Cell : The Unit of Life

1. Introduction to the Cell

• The cell is the fundamental structural and functional unit of all living beings.
• It is the smallest part of an organism capable of independent existence and performing essential life functions.
• Every organism, including humans, starts its life as a single cell (the egg). Old and weak cells continually die and are replaced by new ones.

2. Invention of the Microscope & Discovery of the Cell

• Antony van Leeuwenhoek: Constructed the first simple microscope using a single biconvex lens (magnifying up to 200 times).
• Robert Hooke: Developed a compound microscope using two lenses. He examined a thin slice of cork and observed "boxlike" compartments, coining the term "cells" (meaning empty rooms).
• Electron Microscope: Uses beams of electrons bent by magnets, achieving magnifications over 200,000 times.

3. The Cell Theory

• Proposed by Matthias Schleiden (Botanist) and Theodor Schwann (Zoologist) in 1839.
• Rudolf Virchow later added that all cells arise from pre-existing cells.
• Three Major Points:
  • The cell is the smallest structural unit of all living things.
  • The cell is the functional unit of all living things.
  • All cells arise from pre-existing cells.

4. Cell Numbers, Sizes, and Shapes

• Numbers: Organisms can be single-celled (bacteria, amoeba), few-celled (Spirogyra), or multi-celled (humans, mango trees).
• Sizes:
  • Smallest: Bacteria and Red Blood Cells.
  • Longest: Nerve cells.
  • Largest: Birds' eggs (ostrich egg is the largest single cell).
• Why cells are small: Smaller cells have a larger surface area-to-volume ratio, ensuring rapid communication and greater diffusion of nutrients and respiratory gases.
• Shapes to suit functions:
  • Red Blood Cells: Circular and biconcave to transport oxygen through narrow capillaries.
  • White Blood Cells: Amoeboid to squeeze through capillary walls.
  • Nerve Cells: Long to conduct impulses.
  • Muscle Cells: Long and contractile to pull or squeeze.
  • Guard Cells: Bean-shaped to open and close leaf pores.

5. Structure of a Generalized Cell

A generalized cell consists of three essential parts: the cell membrane, the nucleus, and the cytoplasm.

Cell Boundaries

• Cell Membrane: A living, thin, flexible, and selectively semi-permeable boundary enclosing the cell.
• Cell Wall (Plants Only): A non-living, rigid layer made of cellulose. It gives shape, provides rigidity, and is freely permeable.

Cytoplasm & Organelles

• Cytoplasm: A semi-liquid substance where metabolic activities take place.
• Endoplasmic Reticulum (ER): Irregular network of tubular double membranes. Acts as a supportive framework and transport system. Can be rough (with ribosomes) or smooth.
• Ribosomes: Small granules that act as the sites of protein synthesis.
• Mitochondria: The "powerhouses of the cell". Release energy in the form of ATP during respiration. They have inner folds called cristae.
• Golgi Apparatus: Stacks of flattened sacs involved in the synthesis and secretion of enzymes and hormones.
• Lysosomes: The "suicide bags". Intracellular digestive centers that destroy foreign substances and rapidly dissolve old or injured cells.
• Centrosome (Animals Only): Contains one or two centrioles. Initiates and regulates cell division.
• Plastids (Plants Only):
  • Leucoplasts: Colourless, store starch.
  • Chromoplasts: Coloured (yellow, orange, red), impart colour to flowers and fruits.
  • Chloroplasts: Green, contain chlorophyll to trap solar energy for photosynthesis.

The Nucleus & Non-Living Inclusions

• Nucleus: The most important organelle; regulates and coordinates all life processes. Contains a network of thread-like structures called chromatin fibres, which condense into chromosomes during cell division.
• Chromosomes & Genes: Bearers of heredity. Made of DNA. Humans have a definite number of 46 (23 pairs) of chromosomes.
• Nucleolus: Present inside the nucleus; participates in protein synthesis and ribosome production.
• Vacuoles: Clear spaces containing water and solutions. Plant cells have large, prominent vacuoles containing cell-sap, which give turgidity. Animal vacuoles are small and temporary.
• Granules: Small particles storing food like starch, glycogen, and fats.

6. Key Differences: Plant vs. Animal Cells

• Plant Cells: Definite cell wall (cellulose), no centrosome, large prominent vacuoles, contain plastids, usually larger with distinct outlines, cytoplasm pushed to the periphery.
• Animal Cells: No cell wall, centrosome is present, small/temporary vacuoles (if any), no plastids, smaller size, dense and granular cytoplasm filling almost the entire cell.

7. Protoplasm

• Protoplasm is defined as the living substance of an organism, encompassing both the cytoplasm and the nucleus.
• It is a complex mixture of water, proteins, carbohydrates, fats, and mineral salts.
• An accurate chemical analysis is impossible because protoplasm ceases to be protoplasm as soon as it is removed from the organism.

8. Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells (e.g., Bacteria): Primitive cells lacking a well-defined nucleus (no nuclear membrane). Contain a single length of DNA, small ribosomes, and lack other complex cell organelles.
• Eukaryotic Cells (e.g., Plants, Animals): Have a well-defined nucleus with a nuclear membrane. Contain multiple chromosomes, larger ribosomes, and various cell organelles (mitochondria, ER, etc.).

9. Cellular Activity as Organism Activity

Every activity of a living organism is ultimately the outcome of cellular activity. Examples include:

• Growth: Due to an increase in cell size and number.
• Movement: Due to contractility of muscle cells or unequal growth in plant cells.
• Nutrition: Secretion of enzymes, absorption of digested food, and storage by cells.
• Circulation & Respiration: Muscle contractions pump blood; red blood cells transport respiratory gases.
• Protection & Senses: White blood cells destroy germs; sensory cells allow feeling, seeing, tasting.
• Reproduction & Inheritance: Production of young ones via eggs/sperms; parental traits are passed down via chromosomes.