Photosynthesis Vocabulary

Photosynthesis

Energy within light is captured and used to synthesize sugars (organic molecules that store chemical energy) and produce oxygen.
Occurs in plants, algae, certain protists, and some prokaryotes.

Key Concepts

Overview of Photosynthesis
Reactions That Harness Light Energy
Molecular Features of Photosystems
Synthesizing Carbohydrates via the Calvin Cycle
Variations in Photosynthesis

Photosynthesis Details

Production of glucose (anabolic) is endergonic.
Energy from light drives this endergonic reaction.
$CO2$ is reduced (gains electrons) à $C6H{12}O6$
$H2O$ is oxidized (losses electrons) à $O2$

Autotrophs vs Heterotrophs

Heterotrophs: must consume organic molecules from the environment.
Autotrophs: produce organic molecules from inorganic molecules ( $CO2$ and $H2O$ ).
Photoautotrophs: use the energy of sunlight to make organic molecules.
Photosynthesis powers the biosphere.
Organisms not only feed themselves but also make most of the organic molecules in the living world.
Energy cycle: cells use organic molecules for energy, and plants replenish those molecules using photosynthesis.

Chloroplast

Chloroplast: organelle in plants and algae that carries out photosynthesis.
Green pigment is chlorophyll.
The majority of photosynthesis occurs internally in leaves, in the mesophyll.
$CO2$ enters, and $O2$ exits the leaf through pores called stomata.

Chloroplast Anatomy

Outer and inner membrane separated by intermembrane space.
A third membrane, the thylakoid membrane, contains pigment molecules.
Membrane forms thylakoids.
Enclose thylakoid lumen.
Granum: stack of thylakoids.
Fluid-filled region between thylakoid membrane and inner membrane is the stroma.

Two Stages of Photosynthesis

Light reactions
- Thylakoid membrane
- Use light energy; converts light energy to chemical energy.
- Produce ATP, NADPH, and $O_2$
Calvin cycle
- Occurs in stroma
- Uses ATP and NADPH to incorporate $CO_2$ into carbohydrate sugars

Light Energy

Light is a form of electromagnetic energy (electromagnetic radiation).
Light travels in rhythmic waves.
- Short to long wavelength.
- Shorter wavelengths have more energy.
Light is composed of photons (massless particles traveling in a wavelike pattern).

Electromagnetic Spectrum

Photosynthesis is powered by visible light.
Visible light = 380 to 740 nm.
Shorter wavelength radiation carries more energy than longer wavelength.
The effect of light is dependent on the energy of photons that reach living organisms.
- Gamma, X-rays, UV (high energy) à can cause mutations in DNA.
- Visible light = lower energy.

Photosynthetic Pigments

Light can be: reflected, transmitted, or absorbed.
Pigments are a substance that can absorb visible light.
When light strikes a leaf, some wavelengths are absorbed, and others are reflected.

Light Absorption

Light energy can be absorbed by an atom when it boosts an electron to a higher energy level (orbital).
Absorption boosts electrons to higher energy levels.
After an electron absorbs energy, it is an excited state and usually unstable.

Electron Stability

To become stable:
- e- can drop to a lower energy stateà release heat.
- e- can release energy in the form of light.
- e- can transfer extra energy to molecule (resonance energy transfer).
- e- can be transferred to another molecule where it is captured (occurs in photosynthetic pigments).
Captured light energy can be transferred to other molecules to ultimately produce energy intermediates for cellular work.

Photosynthetic Pigments as Light Receptors

Chlorophyll a is the main photosynthetic pigment.
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis.
Carotenoids are another type of pigment and produce the yellows, oranges, and reds of autumn foliage.

Absorption vs Action Spectrum

Absorption spectrum:
- Wavelengths that are absorbed by different pigments.
- Chlorophylls a and b absorb red and violet and reflect green light.
Action spectrum:
- Rate of photosynthesis by whole plant at specific wavelengths.

Photosystem

The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center.
A reaction-center complex surrounded by light-harvesting complexes.
- The reaction-center complex is an association (complex) of proteins.
In the reaction center, a special pair of chlorophyll a molecules transfers an excited electron to the primary electron acceptor.
- Redox reaction is one of the first steps of the light reactions.

Primary Electron Acceptor

A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a.
The only pigment oxidized during light capture is the reaction center chlorophyll.
Energy (not electrons) is transferred in the light-harvesting complexes.

Photosystems I & II

There are two types of photosystems embedded in the thylakoid membrane.
Photosystem II (PS II)
- Reaction-center chlorophyll a is called P680
- P680 is a special pigment molecule
- Best at absorbing a wavelength of 680 nm
- Absorb light energyà excites e- Movement of electrons
 - Excited e- are relatively unstable
 - Transfers e- to primary electron acceptor
 - e- are removed from water to replenish e- removed from P680
 - Oxidation of $H2O$ yields $O2$
 - Adds $H^+$ to the lumen.
Photosystem I (PS I)
- Best at absorbing a wavelength of 700 nm
- Reaction-center chlorophyll a is called P700
- Ferrodoxin (Fd, protein)à
  - accepts 2 high energy e-
  - then transfer the e- to the enzyme NADP+ reductase
  - Reduces NADP+ to NADPH
  - Production of NADPH

Electron Transport Chain

Electrons exit PSII and enter ETC.
Electrons are transferred to a series of electron carriers (Pq, cytochrome complex, Pc).
Electrons release energy
- Some of their energy is used to harness and pump $H^+$ into thylakoid lumen.
- The remaining energy is transferred to the next component in ETC.
ETC in chloroplast function similarly to ETC in mitochondria.

Electron Flow

Electrons flow through a transport chain and are finally accepted by NADP+à NADPH
- Photosynthesis involves increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH
  - Zigzag shape of energy curve
  - Electron on a nonexcited pigment molecule in PSII starts with the lowest energy
  - Light excites the electron in PSII
  - Photosystem I boosts the electron to an even higher energy level

ATP Formation

ATP synthesis in chloroplasts
- Achieved by chemiosmotic mechanism called photophosphorylation
- Driven by flow of $H^+$ from thylakoid lumen into stroma via ATP synthase
- $H^+$ gradient generated three ways:
  1. Splitting of water places $H^+$ in thylakoid lumen
  2. Movement of e- from PSII to PSI pumps $H^+$ thylakoid lumen
  3. Formation of NADPH consumes $H^+$ in the stroma

Three Chemical Products of Light Reactions

Oxygen gas, $O_2$
- Produced in thylakoid lumen by oxidation of $H_2O$ by PSII
  - Two electrons transferred to P680+ molecules
NADPH
- Produced in the stroma from high-energy electrons that start in PSII and are boosted in PSI
  - NADP+ + 2 electrons + $H^+$ à NADPH
ATP
- Produced in stroma by ATP synthase using the $H^+$ electrochemical gradient

Linear Electron Flow

Electrons begin at PSII and eventually transfer to NADP+ àNADPH
Linear process produces ATP and NADPH in equal amounts
Linear electron flow is the combined action of photosystem II and I as electrons move linearly from PSII to PSI
Reaction-center chlorophyll a (P680) transfers e- to primary electron acceptor
Removes e- from water to replace oxidized P680
Oxidation of water yields oxygen gas
Electron moves down an ETC to more electronegative atoms (similar to the one in mitochondria)
- The energy released is harnessed to pump $H^+$ into the thylakoid space
- Pq = plastoquinone
- Pc = plastocyanin
Electron flows to PSI where light excites a high- energy electron in P700Primary.
Reaction-center chlorophyll a (P700) transfers e - to primary electron acceptor
Electrons are passed down an electron transport chain from the primary electron acceptor of PSI to the protein ferredoxin (Fd)
- Fd = ferredoxin

Noncyclic and Cyclic Electron Flow

Noncyclic electron flow
- Electrons begin at PSII and eventually transfer to NADPH, a linear process (linear electron flow)
- Produces both ATP and NADPH in equal amounts
Cyclic photophosphorylation (cyclic electron flow)
- Path of e- is cyclic
- Produces only ATP (Calvin cycle uses more ATP than NADPH)
- Favored when the level of NADP+ is low, and NADPH is high

Cyclic Electron Flow

Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH or oxygen.
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle.
Key difference: when light strikes PSI, e- are sent to Fd then to Qb, producing no NADPH

Evolution

Descent with modification – related genes play similar but specialized roles in cells.
Homologous genes are similar because they are derived from a common ancestral gene.
The electron transport chains of mitochondria and chloroplasts use homologous genes.
Family of cytochrome b proteins plays similar but specialized roles in both mitochondria and chloroplasts.

Chemiosmosis

Both use energy generated by ETC to create a proton ( $H^+$ ) gradient across a membrane.
Chloroplast utilizes two other mechanisms to create $H^+$ gradient.
Both rely on the diffusion of protons through ATP synthase to drive the synthesis of ATP.
Different energy sources:
- Mitochondria transfer chemical energy from food to ATP.
- Chloroplasts transform light energy into the chemical energy of ATP.

The Light Reactions

Generate ATP
Increase the potential energy of electrons by moving them from $H_2O$ to NADPH
Release $O_2$ as a by-product
The Calvin cycle uses ATP and NADPH to power the synthesis of sugar from $CO_2$

Calvin Cycle

The Calvin cycle uses ATP and NADPH to fix $CO_2$ into sugar.
ATP provides the free energy, and NADPH provides the reducing power.
Carbon enters the cycle as $CO_2$ and leaves as a sugar named glyceraldehyde-3-phospate (G3P).
For the net synthesis of one G3P, the cycle must take place three times, fixing three molecules of $CO_2$ .
Requires massive input of energy: for every 6 $CO_2$ incorporated, it consumes 18 ATP and 12 NADPH (glucose is not directly made).
The Calvin cycle is anabolic and requires an input of energy
3 phases:
1. Carbon fixation
2. Reduction and carb production
3. Regeneration of RuBP
The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.

Three Phases of the Calvin Cycle

$CO_2$ is incorporated into RuBP (5C sugar)
- Rxn is catalyzed by enzyme = rubisco
Forms a 6C molecule by combining $CO_2$ and RuBP (short-lived intermediate)
6C intermediate splits to form 3PG
- Six-carbon immediately splits into 3PG

Phase 1: Carbon fixation
- Carbon removed from the atmosphere and incorporated into an organic molecule

ATP converts 3PG to 1,3-BPG
1,3-BPG is then reduced to G3P
12 ATP and 12 NADPH are required to produce 12 molecules of G3P
Only two G3P exit the cycle for use by the cell
The 10 remaining G3P are used to regenerate RuBP
6 ATP are required to power this step

For every 6 CO2 incorporated:

18 ATP and 12 NADPH are oxidized

Calvin Cycle Summary

Phase 1 – Carbon fixation
- $CO_2$ incorporated into RuBP using rubisco
- Reaction product is a 6C intermediate that splits into 3-phosphoglycerate molecules (3PG)
Phase 2 – Reduction and carb production
- ATP is used to convert 3PG into 1,3- BPG
- NADPH electrons reduce it to (G3P)
- 6 $CO_2$ → 12 G3P
- Only 2 G3P molecules are used for carbohydrates
- 10 G3P molecules must be used for the regeneration of RuBP
Phase 3 – Regeneration of RuBP
- 10 G3P are converted into 6 RuBP using 6 ATP

Synthesizing Carbohydrates via the Calvin Cycle

$CO_2$ incorporated into carbohydrates
Precursors to other organic molecules
Energy storage
The product is glyceraldehyde-3-phosphate (G3P).
Glucose is later made from G3P in a separate process.
Requires massive input of energy
- For every 6 $CO_2$ incorporated, 18 ATP and 12 NADPH must be used

Alternative Mechanisms of Carbon Fixation in Hot, Arid Climates

On hot, dry days, plants close stomata, which reduces evaporative water loss, but also prevents gas exchange
When stomata are closed:
- $CO_2$ levels are reduced
- $O_2$ accumulates
- These conditions favor a wasteful process called photorespiration

Photorespiration

In C3 plants (most plants), fixation of $CO_2$ , via rubisco, forms a three-carbon compound (3PG)
In photorespiration, rubisco fixes $O2$ instead of $CO2$ and produces a two-carbon compound
Photorespiration:
- occurs in the light (photo)
- consumes $O2$ , while releasing $CO2$ (respiration)
- uses ATP; does not produce it!
- produces NO sugars