Download Ch 3 Plasma Membrane Notes [Compatibility Mode]

P.M. Functions – general
• Cells are “bathed” in interstitial fluid –
– Contains important substances cells need to survive
– Cells extract nutrients to remain healthy
– Where the cells eject their waste
• The plasma membrane is a selectively permeable
barrier, regulating how substances pass into and out
of the cell
– Allows some substances to pass while excluding
others
P.M. Functions cont.
• Solute –
– Substance being dissolved within a solution
– Ex. Sugar, salt, calcium, etc.
• Solvent –
– Substance in which solutes are dissolved
– “vat” in which substances are dissolved
– Typically water
• Concentration gradient –
– The difference in solute concentration inside and
outside the cell
Passive processes
• Passive processes –
– Does not require energy (ATP)
– Move substances down (or with) a concentration
gradient
• Substances move from areas of higher concentration to
areas of lower concentration
– Types –
•
•
•
•
•
Diffusion
Simple diffusion
Facilitated diffusion
Osmosis
Filtration
Passive processes
• Diffusion –
– The tendency of molecules/ions to scatter
evenly throughout the environment
– Substances move directly through the plasma
membrane if its…
• Lipid soluble
• Small enough to pass through membrane channels
• Assisted by carrier proteins
– Goal of diffusion – to reach equilibrium –
where molecules are moving equally in all
directions – there is no net movement –
molecules are balanced on each side
Passive processes cont.
• Diffusion cont –
– Move from areas of higher concentration to areas of
lower concentration – move down the concentration
gradient
• Molecules move very quickly & erratically
• Bounce off each other
• They don’t like to be close they naturally move to areas
where numbers are lower (move from high to low)
• The greater the concentration difference the faster the
diffusion process
• Movement occurs due to the kinetic energy of the molecules
– The smaller the molecule the faster the diffusion
– Warmer the temp the faster the diffusion
Diffusion
Passive processes cont.
• Diffusion cont. –
– Simple diffusion –
• Unassisted diffusion of lipid-soluble or very small particles
– Substances that are nonpolar
– Oxygen, carbon dioxide, alcohol, fat-soluble vit
• Down the concentration gradient
– Facilitated diffusion • Substances are moved through the plasma membrane by
binding to protein carriers or by membrane channels
• Transport proteins change shape to allow substances
(glucose & simple sugars) through
• Transported down the concentration gradient
Passive processes
Passive processes cont.
• Osmosis –
– Diffusion of water
– Water is polar, but it is a small molecule that can slip
through the plasma membrane as the membrane
changes shape
– Occurs until solute concentrations are balanced
– Movement occurs due to solute concentrations
• Water moves when solute concentrations differ
• Requires less energy to move water than a solute
• Osmolarity – the total number of all solute particles w/in a
solution
• Osmotic pressure – the cells ability to resist further (net)
water movement – when solute concentrations are equal
Osmosis
Passive processes cont
• Osmosis cont. –
– Tonicity –
• A solutions ability to change the tone or shape of cells by
altering their internal water volume
• The number of nonpenetrating solutes
– Isotonic –
• Solutions with the same concentration of nonpenetrating
solutes on either side of the cell or the total solute
concentration
• Solutes are equal inside and outside the cell
• Cells keep their “normal” shape because there is no net
movement of water
• Body fluids = isotonic
Passive process cont.
• Osmosis cont. –
– Hypertonic –
• Solutions with high concentrations of solutes
• Cells loose water trying to equalize the solutes
• Loosing water causes them to shrink or crenate
– Hypotonic –
• Solutions that contain fewer solutes
• The solution bathing the cell is more dilute than the cell itself
• Cells take on water as it tries to equalize the lower number
of solutes on the inside of the cell
• Cells take on so much water they eventually burst or lyse
• Rehydrating – sports drinks, cola, and apple juice are
hypotonic increasing the amount of water cells absorb,
causing rehydration
Tonicities
Passive processes cont.
• Filtration –
– A pressure-driven process that forces water and
solutes through a membrane or capillary wall by fluid
or hydrostatic pressure
– Passive processes
– Involves a pressure gradient that pushes the solute
fluid (filtrate) from higher-pressure areas to lowerpressure areas
– Not a selective process
– Anything that is small enough will be pushed, by
pressure, through the plasma membrane