Passive Transport and Active Transport (7.3 and 7.4)

Describe the steps of the sodium potassium pump.

1) Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape. 2) Na+ binding stimulates phosphorylation (addition of a phosphate group) of the protein by ATP. 3) Phosphorylation causes the protein to change its shape, decreasing its affinity for Na+, which is expelled to the outside. 4) The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group. 5) Loss of phosphate restores the protein's original shape, which has a lower affinity for K+. 6) K+ is released affinity for Na+ is high again, and the cycle repeats. (translocates three sodium ions out for every two potassium ions pumped in)

Plasmolysis

A cell wall does not give any advantages to the cell if it is immersed in a hypertonic environment. A plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant shrivels, its plasma membrane pulls away from the wall. This is plasmolysis, and it causes the plant to wild and die. This also occurs to the walled cells of bacteria and fungi.

Diffusion

A result of thermal motion (heat energy) is diffusion, the movement of molecules of any substance so that they spread out evenly into the available space. Each molecule moves randomly, but diffusion of a population of molecules may be directional.

Cotransport

A single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport. A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion, analogous to water that has been pumped uphill and performs work as it flows back down. Another transport protein, a contransporter, separate from the pump, can couple the "downhill" diffusion of this substance to the "uphill" transport of a second substance against its own concentration gradient. Ex: A plant cell uses the gradient of hydrogen ions generated by its protein pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell. One transport protein couples the return of hydrogen ions to the transport of sucrose into the cell. This protein can translocate sucrose into the cell against a concentration gradient, but only if the sucrose molecule travels in the company of a hydrogen ion. The hydrogen ion uses the transport protein as an avenue to diffuse down the electrochemical gradient maintained by the proton pump. Plants use sucrose-H+ cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves. The vascular tissue of the plant can then distribute the sugar to nonphotosynthetic organs, like roots.

Electrogenic Pump

A transport protein that generates voltage across a membrane. The sodium potassium pump is the major electrogenic pump of animal cells. By generating voltage across membranes, electrogenic pumps store energy that can be tapped for cellular work.

Sodium-Potassium Pump

ATP can power active transport by transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner than translocates a solute bound to the protein across the membrane. One transport system that works this way is the sodium potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells.

Voltage

All cells have voltages across their plasma membranes. Voltage is electrical potential energy--a separation of opposite charges.

Osmoregulation

Animals or other organisms without rigid cell walls living in a hypertonic or hypotonic environment must have special adaptations for osmoregulation, the control of water balance. The protist Paramecium lives in pond water, which is hypotonic to the cell. Paramecium has a plasma membrane that is much less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. The Paramecium cell doesn't burst because it is also equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis.

Ion Channels and Gated Channels

Another group of channel proteins are ion channels, many of which function as gated channels. Gated channels open or close in response to a stimulus. The stimulus us a substance other than the one to be transported.

Turgid and flaccid

Cells of plants, prokaryotes, fungi, and some protists have walls. When such a cell is immersed in a hypotonic solution (ex: bathed in rainwater) the wall helps maintain the cell's water balance. Like an animal cell, the plant cell swells as water enters by osmosis. However, the relative inelastic wall will only expand so much before it exerts a back pressure on the cell that opposes further water uptake. At this point, the cell is turgid (very firm), which is healthy for the plant cells. If a plant's cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp).

Why is facilitated diffusion considered passive transport?

Despite the help of transport proteins, facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient. Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane, but it does not alter the direction of transport.

Isotonic

If a cell without a wall, like an animal cell, is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane. Water flows across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable.

Hypertonic

If a solution is hypertonic to the cell, the cell will lose water to its environment, shrivel, and probably die. This is why an increase in salinity of a lake can kill animals there (If the lake water becomes hypertonic to the animal's cells, the cells might shrivel and die.)

Hypotonic

If we place an animal cell in a solution that is hypotonic to the cell, water will enter the cell faster than it leaves, and the cell will swell and lyse.

Cystinuria

In certain inherited diseases, specific transport systems are either defective or missing altogether. Cystinuria is a human disease characterized by the absence of a carrier proteins that transports cysteine and other amino acids across the membranes of kidney cells. Kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an individual afflicted with cystinuria develops painful stones from amino acids that accumulate and crystalize in the kidneys.

Concentration Gradient

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance decreases. No work occurs for this to happen; diffusion is a spontaneous process (no energy).

Facilitated Diffusion

Many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This is called facilitated diffusion. Most transport proteins are very specific.

Tonicity

The ability of a solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane, relative to that inside the cell. If there is a higher concentration of non-penetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa.

An ion diffuses not simply down its concentration gradient, but down its electrochemical gradient.

The concentration of sodium ions inside a resting nerve cell is much lower inside than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions "fall" down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side of the membrane. Both electric an chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary.

Membrane Potential

The cytoplasm is negative in charge relative to the extracellular fluid because of an unequal distribution of anions and cations on opposite sides of the membrane. The voltage across a membrane, the membrane potential, ranges from -50 to -200 millivolts.

Passive Transport

The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient represents potential energy and drives diffusion. However, membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. Aquaporins allow water to diffuse very rapidly.

Osmosis

The diffusion of water across a selectively permeable membrane is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

Proton Pump

The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports hydrogen ions (protons) out of the cell. The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution.

Electrochemical Gradient

The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the cell membrane. Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (ion's concentration gradient) and an electrical force (effect of the membrane potential on the ion's movement). This combination of forces acting on an ion is called the electrochemical gradient.

Active Transport

To pump a solute across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against a concentration gradient are all carrier proteins. Active transport enabbles a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. An animal cell has a much higher concentration of sodium ions. The plasma membrane maintains these gradients by pumping sodium out of the cell and potassium into the cell.

Describe an example of diffusion

Uptake of oxygen by a cell performing cellular respiration: Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 that enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.