are distinguished from carriers based on how much and how fast molecules move through them Channels transport materials faster and in greater quantities Carriers ID: 784910
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Slide1
Slide2Channels vs Carriers
Channels
are
distinguished from carriers based on how much and how fast molecules move through them. Channels transport materials faster and in greater quantities.Carriers – Uniport one substance or Symport two substances same direction or Antiport two substances opposite directions
2
Slide3Aquaporins
–
transport
water only.A single human aquaporin-1 channel facilitates water transport at a rate of roughly 3 billion water molecules per second. 10 human aquaporins have been discovered
3
Slide44
What determines the rate of diffusion?
There 4 factors:
The steepness of the concentration gradient. The bigger the difference between the two sides of the membrane the quicker the rate of diffusion.
Temperature. Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster.The surface area. The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment.The type of molecule or ion diffusing. Large molecules need more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails.
Slide55
Passive Transport
Passive transport
is diffusion of a substance across a membrane with no energy investment
CO2, H2O, and O2 easily diffuse across plasma membranesDiffusion of water is known as Osmosis
Slide66
Simple Diffusion
Diffusion
Is the tendency for molecules of any substance to spread out evenly into the available spaceMove from high
to low concentrationDown the concentration gradient
Slide77
Effects of Osmosis on Water Balance
Osmosis
Is the movement of water across a semipermeable membrane
Is affected by the concentration gradient of dissolved substances called the solution’s
tonicity
Slide8Figure 7.11
Lower concentration
of solute (sugar)
Higher concentration
of solute
More similar
concentrations of solute
Sugar
molecule
H
2
O
Selectively
permeable
membrane
Osmosis
Slide99
Water Balance of Cells Without Walls
Tonicity
Is the ability of a solution to cause a cell to gain or lose waterHas
a great impact on cells without walls
Slide1010
Three States of Tonicity
Slide1111
Isotonic Solutions
If a solution is
isotonicThe concentration of solutes is the same as it is inside the cellThere will be
NO NET movement of WATER
Slide1212
Hypertonic Solution
If a solution is
hypertonicThe concentration of solutes is greater than it is inside the cellThe cell will
lose water (PLASMOLYSIS)
Slide1313
Hypotonic Solutions
If a solution is
hypotonicThe concentration of solutes is less than it is inside the cellThe cell will
gain water
Slide1414
Water Balance in Cells Without Walls
Animal cell.
An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.
Slide1515
Water Balance of Cells with Walls
Cell Walls
Help maintain water balanceTurgor pressureIs the pressure of water inside a plant cell pushing outward against the cell membrane
If a plant cell is turgidIt is in a hypotonic environmentIt is very firm, a healthy state in most plantsIf a plant cell is flaccidIt is in an isotonic or hypertonic environment
Slide1616
Water Balance in Cells with Walls
Plant cell.
Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.
Slide17Video: Plasmolysis
Slide1818
How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of fructose per ml How will the water molecules move?
There will be
no net movement of water since the concentration of solute in each solution is equal
Slide1919
How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 75 molecules of fructose per ml How will the water molecules move?
There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute
Slide2020
How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of NaCl per ml How will the water molecules move?
Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a
net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute
Slide21http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Slide2222
Facilitated Diffusion
Facilitated diffusion
Is a type of Passive Transport Aided by ProteinsIn facilitated diffusion
Transport proteins speed the movement of molecules across the plasma membrane
Slide2323
Facilitated Diffusion & Proteins
Channel proteins
Provide corridors that allow a specific molecule or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Slide2424
Facilitated Diffusion & Proteins
Carrier proteins
Undergo a subtle change in shape that translocates the solute-binding site across the membraneA carrier protein
alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
Slide2525
Active Transport
Active transport
Uses energy to move solutes against their concentration gradientsRequires energy, usually in the form of
ATP
Slide2626
The
sodium-potassium pump
Is one type of active transport systemActive Transport
P
P
i
EXTRACELLULAR
FLUID
Na+ binding stimulates
phosphorylation by ATP.
2
Na
+
Cytoplasmic Na
+
binds to
the sodium-potassium pump.
1
K
+
is released and Na
+
sites are receptive again;
the cycle repeats.
3
Phosphorylation causes the
protein to change its conformation, expelling Na
+
to the outside.
4
Extracellular K
+
binds to the
protein, triggering release of the
Phosphate group.
6
Loss of the phosphate
restores the protein’s
original conformation.
5
CYTOPLASM
[Na
+
] low
[K
+
] high
Na
+
Na
+
Na
+
Na
+
Na
+
P
ATP
Na
+
Na
+
Na
+
P
ADP
K
+
K
+
K
+
K
+
K
+
K
+
[Na
+
] high
[K
+
] low
Slide2727
Comparison of Passive & Active Transport
Passive transport
. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport
. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
Diffusion.
Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the lipid bilayer.
Facilitated diffusion.
Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,
either channel or carrier proteins.
ATP
Slide2828
Maintenance of Membrane Potential by Ion Pumps
Membrane potential
Is the voltage difference across a membraneAn electrochemical gradientIs caused by the concentration electrical gradient of ions across a membrane
An electrogenic pumpIs a transport protein that generates the voltage across a membrane
Slide2929
Proton Pump
EXTRACELLULAR
FLUID
+
H
+
H
+
H
+
H
+
H
+
H
+
Proton pump
ATP
CYTOPLASM
+
+
+
+
–
–
–
–
–
+
Slide3030
Cotransport
Cotransport
Occurs when active transport of a specific solute indirectly drives the active transport of another soluteInvolves transport by a membrane protein
Driven by a concentration gradient
Slide3131
Example of Cotransport
Cotransport: active transport
driven by a concentration gradient
Slide3232
Bulk Transport
Bulk transport across the plasma membrane occurs by
exocytosis and endocytosisLarge proteinsCross the membrane by different mechanisms
Slide3333
Exocytosis & Endocytosis
In
exocytosisTransport vesicles migrate to the plasma membrane, fuse with it, and release their contentsIn endocytosisThe cell takes in macromolecules by
forming new vesicles from the plasma membrane
Slide3434
Endocytosis
Slide3535
In
phagocytosis
, a cellengulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane-
enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Three Types of Endocytosis
PHAGOCYTOSIS
In
pinocytosis
,
the cell
“gulps” droplets of
extracellular fluid
into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.
Slide3636
0.25 µm
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Ligand
Coat protein
Coated
pit
Coated
vesicle
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).
Plasma
membrane
Coat
protein
Receptor-mediated endocytosis
enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.
Slide3737
Exocytosis
Slide38http://www.teachersdomain.org/asset/tdc02_int_membraneweb/