Membranes, Diffusion, Osmosis

functions of membranes

-Regulate the passage of substances into and out of cells and between cell organelles and cytosol

*regulated by proteins on the membrane; membranes separates ICF and ISF

-Detect chemical messengers arriving at the cell surface

*hormones, NT (usually outside the cell and bind to receptors on cell surface to cause reaction inside side)

-Link adjacent cells together by membrane junctions

-Anchor cells to the extracellular matrix

*membrane proteins

plasma membrane structure

-Phospholipid bilayer

•Hydrophilic heads: toward watery environment/ISF/cytosol, both sides

*water soluble

•Hydrophobic fatty-acid tails: inside membrane

•Ions and water-soluble molecules cannot pass through hydrophobic membrane interior

membrane structures and protein types

-Cholesterol: so more fluid and flexible, scattered throughout

-Channel proteins

-Integral membrane proteins: embedded

-Peripheral membrane proteins: don’t go all the way across

-Glycoproteins: proteins with CHO chains attached (so immune system can identify)

-Glycolipids: phospholipids with a CHO attached

-Plasma membrane itself relatively thin

fluid mosaic model

to describe the plasma membrane bc made of lots of little pieces the make up the whole thing, and is flexible

Membrane junctions

linking cells

-Desmosomes: strong attachment between cells or cells and connective tissue

-Tight junctions: form a barrier (so water, etc. can’t pass through)

-Gap junctions: allow ions and some other molecules to pass

desmosomes

-provide strong attachments between cells

-cytoskeleton: web of proteins that helps hold the shape

-proteins interlock so strong

-ex: skin

-Hemidesmosome: at the junction of connective tissue and the basement membrane; under epithelial cells

Tight Junctions

-prevent most substances from leaking between cells

-makes the epithelial layer less leaky

-on the apical side/outside

-ex: kidney nephron

Gap Junctions

-allow for communication between neighboring cells

-Connexons: two hemi-channels

-ex: heart muscle (AP can pass cell to cell)

Brownian Motion

-random molecular motion that drives diffusion

Diffusion

-Movement of particles down a concentration gradient, from area of high concentration toward area of low concentration

-Driven by random molecular motion (aka Brownian Motion)

-Over time, reaches equilibrium:

• particles evenly distributed

• particles still moving, but no net movement (Brownian motion still there)

*ex: BG, O2 and other gases

“net flux”

-In this example, membrane is freely permeable to particles

-Random motion causes crossing of membrane (flux) in both directions

*”flux”= move into cell

-Net flux is from high to low

*no net flux last picture

Factors affecting diffusion

-Many factors can affect diffusion, but few of them change significantly during normal life processes

• Temperature: measure of molecular motion, but not to relevant to phys since pretty constant

• Medium/solvent: (water) not much change in phys

• Molecular weight: occasionally relevant (larger molecules diffuse slower)

• Surface area: increase SA= increase diffusion rate, relatively constant in phys (but theoretically could effect, ex: remove part GI villi, alveoli blocked could lead to pneumonia)

• Membrane permeability: from freely → non-permeable, channel proteins, good only under certain conditions

diffusion through membranes

-Because the interior of the lipid bilayer is nonpolar, polar molecules cannot diffuse through

• Includes ions, most drugs & nutrients, water

-Nonpolar molecules are able to diffuse (*don’t need a special carrier)

• Steroid hormones

• Oxygen

• Carbon dioxide

simple diffusion

-through the membrane though the lipid bilalyer (if nonpolar) OR through a channel (if small, polar molecules)

carrier-mediated transport

-Carrier-mediated transport: moves large molecules or transports against the concentration gradient (low → high)

• Facilitated diffusion: substances move down concentration gradient (no energy required, similar to simple diffusion where flow is down concentration gradient, but needs a carrier protein)

• Active transport: substances move up concentration gradient (energy required)

•Primary: pumps that use ATP

•Secondary: energy comes from concentration gradient of a different solute

different types of channels and how used

-Ions diffuse across the membrane by passing through channels

-Presence and state (open/closed) of channels determines permeability (*based on stimuli)

*ex: Na+

-Channel structure determines specificity

picture… both=no energy needed

Simple Diffusion (top)

-can move rapidly, speed determined by the concentration gradient

Facilitated Diffusion (bottom)

-molecules have to temporality bind to the carrier, then the carrier changes shape and transports the molecule to the other side

-rate-limited bc of the binding!!!

Osmosis

-Movement of water into and out of cells

-Special channels called aquaporins

-Like any other substance, water moves down concentration gradient (high water conc → low water conc)

*special case of diffusion; some water can go across, but usually needs an aquaporin

pure water vs. water with solutes

-solutes take up space so have lower water concentration than pure water

why we care about osmolarity and tonicity

-Homeostasis: the maintenance of fluid volume and electrolyte concentration is essential for normal body function and survival

-In medical settings, it is common to give intravenous fluids

• The wrong choice of fluid carries significant risk of patient harm

• Certain fluids can cause cells to swell or shrink, depending on what is dissolved in the fluid

• Example complication: brain swelling

*saline= 0.9% NaCl = same as plasma and ICF; want to give patient an isotonic solution

osmolarity

-A measure of concentration

• Defined as the total amount of dissolved particles per liter of solution

• Expressed as osmoles per liter (osmol/L or OsM)

• 1 osmol is one mole of particles

-Related to molarity, but not necessarily the same – depends on dissociation

-For substances that do not dissociate in solution:

• 1 mol dissolves in 1 liter to form a 1 osmol/L solution

• In this case molarity and osmolarity are the same

*ex: glucose

-For substances that do dissociate in solution:

• Form multiple particles

• Example: 1 mol NaCl dissociates to form 1 mol Na+ and 1 mol Cl- . A 1 molar NaCl solution has an osmolarity of 2 osmol/L (assuming full dissociation)

-The osmolarity of a solution does not tell us whether it will affect cell volume – we need to know what is dissolved, in addition to the quantity

-are most membranes semi-permeable, fully permeable, or non-permeable?

-what is a limit of biological systems when it comes to osmosis?

-semi-permeable

-membranes have limited ability to change volume

nonpentrating solutes

-These substances that cannot cross the membrane without assistance are called nonpenetrating solutes

-Sodium is functions as a nonpenetrating solute because it is pumped out of a cell as rapidly as it enters (Na+ stays on the outside of a cell)

tonicity

-A description of how a solution affects on cell volume (swelling or shrinking)

-Relevant where two compartments are separated by a semipermeable membrane (e.g. extracellular and intracellular fluid)

• Penetrating solutes can freely diffuse across the membrane

• Nonpenetrating solutes cannot freely diffuse across the membrane

  • Some cannot cross at all

  • Others diffuse across the membrane but are rapidly transported back in the opposite direction – e.g. a small amount of Na+ leaks into cells, but it is removed by the Na+ /K+ -ATPase

-The concentration of nonpenetrating solutes is what determines tonicity – water moves by osmosis toward the compartment with a higher concentration of nonpenetrating solutes

what is the osmolarity of ctysol?

~300 mOsm/L

*saline=0.9% NaCl and is also the same

hypertonic vs isotonic vs hypotonic

-hypertonic: water moves out of the cell, and the cell shrinks/”crenation”

-isotonic: concentration dissolved outside=inside, no net movement of water

-hypotonic: water moves into the cell, and the cell swells and could potenentilly burst

*ex: if drink distilled/plain water