5.1 Membrane Structure

• Membranes have three components

  • Phospholipids: forms basic matrix

  • Proteins: embedded into or around membrane, aids in transport

  • carbohydrates: may be attached to lipid or protein

Organization of a membrane

• Framework of membrane called Phospholipid Bilayer

  • made of two layers of phospholipids

  • Non-polar tails face inward, polar heads face outwards

Proteins in membrane

• Three types of proteins

  • Transmembrane Proteins: embedded inside phospholipid bilayer

  • Lipid-anchored proteins: lipid tail embedded inside phospholipid bilayer

  • peripheral membrane proteins: non-covalently bound to proteins near or on the surface of membrane

5.2 Fluidity of Membranes

Membranes are semifluid

• Allows for movements in two dimensions

• Two types of movement

  • Lateral/Rotational Movement: keeps nonpolar tails within hydrophobic interior; energetically favorable

  • Flip-Flop: polar head travels through hydrophobic region, requires flippase and ATP; energetically unfavorable

Lipids can affect fluidity

• Length of nonpolar tails: longer tail lengths can create van der Waal forces, decreasing fluidity

• Prescence of double bonds: doubles bonds create a bend in the structure, makes it more fluid (think unsaturated fats)

• Prescence of Cholesterol: Cholesterol stabilizes membrane, temp can control how fluid it is

  • High temp = more fluid

  • Low temp = rigid

• Membranes need the right level of fluidity

Transmembrane Proteins can be limited in movement

• Cannot move much due to their large size

5.3 Membrane Transport

• Membrane Transport

  • movement of ions & molecules across membranes

• Membranes are selectively permeable

Phospholipid bilayer bans simple diffusion of hydrophilic solutes

• Hydrophobic interior doesn’t allow for diffusion of ions and hydrophobic molecules

• Four factors affect ability of a substance to diffuse

  • Size: smaller = faster diffusion

  • Polarity: Non-polar substance = faster diffusion

  • Charge: Non-charged substance = faster diffusion

  • Concentration: rate of movement is higher if concentration is higher

Cell Membranes have a gradient

• Transmembrane gradient: concentration of solutes is higher on one side and lower on the other

• Electrochemical Gradient: a gradient with electrical & chemical components (ions)

Osmosis

• Isotonic: When concentrations of solutes on both sides are equal

• Hypertonic: Concentration of solutes is higher outside than interior

• Hypotonic: Concentration of solutes is lower outside than interior

• Osmosis: The movement of water from lower concentration to higher concentration

5.4 Proteins carry out membrane transport

Channels and Passageways

• Open passageways facilitate diffusion

  • when open, molecules freely move through to inside of cell

• Most are gated to control flow of solutes

Transporters

• Transmembrane proteins bind to one or more solutes and bring it to the inside of the cell

• Provides pathway for cellular uptake of organic molecules

Active Transport

• Movement of a solute against a gradient

  • From lower concentration to higher; requires ATP

• Two types of active transport

  • Primary Active Transport: involves a pump, pumps one solute across membrane

  • Secondary Active Transport: uses pre-existing pump to transfer two solutes across membrane

ATP-Driven pumps generate electrochemical Gradients

• In animal cells Na+ concentration is lower inside cell, while K+ concentration is higher inside cell

  • Na+ is exported from cell, K+ is imported into cell

• Pumps are called Na+/K+ ATPase pumps

• Four steps for pumping mechanism

  1) 3Na+ binds to E1 conformation, ATP is hydrolyzed to ATP & phosphate group

  phosphate binds to pump, changing pump to E2 Conformation

  2) 3Na+ is released into the extracellular environment due to lack of affinity with E2 conformation

  3) 2K+ binds from outside

  4) Binding causes release of phosphate, switching conformation from E2 to E1

5.5 Intercellular Channels

• Multicellular organisms have intercellular channels

  • allows for direct movement of substances between adjacent cells

Gap Junctions in Animal Cells

• Gap Junctions: Small gaps between the plasma membrane

• Found in tissues & organs where cells need to communicate with each other

  • Allows for passage of ions and small molecules

Plasmodesma are in plant cells

• Intercellular channels in plant cells

5.6 Exocytosis and Endocytosis

• Ways to transport larger molecules

• Exocytosis: Materials are packed inside vesicles & vacuoles and are excreted into extracellular environment

• Endocytosis: When the plasma membrane folds inward to bring in material

  • three types

    • Receptor-Mediated Endocytosis: receptor is specific for a specific molecule

    • Pinocytosis: When cells bring in fluids

    • Phagocytosis: when cells bring in solids

5.7 Cell Junctions

• Junctions are how cells physically adhere to each other and/or extracellular matrix (ECM)

Anchoring Junctions link cells and ECM

• Anchoring Junctions: junctions between adjacent cells and ECM

• Separated into four categories

  • Adheren Junctions: connects cells to each other via cadherins; helps binds to actin filaments

  • Desmosomes: connects cells to each other via cadherins; binds to intermediate fibers

  • Hemidesmosomes: connects cells to ECM via integrins; binds to intermediate fibers

  • Focal Adhesions: connects cells to ECM via integrins; binds to actin fibers

Tight Junctions

• Creates a tight seal that prevents materials from leaking or coming in