Photosynthesis, Big Questions and Answers

Photosynthesis, Light-Driven Process Produces Sugar, ATP and Oxygen

Aug 9, 2009 Donald Reinhardt

It is called the largest biochemical process on the planet and all realize that without plants, life on earth as it is currently known would not be.

A scientific inquiry of plant photosynthesis is intriguing, useful and important.

Q. Why Are Plant Leaves Green?

A. Plants contain chlorophyll pigments in their chloroplasts. Chlorophylls are green pigments.

Sunlight, or white light, is a spectrum of colors: red, orange, yellow, green, blue, indigo and violet. Green leaves, exposed to light, absorb and reflect light. Chlorophyll molecules interact with light and remove red and blue-violet light, but not the green. The green light is reflected back to the eye and green is seen. Remember, white objects reflect back most of the light that strikes their surfaces, and black objects absorb most of the light that strikes them.

Q. What Really Happens in Photosynthesis?

A. Photosynthesis is a physical and chemical process. The light energy in the red and blue regions of the spectrum (680 and 700 nanometers) is captured or trapped by two, distinct chlorophyll reaction centers, PS2 (P680) and PS1 (P700). The chlorophyll systems produce chemical energy from the light energy. The raw materials of carbon dioxide and water are used to make sugar and oxygen.

The universal formula is 6 CO2 + 6 H2O---> C6H12O6 + 6 O2 (see also photo below, click to enlarge; note also that italized numbers next to letters in formula represent subscripts).

The trapping of light energy and subsequent events are unique:

• The light energizes chlorophyll molecules and enables the chlorophyll 680 (PS2) to split water (photolysis).
• When water is split, there is a release of 2 hydrogen ions = (protons) = 2 H+, and 2 electrons = (2e-), plus oxygen= O (written as 1/2 O2). The oxygen is a byproduct released from the plant.

Photosystem 2 absorbs light at 680, can split water and energize electrons. Photosystem 1 absorbs light at 700, cannot split water, receives energized electrons and further energizes them before passing them on.

Next, is the fate of the hydrogens or protons which are pumped by ATP synthetase, an enzyme, from inside the chloroplasts's grana (double click photo below) to the stroma (protoplasm) just outside the grana. When this occurs (due to the proton and electron gradient), ATP is generated.

During the generation of ATP, the electrons and H+ are not wasted. Instead, energized electrons from PS1, P700 chlorophyll molecules are passed outward, and the internalized protons are pumped out from inside the grana to react with NADP and form NADPH.

Q. How Does a Plant Make Sugar from Simple Molecules of Carbon Dioxide?

The carbon dioxide is processed in the stroma which surrounds the chloroplast 's thylakoid membranes (the grana). The carbon dioxide is actually captured by a 5-C (5 carbon-sized) molecule termed ribulose biphosphate (RBiP) to form an unstable 6-C that immediately breaks down into 2 molecules of 3-C phosphoglyceric acid.

Next, in a series of quick steps, the H+, electrons and ATP are used to make diphosphoglyceraldehdye and two molecules are combined to form glucose. These reactions require only the chemical energy furnished by the light reaction. This process is called the Calvin cycle and it is known as the dark phase of photosynthesis, even though it occurs in the light.

Q. Is Sugar, or Glucose, the Only Product of Photosynthesis?

A. Glucose is the prime, singular sugar made by photosynthesis. Oxygen, the other product, is released from leaves, and also may be used within the plant cells for mitochondrial activity.

Plants can manufacture starch, oils, proteins, nucleic acids — essentially, everything they need for survival. However, these products are made from the energy of photosynthesis and mitochondria and are not considered direct products of photosynthesis.

See more about science inquiry and science papers .

Photosynthesis makes the plant world and animal kingdom possible. Plant careers abound: agriculture, agronomy, botany, forestry, greenhouse and nursery retail and wholesale businesses are just a few possibilities. Consider a plant-oriented career.

Sources

Alters, S and B. Alters. 2006, Biology. John Wiley & Sons Inc., Hoboken, N.J. 755 pp

The copyright of the article Photosynthesis, Big Questions and Answers in Scientific Inquiry is owned by Donald Reinhardt. Permission to republish Photosynthesis, Big Questions and Answers in print or online must be granted by the author in writing.

Visible Light Spectrum	Photosynthesis, An Overview Big Picture of Events	Chloroplast Grana Thylakoid Discs Stroma
Chloroplast Thylakoid Discs Electron Micrograph	Photosynthesis Energy Generation Thylakoid Disc