Active Transport
Active transport is a process in which cells use energy to move molecules or ions across a cell membrane against their concentration gradient. This allows the cell to accumulate substances that are in low concentration outside the cell. Active transport is essential for maintaining proper cellular function and is carried out by specific transport proteins embedded in the cell membrane.
7 Key excerpts on "Active Transport"
- Thomas P. Colville, Joanna M. Bassert(Authors)
- 2015(Publication Date)
- Mosby(Publisher)
...Some molecules are unable to enter the cell via the passive routes, perhaps because (1) they are not lipid soluble and therefore cannot penetrate the lipid bilayer, (2) they are too large to pass through a membrane pore, or (3) they are on the wrong side of the concentration gradient. Regardless of the reason, these substances must rely on an active cellular process to enter the cell. Substances can be actively moved into or out of the cell by two processes: Active Transport and cytosis. Active Transport Some amino acids and ions must enter and exit cells without the assistance of a concentration gradient. They cannot move through the plasma membrane passively and must rely on energy, in the form of ATP, to assist in their transport across the cell membrane. Like facilitated diffusion, the Active Transport of a substance relies on a carrier protein with a specific binding site, but unlike facilitated diffusion, it does not require a concentration gradient. All cells demonstrate the Active Transport of electrolytes, specifically, sodium (Na +), potassium (K +), calcium (Ca 2+), and magnesium (Mg 2+). In addition, specialized cells can transport iodide (I −), chloride (Cl −), and iron (Fe 2+). Many Active Transport systems move more than one substance at a time. If all of the substances are moved in the same direction, the system is called a symport system. However, if some substances are moved in one direction and others moved in the opposite direction, the system is called an antiport system. One of the best understood examples of Active Transport is the antiport sodium–potassium pump. Na + and K + are the most common cations in the cell, and Active Transport sites for them can be found speckled throughout the plasma membrane. Normally, the concentration of potassium in the cell is 10 to 20 times higher than it is outside the cell. Conversely, sodium is 10 to 20 times higher outside the cell than it is inside...
eBook - ePubFundamentals of Anatomy and Physiology
For Nursing and Healthcare Students
- Ian Peate, Suzanne Evans, Ian Peate, Suzanne Evans(Authors)
- 2020(Publication Date)
- Wiley-Blackwell(Publisher)
...Being aware of what water and electrolytes do and where they go in the body is the first step to understanding how watery creatures like humans function, and to ensuring that they are safe when in your care. Glossary Active Transport: The energy‐requiring process by which substances are moved up their concentration gradient (i.e. from an area with lower concentration to one with a higher concentration). Adenosine diphosphate (ADP): The product produced when Adenosine triphosphate (ATP) is metabolised with the breaking of its high energy phosphate bond to release energy. Adenosine triphosphate (ATP): High‐energy compound which provides energy for the cell when its high energy bonds with phosphate groups are broken. Compartments: Spaces within the body that are separated by living membranes. Cytoplasm: The fluid and contents of a cell. Diffusion: The movement of substances down their concentration gradients from a higher to a lower concentration. Electrolytes: Substances that dissociate in water to form ions. Endocytosis: Bulk movement into a cell. Extracellular: Outside cells. Exocytosis: Bulk movement out of a cell. Facilitated diffusion: Diffusion with the aid of a transport protein. Hydrophilic: Water‐loving, soluble in water. Hydrophobic: Water‐hating, insoluble in water and soluble in lipids Hypertonic: A solute concentration higher than that of intracellular fluid. Cells placed in a hypertonic solution would shrink, as water would flow out into the hypertonic solution by osmosis. Hypotonic: Solute concentration lower than that of intracellular fluid...
- Ian Peate, Muralitharan Nair, Ian Peate, Muralitharan Nair(Authors)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
...The cell must expend energy which is released by splitting ATP into ADP and phosphate. ATP is a compound made up of a base, a sugar and three phosphate groups (triphosphate). These phosphate groups are held together by high-energy bonds, which when broken release a high level of energy. Once one of these phosphate bonds has been broken and a phosphate group has been released, that compound now has only two phosphate groups (diphosphate) leaving a spare phosphate group, which in turn will join up with another adenosine diphosphate group, so forming another molecule of ATP (with energy stored in the phosphate bonds), and the whole process continues (Vickers 2009). In the human body, the four main Active Transport systems considered when discussing cellular energy are: The sodium-potassium pump: sodium and potassium concentration gradients are generated to produce electrical energy The calcium pump: calcium ions essential for muscle contraction are transported into muscle cells The sodium-linked cotransporter: glucose and amino acids are actively transported into the cells and at the same time sodium moves passively via the cotransporter The hydrogen-linked cotransporter: glucose is actively transported into the cell and at the same time hydrogen ions move into the cell via the cotransporter. Endocytosis Endocytosis is the process by which cells take in molecules such as proteins from outside the cell by engulfing them with their cell membrane. It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. During endocytosis only a small section of the cell membrane plays a part to form a fold and a new intracellular pod is formed containing the substance. There are three types endocytosis: Pinocytosis (‘cell drinking’)...
eBook - ePub- Anne Feltz, Anne Feltz(Authors)
- 2020(Publication Date)
- Garland Science(Publisher)
...It is obvious that the ionic currents, I Na and I K, correspond to passive influx of Na + into the cell and passive efflux of K + toward the extracellular medium. For each ionic species that is not at thermodynamic equilibrium, reaching an actual stationary condition requires an Active Transport that compensates for its passive flux. Because an Active Transport requires energy, it must be coupled to a source of metabolic energy either directly (as a primary Active Transport) by using chemical energy from an exergonic reaction or indirectly (as a secondary Active Transport) by dissipating the potential energy already stored in the electrochemical gradient of a “driving” ion (Figure 3.1). For the most primary Active Transports, the energy is provided by hydrolysis of the high energy bond of the third phosphate of ATP. A “pump”, therefore, is a membrane protein with a cytosolic ATPase function that couples ATP hydrolysis with an Active Transport. In this chapter, we will mainly describe the Na +,K + -ATPase, often named the Na +,K + -pump, found in the plasma membrane of all animal cells. This pump provides an Active Transport of Na + (from inside to outside the cell) and K + (from outside to inside the cell) that maintain stable intracellular concentrations of Na + and K +. Figure 3.1 Diagram showing the Active Transports of Na +, K +, Ca 2+, and glucose (G) against their electrochemical gradients. Uniports facilitate substrate permeation across a membrane. The transport direction, towards the cytoplasm in our example, is imposed by the electrochemical gradient of the substrate, which is more concentrated externally. Active Transport requires metabolic energy. When the ion transport is directly coupled to ATP hydrolysis yielding ADP and one inorganic phosphate, P i (which is the case for the Na +,K + -pump), it is said to be a primary transport...
eBook - ePubCell Physiology
Source Book
- Nicholas Sperelakis, Nicholas Sperelakis(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
...Natural and synthetic iono-phores facilitate the movement of small molecules across biological membranes. These ionophores mimic some of the properties of biological transport proteins in that they promote the movement of specific ions by increasing the rates of movements of specific ions across membranes. The larger biological transport proteins are involved in the movement of virtually all substances which cross the membrane (with the exception of some gasses and very hydrophobic substances). Such proteins carry out facili-tated diffusion wherein the solute is equilibrated across the membrane; secondary Active Transport, where solute concentration gradients are produced by coupling to ion or charge gradients; and primary Active Transport, where the solute is transported against a concentration gradient by using the energy of ATP. Biological transport proteins have been identified at the molecular level using tightly bound inhibitors, protein purification, molecular cloning, and expression of the cloned proteins. The primary structure of biological transport proteins determines the secondary and tertiary structure of the protein. The solute and ATP binding sites and pores formed by the folding of the amino acids of the transport protein are responsible for the binding and transport of solute. Delineation of these structures and the dynamics of changes in these structures during the reaction cycle will be important next stages in the understanding of the transport process. The transport process is usually the rate-limiting step in the utilization of substrate, and in the case of ion pumps, is responsible for utilization of the bulk of the cellular ATP. Transport proteins are therefore often under regulatory control. Defects in the transport regulation or transport proteins lead to diseases such as diabetes and cystic fibrosis...
eBook - ePub- Antonio Blanco, Gustavo Blanco(Authors)
- 2017(Publication Date)
- Academic Press(Publisher)
...Twenty different forms have been identified, each of which are designated CX with a number indicating their molecular mass. Each connexon is made of the same type of connexins or by several different types, but each class only assembles connexin channel subunits within the same tissue type. For example, an endothelial cell connexin cannot form a pore with connexins from muscle cells. Active Transport Simple and facilitated diffusion occurs following a gradient and has a negative ∆ G. When the flow of a substance is in the direction opposite to a gradient, the ∆ G is positive and the flow can only take place if energy is supplied. This constitutes Active Transport. Coupling of an exergonic reaction supplies the energy needed to drive the flow of solutes or ions in the thermodynamically unfavorable direction. In most cases, the transport is coupled to the hydrolysis of ATP. Approximately 50% of the ATP produced in cells is invested in Active Transport activities, which indicates the physiological relevance of these processes. Active Transport is mediated by carrier proteins that have the same characteristics of specificity and saturability described for facilitated diffusion. A main example of Active Transport is the maintenance of ion gradients across cell membranes, which depends on the function of specific carriers or “ion pumps.” These ion transporters are grouped into three main classes, known as ATPases P, F, and V. The most important will be considered. P-Type ATPases The basic structure of all the ATP transporters is similar. They are composed of a subunit of approximately 100 kDa, which is responsible for the ATPase activity of the transporter. Specifically, the ATPase P contains a subunit with an ATP-binding site and becomes transiently phosphorylated during the transport process, giving these ATPases its P-type designation...
eBook - ePubCell Boundaries
How Membranes and Their Proteins Work
- Stephen White, Gunnar von Heijne, Donald Engelman(Authors)
- 2021(Publication Date)
- Garland Science(Publisher)
...12 Primary Transporters Transport against Electrical and Chemical Gradients DOI: 10.1201/9780429341328-13 As we learned in Chapter 11, the lipid bilayer–based permeability barrier to ions and polar molecules allows the interiors of cells to have chemical compositions that differ from their surroundings, enabling such diverse functions as synaptic signaling, nutrient uptake, and energy interconversions. Pores and channels control the flow of ions and other molecules across membrane barriers, utilizing electrochemical gradients. In nerve axons, for example, the intracellular concentration of potassium ions is very high relative to the extracellular space, whereas the intracellular concentration of sodium ions is relatively very low. Voltage-dependent ion channels tightly control the flow of sodium and potassium ions down their electrochemical gradients to produce the resting membrane potential (Box 11.1) and the action potential (Box 11.2). Physiologists have long recognized that electrochemical gradients can only exist due to the expenditure of cellular metabolic energy. An early question was exactly how metabolic energy was tapped to maintain the gradient. Another question was whether the expenditure of metabolic energy is involved directly in the production of action potentials. One idea was that the rising phase of the action potential was due to the passive influx of sodium ions, while the falling recovery phase was due to active pumping of sodium ions out of the cell. Alan Hodgkin and Richard Keynes tested the idea using the squid axon. Without understanding at the time exactly why, they knew that dinitrophenol (DNP) interrupted cell metabolism and should consequently inhibit the Active Transport of ions across the membrane. They showed that while DNP caused a gradual decline (minutes to hours) in the amplitudes of the resting and action potentials, it had no effect on the millisecond time scale of the action potential (Figure 12.1A)...
