Chapter Seven: Photosynthesis: Using Light to Make Food

Questions:

1. What are the similarities and differences between the two photo systems in photosynthesis?
2. Why does a poison that inhibits an enzyme of the Calvin cycle also inhibit the light reactions?
3. How might phytochemicals benefit a cell?

Answers:

1. Both photosystems have primary accepters and both accept light power from electrons.  They both  take place in the thylakoid membrane.  They both use ATP.  Photosystem 2 takes place before Photosystem 1.  They also use different wavelengths.  Photosystem 2 takes in water and releases oxygen and hydrogen.  Photosystem 1 does not do this.    
2. The light reactions require ADP and NADP+, which are not recycled from ATP and NADPH when the Calvin cycle stops.  
3. Phytochemicals can act as antioxidants that protect from reactive forms of oxidative molecules.

Important Facts:

1. Photosynthesis occurs in two metabolic stages.  One stage involves the light reactions.  H+ ions reduce NADP+ to NADPH, which is an electron carrier similar to NADH.  The Calvin cycle is the second stage that occurs in the stroma.  It is a cyclic series of reactions that builds sugar molecules from CO2......NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle.  ATP from the light reactions provides chemical energy for the Calvin cycle, which is often known as a dark reaction.
2. In photosynthesis, electrons gain energy by being boosted up an energy hill.  Light energy captured by chlorophyll molecules provides the boost for the electrons.  As a result, light energy is converted to chemical energy, which is stored in the chemical bonds of sugar molecules.
3. Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule.  This is accomplished by oxidizing the sugar and reducing O2 to H2O.  The electrons lose potential as they travel down an energy hill, the electron transport system.  In contrast, the food producing redox reactions of photosynthesis reverse the flow and involve an uphill climb.
4. Chemiosmosis is the mechanism that not only is involved in oxidative phosphorylation in mitochondria but also generates ATP in chloroplasts.  ATP is generated because  the electron transport chain produces a concentration gradient of hydrogen ions across a membrane.  ATP synthase couples the flow of H+ to the phosphorylation of ADP.  The chemiosmotic production of ATP in photosynthesis is called phosphorylation.
5. The energy released by electrons is conserved as it is passed from one molecule to another.  All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems.  Photosystems are light-harvesting complexes surrounding a reaction center complex.  Energy is passed from molecule to molecule within the photosystem.  The energy finally reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced.  This solar- powered transfer of an electron from the reaction center pigment to the primary electron acceptor is the first step of the light reactions.
   

Key Terms: 
Photosynthesis- Process that converts solar energy to chemical energy.  Plants use water and atmospheric carbon dioxide to produce a simple sugar and liberate oxygen.  This sugar is food for humans for and animals.
Autotroph- an organism that makes its own food, thereby sustaining itself without eating other organsims or their molecules.
Cloroplasts- organelles consisting of photosynthetic pigments, enzyme, and other molecules grouped together.
Chlorophyll- a green pigment located within the chloroplasts of plants, algae, and certain prokaryotes.
Electromagnetic Spectrum- the entire spectrum of radiation.
Mesophyll- the middle layer of tissue inside a leaf.
Photoautograph- an organism that obtains energy from sunlight and carbon from CO2 by photosynthesis.
Thylakoid- one of the number of disk-shaped membranous sacs inside a chloroplast.
Stomata- tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit.
Stroma- the dense fluid within the chloroplast that is contained in two membranes.

Diagram: The light-dependent reactions are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this proces, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH.

Relevant Video: http://www.youtube.com/watch?v=GR2GA7chA_c

Summary: This chapter begins by detailing the stages of photosynthesis.  It then continues to discuss the light reactions and the converting of solar energy to chemical energy.  It mentions photosystems and photophosphorylation.  The chapter then discusses the Calvin cycle and the converting of CO2 to sugars.  It details C3, C4, and CAM plants before discussing solar radiation and the Earth's atmosphere.