BIO101: Introduction to Molecular and Cellular Biology

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?

We can ecologically classify any living organism as an autotroph or a heterotroph. Autotrophs ("self-feeders") are also known 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.

We 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.

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 Overview of Photosynthesis, pay close attention to the summary reaction for photosynthesis (figure 5). Notice that photosynthesis is the reverse of the summary reaction for glycolysis and cellular respiration.

 

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

• What are the inputs and outputs of photosynthesis?

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.

 

 

Keep these inputs and outputs in mind as you review the biochemical components of photosynthesis in The Light-Dependent Reactions of Photosynthesis and The Calvin Cycle.

 

6c. 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?

Although cells can use many organic molecules for fuel, carbohydrates are the principal fuel for cells (this includes glucose). 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.

Incorporating carbon from an inorganic source, such as carbon dioxide, into organic compounds, such as glucose, is called carbon fixation. This 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). This process requires an input of energy.

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

To review, see The Light-Dependent Reactions of Photosynthesis and The Calvin Cycle.

 

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

• 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?

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

The overall purpose of photosynthesis 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 in the forms of ATP and NADPH molecules. The Calvin cycle uses ATP and NADPH 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 Photophosphorylation.

 

6e. 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).

By 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.

To review, see The Calvin Cycle and Photophosphorylation.

 

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

• How is energy transferred stepwise from sunlight to carbohydrate molecules?
• What energy-carrying compounds are used as intermediates in the process?
• What is the fate of the products of photosynthesis?
• How can plants store usable energy that is incorporated during photosynthesis?
• How does photosynthesis contribute to the accumulation of biomass?
• What part of photosynthesis directly produces biomass?

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, are not able to use sunlight directly to power the fixation of carbon dioxide into carbohydrates. 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.

Energy Storage in Plants

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 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 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.

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.

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.

To review, see The Light-Dependent Reactions of Photosynthesis and The Calvin Cycle.

 

6g. 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. RuBisCO (ribulose bisphosphate carboxylase oxygenase) 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 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 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.

To review, see C-4 and CAM Photosynthesis.

 

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

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

Both C4 plants and CAM plants have evolved mechanisms to minimize the costly and wasteful occurrence of photorespiration. Both types of plants accomplish their avoidance of 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), whereas 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 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 compare C4 plants to CAM plants and review the avoidance of photorespiration, 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.

To review, see C-4 and CAM Photosynthesis.

 

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 carbon cycle refers to the many chemical transformations that involve compounds containing carbon. The carbon cycle 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).

The carbon in these organic compounds gets 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.

Review this material in The Carbon Cycle.

 

Unit 6 Vocabulary

You should be familiar with these terms as you prepare for the final exam.

• 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
• G3P
• glucose
• glycolysis
• heterotroph
• inorganic
• light-dependent reactions
• light-independent reactions
• lipid
• mesophyll cell
• monosaccharide
• NADP+
• NADPH
• nutrient cycling
• organic
• oxidation
• oxygen
• photoautotroph
• photorespiration
• photosynthesis
• producer
• reduction
• RuBisCO
• sucrose
• stomata
• water