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By the end of this section, you will be able to:
  • Understand how electrochemical gradients affect ions
  • Distinguish between primary active transport and secondary active transport

Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Electrochemical gradient

We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference 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 same time, cells have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than does the extracellular fluid. So in a living cell, the concentration gradient of Na + tends to drive it into the cell, and the electrical gradient of Na + (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K + , a positive ion, also tends to drive it into the cell, but the concentration gradient of K + tends to drive K + out of the cell ( [link] ). The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient    .

Art connection

This illustration shows a membrane bilayer with a potassium channel embedded in it. The cytoplasm has a high concentration of potassium associated with a negatively charged molecule. The extracellular fluid has a high concentration of sodium associated with chlorine ions.
Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. (credit: “Synaptitude”/Wikimedia Commons)

Injection of a potassium solution into a person’s blood is lethal; this is used in capital punishment and euthanasia. Why do you think a potassium solution injection is lethal?

Moving against a gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy is harvested from ATP generated through the cell’s metabolism. Active transport mechanisms, collectively called pumps , work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. (Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) 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.

Questions & Answers

What is Staining?
Fazal Reply
what is biology
Biology is the study of life
what is biology
Ysabella Reply
biology is a study of living things
Biology is a diverse branch of science that deals with mostly living things
What happen when inhibit the transcription?
what is the effect of not doing sexual intercourse
what is the mechanism of cellular respiration
Rita Reply
what is enzyme
garry Reply
They are organic catalysts that alter the rate of chemical reactions in the body.
what is a cell
Praize Reply
Basic Functional Unit of Life
what is biology
Mordi Reply
biology is the study of living organisms and their interactions with one another and their environments
which of the following event does not occur during some stages of interface?
Bangha Reply
What is microfilaments
KHalid Reply
What is multicellular organisms
Ovie Reply
these are organisms with more than two cells
the process when a male toad fertilizer a female eggs is called what?
Ahrebe Reply
how did unicellular organisms form plants and animals or is it that different unicellular organisms formed plants and animald
YXNG Reply
name the components of faeces
undigested carbohydrate, fibre
what are unicellular organisms..?
they have only one cell
faeces contains many undigested food materials, after the food has been digested then it will be absorbed in the blood stream for assimilation.,......... but the remains toxic materials are stored in the rectum these toxic materials are the faeces and it contains bile salts, the polysaccharides .
unicellular organisms are the ones with only single cell.
thanks for your answers guys.
what is class bryophyta
Emefa Reply
how many stages do we have in glycolysis?
10 stages
the presence of a membrane enclosed nuclosed is a characteristics of what
Addai Reply
eukaryotic cell

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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