Cell Physiology Lecture 2 - Cell Transport

Vesicular Transport

Transport that uses vesicles to move substances across biological membranes. Includes endocytosis and exocytosis

Endocytosis

Uptake of material into cells using vesicles that contain material that pinch off from the plasma membrane and enter the cytoplasm of the cell

3 types of endocytosis

Phagocytosis, Pinocytosis, Receptor-mediated endocytosis

Exocytosis

Release of material from the cell using vesicles containing material that fuse with the plasma membrane and release their contents into the extracellular fluid

Endocytosis vs. Exocytosis

• Endocytosis - materials brought into the cell using vesicles

• Exocytosis - materials are released from the cell using vesicles

Phagocytosis

Cell eating - pseudopdia surround material being brought into cells

• Used to bring large particles into cell, such as bacteria or cell debris

• Processed used by WBCs and Macrophages

  

Pinocytosis

Cell drinking - plasma membrane indents below particles to bring them into cell (no psuedopodia)

• Non-specific - engulfs in the ECF and all solutes

• Used to ingest small molecules

Steps of Phagocytosis

1. Recognition of substance to be ingested on surface membrane

2. Attachment of phagocyte to substance to be ingested

3. Pseudopodia reach around substance to form a phagosome

4. Fusion of phagosome to lysosome to form a phagolysosome

5. Destruction of ingested substance by lysosomal enzymes

6. Release of end products into the cell or out of the cell via exocytosis

Receptor-mediated endocytosis

Phagocytosis that involves binding specific ligands to be brought into the cell. Requires the use of clathrin coated pits

Steps of receptor-mediated endocytosis

1. Extracellular molecules bind to receptors on plasma membrane; receptors cluster together

2. Plasma membrane sinks inward, forming clathrin coated pit

3. Pit separates from the plasma membrane to form a clathrin-coated vesicle. Vesicle contains concentrated molecules from the ECF

4. Clathrin is released from the vesicle and is recycled back to the membrane

5. Vesicle will then travel to specific organelles ot be modified, processed, or released back out of the cell

Exocytosis functions

• Secrete specifc substances

• Release waste products

• Add components of the membrane (lipids/proteins) to the plasma membrane when the vesicles fuse with the membrane

Driving forces for non-vesicular transport

Chemical, electrical, electrochemical

Chemical driving force

Molecules will move passively down their chemical concentration gradient

• As the size of the gradient increases, the rate of transport of the substance increase

  

Electrical Driving force

Membrane potential is a difference in electrical potential or voltage across a cell membrane. Charged molecules will move to an area with a higher concentration of the opposite charge

Example of a seperation of charge

Na+/K+ pump. - 3 Na+ move out of the cell, 2 K+ move into the cell

Electrochemical driving force

Sum of the electrical and chemical driving forces acting on an ion

• If a chemical and electrical driving force push the ion in different directions, the electrochemical driving force is the sum

  • Electrochemical driving force depends on the net direction of the driving forces

Simple diffusion

Passive movement of molecules through a lipid bilayer. Does not require the input of energy. Molecules move from an area of high concentration to low concentration (down the concentration gradient)

Factors that affect how well a substance crosses the bilayer

• Solubility (polar or non-polar?)

• Size (smaller moves easier)

• Charge (ions do not move by simple diffusion)

  

What substances will move by simple diffusion?

• Small, non-polar uncharged

• Small molecules that are polar and uncharged may pass via SD

What substances will not move by simple diffusion?

• Large particles

• Charged particles

Factors that influence the rate of simple diffusion

• Magnitude of the driving force - greater concentration difference = greater rate of SD

• Membrane SA - more SA of membrane = greater rate of SD

• Membrane Permeability - higher permeability = more difusion. Depends on (ranked)

  • Lipid solubility of diffusing substance (non-polar = more diffusion)

  • Size and shape of diffusing substance (small regular shapes = more diffusion)

  • Temperature (higher Temp = more diffusion)

  • Diffusing distance (smaller distance = faster diffusion)

Osmosis

Passive movement of water across a selectively permeable membrane driven by a difference in solute concentration

Which way does water flow

From an area of low solute concentration to an area of high solute concentration

Can water move via simple diffusion?

Yes. Water is a small, polar molecule

Aquaporins

Water channels on the plasma membrane

Carrier-mediated facilitated diffusion

passive movement of molecules across a membrane via carrier transport proteins which have a specific binding site for the substance being transported

How does carrier mediated facilitated diffusion work?

Specific molecules bind to the transporter protein binding sites, causing a conformational change in the protein to expose the molecule to the other side of a membrane

Example of a transporter protein

GLUT proteins - move glucose down the concentration gradient

• Glucose is nonpolar and uncharged, but is too big for simple diffusion

Channel-mediated facilitated diffusion

Passive movement of a molecule down its gradient via protein channels. Protein channels are specific

Gating of ion channels

Opening (activation) or closing (deactivation) of ion channels. Gating is the process of the ion channel transforming between conducting and non-conductive states

Types of gates for protein channels

• Voltage gated - change in voltage opens/closes channels

• Ligand gated - binding of ligand to binding site opens/closes channels

• Mechanically gated - stretching or swelling of cell opens/closes channels

Mediated transport

The use of a protein to cross the cell membrane

2 forms of mediated transport

Facilitated diffusion and active transport

Active transport

The movement of molecules against their electrochemical/concentration gradient. Requires an energy input. The proteins that facilitate this transport are called pumps

Primary active transport

ATP hydrolysis is the energy source used to drive actin transport

• ATP —> ADP + Pi + energy

Example of primary active tranpsort

Na+/K+ pump (Na+/K+ ATPase)

Steps of the Na+/K+ pump

Na+/K+ pump moves 3Na+ ions out of the cell and 2K+ ions into the cell for every molecule of ATP hydrolyzed

• Both molecules go against their concentration gradient

• Protein is activated by phosphorylation (non-covalent modification)

Functions of the Na+/K+ pump

• Establishes and Maintains membrane potential of the cell

• Maintains Na+ and K+ concentration gradients

Secondary active transport

Couples movement of an ion down it’s electrochemical gradient with a substance moving up it’s gradient

Examples of secondary active transport

Na+/glucose symporter, and the Na+/H+ antiporter

Na+/glucose symporter steps

1. Sodium moves down its chemical gradient, moving into the cell

2. Glucose transport is coupled , moving into the cell up its concentration gradient

Movement is electrogenic (positive charge movement into the cell)

Na+/H+ antiporter steps

1. Sodium moves down its chemical gradient, moving into the cell

2. Proton transport is coupled, moving out of the cell down it’s concentration gradient

Movement is electroneutral (no net charge movement

Symport vs. Antiport

• Symport - transport protein moves both molecules in/out of the cell

• Antiport - transport protein moves one molecule into the cell, and the other molecule out of the cell

Transport Rate of Diffusion

Unsaturable transport process - no maximum filled binding sites because there is no binding sites

• Flux is proportional to solute concentration

  

Transport Rate of Mediated Transport

Saturable transprot process - each cell has limited binding sites for a substance

• Transport rate will plateau when all binding sites are saturated

  

Can you plateau a ion channel for maximum flux?

No. Not at physiological conditions