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C7. Energy is obtained in physical or chemical form from the cell’s environment, and energy transformations occur within the cell.


Student Outcome: C7.1

Know that the Sun is the main source of energy for life.



Solar radiation is light energy from the Sun. So you've got the Sun. Millions of kilometers away from the Earth it sits there with all sorts of nuclear reactions going on. It's constantly giving off a huge amount of energy and radiation. By the time the energy and light reach Earth, there isn't much energy left. Scientists have figured out something called a solar constant. The constant is the amount of radiation that actually hits the Earth. They say it's about 29.4 MJ (Megajoules, a unit of energy) for every square meter for each day.



the Earth's atmosphere absorbs much of the incoming energy from the sun. The flow of energy to the Earth is constant. That constant flow is just the amount of EM radiation hitting the outside of the atmosphere. The energy from the Sun has changed by the time it hits the surface of the Earth. At the top of the atmosphere you have the 29.4 MJ. We only see about 17 MJ of energy at the surface of the Earth. Something had to happen to the EM radiation between the top of the atmosphere and the surface of the Earth. Energy and certain types of EM radiation (Infrared and Ultra Violet) have been filtered away.


Source: http://www.geography4kids.com/files/en_solarrad.html



Source: http://rst.gsfc.nasa.gov/Sect16/Sect16_8.html


Student Outcome: C7.2

Understand that light energy can be used by some cells in photosynthesis.


Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems. Photosynthesis is carried out by many different organisms, ranging from plants to bacteria. The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans. All these organisms convert CO2 (carbon dioxide) to organic material by reducing this gas to carbohydrates in a rather complex set of reactions. Electrons for this reduction reaction ultimately come from water, which is then converted to oxygen and protons. Energy for this process is provided by light, which is absorbed by pigments (primarily chlorophylls and carotenoids). Chlorophylls absorb blue and red light and carotenoids absorb blue-green light, but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.


Source: http://photoscience.la.asu.edu/photosyn/education/learn.html


Here is a simple introduction to the word equation for photosynthesis:

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Movie introducing photosynthesis - good graphics but don't worry about the details!



Here is another movie on photosynthesis - unfortunately I can't embed it.


Student Outcome: C7.3

Know that some molecules contain energy that can be released when chemical bonds are broken and new bonds are formed.


Exergonic reaction


  • An exergonic reaction net-generates (gives off) energy (e.g., heat)


  • That is, the products of such a reaction possess less stored energy than do the reactants
  • Only exergonic reactions occur spontaneously.


Source: http://www.mansfield.ohio-state.edu/~sabedon/campbl06.htm


Student Outcome: C7.4

Explain how the ATP/ADP conversion provides energy for use in cells.


ATP functions as a carrier of energy in all living organisms from bacteria and fungi to plants and animals including humans. ATP captures the chemical energy released by the combustion of nutrients and transfers it to reactions that require energy, e.g. the building up of cell components, muscle contraction, transmission of nerve messages and many other functions. ATP has been termed the cell's energy currency.


Adenosine triphosphate (ATP) consists of the nucleoside adenosine linked to three phosphate groups. On removal of the outermost phosphate group, adenosine diphosphate (ADP) is formed while at the same time the energy released can be employed for other reactions. Conversely, with the help of energy, an inorganic phosphate group can be bound to ADP and form ATP. Considerable quantities of ATP are formed and consumed. At rest, an adult converts daily a quantity of ATP corresponding to about one half body-weight, and during hard work the quantity can rise to almost a ton. Most of the ATP synthesis is carried out by the enzyme ATP synthase. At rest Na + , K + -ATPase uses up a third of all ATP formed.


Source: http://nobelprize.org/nobel_prizes/chemistry/laureates/1997/press.html


Check out the video below about ATP synthase - incredible molecular machine!

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Student Outcome: C7.5

Explain why energy pathways involve many small regulated steps.


Metabolism is the set of chemical rections that occur in a cell, which enable it to keep living, growing and dividing. Metabolic processes are usually classified as:

  • catabolism - obtaining energy and reducing power from nutrients.
  • anabolism - production of new cell components, usually through processes that require energy and reducing power obtained from nutrient catabolism.

    There is a very large number of metabolic pathways. In humans, the most important metabolic pathways are:

  • glycolysis - glucose oxidation in order to obtain ATP
  • citric acid cycle (Krebs' cycle) - acetyl-CoA oxidation in order to obtain GTP and valuable intermediates. Go here to see more detail about the Krebs Cycle. (Note it is Wikipedia but it won't bite. Brilliant diagram though).
  • oxidative phosphorylation - disposal of the electrons released by glycolysis and citric acid cycle. Much of the energy released in this process can be stored as ATP.
  • pentose phosphate pathway - synthesis of pentoses and release of the reducing power needed for anabolic reactions.
  • urea cycle - disposal of NH4+ in less toxic forms
  • fatty acid b-oxidation - fatty acids breakdown into acetyl-CoA, to be used by the Krebs' cycle.
  • gluconeogenesis - glucose synthesis from smaller percursors, to be used by the brain.


Student Outcome: C7.6

Describe how a metabolic pathway is controlled by a specific enzyme at each step.

Here is an example of how enzymes control the pathways in the liver

Student Outcome: C7.7

Understand that each step produces intermediate compounds and loses some energy as heat.


  • The process of respiration occurs in a number of steps and an enzyme controls each step. This is referred to as a metabolic pathway.
  • A specific enzyme controls each step.
  • Metabolic pathways occur in small steps because:
    • large steps produce unfavourable conditions such as high temps, pH
    • small steps release small quantities of energy for ATP
    • each step is catalysed by an enzyme
    • energy pathway can be controlled and directed easily
    • small steps provide intermediate compounds (pyruvic acid) that can be used as starting points for other reactions.
  • during each reaction some energy is lost as heat (important in warm blooded animals).


Source: Notes from Cells Theme notes.


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