Physiology Ch.4 Membrane Transport

Body fluid compartments

• Subdivided into intracellular and extracellular fluid

- Separated by cell membrane

• Contains Na+, K+, Cl-, HCO3-, Proteins

Intracellular fluid

• Fluid inside of the cells

• Majority subdivision of body fluid

- 66% of the body’s fluid

• Very low in Na+ and high in K+

- Caused by sodium (Na+) potassium (K+) pump

• Very low in Cl2+

- Caused by calcium (Cl2+) pump

• Very low in HCO3-

• High in proteins

- Caused by protein synthesis in the cells

Extracellular fluid

• Fluid on the outsides of the cells, between cells

• 33% of the body’s fluid

• Subdivided into interstitial fluid and plasma

- Separated by leaky exchange epithelial of capillary wall

• Regulated by body to being the internal environment back to normal

Interstitial fluid

• The majority subdivision of extracellular fluid

- 93% of extracellular volume

• Very high in Na+

• Very low in K+

• High in Cl-

• Very low in HCO3-

• No protein

Plasm

• The minority subdivision of extracellular fluid

- 7% of volume

• Very high in Na+ and very low in K+

- Caused by permeability of capillary tissue

• High in Cl-

- Caused by permeability of capillary tissue

• Very low in HCO3-

• Moderate in protein

- Caused by ingestion and digestion of protein

Passive transport

• Moves solutes between body fluids without using ATP

- No need for ATP because movement is down the concentration gradient, the way particles want to move naturally

• Subdivided into simple and facilitated diffusion Simple

Active transport

• Moves solutes between body fluids with the use of ATP

- ATP needed because movement is up the concentration gradient, against the way particles want to move naturally

• Subdivided into primary and secondary active transport

Simple diffusion

• Type of passive transport

• Plasma membrane does not hinder movement of solute in any way

- Solute passes through as if the plasma membrane does not exist

• Used for movement of O2, CO2, fatty acids, steroid hormones, thyroid hormones, fat-soluble vitamins, ethanol

Facilitated diffusion

• Type of passive transport that uses proteins to help diffusion

• Solutes must move with the help of a channel or carrier / transport protein

• Rate of diffusion affected by

- Number of channels or transport protein

- Open or closes channels

- Set transport rate of channels

Net diffusion

• The way that most of a substance is diffusion

- Diffusion is always done in both directions but one is normally greater than the other

Permeable membrane

• A membrane that allows the passage of other substances

Driving force

• Power solvents to move across a plasma membrane

• Subdivided into chemical, electrical, and electrochemical forces

Chemical driving force

• Caused by solvents’ concentration gradient

• Affects all solvents

Electrical driving force

• Caused by the separation of charges across the plasma membrane

• Only affects charged solvents

Electrochemical driving force

• Caused by both the concentration gradient and electrical difference in the plasm membrane

• Only affects ions

• The ion moves with the net effect of the two kinds of forces

- Sometimes they are in opposite direction, in that case the stronger one wins

Membrane potential

• Separation of electrical charges across plasma membrane

• Results in a negative 70mV charge on the inside of the cell

• Necessary for the electrical force

Rate of diffusion

• How many particles are diffusing across a membrane in a set time

Mean diffusion time

• Average amount of time it takes for a solute to diffuse

• When there is less distance to travel this process is done faster

• Affecting factors

- Mass of the molecule: Smaller moves faster

- Surface area: Larger surface are moves faster

- Medium of diffusion: Faster in gas than liquid

- Concentration difference: Higher difference causes a higher driving force and faster diffusion

- Permeability: Higher means faster diffusion

- Thickness: Thiner means faster

- Temperature: Hotter means faster

Ligand-gated ion channels

• An ion channel that is opened or closed based on a ligand binding to it

- Binding changes that shape of the protein, therefore it’s function

Voltage-gated ion channels

• An ion channel that is opened or closed based on an electrical charge

- Charge change changes shape of the protein, therefore it’s function

Mechanically-gated ion channels

• An ion channel that is opened or closed by a physical stimuli

- Physical stimuli changes the shape of the partition, therefore the function

Aquaporins

• Chanels that move water past the cell membrane

• Abundant in nearly all cells

Facilitated diffusion via transport protein

• Used for large and polar molecules

• Solute binds to transport protein, causing change in protein

• Change in protein causes the protein to flip and expose the bound solute to the opposite side of the cell membrane

• Factors effecting rate

- Transport rate of protein

- Number of carrier proteins

- Magnitude of concentration gradients

GLUT4

• Transport protein that comes from the inside of the cell and connects to the cell membrane

• Allows for the ingestion of glucose from the extracellular fluid

• Stimulated by insulin

• Clinical application: Diabetes does not allow for these proteins to attach, glucose never gets into the cells

Primary Active Transport

• Active transport that directly uses ATP

- Uses an enzyme that catalyzes an ATP molecule through hydrolysis

Secondary Active Transport

• Uses an electrochemical gradient that drives the transport of a solvent

Sodium (Na+) Potassium (K+) Pump

• Located in almost every cell in the body

• Uses active transport to move Na and K against their concentration gradient

- 3 Na moved out of the cell, intracellular to extracellular

- 2 K moved into the cell, extracellular to intracellular fluid

• Process

- Step 1: 3 Na and 1 ATP bind to intracellular side of pump

- Step 2: Protein changes shape and flips intracellular side to face the extracellular fluid

- Step 3: 2 K bind to the originally intracellular side of the pump

- Step 4: Protein changes shape and flips back to original configuration

- Step 5: K is released into inside of the cell

Ca2+ pump

• Located in the plasma membrane of cells in the endoplasmic reticulum

• Removed Ca2+ from a cytoplasm and dumps it into the extra cellular fluid or endoplasmic reticulum

• Used in muscle cells for fast communication

Secondary Active Transport

• A substance moving down its concentration gradient is coupled with a substance moving up it’s concentration gradient

- Substance moving from high to low concentration gradient powers the movement of the substance from low concentration to high concentration

• Subdivided into cotransport and anti port

Cotransport

• Type of secondary active transport

• Both substances are moving from the same side of the plasma membrane to the other side of the plasma membrane

Antiport

• Type of secondary active transport

• The two substances involved in the transport start on opposite ends of the membrane and move in opposite direction

Osmosis

• Diffusion that involves water

- Often facilitated but sometimes simple

Osmolarity

• The total amount of solute dissolved in a solution

• When osmolarity raises water concentration falls

- Water concentration falling triggers water to diffuse to the area from a high concentration water area

• Normal osmolarity 300mOsm

Osmotic Pressure

• Indirect measurement of a solution’s total solute concentration

• Total osmotic pressure increases then osmotic pressure increases

Tonicity

• The effect the osmolarity of a solution has on cells that are submerged in it

Isotonic

• Related to tonicity

• When the non penetrating solutes on the intracellular and extracellular sides are the same

Hypotonic

• Related to tonicity

• When the nonpenetrating solutes on the intracellular fluid is lower than the extracellular

Hypertonic

• More solute in the solution, compared to another solution

• In respect to red blood cells a hypertonic solution caused blood to be pulled out of the cell

Endocytosis

• Region of the plasma membrane folds into the cell

• Small pockets are formed that then pinch into the cells

• An intracellular membrane bound vesicle is born

Exocytosis

• Region of the plasma membrane pushes out into the extracellular fluid

• Small pockets are formed that then pinch into extracellular fluid

• An extracellular membrane is born

Pinocytosis

• Type of endocytosis

• Non-specific

• ECF + dissolved solutes (water, ions, nutrients, small molecules)

Phagocytosis

• Type of endocytosis

• Ingests bacteria or large particles

• Fuse with lysosomes in cytoplasm

Receptor-mediated endocytosis

• Type of endocytosis

• Allows for cells to take specific molecules in

- Certain molecules bind to plasma membrane receptors, concentrated area becomes an endocytosis

Paracellular transport

• Diffusion between epithelial cells

Transcellular transport

• Substances move into epithelial cells at the apical or basal surface then exits on the opposite surface

- Moves through epithelial cell