3. Transport across Cell Membranes

45

cytoplasm

Extracellular space

Biomembranes

Fluid Mosaic Model by Singer and Nicolson (1972)

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Anchoring Proteins in Biomembranes

Transmembrane α helix

Extracellular space

or compartment lumen

cytoplasm

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Anchoring Proteins in Biomembranes

β barrel (e.g., porins in outer mitochondrial membrane)

Self assembly

into trimers

Side viewTop view

(perpendicular to membrane)

Side view

(in membrane plane)

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• prenylated proteins

Lipid-linked transmembrane proteins

• myristoylated proteins

Anchoring Proteins in Biomembranes

C14

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• Glycosyl-phosphatidyl-inositol (GPI) anchor

Anchoring Proteins in Biomembranes

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Membrane Transport of Ions

and small, hydrophilic Molecules Very high activation barrier for diffusion of ions and hydrophilic molecules through the

hydrophobic interior of the lipid bilayer

→ Transporter proteins catalyze diffusion of ions and hydrophilic molecules by

providing a hydrophilic path through the membrane

Free diffusion

Catalyzed

diffusion

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Catalyzed Diffusion is ion/molecule selective, and avoids the formation of a

continuous pore

→ Example: Glucose Transporter

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Catalyzed Diffusion of an Ion/hydrophilic molecule S1 through biomembranes

against a concentration gradient requires energy

→ Energie für Transport von S1 kann geliefert werden durch

a) ATP Hydrolyse

b) Transport eines Ions/Moleküls S2 entlang seines Kontzentrationsgradienten

ATP+

H2O

ADP+

H2PO4-

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2nd Law of Thermodynamics: Only those process can occurs spontaneously that

increase the entropy (“disorder”) of the universe.

Gradients of ions/molecules are energy rich states as they are more ordered

than random (= equal) distributions of ions/molecules

processes that build up gradients require the input of energy

processes that diminish gradients release energy

Ion gradients: Ions are particles that carry electric charge. Therefore, biological

ion gradients exhibit unequal distribution of both matter and electrical charge

across the biomembrane. The electrical gradient is termed electrical potential DΨ.

▪ When moving an ion from one side of the biomembrane to the other the energy

of both the gradient of matter and the gradient of charge needs to be

considered: The Gibbes free energy for the movement of ions is termed

electrochemical potential Dµ

DµIon A = R ∙ T ∙ ln + z ∙ F∙ DΨ[Ion A]Seite1

[Ion A]Seite2 electric charge

of the ion 54

Thermodynamics of Ion Gradients

[Na+] = niedrig

[K+] = hoch

[Glucose] = hoch

The functionality of the multiple transport processes across biomembranes in cells

and tissues is achieved through the interplay of multiple transport proteins

→ Example: transfer of glucose from the samm intestine to the blood stream

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Intracellular Transport of Proteins

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Compartment-specific proteins that are encoded in the nuclear genome need to be

imported through the compartment membrane. This can occur through 3 different

mechanisms:

• Co-translational import: ER

• Post-translational import: Mitochondrium, Plastid, Peroxisome, Nucleus, Vacuole

• Vesicle transport: Golgi apparatus, Lysosome, Plasma membrane,

cell wall/extracellular matrix

Intracellular Transport of Proteins

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