Chapter 8: Photosynthesis and Respiration
Study Plan
» Getting Started » A Guide to the Reading » Tying it all together
Getting Started
Below are a few questions to consider prior to reading Chapter 8. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.
- In terms of energy requirements, what is the consequence of the large brain size to body weight ratio observed in humans?
- What is phosphorylation and how does it affect proteins?
- Why are the leaves of most plants colored green?
- How are proton gradients used in cells to generate chemical energy?
- Under what conditions are the products ethanol and lactic acid produced by organisms?
- What occurs during the process of developing bread dough?
A Guide to the Reading
When exploring the content in Chapter 8 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.
Phosphorylation
The energy carrier molecule ATP is often used to transfer energy to organic compounds within the cell. When one of the phosphate atoms from the ATP molecule is transferred to an organic compound, this compound is said to be “phosphorylated”. Phosphorylation of molecules can result in changes in their shape or activation/deactivation of enzymatic activity. This process plays a key role in regulating the activities of enzymes within the cell. It is important to note that the cell utilizes a process called “oxidative phosphorylation” to regenerate ATP during aerobic respiration. In this process, the energy stored in the proton gradient established across the inner mitochondrial membrane is used to drive the activity of the enzyme ATP synthase. ATP synthase is responsible for phosphorylating ADP molecules to generate ATP.
For more information on this concept, be sure to focus on:
- Section 8.1, Energy Carriers: Powering All Activities of the Cell
- In Section 8.3, Oxidative phosphorylation uses oxygen and NADH to produce ATP in quantity
- Figure 8.11, Oxidative Phosphorylation
Light vs. Dark Reactions of Photosynthesis
Photosynthesis, which occurs in the chloroplast, can be broken down into two interrelated processes – light reactions and dark reactions. The light reactions of photosynthesis work to capture the energy of sunlight inside the chloroplast. As discussed in the chapter, the pigment chlorophyll is green in color – this is because it reflects green light, causing our eyes to see green when we look at a leaf. Consequently, the pigment is efficient at absorbing both red-orange and blue-violet light. When light is absorbed, electrons present in the chlorophyll molecule become “energized” and are then passed on to an electron transport chain (ETC) which is arranged in one of two types of photosystems (I and II). Photosystem I accepts high energy electrons and produces NADPH, a compound used in the cell to reduce compounds. Photosystem II accepts electrons and produces the high energy molecule ATP. In either case, the transport of electrons in the ETC helps to generate a proton gradient across the thylakoid membrane in the chloroplast, which can then be used to generate ATP in the cell. The ATP and NADPH produced by the light reactions of photosynthesis are utilized by the cell to drive the dark reactions of photosynthesis. In the dark reactions, carbon dioxide from the air is combined with water to produce glucose molecules and oxygen. This process requires energy obtained from both ATP and hydrogen ions donated by NADPH. As indicated by its name, the dark reactions of photosynthesis do not require light in order to proceed.
For more information on this concept, be sure to focus on:
- In Section 8.2, The light reactions capture energy from sunlight
- In Section 8.2, The dark reactions manufacture sugars
- Figure 8.4, The Arrangement of Photosystems in the Thylakoid Membrane
- Figure 8.5, Production of Energy Carriers by the Light Reactions
- Figure 8.6, The Dark Reactions Fix Carbon
The Citric Acid Cycle
uring aerobic respiration, the process of glycolysis is followed by a stepwise series of reactions which serve to donate high energy electrons to the compound NAD+, generating NADH. As discussed in the chapter, the process of glycolysis results in the generation of two molecules of pyruvate for every glucose molecule used. The three-carbon pyruvate releases a single CO2 molecule and joins to a compound called coenzyme A. The resulting molecule is called acetyl CoA. Acetyl CoA then enters the citric acid cycle, which is a series of eight oxidation reactions that occur within the mitochondrial matrix. As the citric acid cycle proceeds, carbon atoms are released as CO2 (which are then exhaled) and high energy electrons are stored in NADH. The NADH molecules produced from the citric acid cycle are then used to drive the final stage of respiration – oxidative phosphorylation – which produces ATP. It is important to note that very little ATP is produced during the citric acid cycle. The primary product is in fact the high energy NADH molecules.
For more information on this concept, be sure to focus on:
- In Section 8.3, The citric acid cycle produces NADH and carbon dioxide and
- Figure 8.10, The Citric Acid Cycle
Tying it all together
Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:
Mitochondrial and Chloroplast Structure
- Chapter 5 – Section 5.4, The Specialized Internal Compartments of Eukaryotic Cells
Diffusion
- Chapter 6 – Section 6.1, The Plasma Membrane Is Both Gate and Gatekeeper
Oxidation-Reduction Reactions
- Chapter 7 – in Section 7.2, Capturing energy from foods requires the transfer of electrons
Metabolism
- Chapter 7 – Section 7.2, Using Energy from the Controlled Burning of Food
- Chapter 20 – Maintaining the Internal Environment
Digestion and the Breakdown of Food
- Chapter 21 – Nutrition and Digestion
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