of osmosis: absorption of water by plant roots.
2. Why is the plasma membrane called a selectively permeable membrane?
The plasma membrane allows some substances to pass through while preventing others, so it is described as selectively permeable. It permits useful molecules like water and certain small, uncharged particles to enter, but blocks larger or charged molecules, thereby maintaining the internal composition of the cell.
3. A cell kept in a hypertonic solution shrinks. Why? Explain with the effect on water movement.
In a hypertonic solution, the concentration of solutes outside the cell is higher than inside, meaning the water concentration outside is lower. Water therefore moves out of the cell across the plasma membrane by osmosis, reducing the cell's volume and causing it to shrink or become flaccid.
4. What would happen if an animal cell is placed in distilled water? Why?
Distilled water is hypotonic compared to the cell's cytoplasm. Water will move into the cell by osmosis because the water concentration outside is higher. The cell will swell and may eventually burst because animal cells lack a cell wall to prevent excessive expansion, a phenomenon called lysis.
5. Why do plant cells not burst in a hypotonic solution?
Plant cells have a rigid cellulose cell wall surrounding the plasma membrane. When water enters by osmosis and the cell swells, the cell wall exerts an outward pressure that limits further expansion, so the cell becomes turgid but does not burst. The cell wall protects the plasma membrane from bursting.
6. State the main functions of the following organelles: mitochondria, lysosomes, and Golgi apparatus.
Mitochondria are the sites of aerobic respiration and ATP (energy) production, earning them the name 'powerhouse of the cell'. Lysosomes contain digestive enzymes that break down waste materials, dead cell parts, and foreign invaders, acting as the cell's 'suicide bags' when damaged. The Golgi apparatus modifies, sorts, and packages proteins and lipids synthesized in the ER into vesicles for secretion or use within the cell.
7. What is the role of the nucleus in a eukaryotic cell?
The nucleus contains the genetic material in the form of chromatin, which condenses into chromosomes during cell division. It directs and controls all cellular activities, such as metabolism, growth, and reproduction, by regulating gene expression and protein synthesis. It also takes part in cell division by duplicating its DNA.
8. How are prokaryotic cells different from eukaryotic cells? List any three points.
Prokaryotic cells lack a true nucleus, while eukaryotic cells have a membrane-bound nucleus. Prokaryotes do not have membrane-bound organelles (like mitochondria or Golgi), whereas eukaryotes possess many such organelles. Prokaryotic cells are generally smaller and simpler, with DNA lying free in the cytoplasm, while eukaryotic cells are larger with DNA enclosed in the nucleus.
9. If a plant cell is placed in a sugar solution that is isotonic, what will be the net movement of water? Give a reason.
There will be no net movement of water because the water concentration inside the cell is equal to that in the isotonic solution. The rates of water entering and leaving the cell are equal, so the cell remains unchanged in size and shape. This condition is ideal for maintaining cell volume, as seen in the transport of materials in plants.
10. Why are lysosomes known as the 'suicide bags' of the cell?
Lysosomes contain powerful digestive enzymes that can break down proteins, lipids, and carbohydrates. If a cell is damaged or becomes old, its lysosomes may rupture, releasing these enzymes into the cytoplasm and digesting the cell's own contents, leading to cell death. Hence, they are called 'suicide bags'.
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