Unit 6 Study Guide: Photosynthesis

6a. Explain the role of photosynthesis.

  • What are the two ecological categories of organisms?
  • What type of organism is capable of photosynthesis?
  • How does photosynthesis relate to nutrient cycling?

Any giving organism can be ecologically classified as either an autotroph or a heterotroph. Autotrophs ("self-feeders") are also known as producers, because they produce organic compounds from inorganic materials. They therefore make their own food. Autotrophs require energy to do so, and most autotrophs use light energy in the process of photosynthesis.

Heterotrophs ("feeders on others") are also known as consumers, because they use organic compounds (produced other organisms) as their food. Maximally extracting energy from an organic fuel (food) involves the complete oxidation of the fuel (including glycolysis and cellular respiration), and this leaves inorganic carbon dioxide. Photosynthesis reverses this 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 feeds not only the photosynthetic organisms (the producers) but also the consumers of the world. As you review photosynthesis in this section, pay close attention to the summary reaction for photosynthesis (Figure 5), and notice that it is the reverse of the summary reaction for glycolysis and cellular respiration.


6b. Describe where matter originates and ends up during photosynthesis.

Recall that photosynthesis has a reciprocal relationship with the complete oxidation of glucose (glycolysis and cellular respiration). This means that the summary reaction for each process is the reverse of the summary reaction for the other process. As you refresh your knowledge of the inputs and outputs of photosynthesis, appreciate that 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.




Water (Includes oxygen and hydrogen)

Carbon Dioxide (inorganic carbon source)

Energy (in the form of sunlight)


Glucose (organic carbon product; includes hydrogen from water)

Oxygen (from water after hydrogen is extracted)

Glucose (organic carbon source; includes hydrogen)


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)

Keep these inputs and outputs in mind as you review the biochemical components of photosynthesis (the light-dependent reactions and the Calvin cycle).


6c. Explain how photosynthesis converts low-energy molecules into energy-rich carbohydrates.

Although many organic molecules can be used by as fuel by cells, the principle fuels for cells are carbohydrates (including glucose). Carbohydrates make excellent fuels because they contain many C-H bonds. Whereas 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.

  • 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?

Incorporating carbon from an inorganic source like carbon dioxide into organic compounds like glucose is called carbon fixation, and that 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). Doing so requires an input of energy. The input of energy for the carbon fixation that occurs during photosynthesis is energy in the form of sunlight. Powered by that light energy, water molecules are split into oxygen and hydrogen atoms, and the hydrogen atoms from the water end up bonded to carbon atoms from carbon dioxide molecules to form high-energy carbohydrates. This occurs in two major pathways that comprise photosynthesis. The light dependent reactions split the water, and the Calvin cycle builds the carbohydrate molecules.


6d. Explain how energy is transformed and transferred during photosynthesis.

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

  • How is energy transferred stepwise from sunlight to carbohydrate molecules?
  • What energy-carrying compounds are used as intermediates in the process?

Photosynthetic organisms like plants, algae, and cyanobacteria are not able to use sunlight directly to power the fixation of carbon dioxide into carbohydrate. The carbon fixation requires stored energy in the bonds of two important energy-carrying compounds:

  • ATP is important as a general energy currency in cells, and it’s also 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 highly 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.


6e. Explain how energy is stored in plants.

  • What is the fate of the products of photosynthesis?
  • How can plants store usable energy that is incorporated during photosynthesis?

The major accomplishment of photosynthesis is carbon fixation, the production of 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 build 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 the two monosaccharides, glucose and fructose. Sucrose is a major 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.


6f. Explain how plants accumulate biomass.

Biomass is matter produced by living organisms. It consists of the material making up both living and dead organisms. As such, biomass is organic material.

  • How does photosynthesis contribute to the accumulation of biomass?
  • What part of photosynthesis directly produces biomass?

Since biomass is organic matter, any biological process that transforms inorganic matter into organic matter – any process that fixes carbon – is a process that produces biomass. Carbon fixation occurs in all autotrophs (producers), but the vast majority of autotrophs are specifically photoautotrophs, because their method for carbon fixation is photosynthesis. The Calvin cycle is the component of photosynthesis that actually fixes the carbon from carbon dioxide into the organic form of carbohydrates that are 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 particular attention to the fact that inorganic carbon dioxide enters the process and organic carbohydrate molecules (G3P) exit the process.


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

Photosynthesis consists of two major components: the light-dependent reactions and the Calvin cycle.

  • What is the function of the light-dependent reactions?
  • What are the requirements for (inputs into) the light-dependent reactions?
  • What are the products of (outputs from) the light-dependent reactions?

The overall purpose of photosynthesis is to build carbohydrate molecules. This process requires energy from sunlight, but energy in the form of sunlight cannot be used directly by photosynthetic organisms to build these carbohydrates. Instead, the energy of sunlight must be transformed into chemical energy that is temporarily stored in the forms of ATP and NADPH molecules, both of which can then be used by the Calvin cycle as energy sources to build carbohydrate molecules. So, the main function of the light-dependent reactions is to produce ATP and NADPH. Overall, the light-dependent reactions require sunlight, water, NADP+, and ADP. The products are heat, oxygen, NADPH, and ATP. Make sense of these inputs and outputs as you review this lecture on the light-dependent reactions.


6h. Explain the role of the light-independent phase of photosynthesis, and describe how it is related to the light-dependent reactions.

  • What is the function of the Calvin cycle?
  • What are the requirements for (inputs into) the Calvin cycle?
  • What are the products of (outputs from) the Calvin cycle?
  • What is the relationship between the Calvin cycle and the light-dependent reactions?

The Calvin cycle, which is the light-independent reactions, is the component of photosynthesis where carbon fixation takes place. In other words, during the Calvin cycle inorganic carbon dioxide gets 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. Those G3P molecules can then be 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 in this lecture, ensure that you understand how the light-dependent reactions must operate first if the Calvin cycle is going to operate at all.


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

  • What is the carbon cycle?
  • Why is it a cycle?
  • How does the carbon cycle exemplify conservation of matter?

The phrase "carbon cycle" refers to the many chemical transformations that occur involving compounds containing carbon. The carbon cycle is cyclic, because there is a continuous alternation between carbon of organic compounds and 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). The carbon in these organic compounds gets passed from organism to organism as they feed on each other. Some of the organic molecules get used as fuel by the organisms, 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 only transferred and transformed.


6j. 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 are able to minimize the occurrence of photorespiration?
  • How do plants minimize the occurrence of photorespiration?

A crucial step in the Calvin cycle is the fixation step, which takes in carbon dioxide and joins it with an intermediate compound (ribulose bisphosphate), thus incorporating inorganic carbon dioxide into an organic compound. The enzyme that catalyzes this step is called RuBisCO (ribulose bisphosphate carboxylase oxygenase). 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. This counterproductive process (incorporating oxygen instead of carbon dioxide) is called photorespiration. Two major categories of plant species have evolved ways around this problem:

  • C4 plants separate the process of carbon dioxide intake (which occurs in superficial cells called mesophyll cells) from the process of 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 more efficiently operate the Calvin cycle, because photorespiration is minimized. As you review C4 plants and CAM plants, notice that they accomplish the same thing in two different ways.


6k. Identify the differences in photosynthesis in reference to CAM and C4 plants.

Both C4 plants and CAM plants have evolved mechanisms to minimize the costly and wasteful occurrence of photorespiration.

  • How is a C4 plant different from a CAM plant?

Both types of plants accomplish their avoidance of photorespiration by separating two processes:

  • Intake of carbon dioxide
  • Operation of the Calvin cycle

The major difference between C4 and CAM plants is that species of one type (C4) accomplish their avoidance of photorespiration by spatially separating the two processes (having them occur in separate spaces), whereas CAM species accomplish their avoidance of photorespiration by temporally separating the two processes (by having them occur at separate times).

A C4 plant takes carbon dioxide into mesophyll cells, then it transfers that carbon dioxide into different cells (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 review the avoidance of photorespiration by comparing C4 plants to CAM plants, keep in mind that a typical plant (C3) operates less efficiently, because there is no separation of the processes of carbon-dioxide intake and Calvin-cycle operation.


Unit 6 Vocabulary

This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.

Try to think of the reason why each term is included.

  • ADP
  • ATP
  • Autotroph
  • Biomass
  • Bundle Sheath Cell
  • C3 Plant
  • C4 Plant
  • Calvin Cycle
  • CAM Plant
  • Carbohydrate
  • Carbon Dioxide
  • Carbon Fixation
  • Cellular Respiration
  • Conservation Of Matter
  • Consumer
  • Disaccharide
  • Energy
  • Fat
  • G3P
  • Glucose
  • Glycolysis
  • Heterotroph
  • Inorganic
  • Light-Dependent Reactions
  • Light-Independent Reactions
  • Lipid
  • Mesophyll Cell
  • Monosaccharide
  • NADP+
  • Nutrient Cycling
  • Oil
  • Organic
  • Oxidation
  • Oxygen
  • Photoautotroph
  • Photorespiration
  • Photosynthesis
  • Producer
  • Reduction
  • RuBisCO
  • Sucrose
  • Water
Last modified: Wednesday, July 17, 2019, 5:48 PM