Membrane transport

Dr. Narendhirakannan RT Assistant Professor Department of Biochemistry Types of Membrane transport Active transport, symport, antiport, exchanger, carrier, passive diffusion, facilitated diffusion, ATPase, ion channel, ion pump, permease, selectivity filter, gating, membrane potential, electrogenic, electroneutral, ABC transporter Principles of membrane transport

(1) Membranes are barriers to movement of water- soluble molecules. Principles of membrane transport

(2) Two classes of transport -channels -carriers (=transporters, carriers, exchangers)

(3) Type of transport distinguished based upon how energy is used

Principles of membrane transport

(4) Kinetics (flux in relation to concentration) Passive and facilitated diffusion

• Net movements of molecules from one site from high concentration to low concentration is diffusion • Passive diffusion is unassisted • Its facilitated diffusion if a protein allows diffusion across a membrane barrier • Channels (with selective permeability) allow diffusion of ions down their concentration gradient • If molecules are charged, membrane potential is also an important force Active transport

Cells can use various forms of energy to drive transport against concentration or charge gradients

What kind of energy is used? –transport of some molecules fueled by the energy of ATP –membrane potential can drive the transport of charged molecules –Light Transport that uses energy directly is considered primary active transport Three Forms of Transport Across the Membrane

• Passive Transport Active Transport Passive Transport: Simple Diffusion • Simple Diffusion – Doesn’t require energy – Moves high to low concentration • Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out. Passive Transport: Facilitated Diffusion

• Facilitated Diffusion – Does not require energy – Uses transport proteins to move high to low concentration • Examples: Glucose or amino acids moving from blood into a cell. Proteins are Crucial to Membrane Function Facilitated Diffusion

Molecules will randomly move through the pores in Channel Proteins. Types of Transport Proteins • Channel proteins are embedded in the cell membrane & have a pore for materials to cross • Carrier proteins can change shape to move material from one side of the membrane to the other Facilitated Diffusion

• Some carrier proteins do not extend through the membrane. – They bond and drag molecules through the lipid bilayer and release them on the opposite side. Active Transport

• Active Transport – Requires energy or ATP • Moves materials from LOW to HIGH concentration – AGAINST concentration gradient Active Transport

• Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in—against concentration gradients. – Called the Sodium- Potassium Pump. Sodium-Potassium Pump

• 3 Na+ pumped in for every 2 K+ pumped out; creates a membrane potential. Active Transport--Exocytosis Exocytosis Type of active transport Moving things OUT Molecules are moved out of the cell by vesicles that fuse the with the plasma membrane. This is how many hormones are secreted and how nerve cells communicate with each other. Exocytosis Exocytic vesicle immediately after fusion with plasma membrane. Active Transport--Endocytosis

• Large molecules move materials into the cell by one of three forms of endocytosis. – Pinocytosis – Receptor-mediated endocytosis – Phagocytosis Active Transport-Pinocytosis

• Most common form of endocytosis. – Takes in dissolved molecules as a vesicle. Active Transport-Pinocytosis

• Cell forms an invagination – Materials dissolve in water to be brought into cell – Called “Cell Drinking” Example of Pinocytosis

• Transport across a capillary cell (blue). Receptor-Mediated Endocytosis

Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc. Active Transport--Receptor- Mediated Endocytosis Active Transport--Phagocytosis

Used to engulf large particles such as food, bacteria, etc. into vesicles Called “Cell Eating” Phagocytosis About to Occur Phagocytosis

Phagocytosis - Capture of a parasite (green) by Membrane Extensions of an Immune System Cell (orange)

parasite macrophage

Active transport (secondary)

-others are transported against concentration gradients using the energy of an even greater gradient of a second molecule Active Transport

• Three main mechanisms: – coupled carriers: a solute is driven uphill compensated by a different solute being transported downhill (secondary) – ATP-driven pump: uphill transport is powered by ATP hydrolysis (primary) – Light-driven pump: uphill transport is powered by energy from photons (bacteriorhodopsin) Active Transport

• Energy is required ATPases

(3) ABC Transporters Many proteins have an “ATP-Binding Cassette” and move large molecules across membranes Endocytosis and Exocytosis

• Exocytosis - membrane vesicle fuses with cell membrane, releases enclosed material to extracellular space. • Endocytosis - cell membrane invaginates, pinches in, creates vesicle enclosing contents Receptor Mediated Endocytosis The Cytoskeleton • The cytoskeleton, a component of structural functions, is critical to cell motility. • Cells have three types of filaments that are distinguishable by the diameter. • Actin filaments (microfilaments): 5-9 nm diameter with twisted strands. Intermediate Filaments: 9-nm diameter

Microtubules: hollow tube-like structure ~ 24 nm diameter Cell Locomotion

Why do we care about cell locomotion? Host defense Angiogenesis Wound healing Cancer metastasis Tissue engineering Steps: Protrusion Adhesion Traction • External signals must dictate the direction of cell migration. • Cell migration is initiated by the formation of large membrane protrusion. • Video microscopy showed that G-actin polymerizes to F-actin. (Drugs can alter this process). • Actin exists as a globular monomer (G-actin) and; A filamentous polymer (F-actin) protein. • The addition of Mg2+, K+ or Na+ to a solution of G-actin induces the formation of F-actin and this process is reversible. • Elastic mechanical property of actin filament.

ATPases

(3) ABC Transporters

-Multi-drug resistant protein can pump drugs out of cells. Some cancers become resistant to cancer therapy by increasing expression of MDR, either by gene duplication or increased transcription