Comprehensive Notes on Photosynthesis
Photosynthesis: An Overview
Definition and Basic Process
Photosynthesis is the fundamental process by which plants, certain bacteria, and protistans convert visible light energy from the sun into chemical energy.
This conversion uses raw materials: carbon dioxide (CO2) and water (H2O).
The primary chemical product is glucose (C6H{12}O_6), a form of sugar.
Oxygen (O_2) is released as a byproduct.
Glucose can be subsequently converted into pyruvate, which then releases adenosine triphosphate (ATP) through cellular respiration.
The overall process can be summarized by the word equation:
carbon ext{ }dioxide + water
ightarrow glucose + oxygen
Chlorophyll: The Key Pigment
The conversion of sunlight energy into chemical energy is directly linked to the action of chlorophyll.
Chlorophyll is a complex green pigment molecule essential for photosynthesis.
Different modifications of chlorophyll exist among photosynthetic organisms; however, all photosynthetic organisms contain chlorophyll a.
Accessory pigments assist by absorbing light energy wavelengths that chlorophyll a does not absorb, broadening the spectrum of usable light.
Examples include chlorophyll b (and c, d, e in algae and protistans), xanthophylls, and carotenoids (e.g., beta-carotene).
Chlorophyll a primarily absorbs energy from the violet-blue and reddish orange-red wavelengths of the light spectrum.
It absorbs very little energy from the intermediate green-yellow-orange wavelengths; this is why plants appear green, as these wavelengths are reflected.
Structure of Chlorophyll
All chlorophyll molecules share two main components:
A lipid-soluble hydrocarbon tail with the chemical structure (C{20}H{39}-). This tail anchors the molecule within the photosynthetic membranes.
A flat hydrophilic head containing a magnesium ion (Mg^{2+}) at its center. Variations in side-groups on this head distinguish different types of chlorophyll.
The hydrocarbon tail and the hydrophilic head are connected by an ester bond.
Leaves and Leaf Structure: Solar Collectors
Leaves are specialized organs unique to plants (though not all plants possess them) that function as solar collectors, densely packed with photosynthetic cells.
They facilitate the exchange of raw materials and products of photosynthesis:
Water and carbon dioxide enter the leaf cells.
Sugars (glucose) and oxygen are produced and leave the leaf.
Water transport:
Water absorbed by the roots is transported upwards to the leaves through specialized vascular tissues called xylem vessels.
Gas exchange and water regulation (Stomata):
Terrestrial plants have evolved stomata (singular: stoma) to regulate gas exchange and prevent excessive water loss.
The leaf's protective outer layer, the waxy cuticle, is impermeable to gases like CO2 and O2.
CO_2 can only enter the leaf through the open stoma, which are flanked by two guard cells that regulate their opening and closing.
Likewise, O_2 produced during photosynthesis can only exit the leaf through these open stomata.
A significant drawback of open stomata is considerable water loss through transpiration. For example, cottonwood trees can lose $100$ gallons (approximately 450 ext{ } dm^3) of water per hour during hot desert conditions.
Structure of the Chloroplast and Photosynthetic Membranes
The thylakoid is the fundamental structural unit of photosynthesis.
Both photosynthetic prokaryotes and eukaryotes possess these flattened membranous sacs or vesicles, which house the photosynthetic pigments and enzymes.
Only eukaryotic cells enclose their thylakoids within specialized organelles called chloroplasts, which are surrounded by their own membrane.
Within chloroplasts, thylakoids are organized into stacks resembling pancakes, collectively known as grana (singular: granum).
The fluid-filled space surrounding the grana within the chloroplast is called the stroma.
Chloroplasts are distinctive for having three membrane systems, which define three distinct compartments, in contrast to mitochondria which have two.
Stages of Photosynthesis
Photosynthesis is broadly divided into two main stages:
Light-Dependent Reactions
Light-Independent Reactions (Calvin Cycle / Dark Reaction)
Photoactivation of Chlorophyll
When chlorophyll a absorbs light energy, one of its electrons gains energy and becomes 'excited', moving to a higher energy level.
This excited electron is then transferred to an adjacent molecule known as a primary electron acceptor.
The chlorophyll molecule, having lost an electron, becomes oxidized and acquires a positive charge.
This process, termed photoactivation of chlorophyll a, initiates a cascade of reactions that ultimately leads to the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP), specifically NADPH.
Key chemical reactions involved include:
Condensation reactions: These are responsible for the splitting out of water molecules and the formation of bonds, including phosphorylation (the addition of a phosphate group to an organic compound).
Oxidation/reduction (redox) reactions: These reactions involve the transfer of electrons, crucial for energy conversion.
1. The Light-Dependent Reactions
Location: Occur in the grana (thylakoid membranes) of the chloroplasts.
Requirement: Directly require light energy.
Purpose: To convert light energy into chemical energy in the form of energy-carrier molecules (ATP and NADPH).
Key processes:
Photophosphorylation: Light energy captured by chlorophyll is used to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
Photolysis of Water: Water molecules are split by light energy, releasing oxygen, hydrogen ions (H^+), and free electrons (e^-). The equation is:
2H2O ightarrow 4H^+ + O2 + 4e^-
Reduction of NADP+: The electrons released from water, along with hydrogen ions, react with the electron carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), reducing its oxidized state (NADP^+) to its reduced state (NADPH). The equation is:
NADP^+ + 2e^- + 2H^+
ightarrow NADPH + H^+
2. The Light-Independent Reactions (Calvin Cycle)
Location: Occur in the stroma of the chloroplasts.
Requirement: Do not directly require light; they utilize the ATP and NADPH produced during the light-dependent reactions.
Purpose: To fix carbon dioxide from the atmosphere into carbohydrates.
Initial product: Glyceraldehyde 3-phosphate (GALP), a 3-carbon atom molecule.
Detailed Mechanisms of Light-Dependent Reactions
Photoexcitation and Photoionisation
When a chlorophyll molecule absorbs light energy, its electrons are energized and jump to higher energy levels (this is photoexcitation).
If sufficient energy is absorbed, the molecule becomes ionized, and an electron is 'freed' from the chlorophyll, leaving behind a positively charged chlorophyll ion. This process is called photoionisation.
Photosystems and Electron Transfer
Within the thylakoid membranes, each chlorophyll molecule is functionally associated with an electron acceptor and an electron donor. These three components form the core of a photosystem.
Two electrons from a photoionised chlorophyll molecule are transferred to the electron acceptor.
The positively charged chlorophyll ion then regains a pair of electrons from a neighboring electron donor, such as water.
An electron transfer system (a series of chemical reactions involving electron carriers) transports these electrons back and forth across the thylakoid membrane.
Two main photosystems drive the electron transfer process:
Photosystem II (PSII), also referred to as P680 (indicating its optimal absorption wavelength of 680 ext{ } nm).
Photosystem I (PSI), referred to as P700 (optimal absorption wavelength of 700 ext{ } nm).
It is important to note that PSII occurs before PSI in the overall electron flow, despite its numerical designation (it was discovered second).
The energy changes throughout the electron transfer process can be depicted visually as a 'Z' shape, giving rise to the term Z scheme.
A critical outcome of this electron transfer is the release of enough energy to synthesize ATP from ADP and inorganic phosphate.
ATP Synthesis from ADP: Phosphorylation
The synthesis of ATP involves a condensation reaction between phosphoric acid and ADP.
During this reaction, a molecule of water (H_2O) is eliminated, and a phosphate group is added to ADP, forming ATP.
This process is a form of phosphorylation.
Non-Cyclic Phosphorylation (The Z Scheme) Components and Process
Both ATP and NADPH are produced in non-cyclic phosphorylation.
The components for this process are located within the thylakoid membranes of the chloroplast.
In Photosystem II (PSII):
Photoionisation of chlorophyll transfers excited electrons to an electron acceptor.
Photolysis of water (which acts as an electron donor) occurs, yielding oxygen molecules, hydrogen ions, and electrons. These electrons are then transferred to the positively-charged chlorophyll molecule to replenish its lost electrons.
The electron acceptor passes the electrons to the electron transport chain.
The electron transport chain carries electrons to Photosystem I (PSI).
In Photosystem I (PSI):
Further absorbed light energy re-energizes the electrons.
This increased energy is sufficient to facilitate the reduction of NADP^+ to NADPH.
Forms of NADP:
Oxidized form: NADP^+
Reduced form: NADPH
Chemiosmosis and ATP Synthesis in Detail
As electrons traverse the electron transport chain within the thylakoid membranes, they release energy.
This energy is used to actively pump H^+ ions (protons) from the stroma (the fluid-filled space outside the thylakoids) into the thylakoid compartment (the space inside the thylakoids).
This pumping action creates a significantly higher concentration of H^+ions inside the thylakoid compartment compared to the stroma, establishing an electrochemical gradient.
H^+ ions then diffuse back out of the thylakoid compartment, moving from a region of high concentration to low concentration, through specific protein channels.
This diffusion of protons down their electrochemical gradient powers an enzyme called ATP synthase, which synthesizes ATP from ADP and inorganic phosphate.
This mechanism of ATP production, driven by a proton gradient, is known as chemiosmosis in the context of photophosphorylation within a chloroplast.
Cyclic Phosphorylation
Purpose: While non-cyclic phosphorylation produces ATP and NADPH, often more ATP is required to drive the light-independent reactions than non-cyclic electron flow can provide.
Mechanism: This extra ATP is generated through cyclic phosphorylation, which exclusively involves Photosystem I (PSI).
Excited electrons generated by PSI are transferred to the electron transport chain situated between PSII and PSI.
Crucially, these electrons are not passed to NADP^+; therefore, no NADPH is formed in this process.
The electrons are then transported back to PSI by the electron transport system, completing a cycle.
This cyclic flow generates additional ATP without producing NADPH or releasing oxygen.
The Light-Independent Reactions (Calvin Cycle)
Also referred to as the Dark reaction, this phase involves a series of light-independent reactions that fix carbon dioxide into organic compounds.
Carbon fixation is the process of incorporating atmospheric carbon dioxide (or dissolved CO_2 for aquatic organisms) into organic molecules.
The energy required for these reactions comes entirely from the ATP and NADPH produced during the light-dependent reactions.
Living systems cannot directly utilize light energy for building macromolecules; instead, they convert it into C-C bond energy within carbohydrates, which can then be released through metabolic processes like glycolysis.
Steps in the Calvin Cycle
Carbon Fixation: Carbon dioxide (CO_2) combines with a 5-carbon sugar called ribulose 1,5-biphosphate (RuBP).
This combination initially forms an unstable 6-carbon sugar.
The unstable 6-carbon sugar immediately breaks down into two molecules of a 3-carbon compound: glycerate 3-phosphate (GP) (also known as phosphoglycerate or PGA).
Reduction Phase: The glycerate 3-phosphate (GP) molecules are then phosphorylated by ATP (using energy from the light reactions) to form glycerate diphosphate molecules.
These glycerate diphosphate molecules are subsequently reduced by NADPH (also from the light reactions) to form two molecules of glyceraldehyde 3-phosphate (GALP) (also known as phosphoglyceraldehyde or PGAL).
Fate of GALP and Regeneration of RuBP
For every pair of GALP molecules produced:
One molecule represents the initial net product of photosynthesis. It is quickly converted into glucose and other essential carbohydrates, lipids, or amino acids for the plant.
One molecule is used in a series of chemical reactions to regenerate RuBP, ensuring the cycle can continue.
To synthesize one glucose molecule ($C6H{12}O_6$), the Calvin cycle must run multiple times, requiring more than two GALP molecules to exit the cycle.
Specifically, out of 12 molecules of glyceraldehyde phosphate (GALP) eventually formed, two molecules are removed from the cycle to construct one glucose molecule.
The remaining GALP molecules are then converted, using energy from ATP, to reform six RuBP molecules, thus restarting the Calvin cycle.
Factors Affecting the Rate of Photosynthesis
The rate of photosynthesis is influenced by several limiting factors, primarily:
Light Intensity
Carbon Dioxide Concentration
Temperature
1. Light Intensity
As light intensity increases, the rate of the light-dependent reactions, and consequently the overall rate of photosynthesis, increases proportionally.
However, beyond a certain point, the rate of photosynthesis will cease to increase with further light intensity, as it becomes limited by another factor (e.g., CO_2 or temperature).
The wavelength of light is also crucial:
PSI absorbs energy most efficiently at 700 ext{ } nm.
PSII absorbs energy most efficiently at 680 ext{ } nm.
Light with a high proportion of energy concentrated in these optimal wavelengths will result in a higher rate of photosynthesis.
2. Carbon Dioxide Concentration
An increase in CO_2 concentration directly increases the rate at which carbon is incorporated into carbohydrates during the light-independent reactions.
Therefore, the overall rate of photosynthesis increases with rising CO_2 concentration until it becomes limited by another factor (e.g., light intensity or temperature).
3. Temperature
Photosynthesis is a process catalyzed by enzymes, which are highly sensitive to temperature.
As the temperature increases and approaches the optimum temperature for the photosynthetic enzymes, the overall rate of photosynthesis increases.
However, if the temperature rises above the optimum, the enzymes begin to denature, causing the rate of reaction to decrease sharply and eventually stop entirely.