Osmosis, Diffusion, and Membrane Transportnapavalley.edu/.../Documents/Lec6_Osmo_Diff_Mem_219.pdfOsmosis, Diffusion, and Membrane Transport Bio 219 Napa Valley College Dr. Adam Ross

Osmosis, Diffusion, and Membrane Transport

Bio 219Napa Valley College

Dr. Adam Ross

Overview

In order to understand how cells regulate themselves, we must first understand how things move into and out of cells

Diffusion

• Diffusion is the movement of particles from an area of high charge or concentration to an area of lower charge or concentration

• Referred to as moving “down” a charge or concentration gradient• Ex. H+ ions in mitochondria moving through ATP synthase

• Result of random molecular motion

• Fick’s Law of Diffusion gives rate of diffusion:• Rate = P A (Cout – Cin) / (x)

• Rate is proportional to permeability (P), surface area (A), concentration gradient (Cout – Cin); inversely proportional to diffusion distance or membrane thickness (x)

Gradients

• Concentration• Caused by unequal distribution of a substance on either side of the

membrane

• If the inside of a cell is negative, it will attract positively charged things

• Electrical (charge)• Caused by unequal distribution of charge on either side of the membrane

Diffusion

Osmosis

• Osmosis is the movement of solvent through a semi permeable membrane in order to balance the solute concentration on either side of the membrane.

• In cells the solvent is water• Water can cross membranes

Osmosis

Osmolarity

• Total concentration of all solutes in a solution• 1 Osm = 1 mole solute/ L

• Have to account for both atoms in salts• 1M NaCl +1 L H2O → 1M Na+ + 1M Cl ≈ 2 Osm

• Plasma = 290 mOsm

Osmotic pressure

• This is the actual driving force for net water movement

• Depends on difference in total solute concentration

• Negative pressure pulls water from dilute to concentrated solution

Tonicity

• Extracellular solute has an effect on water in the cell• Hypotonic: ECF is less concentrated than ICF

• Water moves into cell

• Hypertonic: ECF is more concentrated than ICF• Water moves out of cell

• Isotonic: ECF and ICF have the same [solute]• No net water movement

Osmosis

Hypotonic solution Hypertonic solution Isotonic solution

Membrane permeability

• Highly permeable:• Oxygen, CO2, fatty acids, steroids, water (variable via AQP)

• Less permeable:• Na+, K+, Cl- (via channels)

• Glucose and AA (via carrier)

• Impermeable:• Proteins (except via vesicle)

• ATP

• DNA, RNA

Permeability

• Ability of a particular substance to pass through the lipid bilayer• Can either pass directly through (fats, water)

• Small, lipophilic molecules pass through more quickly than large hydrophilic molecules

• Or via pore/ transporter (ions, glucose)

Diffusion through channels

• Ion channels are pores that allow ion passage into and out of cells• Most are selective for certain ions

• Ions diffuse down electrochemical gradient• Determined by concentration AND charge

• Channels can be gated (opened by ligand or voltage) or non-gated (always open)

• Aquaporins (AQP) are water channels found in the membranes of most cells.

Ligand gated ion channel

© 2010 Nature Education

nAChR activation by Ach (blue) and entry of Na+ ions (red) into cell

Facilitated Diffusion

• Carrier Proteins• Mediated diffusion down concentration gradient

• No energy required

• Rate is limited by the number of proteins in the membrane

• GLUT family of carrier proteins transport glucose across cell membranes• GLUT4 is insulin sensitive

Via WikiMedia Commons

Primary active transport

• Pumps are transport proteins that use energy from ATP directly• Called ATPases

• Transport ions uphill against electrochemical gradients

• Na/K ATPase transports Na+ out and K+ in• Maintains ionic composition of ICF and ECF

• K+ and Na+ gradients are the basis of electrical properties of cells (membrane potentials)

• Na+ gradient provides energy for use in transporting other molecules

• Others: Ca2+ ATPase, H+ ATPase in stomach

Primary active transport

Via WikiMedia Commons

Secondary active transport

• Uses energy stored in ionic gradients to transport other molecules

• Transport protein couples downhill flow of ion to uphill flow of other molecule

• Cotransport• Both molecules move in same direction

• Na+/ Glucose

• Exchange/ countertransport• Molecules move in opposite direction

• Na+/H+ exchanger

Secondary active transport

Via WikiMedia Commons

Vesicular transport

• Transport via membrane bound vesicles• Endocytosis- taking things into the cell

• Phagocytosis- solid matter

• Pinocytosis- ECF and solutes

• Exocytosis- secretion of products from the cell• Mucus, proteins, neurotransmitters, hormones

Exocytosis

Epithelia transport

• Apical membrane: faces lumen

• Basolateral membrane: faces ECF, attached to basement membrane

• Tight junctions- prevent leakage between cells.

Transepithelial transport

• NaCl, glucose and H2O in the small intestine and kidneys• Requires openings on both apical and basolateral membranes of cells.

• NaCl• Apical: enters via diffusion through ion channels

• Baso: primary active transport (Na/K pump) moves Na+ to ECF

• Cl- passively follows Na+

• Glucose• Apical: secondary active transport (co-transport with Na+ via SGLT)

• Baso: facilitated diffusion into ECF (GLUT)

• Water• Apical: osmosis brings water into cell

• Baso: Water follows solute out of cell via osmosis (AQP)

Transepithelial transport

Membrane as a capacitor

• A capacitor is something that can store charge

• Because the lipid bilayer is impermeable to charged ions, membrane pores can be used to regulate the internal and external environments

• By regulating the ionic composition of the ECF and ICF, the cell can create an electrochemical potential between the inside and outside of the membrane

Gradients

• Charge and concentration gradients may be in the same direction (Na+) or opposite directions (K+)• Na+ concentration and electrical gradients pull it in to the cell

• K+ concentration pulls out of cell

• K+ electrical pulls into cell

At equilibrium:

• Electrical and chemical potential differences are equal and opposite

• Δμ𝑥 = 𝑅𝑇 𝑙𝑛𝑋

𝑖

𝑋𝑜

+ 𝑍𝑥 𝐹(ψ𝑖 − ψo)

• Chemical energy difference

• = 𝑅𝑇 𝑙𝑛𝑋

𝑖

𝑋𝑜

• T = temp, R = gas constant, X = ion

• Electrical energy difference• = 𝑍𝑥 𝐹(ψ𝑖 −ψo)

• Z is the valence (or charge) of x• F is Faraday’s constant• (ψ𝑖 − ψo) is charge difference across the membrane

• Also known as membrane potential (Vm)

Net force on a substance