The Cell — Fundamental Unit of Life

• Why the cell is called the fundamental unit of life.
• Prokaryotic vs eukaryotic cells.
• Structure and function of all major cell organelles.
• Differences between plant and animal cells.
• Cell division — mitosis and meiosis basics.

What is a cell?

• Cell: smallest structural and functional unit of life.
• First observed by Robert Hooke (1665) in cork (dead plant cells) using his compound microscope; named them "cells" (resembled small rooms in a monastery).
• First living cells observed by Antonie van Leeuwenhoek (1670s).
• Cell Theory (Schleiden, Schwann, Virchow — 1830s–1850s):
  1. All living things are made of cells.
  2. The cell is the basic unit of life.
  3. All cells arise from pre-existing cells.

Cell size and shape

• Most cells: 1–100 micrometres (μm); invisible to naked eye.
• Exceptions: ostrich egg yolk (largest single cell; ~15 cm); human nerve cells (longest; up to 1 metre).
• Shape varies by function: disc (RBC), elongated (muscle), branched (nerve), spherical (egg).

Prokaryotic vs Eukaryotic cells

The plasma membrane (cell membrane)

• Structure: phospholipid bilayer with embedded proteins (Fluid Mosaic Model — Singer & Nicolson, 1972).
• Function:
  • Controls what enters and leaves the cell (selectively permeable).
  • Maintains cell shape.
  • Cell-to-cell communication.
• Osmosis: movement of water from high water concentration to low water concentration through a semi-permeable membrane.
  • Plasmolysis: plant cell placed in concentrated salt solution → water leaves → cytoplasm shrinks away from cell wall.
  • Turgidity: plant cell in water → water enters → cell swells → presses against wall → turgor pressure → plant stays upright.

Cell wall (plants only)

• Made of cellulose (a polysaccharide).
• Rigid; provides structural support; prevents over-expansion.
• Absent in animal cells.
• Fully permeable (lets everything through; plasma membrane controls entry).

The Nucleus

• Control centre of the cell.
• Bounded by nuclear membrane (double membrane with pores).
• Contains chromosomes (made of DNA + histone proteins).
• Nucleolus: dense region inside nucleus; makes RNA for ribosomes.
• DNA: carries genetic information; instructions for all cell activities.
• Human cells: 46 chromosomes (23 pairs).

Cell organelles

Endoplasmic Reticulum (ER)

Golgi Apparatus (Golgi Body)

• Stack of flattened membrane sacs (cisternae).
• Function: receives proteins from Rough ER → modifies, packages, sorts → ships to destination (cell membrane, lysosomes, outside cell).
• Called the "post office" or "traffic police" of the cell.

Mitochondria

• Double membrane: outer (smooth) + inner (folded into cristae to increase surface area).
• Matrix: fluid inside inner membrane; contains enzymes + mitochondrial DNA (own DNA!).
• Function: cellular respiration — breaks down glucose → releases ATP (energy).
• Called "powerhouse of the cell".
• Semi-autonomous: has its own DNA and ribosomes → evidence it was once a free-living bacterium (endosymbiotic theory).

Chloroplasts (plants only)

• Double membrane + internal membrane system (thylakoids stacked into grana).
• Contains chlorophyll (green pigment).
• Function: photosynthesis — converts CO₂ + H₂O + light → glucose + O₂.
• Also semi-autonomous (own DNA).

Ribosomes

• Tiny; no membrane; made of RNA + protein.
• Function: protein synthesis (translation of mRNA).
• Found: free in cytoplasm (make proteins for cell use) OR on Rough ER (make proteins for export).
• All cells — prokaryotic and eukaryotic — have ribosomes.

Lysosomes

• "Suicide bags of the cell" (called so by Christian de Duve).
• Membrane-bound sacs containing digestive enzymes.
• Function:
  • Digest food particles taken in by the cell.
  • Break down old/damaged organelles (autophagy).
  • During cell death — rupture and digest the cell itself (autolysis).
• Common in animal cells; rare in plants.

Vacuoles

Cell Centrosome / Centrioles

• Present in animal cells (absent in plant cells).
• Involved in cell division — form the spindle fibres that pull chromosomes apart.

Plant cell vs Animal cell

Cell division — basics

Mitosis

• Type of division that produces 2 identical daughter cells.
• Used for: growth, repair, replacement of old cells.
• Daughter cells have same chromosome number as parent (46 in humans).
• Phases: PMAT — Prophase → Metaphase → Anaphase → Telophase → Cytokinesis.

Meiosis

• Type of division that produces 4 daughter cells with half the chromosomes (23 in humans).
• Used for: producing sex cells (gametes) — sperm and egg.
• Ensures offspring have correct chromosome number after fertilisation.
• Crossing over during meiosis creates genetic variation.

• Who first observed cells? What did he observe?
• What is the difference between prokaryotic and eukaryotic cells? Give one example of each.
• Name three organelles found in plant cells but NOT in animal cells.
• What is the function of mitochondria? Why is it called the powerhouse of the cell?
• What is osmosis? What is plasmolysis?

Open the Practice tab for graded questions on the Cell.