8.5: Energy

Photosynthesis and Respiration

Photosynthesis

Photosynthesis is the process in which plants, some bacteria and some protistans (the simplest eukaryotes – single or multicellular organisms) use energy provided by the sun, carbon dioxide and water to produce sugar (glucose). The plant cells then respire (“breathe”) and convert these sugars to ATP (which is adenosine triphosphate) the ‘fuel’ used by all living things. Oxygen is released as a waste product from the splitting of water in the reaction (see the equations below). This is an essential component as it is this release of oxygen that created the concentration of oxygen in our breathable atmosphere we have today.

 

A general balanced equation for photosynthesis is:

 

6 CO2(gas) + 12 H2O(liquid) + photons (sun energy)  ----> C6H12O6(aqueous) + 6 O2(gas) + 6 H2O(liquid)

 

carbon dioxide + water + light energy  ---> glucose + oxygen + water

 

This equation can also be presented in a more simplified form:

 

6 CO2(gas) + 6 H2O(liquid) + photons (sun energy) ---> C6H12O6(aqueous) + 6 O2(gas)

 

carbon dioxide + water + light energy ---> glucose + oxygen

 

The picture below shows a close-up of a leaf. Leaves are the primary site of photosynthesis in plants. You can see the cells in the leaf and the veins which transport the water into the leaf and the ATP back out to feed the rest of the plant.

Figure 1:  The Cells of a Leaf
(Image source: http://en.wikipedia.org/wiki/Image:Leaf_1_web.jpg)

Cellular Respiration

This is the process that takes place inside a cell and involves the oxidising of food molecules, such as glucose, into carbon dioxide and water. Oxidation is the combination of a substance with oxygen and involved the LOSS of electrons from an element or compound. The most common or well known oxidation reaction is rust. In this case iron (Fe) loses electrons.  The equation below explains this process:

Fe(s) ---> Fe2+(aq) + 2e-

This is an abbreviated equation just showing the iron’s participation in the oxidation reaction. The 2e- in this equation is the two electrons that have been lost by the solid iron when it has reacted with oxygen.

The energy released from cellular respiration is trapped in the form of ATP (adenosine triphosphate) for use by all of the functions of the cell which require energy. The process of cellular respiration happens in two phases:

  • Glycolysis – this is the breakdown of glucose (sugar) into pyruvic acid (CH3COCO2H is an alpha-keto acid which plays an important role in biochemical processes)
  • The complete oxidation of pyruvic acid into carbon dioxide and water

 

Respiration can be described using the following word equation:

 

glucose + oxygen---> carbon dioxide + water + energy 

 

C6H12O6 + 6 O2  ---> 6 CO2 + 6 H2O + energy

 

A basic summary of this process is:

Respiration:

  • The release of energy from food
  • Takes place in plant and animal cells

Respiration requires:

  • Glucose
  • Oxygen

Respiration Produces:

  • Energy
  • Carbon Dioxide
  • Water

 

The respiration of plants is fairly similar to how humans respire.  Breathing in humans also happens in two phases called:

  • Inspiration                                                                     
  • Expiration

In inspiration oxygen is inhaled into the lungs and placed in microscopic air sacks called alveolus. It is in these sacks that respiration occurs. Basically there is an exchange of oxygen and carbon dioxide between the air and the blood. When we breathe out (or expire) the carbon dioxide is expelled from our bodies into the atmosphere.

 

Australian Curriculum links
(ACSSU225) Chemical change involves substances reacting to form new substances

(ACSBL050)  Biochemical processes in the cell are controlled by the nature and arrangement of internal membranes, the presence of specific enzymes, and environmental factors

(ACSBL052)  Photosynthesis is a biochemical process that in plant cells occurs in the chloroplast and that uses light energy to synthesise organic compounds; the overall process can be represented as a balanced chemical equation

(ACSBL053)  Cellular respiration is a biochemical process that occurs in different locations in the cytosol and mitochondria and metabolises organic compounds, aerobically or anaerobically, to release useable energy in the form of ATP; the overall process can be represented as a balanced chemical equation

Science Understanding  - Energy for Earth processes

(ACSES045)  Processes within and between Earth systems require energy that originates either from the sun or the interior of Earth

(ACSES046)  Thermal and light energy from the Sun drives important Earth processes including evaporation and photosynthesis

Science Understanding  - Energy for biogeochemical processes

(ACSES053)  Photosynthesis is the principal mechanism for the transformation of energy from the sun into energy forms that are useful for living things; net primary production is a description of the rate at which new biomass is generated, mainly through photosynthesis