nutrient transport across m embranes
DESCRIPTION
Nutrient Transport across M embranes. Membranes as barriers. Except for H 2 O, most polar molecules do NOT move across the lipid bilayers. Relative speed of nutrient movement across bacterial membranes. substance. permeability. water glycerol T ryptophan ( 色氨酸 ) glucose Cl - K + - PowerPoint PPT PresentationTRANSCRIPT
Shixue Yin (Prof Dr) Yangzhou University
Nutrient Transport across Membranes
Shixue Yin (Prof Dr) Yangzhou University
Except for H2O, most polar moleculesdo NOT move across the lipid bilayers
Membranes as barriers
Shixue Yin (Prof Dr) Yangzhou University
Relative speed of nutrient movement across bacterial
membranes substance permeability
waterglycerolTryptophan ( 色氨酸 )glucoseCl-
K+
Na+
1000.10.0010.00110-6
10-7
10-8
Note that many nutrients are polar
Shixue Yin (Prof Dr) Yangzhou University
Membrane transport systemsare the systems to move nutrients andwaste products across membranes
Uniporter transport
Passive
Active Antiport
Passive diffusion
Facilitated diffusionChannel proteins
SymportABC systemgroup translocation
Shixue Yin (Prof Dr) Yangzhou University
1.Molecules move along a concentration gradient (from region of higher concentration to lower concentration)
2.Movement is driven by random thermal action - no energy output by organism
3.Concentration reaches equilibrium
4.Water, gasses, lipids, small uncharged polar molecules
5.Not primary mode for hydrophilic ions
What is diffusion?What is diffusion?
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Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.
Passive 1 Passive diffusion
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Channel proteins 蛋白通道 transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient, in the case of certain ions from an area of higher concentration to lower concentration.
While water molecules can directly cross the membrane by simple diffusion, as mentioned above, their transport can be enhanced by channel proteins called aquaporins( 运水蛋白 ).
Passive 2. Channel proteins
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1. Diffusion aided by a carrier protein - permease - in cell membrane
2. Carrier provides specificity and increases rate to equilibrium
3. is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
4. Not highly important in prokaryotes
What is facilitated diffusion?What is facilitated diffusion?
Passive 3. facilitated diffusion
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Passive 3 Facilitated diffusion
Diffusion facilitatorprotein Nutrient molec in high conc
Specific binding of nutrient to facilitator protein
Protein conformation (change shape)
Release of nutrients intothe cytoplasm of the cell
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Important points about facilitated diffusion
1. Most transport proteins are specific for a single nutrient.
2. Over time facilitated diffusion results in an equal concentration of nutrient inside and outside the cell
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Uniporters are transport proteins that transport a substance across a membrane down a concentration gradient from an area of greater concentration to lesser concentration.
Uniporter transport is powered by the potential energy of a concentration gradient and does not require metabolic energy.
Passive 4 Uniport
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Passive 4 Uniport
outside
inside
Transport protein
Nutrientin high conc
e.g. potassium uniporter K+
Nutrientin low conc
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1.Movement against a concentration gradient. It can produce and intracellular nutrient concentration 1000x greater than that of the same nutrient outside the cell
2.Aided by a carrier protein
3.Requires energy from cell (ATP - primary or PMF-secondary/simple)
4.Many amino acids and sugars accumulated by this method
What is active transport?What is active transport?
Active transport
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Trans-porters on mem-brane
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Transport proteins include:
1). uniport单运体
2). Symport 共运体
3). Antiport 反运体
In prokaryotic environments, nutrients are often scarce.
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Antiporters are transport proteins that transport one substance across the membrane in one direction while simultaneously transporting a second substance across the membrane in the opposite direction.
Antiporters in bacteria generally use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient. Sodium ions (Na+) and protons (H+), for example, are co-transported across bacterial membranes by antiporters.
Active 1 Antiport
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3. Antiport
outside
inside
transport protein
H+H+
H+H+
H+
H+
H+
H+H+
Nutrient in high conc
compound 1 (nutrient)
in low conc; glucose, and amino acids
compound 2Na+, H+ as anti-anion
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Alternative way to look at Antiport
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ABC system is an example of an ATP-dependent active transport found in various gram-negative bacteria. It involves substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membrane. The periplasmic-binding protein picks up the substance to be transported and carries it to a membrane-spanning transport protein. Meanwhile, an ATP-hydrolyzing protein breaks ATP down into ADP, phosphate, and energy. It is this energy that powers the transport of the substrate, by way of the membrane-binding transporter, across the membrane and into the cytoplasm. Examples of active transport include the transport of certain sugars and amino acids. Over 200 different ABC transport systems have been found in bacteria.
ATP-Binding Cassette (ABC) ATP-Binding Cassette (ABC) transporttransport
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ATP-Binding Cassette (ABC) transportATP-Binding Cassette (ABC) transport
only in Gram only in Gram negative prokaryotesnegative prokaryotes
The transporters are a family of periThe transporters are a family of periplasmic binding proteins that have hiplasmic binding proteins that have high affinity for substrate gh affinity for substrate
Transport channelTransport channel
Supplying energySupplying energy
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Active Transport, The "ABC" System of Transport Step 1This form of active transport involves both transporter proteins and the energy provided by the hydrolysis of ATP. A specific periplasmic-binding protein carries the substance to be transported to a membrane-spanning
Step 2 The molecule to be transported across the membrane enters the transporter protein system
and a molecule of ATP enters the ATP binding site of the ATP-hydrolyzing protein.
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Step 3Energy provided by the hydrolysis of ATP into ADP, phosphate, and energy moves the molecule across the membrane.
Step 4 The carrier protein releases the molecule being transported and the transporter system is ready to be used again.
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Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction.
Symporters use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient. Sulfate (HSO4
-) and protons (H+) as well as phosphate (HPO4
-) and protons (H+) are co-transported across bacterial membranes by symporters.
4. Symport
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4. Symport
outside
inside
transport protein
compound 2compound 1
(nutrient)
e.g. lactose H+
lac permease
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Alternative way to look at symport
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C. group translocationThe phosphotransferase system (PTS)The nutrient is chemically alteredduring transport.
Phosphoenolpyruvate ( 磷酸烯醇丙酮酸 PEP) (a "high energy compound") supplies the energy.Phosphate is transferred to the nutrient by a series of phosphorylation dephosphorylation reactions
Materials such glucose; mannose; fructose; NAG; purines; pyrimidines are transported through this mechanism
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The phosphotransferase system (PTS)
Phosphoenolpyruvate undergoes a series of phosphorylation and dephosphorylation reactions till EnzIIc receives the phosphate
1
Glucose is phosphorylated on EnzIIc and is transported into the inside of the cell.
2
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Enzyme IIc
IIbIIa
Hpr Ipyruvate
PEPP
Glucose uptake by the PTS
P
Pglucosephosphate
Glucose
Phosphoenolpyruvate
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VI. Proton motive force (PMF): an energy source for active transport
A. proton pumping
B. proton gradient
C. charge gradient
D. symport with H+
E. symport with Na+
Note: PMF is also a general energy source
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A. proton pumping
cytoplasmic membrane
H+
H+
H+
H+
H+H+
H+
H+
H+
Most cells "pump" protons out.
This creates two sources of energy:a proton gradient and a charge gradient.
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cytoplasmic membrane
H+
H+
H+
H+
H+ H+
H+
H+
H+
The cell membrane is a barrier that holds the protons back the way a dam holds back water.
The controlled movement of protons back into thecell can be used as energy for nutrient concentration.
B. proton (pH) gradient
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cytoplasmic membrane
++
+
++ +
+
++
When the cell membrane holds back protons it also holds back a charge.
The controlled movement of charge across the cell membrane can also provide energy for nutrient concentration.
C. charge gradient
-----
---
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Thus, proton pumping provides two sources of energy:
This dual energy source is calledproton motive force (PMF).
1) a proton (pH) gradient
2) a charge gradient
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outside
inside
Transport protein
compound 1 (nutrient)H+
H+
D. Symport with H+ allows the
Usually, one proton is used per nutrient molecule
concentration of nutrients using PMF
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outside
inside
transport protein
compound 1 (nutrient)Na+
E. Symport with Na+ also allows the
Usually, one sodium is used per nutrient molecule
concentration of nutrients.
Na+
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Symport with H+ uses both the proton gradient and the charge gradient as energy, while symport with Na+ usesonly the charge gradient.
During symport, uptake of the nutrient and the second molecule is "coupled",
that is they must be taken up together.
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Membrane transport proteinsMembrane transport proteins
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