Botany Lecture 2 - Notes
Introduction to Photosynthesis
Photosynthesis is the process through which photosynthetic organisms convert radiant energy from the sun into chemical energy
The following materials are used: CO2 (carbon dioxide) and HOH (water)
The complex organic molecules produced are used for:
Energy source for cellular metabolism
Carbon skeleton intermediates for production of complex organic molecules
Components of various metabolic pathways
Energy storage
Maintaining the growth and development of the plant body
Photosynthesis releases oxygen (O2), produces ATP, NADPH, and converts CO2 into carbohydrates
It is the largest synthetic process on Earth, incorporating nearly all energy into the biosphere
Annually, an estimated 100 billion tons of carbohydrates are formed by photosynthetic organisms globally
Equations for Photosynthesis
The basic chemical reactions for photosynthesis:
The primary sugar typically referenced in photosynthesis equations is glucose (C6H12O6), however, the primary carbohydrates produced in photosynthetic cells are triose sugars (3-carbon sugars):
Phosphoglycerate (PGA) or Phosphoglyceric acid
Phosphoglyceraldehyde (PGAL), the reduced form of PGA
More accurate representation of photosynthesis:
Photosynthetic Pigments
Light must be absorbed by pigments to be utilized for photosynthesis
Definitions:
Pigment: A substance that absorbs light
Pigments absorbing all wavelengths appear black; those reflecting all appear white
Most pigments absorb specific wavelengths and reflect/transmit others
Visible Light Spectrum: Represents the range of light used by plants for photosynthesis
Range: 400 nm to 750 nm
Major colors and their respective wavelengths:
Violet (400nm), Blue (450nm), Green (550nm), Yellow (600nm), Orange (650nm), Red (700nm), Far Red (750nm)
Nature of Light:
Light has both wave and particle characteristics
The energy of a photon is inversely related to its wavelength (longer wavelengths possess less energy)
Each pigment has an absorption spectrum indicating the wavelengths absorbed
Chlorophyll, the green pigment of plants, primarily absorbs light in violet, blue, and red wavelengths, reflecting/transmitting green and yellow wavelengths
Excitation of Electrons in Pigments
Upon light absorption, electrons in pigment molecules enter an excited state
Possible fates for excited state energy:
Converted to heat (thermal deactivation) or light (fluorescence, which is usually wasted)
Transferred to a neighboring pigment molecule (resonance energy transfer)
The electron may be transferred to a neighboring pigment molecule (useful for plants)
The process of energy transfer is through inductive resonance, following an excitation energy gradient towards the reaction center pigment
Order of excitation transfer: Carotenoids → Chlorophyll B → Chlorophyll A → Reaction Center Pigment (P680 or P700)
Types of Pigments in Plants
Chlorophyll (main photosynthetic pigment):
Multiple forms exist:
Chlorophyll A:
Present in all photosynthetic eukaryotes and cyanobacteria
Essential for oxygen-generating photosynthesis
Blue-green in color with a terminal methyl group
Chlorophyll B:
Accessory pigment with a different absorption spectrum
Yellow-green in color with a terminal aldehyde group
Broadens light absorption range; transmits energy to chlorophyll A
Chlorophyll C: Found in certain algae, replaces chlorophyll B, and is absent in higher plants
Chlorophyll D and E: Not found in higher plants
Difference in structures between Chlorophyll A & B: Chlorophyll A has a methyl group (-CH3) at the C3 position, while Chlorophyll B has a formyl group (-CHO) at the same position, which slightly alters their light absorption characteristics.
Chlorophyll A: Contains a porphyrin ring with a central magnesium ion and a long phytol tail, enabling it to capture solar energy efficiently.
Chlorophyll B: Has a similar porphyrin structure but differs in its side group, which contains a carbonyl group, allowing it to absorb light in different wavelengths.
Photosynthetic Bacteria Pigments:
Bacteriochlorophyll
Chlorobium Chlorophyll (found in green sulfur bacteria; uses H2S as electron source, does not produce O2)
Carotenoids:
Types include:
Carotenes: Orange/red/purple, oxygen-free structure
Xanthophylls: Yellow pigments containing oxygen
Phycobilins: Water-soluble, red/blue pigments found in red algae and cyanobacteria
Accessory pigments such as Chlorophyll B, Carotenoids, and Phycobilins aid in capturing light for photosynthesis
Functions of Carotenoids
Act as accessory pigments, increasing light absorption and reducing photo-oxidative damage
Stabilize chlorophyll against excess light by acting as antioxidants
Two mechanisms to quench excess energy from chlorophyll: 1) Physical Quenching and 2) Chemical Quenching
Physical Quenching: Dissipates excess energy in the xanthophyll cycle, forms zeaxanthin from violaxanthin, and helps release excess energy as heat
Chemical Quenching: Involves a reaction between a quencher and singlet excited oxygen (1O2), generating carotenoid endoperoxides as a by-product
The Light-Dependent Reactions of Photosynthesis
Photosystems:
Photosynthesis consists of two stages: Light-dependent reactions and Light-independent reactions (Calvin cycle)
Light-dependent reactions involve:
Oxygen evolving complex (OEC)
Photosystem II (PSII)
Cytochrome complex
Photosystem I (PSI)
ATP, NADPH, and O2 are produced during these reactions
Structure and Function of PSII:
PSII is a dimeric complex with multiple reaction centers and antenna complexes
The oxygen-evolving complex performs oxidation of water during photolysis
Importance of manganese ions (Mn) in the OEC for electron transfer and O2 release
The flow of electrons during photosynthesis is orchestrated through a series of redox reactions
Photolysis:
Involves splitting water molecules to provide protons and electrons, contributing to the proton gradient and O2 release
The overall reaction:
Important soluble proteins, such as plastocyanin and ferredoxin, facilitate electron transport
Photosystem I (PSI)
PSI processes electrons transported from plastocyanin to generate NADPH
Incorporates P700 as a reaction center pigment, with additional modifications at various points of the chain to facilitate electron transfer
dPSI is associated with Light Harvesting Complex I (LHCI)
Noncyclic and cyclic electron transport pathways describe the movement of electrons and production of ATP during photosynthesis
Calvin Cycle (Light-Independent Reactions)
The Calvin cycle occurs in the stroma, where CO2 is fixed into carbohydrates
Key stages:
Carboxylation: CO2 is incorporated into ribulose bisphosphate (RuBP) through rubisco, forming 3-PGA
Reduction: ATP phosphorylates 3-PGA to form 1,3-bis-PGA which is reduced to 3-PGAL by NADPH
Regeneration of RuBP: Uses additional ATP to regenerate RuBP from ribulose-5-phosphate
For every CO2 molecule fixed, 2 NADPH and 3 ATP are required
C4 and CAM Pathways
C4 Plants:
Have evolved a separate pathway for CO2 fixation that captures CO2 in the form of bicarbonate
Processes occur in mesophyll and bundle sheath cells with spatial separation
C4 pathway facilitates higher photosynthetic efficiency in high temperatures and low water conditions
CAM Plants:
Temporally separate carbon fixation, fixing CO2 at night via PEP carboxylase
Store malate in vacuoles, releasing CO2 during the day for use in the Calvin cycle
Extremely efficient in water-limited environments, with reduced water loss during day-time stomatal closure
Photorespiration
Occurs when rubisco binds O2 instead of CO2, resulting in the production of phosphoglycolate
Phosphoglycolate is metabolized in a series of reactions releasing CO2, while recovering some carbon within the cycle
Although photorespiration does not yield ATP or NADPH, it serves several beneficial functions, including protection from photo-oxidation
Aerobic Respiration
The oxidation of organic molecules produced through photosynthesis to yield ATP
Stages of cellular respiration: glycolysis, Krebs cycle, oxidative electron transport chain
Basic Equation:
Glycolysis leads to pyruvate, the Krebs cycle processes the pyruvate, and ultimately ATP synthesis occurs via chemiosmosis in electron transport
Summary of Photosynthetic Processes
Noncyclic and cyclic electron transport chains collaborate to generate the electrochemical gradients necessary for ATP production
The two photosystems cooperate to provide for efficient electron transfer during photosynthesis
The Calvin cycle enables the synthesis of carbohydrates using the ATP and NADPH generated from light reactions
C4 and CAM pathways represent adaptations in specific environments to enhance photosynthetic efficiency and limit water loss.
The photosynthetic electron transport chain, which directly requires light to drive, happens along the thylakoid membranes of chloroplasts, where light energy is converted into chemical energy through a series of redox reactions. Water is oxidized to extract the electrons, which ultimately replace the electrons lost by chlorophyll during light absorption, ensuring a continuous flow of energy necessary for the establishment of a proton gradient that drives ATP synthesis. They are initially restored as redox energy. In the process of extracting electrons from water, oxygen is released as a byproduct, playing a crucial role in maintaining Earth's atmosphere and supporting aerobic life forms. It starts in the lumen and thyakoid space, where protons accumulate as water is split. This creates a proton motive force that drives ATP synthase, ultimately resulting in the conversion of ADP and inorganic phosphate into ATP, which is then utilized in the Calvin cycle for the synthesis of glucose and other carbohydrates. :
Oxygen gas
ATP
NADPH
Then, the light-independent reactions, calvin cycle, c3 cycle or carbon fixation, represents the energy in sunlight from. It happens in the stroma of chloroplasts and utilizes the ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose.
What is 3-PGA? 3-PGA, or 3-phosphoglycerate, is a three-carbon compound that serves as the first stable product of carbon fixation in the Calvin cycle, playing a crucial role in the synthesis of glucose during photosynthesis. It is the final sugar.
3-PGA is formed when carbon dioxide is added to ribulose bisphosphate (RuBP) by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO).
Following the production of 3-PGA, it undergoes a series of transformations involving ATP and NADPH, eventually leading to the generation of glucose through the reduction and regeneration phases of the Calvin cycle.
Photosystem II (water-plastoquinone oxidoreductase)
If the2
Anthocyanins and betacyanins or betalins
What are they?: These are pigment compounds found in various plants, playing a crucial role in determining flower and fruit colors, while also providing protection against UV radiation and herbivory.
These two classes of pigments are water-soluble:
Anthocyanins:
Type: Flavonoids
Colors: Usually red, blue, or violet (rarely yellow)
Betacyanins:
Type: Aromatic, nitrogenous compounds
Colors: Mostly yellow or red
Both classes of pigments are generally not considered photosynthetic.
Cyclic electron transport is a critical component of the photosynthetic process, primarily occurring in the thylakoid membranes of chloroplasts. This process is distinct from noncyclic electron transport and serves several key functions:
Functionality: Cyclic electron transport allows for the regeneration of ATP without the production of NADPH or oxygen. This is particularly important when the plant requires more ATP than NADPH, which can happen during certain periods of photosynthesis.
Mechanism: During the cyclic electron transport, electrons from photosystem I (PSI) are redirected back to the electron transport chain instead of moving toward NADP+ for reduction. The cycle begins when photons excite electrons in PSI, specifically in the P700 pigment complex. Once energized, these electrons enter a series of electron carriers, including plastoquinone and cytochrome b6f, which work to pump protons (H+) into the thylakoid lumen.
Proton Gradient Generation: The movement of protons into the lumen contributes to the development of a proton motive force, which is essential for ATP synthesis via ATP synthase. When the proton gradient reaches a threshold, protons flow back across the thylakoid membrane through ATP synthase, leading to the phosphorylation of ADP to ATP.
Cyclic Flow Illustration: In a typical diagram of cyclic electron flow, the flow of electrons illustrates this process: excited electrons return from the electron carriers back to PSI, forming a closed loop that enables efficient ATP generation without the production of other byproducts, as seen in noncyclic transport.
Adaptation and Importance: Cyclic electron transport is particularly advantageous for the plant under conditions where light intensity is high, and the demand for ATP increases. It can also prevent over-reduction and potential damage caused by excessive NADPH formation, thus maintaining homeostasis in the photosynthetic apparatus.
Connection to Overall Photosynthesis: Together with noncyclic electron transport, cyclic electron transport plays an integral role in the light-dependent reactions of photosynthesis, with cyclic pathways balancing the production of ATP and maintaining the flow of energy essential for the Calvin cycle and subsequent carbohydrate synthesis during light-independent reactions.
Respiration Aerobic (oxygen-using) respiration is a critical metabolic process in plants where photoassimilates—high-energy molecules such as glucose, produced during photosynthesis—are oxidized to release ATP energy. This energy is essential for various cellular functions, including growth, development, and maintenance of cellular structures, as well as for supporting overall plant metabolism.
Most of it takes place in the mitochondria, where the Krebs cycle and oxidative phosphorylation occur. These processes are crucial for converting energy stored in photoassimilates into usable ATP, illustrating the interconnectedness of photosynthesis and respiration in sustaining plant life.
Key Stages of Aerobic Respiration
Glycolysis:
This stage occurs in the cytoplasm and is the first step in the oxidation of glucose.
One glucose molecule is converted into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH molecules through substrate-level phosphorylation.
Glycolysis does not require oxygen and serves as an anaerobic pathway.
Krebs Cycle (Citric Acid Cycle):
The Krebs cycle takes place in the mitochondrial matrix, where acetyl-CoA derived from pyruvate enters the cycle.
For each acetyl-CoA molecule that enters the cycle, the following products are generated:
3 NADH
1 FADH₂
1 GTP (or ATP)
2 CO₂ (released as byproducts)
The NADH and FADH₂ produced carry high-energy electrons to the next stage of respiration.
Oxidative Phosphorylation:
This process occurs across the inner mitochondrial membrane and involves the electron transport chain (ETC).
NADH and FADH₂ donate electrons to the ETC, where they pass through a series of protein complexes, releasing energy that pumps protons (H⁺ ions) into the intermembrane space, creating a proton gradient.
The stored energy in the proton gradient is then used by ATP synthase to produce ATP as protons flow back into the mitochondrial matrix.
Oxygen acts as the final electron acceptor, combining with electrons and protons to form water, which is crucial for completing the respiration process.
Importance of Aerobic Respiration in Plants
Energy Production: Aerobic respiration is far more efficient in terms of energy yield compared to anaerobic pathways, producing up to 38 ATP molecules per glucose molecule, whereas anaerobic respiration (fermentation) yields only 2 ATP.
Metabolic Balance: It maintains the balance of energy within the plant cells, ensuring that the energy demands for activities like nutrient transport, biosynthesis, and growth are met adequately.
Integration with Photosynthesis: The products of photosynthesis, such as glucose, directly fuel aerobic respiration, while the byproducts of respiration, such as CO₂ and water, are utilized again in photosynthesis, forming a cyclical relationship essential for plant survival.
Adaptation to Environmental Conditions: Aerobic respiration allows plants to thrive in oxygen-rich environments and is critical for adaptation during periods of insufficient light for photosynthesis, when stored glucose needs to be metabolized for energy.