Cell: The Building Block of Life

• Life is widely accepted to have originated in water. Some researchers think it began in small water pools with changing conditions (like hot springs), not in the oceans.
• Puga Valley (Ladakh): hot springs stay near boiling point even in a cold climate, similar to the early Earth (about 3.5 billion years ago). They are home to heat-loving, unicellular bacteria called thermophiles.
• Scientists of the Birbal Sahni Institute of Palaeosciences, Lucknow found calcium carbonate forming rapidly around these springs. It may have protected early organic molecules from radiation and helped form the first protective membrane, the barrier that defines a cell.
• All living organisms are made of cells, the basic level at which life exists. Unicellular: bacteria, yeast. Multicellular: plants, fish, birds, humans.

• Tissue: a group of similar cells doing similar functions. Different tissues form an organ; several organs working together form an organ system.

• A convex lens, or an objective lens + eyepiece combination, magnifies objects.
• Robert Hooke (1665): first to observe cells, using a self-made microscope (about 200–300X) on a thin slice of cork. He saw box-like compartments and named them 'cells'.
• Parts of a light microscope: eyepiece, body tube, objective lens, stage, mirror, coarse and fine adjustment knobs, handle, base. School objective lenses are 10X, 40X.
• Microscopes improved in three features: resolution (clarity), contrast (difference in brightness between parts) and magnification.
• Electron microscope: uses a beam of electrons instead of light; shows cell structure at the nanometre scale. Most organelles are visible only with it.

• Unit conversion: $1\ \text{mm} = 1000\ \mu\text{m}$; $1\ \mu\text{m} = 1000\ \text{nm}$.
• Example: field diameter $5$ mm $= 5000\ \mu\text{m}$; $25$ cells across → cell size $= 5000/25 = 200\ \mu\text{m}$. With $10\text{X}$ eyepiece and $10\text{X}$ objective, magnification $= 100\text{X}$.
• Method: measure the field with a transparent mm ruler first, then count cells in a straight line along the same diameter.

• Structure: about 7–10 nm thick, made of lipids and proteins.
• Interactions with other cells and the environment (exchange of materials, response) occur at the membrane, even in unicellular organisms.

• Activity 2.2 (potato pieces): piece in plain water swells (weight increases); piece in 20% salt/sugar solution shrinks (weight decreases). Water moves through the membrane, but the salt/sugar molecules do not.
• Fair test: cut the two pieces of roughly equal size and record initial weights so changes can be compared.
• Application: adding a lot of salt or sugar (pickles, murabbas, sharbat) makes the surroundings hypertonic, so water leaves microbes and spoilage is prevented.

• Present in: plant, fungal and bacterial cells, as an extra layer outside the cell membrane. Absent in animal cells.
• Functions: rigid support against wind and rain; keeps leaves and flowers firm; maintains shape; helps plants stay upright; protection and structural support in fungi and bacteria.
• Permeable: water and some dissolved minerals pass through. With the selective membrane it helps roots absorb water and nutrients.
• Composition (plants): mainly cellulose, a carbohydrate made of many glucose units. Cellulose in our diet is roughage and helps digestion.

• No rigid wall lets animal cells change shape easily, supporting movement and tissue function.

• Most cells have three basic parts: a plasma membrane, cytoplasm (semi-fluid, jelly-like) and a nucleus (or nucleoid). The cytoplasm also holds organelles (sub-cellular components).
• Prokaryotic (pro = primitive, karyon = nucleus): no well-defined nucleus and no membrane-bound organelles; activities occur directly in the cytoplasm. Eg: bacteria.
• Eukaryotic (eu = true): well-defined nucleus and membrane-bound organelles. Eg: plant and animal cells.

• Beyond: the cytoskeleton (network of fine fibres in eukaryotes) gives support, shape, movement and internal transport. Cell inclusions are stored substances: starch (plant cells), crystals of calcium oxalate or silica.
• Acellular agents: viruses (genetic material + protein coat), viroids (no protein coat) and prions (misfolded proteins, no genetic material) are not cells and are too small for a light microscope.

• Nuclear membrane: double-layered, with nuclear pores for transfer of material between nucleus and cytoplasm.
• Nucleolus: dense round body; site of synthesis of ribosomal subunits. They leave for the cytoplasm, where one large and one small subunit assemble into a ribosome.
• Chromatin: entangled thread-like mass in a non-dividing cell. Before division it organises into rod-shaped chromosomes.
• Chromosomes = DNA + specific proteins; carry inheritance from parents to offspring. DNA (deoxyribonucleic acid) holds genetic information; its functional segments are genes.

• Prokaryotes: DNA is a single circular molecule with proteins in a region called the nucleoid.
• Specialised cells: mature human RBCs have no nucleus (enucleate). This leaves more room for haemoglobin to carry oxygen, but they cannot repair or divide, so they live only about 120 days.
• Synthetic DNA experiment (J. Craig Venter, 2010): a synthetic copy of Mycoplasma mycoides DNA was put into another bacterium whose DNA had been removed; the cell grew and divided following the new DNA. It shows that DNA controls the structure and activities of a cell. Only the DNA was synthetic; the rest of the cell was already living.

A eukaryotic cell is like a tiny living factory: organelles build materials, remove waste and supply energy, and work together.

• Camillo Golgi (1898): an Italian scientist who first saw the Golgi apparatus in nerve cells of a barn owl using special stains. Early microscopes could not resolve it, so many doubted it; electron microscopy confirmed it decades later and it was named in his honour.
• Sperm: human sperm cells contain lysosomal enzymes that help break down the outer layer of the egg during fertilisation.

• Similarity of mitochondria and plastids to bacteria: both have their own DNA and ribosomes and can make some of their own proteins, suggesting a shared evolutionary history with single-celled organisms.
• Mitochondria vs chloroplasts: both are double-membrane organelles with their own DNA; mitochondria release energy (respiration) and are in plant and animal cells, chloroplasts make food (photosynthesis) and are only in plant cells.

• Mature plant cell: usually one large central vacuole bounded by a single selectively permeable membrane, filled with cell sap.
• Stores: water, minerals, sugars and waste material.
• Support: stored water maintains pressure inside the cell, keeping it firm. When water is scarce, the vacuole loses water, cells become less firm and the plant wilts.
• Animal cells: vacuoles are sometimes present, small, for temporary storage.

• Why cells divide: to replace old, dead or damaged cells (healing a cut, new hair) and for growth. Growth is mainly due to cell division, not cells growing bigger; cells can only grow to a certain size.
• Activity 2.5 (onion root tip): cells of a growing root tip divide continuously, so slides show cells in different stages of division. Steps: grow roots in water (5–6 days), fix in aceto-alcohol (glacial acetic acid : ethanol = 1 : 3) and preserve in 70% ethanol, soften with dilute HCl, stain with aceto-carmine, warm gently, squash under a coverslip, observe.
• Hundreds of billions of cells (about 1% of body cells) are replaced every day.
• Both prokaryotic and eukaryotic cells divide; eukaryotic cells divide in an orderly, controlled way through the cell cycle (stages studied in higher grades).

• Meiosis details: division 1 halves the chromosome number (2 cells); division 2 is like mitosis (4 cells). On fertilisation two gametes combine and restore the original chromosome number. Variation is why children resemble parents but are not identical.
• A human starts as one fertilised egg that divides repeatedly (mitosis) into trillions of cells.
• Errors in mitosis: uncontrolled division, tumours, abnormal chromosome number in body cells. Errors in meiosis: genetic disorders, early pregnancy loss, reduced fertility.
• Cell culture: growing plant or animal cells outside the body in a nutrient-rich medium under sterile conditions with suitable temperature, pH and moisture; used to study cells and make biochemicals, food, medicines and vaccines.
• Haberlandt (1902) and totipotency: any living plant cell can grow into a complete plant with suitable nutrients and conditions; this ability is totipotency and led to Plant Tissue Culture Technology.
• Arun Kumar Sharma: Indian botanist known for work on chromosomes and for lab methods to study plant chromosomes (Shanti Swarup Bhatnagar award, Padma Bhushan).

• The theory unifies all biology, from bacteria to humans, and explains the continuity of life through cell division.
• Every cell has a definite life span. Cells grow and divide in a controlled way, stay in place, do their job and die when no longer needed; dead cells are replaced.
• Contact inhibition: in many animal cells, division stops when cells touch neighbouring cells. Plant cells do not show it because of rigid cell walls; they follow a different growth pattern.
• Cancer cells lose this control and divide uncontrollably, forming tumours (benign or malignant). Malignant tumours invade nearby tissue and spread to other parts.
• Programmed Cell Death (PCD): a genetically regulated, organised destruction of cells, essential for development, quality control and immunity. Eg: it removes cells between developing fingers; without it we would have webbed hands.