Selectively permeable: • property of biological membranes
which allows some substances to pass more easily than others
Protein: too big to pass through
Selectively Permeable Membrane
Water molecule: can pass through pore
Enlargement of Membrane
Transport proteins:
• membrane proteins that transport specific molecules or ions across biological membranes:– may provide hydrophilic tunnel
through membrane– may bind to a substance and physically
move it across the membrane– are specific for the substance they
move
Movement across the cell membrane can be:
1. Passive • cell does not have
to expend energy
2. Active• energy-requiring process during which a
transport protein pumps a molecule across a membrane, against its conc. gradient; is energetically “uphill”
Passive Transport:
DIFFUSION • net movement of a substance
down a concentration gradient – results from KE of molecules– results from random molecular
movement– continues until equilibrium is
reached (molecules continue to move but there is no net directional movement)
Passive Transport: OSMOSIS
• diffusion of water across a selectively permeable membrane; water moves down its concentration gradient – continues until equil. is reached– at equil. water molecules move in
both directions at same rate
– relationship between solute concentration and movement of water can be described as:
• HYPERTONIC: a solution w/a greater solute concentration than that inside a cell
• HYPOTONIC: a solution w/a lower solute concentration compared to that inside a cell
• ISOTONIC: a solution w/an equal solute concentration compared to that inside a cell
• Osmotic potential: A measure of the potential of water to move between regions of differing concentrations across a water-permeable membrane – osmotic potential of pure water = 0– more solute = negative osmotic
potentialHI LO
EX: Ψ= -0.40 (sol’n)
Ψ= -0.23 (cell)Water out
In cells with cell walls: • in a hypertonic environment,
PLASMOLYSIS occurs; cells shrivel and usually die
• in a hypotonic environment, water moves into cell, causing it to swell; cell becomes more TURGID.
Passive Transport: FACILITATED DIFFUSION
• diffusion of solutes across a membrane, with the help of transport proteins; passive transport because it is movement down a concentration gradient
Examples of Active Transport protein
“pumps”:1. Sodium-Potassium Pump:
– actively pumps Na+ ions out / K+ ions in
– in every pump cycle, 3 Na+ leave and 2 K+ enter cell
– Na+ and K+ are moved against their gradients (both concentration and electric potential!)
• Membrane Potential: voltage across membrane; cell’s inside is negatively charged w/respect to outside – favors diffusion of cations into
cell and anions out of cell
• Electrochemical Gradient: diffusion gradient resulting from the combined effects of membrane potential and conc. gradient
2. Proton Pump: pumps protons (H+ ions) out of the cell, creating a proton gradient (protons are more concentrated outside the membrane than inside); – protons then diffuse back into cell – the force of the proton pushing back
through the membrane is used to power the production of ATP
AND…
3. Cotransport / Coupled Channels: process where a single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their conc. gradients.
– Example: plants use a proton pump coupled with sucrose-H+ transport to load sucrose into specialized cells
ACTIVE TRANSPORT: EXOCYTOSIS & ENDOCYTOSIS
• transport of large molecules (e.g. proteins and polysaccharides) across cell membrane
Exocytosis Endocytosis
* exporting macromolecules by fusion of vesicles w/the plasma membrane* vesicle buds from ER or Golgi and migrates to plasma membrane* used by secretory cells to export products (e.g. insulin in pancreas)
* importing macromolecules by forming vesicles derived from plasma membrane* vesicle forms in localized region of plasma membrane* used by cells to incorporate extracellular substances
Three types of Endocytosis:
1) Phagocytosis: solid particles (“cell eating”)
2) Pinocytosis: fluid droplets (“cell drinking”)
3) Receptor-Mediated Endocytosis:
importing of specific macromolecules by
inward budding of vesicles formed from
COATED PITS
What type of transport does this illustrate?
A. Diffusion
B. Facilitated diffusion
C. Active transport
D. Osmosis