College Biology Cells on 11 March 2025 at 13.40.46 PM

Eukaryotic Organelles

• Nucleus

  • Stores DNA

  • Double nuclear envelope with nuclear pores for selective access

  • Contains nucleolus: produces ribosomal RNA and machinery for protein synthesis

  • DNA is organized into chromosomes (46 in humans) containing genes (recipes for proteins)

  • Chromosomes are made of DNA wrapped around histone proteins forming nucleosomes

Membrane Bound Organelles

• Mitochondria

  • Powerhouse of the cell, produces energy in the form of ATP through cellular respiration

  • Contains double membranes: outer membrane and highly folded inner membrane

  • Uses glucose and oxygen to produce energy

• Chloroplasts

  • Site of photosynthesis, converting sunlight, carbon dioxide, and water into sugars (energy)

  • Contains double membranes and internal structures called thylakoids, where light reactions occur

  • Stroma is the fluid inside the chloroplast where the Calvin cycle occurs

• Ribosomes

  • Sites of protein synthesis, can be found free in cytoplasm or bound to the rough endoplasmic reticulum (RER)

  • Rough ER: Studded with ribosomes; site for protein modification and folding

  • Smooth ER: No ribosomes; involved in lipid synthesis and calcium storage, particularly in muscle cells (known as sarcoplasmic reticulum)

• Golgi Apparatus

  • Modifies, sorts, and packages proteins for secretion or use within the cell

• Lysosomes

  • Contain enzymes for digestion of waste materials and cellular debris

• Vacuoles

  • Large storage organelles, especially in plant cells, for water, nutrients, and waste products

Organelles Related to Energy Production

• Mitochondria generate ATP for animal cells.

• Chloroplasts synthesize sugars in plants.

Membrane Properties

• Fluid Mosaic Model: Describes the membrane structure, made of phospholipids and proteins that can move laterally

  • Phospholipids: Amphipathic, containing a hydrophilic phosphate head and hydrophobic fatty acid tails

  • Proteins:

    • Integral proteins: Span the membrane, allow transport and act as channels

    • Peripheral proteins: Attach to the outer or inner membrane surface

Transport Mechanisms

• Passive Transport: No energy required; moves down concentration gradient

  • Diffusion: Molecules move directly through the membrane unassisted

  • Facilitated Diffusion: Requires protein channels for larger or polar molecules

• Active Transport: Requires energy; moves against concentration gradient

  • Uses ATP to move substances into areas of higher concentration

Endosymbiotic Theory

• Mitochondria and chloroplasts likely originated from free-living bacteria that were engulfed by ancestral eukaryotic cells

• Evidence:

  • Both organelles possess their own circular DNA similar to bacterial DNA, reproduce independently, and have ribosomes similar to bacteria

Cell Communication

• Extracellular Matrix (ECM): Components secreted by cells that aid in cell adhesion, movement, and communication in multicellular organisms

  • Contains proteins like collagen which provide structure and support

• Cell junctions facilitate communication:

  • **Plasmodesmata (Plant)*: Channels between plant cell walls for nutrient sharing

  • Gap Junctions (Animal): Channels allowing small molecules and ions to pass freely between adjacent cells

  • Tight Junctions (Animal): Create impermeable barriers between cells protecting from external environmental exposure

  • Desmosomes (Animal): Anchoring junctions that hold tissues together during stretching

Summary of Functions

• Organelles work separately but collectively for cellular functions, energy production, and synthesis of necessary biomolecules.

• Collaboration of nucleus, ribosomes, ER, Golgi apparatus, and mitochondria/chloroplasts is essential for effective metabolism and cellular maintenance.