The Fundamental Unit of Life — NCERT Solutions, AI Tutor & Practice

Understand: The cell is the basic structural and functional unit of life.

Discovery:

• In 1665, Robert Hooke examined a thin slice of cork under a self-designed microscope.
• He observed tiny compartments resembling a honeycomb and named them 'cells' (from Latin cellula = little room).
• What he actually saw were the dead cell walls of cork tissue.

Cell Theory (given by Schleiden 1838 and Schwann 1839, later added to by Virchow 1855):

1. All living organisms are made up of cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (Virchow's contribution).

Conclude: Hooke's observation of cork led to the discovery of the cell, and the cell theory established that cells are the fundamental units of all life.

Understand: Cells are classified based on the presence or absence of a membrane-bound nucleus.

| Feature | Prokaryotic Cell | Eukaryotic Cell | |---------|------------------|-----------------| | Nucleus | No true nucleus; nuclear region (nucleoid) not membrane-bound | True nucleus with nuclear membrane | | Size | Generally small ($1$–$10\ \mu m$) | Generally large ($5$–$100\ \mu m$) | | Membrane-bound organelles | Absent (no mitochondria, ER, etc.) | Present | | Chromosome | Single, circular | More than one, linear | | Examples | Bacteria, blue-green algae | Plant cells, animal cells, fungi |

Conclude: The key difference is the presence of a membrane-bound nucleus and organelles in eukaryotes, absent in prokaryotes.

Understand: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).

Analyze — Effect on a plant cell:

(a) Hypotonic solution (outside more dilute than cell):

• Water enters the cell by osmosis.
• Cell swells and becomes turgid (but does not burst due to rigid cell wall).

(b) Isotonic solution (equal concentration):

• Water enters and leaves at equal rates.
• No net change; cell size remains the same.

(c) Hypertonic solution (outside more concentrated):

• Water leaves the cell by osmosis.
• Cell shrinks; the cell membrane pulls away from the wall — this is plasmolysis.

Conclude: The direction of water movement in osmosis depends on the relative concentration of the solutions.

Understand: The plasma membrane is the outermost living boundary of the cell, made of lipids and proteins.

Analyze:

• It allows the passage of only certain substances into and out of the cell.
• Some molecules (like oxygen, carbon dioxide, water) can pass freely.
• Other substances are blocked or allowed selectively depending on the cell's needs.
• It prevents the free movement of all materials, thus controlling entry and exit.

Conclude: Because it permits some substances to cross while restricting others, the plasma membrane is called selectively permeable (or semi-permeable).

Understand: Both are eukaryotic cells but differ in certain structures.

| Feature | Plant Cell | Animal Cell | |---------|------------|-------------| | Cell wall | Present (made of cellulose) outside plasma membrane | Absent | | Vacuole | Large, central vacuole | Small or absent | | Plastids | Present (e.g., chloroplast) | Absent |

Other points: Animal cells often have a centriole, usually absent in plant cells.

Conclude: The presence of a cell wall, large vacuole, and plastids distinguishes plant cells from animal cells.

Understand: Mitochondria are double-membrane organelles found in eukaryotic cells.

Analyze:

• They release energy required for various chemical activities of life through the process of cellular respiration.
• The energy released is stored in the form of ATP (Adenosine Triphosphate).
• ATP is known as the energy currency of the cell.
• This ATP is used by the body wherever energy is needed.
• Mitochondria have their own DNA and ribosomes, so they can make some of their own proteins.

Conclude: Because they generate energy (ATP) that powers all cellular functions, mitochondria are called the powerhouse of the cell.

Understand: The nucleus is a large, spherical, membrane-bound organelle usually located at the centre of the cell.

Structure:

• Bounded by a nuclear membrane with pores (nuclear pores) allowing transfer of materials.
• Contains a dense structure called the nucleolus.
• Contains chromatin material made of DNA and proteins, which condenses into chromosomes during cell division.

Functions:

1. Controls all the activities of the cell (control centre).
2. Contains chromosomes which carry genetic information (genes) from one generation to the next.
3. Plays a key role in cell reproduction/division.

Conclude: The nucleus acts as the control centre and carrier of hereditary information in the cell.

Understand: Lysosomes are small, membrane-bound sac-like organelles containing powerful digestive enzymes.

Functions:

• They act as the waste disposal system of the cell.
• They digest worn-out organelles, foreign materials, and food particles.
• They keep the cell clean.

Why 'suicidal bags':

• When a cell gets damaged or during disruption, lysosomes may burst and release their digestive enzymes.
• These enzymes then digest (destroy) the cell's own material, causing the cell to die.
• Hence lysosomes are called the 'suicidal bags' of the cell.

Conclude: Lysosomes clean the cell but can also self-destruct it by releasing their enzymes.

Understand: Each organelle has a specific role; removing it disrupts related functions.

(a) If the plasma membrane ruptures:

• The membrane controls entry and exit of substances.
• Without it, the cell loses control over exchange of materials with the surroundings.
• Cell contents leak out, and the cell will die.

(b) If the Golgi apparatus is removed:

• The Golgi apparatus stores, modifies, packages, and dispatches materials made in the ER.
• It also forms lysosomes.
• Without it, the cell cannot secrete or package substances properly, and lysosome formation stops.

Conclude: Removing any key organelle disrupts the cell's normal life processes and can cause cell death.

Understand: Both are types of passive movement of molecules.

| Feature | Diffusion | Osmosis | |---------|-----------|---------| | Definition | Movement of molecules from higher to lower concentration | Movement of water molecules from higher to lower water concentration through a selectively permeable membrane | | Membrane | Not necessarily required | Selectively permeable membrane required | | Molecules | Any substance (solid, liquid, gas) | Only solvent (water) | | Example | Exchange of $CO_2$ and $O_2$ across cell | Absorption of water by roots |

Conclude: Osmosis is a special type of diffusion involving only water across a selectively permeable membrane.

Understand: The endoplasmic reticulum (ER) is a network of membranous tubules and sheets connected to the nuclear membrane.

Two types: | Feature | Rough ER (RER) | Smooth ER (SER) | |---------|----------------|-----------------| | Ribosomes | Present on surface (giving rough look) | Absent (smooth surface) | | Main function | Manufactures proteins | Manufactures lipids (fats) |

Functions of ER:

1. Serves as channels for transport of materials (proteins, lipids) within the cell.
2. RER helps in protein synthesis (ribosomes on it).
3. SER helps in fat/lipid synthesis and detoxification of poisons in liver cells.
4. Provides a surface (cytoplasmic framework) for biochemical activities.

Conclude: RER makes proteins and SER makes lipids; together the ER transports and processes materials in the cell.

Understand: Every living organism is made up of cells.

Structural unit:

• All living organisms — whether unicellular (like Amoeba) or multicellular (like humans) — are made up of cells.
• Cells are the building blocks that make up the body structure of every organism.

Functional unit:

• All the basic life processes (respiration, nutrition, excretion, growth, reproduction) take place inside cells.
• Each cell can independently perform functions needed to keep itself alive.
• Division of labour among organelles within the cell carries out all functions of life.

Conclude: Since cells both build the body (structural) and carry out all life activities (functional), the cell is called the structural and functional unit of life.