Lecture 6 week3 Membrane Transport

Membrane Transport Proteins Many molecules must move back and forth from inside and outside of the cell Most cannot pass through without the assistance of proteins in the membrane bilayer Each cell has membrane has a specific set of proteins depending on the cell

Definitions Solution – mixture of dissolved molecules in a liquid Solute – the substance that is dissolved Solvent – the liquid Isotonic – equal solute on each side of the membrane Hypotonic – less solute outside cell, water rushes into cell and cell bursts Hypertonic – more solute outside cell, water rushes out of cell and cell shrivels

Movement of Small Molecules Specialized membrane transport proteins are responsible for transferring small water-soluble molecules across cell membranes.

Small nonpolar molecules The rate at which a molecule diffuses across a synthetic lipid bilayer depends on its size and solubility. Small nonpolar molecules Uncharged polar molecules specialized membrane transport proteins are required ,

Ion Concentrations The maintenance of solutes on both sides of the membrane is critical to the cell

Membrane Transport Proteins Fall into Two Classes: Transporters and Channels -transport molecules by changing configuration -Solutes pass if they have the right size and charge affected by temperature and can be saturated -transport ions or very small molecules down a concentration gradient, and are more like a hollow tube that can open or shut. They filter and transport molecules based on size and charge. -cannot be saturated. -Do not change shape????

Passive vs Active Transport

Active transport: energy from hydrolysis of ATP is directly coupled to the movement of a specific substance across a membrane

Passive transport

Comparison chart Active Transport versus Passive Transport comparison chart Active Transport Passive Transport Definition Active Transport uses ATP to pump molecules. Transport occurs from a low concentration of solute to high concentration of solute. Requires cellular energy. Movement of molecules DOWN the concentration gradient. It goes from high to low concentration, in order to maintain equilibrium in the cells. Does not require cellular energy. Types of Transport Endocytosis, cell membrane/sodium-potassium pump & exocytosis Diffusion, facilitated diffusion, and osmosis. Functions Transports molecules through the cell membrane against the concentration gradient so more of the substance is inside the cell (i.e. a nutrient) or outside the cell (i.e. a waste) than normal. Disrupts equilibrium established by diffusion. Maintains dynamic equilibrium of water, gases, nutrients, wastes, etc. between cells and extracellular fluid; allows for small nutrients and gases to enter/exit. No NET diffusion/osmosis after equilibrium is established. Types of Particles Transported proteins, ions, large cells, complex sugars. Anything soluble (meaning able to dissolve) in lipids, small monosaccharides, water, oxygen, carbon dioxide, sex hormones, etc. Examples phagocytosis, pinocytosis, sodium/potassium pump, secretion of a substance into the bloodstream (process is opposite of phagocytosis & pinocytosis) diffusion, osmosis, and facilitated diffusion. Importance In eukaryotic cells amino acids, sugars and lipids need to enter the cell by protein pumps, which require active transport. These items either cannot diffuse or diffuse too slowly for survival. It maintains equilibrium in the cell. Wastes (carbon dioxide, water, etc.) diffuse out and are excreted; nutrients and oxygen diffuse in to be used by the cell.

TRANSPORTERS AND THEIR FUNCTIONS Each cellular membrane contains a set of different transporters appropriate to that particular membrane Each transporter is highly selective, often transferring just one type of molecule

Type of transporters the solutes transferred in the same direction carry a single solute across the membrane opposite direction

Electrochemical Gradient This gradient determines the direction of the solute during passive transport

Active Transport 3 main methods to move solutes against an electrochemical gradient Coupled transporters – 1 goes down gradient and 1 goes up the gradient ATP-driven pumps – coupled to ATP hydrolysis Light-driven pumps – uses light as energy, bacteria

The Na+-K+ pump plays a central role in membrane transport in animal cells. this transporter uses the energy of atp hydrolysis to pump Na+ out of the cell and K+ in, both against their electrochemical gradients, although the electrochemical gradient for K+ is close to zero.

Na+-K+ Pump is a Cycle

Epithelial cells possess a glucose–Na+ symport, which they can use to take up glucose from the gut lumen by active transport, even when the concentration of glucose is higher inside the cell than in the gut

Two types of glucose transporters enable gut epithelial cells to transfer glucose across the gut lining. to keep the concentration of Na+ in the cytosol low, Na+ that enters the cell via the Na+-driven glucose symport is pumped out by Na+-K+ pumps. there is ample Na+ in the gut lumen, provided by the diet.

Osmosis The movement of water from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration) is called osmosis.

Cells have several mechanisms to cope with osmotic challenges