Download Passive Transport across Plasma Membrane

Passive Transport across Plasma
Membrane
Plasma Membrane Functions
• Membrane Transport
• Cells bathed in interstitial fluid – rich in nutrients (amino acids,
sugars, fatty acids, vitamins, hormones & neurotransmitters, salts,
wastes)
• Selectively Permeable – allows some things into
cell, keeps others out, also keeps substances in the
cell but wastes can exit.
– Damaged cells (burns) – ruin permeability – fluids,
proteins, ions exit dead or damaged cell.
2 types of membrane transport
1. Passive transport – move across membrane
without energy input
– Diffusion – transport for every cell.
– Filtration – across capillary walls
2. Active Transport – cell provides ATP to move
substances across membrane.
Diffusion
• Molecules & ions scatter evenly throughout - due to
kinetic energy.
– Movement from high concentration to low
concentration (down or along the concentration
gradient)
– Faster diffusion with greater difference in gradient
• Molecules diffuse through physical barrier if…
1. Lipid soluble
2. Small enough to pass through channels
3. Assisted by carrier molecule
Passive Membrane Transport:
Diffusion
• Simple diffusion – nonpolar and lipid-soluble
substances
– Diffuse directly through the lipid bilayer
– Substances Diffuse through channel proteins
– Oxygen, carbon dioxide, fat-soluble vitamins.
PLAY
Diffusion
Passive Membrane Transport:
Diffusion
• Facilitated diffusion
– Transport of glucose, amino acids, and ions
– Not transported through lipid bilayer
– Transported substances bind carrier proteins or
pass through protein channels
– Follows concentration gradient
Carrier Proteins – Part of Facilitated
Diffusion
• Are integral transmembrane integral proteins
• Show specificity for certain polar molecules
too large to pass through channels (including
sugars and amino acids )
• Oxygen, water, glucose, ions = passive
transport to save ATP.
Diffusion Through the Plasma
Membrane
Extracellular fluid
Lipidsoluble
solutes
Lipid-insoluble
solutes
Small lipidinsoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7
Diffusion Through the Plasma
Membrane
Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
Figure 3.7
Diffusion Through the Plasma
Membrane
Lipid-insoluble
solutes
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
Figure 3.7
Diffusion Through the Plasma
Membrane
Small lipidinsoluble
solutes
(c) Channel-mediated facilitated
diffusion through a channel
protein; mostly ions selected
on basis of size and charge
Figure 3.7
Diffusion Through the Plasma
Membrane
Water
molecules
Lipid
bilayer
(d) Osmosis, diffusion through
a specific channel protein
(aquaporin) or through the
lipid bilayer
Figure 3.7
Diffusion Through the Plasma
Membrane
Extracellular fluid
Lipidsoluble
solutes
Lipid-insoluble
solutes
Small lipidinsoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7
Passive Membrane Transport: Osmosis
• Occurs when the concentration of a solvent is
different on opposite sides of a membrane
• Diffusion of water across a semipermeable
membrane
• Osmolarity – total concentration of solute
particles in a solution
• Tonicity – how a solution affects cell volume
PLAY
Osmosis
Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8a
Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8b
Passive Membrane Transport: Filtration
• The passage of water and solutes through a
membrane by hydrostatic pressure (still passive
but through pressure gradient)
• Pressure gradient pushes solute-containing fluid
from a higher-pressure area to a lower-pressure
area
• Application – water forced out of capillaries,
urine forced out of glomerulus. (Concentration
gradient may bring molecules back into
capillaries)
Effects of Solutions of Varying Tonicity
• Isotonic – solutions with the same solute
concentration as that of the cytosol
– Not net gain of loss of water in or out of cell
• Hypertonic – solutions having greater solute
concentration than that of the cytosol
– Cell in strong saline solution, cell loses water, shrinks
• Hypotonic – solutions having lesser solute
concentration than that of the cytosol
– Cell in Distilled Water, cell fills up, can burst