Lecture 2 - Cell Biology; PM, Diffusion, Osmosis

Cell Composition

• Cells are made of mostly water, organic molecules, and inorganic ions

Basic cell structures

Plasma membrane functions

1. Helps maintain composition of intra- and extracellular fluids

   1. Controls inward and outward traffic

2. Forms a framework for protein components of cell

3. Detects chemical messengers at cell surface

4. Links adjacent cells together

Epithelial cells* (be familiar)

• Epithelial cell function depends on the asymmetrical distribution of transporters within the cell

• Apical cell membrane - exposed to the outside

• Basolateral cell membrane - faces inward

• They are interconnected by protein linkages - tight junctions

  • Limits movement of solutes and water around cells, and restricting free movement of the membrane proteins between the apical and basolateral regions

• Ion transport demands lots of energy - Epithelial cells have abundant mitochondria to product ATP

• Transcellular transport - the movement of solutes (or water) through epithelial cells

• Paracellular transport - the movement of solutes (or water) between adjacent cells)

  • Epithelial cells can control movement in/out of an organ

• Small molecules (water, ions) can cross through the membrane connections/tight junctions via paracellular transport, though large molecules cannot

• Leaky epithelia - permit paracellular transport

• Tight epithelia - minimal paracellular transport

Tight Junctions

• Impermeable barrier

• In between 2 sets of cells (like epithelial cells)

• Prevents paracellular movement (between 2 cells; few molecules selective), so molecules can only do transcellular movement (through a cell) if permitted

Desmosomes (spots) / Adheren junctions (band, velco-like)

• For anchoring

  • Desmosomes - keep cells from separating apart, very strong and rigid

    • Tend to be in tension-bearing organs

      • Skin, heart, bladder, esophagus

  • Adherens - Hold cells together while allowing some flexibility

    • Tend to be in flexible tissues/organs

      • Intestinal lining, blood vessels

Gap junctions

• Communication

• Tunnels that allow movement of chemicals or charge from one cell to another

• Cell to cell

• Like in cardiac cells (charge)

Plasma membrane structure

• Phospholipids - polarity

  • Polar, hydrophilic phosphate group heat

  • Nonpolar, hydrophobic fatty acid tails

• Channel proteins

• Peripheral proteins - receptors, tags

• Glycoproteins

• Cholesterol - temperature / thermal regulation

Plasma membrane relative permeability / diffusion through plasma membrane

1) Hydrophobic molecules (nonpolar)

• Easily pass through via diffusion

  • O2, CO2, N2 → small, nonpolar

  • Steroids

    • Testosterone, cortisol, estrogen, Vitamins A/E/D

  • Large nonpolar/hydrophobic molecules can pass through

2) Small, uncharged polar molecule

• Glycerol

3) Large, uncharged polar molecules

• Unless hydrophobic (nonpolar), big molecules have a harder time passing through

  • Glucose, sucrose, slightly impermeable

4) Ions

• Charge = very difficult to pass through

  • H+, Na+, K+, etc

4) Charged polar molecules

• Amino acids, ATP

Molecules that can EASILY DIFFUSE through the phospholipid bilayer

1) Hydrophobic (nonpolar) molecules

• O2, CO2, N2

• Steroids

2) Small, uncharged polar molecules

• Glycerol

Molecules that HAVE DIFFICULTY DIFFUSING through the PM

1) Large, uncharged polar molecules

• Glucose, sucrose

Molecules that are IMPERMEABLE to the PM

1) Ions

• Na+, Ca2+, Cl-

2) Charged polar molecules

• Amino acids, ATP

Movement of molecules across the PM

• Simple (passive diffusion)

  • Through PM

• Facilitated diffusion

  • Through Protein channels

• Active Transport

  • Through protein channels, ATP

• Bulk Transport

Simple Diffusion

• Concentrated area to less concentrated region to reach equilibrium through time

  • Electrochemical gradients

    • Form of energy storage

• Random walk process due to random thermal motion

  • Molecules collide with other molecules and change direction unpredictably (random direction and length), but the net diffusion is from high to low concentration

    This produces a random walk.

• Small and nonpolar molecules can diffuse through the PM

Electrochemical gradient

• Gradients can be chemical, electrical, or both

  • Non-charged molecules only have movement due to concentration, so they diffuse independent of membrane charge

• Form of energy storage (potential energy)

In electrochemical gradients:

• Ions are held apart by membranes

• Their movement is restricted

• Energy is stored in:

  • Unequal concentrations

  • Separated charges

    • When channels open, ions move spontaneously, releasing that stored energy.

Flux & Net flux

• Flux - measure of diffusion rate

• Net Flux - difference between the 2 one-way fluxes

  • Measure of net gain of molecules by one side and net loss form the other side

  • Diffusion still occurs while net flux = 0 ; equal movement in and out

Passive Diffusion (for 1 substance) *NO CHARGE

• Movement of molecules due to the intrinsic kinetic energy of molecules

• No metabolic energy (ATP) used - Passive

• Movement from higher to lower concentration

• At equilibrium, Net flux (Rate of diffusion) = 0

Passive Diffusion (for 2 substances)

• Each substance diffuses down its OWN concentration gradient, independent of the concentration gradients of other substances

• At equilibrium, equal number of each molecule on either side

Passive Diffusion - Net Flux direction and magnitude

• Net flux direction and magnitude depend on:

1. Permeability

   1. Change by opening and closing channels

2. Concentration gradient

3. Temperature

   1. Higher T = higher diffusion rate

4. Surface area

   1. Big SA increase diffusion

5. Size of molecule

   1. Smaller molecules faster than bigger

6. Distance

   1. How thick the membrane is, how far a molecule has to diffuse

Water and PM Diffusion

• Water is excluded by the hydrophobic tails of the phospholipid bilayer

• Can diffuse, but not via simple → FACILITATED

  • Majority of water diffusion is via aquaporins (facilitated), only a bit through PM (Simple)

Osmosis

• Passive diffusion of water

• Net diffusion of water from a region of high water concentration to a region of low water concentration

• Facilitated by aquaporins

Osmolarity

• Total concentration of solutes in a solution

• Depends on the total number of molecules, not individual type

  • Isosmotic

  • Hyperosmotic

  • Hyposmotic

Hyposmotic solution

• Solution has less solute than another solvent

• If solution is hyposmotic to the sample, the sample is hyperosmotic to the solution, causing the sample to swell

• Sample will shrink if it is hyposomotic to the solution

Hyperosmotic

• Solution has higher concentration that another solvent

• If solution is hyperosmotic to the sample, the sample is hypoosmotic to the solution, causing the sample to shrink

• Sample will swell if it is hyperosmotic to the solution

Isosmotic

• 2 solutions have same concentration of solutes

  • Solutes don’t need to be the same

• Sample will stay the same size

Direction of water movement through a permeable membrane

• Water will move to the hypertonic side, against its concentration gradient, to decrease the salt gradient

Osmotic Pressure

• Pressure generated by water moving based on osmolarity

  • Osmotic pressure is the pulling force that draws water into a compartment because of solute concentration differences across a semipermeable membrane.

    • (Why water wants to move)

Hydrostatic Pressure

• Pressure exerted by the standing column of water - gravity

• The pushing force exerted by a fluid on the walls of its container or membrane; caused by gravity

Osmotic pressure and Hydrostatic pressure

• Water would want to move through an impermeable membrane forever towards the more concentrated side due to osmotic pressure, but it cannot, due to external hydrostatic pressure (gravity)