Chapter 2: Cell 🔬
1. The Origin of Life & Introduction
It is widely accepted amongst the scientific community that life originated in water. Some researchers believe it began in small water pools with changing environmental conditions rather than vast oceans.
- Puga Valley, Ladakh: The hot springs here maintain near-boiling temperatures even in a cold climate, mimicking the early Earth (about 3.5 billion years ago).
- Thermophiles: Heat-loving, mostly unicellular bacteria found in these hot springs.
- Birbal Sahni Institute of Palaeosciences (Lucknow): Scientists studied these springs and found rapid calcium carbonate deposition. These deposits likely protected early organic molecules from harmful radiation, aiding the formation of the first protective membrane (the barrier defining a cell).
Hierarchical Organisation: Cells → Tissues → Organs → Organ Systems (e.g., nasal pores, cavity, trachea, and lungs form the respiratory system). The cell remains the fundamental unit of structure and function.
2. How to Study Cells? (Microscopy) 🔍
The limit of resolution of the human eye is 0.1 mm (when viewed from the near point of 25 cm). Objects closer or smaller appear as a single point. To study cells, microscopes are essential:
Types of Microscopes
1. Light Microscope: Uses visible light and a combination of lenses (objective and eyepiece). Magnification is the product of both lenses (e.g., 10X eyepiece × 10X objective = 100X total magnification). It improves resolution, contrast, and magnification.
2. Electron Microscope: Uses a powerful beam of electrons instead of light to produce highly magnified images, revealing fine details at the nanometre scale (1 nm = one-billionth of a metre).
🧪 Activity 2.1: Estimating the Size of a Cell
Procedure: Focus a transparent millimetre ruler under a light microscope. Measure the diameter of the circular field of view in mm and convert to micrometres (1 mm = 1000 µm). Replace the ruler with an onion peel slide. Count the number of cells across the diameter.
Calculation Formula:
Size = 5000 / 25 = 200 µm per cell!
Robert Hooke (1665) observed a thin slice of cork under a self-designed microscope (200-300X magnification). He saw box-like compartments and coined the term “cells”.
3. Structure of a Cell: The Boundary
a. Cell Membrane (Plasma Membrane)
The universal, extremely thin (7 to 10 nm thick) boundary that protects cellular contents and defines the cell’s individuality.
- It is selectively permeable: allows some substances to pass while blocking others.
- Fluid-Mosaic Model: Explains its structure. It consists of a lipid bilayer (fats with water-attracting heads facing outwards and water-repelling tails inwards) with embedded proteins.
- It is “fluid” because molecules can move sideways, flip, and rotate. The proteins act as gatekeepers facilitating the passage of specific substances.
Diffusion vs. Osmosis
Diffusion: The net movement of particles from a region of higher concentration to lower concentration (concentration gradient). *E.g., O₂ and CO₂ exchange in alveolar cells.*
Osmosis: The diffusion of water across a selectively permeable membrane from a dilute solution (more water, less solute) to a concentrated solution (less water, more solute).
| Extracellular Solution Type | Condition | Result on Cell |
|---|---|---|
| Isotonic | Outside concentration = Inside concentration | No net movement. Cell stays the same. |
| Hypotonic | Outside is dilute (less solute, more water) | Water enters cell. Cell swells. |
| Hypertonic | Outside is concentrated (more solute, less water) | Water leaves cell. Cell shrinks. |
🧪 Activity 2.2: Potato Osmometer
Two roughly equal potato pieces are weighed. Piece A is placed in plain water (Hypotonic); Piece B is placed in a 20% salt/sugar solution (Hypertonic). After an hour, Piece A swells (gains weight due to endosmosis), while Piece B shrinks (loses weight due to exosmosis).
b. Cell Wall (Outer Covering)
Plants, fungi, and bacteria possess an additional covering outside the cell membrane called the cell wall. Plants require this rigid structure to withstand environmental stresses (wind, rain) since they are immotile.
- In plants, it is primarily composed of Cellulose (a carbohydrate of linked glucose units that acts as roughage in human digestion).
- It helps leaves/flowers remain firm, maintains shape, and keeps plants upright.
- Unlike the membrane, the cell wall is permeable, allowing water and dissolved minerals to pass through.
🧪 Activity 2.3: Onion/Rhoeo vs. Human Cheek Cells
Observations: Onion peel or Rhoeo (Cradle lily) leaf cells are box-shaped and regularly arranged. Human cheek cells are irregularly arranged because animal cells lack a rigid cell wall. When 20% sugar solution is added to Rhoeo cells, the inner contents shrink away from the rigid cell wall (plasmolysis), but the overall boundary size remains the same.
4. Prokaryotic vs. Eukaryotic Cells 🦠
All cells possess a plasma membrane, cytoplasm (semi-fluid, jelly-like substance), and genetic material. Based on nuclear organisation, cells are classified as:
| Characteristics | Prokaryotic Cell (e.g., Bacteria) | Eukaryotic Cell (e.g., Plants, Animals) |
|---|---|---|
| Nucleus | Primitive. Genetic material (single circular DNA) lies in an undefined region called the Nucleoid. | True, well-defined nucleus enclosed by a double nuclear membrane. |
| Size | Typically 1 to 10 µm. | Typically 10 to 100 µm. |
| Organelles | Lacks membrane-bound organelles. Cellular activities occur directly in cytoplasm. | Contains specialised membrane-bound organelles (Mitochondria, ER, etc.). |
| Organism Type | Usually unicellular. | Can be unicellular or multicellular. |
Viruses: Genetic material enclosed in a protein coat.
Viroids: Infectious RNA lacking a protein coat.
Prions: Infectious, misfolded proteins lacking genetic material.
5. The Cell Interior: Organelles ⚙️
Eukaryotic cells are like tiny living factories. They use a network of fine fibres called the Cytoskeleton for structural support, shape maintenance, internal transport, and movement. Cells may also contain Cell Inclusions (stored inactive materials like starch, calcium oxalate, or silica crystals in plants).
🧠 1. Nucleus: House of Coded Instructions
Surrounded by a double-layered nuclear membrane with pores to allow material transfer to the cytoplasm. It contains a dense round body, the Nucleolus, which is the site of ribosomal subunit synthesis.
- Contains DNA (Deoxyribonucleic acid) which holds genetic information. Functional segments of DNA are called genes.
- In a non-dividing cell, DNA is an entangled mass called Chromatin.
- When dividing, chromatin organises into rod-shaped Chromosomes (composed of DNA and specific proteins).
🏭 2. Endoplasmic Reticulum (ER) & 🔵 3. Ribosomes
Ribosomes are tiny structures (free in cytoplasm or attached to ER) responsible for protein synthesis.
The Endoplasmic Reticulum is a large continuous network of tubes connected to the outer nuclear envelope. It transports and synthesises materials.
- Rough ER (RER): Appears rough under an electron microscope due to attached ribosomes. Synthesises and secretes proteins (e.g., in pancreatic gland cells).
- Smooth ER (SER): Lacks ribosomes. Involved in the synthesis and storage of fats (lipids) and certain hormones.
📦 4. Golgi Apparatus
Observed first by Camillo Golgi (1898) in the nerve cells of a barn owl using special staining techniques. It consists of stacks of flattened, sac-like structures. It acts as the cell’s post office: it modifies, sorts, and packages proteins and lipids into vesicles for transport, secretion, or lysosome formation.
🗑️ 5. Lysosomes: The Clean-up System
Single membrane-bound sacs filled with powerful digestive enzymes. They break down unwanted proteins, carbohydrates, fats, worn-out organelles, and damaged parts, keeping the cell clean.
*Fun Fact: Human sperm cells contain lysosomal enzymes to break down the outer layer of the egg, allowing fertilisation!
⚡ 6. Mitochondria: Powerhouse of the Cell
Responsible for energy supply via cellular respiration (breaking down glucose to release energy). The energy is stored as ATP (Adenosine Triphosphate), the energy currency of the cell.
- Surrounded by two membranes. The outer is smooth and porous.
- The inner membrane is folded into finger-like projections called Cristae, which drastically increase the surface area for energy-producing chemical reactions.
- Unique feature: They have their own DNA and ribosomes!
🌿 7. Plastids (Only in Plant Cells)
Used for food synthesis and storage. Like mitochondria, plastids possess their own DNA and ribosomes, suggesting a shared evolutionary history with bacteria.
- Chloroplasts: Double-membrane bound. Contain a semi-fluid stroma and disc-shaped structures bearing green chlorophyll. They absorb light energy for photosynthesis. Sugars synthesised are stored in the stroma as starch granules.
- Chromoplasts: Contain pigments (yellow, orange, red) providing bright colours to flower petals and fruits to attract pollinators and seed-dispersing animals.
- Leucoplasts: Colourless plastids that store food materials like starch, oils, or proteins (e.g., found in potato and Colocasia/taro).
💧 8. Vacuoles: Storage and Support
In mature plant cells, there is usually one large central vacuole surrounded by a single selectively permeable membrane. It is filled with a watery fluid called cell sap.
- Stores water, minerals, sugars, and waste.
- By storing large amounts of water, it maintains internal pressure, keeping the plant cell firm (turgid). Loss of water causes the plant to wilt.
- In animal cells, vacuoles are smaller and used only for temporary storage.
His team sequenced the DNA of a bacterium (*Mycoplasma mycoides*). They chemically synthesised an exact copy of this DNA in a lab and inserted it into a closely related cell (whose own DNA was removed, keeping the cytoplasm and membrane intact). The cell grew and divided following instructions from the synthetic DNA! This proved that DNA masterminds the structure and activities of the cell.
6. How do Normal Cells Grow and Divide? 🧬
Every day, an estimated hundreds of billions of cells in our body are replaced (almost 1% of total cells). Cells cannot grow indefinitely in size; growth occurs through cell division. In eukaryotes, this is an orderly process called the Cell Cycle.
🧪 Activity 2.5: Onion Root Tip Experiment (Studying Cell Division)
1. Grow an onion bulb over a water jar.
2. Cut the freshly grown root tips (2-3 cm) and place them in freshly prepared aceto-alcohol (glacial acetic acid : ethanol :: 1:3) for 24 hours. Transfer to 70% ethanol for preservation.
3. Soften the tissue with dilute Hydrochloric acid (HCl) for 10-15 mins.
4. Add 2-3 drops of aceto-carmine stain, warm gently over a spirit lamp.
5. Squash the coverslip to spread the cells and observe. The cells of a growing tip divide continuously, displaying various structural stages of cell division.
A. Mitosis
Purpose: Normal growth, repair of damaged tissues, maintenance, and asexual reproduction.
- One parent cell divides once to form two genetically identical daughter cells.
- Each new cell gets the same DNA and the same number of chromosomes as the parent cell.
B. Meiosis
Purpose: Occurs only in reproductive organs to produce gametes for sexual reproduction (creates genetic diversity!).
- In animals: occurs in male testes (produces sperm) and female ovaries (produces eggs).
- In plants: occurs in anthers (pollen/sperm) and ovaries (eggs).
- It is a two-step division resulting in four daughter cells.
- The number of chromosomes in each gamete is reduced to half. When gametes fuse during fertilisation, the original number is restored.
Arun Kumar Sharma was a famous Indian botanist renowned for his work on chromosomes, plant taxonomy, and evolution. He invented useful lab methods to study plant chromosomes and received the Shanti Swarup Bhatnagar award and Padma Bhushan.
7. Cell Theory & Regulatory Mechanisms ⚖️
The Classical Cell Theory
Formulated by combining the findings of three prominent scientists:
- Matthias Schleiden (1838, Botanist): All plants are made of cells.
- Theodor Schwann (1839, Zoologist): All animals are made of cells.
- Rudolf Virchow (1855): Proposed that all cells arise from pre-existing cells.
Totipotency (Gottlieb Haberlandt, 1902): Proposed that any living plant cell (even a mature one) can develop into a complete plant if provided suitable nutrients and conditions. This is the foundation of modern Plant Tissue Culture Technology.
Do cells grow and reproduce forever?
No. Cells grow in a controlled way, carry out functions, and die when no longer needed. They have a definite lifespan.
- Contact Inhibition: In many animal cells, cell division stops when they physically touch neighbouring cells. Plant cells grow differently due to rigid walls and do not show contact inhibition.
- Tumours & Cancer: When normal regulatory systems fail (often due to errors in mitosis), cells lose contact inhibition and divide uncontrollably, forming tumours (benign or malignant). Cancerous cells can invade nearby tissues and spread.
- Programmed Cell Death (PCD): A genetically regulated, highly organised process of selective cell destruction. Essential for quality control, immune function, and normal development (e.g., PCD eliminates cells between digits during embryonic development to prevent webbed hands).
- Errors in Meiosis: Faulty meiosis can lead to abnormal chromosome numbers, causing genetic disorders, distinctive physical features, early pregnancy loss, or reduced fertility.
🔬 Bridging Science and Society: Cell Culture
Scientists have developed methods to grow plant and animal cells outside the body in special conditions, known as cell culture. Cells are placed in a nutrient-rich medium maintaining specific temperature, pH, moisture, and sterile conditions. It is crucial for producing biochemicals, food, medicines, and vaccines.