Lecture 5: Molecules Cross Cell Membranes in Many Ways

What are membrane potentials? How do they influence the flow of ions across a membrane?

Membrane potentials exacerbate or ameliorate the flow of ions across the membrane; Drives K+ back in -concentration gradient lets Ka+ out -inside the cell creates a negative charge -Ka+ outside cell is attracted to the negative charges inside the cell

What distinguishes channel proteins from transporter proteins in cellular membranes?

TRANSPORTERS: bind to solute, undergo conformational changes to transfer it over the membrane; moves against electrochemical gradient (low--> high); required energy (i.e. ATP) CHANNELS: Form aqueous pores across membrane that ions go thru; ions move across a gradient (does not require energy)

How does water move through biomembranes? What controls the net directionality of its movement? How do cells handle osmotic effects?

Water crosses the cell membrane via aquaporin channel proteins. Osmotic affects created by high concentration of solutes inside the cell determines net direction of molecular flow. Cells have adapted by having active ion pumps, building cell walls, turgor pressure, water extrusion via vacuoles

How are potassium ions filtered as they enter a potassium channel? What prevents sodium ions from diffusing through the same channel?

Water molecules are replaced from K+ ions by energetically equivalent interactions with carbonyl groups in the constrictor neck of the channel. With water bonds, K+ ions cant flow because water holds onto the molecule. Sodium ions are too small to fit the channel geometry in order to undergo the dehydration process, therefore, passage is energetically unfavorable.

How are water molecules filtered as they enter aquaporin channels? How is the flow of protons across the water channel prevented?

Water molecules bind to carbonyl groups that help them enter channels Asparagine (Asn) side chains block unwanted relay of protons between water molecules

What ensures the directionality of nerve signal propagation along the body of a neuron?

A delay in the structural reactivation of voltage-gated sodium channel guarantees directionality in signal propagation.

What forms of energy can drive active transport via pumps?

ATP, light or redox drive pumps to drive active transport

Why is it important to gate all ion channels in cells?

Because if hydronium ions go through water channels, pH can change.

What are couple transporters (antiport and symport)? Why are they important for cells?

Couple transporters USE ENERGY to transport molecules across a membrane. Antiport: cotransport/integral membrane protein actively transporting 2 or more different molecules or ions across a membrane in OPPOSITE DIRECTIONS Symport: transportation of molecules of more than one substance AT THE SAME TIME

How does glucose enter and exit the cytoplasm of the epithelial cell lining the gut? What energy sources drive this vital cellular process?

Glucose enters by a coupled transporter and Na+ as an energy source (Na+ gradient powered symport) Glucose exits the cells via uniporter down a concentration gradient

How do oxygen and carbon dioxide enter and exit red blood cells? How and why do they "trap" oxygen and carbon dioxide intracellularly?

In capillaries of tissues, carbon dioxide go down concentration gradient and diffuses into the red blood cell. There, the enzyme "carbonic anhydrase" by converting it to bicarbonate. Oxygen exits the cells by simple diffusion after its release from hemoglobin in the cytoplasm

How is transport of large polar molecules across biomembranes achieved? What governs the net direction of flow?

Large molecules enter a cell by ENDOCYTOSIS (small piece of cell wrap around the particle and brought into the cell). Concentration gradient governs net direction of molecular flow

What makes channel proteins selective for transport of a particular ion or small molecule?

They have gated systems and selectivity filters. Many can be temporarily de-sensitized/inactivated, until stimulus disappears.

How does the Na+/Ka+ pump in human cells create an electrochemical gradient that contributes to a cell's membrane potential?

This Na+/Ka+ pump pumps more negative charges out of the cell and lots of K+ (positive ions) inside the cell creating this electrochemical gradient

What are the differences between simple diffusion, facilitated diffusion, and active transport mechanisms?

simple diffusion: particle passes through without channels or aid from integral membrane proteins Facilitated diffusion: process of passive transport of molecules across membrane via integral proteins active transport mechanisms: ATP hydrolysis, symport and antiport (couple transport).