Homework questions to be emailed to biovcc@yahoo.com

0        List the wavelengths of light that are most effective for photosynthesis

2        Compare cyclic and noncyclic electron flow and explain the relationship between these components of the light reaction

3        Describe the role of ATP and NADPH in the Calvin cycle

4        Describe what happens to rubisco when the O2 concentration is much higher than CO2

5        Describe two important photosynthetic adaptations that minimize photorespiration

 

LECTURE Notes Photosynthesis and Energy

 

 

Photosynthesis

 

WHY STUDY PHOTOSYNTHESIS?

•Photosynthesis is arguably the most important biological process on earth.

• By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today.

 

•. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials.

•The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world.

 

 

. This being the case, scientific research into photosynthesis is vitally important.

 If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to better use our lands.

 

 

. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new, efficient ways to collect and use solar energy.

 

 

 

Respiration is an Oxidation-Reduction process

 

–    Loss of electrons from one substance = oxidation.

–    Addition of electrons to a substance = reduction.

–    Oxidizing agent - accepts electrons.

–    Reducing agent - gives up electrons.

•       Oxygen - very strong oxidizing agent (hence: “oxidizing” or “oxidation”)

Redox reactions

E.g. Na + Cl -> Na+ + Cl-

 

 

According to the first law of thermodynamics, energy cannot be created or destroyed, but it can be transferred from one place to another and transformed from one form to another. During photosynthesis, energy in the form of light is transferred from the sun, some 92 million miles away, to a pigment molecule in a photosynthetic organism such as a plant. What follows is an interesting series of energy transformations in which light energy is transformed into electrochemical energy and then into energy stored within chemical bonds.

 

Albert Einstein formulated the photon theory of light in which he proposed that light is composed of discrete particles called photons—massless particles traveling in a wavelike pattern and moving at the speed of light. Each photon contains a specific amount of energy. An important difference between the various types of electromagnetic radiation is the amount of energy found in the photons.

Shorter wavelength radiation carries more energy per unit of time than longer wavelength radiation. For example, the photons of gamma rays carry more energy than those of radio waves.

The photons found in gamma rays, X-rays, and UV rays have very high energy. When molecules in cells absorb such energy, the effects can be devastating. Such types of radiation can cause mutations in DNA and even lead to cancer. By comparison, the energy of photons found in visible light is much milder. Molecules can absorb this energy in a way that does not cause permanent harm. Next, we will consider how molecules in living cells absorb the energy within visible light.

 

Pigments Absorb Light Energy

When light strikes an object, one of three things will happen.

•        First, light may simply pass through the object.

•       Second, the object may change the path of light toward a different direction.

•       A third possibility is that the object may absorb the light.

 

The term pigment is used to describe a molecule that can absorb light energy. When light strikes a pigment, some of the wavelengths of light energy are absorbed, while others are reflected.

For example, leaves look green to us because they reflect radiant energy of the green wavelength. Various pigments in the leaves absorb the other light energy wavelengths. At the extremes of color reflection are white and black.

 

A white object reflects nearly all of the visible light energy falling on it, whereas a black object absorbs nearly all of the light energy. This is why it’s coolest to wear white clothes on a sunny, hot day.

What do we mean when we say that light energy is absorbed?

 In the visible spectrum, light energy is usually absorbed by boosting electrons to higher energy levels .

 

Recall from Chapter 2 that electrons are located around the nucleus of an atom. The location in which an electron is likely to be found is called its orbital. Electrons in different orbitals possess different amounts of energy.

Figure 2.9  Energy levels of an atom’s electrons

 

For an electron to absorb light energy and be boosted to an orbital with a higher energy, it must overcome the difference in energy between the orbital it is in and the orbital to which it is going.

For this to happen, an electron must absorb a photon that contains precisely that amount of energy.

 

In the case of photosynthetic pigments, however, a different event happens that is critical for the process of photosynthesis.

Rather than releasing energy, an excited electron in a photosynthetic pigment is removed from that molecule and transferred to another molecule where the electron is more stable. When this occurs, the energy in the electron is said to be “captured,” because the electron does not readily drop down to a lower energy level and release heat or light.

 

 

 

The role of the reaction center is to quickly remove the high energy electron from P680  and transfer it to another molecule, where the electron will be more stable. This molecule is called the primary electron acceptor.

 The transfer of the electron from P680 to the primary electron acceptor is remarkably fast.  It occurs in less than a few picoseconds! (One picosecond equals one-trillionth of a second, also noted as 10-12 s.)

 

Because this occurs so quickly, the excited electron does not have much time to release its energy in the form of heat or light.

After the primary electron acceptor has received this high energy electron, the light energy has been captured and can be used to perform cellular work.

As discussed earlier, the work it performs is to synthesize the energy intermediates ATP and NADPH.

 

 

The Splitting of Water

•       Chloroplasts split water into

–           Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

 

 

•       The light reactions produce three chemical products: ATP, NADPH, and O2.

•        ATP and NADPH are energy intermediates that provide the needed energy and electrons to drive the Calvin cycle.

•        Like NADH, NADPH (nicotinamide adenine dinucleotide phosphate) is an electron carrier that can accept two electrons. Its structure differs from NADH by the presence of an additional phosphate group.

 

 

•       As we have just seen, the Calvin cycle begins by using carbon from an inorganic source, that is, CO2, and ends with organic molecules that will be used by the plant to make other compounds.

•       You may be wondering why CO2 molecules cannot be directly linked to form these larger molecules.

•       The answer lies in the number of electrons that orbit carbon atoms.

•        In CO2, the carbon atom is considered electron poor.

 

•       Oxygen is a very electronegative atom that monopolizes the electrons it shares with other atoms.

•       In a covalent bond between carbon and oxygen, the shared electrons are closer to the oxygen atom.

•        By comparison, in an organic molecule, the carbon atom is electron rich.

•       During the Calvin cycle, ATP provides energy and NADPH donates high-energy electrons, so the carbon originally in CO2 has been reduced.

 

•       The Calvin cycle combines less electronegative atoms with carbon atoms so that C—H and C—C bonds are formed.

•       This allows the eventual synthesis of larger organic molecules including glucose, amino acids, and so on.

•        In addition, the covalent bonds within these molecules are capable of storing large amounts of energy.

 

 

The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration

 

•           The Calvin cycle

Carbon mobilization in vascular plants

 

 

We learned that rubisco functions as a carboxylase because it adds a CO2 molecule to RuBP, an organic molecule, to create two molecules of 3 phosphoglycerate (3PG).

         RuBP +CO2 → 2 3PG

For most species of plants, the incorporation of CO2 into RuBP is the only way for carbon fixation to occur. Because 3PG is a three-carbon molecule, these plants are called C3 plants. Examples of C3 plants include wheat and oak trees.

 

About 90% of the plant species on Earth are C3 plants. Researchers have discovered that the active site of rubisco can also function as an oxygenase, although its affinity for CO2 is over 10-fold better than that for O2. Even so, when O2 levels are high and CO2 levels are low, rubisco adds an O2 molecule

to RuBP. This creates only one molecule of 3-phosphoglycerate and a two-carbon molecule called phosphoglycolate. The  phosphoglycolate is then dephosphorylated to glycolate and released from the chloroplast.

 

In a series of several steps, the two-carbon glycolate is eventually oxidized in other organelles to produce an organic molecule plus a molecule of CO2.

RuBP  O2 → 3-phosphoglycerate

Phosphoglycolate → Glycolate + An organic molecule  CO2

This process, called photorespiration, uses O2 and liberates CO2.

 Photorespiration is considered wasteful because it reverses the effects of photosynthesis.

 

This reduces the ability of a plant to make carbohydrates and thereby limits plant growth.

Photorespiration is more likely to occur when plants are exposed to a hot and dry environment.

Under these conditions,the stomata of the leaves close, inhibiting the uptake of CO2 from the air and trapping the O2 that is produced by photosynthesis.

When the level of CO2 is low and O2 is high, photorespiration is favored.

 

When rubisco first evolved some 3 billion years ago, the atmospheric oxygen level was low, so photorespiration would not have been a problem.

Another view is that photorespiration may have a protective advantage. On hot and dry days when the stomata are closed, CO2 levels within the leaves will fall, and O2 levels will rise. Under these conditions, highly toxic oxygen-containing molecules such as free radicals may be produced that could damage the plant.                              ??????

 

 

. Many plants that evolved in arid or hot environments display an adaptation known as C4 photosynthesis, which improves productivity by aiding CO2 absorption. An amazing 30,000 plant species utilize C4 photosynthesis, which is thought to have evolved on about 70 separate occasions. In addition, 7,500 other plant species possess a variation of C4 photosynthesis known as CAM (crassulacean acid metabolism; see Chapter 8). However, many other plants that grow in hot environments lack C4 photosynthesis. Rice, a staple crop for much of the world’s population, is a prominent example. Agricultural scientists envision using genetic engineering techniques to endow rice with C4 photosynthesis with the goal of increasing this crop’s productivity.

 

 

•          The CAM pathway is similar to the C4 pathway

 

8.14  Crassulacean acid metabolism (CAM) (Part 1)

8.14  Crassulacean acid metabolism (CAM) (Part 2)

 

 

•                  

other   

I. Photosynthesis: Light is important to plants

II. Properties of Light 

          X-ray---UV--380nm-------------------------760nm---IR---TV

 appears      black   violet blue green yellow orange red  black         

               tanning                                   heat lamps

                     (nanometer - nm = 1 billionth of a meter)

IV. Summary of Photosynthesis

Links relating to photosynthesis

Why Study Photosynthesis: An essay on the significance of photosynthesis on living organism, especially humans
Photosynthesis from Newton's Apple
Photosynthesis: An excellent review
Photosynthetic Antennas: Advanced information
How a rainbow is formed
The Light Reaction: Detailed but most useful
The Dark Reaction: Detailed but most useful