Notes for Chapter 7: Photosynthesis

Section 7.1

Photosynthesis is the process by which photoautotrophs convert solar energy into stored chemical energy. As you study the specifics of photosynthesis, keep this overview of the process in mind: photosynthesis can be divided into two separate reactions, the light-dependent reaction and the light-independent reaction.

BASIC OVERVIEW -

Light-dependent –

Input: light energy, ADP and phosphate, NADP+, water

Output: ATP, NADPH, Oxygen

(Occurs at the thylakoid membrane)

Light-independent –

Input: ATP, NADPH, Carbon Dioxide

Output: ADP and P, NADP+, glucose (or other stored energy)

(Occurs in the stroma)

Section 7.2 Sunlight as Energy

Never forget that light is a form of energy: photosynthesis never creates energy, it just changes the form. Pigments are molecules capable of absorbing specific wavelengths of light. An absorption spectrum, such as those on page 117, represent which colors a pigment absorbs. Thus, chlorophyll, which appears green to our eyes, has high points in the blue and red region but a flat line at the green region.

Section 7.3 Pigments

Pigments are molecules that absorb specific wavelengths of light. In a photosynthetic cell, pigments are arranged in photosystems. Within a photosystem, there are accessory pigments, such as carotenoids, anthocyanins and phycobilins. These capture light and pass excited electrons onto the next pigment molecule. Ultimately, the electrons will reach a reaction center, usually a chlorophyll a molecule. This molecule doesn’t pass on the electron to the next pigment molecule but rather uses the available energy.

Section 7.4 Specifics of Light –Dependent Reaction

Light is collected by a variety of pigments. Many pigments are accessory pigments, meaning they harvest light energy and pass the excited electrons on to the reaction center. These accessory pigments help photoautotrophs collect a greater range of the light spectrum and can also protect photoautotrophs from the effects of UV rays. The reaction center (a chlorophyll a molecule) doesn’t pass its electron on to another pigment molecule. When the electrons reach the reaction center, the electrons are used to produce ATP or NADPH. All of the electron transport systems are located on the thylakoid membrane.

The light-dependent reaction is divided into two processes: photosystem I and photosystem II. At PSII, water is divided (a process called photolysis), providing oxygen, hydrogen ions and electrons. The electrons from PSII are then passed to PSI (the numbers are backwards because PSI was discovered first and named before science understood the correct order in which they function within a cell) to fuel the production of NADPH.

Section 7.5 ATP Production

When photolysis occurs at PSII, the H+ ions build up inside the thylakoid membrane. A high concentration of ions has the ability to do work, and this is exactly how ATP formation proceeds inside a cell. When the ions flow through the thylakoid membrane into the stroma, the energy of the moving ions is used to fuel the formation of ATP, a process called chemiosmotic ATP formation.

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Section 7.6 Light-Independent Reaction

This reaction is also called the Calvin-Benson cycle. Remember, it requires ATP, NADPH and CO2 in order to begin. It doesn’t require pigments or light energy because ATP and NADPH provide the needed energy. The main goal of the light-independent reaction is the fixation of energy within carbon-containing compounds. Carbon fixation occurs when carbon from CO2 is incorporated into a stable organic molecule, such as glucose.

Here are the steps of the Calvin-Benson Cycle: In the stroma, CO2 is attached to an enzyme called Rubisco (a 5-carbon protein, this is by far the most common protein in the universe). The resulting 6-carbon intermediate is unstable and degrades into 2 PGA molecules (each with 3-carbon atoms). When a P is added from ATP, the resulting molecule is PGAL (a phosphoralated 3-carbon atom). Two PGAL molecules can combine to form a molecule of glucose (with an attached phosphate, making it very reactive and primed to enter new reactions). "Leftover" PGAL can be reformed back into Rubisco, allowing the process to cycle. Study figure 7.15 in your textbook.

 The four possible essays to test your knowledge of photosynthesis:

1) Given a pigment absorption spectrum or light spectrum, be able to match up the ideal pigment/light combination and provide an explanation as to why the combination works.

2) Be able to explain the role played by light, pigments, ATP, and NADPH is photosynthesis. Include details such as where each is produced/collected, where they react or where they are used, and explain if are part of the light-dependent/light-independent reaction.

3) Be able to trace carbon, oxygen and hydrogen in the process of photosynthesis. Be specific and follow each from its source, any intermediates and the final product.

4) NADPH, in nature, acts as a carrier for electrons and hydrogen ions. In our lab 4b, we use DPIP in its place because it exhibits a change in color as it is reduced in the light reaction. Within a chloroplast, explain where the light reaction occurs and where the light-independent reaction occurs. Be able to draw a diagram of a chlorophyll and label outer membrane, thylakoid stacks, thylakoid membranes, inner thylakoid space and the stroma.