Photosynthesis: The Process of Converting Light Energy into Chemical Energy, Summaries of Cellular and Molecular Biology

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M. Jones, G. Jones,

Cellular biology

Reading Notes Chapter 10 - Photosynthesis Photosynthesis - the process of capturing light energy from the sun and convert it to chemical energy that is stored in sugar and organic molecules Autotrophic Nutrition

• <self-feeders=
• Sustain themselves without eating anything derived from other living begins
• Produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment
• Almost all plants are autotrophs (specifically photoautotrophs) Heterotrophic Nutrition
• <other-feeding=
• Unable to make their own food, they live on compounds produced by other organisms
• Decomposers (fungi and most prokaryotes) are heterotrophs Chloroplasts: The Sites of Photosynthesis in Plants
• Leaves are the major sites of photosynthesis in most plants
• Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf
• A typical mesophyll cell has about 30- 40 chloroplasts
• CO2 enters the leaf and oxygen exits through microscopic pores called stomata
• A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma
• Suspended within the stroma is a third membrane system made up of sacs called thylakoids, which segregates the stroma from the thylakoid space
• Chlorophyll is the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast
• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

The Nature of Sunlight

• Light is a form of electromagnetic energy
• Wavelength- the distance between crests of the waves
• The entire range of radiation is known as the electromagnetic spectrum (the segment most important to life is the from about 380nm to 750 nm, known as visible light)
• Light behaves as though it consists of discrete particles called photons
• Photons act like objects in that each of them has a fixed quantity of energy
• The amount of energy is inversely related to wavelength- the shorter the wavelength, the greater the energy of each photon of that light
• Visible light is the radiation that drives photosynthesis
• Substances that absorb visible light are known as pigments- different pigments absorb light of different wavelengths
• We see green when we look at a leaf because chlorophyll absorbed violet-blue and red light while transmitting and reflecting green light
• Violet-blue and red light work best for photosynthesis since they are absorbed, and green is the least effective color
• Three types of pigments in chloroplasts:
  1. Chlorophyll a - the key light-capturing pigment that participates directly in the light reactions
  2. Chlorophyll b - the accessory pigment
  3. Carotenoids- a separate group of accessory pigments. Has the function of photoprotection, these compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll Excitation of Chlorophyll by Light
• The colors corresponding to the absorbed wavelengths disappear form the spectrum of the transmitted and reflect light, but energy cannot disappear
• When a molecule absorbs a photon of light, one of the electrons is elevated to an orbital where it has more potential energy (the pigment molecule)
• The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground and excited state
• Once the electron is raised to an excited state, it cannot stay there. It drops back down to the ground- state orbital and releases excess energy as heat.
• The conversion of light energy to heat is what makes the top of an automobile so hot on a sunny day Photosystems
• In the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems
• A photosystem is composed of a reaction-center complex surrounded by several light- harvesting complexes
• The reaction-center complex consists of various pigment molecules (like chlorophyll a, chlorophyll b, and multiple carotenoids) that are bound to proteins

• When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex
• The reaction-center complex also contains a molecule capable of accepting electrons and becoming reduced, this is called the primary electron acceptor
• The solar powered transfer of an electron from the reaction-center chlorophyll a to the primary electron acceptor is the first step of the light reactions
• As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it
• The thylakoid membrane contains 2 types of photosystems that cooperate in the light reactions of photosynthesis 1. Photosystem II (PS II)- chlorophyll a in this photosystem is best at absorbing light having a wavelength of 680nm 2. Photosystem I (PS I)- chlorophyll a in this photosystem is best at absorbing light having a wavelength of 700nm Linear Electron Flow
• Light drives the synthesis of ATP and NADPH by energizing the two photosystems embedded in the thylakoid membranes
• Linear electron flow occurs during the light reactions of photosynthesis

1. A photo of light strikes one of the pigment molecules in a light-harvesting complex of PS II. One of its electrons is energized, when it falls back to ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. The process continues until it reaches the P pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state
2. This electron is transferred from P680 to the primary electron acceptor. The resulting form of P680 is missing an electron, and becomes P680+
3. 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+ pair, each electron replacing one transferred to the primary electron acceptor. The H+ are released into the thylakoid space. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O
4. Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain. This chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc)
5. The exergonic <fall= of electrons to a lower energy level provides energy for the synthesis of ATP. As electrons pass though the cytochrome complex, H+ are pumped into the thylakoid space, contributing to the proton gradient
6. Meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a. The photoexcited electron is then transferred to

Comparing Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis
• An electron transport chain pumps H+ protons across a membrane as electrons are passed through a series of carriers that are progressively more electronegative. Thus, electron transport chains transform redox energy to a proton-motive force
• An ATP synthase complex couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP
• In chloroplasts, the high-energy electrons dropped down the chain come from water. Chloroplasts do not need molecules from food to make ATP, their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain
• The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (interior) which functions as the H+ reservoir. ATP is synthesized as the H+ ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes. Thus, ATP is formed in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle Summary of Light Reactions: Electron flow pushes electron from water, where they are at a low state of potential energy, ultimately to NADPH, where they are stored at a high state of potential energy. The light-driven electron flow also generates ATP. Thus, the equipment of the thylakoid membrane convers light energy to chemical energy stored in ATP and NADPH (oxygen is a byproduct) Light reactions capture solar energy and use it to make ATP and transfer electrons from water to NADP+, forming NADPH The Calvin Cycle
• The cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make sugar

• Carbon enters the cycle in the form of CO2 and leaves in the form of sugar
• The carbohydrate produced is glyceraldehyde 3-phosphate (G3P)
• The cycle must take place 3 times to make one molecule of G3P
• Phase 1: Carbon Fixation
  1. Incorporates CO2 molecules one at a time by attaching a five-carbon sugar, ribose biphosphate (RuBP). The enzyme that catalyzes this is rubisco
  2. The product is a six-carbon intermediate that is then split in half, forming two molecules of 3-phosphoglycerate
• Phase 2: Reduction
  1. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate
  2. A pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming G3P (a sugar) Note: for every three molecules of CO2 that enter the cycle, there are six molecules on G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to complete the cycle
• Phase 3: Regeneration of the CO2 Acceptor (RuBP)
  1. The carbon skeletons of five molecules of G3P are rearranged into three molecules of RuBP. The cycle spends three more molecules of ATP to accomplish this. The RuBP is now prepared to receive CO2 again, and the cycle continues
• For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules of NADPH Summary of the Calvin Cycle: ATP and NADPH are used to produce sugar from CO2. The energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds.