Chapter 7: Membranes


Membranes

  • needed for cells to function

  • Major Functions: to separate and comparmentalize

  • Main Characteristics:

    • membranes are made of phospholipids

    • membrane lipids form a lipid bilayer

    • Amphipathic Molecule- hydrophilic + hydophobic = Lipid Bilayer

  • Membranes also include Cholesterol.

  • Membranes are basicallt fluid; there is surface movement. (they are laterally mobile)

  • The fluidity is dependent on degree of saturation of fatty acids

    • Saturated- Fatty acids are tightly packed together (linear).

    • Unsaturated- Fatty acids are more fluid and can bend.

  • Cholestrol- other membrane lipid

    • At low temps— > cholestrol can disrupt tight packing

    • At high temps—> cholestrol packs unsaturated fatty acids, decreasing fluidity


Proteins in Membranes

  • 75% of membrane is protein

  • Peripheral Protein- proteins that are on membrane

    • they are bound to membrane lipids or other proteins in membrane, they are non-covalent.

    • Peripheral proteins can come on and off membrane


Integral Proteins

  • proteins where at least some portion protein inside lipid bilayer

    • Ex: Transmembrane protein- protein that spans entire length of membrane. There are associated with membrane

  • Major Functions of Transmembrane Protein

    • Transporters- moving substances in and out of cell

    • transmembrane proteins can be enzymes. They are involved in cell signaling (responding to stimulus)

    • Intercellular Joining—→Cell-Cell recognition


Fluid Mosaic Model

  • membrane is composed of fluid lipid bilayer with different lipids & proteins


Membranes have Different Sides

  • when vesicle pinches off of endomembrane system it fuses with plasma membrane

  • outside layer of plasma membrane is continuous with inside organelles of endomembrane system


Membrane are Selectivly Permeable

  • membranes are semi-permeable. Only certain substances can move through membrane

  • permeability is based on size & polarity

  • Polar/ Non-Polar- Hydrophobic molecule—→ permeable polar molecules.

    • Small molecules can move easily through membrane

    • Larger hydrophobic molecules have low permeability

    • Charged ions/ compound/ molecules: are NOT permeable

  • Osmosis- Diffusion of a solvent across selectively permeable membrane. (Ex: H2O)


Movement of Water Through Membrane

  • Osmosis- Diffusion of a solvent through the membrane.

  • Tonicity- Ability of a cell to gain or lose water depending on solute concentration of surrounding solution

  • Hypotonic- Lower concentration of solute, more water (More water)

  • Isotonic- Same concentration as cell

  • Hypertonic- Higher concentration of solute. (Less water)


For Large Hydrophilic Molecules/Ions

  • they need proteins to transport through membrane

  • Passive Transport- Moving substances from a higher concentration to a lower concentration (used in Facilitated Diffusion)

    • Proteins—→ Channels + Protein Carriers = passive transport

    • Channels are hydrophilic

    • Proteins carriers bind to substances non-covalently

  • Active Transport- moving substances from a lower concentration to a higher concentration

    • this required ATP

  • Type of Active Transport

    • Co-Transport: use energy from substances going down concentration gradient to couple with moving substance against its concentration gradient.

    • Bulk Transport- use of vesicles to move “cargo”

      • From one organelle to another (in endo membrane system) in and out of cell.

  • Exocytosis- movement out of cell using vesicles

  • Endocytosis- movement into cell. Making a vesicle

    • Types of endocytosis are phagocytosis, pinocytosis, receptor-mediated endocytosis

    • Phagocytosis- taking up large substances into cell, “cellular eating”

      • - creates a vesicle called phagosome

      • -Food vacuole fuses with lysosome to break down components

      • -Basically phagocytosis takes in large substances to break it down

    • Pinocytosis- taking up small substances, “cellular drinking”

    • Receptor-Mediated Endocytosis- only occurs when receptor protein on plasma membrane binds to specific substance