1 Cell membrane; transport; osmolarity

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Proteins in the cell membrane

1. Integral proteins: Anchored to and imbedded in the membrane through hydrophobic interactions (ion channels, transport and receptor proteins, guanosine 5'triphosphate-binding proteins (G-proteins))
2. Peripheral proteins: Not imbedded and are not coavlently bound to membrane components, but are attached to cell membrane by electrostatic interactions

Intercellular connections

1. Tight junctions (zonula occludens): Attachments between cells. May be a intercellular pathway for solutes (depending on size and charge) or impermeable
2. Gap junctions: Attachments between cells that permit interceullar communication (e.g. between myocardial cells)

Modes of transport across the cell membrane

1. Simple diffusion
2. Facilitated diffusion
3. Active transport (primary and secondary)
Carrier-mediated transport includes facilitated diffusion and active transport

Simple diffusion (characteristics; calculate)

1. Together with an electrochemical gradient; passive
2. Diffusion can be measured: J = -PA(C1-C2)
J=Flow mmol/sec; P=Permeability cm/sec; A=Area cm2; C=Concentration mmol/L

Factors that increase permeability

1. Increased Oil/water partition coefficient increases solubility in the lipid
2. Lesser radius of the solute increases diffusion coefficient and speed of diffusion
3. Lesser membrane thickness decreases the diffusion distance

Oxygen and Na+ through a membrane - which factors affect the permeability

1. Oxygen is a small hydrophobic solute - they have the highest permeability
2. Na+ are hydrophilic and must cross through water-filled channels or pores. The flux depends on both concentration gradient AND electrical potential difference

What is special about the carrier-mediated transport

1. Stereospecificity: e.g. D-glucose can be transported this way, L-glucose cannot
2. Saturation: Transport rate increases with concentration of the solute until the carrier is saturated. This transport maximum is analogus to the maximum velocity in enzyme kinetics
3. Competition: Similar substances compete for transport. E.g. Galactose and Glucose

Characteristics for facilitated diffusion

Occurs down an electrochemical gradient; passive; more rapid than simple diffusion; carrier-mediated (stereospecificity, saturation and competition)

Characteristics for primary active transport (with examples+drugs affecting)

1. Against concentration gradient; requires energy from ATP; carrier-mediated
2. Sodium-potassium pump (3 sodium out and 2 potassium in). Inhibitors are ouabain and digitalis (cardiac glycoside drugs)
3. Calcium pump (in ER and SR it is called SERCA)
4. Hydrogen-potassium pump (proton pump). Inhibited by proton pump inhibitors such as omeprazole

Characteristic for secondary active transport (inhibition of it)

1. Coupled transport
2. One (usually sodium) is with the c gradient and provides energy for the other substance (goes against gradient)
3. Can be cotransport/symport (e.g. Sodium-glucose transport) or counter-transport/exchange or antiport (e,g, Sodium-proton exchange)
Inhibition of Sodium-potassium pump will decrease sodium transmembrane gradient and inhibit secondary active transport

Explain the principles behind Sodium-Calcium countertransport

Calcium goes out of the cell against the c.gradient. Sodium goes into the cell with the c.gradient. Energy is derived from the sodium movement. The Sodium gradient is maintained by the Sodium-Potassium pump

Definition, measuring and calculation of osmolarity

1. The concentration of osmotically active particles in a solution
2. Is a colligative property that can be measured by freezing point depression
3. Can be calculated: Osmolarity = g x C
Osm. = osm/L
g = number of different particles (osm/mol)
C = concentration mol/L

Definition and calculation osmotic pressure

1. Two solutions with water are separated with a semipermeable membrane where one has a higher osmolarity. This causes an osmotic pressure which depends upon the c. of osmotically active particles
2. Can be calculated with van't Hoff's law:
pi = g x C x RT
pi=Osmotic pressure (atm/mmHg)
R=gas constant (0.082)
T=Absolute temperature

How do we term different osmolarity and osmotic pressure between two solutions

1. Differences in osmolarity: Isosmotic or hyper/hypoosmotic
2. Differences in osmotic pressure: Isotonic or hyper/hypotonic

What is the term used for osmotic pressure created by proteins?

Colloidosmotic pressure (oncotic pressure)

Between solutions with different osmotic pressure - where does the water go? How to increase flow?

Water flows from hypotonic to hypertonic and increases in flow when the solute concentration increases (increase in osmotic pressure)

Reflection coefficient

Number between 0 and 1 that describes the ease with which a solute permates a membrane:
1. If 1 - solute is impermeable. E.g. Serum albumin
2. If 0 - solute is permeable. Causes no osmotic effect. E.g. Urea has a reflection coefficient close to zero and is an ineffective osmole

Effective osmotic pressure

It is the osmotic pressure (calculated by van't Hoff's law) multiplied by the reflection coefficient

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