Cell: The Unit of Life

Cells are the basic structural and functional units of life. Modern cell theory states that living organisms are composed of cells and their products, and that new cells arise from pre-existing cells.

• Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles. Examples: bacteria, cyanobacteria.
• Eukaryotic cells contain a true nucleus and specialised organelles. Examples: plant cells, animal cells, fungi.
• Ribosomes occur in both groups, but internal organisation is far more elaborate in eukaryotes.

The plasma membrane follows the fluid mosaic model: a phospholipid bilayer with embedded proteins. It is selectively permeable and regulates all transport into and out of the cell.

• Diffusion moves substances down a concentration gradient without energy.
• Osmosis is movement of water across a selectively permeable membrane.
• Active transport uses ATP to move substances against a concentration gradient.

Plant cells also have a rigid cell wall made mainly of cellulose. The middle lamella (calcium pectate) holds adjacent cells together.

The nucleus stores hereditary material (DNA). The nucleolus is the site of rRNA synthesis and ribosome assembly.

• RER (Rough Endoplasmic Reticulum) — ribosomes on surface; helps in protein synthesis and transport.
• SER (Smooth Endoplasmic Reticulum) — no ribosomes; linked with lipid synthesis and detoxification.
• Golgi apparatus — modifies, packages, and dispatches biomolecules. The cis face receives vesicles from ER; the trans face sends vesicles to the membrane or lysosomes.
• Lysosomes — contain hydrolytic enzymes; responsible for intracellular digestion.

Mitochondria are the powerhouses of the cell. They produce ATP via cellular respiration. Each mitochondrion has an outer membrane and an inner membrane with cristae. They contain their own circular DNA and 70S ribosomes — evidence for the endosymbiotic theory.

Plastids are characteristic of plant cells and some algae:

• Chloroplasts — contain chlorophyll and carry out photosynthesis. Also have 70S ribosomes and circular DNA.
• Chromoplasts — provide non-green pigments (yellow, orange, red) to petals and fruits.
• Leucoplasts — colourless; mainly store starch (amyloplasts), oils (elaioplasts), or proteins (aleuroplasts).

Vacuoles store cell sap in plants and maintain turgor pressure.

The cytoskeleton provides structural support, maintains cell shape, and facilitates movement:

• Microfilaments — made of actin; involved in cell motility and contraction.
• Intermediate filaments — provide tensile strength.
• Microtubules — made of tubulin; form the spindle apparatus during cell division.

Cilia and flagella in eukaryotes show the 9+2 arrangement — 9 peripheral doublet microtubules + 2 central singlets (axoneme).

Cell connections:

• Plasmodesmata — cytoplasmic channels connecting adjacent plant cells.
• Desmosomes — strong junctions between animal cells.
• Tight junctions — seal cells together to prevent leakage between them.
• Gap junctions — allow ions and small molecules to pass between cells.