active transport

Examples of active transport in the following topics:

• Secondary Active Transport

  • In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
  • Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.
  • Both antiporters and symporters are used in secondary active transport.
  • Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
  • An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

• Transport of Electrolytes across Cell Membranes

  • Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
  • For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.
  • The mechanisms that transport ions across membranes are facilitated diffusion and active transport.
  • All movement can be classified as passive or active.
  • Active transport requires additional energy as particles move against their gradient.

• Electrochemical Gradient

  • To move substances against the membrane's electrochemical gradient, the cell utilizes active transport, which requires energy from ATP.
  • Because active transport mechanisms depend on a cell's metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.
  • Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.
  • Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.
  • An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement.

• Primary Active Transport

  • The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur .
  • The secondary transport method is still considered active because it depends on the use of energy as does primary transport.
  • Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport).

• Transportation of Photosynthates in the Phloem

  • These sugars are transported through the plant via the phloem in a process called translocation.
  • The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient.
  • They assist with metabolic activities and produce energy for the STEs .
  • Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration.
  • Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements.

• Components of Plasma Membranes

  • The movement of a substance across the selectively permeable plasma membrane can be either "passive"—i.e., occurring without the input of cellular energy—or "active"—i.e., its transport requires the cell to expend energy.
  • The cell employs a number of transport mechanisms that involve biological membranes:
  • Transmembrane protein channels and transporters: transports small organic molecules such as sugars or amino acids
  • Endocytosis: transports large molecules (or even whole cells) by engulfing them
  • These proteins can be receptors, which work as receivers of extracellular inputs and as activators of intracellular processes, or markers, which allow cells to recognize each other.

• Electron Transport Chain

  • The most vital part of this process is the electron transport chain, which produces more ATP than any other part of cellular respiration.
  • Electron transport is a series of redox reactions that resemble a relay race.
  • A prosthetic group is a non-protein molecule required for the activity of a protein.
  • Once it is reduced to QH2, ubiquinone delivers its electrons to the next complex in the electron transport chain.
  • The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen.

• Vascular Tissue: Xylem and Phloem

  • Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant.
  • Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity.
  • The activity of the sieve tubes is controlled by companion cells through plasmadesmata.
  • Xylem and phloem tissue make up the transport cells of stems.
  • The direction of water and sugar transportation through each tissue is shown by the arrows.

• Regulatory Mechanisms for Cellular Respiration

  • Access of glucose to the cell can be regulated using the GLUT proteins that transport glucose .
  • This site has an effect on the enzyme's activity, often by changing the conformation of the protein.
  • These regulators, known as allosteric effectors, may increase or decrease enzyme activity, depending on the prevailing conditions, altering the steric structure of the enzyme, usually affecting the configuration of the active site.
  • GLUT4 is a glucose transporter that is stored in vesicles.
  • A cascade of events that occurs upon insulin binding to a receptor in the plasma membrane causes GLUT4-containing vesicles to fuse with the plasma membrane so that glucose may be transported into the cell.

• Control of Catabolic Pathways

  • Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).
  • High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme's activity.
  • The enzyme's activity is increased when fructose-1,6-bisphosphate levels increase.
  • Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP .
  • Levels of ADP and ATP affect the rate of electron transport through this type of chain transport.