The Fundamental Unit of Life
Detailed NCERT Solutions & Cell Structure
In-Text Questions (Page 59)
Q1. Who discovered cells, and how?
Robert Hooke discovered cells in 1665. He examined a thin slice of cork under a self-designed primitive microscope and observed that the cork resembled the structure of a honeycomb consisting of many little compartments. He called these boxes 'cells'.
Q2. Why is the cell called the structural and functional unit of life?
1. Structural Unit: All living organisms are made up of cells. It provides
structure to the body.
2. Functional Unit: All the fundamental functions of life (respiration, nutrition,
excretion, etc.) are performed by the cells.
In-Text Questions (Page 61)
Q1. How do substances like carbon dioxide and water move in and out of the cell? Discuss.
Carbon Dioxide (CO2): Moves by the process of diffusion. It moves from a region of high concentration (inside the cell) to a region of low concentration (outside the cell).
Water (H2O): Moves by the process of osmosis through the selectively permeable plasma membrane. It moves from high water concentration to low water concentration.
Q2. Why is the plasma membrane called a selectively permeable membrane?
It allows or permits the entry and exit of some materials in and out of the cell. It also prevents the movement of some other materials. Therefore, it is called a selectively permeable membrane.
In-Text Questions (Page 63)
Q1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| 1. Size: generally small (1-10 µm) | 1. Size: generally large (5-100 µm) |
| 2. Nuclear region: Not well defined (called Nucleoid) | 2. Nuclear region: Well defined and surrounded by a nuclear membrane |
| 3. Chromosome: Single | 3. Chromosome: More than one |
| 4. Membrane-bound organelles absent | 4. Membrane-bound organelles present |
In-Text Questions (Page 65)
Q1. Can you name the two organelles we have studied that contain their own genetic material?
1. Mitochondria
2. Plastids (in plant cells)
Q2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
The cell will not be able to perform its basic functions like respiration, nutrition, excretion, etc. This may result in the death of the cell. Lysosomes may burst and digest their own cell.
Q3. Why are lysosomes known as suicide bags?
Lysosomes contain powerful digestive enzymes. If the cell gets damaged, the lysosome may burst, and the enzymes digest their own cell. Hence, they are called suicide bags.
Q4. Where are proteins synthesized inside the cell?
Proteins are synthesized in the Ribosomes.
Main Textbook Exercises
Q1. Comparison between Plant cell and Animal cell.
| Plant Cell | Animal Cell |
|---|---|
| Cell wall present. | Cell wall absent. |
| Plastids present. | Plastids absent. |
| Large, central vacuole. | Small, temporary vacuoles (if any). |
Q3. What would happen if the plasma membrane ruptures or breaks down?
The constant internal chemical composition of the cell will involve exchange of materials from the surroundings uncontrollably. The cell will lose its ability to maintain homeostasis and will eventually die.
Q4. What would happen to the life of a cell if there was no Golgi apparatus?
1. Packaging and dispatching of materials synthesized by ER would stop.
2. Formation of lysosomes would not happen.
3. Accumulation of waste and synthesized products would occur.
Q5. Which organelle is known as the powerhouse of the cell? Why?
Mitochondria is called the powerhouse of the cell because energy required for various chemical activities is released by mitochondria in the form of ATP (Adenosine Triphosphate) molecules.
The Fundamental Unit of Life
Detailed Focus on Cell Organelles
1. What is a Cell?
The cell is the smallest structural and functional unit of life. It is the building block of all living organisms.
Single cell constitutes the whole organism. Ex: Amoeba, Chlamydomonas, Paramecium, Bacteria.
Many cells group together to form tissues, organs, etc. Ex: Fungi, Plants, Animals.
2. Structural Organization
Outermost covering of the cell. It is made of lipids and proteins. It is flexible and selectively permeable.
- Diffusion: Spontaneous movement of substance from high to low concentration (e.g., CO2, O2).
- Osmosis: Movement of water molecules through a selectively permeable membrane from high water concentration to low water concentration.
Present only in plant cells. Rigid outer covering lying outside plasma membrane. Composed mainly of cellulose. It provides structural strength to plants.
Plasmolysis: Shrinkage of contents of the cell away from the cell wall when a living plant cell loses water through osmosis.
Wait, this is the brain of the cell! It has a double layered covering called nuclear membrane. Contains chromosomes (DNA + Protein). Functional segments of DNA are called Genes.
3. Cell Organelles
Large network of membrane-bound tubes.
- RER (Rough ER): Has ribosomes attached. Synthesizes proteins.
- SER (Smooth ER): No ribosomes. Synthesizes fat molecules (lipids). Helps in detoxifying poisons in liver cells of vertebrates.
- Membrane Biogenesis: Process of building cell membrane using proteins and lipids from ER.
System of membrane-bound vesicles (flattened sacs). Discovered by Camillo Golgi.
Functions: Storage, modification, and packaging of products in vesicles. Formation of lysosomes.
Membrane-bound sacs filled with digestive enzymes (made by RER). Waste disposal system of the cell. Known as 'Suicide Bags'.
Powerhouse of the cell. Two membrane coverings (outer porous, inner deeply folded). Generates ATP. Has its own DNA and ribosomes.
Present only in plant cells. Have their own DNA/Ribosomes.
- Chromoplasts: Colored plastids (Chloroplasts contain chlorophyll - photosynthesis).
- Leucoplasts: White or colorless (store starch, oils, protein granules).
Storage sacs for solid or liquid contents. Small in animal cells, very large in plant cells (50-90% volume). Provide turgidity and rigidity.
Key Facts & Definitions
50+ Important Points to Remember
Basic structural and functional unit of life.
Discovered cell in 1665 in a cork slice.
Discovered free living cells in pond water (1674).
Discovered nucleus in the cell (1831).
Coined the term 'Protoplasm' for the fluid substance of the cell (1839).
Proposed by Schleiden (1838) and Schwann (1839). All plants and animals are composed of cells.
Expanded cell theory: "Omnis cellula-e cellula" (All cells arise from pre-existing cells) in 1855.
Organisms made of single cell (Amoeba, Paramecium).
Organisms made of many cells (Humans, Fungi).
Selectively permeable outer covering of the cell.
Movement of substance from high concentration to low concentration.
Movement of water across a semi-permeable membrane.
Medium has higher water concentration than cell. Cell gains water (swells).
Medium has same water concentration as cell. No net movement.
Medium has lower water concentration than cell. Cell loses water (shrinks).
Process by which Amoeba engulfs food using flexible membrane.
Rigid outer covering in plant cells made of cellulose.
Shrinkage of cell contents away from cell wall in plants due to osmosis.
Control center of the cell containing genetic material.
Rod-shaped structures visible during cell division; contain DNA.
Deoxyribo Nucleic Acid. Contains information for inheritance.
Functional segments of DNA.
Organisms whose cells lack a nuclear membrane (e.g., Bacteria).
Undefined nuclear region containing only nucleic acids in prokaryotes.
Fluid content inside plasma membrane containing organelles.
Network of tubes involved in transport and synthesis.
Has ribosomes; synthesizes proteins.
Manufactures fat molecules/lipids. Detoxifies poisons.
Building of cell membrane using ER proteins and lipids.
Packages and dispatches materials. Forms lysosomes.
Dispose of waste. Contain digestive enzymes. 'Suicide Bags'.
Powerhouse of the cell. Generates ATP.
Adenosine Triphosphate. Energy currency of the cell.
Present only in plant cells. Have own DNA/Ribosomes.
Colored plastids.
Green plastids covering chlorophyll. Sites of photosynthesis.
White/colorless plastids. Store starch, oils.
Storage sacs. Small in animals, large in plants (turgidity).
Process by which new cells are made.
Cell division for growth. Mother cell divides into 2 identical daughter cells.
Cell division for gamete formation. Produces 4 new cells with half chromosomes.
Reproductive cells having half the number of chromosomes.
Site of protein synthesis. Attached to RER or free in cytoplasm.
Mitochondria and Plastids (Have own DNA/Ribosomes).
Complex substance providing structural strength to plants.
Described Golgi Apparatus. Nobel prize in 1906.
Folds of inner mitochondrial membrane (increase surface area).
Matrix of the plastid.
State of being swollen due to high fluid content (provided by vacuole).
1-10 micrometers.