Cell - The Structural and Functional Unit of Life

ICSE Class 10 Biology Detailed Master Notes

1. Cells - A Basic Study in Biology

Cell: The cell is the fundamental, structural, and functional unit of all living beings. Every organism begins its life as a single cell.

The Cell Theory

The formulation of the Cell Theory is a cornerstone in biology, postulated by three scientists:

Matthias Schleiden (1838): A botanist who announced that all plants are made of cells.
Theodor Schwann (1839): A zoologist who declared that all animals are made of cells.
Rudolf Virchow (1858): Added the final touch by stating "Omnis cellula e cellula" (All cells arise from pre-existing cells).

2. Cells - How Numerous?

The overall size of an organism depends entirely on the number of cells it contains, not on the size of its individual cells.

Single-celled (Unicellular): Made up of just one single cell. Examples: Bacteria, yeast, and amoeba.
Few-celled: Made up of just a few hundred or a few thousand cells. Examples: Spirogyra, Volvox.
Multi-celled (Multicellular): Made up of millions and billions of cells. Examples: Humans, mango trees.

3. Cells - How Small?

Most cells are microscopic. Being small provides a high Surface Area to Volume ratio, which is essential for the rapid exchange of materials (nutrients in, waste out) across the cell membrane.

Smallest cells: Mycoplasma (PPLO), bacteria, and red blood cells (RBCs) in the human body.
Longest cells: Nerve cells (Neurons) - can be over a meter long.
Largest cells: Birds' eggs (specifically the central yellow yolk). The ostrich egg is the largest single living cell.

Fascinating Facts (Adult Human Body):

1000 million million cells in the whole body.
10,000 million nerve cells in the brain cortex.
25 million million red blood cells.
30 thousand million white blood cells.

4. Cell Shapes and Their Functions

The shape of a cell is intimately related to the specific function it performs in the body:

Human Red Blood Cells (RBCs): Circular and biconcave. This shape increases surface area for oxygen absorption and allows them to squeeze through narrow capillaries.
White Blood Cells (WBCs): Amoeboid (capable of changing shape). This allows them to squeeze out through capillary walls (a process called diapedesis) to fight infections.
Nerve Cells (Neurons): Long and branched. Designed to conduct electrical impulses from distant parts of the body to the central nervous system.
Guard Cells: Bean-shaped in dicots. Regulates the opening and closing of stomata for gas exchange and transpiration.

5. Gross Structure of Cell - The Essential Parts

Fig. 1.1: A generalised animal cell and a generalised plant cell.

1. Cell Membrane (Plasma Membrane)

It is a living, extremely thin, and flexible membrane made of lipoproteins.
It is Selectively Permeable (or differentially permeable), meaning it strictly regulates the entry and exit of specific substances.
In plant cells, an outermost, non-living, rigid layer called the Cell Wall surrounds the membrane.
- Made primarily of cellulose.
- It provides shape, rigidity, and protection.
- It is Freely Permeable (allows everything in solution to pass through).

2. Cytoplasm

The semi-liquid jelly-like substance occupying most of the cell space inside the cell membrane.

It is the site where all metabolic activities (chemical reactions) take place.
It contains embedded living structures called Organelles.
The liquid part of the cytoplasm, minus the organelles, is called the Cytosol.

3. Nucleus (The Brain of the Cell)

A dense, spherical body that serves as the command center of the cell.

Surrounded by a double-layered Nuclear Membrane containing pores.
Filled with a semi-solid ground substance called Nucleoplasm.
Contains a dark, round body called the Nucleolus (which produces ribosomes).
Contains a tangled mass of thread-like structures called the Chromatin Network. During cell division, these condense to form thick structures called Chromosomes (the bearers of hereditary genes).

Protoplasm vs Cytoplasm & Protoplast:
Protoplasm = Cytoplasm + Nucleus. It refers to the entire living substance of the cell.
Protoplast is the term more appropriately used for a plant cell after the removal of its non-living cell wall.
Cytoplasm is only the living substance outside the nucleus but inside the cell membrane.

Fig. 1.2: Structure of nucleus.
Fig. 1.3: An early stage of cell division in an animal cell showing chromosomes condensing from the chromatin network.

6. Finer Structure of Cell - The Organelles

Organelles: The specialized, membrane-bound, living structures suspended in the cytoplasm, each concerned with a definite function (just like organs in our body).

Cell Organelles: ICSE Board Keywords & Functions

Organelle	Characteristics	Chief Function(s) [ICSE Focus]
Endoplasmic Reticulum (ER)	Irregular network of double-membrane tubules. Can be Smooth (SER) or Rough (RER - with attached ribosomes).	Acts as the Supportive framework of the cell. Helps in intracellular transport and protein/fat synthesis.
Mitochondria	Sausage-shaped, double-walled. Inner wall has finger-like folds called cristae. Has its own DNA.	Powerhouse of the cell. Site of cellular aerobic respiration. Releases energy in the form of ATP.
Golgi Apparatus (Dictyosomes in plants)	Stacks of flattened membrane sacs (cisternae) and vesicles.	Synthesis and Secretion of enzymes and hormones. Forms the acrosome of the sperm.
Ribosomes	Dense, spherical, non-membrane bound bodies made of RNA. Found free or on RER.	Protein factories. Site of protein synthesis.
Lysosomes	Membranous sacs containing powerful hydrolytic (digestive) enzymes.	Intracellular digestion. Known as Suicide bags because they burst to destroy old/injured cells.
Centrosome (Animal cells only)	Located near the nucleus. Contains two centrioles surrounded by aster rays.	Initiates cell division by forming Spindle fibres in animal cells.
Plastids (Plant cells only)	Double-membraned. Types: 1. Chloroplasts (Green) 2. Chromoplasts (Red/Yellow/Orange) 3. Leucoplasts (Colourless).	1. Traps solar energy for photosynthesis. 2. Imparts colour to flowers/fruits for pollination. 3. Stores starch.
Vacuoles (Non-living inclusion)	Clear spaces filled with water/cell sap. Surrounded by a membrane called Tonoplast. Large in plants, small/absent in animals.	Stores water, cell sap, and provides Turgidity to plant cells.
Granules (Non-living inclusion)	Small particles, crystals, or droplets scattered in the cytoplasm.	Serve as food storage for the cell: Starch (in plants), Glycogen (in animals), and fat droplets.

Note on Pigments: Chlorophyll is found in Chloroplasts. Xanthophyll (yellow) and Carotene (orange) are found in Chromoplasts. However, Anthocyanin (blue/purple pigment of many petals) is NOT found in plastids; it is dissolved in the cell sap of the vacuole.

Crucial Differences: Plant Cell vs Animal Cell

Feature	Plant Cells	Animal Cells
1. Cell wall	Present (made of cellulose); rigid.	Absent.
2. Centrosome	Absent. (Cell division driven by analog structures).	Present (contains centrioles to form spindle fibres).
3. Vacuoles	Large, prominent, usually a single central vacuole.	Small, temporary, or completely absent.
4. Plastids	Present (Chloroplasts, Chromoplasts, etc.).	Absent.
5. Cytoplasm	Pushed to the periphery as a thin lining due to the large vacuole.	Dense, granular, and fills the entire cell.

7. Microscopic Examination of Onion Peel (Practical Application)

A classic biological experiment involves preparing a temporary mount of an onion epidermal peel to observe plant cells.

Procedure: A thin, transparent strip is torn from the inner concave side of an onion scale leaf. It is mounted on a slide in a drop of water.
Staining: The peel is stained using Iodine or Eosin solution. Reason: This stains the cell components, specifically making the nucleus more distinct under the microscope.
Observations under Microscope:
- The cells have a somewhat regular, linear or rectangular shape.
- They are firmly bound together with a prominent, thick cell wall.
- A large central vacuole is present.
- Due to this large vacuole, the nucleus is pushed to the periphery (one side) of the cell.

Fig. 1.4 & 1.5: Preparation of an onion peel slide.
Fig. 1.6: Cells from an onion scale leaf peeling seen under a low-power microscope, showing the nucleus pushed to the side.

8. The Nucleus - Key to the Life of a Cell

To prove that the nucleus is the absolute master of the cell, scientists conducted a famous experiment on the Amoeba.

Observation 1: An intact Amoeba grows, functions, and divides normally.
Observation 2: If the nucleus is carefully surgically removed (enucleated), the Amoeba immediately stops growing and dies.
Observation 3: If a nucleus from a donor Amoeba is transplanted into the enucleated Amoeba before it dies, the recipient Amoeba recovers, survives, and divides normally.

Conclusion: The nucleus regulates all metabolic and reproductive activities of the cell. Without it, the cell cannot survive.

Fig. 1.7: An experiment on amoeba to show that nucleus is essential for normal life.

9. ICSE Board Exam Practice Questions (PYQ Patterns)

The following questions are strictly designed per the ICSE Class 10 Board Examination pattern to help you secure maximum marks in Section A and Section B.

Type A: Name the Following

The structural and functional unit of life.
Ans: Cell
The organelle known as the 'suicide bag' of the cell.
Ans: Lysosome
The organelle known as the 'powerhouse' of the cell.
Ans: Mitochondria
The network of thread-like structures present in the nucleoplasm that condenses during cell division.
Ans: Chromatin fibres (or Chromatin network)
The plastid which stores starch.
Ans: Leucoplast
The non-living granules in animal cells that store reserve food.
Ans: Glycogen granules
The blue-violet pigment dissolved in the cell sap.
Ans: Anthocyanin
The non-living, rigid layer surrounding the plasma membrane in plant cells.
Ans: Cell wall
The membrane surrounding the large central vacuole of a plant cell.
Ans: Tonoplast
The specific stain used to make the nucleus of an onion peel distinct.
Ans: Iodine or Eosin solution

Type B: State the Exact Location & Function

ICSE FAVOURITE State the exact location and the main function of the following:

Centrosome:
- Location: In the cytoplasm, near the nucleus, found only in animal cells.
- Function: It initiates cell division by forming spindle fibres.
Ribosomes:
- Location: Scattered freely in the cytoplasm or attached to the rough endoplasmic reticulum.
- Function: They are the sites of protein synthesis.
Chloroplasts:
- Location: In the cytoplasm of plant cells (specifically in the green parts/leaves).
- Function: Traps solar energy and helps in photosynthesis.

Type C: Give Biological Reasons

Why is the cell membrane called a selectively permeable membrane?
Ans: It allows only certain selective substances (solvent and some solutes) to pass through it while preventing the passage of other harmful or large substances.
Why is the nucleus mostly pushed to the periphery in a plant cell?
Ans: Plant cells contain a single, large, and prominent central vacuole filled with cell sap, which occupies most of the space and pushes the cytoplasm and nucleus to the periphery.
Why are lysosomes called 'suicide bags'?
Ans: Lysosomes contain powerful digestive enzymes. When a cell gets damaged, old, or infected, the lysosomes burst and their enzymes digest their own cell, hence the name.
Why are mitochondria called the 'powerhouses' of the cell?
Ans: They are the sites of cellular aerobic respiration where glucose is broken down to release energy. This energy is stored in the form of ATP molecules for cellular activities.
Why do plant cells have a rigid shape compared to animal cells?
Ans: Plant cells possess an outermost, non-living, and rigid cell wall made of cellulose that provides a fixed shape, which is absent in animal cells.

Type D: Differentiate Based on Given Parameters

CRITICAL THINKING Differentiate between the following pairs based on the criteria given in the brackets:

Cell Wall and Cell Membrane (Permeability):
Ans: Cell wall is freely permeable, whereas the cell membrane is selectively permeable.
Plant Cell and Animal Cell (Centrosome):
Ans: Plant cells lack a centrosome, whereas animal cells contain a centrosome near the nucleus.
Chromoplast and Leucoplast (Function):
Ans: Chromoplasts impart colour to flowers and fruits to attract pollinators, whereas leucoplasts store food primarily in the form of starch.
Cytoplasm and Protoplasm (Composition):
Ans: Cytoplasm is the living substance excluding the nucleus, whereas protoplasm is the entire living substance including the cytoplasm and the nucleus.

10. Visual / Diagram-Based Assessment

Identify the Organelles

VISUAL EXERCISE In the ICSE boards, you are frequently asked to identify organelles based on structural diagrams. Study the organelle structures below (e.g., stacked cisternae, double membrane with folds) and recall their functions.