General Biology I - BSC 1010C - Chapter 10: Photosynthesis

How many CO2 molecules must enter the Calvin cycle to produce one G3P? How many times must the cycle occur to produce one G3P? What is G3P and what do photosynthetic organisms synthesize with G3P?

3 CO2 molecules must enter the Calvin cycle to produce 1 G3P. To produce 1 G3P, the Calvin cycle must occur 3 times. G3P, or glyceraldehyde 3-phosphate, is a three-carbon carbohydrate that is the direct product of the Calvin cycle. Photosynthetic organisms can synthesize glucose and other organic molecules with G3P.

Describe the events that occur at photosystem II. Be sure to address: water, energy, electrons, oxygen gas, light harvesting complexes, chlorophyll, reactions center, and primary electron acceptor.

A photon of light strikes one of the pigment molecules in a light-harvesting complex of PS II, boosting one of its electrons to a higher level. Energy is relayed to other pigment molecules until it reaches the P680 pair of chlorophyll-a molecules in the PS II reaction center complex. It excites an electron in the pair of molecules to a higher energy state, which is transferred from the excited P680 to the primary electron acceptor. An enzyme catalyzes the splitting of a water molecule into two electrons, two H+ ions, and an oxygen atom. The electrons are supplied one by one to the P680 acceptor, each electron replacing one transferred to the primary electron acceptor. The H+ ions are released into the thylakoid space, and the oxygen atom combines with an oxygen atom generated by the splitting of another water molecule, forming the waste product of oxygen gas or O2.

What is a pigment, how are pigments utilized by photosynthetic organisms? What types of pigments are used by photosynthetic organisms? Compare and contrast the absorption spectrum of each pigment.

A pigment is a substance that absorbs light at a particular wavelength. Chlorophyll, for example, absorbs violet-blue and red light while transmitting and reflecting green light. There are three main types of pigments in chloroplasts. Chlorophyll a is the key light-capturing pigment that participates directly in the light reactions. Chlorophyll b is an accessory pigment that transfers energy to chlorophyll a. Carotenoids are orange or yellow accessory pigments; by absorbing wavelengths of light that chlorophyll cannot, carotenoids broaden the spectrum of colors that can drive photosynthesis. Chlorophyll a best absorbs violet-blue and red light, chlorophyll b best absorbs blue and orange light, and carotenoids best absorb blue and blue-green light.

What membrane do all photosynthetic organisms execute the light reactions on? What proteins are integral to the light reactions? What is the role of the thylakoid space and the stroma in the light reactions?

All photosynthetic organisms execute the light reactions on the thylakoid membrane. The proteins that are integral to the light reactions are PSII and PSI. The thylakoid membrane uses the flow of electrons through the transport chain between PSII and PSI to pump hydrogen ions from the stroma into the thylakoid space, generating an electrochemical gradient, with the H+ concentration in the thylakoid space exceeding that in the stroma. At the end of the process, the stroma contains the end products of the light-dependent reactions - ATP and NADPH - setting the stage for the next steps in photosynthesis.

What is the difference between heterotrophs and autotrophs?

Autotrophs are organisms that obtain organic food molecules without eating other organisms or substances derived from other organisms; they use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones. Heterotrophs are organisms that obtain organic food molecules only by eating other organisms or substances derived from them.

What are C4 plants, and how do they function? Describe their carbon fixation system and compare their carbon fixation enzyme to rubisco.

C4 plants preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. When a C4 plant's stomata close on a hot day, O2 levels rise and CO2 levels fall. Then, PEP carboxylase adds CO2 to PEP, forming the four-carbon product oxaloacetate; it has a much higher affinity for CO2 than does rubisco and no affinity for O2. C4 plants separate the atmospheric uptake of gases and oxygen synthesis from the Calvin cycle by performing each in different cell types: mesophyll and bundle sheath cells.

What are CAM plants, and how do they function? Describe their carbon fixation system.

CAM plants open their stomata during the night and close them during the day. This helps desert plants conserve water, but it also prevents CO2 from entering the leaves. During the night, when their stomata are open, CAM plants take up CO2 and incorporate it into a variety of organic acids. The mesophyll cells of CAM plants store these organic acids they make during the night in their vacuoles until morning, when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids to become incorporated into sugar in the chloroplasts.

How does cyclic electron flow differ from linear electron flow? Does the ATP that is produced go to the Calvin cycle?

Cyclic electron flow uses photosystem I but not photosystem II. The electrons cycle back from ferredoxin (Fd) to the cytochrome complex, then via a Pc molecule to a P700 chlorophyll in the PS I reaction-center complex. There is no production of NADPH and no release of oxygen that results from this process, but ATP is generated; this ATP does not go to the Calvin cycle.

Describe how scientists used stable isotopes to determine the fate of oxygen in the photosynthetic equation.

During the 1950s, scientists tested C.B. van Niel's hypothesis that plants split H2O as a source of electrons from hydrogen atoms and released O2 as a by-product by using oxygen-18 as a tracer to follow the path of oxygen atoms during photosynthesis. The experiments showed that a significant result of the shuffling of atoms during photosynthesis was the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, O2, was released into the atmosphere, confirming the hypothesis.

Describe the action of the electron transport chain between photosystems I and II. How does it differ with the mitochondria in terms of the hydrogen ion concentration gradient?

Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain, which is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc). Each component carries out redox reactions as electrons flow down the ETC, releasing free energy that is used to pump H+ protons into the thylakoid space, creating an electrochemical gradient across the thylakoid membrane. While in cellular respiration, the H+ concentration builds up in the intermembrane complex outside of the mitochondrial matrix, the electrons in the light reactions travel from the outer stroma to the thylakoid space, creating a high H+ concentration on the interior of the chloroplast.

Where does photosynthesis occur in macroalgae, unicellular photosynthetic eukaryotes, and prokaryotes?

In prokaryotes, the reactions of photosynthesis will take place along membranes that lie along the cell membrane. Regardless of cell type, the reactions of photosynthesis will always occur on a thylakoid membrane; in eukaryotes, the membrane will be contained within the chloroplast, while in prokaryotes, the membrane will lie along the cell membrane and is not considered an organelle.

What alternative carbon fixation systems have evolved, and how does their carbon fixation enzyme compare to Rubisco in sensitivity to oxygen?

In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle - even in hot, arid climates. The two most important are C4 photosynthesis and CAM (crassulacean acid metabolism).

In linear electron flow, what is the action of photosystem I and how does it send electrons to the Calvin cycle? How does the ATP synthase synthesize ATP through chemiosmosis, where is the ATP used?

Light energy is transferred via light-harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll-a molecules located there. The electron is transferred to the PS I's primary electron acceptor and then down a second ETC through the protein ferredoxin (Fd). The enzyme NADP+ reductase catalyzes the transfer of electrons of Fd to NADP+, and NADP+ is reduced to NADPH and sent to the Calvin cycle. Through the process of chemiosmosis, the ATP synthase couples the diffusion of hydrogen ions down their gradient with the photophosphorylation of ATP. The ATP is sent to be used in the Calvin cycle.

What type of energy is light, where does it originate, how does it compare to other types of electromagnetic radiation? What is the visible light spectrum?

Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic waves originate from the vibration of charged particles, and unlike other waves, they can travel without a medium. The visible light spectrum is that portion of the electromagnetic spectrum that can be detected as various colors by the human eye, ranging in wavelengths from about 380 nm to about 750 nm.

What types of organisms are classified as photoautotrophs? How did they earn this distinction?

Photoautotrophs include plants, unicellular and multicellular algae, some non-algal unicellular protists, cyanobacteria, and photosynthetic prokaryotes. All of these organisms are classified as such because they all use light energy to drive the synthesis of organic molecules from carbon dioxide and water.

In what way does the entire ecosystem depend on photoautotrophs?

Photoautotrophs use light energy to drive the synthesis of organic molecules from carbon dioxide and water. The entire ecosystem depends on photoautotrophs because they provide a constant source of the basic building block required for life. They use energy from sunlight to construct the basic organic molecules that all other heterotropic life relies on.

Explain what "photorespiration" is. How does it affect the plant's carbon-fixing efficiency?

Photorespiration is a metabolic pathway that consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output; it is considered a wasteful process. It generally occurs on hot, dry, bright days, when stomata close and the O2/CO2 ratio in the leaf increases, favoring the binding of O2 rather than CO2 by rubisco.

How is aerobic cellular respiration tied to photosynthesis?

Photosynthesis and aerobic cellular respiration are the two main processes performed by most living organisms to obtain usable energy from nature. While photosynthesis is performed by most plants who can prepare their own food, most animals fulfill their energy requirements through aerobic cellular respiration.

How does the chemical reaction of photosynthesis compare with the chemical reaction that summarizes aerobic cellular respiration? Do both of these processes occur in plants? Do both processes occur at night?

Photosynthesis and respiration are reactions that complement each other in the environment. They are in reality the same reactions but occurring in reverse. While in photosynthesis carbon dioxide and water yield glucose and oxygen, in cellular respiration glucose and oxygen yield carbon dioxide and water. Both of these processes occur in plants, but only cellular respiration occurs at night due to photosynthesis requiring light.

Where does the Calvin Cycle occur, and what does it accomplish? Does this process utilize or generate ATP? What products do the light reactions contribute to it? What is the role of ribulose 1,5 bisphosphate? What is Rubisco, and what does it do?

The Calvin cycle takes place in the stroma of chloroplasts in photosynthetic organisms. This process utilizes ATP. The light reactions contribute NADPH and ATP to the process; the Calvin cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar. Ribulose 1,5-bisphosphate, or RuBP for short, is a five-carbon sugar that accepts CO2 molecules and is regenerated in the third phase of the Calvin cycle. Rubisco, or RuBP carboxylase-oxygenase, is the enzyme that normally catalyzes the first step of the Calvin cycle, which is the addition of CO2 to RuBP; it is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.

What is the chemical reaction that summarizes the process of photosynthesis? Identify the products and the reactants.

The equation for photosynthesis states that the combination of carbon dioxide, water and light energy produces a carbohydrate (glucose) and oxygen. The chemical formula is 6 CO2 + 6 H2O + light energy = C6H12O6 + 6 O2.

List and describe the organs, tissues, cells, organelles, and organelle components of a plant that are relevant to photosynthesis.

The leaf is the major site of photosynthesis in most plants. Chloroplasts, cellular organelles that conduct photosynthesis, are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata. A chloroplast has two membranes surrounding a dense fluid called the stroma. Thylakoids are flattened, membranous sacs within a chloroplast that exist in stacks called grana; their membranes contain molecular "machinery" used to convert light energy to chemical energy. Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.

What are the two general functions that the light reactions and the light-independent reactions (Calvin cycle) collectively achieve for a photosynthetic organism?

The light reactions convert solar energy to the chemical energy of ATP and NADPH. The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar.

What general functions correspond with the light reactions of photosynthesis versus the light independent reactions (or Calvin cycle)?

The light reactions of photosynthesis convert solar energy to chemical energy; they reduce NADP+ to NADPH and make ATP through a process called photophosphorylation. The Calvin cycle performs carbon fixation, incorporating CO2 from the air into organic molecules, which are then converted to sugar.

What is chlorophyll excitation, what visible evidence can be observed when chlorophyll gets excited?

When a chlorophyll molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital where it has more potential energy; the pigment molecule is now said to be in an excited state. If the illuminated molecule exists in isolation, its excited electron(s) immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence (light). When a chlorophyll solution is excited with ultraviolet light, it fluoresces with a red-orange glow.