The Fundamental Unit of Life

Historical Discovery of the Cell

• Robert Hooke (1665): While examining a thin slice of cork (a substance derived from the bark of a tree) using a self-designed, primitive microscope, Hooke observed that the cork resembled a honeycomb structure consisting of many small compartments. He named these boxes "cells," which is the Latin word for "a little room."

• Significance: Although seemingly insignificant, this was the first recorded instance of someone observing that living things are composed of separate units. The term "cell" remains the standard biological descriptor to this day.

• Leeuwenhoek (1674): Utilizing an improved microscope, he discovered free-living cells in pond water for the first time.

• Robert Brown (1831): Discovered the nucleus within the cell.

• Purkinje (1839): Coined the term "protoplasm" to describe the fluid substance found inside the cell.

• Schleiden (1838) and Schwann (1839): These two biologists presented the Cell Theory, stating that all plants and animals are composed of cells and that the cell is the basic unit of life.

• Virchow (1855): Expanded the cell theory by suggesting that all cells arise from pre-existing cells.

• Electron Microscope (1940): The discovery of the electron microscope allowed for the detailed observation and understanding of the cell's complex structure and its various organelles.

Understanding Living Organisms

• Activity 5.1 (Onion Peel Mount):     * Procedure: Extract a small piece from an onion bulb. Use forceps to peel the skin (epidermis) from the concave inner layer. Immediately place the peel in a watch-glass of water to prevent folding or drying. Transfer a small piece to a glass slide with a drop of water, ensuring it is flat (using a paintbrush if needed). Add a drop of safranin solution (stain) and apply a cover slip using a mounting needle, avoiding air bubbles. Observe under a compound microscope.     * Microscope Components: Eyepiece, coarse adjustment, fine adjustment, arm, objective lens, stage, swivel, mirror, base, body tube, clip, microscope slide, and condenser.     * Observation: One sees small, similar structures (cells) with a nucleus and clear boundaries.

Unicellular and Multicellular Organisms

• Unicellular Organisms: A single cell constitutes the entire organism. Examples include Amoeba, Chlamydomonas, Paramoecium, and bacteria.

• Multicellular Organisms: Many cells group together in a single body to assume different functions and form various body parts. Examples include fungi, plants, and animals.

• Origin of Multicellularity: Every multicellular organism begins as a single cell. Cells divide to produce cells of their own kind; thus, all cells come from pre-existing cells.

• Variety of Human Cells: The human body contains cells of different kinds, such as:     * Sperm cells     * Ovum     * Bone cells     * Blood cells     * Smooth muscle cells     * Nerve cells (typically have a fixed, peculiar shape)     * Fat cells

Structural Organization of the Cell

Every cell typically exhibits three main features regardless of its specific function:

1. Plasma Membrane (or Cell Membrane)

2. Nucleus

3. Cytoplasm

5.2.1 Plasma Membrane (Cell Membrane)

• Definition: The outermost covering of the cell that separates cell contents from the external environment. It is composed of organic molecules called lipids and proteins.

• Function: It permits the entry/exit of some materials while preventing others. It is therefore described as a "selectively permeable membrane."

• Diffusion: The spontaneous movement of substances (like $CO_2$ or $O_2$) from a region of high concentration to a region of low concentration.     * Example: $CO_2$ (cellular waste) accumulates in high concentrations inside the cell. Because the external environment has a lower concentration, $CO_2$ moves out by diffusion. Oxygen behaves similarly, entering the cell when internal levels decrease.

• Osmosis: The passage of water from a region of high water concentration through a selectively permeable membrane to a region of low water concentration until equilibrium is reached.

• Sugar/Salt Solutions and Osmosis:     1. Hypotonic Solution: The medium has higher water concentration (dilute) than the cell. The cell will gain water and swell.     2. Isotonic Solution: The medium has the exact same water concentration as the cell. There is no net movement of water; the cell stays the same size.     3. Hypertonic Solution: The medium has a lower concentration of water (concentrated) than the cell. The cell will lose water and shrink.

• Applications of Osmosis: Unicellular freshwater organisms and most plant cells gain water through osmosis. Plant roots absorb water via osmosis.

• Endocytosis: The flexibility of the plasma membrane allows the cell to engulf food and other materials from the environment. This is how Amoeba acquires its food.

5.2.2 Cell Wall

• Location: Found in plant cells, located outside the plasma membrane. It is a rigid outer covering.

• Composition: Mainly composed of cellulose, a complex substance providing structural strength to plants.

• Plasmolysis: When a living plant cell loses water via osmosis, the cell contents shrink or contract away from the cell wall.

• Function: Permits plant, fungi, and bacterial cells to withstand hypotonic external media without bursting. The cell swells and builds pressure against the wall, and the wall exerts equal pressure back.

5.2.3 Nucleus

• Structure: Has a double-layered covering called the nuclear membrane. It contains pores to allow the transfer of material to the cytoplasm.

• Chromosomes: Rod-shaped structures visible only during cell division. They are composed of $DNA$ (Deoxyribo Nucleic Acid) and protein.

• DNA and Genes: $DNA$ molecules contain information for inheritance and cell organization. Functional segments of $DNA$ are called genes.

• Chromatin: In non-dividing cells, $DNA$ exists as chromatin, an entangled mass of thread-like structures.

• Function: The nucleus directs chemical activities, plays a central role in cellular reproduction (division), and determines cell development and maturity.

• Prokaryotic vs. Eukaryotic Nucleus:     * Prokaryotes: Lack a nuclear membrane. The undefined nuclear region containing only nucleic acids is called a nucleoid.     * Eukaryotes: Possess a well-defined nuclear membrane.

5.2.4 Cytoplasm

• Definition: The fluid content inside the plasma membrane. It contains many specialized cell organelles.

• Organelles: Membrane-bound structures that perform specific functions. In prokaryotes, membrane-bound organelles are absent.

• Viruses: Viruses lack membranes and do not show characteristics of life until they enter a living host to use its machinery for multiplication.

Cell Organelles

5.2.5 (i) Endoplasmic Reticulum (ER)

• Description: A network of membrane-bound tubes and sheets, similar in structure to the plasma membrane.

• Rough ER (RER): Appears rough due to attached ribosomes. Ribosomes are the sites of protein manufacture.

• Smooth ER (SER): Manufactures fat molecules (lipids). In vertebrate liver cells, SER detoxifies poisons and drugs.

• Membrane Biogenesis: The process where proteins and lipids manufactured by the ER are used to build the cell membrane.

• Function: Serves as a transport channel for materials (especially proteins) and provides a surface for biochemical activities.

5.2.5 (ii) Golgi Apparatus

• Description: First described by Camillo Golgi, it consists of membrane-bound vesicles (flattened sacs) called cisterns arranged in parallel stacks.

• Function: Storage, modification, and packaging of products in vesicles. It may convert simple sugars into complex sugars and is involved in forming lysosomes.

• Camillo Golgi (1843-1926): Developed the "black reaction" (using silver nitrate) to stain individual nerve cells. Shared the Nobel Prize in 1906 with Santiago Ramony Cajal.

5.2.5 (iii) Lysosomes

• Structure: Membrane-bound sacs filled with digestive enzymes (made by RER).

• Function: Waste disposal system. They digest foreign material (bacteria, food) and worn-out organelles by breaking complex substances into simpler ones.

• "Suicide Bags": If a cell is damaged, lysosomes may burst and the enzymes digest the cell itself.

5.2.5 (iv) Mitochondria

• Powerhouse of the cell: They generate energy in the form of $ATP$ (Adenosine triphosphate) molecules, known as the "energy currency" of the cell.

• Structure: Two membrane coverings; the outer is porous, the inner is deeply folded to increase surface area for $ATP$ generation.

• Unique Feature: They possess their own $DNA$ and ribosomes, allowing them to make some of their own proteins.

5.2.5 (v) Plastids

• Location: Found only in plant cells.

• Types:     1. Chromoplasts: Coloured plastids. Chloroplasts are chromoplasts containing chlorophyll (essential for photosynthesis) and other yellow/orange pigments.     2. Leucoplasts: White or colourless plastids used for storing starch, oils, and protein granules.

• Structure: Internal organization consists of membrane layers (thylakoids) embedded in a material called stroma. Like mitochondria, they have their own $DNA$ and ribosomes.

5.2.5 (vi) Vacuoles

• Description: Storage sacs for solids or liquids. Small in animals; very large in plants (occupying $50\text{-}90\%$ of cell volume).

• Function in Plants: Full of cell sap, providing turgidity and rigidity. Store amino acids, sugars, organic acids, and proteins.

• Function in Single-Celled Organisms: Amoeba uses food vacuoles. Some unicellular organisms use specialized vacuoles to expel excess water and waste.

5.3 Cell Division

New cells are formed for growth, replacement of dead/injured cells, and gamete formation for reproduction.

1. Mitosis: The process of division for growth. One mother cell divides into two identical daughter cells with the same number of chromosomes as the mother cell. This helps in growth and tissue repair.

2. Meiosis: Occurs in reproductive organs to form gametes. It involves two consecutive divisions, resulting in four new cells. These new cells have only half the number of chromosomes ($\frac{1}{2}n$) compared to the mother cell.

Questions and Comparison Table

Differences Between Prokaryotic and Eukaryotic Cells

Questions & Discussion

1. Who discovered cells, and how? Robert Hooke discovered cells in 1665 by observing a thin slice of cork under a self-designed microscope.

2. Why is the cell called the structural and functional unit of life? Because all living organisms are composed of cells, and the cell is the smallest unit capable of performing all life-sustaining functions (respiration, nutrition, waste clearance).

3. How do $CO_2$ and water move in/out? $CO_2$ moves by diffusion (high to low concentration). Water moves by osmosis through the selectively permeable membrane.

4. Why is the plasma membrane called selectively permeable? Because it regulates the entry and exit of specific materials while blocking others.

5. Which organelles contain their own genetic material? Mitochondria and Plastids.

6. Why are lysosomes called suicide bags? Because they contain powerful digestive enzymes that can digest the whole cell if it becomes damaged or malfunctions.

7. Where are proteins synthesized? On the ribosomes, which are often attached to the RER.

Summary of Learned Concepts

• The cell is the fundamental organizational unit of life.

• Plasma membranes are composed of lipids and proteins.

• Plant cell walls are made of cellulose; they protect against hypotonic media.

• Nucleus contains $DNA$ and directs life processes.

• ER facilitates transport and manufacturing.

• Golgi apparatus packages and modifies substances.

• Mitochondria provide energy ($ATP$).

• Chloroplasts conduct photosynthesis; Leucoplasts store materials.

• Vacuoles maintain plant turgidity.

• Cells divide via mitosis (growth) or meiosis (gametes).