Cell Phyisology

Transcellular Transport

Through the cell, in at top, out at bottom

Paracellular Transportation

Goes through between cells

Passive Transport

• Does not require energy from the cells

• Materials spontaneously move from a region of higher concentration to an area of lower concentration

• movement down a concentration gradient

Active Transport

• Requires energy input

• materials are actively moved from an area of low concentration to an area of high concentration

• movement against a concentration gradient

simple diffusion, osmosis, facilitated diffusion, bulk filtration

Types of passive transport

Simple diffusion

• The random motion of molecules in a solution

• Passive and random process

• High concentration —> low concentration

• Rapid over short distances

• Net movement till concentrations are uniformly equal

• inversely related to molecular size & fluid viscosity

• Can occur in an open system or across a partition

Osmosis

• The diffusion of water across a selectively permeable membrane

• Free water molecules move from a region of higher H2O concentration to a region of lower H2O concentration, until equilibrium

• Low H2O concentration = high solute concentration

Isotonic

• Equal concentrations of solute

• Equal net flow in & out of cell

Hypertonic

• High concentration of solutes

• water will flow in

Hypotonic

• Low concentration of solutes

• Water will flow out

Osmolarity

Depends on the concentrations of all solutes in the cell or solution

Tonicity

• Refers to the effects of solutions on cells

• Depends on concentrations of only non-permeable solutes

Facilitated Diffusion

• Membrane proteins provide a path for diffusion

• Channels and transporters

Bulk Filtration

• The concentrated movement of large groups of molecules, often including water

• Everything moves in the same direction

• Occurs most often in response to pressure

• Net movement down a concentration gradient due to the random motion of individual molecules

Fick’s Law

describes the flux (movement in 1 direction, over an area)

Movement of molecules

J (Fick’s Law)

Diffusional Coefficient

Ds (Fick’s Law)

Concentration Gradient

-dCs/dx

Factors that increase rate of diffusion

Temperature, Area, membrane permeability, lipid solubility, concentration gradient

Factors that decrease rate of diffusion

Molecular size, membrane thickness

Types of Transporters

Channels, Carriers, Pumps

Characteristics of Ion Channels

• Typically for ions

• Not usually called “transporters”

• Passive: classic facilitated diffusion

Characteristics of Carries

• “Doorman” mechanism

• Uniport, symport, or antiport

• May be passive or active

Characteristics of Pumps

• Typically for ions

• Active: require direct energy usage

Ion Channels

• membrane proteins that act as a passive conduct for ions through the membrane

• Cycle between open and closed conformations

• when open, they permit passive diffusion of ions

• may have selectivity, but still pass ions fast

Carriers

• Facilitate active or passive transport

• Cycle between multiple conformations in which a solute binding site is accessible on one side of the membrane or the other

Alternating Access Model

Open and close one side of a carrier protein before opening the next side

Occlusion State

A condition wherein the solute may not access either inside or outside of a cell

Primary Active Transport

• Directly requires ATP

• The movement of one (or more) solutes against their concentration gradient using chemical energy

Secondary Active Transport

• Utilizes energy stored in a pre-existing gradient

• The movement of one (or more) solutes down their concentration gradients provides energy for the movement of other solutes against their concentration gradient

Uniport

Moves molecules across a membrane independent of other molecules

Symport

Moves two types of molecules in the same direction

Antiport

Moves two types of molecules in opposite directions

2 Na+ out, 1 K+ in, Uses 1 ATP, Primary active transport

Sodium Potassium Pump

Secondary active transport, glucose moves against the gradient, Na+ moves with its gradient

• Glucose Transporter