Chapter 7: Photosynthesis and Energy Transfer

Introduction to Photosynthesis

• Photosynthesis is performed by autotrophs, such as plants, algae, and some bacteria (e.g., cyanobacteria).

• Autotrophs(occurs in places like the leaves and stems): convert inorganic materials into organic matter.

• Photosynthesis: the process in which the energy from light is captured and used to synthesize glucose and other organic molecules. Almost all living organisms rely on photosynthesis for nourishment, either directly or indirectly. Also responsible for producing the oxygen that makes up large portion of Earth’s atmosphere.

Site of Photosynthesis

• Photosynthesis takes place primarily in the green parts of plants (e.g., leaves, stems).

• NOT in roots, as they do not contain the necessary pigments and structures.

• Photosynthesis equation: CO2+2H2O+ Light energy→CH2O+O2+H2O

• 6CO2+12H2O+Light energy→C6H12O+6O2+6H2O ΔG=+686 kcal/mol

• In this redox reaction, the carbon in CO2 is reduced during the formation of glucose, and oxygen in H2O is oxidized during the formation of O2.

• In this case the energy from sunlight ultimately drives the synthesis of glucose

• Autotrophs: sustain themselves by producing organic molecules from inorganic sources

  • Photoautotrophs: a specific type of autotroph that uses light energy to convert carbon dioxide and water into organic molecules through photosynthesis. Most species of bacteria and protists, as well as all species of fungi and animals, are heterotrophs (plants as well)

• Life in the biosphere is largely driven by photosynthetic power of plants, algae, and cyanobacteria.

• Photoautotrophs make a large proportion of the Earth’s organic molecules via photosynthesis using light energy, CO2 and H2O. During this process, they also produce O2.

• To supply their energy needs both photoautotrophs and heterotrophs metabolize organic molecules via cellular respiration

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Chloroplast Structure

• Mesophyll Cells: Contain numerous chloroplasts responsible for photosynthesis.

• Chloroplast Components:

  • Thylakoids: Disc-like structures where light reactions occur; contain pigments.

  • Stroma: Fluid-filled space where the Calvin cycle occurs.

Photosynthesis Process

1. Light Reactions:

   • Occur in the thylakoid membranes.

   • Light energy is converted into chemical energy (ATP and NADPH).

   • Requires photons of light to excite electrons.

   • Water is split to provide replacement electrons, yielding oxygen as a byproduct.

2. Calvin Cycle:

   • Occurs in the stroma using ATP and NADPH from light reactions.

   • Converts carbon dioxide into glucose.

   • Note: Sugars are not produced directly but are built using intermediates.

Light Absorption and Electromagnetic Spectrum

• Electromagnetic Spectrum: Displays the relationship between light energy and wavelength.

  • Longer wavelengths (e.g., red light) have lower energy.

  • Shorter wavelengths (e.g., violet light) have higher energy.

• Visible Spectrum: ROYGBIV (red, orange, yellow, green, blue, indigo, violet) correspond to different wavelengths:

  • Red has a long wavelength and low energy.

  • Violet has a short wavelength and high energy.

Role of Photopigments

• Chlorophyll: The pigment responsible for green color; absorbs red and blue/violet light.

• Carotenoids: Accessory pigments (e.g., beta-carotene) absorb additional light wavelengths.

  • Provide autumn color changes in leaves when chlorophyll breaks down.

  • Absorption spectrum: a graph that plots a pigment’s light absorption as a function of the light’s wavelength

  • Action spectrum: plots the rate of photosynthesis as a function of the wavelength of light

How Light Affects Plants

• Black Surfaces: Absorb all wavelengths of visible light, thus appearing black and getting hot (e.g., asphalt).

• White Surfaces: Reflect most wavelengths, which is why they appear white and remain cool.

Energy Utilization in Photosynthesis

• When light strikes atoms with electrons, they can absorb energy and be excited to higher energy states.

• Excited electrons can:

  • Transfer energy (resonance energy transfer).

  • Be used for the production of NADPH and ATP.

Structure and Function of Photosystems

• Photosystems: Complexes in thylakoid membranes where light reactions occur.

  • Photosystem I (PSI) and Photosystem II (PSII): Important for capturing light and facilitating electron transport.

  • NADP+ Reductase: Enzyme that converts NADP+ to NADPH.

  • ATP Synthase: Enzyme that generates ATP using a proton gradient.

• Electrons are replenished from water splitting, producing oxygen.

Cyclic and Linear Electron Flow

• Linear Flow: Involves both photosystems and produces ATP and NADPH.

• Cyclic Flow: Involves only PSI and produces ATP without NADPH. Used when more ATP is needed for the Calvin cycle.

Final Notes

• The process of photosynthesis mainly takes place during the day, harnessing solar energy efficiently.

• Leaf color change in fall due to decreased chlorophyll and increased carotenoids - both pigment systems contribute to maximizing light absorption.

Photosynthesis and the Calvin Cycle Explained

Overview of Photosynthesis

• Plants convert light energy into chemical energy through photosynthesis, involving two key phases: the light reactions and the Calvin cycle.

• Photosystems: Two types, PSI and PSII, are critical for this process:

  • PSII: Leads in absorbing light and oxidizing water to produce oxygen (O₂).

  • PSI: Absorbs light to aid in the production of NADPH.

  • Photosystem I was discovered before photosystem II but photosystem II is the initial step in photosynthesis

Photosystem II (PSII)

• Components:

  • Light Harvesting Complex: Contains numerous pigment molecules that absorb photons.

  • Reaction Center (P680): Special pigment that absorbs light at 680 nm, crucial for initiating electron transport.

• Process of Energy Transfer:

  • Absorption of light leads to the excitation of electrons within pigment molecules.

  • Resonance Energy Transfer: Energy (not electrons) is passed from one pigment molecule to another until it reaches P680.

  • Excitation of P680 produces P680\* (excited state).

  • Photosystem II harvests light energy and oxidizes water:

    1) light energy is absorbed by a pigment molecule. This boosts an electron in the pigment to a higher energy level

  • 2) energy is transferred among pigment molecules via resonance energy transfer until it reaches P680, converting it to P680*

  • 3) the high energy electron on P680* is transferred to the primary electron acceptor where it is very stable. P680* becomes p680+

  • 4) A low energy electron from water is transferred to P680+ to convert it to P680. O2 is produced

  • One role of the reaction center is to carry out a redox reaction in which the high-energy electron from P680* is transferred to another molecule, where the electron is stable

  • The missing electron of P680 is replaced with a low-energy electron from water

  • In 1960, Robin Hill and Fay Bendall proposed that photosynthesis involves two events in which light is absorbed. According to their model, known as the Z scheme, an electron proceeds through a series of energy changes during photosynthesis

The Calvin Cycle

• Occurs in the stroma of chloroplasts in plants and algae, and in the cytoplasm of cyanobacteria.

• Function: Converts carbon dioxide (CO₂) from the atmosphere into carbohydrates (e.g., glucose).

• Importance:

  • Carbohydrates are essential as precursors for organic molecules and primary energy storage.

• Phases of the Calvin Cycle:

  1. Carbon Fixation: CO₂ is combined with Ribulose bisphosphate (RuBP), catalyzed by the enzyme rubisco.

  2. Reduction Phase: Uses ATP and high-energy electrons from NADPH to convert 3-phosphoglycerate (3PG) into glyceraldehyde-3-phosphate (G3P).

  3. Regeneration of RuBP: Enzymatic reactions transform some G3P back to RuBP to allow the cycle to repeat, utilizing ATP.

     -Although biologists commonly describe glucose as a product of photosynthesis, glucose is not directly made by the Calvin cycle. Instead, glyceraldehyde-3-phosphate (G3P) is the product of the Calvin cycle. G3P molecules are used as starting materials for the synthesis of glucose and other molecules, including sucrose. After glucose molecules are made, they may be linked together to form a polymer of glucose called starch, which is stored in the chloroplast for later use. Alternatively, the disaccharide sucrose may be made and transported out of the leaf to other parts of the plant. The Calvin cycle is divided into three phases: carbon fixation, reduction and carbohydrate production, and regeneration of ribulose bisphosphate

Experimental Insights

• Melvin Calvin's Work: Utilized C-14 isotope for tracing carbon during experiments involving Chlorella algae to elucidate steps of the Calvin Cycle.

• Method:

  • Inject C-14 labeled CO₂, incubate samples, halt reaction, and analyze organic molecules through autoradiography and chromatography.

  • Identified the sequence of products during the Calvin cycle and the incorporation of carbon.

Carbon Reduction and Energy Storage

• CO₂ → Organic Molecules: The Calvin cycle transitions carbon from an inorganic state (CO₂) to an organic state (e.g., G3P), rich in potential energy.

• Bond Types: Conversion from C-O bonds (in CO₂) to C-H and C-C bonds (in organic molecules) enables the formation of energy-rich compounds.

• Carbohydrates: Formation of glucose or other polysaccharides from G3P, which can be stored as starch.

• Certain environmental conditions such as temperature, water availability, and light intensity alter the way in which the Calvin cycle operate

Photorespiration and Adaptations

• Photorespiration: A wasteful process where rubisco reacts with O₂ instead of CO₂, especially in conditions of water stress.

  • Causes loss of carbon fixation potential (up to 50% reduction of photosynthetic output).

  • May provide protection against damage from toxic molecules when stomata are closed (water conservation).

• C3 vs C4 vs CAM Plants:

  • C3 Plants(3-phosphoglycerate or 3-PGA): Majority group, directly fix CO₂ using rubisco, susceptible to photorespiration.

  • C4 Plants(four-carbon compound): Utilize PEP carboxylase to limit photorespiration by separating carbon fixation and Calvin cycle spatially through mesophyll and bundle sheath cells (e.g., sugarcane, corn).

  • CAM Plants(Crassulacean Acid Metabolism): Separate these processes temporally (night/day) to minimize water loss (e.g., cacti).

  • thylakoids: thylakoid membrane forms many flattened, fluid filled tubules

  • thylakoid lumen: enclose a single, convoluted compartment

  • stroma: fluid filled region of the chloroplasts between thylakoid membrane and inner membrane

  • The light reaction take place at the thylakoid membrane and the Calvin cycle occurs in the stroma

  • Electromagnetic Spectrum: Displays the relationship between light energy and wavelength. Longer wavelengths (e.g., red light) have lower energy, while shorter wavelengths (e.g., violet light) have higher energy.

    Visible Spectrum: ROYGBIV (red, orange, yellow, green, blue, indigo, violet) corresponds to different wavelengths: Red has a long wavelength and low energy, whereas violet has a short wavelength and high energy.

    Role of Photopigments: Chlorophyll, the pigment responsible for the green color in plants, absorbs red and blue/violet light. Carotenoids, accessory pigments (e.g., beta-carotene), absorb additional light wavelengths. This absorption is crucial for photosynthesis as it allows photopigments to harness light energy to drive the photosynthetic processes.

  • An excited electron can release heat. For example, on a sunny day, the sidewalk heats up because it absorbs light energy that is released as heat. An electron can release energy in the form of light. Certain organisms, such as jellyfish, possess molecules that make them glow. This glowing is due to the release of light when electrons drop down to lower energy levels, a phenomenon called fluorescence. An excited electron can transfer its extra energy to an electron in a nearby molecule, a process called resonance energy transfer. Page 155 In the case of certain photosynthetic pigments such as P680, which is discussed later, another event can happen that is critical for the process of photosynthesis. Rather than releasing energy or transferring it to another molecule, an excited electron in P680 is removed from that molecule and transferred to another molecule where the electron is stable. When this occurs, the energy in the electron is said to be “captured” because the electron does not readily drop down to a lower energy level and release heat or light

  • Biosphere: The global sum of all ecosystems; the zone of life on Earth, which includes land, water, and the atmosphere, where living organisms interact with each other and their environment.

  • G14 Isotope: A radioactive isotope of carbon, specifically Carbon-14 (C-14), used in scientific research such as tracing carbon compounds through biological processes, including the Calvin Cycle in photosynthesis.

  • G3P (Glyceraldehyde-3-phosphate): A three-carbon sugar phosphate produced in the Calvin Cycle; it is a key intermediate in the synthesis of glucose and other carbohydrates.

  • Thylakoids: Membrane-bound structures within chloroplasts where the light-dependent reactions of photosynthesis take place; they contain chlorophyll and other pigments.

  • Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase): An enzyme that catalyzes the reaction between ribulose bisphosphate (RuBP) and carbon dioxide in the Calvin Cycle; it is crucial for carbon fixation.

  • Carotenoids: Accessory pigments found in plants that absorb light in other wavelengths (e.g., blue and green) and are responsible for the yellow, orange, and red colors in plants; they play a role in photosynthesis and provide photoprotection.

  • PEP Carboxylase: An enzyme that catalyzes the first step of carbon fixation in C4 and CAM plants, converting phosphoenolpyruvate (PEP) and carbon dioxide to oxaloacetate, thereby minimizing photorespiration.

  • Photorespiration: A process where the enzyme rubisco reacts with oxygen instead of carbon dioxide, leading to a wasteful pathway that can significantly reduce the efficiency of photosynthesis under stress conditions such as high temperature and low CO₂ concentration.

  • Z Scheme: A model describing the energy changes of electrons during photosynthesis, outlining the excitation of electrons in photosystems I and II through a series of reactions that ultimately leads to the production of ATP and NADPH.

  • Ribulose bisphosphate(RuBP): 5 carbon sugar that plays a critical role in Calvin Cycle of photosynthesis

    Conclusion

• Photosynthesis is a crucial biological process allowing plants to utilize light energy, producing organic matter essential to life. Understanding its detailed mechanisms, adaptations, and energy dynamics is key to comprehending plant biology and ecology.