Bio

How is diffusion down a concentration gradient, or away from the higher concentration a spontaneous process?

-Because it results in an increase in entropy.

Diffusion Across A Selectively Permeable Membrane Establishes An Equilibrium

-At equilibrium, movement across the membrane does not stop. Instead, these solutes continue to move back and forth across the membrane due to their constant random motion. At this point, there is no longer a net movement of solutes across the membrane because they are equally likely to move in any direction.

What is gated channels?

-A channel protein that opens and closes in response to a specific stimulus, such as the binding of a particular substance or a change in voltage across the membrane. -they open or close in response to a signal, such as the binding of a particular substance or a change in the electrical voltage across the membrane.

What are protocells?

-A hypothetical pre-cell structure consisting of a membrane compartment that encloses replicating macromolecules, such as ribozymes. -Simple vesicle-like strcutures that harbor nucleic acids. -Possible intermediates in the evolution of cell.

What are vesicles?

-A membrane enclosed compartment with an aqueous interior that is often used in cells to transport cargo between organelles or to the plasma membrane for secretion.

Whan is scanning electron microscope (SEM)?

-A microscope that produces surface images by reflecting electrons off a specimen coated with a layer of metal atoms. -Produces images of an object's surface -Use freezing and fracturing the membrane before examining it

What is a detergent?

-A small amphipathic molecule that can form micelles. However, unlike amphipathic lipids, detergents are water soluble. When detergents are added to the solution surrounding a lipid bilayer, the hydrophobic tails of the detergent molecule interact with the hydrophobic tails of the lipids and with the hydrophobic portions of transmembrane proteins. These interactions displace the membrane phospholipids and end up forming water-soluble detergent-protein complexes that can be isolated.

What is carrier proteins?

-A transmembrane protein that facilitates diffusion of a small molecule (e.g.,glucose) across a membrane by a process involving a reversible change in the shape of the protein. Also called carrier or transporter.

What is channel proteins?

-A transmembrane protein that forms a pore in a cell membrane, which may open or close in response to a signal. The structure of most channels allows them to admit just one or a few types of ions or molecules. -Some of these channels are for ions, and others are for small polar molecules. Each channel protein has a structure that permits only a particular type of ion or small molecule to pass through it.

What are ion channels?

-A type of channel protein that allows certain ions to diffuse across a plasma membrane down an elctrochemical gradient.

What are aquaporins?

-A type of channel protein that facilitates the movement of water (osmosis) across a plasma membrane. allow water to cross the plasma membrane but exclude other molecules and most ions. Although water can move across lipid bilayers without aquaporins, they are transported over 10 times faster when these channels are present. This increased rate of transport is particularly important for the absorption of water in your gastrointestinal tract.

What are Liposomes?

-An artifical vescile formed by mixing amphipathic lipids, such as phospholipids,together in an aqueous solution.

What happens after molecules or ions are randomly distributed throughout a solution?

-An equilibrium is established

What is peripheral membrane proteins?

-Any membrane protein that does not span the entire lipid bilayer but instead binds to only one side of the bilayer. -Proteins that bind to membrane lipids or integral membrane proteins without passing through it

What is integral membrane proteins?

-Any membrane protein that spans the entire lipid bilayer. Also called transmembrane protein.

What do Micelles and phospholipid bilayers form?

-Both form spontaneously in water-no input of energy is required. -This fact may come as a surprise because at the level of lipid organization,entropy seemingly decreases-the lipids become less disordered as micelles and phospholipid bilayers form.

What do diffusion and osmosis have to do with the first membranes floating in the prebiotic oceans of early Earth?

-Both processes tend to reduce differences in chemical composition between the inside and outside of membrane-bound compartments. But if liposome-like structures first arose in early Earth oceans, their interiors probably didn't offer a radically different environment from the surrounding solution.

How do phospholipds form?

-Bulkier non polar regions consisting of two hydrocarbon tails, tend to form bilayers.

Broad classes of proteins that affect membrane permeability:

-Channels -Carriers -Pumps

What is Istonic?

-Comparative term designating a solution that, if inside a cell or vescile,results in no net uptake or loss of water and thus no effect on the volume of the membrane-bound structure. This solution has the same solute concentration as the solution on the other side of the membrane. -If solute concentrations are equal on both sides of the membrane, the outside is said to be isotonic. There is no net movement of water, and the vesicle maintains its size and shape.

What is hypertonic?

-Comparative term designating a solution that, if outside a cell or vescile, results in the loss of water and shrinkage of the membrane-bound structure. -This solution has a greater solute concentration than the solution on the other side of the membrane. -Used when the solute is unable to pass through the membrane. -If the solution outside the vesicle has a higher concentration of solutes than the interior has, the solution outside is said to be hypertonic relative to the inside of the vesicle. Water moves out of the vesicle into the solution outside. As water leaves, the vesicle shrinks and the membrane shrivels.

What is Hypotonic?

-Comparative term designating a solution that, if outside a cell or vescile,results in the uptake of water and swelling or even bursting of the membrane-bound structure. -This solution has a lower solute concentration than the solution on the other side of the membrane. -Used when the solute is unable to pass through the membrane. -If the solution outside the vesicle has a lower concentration of solutes than the interior has, the outside solution is said to be hypotonic relative to the inside of the vesicle. Water moves into the vesicle via osmosis. The incoming water causes the vesicle to swell, or even burst.

lipid bilayer

-Created when lipid molecules align in paired sheets. -Basic structural element of all cellular membranes -Consists of a two-layer sheet if phospholipid molecules with their hydrophobic tails oriented toward the inside and their hydrophilic heads toward the outside. -Also called phospholipid bilayer

What is concentration gradient?

-Difference across space (e.g., across a membrane) in the concentration of a dissolved substance. -A difference in solute concentrations

What is passive transport?

-Diffusion of a substance across a membrane. -When this event occurs with the assistance of membrane proteins, it is called facilitated diffusion.

What is Osmosis?

-Diffusion of water across a selectively permeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). For osmosis to occur, the solute would not be able to pass through the membrane.

Packed saturated hydrocarbon tails

-Have fewer spaces and more van der Waals interactions. As the length of saturated hydrocarbon tails increases, the forces that hold them together also increase, making the membrane even denser.

How is the transport of chloride ions involved in mucus consistency?

-If a defective CFTR channel prevents chloride ions from leaving cells surrounding airway passageways, a hypertonic state can't be established on airway surfaces. As a result, water isn't pulled from the cells by osmosis to maintain the proper mucus consistency inside the airways. In effect, the disease results from the mismanagement of osmosis.

What about the permeability of very small substances, such as ions?

-It turns out that charged solutes—even tiny ions like sodium (Na+)(Na+)—do not effectively cross lipid bilayers without "help" from membrane proteins. -Without these proteins, sodium ions would cross the membrane a billion times slower than water.

What happens when unsaturated hydrocarbon tails are packed into a lipid bilayer?

-Kinks created by double bonds produce spaces among the tails. These spaces reduce the number of van der Waals interactions that help hold the hydrophobic tails together, weakening the barrier to solutes.

What molecules move across the bilayers slowly?

-Large polar molecules (Ex: Glucose)

Where are peripheral membrane proteins found?

-Membrane surface facing the interior of the cell, while others are found only on the cell exterior.As a result, the interior and exterior surfaces of the plasma membrane are distinct—the peripheral membrane proteins and the ends of transmembrane proteins differ.

When does Osmosis usually occur?

-Only when solutions are separated by a membrane that permits water to cross, but holds back some or all of the solutes—that is, a selectively permeable membrane.

What is facilitated diffusion?

-Passive movement of a substance across a membrane with the assistance of transmembrane carrier proteins or channel proteins.

What do lipid bilayers consist of?

-Phospholipids

If amphipathic lipids are responsible for the lipid bilayer of a cell membrane, how do proteins fit in? Can a protein be amphipathic too?

-Recall that proteins consist of amino acids, which have side chains that range from highly nonpolar to highly polar or charged. It's conceivable, then, that a protein could have a series of nonpolar amino acid residues in the middle of its primary structure flanked by polar or charged amino acid residues. The nonpolar residues would be stable in the interior of a lipid bilayer, while the polar or charged residues would be stable alongside the polar lipid heads and surrounding water.

Cholesterol molecules are present, to varying extents, in the membranes of every cell in your body. What effect does adding cholesterol have on a membrane?

-Researchers have found that adding cholesterol molecules to artificial membranes dramatically reduces their permeability.

What do carrier proteins allow?

-Selectively pick up a solute on one side of the membrane, then drop it off on the other side.

What molecules move across the bilayers quickly?

-Small nonpolar molecules such as oxygen (O2) -If the small molecules are polar but uncharged, such as water (H2o), the rate of transport decreases.

What is diffusion?

-Spontaneous movement of a substance from one region to another, often with a net movement from a region of high concentration to one of low concentration (i.e., down a concentration gradient)

Are amino acids hydrophilic or hydrophobic?

-The amino acid residues that line a channel's pore are hydrophilic relative to those facing the hydrocarbon tails of the membrane.

How does lipid structure affect membrane permeability?

-The amphipathic nature of phospholipids causes them to spontaneously form into bilayers consisting of two lipid sheets held together by hydrophobic interactions. But not all phospholipid bilayers are the same. The length and saturation state of the hydrocarbon tails, in addition to the presence of cholesterol molecules, profoundly influences the physical properties of a membrane and its permeability.

What is electrochemical gradient?

-The combined effect of an ion's concentration gradient and electrical (charge) gradient across a membrane that affects the diffusion of ions.

What do the terms "hypertonic," "hypotonic", and "isotonic" refer to?

-The effect of water movement into or out of a membrane-enclosed structure, such as a vesicle or cell.

What is the primary difference between channels and carrier proteins?

-The mechanism of transport

What is selective permeability?

-The property of a membrane that allows some substances to diffuse across it much more readily than other substances. -This difference in membrane permeability is a critical issue because controlling what passes between the exterior and interior environments is a key characteristic of cells.

What is permeability?

-The tendency of a structure,such as a membrane, to allow a given substance to diffuse across it

What is the Fluid-Mosaic Model?

-The widely accepted hypothesis that cellular membranes consist of proteins embedded in a fluid phospholid bilayer. -Membranes are a dynamic and fluid mosaic of phospholipids and different types of proteins.

How do micelles form?

-They form from free fatty acids or other simple amphipathic lipids with single hydrocarbon chains.

What are Micelles?

-Tiny spherical aggregates created when the hydrophilic heads of a set of lipids face outward and interact with the water, while the hydrophobic tails interact with each other in the interior, away from the water.

When are lipid bilayers more permeable?

-When they contain many short, kinked, unsaturated hydrocarbon tails. A largely unsaturated membrane allows more materials to pass because its interior is held together less tightly. Bilayers containing mostly long, straight, saturated hydrocarbon tails are much less permeable.

Freeze-Fracture technique

-allows researchers to split cell membranes and view the middle of the structure.

What are fatty acids able to do?

-amphipathic and able to assemble into lipid bilayers and form water-filled vesicles. Their experiments showed that ions, and even ribonucleotides, can diffuse across fatty acid bilayers—meaning that monomers could have been available for RNA synthesis inside early Earth vesicles.

What happens when amphipathic lipids are dispersed in an aqueous solution?

-cages of water form around each of the nonpolar tails.If the tails aggregate to form micelles and bilayers, then only the hydrophilic regions of the lipids are exposed and the water cages will melt. This decrease in water molecule organization results in an overall increase in the entropy of the system.

What is a membrane permeability closely related to?

-its level of fluidity, which is a measure of molecular mobility. As temperature drops, molecules in a bilayer move more slowly and become less fluid. As a result, the hydrophobic tails in the interior of membranes pack together more tightly. At very low temperatures, lipid bilayers even begin to solidify.

What has to happen for a cell to emerge from protocells?

-its membrane must be modified to import solutes necessary for life while excluding those that might damage it.

secondary and tertiary structures of proteins are almost what?

-limitlessly variable, it is possible for proteins to form openings and function as a selective passageway across a lipid bilayer.

What do channel proteins allow?

-movement through a selective pore, much like bridges allow people to cross back and forth over a river.

What do Ion channels form?

-pores, or openings, in a membrane. Ions diffuse through these pores in a predictable direction: from regions of high concentration to regions of low concentration and from areas of like charge to areas of unlike charge.

What is the primary importance of the first lipid bilayers?

-simply to provide a container for replicating the first "living" molecule—thought by many researchers to have been RNA -But ribonucleotide monomers would need to be available for these RNAs to replicate.

What happens when water moves by osmosis?

-the solutions on both sides of the membrane experience a change in volume as well as a change in solute concentration. The greater the initial difference in solute concentration, the greater the volume change will be. However, opposing forces, such as the pressure resulting from the downward pull of gravity, exert resistance to the directional movement of water.

What happens if a solute can't cross the membrane?

-then any associated water molecules are also prevented from crossing. Thus, only unbound water molecules are able to diffuse across the membrane during osmosis. -The overall result is that osmosis dilutes the higher concentration of solute as water diffuses across the membrane. This directional movement is spontaneous because entropy will increase as the difference in solute concentrations decreases.

Can negatively charged ribonucleotides get across lipid bilayers and inside lipid-bounded vesicles?

For lipid bilayers consisting of phospholipids, the answer is no.

lipid bilayer are

Highly selective

Do amphipathic lipids dissolve in water?

No, their hydrophilic heads interact with water but their hydrophobic tails do not, instead of dissolving amphipathic lipids form one of two structures, micelles and lipid bilayers

How do uncharged polar and nonpolar molecules cross membranes?

Readily and spontaneously-without an input of energy.

So if membranes don't simply filter solutes based on size, what is responsible for the differences in permeability?

The leading hypothesis to explain the pattern of permeability described earlier is that charged substances and polar molecules above a certain size are more stable dissolved in water—a polar environment—than they would be in the nonpolar interior of membranes.

How Does Temperature Affect the Fluidity and Permeability of Membranes?

The phospholipids in the plasma membrane of a cell have a consistency resembling olive oil. This fluid physical state allows individual lipid molecules to move laterally within each layer, a little like a person moving about in a dense crowd. By tagging individual phospholipids and following their movement, researchers have clocked average speeds of 2 micrometers (μm)(μm) per second. At these speeds, a phospholipid could travel the length of a small bacterial cell in a second.

How is life affected by osmosis?

When water moves across the membranes of cells and vesicles, the volume and concentration of solutes enclosed within the membrane will change. In cells, a rapid change in amount of water can be catastrophic.

How does the solute affect movement?

solute affects the movement of water across a membrane.

How does hydrophobic interactions occur?

when nonpolar structures become surrounded by a "cage" of highly organized water molecules.