Topic 1: Cell Physiology Lectures 6-10

What’s the difference between endocytosis + exocytosis?

• endocytosis: uptake of material into cell via vesicles that pinch off from plasma membrane + enter cytoplasm

  • phagocytosis

  • pinocytosis

  • receptor-mediated endocytosis

• exocytosis: release of material from cell via vesicles that fuse with plasma membrane

What’s the difference between phagocytosis + pinocytosis?

• phagocytosis: cell eating

  • uses pseudopodia (extensions of plasma membrane) to surround material being brought into cell

  • brings in large particles (bacteria, cell debris)

  • commonly used by white blood cells

• pinocytosis: cell drinking

  • plasma membrane indents below the particles to be brought in + pinches off to form endocytic vesicle

  • non-specific: brings in extracellular fluid + substances dissolved in that fluid (small molecules, ions, nutrients)

What are the steps of phagocytosis?

1. once activated, it moves to substance to be ingested

2. steps

   1. recognition of bacteria or other particle

   2. attachment of bacteria or other particle to phagocyte

      1. this step begins active process of phagocytosis

   3. pseudopodia ingest bacteria or particles into phagosome (vesicle)

   4. fusion of lysosome with phagosomes to form phagolysosome

   5. destruction of bacteria + digestion

   6. exocytosis (release of end products into or out of cell)

What is receptor-mediated endocytosis?

• specific process where receptors bind specific ligands to be brought into cell

• involves clathrin, which may…

  • deposit contents into lumen of organelle

  • travel across cell + fuse with plasma membrane to release contents

  • fuse with endosomes ➡ sorts content to Golgi or lysosomes

• steps

  • extracellular molecules bind to receptors on plasma membrane; receptors cluster together

  • plasma membrane sinks inward, forms clathrin-coated pit

  • pit separates from plasma membrane, forms clathrin-coated vesicle containing concentrated molecules from ECF

What is exocytosis?

• vesicles fuse with plasma membrane + release contents into ECF

• functions:

  • secrete specific substances

  • release waste products

  • add components to plasma membrane + replace portions of plasma membrane removed by endocytosis

• functions balance portions of plasma membrane removed by endocytosis

What types of driving forces exist for non-vesicular transport?

1. chemical

2. electrical

3. electrochemical

How does the chemical driving force work in non-vesicular transport?

• concentration gradient: different concentrations of substance on either side of membrane

• molecules move passively in direction of driving force aka down concentration gradient

  • high to low

• size of gradient increases ➡ rate of transport of substance increases

How does the electrical driving force work in non-vesicular transport?

• membrane potential: difference in electrical potential/voltage across cell membrane

  • separation of charge (unequal distribution of charges across cell membrane)

• only charged substances experience attractive or repulsive forces, membrane potential can push or pull them in different directions depending on charge of substance + polarity of membrane potential

What is the electrochemical driving force in non-vesicular transport?

• sum of electrical + chemical driving forces acting on ion

  • net direction depends on combination of electrical + chemical driving forces

• neutral substances aren’t affected by electrical driving force

What is simple diffusion?

• passive movement of molecules through a biological membrane’s lipid bilayer (doesn’t require energy)

• substances move from high concentration to low concentration (down concentration gradient)

What factors affect how well a substance crosses a phospholipid bilayer?

• lipid solubility

  • polar: water-soluble + lipid-insoluble

  • non-polar: water-insoluble + lipid-soluble

• size

  • smaller substances move more easily

Which of these substances can + can’t cross membrane by simple diffusion: small, polar, uncharged : large, polar, uncharged : charged (+ or -) : ions?

• small, polar + uncharged: yes

• large, polar + uncharged: yes if small enough

• charged: no due to non-polar core of phospholipid bilayer

• ions: no since their charge prevents interaction with non-polar fatty acid tails of membrane (blocks passage)

What factors influence the rate of simple diffusion?

1. magnitude of driving force

   1. greater concentration difference across membrane ➡ greater driving force + faster rate of diffusion

2. membrane SA

   1. larger SA ➡ more molecules can cross at once

3. membrane permeability

   1. depends on:

      1. lipid solubility of diffusing substance

         1. non-polar substances cross membrane more readily

      2. shape + size of diffusing substance

         1. smaller, more regularly-shaped substances diffuse faster

      3. temperature

         1. increasing temperature ➡ increasing diffusion rate

      4. diffusing distance

         1. smaller diffusing distance ➡ faster diffusion

What is osmosis?

• net movement of water across a selectively permeable membrane driven by difference in solute concentrations on the 2 sides of membrane

• water flows from high to low water concentration or from low to high solute concentration

  • high water = low solute : low water = high solute

  • water concentration can be changed by adding solute

What’s considered pure water?

Water with no solute added.

Can water move across membranes by simple diffusion?

• yes since water molecules are small + polar, but simple diffusion of water is limited + finite

• certain tissues (ex. kidneys) have higher water permeability via aquaporins: channels that allow water to easily move across membrane

What is mediated transport + its 2 types?

• use of a protein to move substances across cell membrane

• types:

  • facilitated diffusion

  • active transport

What is facilitated diffusion?

• form of mediated transport that’s passive + moves substances down their concentration gradient

• specific + uses transport proteins:

  • carriers: proteins with binding sites for specific substances

  • channels: selective for a specific ion or type of ion

What is carrier-mediated facilitated diffusion?

• use of carriers with specific binding sites that binds the substance + moves it across plasma membrane from high to low concentration

  • doesn’t need energy

• ex. GLUT family of proteins

  • transports glucose across membrane

    • glucose is too large for simple diffusion even though it’s non-polar + uncharged

What is channel-mediated facilitate diffusion?

• use of channels that are selective for specific ions or types of ions (ex. aquaporins)

• direction + magnitude of ion flux depends on ion’s electrochemical gradient

• channels are open or closed + can be gated via…

  • voltage-gated: opened/closed by changes in voltage

  • ligand-gated: opened/closed by binding of ligand

  • mechanically-gated: opened/closed by mechanical stimuli (stretching or swelling of cell)

What is active transport?

• transport proteins (pumps) with specific binding sites move substances against their electrochemical or concentration gradient (low ➡ high)

  • requires energy

What are the 2 types of active transport?

1. primary

   1. uses ATP hydrolysis (ATP ➡ ADP + P_i) to power uphill transport of substances

      1. ex. Na⁺/K⁺ pump: for each ATP hydrolyzed it moves 3 Na⁺ out of cell + 2 K⁺ into cell

         1. maintains Na⁺ + K⁺ concentration gradients

         2. contributes to membrane potential

2. secondary

   1. uses energy of ion moving down its electrochemical gradient with another substance moving uphill against its gradient

   2. cotransport (symport): 2 substances transported in same direction

   3. countertransport (antitransport): 2 substances transported in opposite directions

What does it mean when secondary active transport is electrogenic or electroneutral?

• electrogenic: transport leads to net movement of electrical charge across membrane

  • ex. Na⁺/glucose cotransporter: since only (+) charges are moved ➡ net charge movement

• electroneutral: transport doesn’t change overall charge balance

  • ex. Na⁺/H⁺ exchanger: exchange of one (+) charge for another ➡ net charge is same

Is simple diffusion + mediated transport saturable or unsaturable?

• simple: unsaturable; doesn’t involve proteins or binding sites

  • increasing substance’s concentration will continuously increase its movement across membrane

• mediated: saturable; depends on transport proteins with limited binding sites

  • as concentration increases, more sites are occupied until maximum rate is met ➡ transport rate plateaus _ doesn’t increase further