Nutrient Transport across M embranes

Shixue Yin (Prof Dr) Yangzhou University

Nutrient Transport across Membranes


Except for H2O, most polar moleculesdo NOT move across the lipid bilayers

Membranes as barriers


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


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


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?


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


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


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


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


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


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


Passive 4 Uniport

outside

inside

Transport protein

Nutrientin high conc

e.g. potassium uniporter K+

Nutrientin low conc


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


Trans-porters on mem-brane


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.


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


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


Alternative way to look at Antiport


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


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


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.


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.


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


4. Symport

outside

inside

transport protein

compound 2compound 1

(nutrient)

e.g. lactose H+

lac permease


Alternative way to look at symport


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


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


Enzyme IIc

IIbIIa

Hpr Ipyruvate

PEPP

Glucose uptake by the PTS

P

Pglucosephosphate

Glucose

Phosphoenolpyruvate


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


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.



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



++

+

++ +

+

++

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

-----

---


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


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


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+


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.


Membrane transport proteinsMembrane transport proteins


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Nutrient Transport across M embranes