Section 3: Cells


Module 3

  1. Cell membrane selectively permeable

-> Carbohydrates for cell recognition

-> Cholesterol for stability & immune response

-> Associated enzyme act as catalysts for reactions

Carriers - Transport molecules across membrane

Identity Markers - Distinguish between self & foreign cells

Pores - Allows water soluble subs in cell

Enzyme - Chemical reactions, breakdown of molecules

Receptor - Attach hormones & neurotransmitters

Diffusion - Electrically charged molecules (Na+) tend to move toward areas of opposite charge (+) (-)

-> Down their electric current gradient

  1. Lipid soluble substances (O2, Co2, steroids) pass through cell membrane with ease
  2. Water soluble substances must use pores/carriers (K+, Na+)

Diffusion Factors

  1. Size of channels (sugar too big), approximately 0.8 nm
  2. Charge on molecule, (+) molecule won't go through (+) channel
  3. Greater the electrochemical gradient, greater its rate of movement down channel/membrane
  4. # channels in membranes, increase in channels, increase in ions in cell

Facilitated Diffusion

  1. Rate of transport limited by number of available proteins
  2. Shows chemical specificity (interact with specific shape)
  3. Shows competitive inhibition

Active Transport - Involves energy

  1. Works against concentration gradient
  2. Requires ATP
  3. Shows competitive inhibition & saturation


  1. Cell has osmolarity 300 milliosmoles (mOsm)
  2. Diffusion of H2O across membrane, net movement H2O down concentration gradient

Solute - Substance being dissolved (ice, sugar, Na+)

Solvent - Liquid doing dissolving (H2O)

Solution - When solute dissolves in solvent

Factors affecting Osmosis

  1. Permeability of membrane to solutes in intra & interstitial
  2. Concentration gradients of solutes in intra & interstitial
  3. Pressure gradient across cell membrane.

Units -> osmole <- # osmotically active particles in solution

  1. Osmolality= # osmoles/ Kg. H2O
  2. Osmolarity= # of osmoles/ L of solution

Tonicity - Ability solution to cause osmosis cross biomembrane

Isotonic (300 mOsm) - Concentrations same on both sides (balance)

Hypotonic (260 mOsm) - Low concentration compared cellular fluids so osmosis into cell (swells)

Hypertonic (360 mOsm) - High concentration compared to cell & osmosis out of cell (cell shrinks)

Concentration Gradients & Membrane Permeabilities

  1. Na+, Ca++, Cl- higher concentration outside cell

-> Concentration gradients make them inside cell

-> Just cause gradient doesn't mean it moves that way

-> All three have very few channels in membrane

  1. K+ higher concentration inside cell

-> Concentration gradient move out of cell

-> Membrane more permeable, some diffuse through

  1. These channels open due to a variety of stimuli

Membrane Potentials

-> Movement of charged particles (ions) affected by electrical gradients (+ & -) attract.

Electrical Potential - Charge difference between two points

Resting Membrane Potential (-70 mv)

  1. Nerves and muscles are excitable cells, they need resting potential
  2. Very minute excess of (-) ions accumulate inner
  3. Cations accumulate outside
  4. Establishment of electrical Potential
  5. All cells in body have one

Equilibrium Potentials

  1. When concentration gradient & electrical gradient are equal in magnitude in opposite directions no net movement

-> Called electrochemical equilibrium

Equilibrium Potential - electrical potential that must be applied to the inside of the cell in order stop concentration gradient movement

  1. Larger the concentration gradient, larger equilibrium potential needed to stop movement of ion

Vertebrate Neuron Equilibrium Potentials

  1. E (K+) = -90mV
  2. E (Na+) = +60 mV

-> Has pores otherwise would diffuse

  1. E (Cl-)= -70mV
  2. Charge needed to keep ion from moving into cell

Sodium Potassium Pump

  1. Pumps 3Na+ ions out & 2K+ in
  2. Helps maintain resting potential -> Electrogenic pump
  3. Pumps against concentration gradient needs ATP
  4. Causes cell to become electronegative
  5. Without it most cells would swell & explode
  6. Cell volume kept constant
  7. Helps with osmosis & solute concentrations

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