3.2 The Process of Photosynthesis

Expectations

• Explain how plant cells transfer the energy from light into energy stored in chemical bonds.
• Identify the raw materials and end products of photosynthesis.

Notes

Photosynthesis is a process that converts carbon dioxide into carbohydrates using light energy from the Sun. Photosynthesis releases oxygen as a waste product. This process stores the energy in the chemical bonds of carbohydrates which the photosynthetic organism or those higher in the food chain can use to carry out their life processes.

Water is the most common electron donor for oxygenic photosynthesis so the chemical equation is:

nCO₂ (carbon dioxide) + nH₂O (water) + photons → (CH₂O)_n (carbohydrate) + nO₂ (oxygen)

So for example the production of glucose is:

6CO₂ (carbon dioxide) + 6H₂O (water) + photons → C₆H₁₂O₆ (glucose) + 6O₂ (oxygen)

The reverse process, cellular respiration releases the energy stored in the chemical bonds of carbohydrates. Both cellular respiration and photosynthesis are actually a series of many different chemical reactions. All chemical reactions involve the release or absorption of energy.

What is Photosynthesis?

The process of photosynthesis involes over 100 chemical reactions which happen in two stages: the photo stage and the synthesis stage. The reactions of the photo stage supply the chemical energy needed to drive the synthesis reactions forward. The reactions of the synthesis stage store chemical energy in the bonds of carbohydrates. The photo stage is also known as light reactions and the synthesis stage is called the light-independent or dark reactions.

What is Light?

Light is the visible spectrum of electromagnetic radiation. The main ideas of the wave model of light are as follows:

• Radiation such as light consists of energy waves with both electrical and magnetic properties. This electromagnetic radiation travels outward from its source in regular waves or pulses of energy, rather than in a constant unchanging flow.
• The entire range of radiation is called the electromagnetic spectrum.
• All forms of electromagnetic radiation travel at the speed of light, 299 792 458 m/s, but properties vary with the frequency of the radiation.
• We perceive different frequencies of visible radiation as different colours. Light with the greatest frequency is perceived as violet while light with the lowest frequencies is perceived as red. A combination of all frequencies is perceived as white.

The wave model of light is useful for explaining properties of light such as reflection and refraction.