Cell Organelles

  • Nucleus: Largest organelle in eukaryotic cells, contains chromosomes; surrounded by nuclear envelope with nuclear pores; contains nucleoli where ribosomes are synthesized.

  • Mitochondria: Powerhouse of the cell; double-membrane structure with inner folds (cristae) for energy production; ATP is energy currency formed from glucose.

  • Ribosomes: Sites of protein synthesis, vary in number by cell type; consist of RNA and proteins; can be free in cytoplasm or fixed to the endoplasmic reticulum (ER).

  • Endoplasmic Reticulum (ER):

    • Smooth ER (SER): Synthesizes lipids and carbohydrates, detoxifies substances.
    • Rough ER (RER): Studded with ribosomes, processes and packages proteins for secretion.

  • Golgi Apparatus: Modifies, packages, and distributes proteins; consists of flattened membranes (cisternae); involved in secretion and membrane renewal.

  • Secretory Vesicles: Carry proteins from Golgi to cell surface for exocytosis or membrane renewal.

  • Lysosomes: Contain digestive enzymes; break down waste and cellular debris; involved in autophagy and endocytosis for defense.

  • Peroxisomes: Smaller than lysosomes; involved in lipid metabolism and detoxification; generate hydrogen peroxide.

Plant Cell Specific Structures

  • Cell Wall: Composed of cellulose in plants; provides structure and protection; freely permeable.

  • Chloroplasts: Sites of photosynthesis; contain thylakoids with chlorophyll; double-membrane with own DNA.

  • Vacuoles: Large storage sacs; maintain turgor pressure, especially in plants.

Cell Membrane Structure and Functions

  • Phospholipid Bilayer: Forms the basic structure; integral and peripheral proteins; cholesterol adds stability.

  • Functions:

    • Physical isolation of the cell; regulates exchange with the environment; sensitivity to external changes; provides structural support.

Membrane Transport Processes

  • Passive Transport: Movement of substances across membranes without energy; includes simple diffusion, facilitated diffusion, and osmosis.

    • Osmosis: Movement of water across a semi-permeable membrane toward higher solute concentration.

  • Active Transport: Requires energy (ATP) to move substances against their concentration gradient; includes sodium-potassium pump and coupled transport mechanisms.

  • Endocytosis/Exocytosis: Mechanisms for bulk transport of materials into and out of cells; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.