Comprehensive Notes on Photosynthesis

Photosynthesis

Overview

Photosynthesis is the process that feeds us all.
Understanding photosynthesis can revolutionize planting methods.
Plants communicate their needs, allowing for optimization of plant qualities by adjusting environmental factors.
Nature provides sunlight, water, organic nutrients, and temperature, which can be optimized in controlled environments.
Photosynthesis is nature's way of providing for us through plants.

Photosynthesis Equation

Photosynthesis Equation: $6CO2 + 6H2O \xrightarrow{\text{solar energy}} C6H{12}O6 + 6O2$
- Carbon dioxide + water yields glucose + oxygen, driven by solar energy.
Cellular Respiration: Glucose + Oxygen yields ATP + Heat. Also, it is powered by chemical energy.

Photosynthesis Process

An overview of photosynthesis includes:
- Light-dependent reaction
- Light-independent reaction
- ATP, NADPH, G3P, and Glucose production.
The key questions are how and where these processes occur.

The Chloroplast

Chloroplast structure includes:
- Outer membrane
- Inner membrane
- Thylakoid: contains chlorophyll
- Thylakoid space (lumen)
- Granum: stack of thylakoids
- Stroma: fluid-filled space around thylakoids

Light-Dependent Reaction

Light is essential for the photolysis of water, providing H+.
This reaction occurs in the presence of pigments that absorb specific wavelengths.
Oxygen ( $O2$ ) is produced and used for the reduction of $CO2$ to glucose in the Calvin cycle.
ATP and NADPH are generated.

Ideal Look of Light-Dependent Reaction

Diagrammatic representation of the light-dependent reaction shows:
- Photosystems II (PSII) and I (PSI)
- Electron flow from water to NADPH
- ATP synthase producing ATP
- H+ gradient across the thylakoid membrane
Key components:
- Water is split to form 1/2 $O_2$ + 2 $H^+$ releasing electrons.
- Protons ( $H^+$ ) accumulate in the thylakoid lumen, creating a high concentration.
- Electrons pass through PSII (P680) and PSI (P700).
- ATP synthase uses the proton gradient to produce ATP from ADP.
- NADP+ is reduced to NADPH using electrons from PSI and protons.
Photoactivation excites electrons to a higher energy level, and they are absorbed by electron acceptors, leading to a positively charged reaction center.
Electrons from PSII replenish PSI, while electrons from water photolysis replenish PSII.
High concentration of $H^+$ in the thylakoid space drives ATP synthesis via chemiosmosis.
Photons strike both photosystems, causing photoactivation.
Electrons from PSI move to primary electron acceptor (FeS) and then to NADP+.
Reduced NADP+ combines with $H^+$ to form NADPH.
Electrons from PSII move along the electron transport chain (Pq → Cyt b → Pc) to PSI.

Major Events in Light-Dependent Reaction

Key events include:
1. Production of NADPH.
2. ATP synthesis through photophosphorylation.
3. Absorption of light and photoactivation.
4. Oxygen generation from water photolysis.

Absorption of Light Energy and Photoactivation

Photoactivation occurs when photosystems absorb photons, exciting electrons to a higher energy level.
Two types of photosystems:
- Photosystem II (PSII): absorbs best at 680 nm; reaction center is called P680.
- Photosystem I (PSI): absorbs best at 700 nm; reaction center is called P700.
Energy is passed from one pigment molecule to another until it reaches the reaction center (P700 or P680).
Electrons in P700 or P680 become excited and move to an electron acceptor in the electron transport chain.
Water photolysis occurs at PSII. (Refer to page 218 for electron transport chain naming)

ATP Synthesis through Photophosphorylation

Chemiosmosis: movement of ions (protons) across a membrane down their electrochemical gradient to drive ATP synthesis.
(Recap: Phosphorylation is adding $PO_4$ to ADP to form ATP; "photo" refers to light)
ATP is produced through chemiosmosis, where ions move across a semipermeable membrane down their electrochemical gradient.
High concentration of $H^+$ ions in the thylakoid space is created by:
- Photolysis of water.
- Electron flow through the electron carrier chain.
Stroma has a low concentration of $H^+$ ions.
Movement of $H^+$ across the thylakoid membrane via ATP synthase creates a proton concentration gradient.
$H^+$ movement through ATP synthase induces phosphorylation of ADP and $P_i$ in the stroma to form ATP.

Types of Photophosphorylation

Non-cyclic photophosphorylation:
- Involves both PSII and PSI.
- Produces ATP and NADPH.
Cyclic photophosphorylation:
- Involves PSI only.
- Produces ATP only.

Non-Cyclic Photophosphorylation

A photon hits a pigment molecule of PSII, exciting P680.
An excited electron from P680 is transferred to the primary electron acceptor.
$H_2O$ is split, and electrons are transferred to P680+; oxygen is released as a byproduct.
Electrons fall down an electron transport chain (ETC) from the primary electron acceptor of PSII to PSI.
Energy released by the ETC drives the creation of a proton gradient across the thylakoid membrane.
In PSI, transferred light energy excites electrons from P700 and captures them with a primary acceptor; P700+ is stabilized by an electron from PSII.
Electrons fall down an ETC from the primary electron acceptor of PSI to ferredoxin (Fd).
Electrons are transferred to NADP+, reducing it to NADPH.

Cyclic Photophosphorylation

Electron in PSI is excited and transferred to ferredoxin (Fd).
Electron travels down the ETC and re-enters PSI.
Energy released by the ETC drives the creation of a proton gradient, which drives ATP synthesis.

Production of NADPH

In non-cyclic photophosphorylation, electrons from PSII are accepted by PSI.
PSI can be photoactivated again after its electrons are replaced.
Excited electrons from PSI are released to the primary electron acceptor, FeS, and then transported through a short electron carrier chain to NADP+ in the stroma.
Reduced NADP+ combines with $H^+$ from water photolysis to form NADPH.
NADPH is a reducing agent used in carbon dioxide fixation during the light-independent reaction.
Equation: $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$

Production of Oxygen from Photolysis of Water

PSII becomes positively charged after releasing excited electrons.
In the presence of PSII, $H_2O$ molecules in the thylakoid space are activated by light energy and dissociate to form $OH^-$ , $H^+$ , and activated electrons.
Activated electrons are accepted by PSII+ to form back PSII.
$OH^-$ combine to liberate oxygen gas.

Differences Between Cyclic and Non-Cyclic Photophosphorylation

Feature	Cyclic Photophosphorylation	Non-Cyclic Photophosphorylation
Photosystems Involved	PSI only	PSI and PSII
Reaction Center	P700	P680
Electron Cycling	Electrons are cycled back	Electrons are not cycled back
Photolysis of Water	Does not take place	Takes place
Products Synthesized	Only ATP synthesized	ATP and NADPH synthesized

Let us refresh section

The questions are self explanatory and are not included here.

7 Steps of Photosynthesis

$CO2$ and $H2O$ enter the leaf.
Light hits the pigment in the thylakoid membrane, splitting $H2O$ into $O2$ .
Electrons move down to enzymes.
Light hits the second pigment molecule, allowing enzymes to convert ADP to ATP, and NADP+ is converted to NADPH.
ATP and NADPH are used by the Calvin cycle as a power source for converting $CO_2$ from the atmosphere into glucose.
The Calvin cycle converts 3 $CO_2$ molecules from the atmosphere to glucose.
The second of two major stages in photosynthesis involves atmospheric $CO_2$ fixation and reduction of the fixed carbon into carbohydrates.

Light vs Dark Reaction

Photosynthesis involves light-dependent and light-independent reactions.
Light-dependent reactions:
- Convert light energy to chemical energy (ATP + NADPH).
- Produce oxygen gas as a waste product.
Light-independent reactions (Calvin Cycle):
- Make sugar using carbon dioxide and the energy-containing products of the light-dependent reactions (ATP + NADPH).

The Nature of Sunlight

Light is a form of electromagnetic energy or electromagnetic radiation.
Light travels in wavelengths.
Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see.

Photosynthetic Pigments

Pigments are substances that absorb visible light.
Different pigments absorb different wavelengths.
Wavelengths that are not absorbed are reflected or transmitted.
Leaves appear green because chlorophyll reflects and transmits green light.

Spectrophotometer

A spectrophotometer measures a pigment’s ability to absorb various wavelengths.
The machine sends light through pigments and measures the fraction of light transmitted at each wavelength.
Chlorophyll absorbs very little green light (high transmittance).
Chlorophyll absorbs most blue light (low transmittance).

Absorption and Action Spectra

An absorption spectrum is a graph of the absorbance of different wavelengths of light by a pigment.
The absorption spectrum of chlorophyll a suggests that violet-blue and red light are most absorbed by plants for photosynthesis.
An action spectrum is a graph of the rate of photosynthesis at different wavelengths of light involved in photosynthesis.

Types of Chlorophyll

Chlorophyll a is the main photosynthetic pigment.
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis.
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll.