Transcript Passive Membrane Transport: Diffusion

Lecture 13
Plasma membrane transport
Passive transport
PP68-73
Membrane Transport
• Our cells are bathed in a extracellular fluid
called Interstitial fluid that is derived from
blood contains all sort of nutrients &waste
products.
• Plasma membrane has Selectivity and
Differentially permeable barrier.
• Substances move through plasma membrane
in two ways: Passive process or active process
Passive Membrane Transport:
Diffusion
• Simple diffusion – nonpolar and lipid-soluble
substances
– Diffuse directly through the lipid bilayer
– Diffuse through channel proteins
5.3 Passive transport is diffusion across a membrane
with no energy investment
• Diffusion is a process in which particles spread
out evenly in an available space
– Particles move from an area of more concentrated
particles to an area where they are less
concentrated
– This means that particles diffuse down their
concentration gradient
– Eventually, the particles reach equilibrium where the
concentration of particles is the same throughout
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5.3 Passive transport is diffusion across a membrane
with no energy investment
• Diffusion across a cell membrane does not
require energy, so it is called passive transport
– The concentration gradient itself represents
potential energy for diffusion
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Molecules of dye
Membrane
Equilibrium
Two different
substances
Membrane
Equilibrium
5.4 Osmosis is the diffusion of water across a
membrane
• It is crucial for cells that water moves across their
membrane
– Water moves across membranes in response to solute
concentration inside and outside of the cell by a process
called osmosis ( Unassisted diffusion)versus facilitated
diffusion)
– Osmosis will move water across a membrane down its
concentration gradient until the concentration of solute
is equal on both sides of the membrane
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Lower
concentration
of solute
Solute
molecule
Selectively
permeable
membrane
Higher
concentration
of solute
Equal
concentration
of solute
H2O
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water
Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8a
Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8b
Membrane transport
Based on
• Solutions are a homogenous mixture of a
fluid or gas with two or more components
• Solvent the substance which is present in
the largest amount in a solution
• Intracellular fluid: solution within cell
• Interstitial fluid: solution outside cell
Membrane Transport
• Plasma membranes are selectively permeable
• Some molecules easily pass through the
membrane; others do not
Types of Membrane Transport
• Passive processes
– No cellular energy (ATP) required
– Substance moves down its concentration gradient
• Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
Passive Transport
• Movement from high concentration to an
area of lower concentration
• Driving force is kinetic energy which is
affected by size and temp
• Solutes which travel by simple diffusion must
be
– lipid soluble to pass thru like lipids, vitamin A, D, E
– Small like glucose
Passive Processes
•
What determines whether or not a
substance can passively permeate a
membrane?
1. Lipid solubility of substance
2. Channels of appropriate size
3. Carrier proteins
Passive Processes
•
•
•
•
Simple diffusion
Carrier-mediated facilitated diffusion
Channel-mediated facilitated diffusion
Osmosis
Passive Membrane Transport:
Diffusion
• Simple diffusion – nonpolar and lipid-soluble
substances such as lipids,Vitamins A,D,E smal
molcules like glucose
– Diffuse directly through the lipid bilayer OR
– Diffuse through channel proteins
Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic)
substances diffuse directly through the
phospholipid bilayer
Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
Figure 3.7a
Facilitated diffusion
• Occurs by simple diffusion and down concentration
gradient but uses protein channels
• This type of transport is needed for non-lipid soluble
or large molecules
• Like other forms of diffusion, no energy used
Passive Processes: Facilitated
Diffusion
• Certain lipophobic molecules (e.g., glucose,
amino acids, and ions) use carrier proteins or
channel proteins, both of which:
– Exhibit specificity (selectivity)
– Are saturable; rate is determined by number of
carriers or channels
– Can be regulated in terms of activity and quantity
Facilitated Diffusion Using Carrier
Proteins
• Transmembrane integral proteins transport
specific polar molecules (e.g., sugars and
amino acids)
• Binding of substrate causes shape change in
carrier
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
Figure 3.7b
Facilitated Diffusion Using Channel
Proteins
• Aqueous channels formed by transmembrane
proteins selectively transport ions or water
• Two types:
– Leakage channels
• Always open
– Gated channels
• Controlled by chemical or electrical signals
Small lipidinsoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
Figure 3.7c
• The cell membrane contains hourglass-shaped
proteins that are responsible for entry and exit
of water through the membrane
– Dr. Peter Agre, a physician at the Johns Hopkins
University School of Medicine, discovered these
transport proteins and called them aquaporins
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Osmosis
• Diffusion or movement of water thru the
plasma membrane via aquaporins
• Water is highly polar therefore unable to pass
thru the membrane itself
• Osmosis of water moves down its
concentration gradient
Passive Processes: Osmosis
• Movement of solvent (water) across a
selectively permeable membrane
• Water diffuses through plasma membranes:
– Through the lipid bilayer
– Through water channels called aquaporins (AQPs)
Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
Figure 3.7d
5.6 Transport proteins may facilitate diffusion
across membranes
• Some proteins function by becoming a
hydrophilic tunnel for passage
– Other proteins bind their passenger, change shape,
and release their passenger on the other side
– In both of these situations, the protein is specific for
the substrate, which can be sugars, amino acids,
ions, and even water
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Carrier Proteins
• Are integral transmembrane proteins
• Show specificity for certain polar molecules
including sugars and amino acids
(a)
Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Membrane
Solute
molecules
(sugar)
Figure 3.8a
(b)
Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Left
compartment
Right
compartment
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
H2O
Membrane
Solute
molecules
(sugar)
Figure 3.8b
Importance of Osmosis
• When osmosis occurs, water enters or leaves a
cell
• Change in cell volume disrupts cell function
Tonicity
• Tonicity: The ability of a solution to cause a
cell to shrink or swell
• Isotonic: A solution with the same solute
concentration as that of the cytosol
• Hypertonic: A solution having greater solute
concentration than that of the cytosol
• Hypotonic: A solution having lesser solute
concentration than that of the cytosol
5.5 Water balance between cells and their surroundings
is crucial to organisms
• Tonicity is a term that describes the ability of a
solution to cause a cell to gain or lose water
– Tonicity is dependent on the concentration of a
nonpenetrating solute on both sides of the
membrane
– Isotonic indicates that the concentration of a solute is the
same on both sides
– Hypertonic indicates that the concentration of solute is
higher outside the cell
– Hypotonic indicates a higher concentration of solute
inside the cell
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(a)
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
(b)
Hypertonic solutions
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
(c)
Hypotonic solutions
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9
5.5 Water balance between cells and their surroundings
is crucial to organisms
• Many organisms are able to maintain water balance
within their cells by a process called
osmoregulation
– This process prevents excessive uptake or excessive loss
of water
– Plant, prokaryotic, and fungal cells have different issues
with osmoregulation because of their cell walls
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Isotonic solution
Hypotonic solution
Hypertonic solution
(A) Normal
(B) Lysed
(C) Shriveled
Animal
cell
Plasma
membrane
Plant
cell
(D) Flaccid
(E) Turgid
(F) Shriveled
(plasmolyzed)
Passive Membrane Transport:
Filtration
• The passage of water and solutes through a
membrane by hydrostatic pressure
• Pressure gradient pushes solute-containing
fluid from a higher-pressure area to a lowerpressure area
Table 3.1
Diffusion Through the Plasma
Membrane
Extracellular fluid
Small lipidinsoluble
solutes
Lipid-insoluble
solutes
Lipidsoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(b) Carrier-mediated facilitated
(a)
Simple diffusion
directly through the
phospholipid bilayer
Channel-mediated (d) Osmosis, diffusion
diffusion via protein carrier
through a specific
facilitated diffusion
specific for one chemical; binding
channel protein
of substrate causes shape change through a channel
(aquaporin) or
protein; mostly ions
in transport protein
through the lipid
selected on basis of
bilayer
size and charge
(c)
Figure 3.7
Summary of Passive Processes
Process
Simple
diffusion
Facilitated
diffusion
Osmosis
Energy
Source
Kinetic
energy
Kinetic
energy
Kinetic
energy
• Also see Table 3.1
Example
Movement of O2 through
phospholipid bilayer
Movement of glucose into
cells
Movement of H2O through
phospholipid bilayer or
AQPs
Thank you