transporting molecules across cell membranes - seattle...
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Transporting molecules across cell membranes Nutrients, wastes, hormones,
ions
Recall phospholipid bilayer • It’s a barrier! • Only a small subset of molecules may pass
through without help
Cell membrane • Sense & respond to changes in environment • Regulate exchange with environment
Cell membranes are selectively permeable
• Some compounds pass uninhibited through membrane (passive diffusion), some require assistance from membrane proteins (facilitated diffusion), and some require assistance AND energy expenditure (active transport)
1. Diffusion – Passive transport or diffusion – Carrier-mediated (facilitated) transport or diffusion
2. Active Transport – Pumps, bulk transport
PM + proteins mediate transport
Passive (Diffusion & Osmosis)…or Active
Simple diffusion; P
Osmosis; P Channel-mediated; P
Carrier- mediated; A or P
Proteins are responsible • Some are receptors that regulate bulk
transport – Induce endocytosis or exocytosis
• Cyto from “kyto” = hollow vessel
Concept Check • Membranes organize cell activities. The proteins
imbedded in the membranes are essential to their function. These membrane proteins have properties that allow them to “float” in the membrane. Which of the following describe those properties? a) The surface region of the protein in the interior
of the membrane is mostly hydrophobic. b) The surface region of the protein in the interior
of the membrane is mostly hydrophilic. c) The surface region exposed to the exterior
environment is hydrophobic. d) The surface region exposed to the interior
environment is hydrophobic.
Answer • Membranes organize cell activities.
The proteins imbedded in the membranes are essential to their function. These membrane proteins have properties that allow them to “float” in the membrane. Which of the following describe those properties? a) The surface region of the protein in the
interior of the membrane is mostly hydrophobic.
So, what is diffusion? • Solvent: The predominant liquid or gas in
a solution • Solute: The stuff that is dissolved in a
solution • Diffusion: The net movement of solute
from a higher to a lower concentration (Concentration gradient), until equilibrium is achieved. Uses intrinsic Kinetic Energy (KE).
Everything has Energy • Energy = The capacity to perform work;
to rearrange matter • 2 forms:
– Potential Energy (PE): stored energy, due to position or structure
– Kinetic Energy (KE): Energy of motion • Heat is KE associated with the movement of
molecules/atoms
Passive Diffusion
Passive Diffusion
Passive diffusion
• Kinetic energy causes particles to move
• Diffusion occurs due to random collisions between these moving particles
Osmosis • Passive diffusion of H20
down its concentration gradient – Due to solutes inside
flask, there’s more H20 outside than in
– Only H20 can pass through pores
– H20 moves down its concentration gradient and INTO flask
Osmosis + Diffusion • Both are happening all the time across
cell membranes • Osmosis (H20) occurs RAPIDLY,
diffusion (solutes) occurs SLOWLY • H20 moves into cells with high solute
concentration and out of cells with low solute concentration
What happens? • If you place a cell into a solution whose
solute concentration is much lower than that of the cell, what will happen to the cell? Will it:
1. Shrink (crenate; as H20 leaves) 2. Swell (as H20 enters) 3. Remain unchanged
Concept Check • This diagram represents osmosis
of water across a semipermeable membrane. The U-tube on the right shows the results of the osmosis. What could you do to level the solutions in the two sides of the right hand U-tube? a) Add more water to the left hand
side. b) Add more water to the right hand
side. c) Add more solute to the left hand
side. d) Add more solute to the right hand
side.
Answer • This diagram represents osmosis
of water across a semipermeable membrane. The U-tube on the right shows the results of the osmosis. What could you do to level the solutions in the two sides of the right hand U-tube?
c) Add more solute to the left hand side.
Tonicity • Tonicity: measure of the solvent concentration
of a solution and it’s osmotic pressure • Hypertonic: Solute concentration is greater
than the cytoplasm = net movement of H2O out of cell
• Hypotonic: Solute concentration of solution is less than the cytoplasm = net movement into cell
• Isotonic: Solute concentration of solution is equal to the cytoplasm = no net movement of H20
Cells in different solutions
Facilitated diffusion
• Diffusion can be simple or facilitated
• Facilitated is Passive – No energy needed to
move molecules. – Molecules move down
their concentration gradient
Active transport
1. requires ATP (energy) to move molecules.
2. Molecules move against their concentration gradient.
Maintains gradient for cotransport
Endo- & Exocytosis • Bulk transport; A • Requires ATP
– Phagocytosis: ingestion of solid particles • “phagein” = to eat
– Pinocytosis: ingestion of liquid
Endocytosis Active; Requires ATP
Exocytosis Active; Requires ATP