MEMBRANE MEMBRANE TRANSPORTTRANSPORT

Overview of membrane transport Overview of membrane transport proteinsproteins

Uniporter-catalyzed transportUniporter-catalyzed transport

Uniporters accelerate a reaction that is already Uniporters accelerate a reaction that is already thermodynamically favored (similar to enzymes)thermodynamically favored (similar to enzymes)

This type of transport is termed facilitated This type of transport is termed facilitated transport or facilitated diffusiontransport or facilitated diffusion

Three main features distinguish uniport transport Three main features distinguish uniport transport (facilitated diffusion) from passive diffusion(facilitated diffusion) from passive diffusion The rate of facilitated diffusion is much higher The rate of facilitated diffusion is much higher

than passive diffusionthan passive diffusion Transport is specificTransport is specific Transport occurs via a limited number of Transport occurs via a limited number of

uniportersuniporters

A comparison of the uptake rate of A comparison of the uptake rate of glucose by facilitated diffusion and glucose by facilitated diffusion and

passive diffusionpassive diffusion

An example uniporterAn example uniporter

Subunit composition of the Subunit composition of the NaNa ++ /K/K++ ATPase ATPase

Proposed model for operation of Proposed model for operation of the two-Nathe two-Na ++ /one-glucose /one-glucose

symportersymporter

Transport across epithelia: the Transport across epithelia: the intestinal epithelium is highly intestinal epithelium is highly

polarizedpolarized

Transepithelial movement of Transepithelial movement of glucose and amino acids requires glucose and amino acids requires

multiple transport proteinsmultiple transport proteins

Tight junctions seal off body Tight junctions seal off body cavities and restrict diffusion of cavities and restrict diffusion of

membrane componentsmembrane components

Bulk transport: Endocytosis (Pinocytosis, Phagocytosis)

Receptor-Mediated Endocytosis

Protein structure determines Protein structure determines functionfunction

Proteins are single, unbranched chains of Proteins are single, unbranched chains of amino acid monomersamino acid monomers

There are 20 different amino acidsThere are 20 different amino acids A protein’s amino acid sequence A protein’s amino acid sequence

determines its three-dimensional structure determines its three-dimensional structure (conformation) (conformation)

In turn, a protein’s structure determines In turn, a protein’s structure determines the function of that proteinthe function of that protein

All amino acids have the same All amino acids have the same general structure but the side chain general structure but the side chain

(R group) of each is different(R group) of each is different

Hydrophilic amino acidsHydrophilic amino acids

Hydrophobic and “special” amino Hydrophobic and “special” amino acidsacids

Peptide bonds connect amino Peptide bonds connect amino acids into linear chainsacids into linear chains