Unit 6: Photosynthesis

6a. Explain the role of photosynthesis and describe its matter and energy inputs and outputs

  • What are the two ecological categories of organisms?
  • What type of organism is capable of photosynthesis?
  • How does photosynthesis relate to nutrient cycling?
  • What is the role of producers in an ecosystem?
  • What are the inputs and outputs of photosynthesis?

We can ecologically classify any living organism as an autotroph or a heterotroph. Biologists call autotrophs (self-feeders) producers because they produce organic compounds from inorganic materials. They make their own food. Autotrophs require energy to do so, and most autotrophs use light energy in the process of photosynthesis.

Biologists call heterotrophs (feeders on others) consumers because they feed on organic compounds produced by other organisms. Maximally extracting energy from an organic fuel (food) involves the complete oxidation of the fuel (including glycolysis and cellular respiration); this leaves inorganic carbon dioxide. Heterotrophs can be of different levels or "degrees," such as secondary heterotroph, tertiary heterotroph, and so on, depending on what level they feed on in the food chain.

Photosynthesis reverses these processes by starting with inorganic carbon dioxide and transforming it into organic compounds that can be used as fuel. In this way, photosynthesis is an important part of carbon cycling because photosynthesis has a reciprocal relationship with glycolysis and cellular respiration.

Photosynthesis is of vital importance to organisms because photosynthesis provides food for photosynthetic organisms (the producers) and the consumers of the world. As you review, pay close attention to the summary reaction for photosynthesis. Notice that photosynthesis is the reverse of the summary reaction for glycolysis and cellular respiration.

Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle.

Figure 8.7 Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make G3P from CO2. Credit: Rao, A., Ryan, K., Fletcher, S., Hawkins, A. and Tag, A. Texas A&M University.


Recall that photosynthesis has a reciprocal relationship with the complete oxidation of glucose (glycolysis and cellular respiration). This means the summary reaction for each process is the reverse of the summary reaction for the other process.

As you review the inputs and outputs of photosynthesis, appreciate this reciprocal relationship by noticing that the inputs into photosynthesis are the outputs from the complete oxidation of glucose, and the outputs from photosynthesis are the inputs into the complete oxidation of glucose.

Inputs

Process

Outputs

  • Water (Includes oxygen and hydrogen)

  • Carbon dioxide (inorganic carbon source)

  • Energy (in the form of sunlight)

  • Photosynthesis

  • Glucose (organic carbon product; includes hydrogen from water)

  • Oxygen (from water after hydrogen is extracted)

  • Glucose (organic carbon source; includes hydrogen)

  • Oxygen

  • Complete oxidation of glucose (glycolysis and cellular respiration)

  • Water (formed when oxygen joins with hydrogen)

  • Carbon dioxide (inorganic carbon product left over after hydrogen is extracted)

  • Energy (in the form of chemical energy in the bonds of glucose)


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6b. Describe how photosynthesis converts low-energy molecules into energy-rich carbohydrates

  • How does photosynthesis transform low-energy, inorganic molecules into high-energy, organic molecules (fuels)?
  • What is the name for the process of incorporating inorganic molecules into organic compounds?
  • What are the inputs and outputs of photosynthesis?
  • Where is the Carbon found for photosynthesis to take place? What form is it in

Although cells use many organic molecules for fuel, carbohydrates (including glucose) are their principal fuel. Carbohydrates make excellent fuels because they contain many C-H bonds. While energy is released by oxidizing an organic fuel molecule into carbon dioxide, it takes energy to reverse the process and reduce carbon dioxide into an organic fuel.

Carbon fixation is the process of incorporating carbon from an inorganic source, such as carbon dioxide, into organic compounds, such as glucose. It is an extremely important function of photosynthesis. Carbon fixation results in products (organic compounds) that contain more chemical energy than the reactants (carbon dioxide molecules), requiring an energy input.

Sunlight provides the energy input for the carbon fixation that occurs during photosynthesis. Powered by light energy, water molecules split into oxygen and hydrogen atoms. The hydrogen atoms bond to the carbon atoms from carbon dioxide molecules to form high-energy carbohydrates. This occurs in the two major pathways of photosynthesis. Light-dependent reactions split the water, while the Calvin Cycle builds the carbohydrate molecules.

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6c. Explain the role of the light-dependent phase of photosynthesis

  • What is the function of the light-dependent reactions, and what are its inputs and products?
  • What is the function of the Calvin cycle, and what are its inputs and products?
  • What is the relationship between the Calvin cycle and the light-dependent reactions?
  • How is energy transferred from sunlight to carbohydrate molecules, and what energy-carrying compounds are used as intermediates in the process?
  • What happens to the products of photosynthesis?
  • How can plants store usable energy incorporated during photosynthesis?
  • How does photosynthesis contribute to the accumulation of biomass, and what part of photosynthesis produces biomass directly?

Photosynthesis has two major components: the light-dependent reactions and the Calvin cycle. Its purpose is to build carbohydrate molecules. This process requires energy from sunlight, but photosynthetic organisms cannot use sunlight energy directly to build these carbohydrates. Instead, the sunlight energy must be transformed into chemical energy that is temporarily stored as ATP and NADPH molecules. The Calvin cycle uses ATP and NADPH as energy sources to build carbohydrate molecules.

In essence, the main function of the light-dependent reactions is to produce ATP and NADPH. The light-dependent reactions require sunlight, water, NADP+, and ADP. The products are heat, oxygen, NADPH, and ATP.

The Calvin Cycle

The Calvin cycle (the light-independent reactions) is the component of photosynthesis where carbon fixation takes place. In other words, during the Calvin cycle, inorganic carbon dioxide is transformed into organic compounds (molecules of glyceraldehyde-3-phosphate, or G3P). This expensive process requires energy in the form of two energy-storing compounds (NADPH and ATP).

Using the energy stored in NADPH and ATP, the Calvin cycle takes in carbon dioxide and (after several rearrangements of atoms, forming several different intermediates) produces a three-carbon compound called G3P. These G3P molecules are chemically transformed into other organic molecules for various uses in the organism.

Light energy indirectly powers the Calvin cycle because the Calvin cycle requires NADPH and ATP, and the light-dependent reactions (using sunlight) produce NADPH and ATP for the Calvin cycle. As you review the particulars of the Calvin cycle, ensure that you understand how the light-dependent reactions must operate first if the Calvin cycle is going to operate at all.

Photosynthesis uses sunlight energy to fix carbon dioxide into carbohydrates. However, the transfer of energy from sunlight to the chemical energy of the carbohydrate product is not direct.

Photosynthetic organisms, such as plants, algae, and cyanobacteria, cannot use sunlight directly to power the fixation of carbon dioxide into carbohydrates. Carbon fixation requires stored energy in the bonds of two important energy-carrying compounds:

  • ATP is the general energy currency in cells. It is required in parts of the Calvin cycle of photosynthesis. The light-dependent reactions transform ADP into ATP, storing some of the energy originally in the sunlight.

  • NADPH temporarily stores energy for use in the Calvin cycle of photosynthesis, just as the related compound NADH stores energy during cellular respiration. The light-dependent reactions transform NADP+ into NADPH, storing some of the energy originally in the sunlight.

As you review the light-dependent reactions and the Calvin Cycle, pay attention to how energy gets transformed from light energy into chemical energy as energy gets transferred from sunlight to ATP and NADPH and finally to the C-H bonds of the carbohydrates produced.

Energy Storage in Plants

The major accomplishment of photosynthesis is carbon fixation, producing organic compounds from inorganic compounds. The direct, organic product of the Calvin cycle is a three-carbon carbohydrate. That organic product can then be used as a precursor to building organic macromolecules, including proteins, lipids, nucleic acids, and polysaccharides, or it can be used to build monosaccharides like glucose.

If a plant needs to store energy in the form of oxidizable fuels, the primary sugar that is produced to store chemical energy is sucrose. Sucrose is a disaccharide consisting of two monosaccharides, glucose and fructose. Sucrose is a significant component of sap that travels through a plant's vessels to deliver that stored energy to different parts of the plant. Plants can also store energy in lipid forms (fats and oils) as occurs, for example, in nuts.

Biomass

Biomass is the matter living organisms produce. Since biomass is the material of living and dead organisms, it is organic material.

Since biomass is organic matter, any biological process that transforms inorganic matter into organic matter – any process that fixes carbon – produces biomass. Carbon fixation occurs in all autotrophs (producers), but the vast majority of autotrophs are photoautotrophs because their method for carbon fixation is photosynthesis.

The Calvin cycle is the component of photosynthesis that fixes the carbon from carbon dioxide into the organic form of carbohydrates produced. So, the Calvin cycle directly contributes to the accumulation of biomass. Recall that organic biomass can be converted back into inorganic carbon dioxide in autotrophs and heterotrophs by oxidizing the organic compounds using glycolysis and cellular respiration. As you review the Calvin cycle, pay attention to the fact that inorganic carbon dioxide enters the process and organic carbohydrate molecules (G3P) exit the process.

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6d. Explain how plants have adapted to deal with the problem of photorespiration

  • What is photorespiration?
  • Why is photorespiration problematic for a plant?
  • What kinds of plants can minimize the occurrence of photorespiration?
  • How do plants minimize the occurrence of photorespiration?

The fixation step is crucial in the Calvin cycle. It takes in carbon dioxide and joins it with an intermediate compound (ribulose bisphosphate), incorporating inorganic carbon dioxide into an organic compound. Ribulose bisphosphate carboxylase oxygenase (RuBisCO) is the enzyme that catalyzes this step. RuBisCO can operate to join either carbon dioxide or oxygen to ribulose bisphosphate. However, joining oxygen instead of carbon dioxide is counterproductive because no carbon fixation (the purpose of the Calvin cycle) takes place.

We call this counterproductive process (incorporating oxygen instead of carbon dioxide) photorespiration. Two major categories of plant species have evolved ways around this problem:

  • C4 plants separate carbon dioxide intake (which occurs in superficial cells called mesophyll cells) from carbon fixation in the Calvin cycle (which occurs in deeper cells called bundle sheath cells).

  • CAM plants take in carbon dioxide and store it in the form of organic acids during the night when their stomata are open. During the day, the organic acids get broken down to release the carbon dioxide for the Calvin cycle while the stomata are closed (preventing oxygen from interfering).

These two types of plants operate the Calvin cycle more efficiently because photorespiration is minimized. As you review C4 plants and CAM plants, notice they accomplish the same thing in two different ways.

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6e. Identify the differences in photosynthesis in CAM and C4 plants

  • How does a C4 plant differ from a CAM plant?
  • What type of plant has more efficient photosynthesis in a hot, dry environment?
  • What type of plant has more efficient photosynthesis in a cool, wet environment?
  • When do C4 plants keep their stomata open, as compared to CAM plants?

C4 plants and CAM plants have evolved mechanisms to minimize the costly and wasteful occurrence of photorespiration. Both types of plants avoid photorespiration by separating two processes: the intake of carbon dioxide and the operation of the Calvin cycle.

The major difference between C4 and CAM plants is that C4 plants avoid photorespiration by separating the two processes spatially (they occur in separate spaces). CAM plants avoid photorespiration by separating the two processes temporally (they occur at separate times).

A C4 plant takes carbon dioxide into mesophyll cells, then it transfers that carbon dioxide into bundle sheath cells that are farther away from the atmospheric oxygen. The Calvin cycle then occurs in this separate space (the bundle sheath cells).

A CAM plant opens its stomata only at night, taking in carbon dioxide that gets stored in acid form until the next day. During daylight, the stomata are closed (disallowing oxygen from entering), and the acids are processed to release the carbon dioxide to the Calvin cycle, which occurs at a separate time compared to the intake of carbon dioxide.

As you compare C4 and CAM plants and review how they avoid photorespiration, keep in mind that a typical plant (C3) operates less efficiently because the carbon dioxide intake process and the Calvin-cycle operation do not occur separately.

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6f. Explain what the carbon cycle is and how it relates to the conservation of matter

  • What is the carbon cycle, and why is it a cycle?
  • How does the carbon cycle exemplify the conservation of matter?
  • Where does the carbon that enters the cycle come from?

The carbon cycle refers to the many chemical transformations that involve compounds containing carbon. It is cyclic because there is a continuous alternation between the carbon of organic compounds and the carbon of inorganic compounds. Inorganic carbon dioxide gets fixed (by autotrophs) into organic compounds. These organic compounds get converted into other organic compounds (including simple organic compounds like monosaccharides, nucleotides, and amino acids, as well as complex macromolecules like polysaccharides, nucleic acids, lipids, and polypeptides).

Carbon in these organic compounds is passed from organism to organism as they feed on each other. Organisms use some of the organic molecules as fuel, and the oxidation of these organic fuels (to provide energy for the organisms) returns the carbon to inorganic form (carbon dioxide) to complete the cycle. In this cycle of transformations, carbon (matter) remains in the ecosystem (it is conserved). It is not destroyed; it is merely transferred and transformed.

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Unit 6 Vocabulary

This vocabulary list includes terms you will need to know to successfully complete the final exam.

  • autotroph
  • biomass
  • C4 plant
  • CAM plant
  • carbon cycle
  • carbon fixation
  • glyceraldehyde-3-phosphate (G3P)
  • heterotroph
  • light-dependent reaction
  • NADPH
  • photorespiration
  • photosynthesis
  • ribulose bisphosphate carboxylase oxygenase (RuBisCO)
  • stomata
  • sucrose