Active Transport

3 Key excerpts on "Active Transport"

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  eBook - ePub

  Principles of Animal Nutrition

  • Guoyao Wu(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Active Transport refers to the energy-dependent, carrier protein–mediated movement of substances through the biological membrane against their concentration or electrochemical gradients. An example of Active Transport is the uptake of glutamine from the plasma (0.5–1 mM) into skeletal muscle (10–20 mM) by transporter N (Xue et al. 2010). The energy required for Active Transport is almost exclusively provided by ATP hydrolysis.

  Secondary Active Transport refers to a form of Active Transport where a substance crossing the biological membrane is coupled with the movement of an ion (typically Na+ or H+ ) down its electrochemical potential (Friedman 2008). Secondary Active Transport requires a carrier protein and is commonly referred to as ion-coupled transport. On the basis of the direction of movement of coupled solutes, transporters of secondary Active Transport are known as either symporters (cotransporters) for the same direction of solute movement or antiporters (exchangers or counter-transporters) for the opposite direction of solute movement. An example of secondary Active Transport is the transport of glucose from the lumen of the small intestine into the enterocyte through sodium–glucose-linked transporter-1 (SGLT1; a symporter) on the apical membrane (brush-border membrane) (Boron 2004). SGLT1 utilizes the co-movement of Na+ down its electrochemical gradient to drive the complete uptake of glucose from the intestinal lumen. In contrast, the Na+ /H+ exchanger, which plays a major role in regulating the intracellular pH and Na+ homeostasis, is an antiporter. Secondary Active Transport does not directly require energy (ATP, GTP, or UTP) (Cooper and Hausman 2016).

  Overview of the Animal System

  An animal is composed of nine systems (nervous, digestive, circulatory, musculoskeletal, respiratory, urinary, reproductive, endocrine, and immune systems) and five sense organs (Dyce et al. 1996). Utilization of dietary nutrients by animals involves the cooperation of all the organs in the body. For example, the nervous system controls the food intake and behavior of animals; the digestive system is required for the digestion and absorption of enteral nutrients in diets; the circulatory system is needed for the transport of absorbed nutrients from the stomach and intestine into the general circulation; the respiratory system is responsible for the supply of oxygen to oxidize fatty acids, glucose, and AAs into CO2 and water; the endocrine system regulates nutrient metabolism under physiological and pathological conditions; the immune system protects the animal from infection and ensures a healthy state; the urinary system excretes metabolites from the body; the musculoskeletal system provides the structure, support, and movement (e.g., walking, chewing, swallowing, and breathing) of the organism, with skeletal muscle being the major component of growth (Davis et al. 2002; Field et al. 2002; Scanes 2009); and the reproductive system ensures the continuous propagation of the animal species (Guyton and Hall 2000). Thus, it is important for nutritionists to understand the complexity and interactions of all the anatomical systems in animals. Figure

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  eBook - ePub

  An Essential Introduction to Cardiac Electrophysiology

  • Ken MacLeod(Author)
  • 2013(Publication Date)
  • ICP

    (Publisher)

  Chapter 5

  Active Transporters

  We have learnt from previous chapters that channel opening allows a particular ion to diffuse across the membrane according to its electrochemical gradient. If this ion movement is not counteracted in some way, the concentration differences across the cell membrane will slowly dissipate and cell excitability will gradually be lost. Cells maintain the chemical gradients upon which the ionic movement depends by using another group of membrane transport proteins known collectively as Active Transporters.

  5.1Chapter objectives

  After reading this chapter you should be able to: •List and compare the properties of Active Transporters with those of channels

  •Describe the structure and function of the Na+ /K+ ATPase, the Na+ /Ca2+ exchanger and the Na+ /H+ exchanger

  •Discuss how the intracellular concentrations of Na+ , H+ and Ca2+ are regulated and why the concentration of one of these ions plays a role in determining the concentration of the others

  5.2What do Active Transporters do?

  These proteins move ions (and some amino acids and sugars) against their electrochemical gradients by forming complexes with them and then changing their conformation to deliver the ion on the other side of the membrane. The processes of ion binding, conformational switching and then unbinding are much slower than moving an ion through a channel. There are different types of Active Transporter largely categorized on whether or not they use ATP as an energy source.

  To move ions against their concentration gradient requires some form of energy input. Some transporters use the energy derived from the hydrolysis of ATP and these are called ATPases or pumps. Other transporters do not use ATP but, instead, use the large electrochemical gradients of some ions as an energy source. The movement of one ion against its concentration (and/or electrical) gradient is coupled to the transport of another ion down

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  eBook - ePub

  Anatomy and Physiology For Dummies

  • Erin Odya, Maggie A. Norris(Authors)
  • 2017(Publication Date)
  • For Dummies

    (Publisher)

  Chapter 9 ). Capillaries are only one cell layer thick, and the capillary wall acts as a filter, controlling the entrance and exit of small molecules. Small molecules dissolved in tissue fluid, such as carbon dioxide and water, are pushed through the capillary wall, sliding between the cells and into the blood, while substances dissolved in the blood, such as glucose and oxygen, do the same in the opposite direction. The pulsating force of blood flow provides a steady force to drive this movement.

  The blood pressure in the capillaries is highest at the arterial end and lowest at the venous end. At the arterial end, blood pressure pushes substances through the capillary wall and into the tissue fluid. At the venous end, lower blood pressure (thus higher net osmotic pressure) pulls water from the extracellular fluid (and anything dissolved in it) into the capillary.

  Does passive transport contradict the idea that the cell controls what comes in and out through the membrane? No. The substances that move by passive mechanisms are “ordinary” small molecules and ions that are always present in abundance within and between every cell and kept within a physiologically healthy concentration range by the forces of homeostasis, the first line of defense against physiological abnormality. If at any time the physiological levels get too high or too low, the cell has pumps that can counteract the passive transport.

  Crossing the membrane actively

  Active Transport allows a cell to control which big, active, biological molecules move in and out of the cytoplasm. Active Transport is a fundamental characteristic of living cells (whereas you can set up a system for diffusion, as we note earlier, in a jar of water).

  Like many matters in cell biology, Active Transport mechanisms are numerous and widely varied. When there is a molecule outside the cell that it needs, a simple Active Transport mechanism is used. Cell membranes have embedded proteins for the Active Transport of a single, specific molecule. They must be activated, or opened, for the molecule to be pumped in or out. This is generally done by a binding site on the same protein, but it can also be triggered by another protein with the membrane.

  With very large molecules, another energy-requiring transport method is used. For example, a large protein made within the cell could require far too much space to exit — effectively rupturing the cell. Instead, the protein is packaged in a vesicle whose outer membrane is the same phospholipid bilayer as the cell membrane. During this transport process, called exocytosis, the lipids realign, letting the protein out of the cell without ever breaching the seal. This same process can occur in reverse, called endocytosis,

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