In Secondary Active Transport In Animal Cells, Which Of The Following Is True?

Structure and Function of Plasma Membranes

24 Agile Transport

Learning Objectives

By the end of this department, you volition be able to do the post-obit:

• Understand how electrochemical gradients affect ions
• Distinguish between principal active send and secondary active send

Active transport mechanisms crave the cell'south free energy, usually in the grade of adenosine triphosphate (ATP). If a substance must move into the cell confronting its concentration slope—that is, if the substance'southward concentration inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the jail cell must use energy to move the substance. Some active send mechanisms motility modest-molecular weight materials, such as ions, through the membrane. Other mechanisms ship much larger molecules.

Electrochemical Gradient

Nosotros have discussed simple concentration gradients—a substance'south differential concentrations across a space or a membrane—just in living systems, gradients are more circuitous. Because ions motion into and out of cells and because cells contain proteins that do not move across the membrane and are more often than not negatively charged, there is besides an electrical gradient, a divergence of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the aforementioned time, cells accept college concentrations of potassium (1000⁺) and lower concentrations of sodium (Na⁺) than the extracellular fluid. Thus in a living prison cell, the concentration gradient of Na⁺ tends to drive information technology into the jail cell, and its electrical gradient (a positive ion) besides drives information technology inward to the negatively charged interior. However, the situation is more than complex for other elements such as potassium. The electrical slope of K⁺, a positive ion, also drives information technology into the cell, only the concentration gradient of Grand⁺ drives K⁺ out of the cell ((Figure)). Nosotros call the combined concentration gradient and electrical accuse that affects an ion its electrochemical gradient.

Visual Connection

Electrochemical gradients arise from the combined effects of concentration gradients and electric gradients. Structures labeled A correspond proteins. (credit: "Synaptitude"/Wikimedia Commons)

Injecting a potassium solution into a person'due south blood is lethal. This is how capital punishment and euthanasia subjects die. Why do you call back a potassium solution injection is lethal?

Moving Against a Slope

To movement substances against a concentration or electrochemical gradient, the prison cell must employ free energy. This energy comes from ATP generated through the prison cell'southward metabolism. Active transport mechanisms, or pumps, piece of work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face up of these passive movements. A cell may spend much of its metabolic free energy supply maintaining these processes. (A cherry claret cell uses most of its metabolic energy to maintain the imbalance betwixt outside and interior sodium and potassium levels that the cell requires.) Considering active transport mechanisms depend on a jail cell's metabolism for energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.

2 mechanisms exist for transporting small-molecular weight cloth and small molecules. Chief agile transport moves ions beyond a membrane and creates a divergence in charge beyond that membrane, which is directly dependent on ATP. Secondary active transport does non directly require ATP: instead, it is the motion of material due to the electrochemical gradient established by primary active transport.

Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: at that place are three protein types or transporters ((Figure)). A uniporter carries one specific ion or molecule. A symporter carries ii different ions or molecules, both in the same management. An antiporter likewise carries two dissimilar ions or molecules, but in different directions. All of these transporters tin also ship small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they practise not crave ATP to work in that process. Some examples of pumps for agile ship are Na⁺-K⁺ ATPase, which carries sodium and potassium ions, and H⁺-Thou⁺ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca²⁺ ATPase and H⁺ ATPase, which conduct only calcium and only hydrogen ions, respectively. Both are pumps.

A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, only in different directions. (credit: modification of work by "Lupask"/Wikimedia Commons)

Primary Active Transport

The primary active ship that functions with the active send of sodium and potassium allows secondary active transport to occur. The 2d transport method is yet agile because it depends on using free energy every bit does chief transport ((Figure)).

Primary active transport moves ions beyond a membrane, creating an electrochemical gradient (electrogenic transport). (credit: modification of work by Mariana Ruiz Villareal)

One of the about important pumps in animate being cells is the sodium-potassium pump (Na⁺-1000⁺ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na⁺ and Grand⁺) in living cells. The sodium-potassium pump moves Thou⁺ into the prison cell while moving Na⁺ out at the same time, at a ratio of 3 Na⁺ for every two M⁺ ions moved in. The Na⁺-K⁺ ATPase exists in two forms, depending on its orientation to the cell's interior or exterior and its affinity for either sodium or potassium ions. The process consists of the following half-dozen steps.

1. With the enzyme oriented towards the cell'southward interior, the carrier has a high affinity for sodium ions. 3 ions bind to the protein.
2. The protein carrier hydrolyzes ATP and a low-free energy phosphate group attaches to it.
3. As a result, the carrier changes shape and reorients itself towards the membrane's exterior. The protein'due south affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape modify increases the carrier's analogousness for potassium ions, and two such ions attach to the poly peptide. Subsequently, the low-free energy phosphate grouping detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the cell's interior.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into the cytoplasm. The protein at present has a higher analogousness for sodium ions, and the process starts again.

Several things have happened equally a issue of this process. At this point, there are more than sodium ions exterior the jail cell than inside and more potassium ions inside than out. For every three sodium ions that motility out, 2 potassium ions move in. This results in the interior being slightly more negative relative to the exterior. This divergence in charge is of import in creating the conditions necessary for the secondary procedure. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Link to Learning

Picket this video to see an active ship simulation in a sodium-potassium ATPase.

Secondary Agile Ship (Co-transport)

Secondary active transport brings sodium ions, and possibly other compounds, into the jail cell. Equally sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel poly peptide exists and is open up, the sodium ions will pull through the membrane. This movement transports other substances that can attach themselves to the ship protein through the membrane ((Effigy)). Many amino acids, as well every bit glucose, enter a cell this way. This secondary process too stores loftier-energy hydrogen ions in the mitochondria of plant and animal cells in order to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy every bit the ions surge through the channel protein ATP synthase, and that energy so converts ADP into ATP.

Visual Connexion

An electrochemical gradient, which chief active transport creates, can motility other substances against their concentration gradients, a process scientists call co-transport or secondary active transport. (credit: modification of work past Mariana Ruiz Villareal)

If the pH outside the prison cell decreases, would you expect the amount of amino acids transported into the cell to increment or decrease?

Section Summary

The combined gradient that affects an ion includes its concentration slope and its electric slope. A positive ion, for example, might lengthened into a new area, downwardly its concentration slope, but if information technology is diffusing into an area of net positive accuse, its electrical gradient hampers its diffusion. When dealing with ions in aqueous solutions, one must consider electrochemical and concentration slope combinations, rather than merely the concentration gradient lonely. Living cells need certain substances that exist inside the cell in concentrations greater than they exist in the extracellular space. Moving substances up their electrochemical gradients requires free energy from the prison cell. Active ship uses free energy stored in ATP to fuel this transport. Active send of small molecular-sized materials uses integral proteins in the cell membrane to motion the materials. These proteins are analogous to pumps. Some pumps, which acquit out primary active send, couple straight with ATP to drive their action. In co-transport (or secondary agile send), free energy from primary send can motility another substance into the prison cell and up its concentration gradient.

Visual Connection Questions

(Figure) Injecting a potassium solution into a person'south blood is lethal. Death penalty and euthanasia utilize this method in their subjects. Why exercise you lot retrieve a potassium solution injection is lethal?

(Figure) Cells typically accept a loftier concentration of potassium in the cytoplasm and are bathed in a high concentration of sodium. Injection of potassium dissipates this electrochemical gradient. In heart muscle, the sodium/potassium potential is responsible for transmitting the signal that causes the muscle to contract. When this potential is dissipated, the indicate can't be transmitted, and the eye stops chirapsia. Potassium injections are as well used to stop the eye from chirapsia during surgery.

(Figure) If the pH outside the cell decreases, would you look the corporeality of amino acids transported into the jail cell to increment or decrease?

(Figure) A subtract in pH ways an increment in positively charged H⁺ ions, and an increase in the electrical gradient beyond the membrane. The transport of amino acids into the prison cell will increment.

Review Questions

Active transport must function continuously because __________.

1. plasma membranes wear out
2. non all membranes are amphiphilic
3. facilitated transport opposes active send
4. improvidence is constantly moving solutes in opposite directions

D

How does the sodium-potassium pump make the interior of the jail cell negatively charged?

1. by expelling anions
2. by pulling in anions
3. by expelling more cations than are taken in
4. past taking in and expelling an equal number of cations

C

What is the combination of an electrical gradient and a concentration gradient called?

1. potential slope
2. electrical potential
3. concentration potential
4. electrochemical gradient

D

Critical Thinking Questions

Where does the cell become energy for active transport processes?

The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps.

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

The sodium-potassium pump forces out iii (positive) Na⁺ ions for every two (positive) Yard⁺ ions it pumps in, thus the cell loses a positive accuse at every cycle of the pump.

Glucose from digested food enters abdominal epithelial cells past active transport. Why would intestinal cells use active ship when nearly trunk cells use facilitated diffusion?

Intestinal epithelial cells utilize active ship to fulfill their specific role every bit the cells that transfer glucose from the digested food to the bloodstream. Intestinal cells are exposed to an environment with fluctuating glucose levels. Immediately after eating, glucose in the gut lumen will be high, and could accrue in intestinal cells by diffusion. However, when the gut lumen is empty, glucose levels are higher in the abdominal cells. If glucose moved by facilitated improvidence, this would cause glucose to catamenia back out of the intestinal cells and into the gut. Active transport proteins ensure that glucose moves into the intestinal cells, and cannot motility back into the gut. It besides ensures that glucose send continues to occur even if high levels of glucose are already present in the intestinal cells. This maximizes the amount of energy the body can harvest from food.

The sodium/calcium exchanger (NCX) transports sodium into and calcium out of cardiac muscle cells. Describe why this transporter is classified equally secondary active transport.

The NCX moves sodium downwards its electrochemical slope into the jail cell. Since sodium's electrochemical slope is created by the Na+/1000+ pump, a transport pump that requires ATP hydrolysis to establish the slope, the NCX is a secondary active transport process.

Glossary

agile transport

method of transporting material that requires energy

antiporter

transporter that carries ii ions or small molecules in different directions

electrochemical slope

a combined electrical and chemical force that produces a gradient

electrogenic pump

pump that creates a charge imbalance

main active transport

active transport that moves ions or small molecules across a membrane and may create a difference in charge beyond that membrane

pump

agile transport mechanism that works against electrochemical gradients

secondary active transport

motion of material that results from principal active ship to the electrochemical gradient

symporter

transporter that carries 2 different ions or small molecules, both in the same direction

transporter

specific carrier proteins or pumps that facilitate move

uniporter

transporter that carries 1 specific ion or molecule

Source: https://opentextbc.ca/biology2eopenstax/chapter/active-transport/

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