Lecture 3 Notes: Cells, Lysosomes, Golgi, Cytosol, Interstitial Fluid, and Membrane Transport

Lysosomes

  • Membranous sacs with digestive enzymes. They break down cellular components; if they burst, enzymes digest other organelles and can destroy the cell.

  • Essential for cell turnover; body recycles leftovers.

  • Purposely burst during fetal development (e.g., to create fingers/toes) and can inappropriately burst in aging brains.

  • Antioxidants can help prevent premature bursting.

  • Root word: lys or lyse means to break or burst.

Golgi apparatus

  • Stacked, flat membranous sacs.

  • Primary function: modify and package proteins for transport outside the cell.

Cytosol and Cytoplasm

  • Cytosol: clear, gel-like material within the cell where organelles float.

  • Cytoplasm: cytosol + organelles.

  • Contents: Water, glucose, O2, CO2, amino acids, proteins, electrolytes, lipids, wastes.

  • Root word: CYT or CYTE means cell.

Interstitial fluid and its role in transport

  • Fluid surrounding cells, outside the cytosol. Its contents resemble cytosol.

  • Delivers nutrients to cells and collects wastes from them.

  • Blood supplies interstitial fluid with nutrients and removes wastes.

  • Substances move into cells from interstitial fluid ($\text{IF} \to \text{PM} \to \text{cytosol}$), and out of cells ($\text{cytosol} \to \text{PM} \to \text{IF}$).

Concentration gradient

  • Difference in the amount of a substance between two points.

  • Molecules tend to move from higher concentration to lower concentration (down the gradient) due to kinetic energy and collisions.

  • Factors affecting diffusion across the plasma membrane (PM): size, charge, and lipid solubility of the material.

Membrane transport: passive vs active

  • Plasma membrane (PM) is selectively permeable, regulating substance passage.

  • Two categories of transport:

    • Passive transport: No ATP (energy) required; driven by diffusion down the concentration gradient.

    • Active transport: ATP required; moves substances against their concentration gradient (low to high).

Passive transport

  • Movement across PM without cellular energy.

  • Simple diffusion: Small, nonpolar, lipid-soluble molecules (e.g., O2, CO2) move directly through PM from high to low concentration.

  • Osmosis: Movement of water across a selectively permeable membrane towards higher solute concentration (lower water concentration).

    • Hypotonic solution: Fewer solutes than inside the cell; water enters the cell (risk of hemolysis).

    • Hypertonic solution: More solutes than inside the cell; water leaves the cell (risk of crenation).

    • Isotonic solution: Equal solute concentration; no net water movement (e.g., normal saline IV).

    • Hemolysis: RBCs burst due to excessive water intake.

    • Crenation: RBCs shrink due to water loss.

    • Edema: Fluid accumulation in tissues, sometimes treated with hypertonic IVs.

  • Facilitated diffusion: Requires carrier proteins in the PM; still passive. Moves substances (due to size or polarity) that cannot diffuse freely.

Active transport

  • Requires cellular energy (ATP) to move substances, often against their concentration gradient.

  • Simple active transport: Uses carrier proteins to move material from low to high concentration (e.g., amino acid uptake).

  • Bulk transport: Movement of large quantities via vesicles; energy-dependent.

    • Exocytosis: Vesicles inside the cell fuse with PM to release contents outside (e.g., mucus secretion).

    • Endocytosis: Materials brought into the cell via vesicles.

      • Phagocytosis (cell eating): PM engulfs large particles in vesicles (immune defense).

      • Pinocytosis (cell drinking): PM invaginates to internalize small droplets.

      • Receptor-mediated endocytosis: Specialized pinocytosis requiring receptors. Important for glucose uptake, where insulin binding promotes PM invagination to bring glucose into cells. Issues lead to elevated blood glucose levels (diabetes).

Summary of key terms and concepts

  • Lysosome: Cellular breakdown, recycling. (Root: lys/lyse = break).

  • Golgi apparatus: Protein modification and packaging.

  • Cytosol/Cytoplasm: Intracellular fluid/fluid + organelles. (Root: Cyt = cell).

  • Interstitial fluid: Extracellular fluid, nutrient/waste exchange.

  • Concentration gradient: Drives diffusion from high to low concentration.

  • Passive transport: Simple diffusion, osmosis, facilitated diffusion (no ATP).

  • Active transport: Simple active, bulk transport (uses ATP, against gradient).

  • Bulk transport: Exocytosis, endocytosis (phagocytosis, pinocytosis, receptor-mediated).

  • Receptor-mediated endocytosis: Crucial for insulin-mediated glucose uptake and blood glucose homeostasis.

This material covers membrane transport and organelles.