movement through the membrane

electrochemical gradient

- ex: a large sodium ion concentration gradient across the membrane favors the diffusion of these ions from the region of higher sodium concentration to the region of lower sodium concentration. But in addition, the solution above the membrane has a net positive charge while the solution below the membrane has a net negative charge. When considered together, concentration and electrical gradients are called an electrochemical gradient. -In response to electrochemical gradients, ions will diffuse in a directional manner if an appropriate channel exists. For example, if a sodium ion channel were inserted into the membrane in Figure 6.21, the net movement of sodium ions would occur down the electrochemical gradient. At equilibrium, sodium ions would continue to move back and forth through the channel, but at equal rates.

how GLUT 1 works

- note that proteins frequently change shape when they bind to other molecules and that such conformational changes are often a critical step in their function -when glucose binds to GLUT-1, it changes the shape of the protein in a way that moves the sugar through the hydrophobic region of the membrane and releases it on the other side. -diffusion drives the movement -GLUT-1 facilitates diffusion by allowing glucose to enter the carrier from either side of the membrane. Glucose will pass through the carrier in the direction dictated by its concentration gradient. A large variety of molecules move across plasma membranes via specific carrier proteins.

concentration gradient

A difference in solute concentrations creates what is called a concentration gradient. Solutes move randomly in all directions, but when a concentration gradient exists, there is a net movement from regions of high concentration to regions of low concentration. Diffusion down a concentration gradient, or away from the higher concentration, is a spontaneous process because it results in an increase in entropy.

step 4

The sodium ions exit the protein and diffuse to the exterior of the cell.

channel proteins

The movement of substances through channel proteins is passive—meaning it does not require an input of energy. Channel proteins simply enable ions K+) or small polar molecules (e.g., water) to diffuse across lipid bilayers efficiently in response to an existing gradient. When transmembrane proteins assist the passive transport of substances that otherwise would not cross a membrane readily, the process is called facilitated diffusion.

carrier proteins

The movement of water and K+ are examples of facilitated diffusion through channel proteins, but facilitated diffusion can also occur through specialized membrane proteins called carrier proteins.

diffusion once at equilibrium

-Once the molecules or ions are randomly distributed throughout a solution, an equilibrium is established -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.

osmosis

-The movement of water is a special case of diffusion -occurs 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. To drive this point home, let's suppose the concentration of a particular solute is higher on one side of a selectively permeable membrane than it is on the other side. Also suppose that this solute cannot diffuse through the membrane to establish equilibrium. What happens? Water will move from the side with a lower concentration of solute to the side with a higher concentration of solute

aquaporins relation to channel selectivity

-To understand what makes channels selective, first look at the structure of the pore formed across the membrane. The amino acid residues that line a channel's pore are hydrophilic relative to those facing the hydrocarbon tails of the membrane. -aquaporins 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. -Key side chains in the interior of the pore function as a filter. The position of these groups across the channel allows only water molecules, which are capable of interacting with all of the functional groups in a precise manner, to pass through to the other side.

secondary cotransport

-With each cycle, three Na+ ions are exported for every two K+ ions imported. In this way, the outside of the membrane becomes positively charged relative to the inside. In other words, the sodium-potassium pump converts energy from ATP to an electrochemical gradient across the membrane that favors a flow of anions (negative ions) out of the cell and a flow of cations (positive ions) into the cell. -The electrochemical gradient established by Na+/K+-ATPase represents a form of stored energy, much like the electrical energy stored in a battery. -Gradients are crucial to the function of the cell, in part because they make it possible for cells to engage in secondary active transport—also known as cotransport. When cotransport occurs, ATP is not directly used to power transport, but instead an ATP pump provides the energy in the form of a gradient that is used to power the movement of a different solute in a directed manner, often against its particular gradient.

sodium potassium pump example of active transport pump

-classic example of how structural changes can lead to active transport is provided in the sodium-potassium pump, or more formally, Na+/K+-ATPase. The Na+/K+ part of the name refers to the ions that are transported, ATP indicates that adenosine triphosphate is used, and -ase identifies the molecule as an enzyme. -occurs in 8 step process

ion channels

-ions routinely cross membranes by way of specialized transmembrane proteins called ion channels. -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.

GLUT 1 example of carrier protein

-lipid bilayers are only moderately permeable to glucose. -After isolating and analyzing many proteins from red blood cell membranes, researchers found one protein that specifically increases membrane permeability to glucose. When they added this purified protein to liposomes, the artificial membrane transported glucose at the same rate as a membrane from a living cell. This experiment convinced biologists that the membrane protein—now called GLUT-1 (short for glucose transporter 1)—was indeed responsible for transporting glucose across plasma membranes.

osmosis dilution and concentration results

-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. -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.

step 3

A phosphate group from ATP is transferred to the pump. When the phosphate group attaches, the pump changes its shape in a way that opens the ion-binding pocket to the external environment and reduces the pump's affinity for sodium ions

active transport and ATP

ATP (the nucleotide adenosine triphosphate) often provides the energy for active transport by transferring a phosphate group to an active transport protein called a pump. Recall that ATP contains three phosphate groups, and that phosphate groups carry two negative charges. When a phosphate group is transferred from ATP to a pump, its negative charges interact with charged amino acid residues in the protein. As a result, the pump's potential energy increases and its shape changes.

channel proteins

Cells have many different types of pore-like channel proteins in their membranes. 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 is responsible for this selectivity?

step 5

In this conformation, the pump has binding sites with a high affinity for potassium ions facing the external environment

step 8

In this conformation, the pump has low affinity for potassium ions. The potassium ions exit the protein and diffuse into the interior of the cell. The cycle then repeats

hypertonic

Left 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.

hypotonic

Middle 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.

other types of pumps

Other types of pumps move protons (H+), calcium ions (Ca2+), or other ions or molecules across membranes in a directed manner, regardless of the existing gradient. This is an important point. If the gradient were to reverse, pumps would continue to use the same energy source to transport the solutes in the same direction, even if it is not against the existing gradient. As a result, cells can import and concentrate valuable nutrients and ions inside the cell despite their relatively low external concentration. They can also expel molecules or ions, even when a gradient favors diffusion of these substances into the cell.

GLUT 1 example of cotransport

Recall that GLUT-1 facilitates the movement of glucose into or out of cells in the direction of its gradient. Can glucose be moved against its gradient? The answer is yes—a cotransport protein in your gut cells uses the Na+ gradient created by Na+/K+-ATPases to import glucose against its chemical gradient. When Na+ ions bind to this cotransporter, its shape changes in a way that allows glucose to bind. Once glucose binds, the cotransporter changes shape and transports both Na+ ions and glucose to the inside of the cell. Note that in this case, Na+ is moving down its gradient and the glucose is moving against its gradient. After dropping off Na+ ions and glucose, the protein's original shape returns to repeat the cycle. In this way, glucose present in the food you digest is actively transported into your body. The glucose molecules eventually diffuse into your bloodstream and are transported to your brain, where they provide the chemical energy you need to stay awake and learn some biology

isotonic

Right 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.

diffusion

Spontaneous movement of molecules and ions is known as diffusion.

step 2

Three sodium ions diffuse from the inside of the cell, bind to these sites, and activate the ATPase activity in the pump.

step 6

Two potassium ions from outside the cell bind to the pump

step 7

When the potassium is bound, the phosphate group is cleaved from the protein and its structure changes in response—back to the original shape with the ion-binding pocket facing the interior of the cell.

step 1

When Na+/K+-ATPase is in the conformation shown here, binding sites with a high affinity for sodium ions are available.

osomsis on hyper/hypo/iso tonic

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. (Remember that osmosis occurs only when a solute cannot pass through a separating membrane.) -the terms "hypertonic," "hypotonic," and "isotonic" are used only in referring to the effect of water movement into or out of a membrane-enclosed structure, such as a vesicle or cell.

voltage gated channels

a potassium channel in closed and open configurations. The electrical charge on the membrane is normally negative on the inside relative to the outside, which causes the channel to adopt a closed shape that prevents potassium ions (K+) from passing through. When this charge asymmetry is reversed, the shape changes in a way that opens the channel and allows potassium ions to cross. The key point here is that in almost all cases, the flow of ions and small molecules through membrane channels is carefully controlled.

gated channels

aquaporins and many other ion channels are gated channels—meaning that 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.

active transport pumps

cells move molecules or ions in a directed manner, often aganst an existing gradient. Accomplishing this task requires an input of energy to counteract the decrease in entropy that occurs when molecules or ions are concentrated. It makes sense, then, that transport against a gradient is called active transport.

osmosis considerations

important to note that the solute affects the movement of water across a membrane. Recall that water molecules interact with charged particles and form hydrogen bonds with polar molecules. 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.

difference in channel and carrier proteins

primary difference between channels and carrier proteins is the mechanism of transport. Channels allow movement through a selective pore, much like bridges allow people to cross back and forth over a river. In contrast, carrier proteins selectively pick up a solute on one side of the membrane, then drop it off on the other side. This would be like a ferry picking up people on one side of a river and then dropping them on the other side.