Physiology Sept. 26th
Quiz Review:
For the Calvin cycle, fixing one carbon dioxide (CO2) to produce a triose phosphate (a 3-carbon molecule) requires three molecules of RuBP (ribulose-1,5-bisphosphate) and three molecules of CO2.
To generate one triose phosphate molecule for sugar production, the cycle consumes 9 ATP and 6 NADH.
NADH vs. FADH2 ATP Yield:
NADH generates more ATP than FADH2 due to differences in electron entry points into the electron transport chain.
NADH electrons enter at Complex I, leading to more proton movement and thus greater proton motive force and ATP generation (multiplier of 1.5).
FADH2 electrons enter at Complex II, resulting in less proton movement and lower ATP yield.
The lower ATP yield from glycolytic NADH (cytosolic) compared to mitochondrial NADH is because cytoplasmic NADH requires a shuttle system to enter the mitochondria. This shuttle system effectively transfers the electrons to FADH2 within the mitochondria, causing them to enter Complex II and yield fewer ATP. This specific detail is not subject to exam questions.
Plant Photoreceptors - General Principles:
Photoreceptors are protein sensors for light.
Most plant photoreceptors achieve light sensing through a chemical prosthetic group called a chromophore. The chromophore is a light-absorbing molecule that undergoes a structural change upon light absorbance, which in turn induces a conformational change in the associated protein receptor.
Exception: The UVR8 UV light receptor is unique in that the protein itself, without a chromophore, directly senses UV light.
Phytochrome (Red and Far-Red Light Sensor):
Forms: Exists in two interconvertible forms:
PR (Phytochrome Red): Absorbs red light. This form is induced by far-red light.
PFR (Phytochrome Far-Red): Absorbs far-red light. This form is induced by red light and is considered the biologically active form.
Photoreversibility: The conversion between PR and PFR is largely reversible by exposure to the respective wavelengths of light.
Action Spectra Overlap: Conversion is not 100\% complete; action spectra for PR and PFR overlap. Even under saturating far-red light or in prolonged darkness, approximately 2\% of the total phytochrome remains in the active PFR state, meaning it's impossible to completely convert all phytochrome to the inactive PR form.
Thermal Conversion: PFR can thermally convert back to PR in the dark, a process sensitive to temperature.
Chromophore: Phytoprobabilin (not required for memorization).
Mechanism of Action:
Light absorption by the chromophore induces its structural rearrangement.
This triggers a conformational change in the phytochrome protein.
The inactive PR form is typically sequestered in the cytosol.
Upon red light activation to PFR, the conformational change allows the PFR form to translocate into the nucleus.
Within the nucleus, PFR primarily drives changes in gene expression. Some cytosolic PFR also regulates other signaling events, such as ion transport across membranes.
Ecological Role - Sensing Red to Far-Red Ratio:
Plants use the ratio of red to far-red light (R:FR) to gain critical information about their environment, establishing an equilibrium between the PR and PFR states.
Shade Avoidance Syndrome: When light passes through a leaf canopy, more red light is absorbed by chlorophyll, while far-red light is significantly reflected. Plants growing under a canopy perceive a lower R:FR ratio (higher proportion of far-red light).
This lower R:FR ratio signals shade, prompting a developmental response where vertical growth (stem elongation) is prioritized. The goal is to grow taller and emerge from the canopy into more direct, full sunlight for optimal photosynthesis.
In horticultural settings, manipulating the R:FR ratio using LEDs can control plant morphology; a higher far-red ratio promotes elongated growth, while a lower ratio favors vegetative (leaf) expansion.
Types of Phytochrome Responses:
Rapid Responses: Biochemical events occurring within seconds to minutes.
Slower Responses: Morphological and developmental processes (e.g., shade avoidance, changes in overall plant anatomy), which can take weeks or months.
Photoreversibility and Reciprocity:
Photoreversibility: A response induced by red light can often be reversed by a subsequent pulse of far-red light.
Law of Reciprocity: For certain responses, the magnitude of the response depends on the total light fluence (number of photons multiplied by time), not the specific duration or intensity (e.g., X fluence for 1 second yields the same response as 0.5X fluence for 2 seconds).
Very Low Fluence Responses (VLF): Occur at extremely low light levels.
Reciprocity applies.
NOT photoreversible (due to the persistent 2\% PFR pool).
Low Fluence Responses (LF): Occur at dim, intermediate light levels.
Reciprocity applies.
IS photoreversible.
High Irradiance Responses (HIR): Occur under continuous high light conditions.
Response is proportional to irradiance, not just total photons.
No reciprocity.
NOT photoreversible.
Example: Photoreversible Seed Germination:
Many seeds require light to germinate; red light stimulates germination.
A pulse of red light induces germination. A subsequent pulse of far-red light can revert this effect, preventing germination.
This effect can be reversed multiple times (e.g., Red
ightarrow Germinate; Red + Far-red
ightarrow No Germination; Red + Far-red + Red
ightarrow Germinate).
Blue Light Responses:
Unlike phytochrome, blue light receptors sense a specific wavelength and do not reverse signals between two different wavelengths.
Responses to blue light can have a delayed onset and continue even after the light is removed.
Cryptochromes:
Chromophore: Flavin Adenine Dinucleotide (FAD).
Mechanism: FAD undergoes a redox change (FAD
ightarrow FADH_2) upon blue light absorption, triggering a crucial conformational change in the cryptochrome receptor protein.Functions:
Mediate membrane depolarization.
Play a key role in coordinating the circadian rhythm, linking light perception to metabolic and developmental processes.
Control de-etiolation: In light, cryptochromes stop hypocotyl elongation, signaling that the seedling has emerged from the soil. In darkness (etiolation), elongation continues to bypass obstacles.
Regulate the production of anthocyanins (purple pigments).
Contribute to phototropic responses (differential growth/orientation towards light).
Influence dynamic chloroplast movement within cells to optimize light capture and minimize photodamage.
Phototropins:
Chromophore: Flavin mononucleotide (FMN), located in characteristic LOVE (Lights-Oxygen-Voltage-sensing) domains.
Mechanism: Unlike other photoreceptors, phototropins are catalytically active enzymes, specifically protein kinases. Light absorption induces a conformational change that activates their kinase activity.
They autophosphorylate (phosphorylate themselves) and phosphorylate other protein substrates by transferring a phosphate group from ATP.
Functions:
Control various phototropic movements, including dynamic chloroplast movement within cells.
Regulate stomatal movement: Blue light is a very strong signal for stomatal opening.
Stomata are pores in leaves that regulate gas (CO_2) exchange and water transpiration.
Phototropin kinase activity leads to phosphorylation of substrates that regulate ion channel movement in the guard cells.
Ion flux alters the turgor pressure of the guard cells: high turgor causes stomata to open, while low turgor leads to closure.
UVR8 (UV-A Receptor):
Unique Feature: Unlike other plant photoreceptors, UVR8 does not require a chromophore. The UVR8 protein directly senses UV light.
Mechanism: UV light, being higher energy, can be directly absorbed by specific amino acid residues, particularly tryptophan, within the protein itself.
In the dark or absence of UV light, UVR8 exists as an inactive dimer, held together by an extensive interaction interface involving salt bridges and tryptophan residues.
Upon illumination with UV light, the tryptophan residues absorb the UV energy, which disrupts the interaction interface.
This causes the dimer to dissociate into two active monomers.
These active monomers then translocate from the cytosol into the nucleus to regulate gene expression.
Evolutionary Significance: UVR8 has deep evolutionary roots (found in some algae), but its regulatory network significantly expanded with the emergence of land plants. Terrestrial growth exposes plants to much higher levels of UV light, necessitating a robust UV sensing and response system.
Functions:
Crucial for UV tolerance, inducing the synthesis of protective compounds like flavonoids and phenolics, which act as antioxidants to scavenge reactive oxygen species (ROS) produced by UV damage.
Regulates the expression of numerous genes, including its own gene (UVR8), and coordinates various developmental processes in response to UV light.
Midterm Information:
Today's lecture material will be covered on the midterm exam next week.
Time for midterm-related questions will be provided at the end of the class. This is the midterm for quizzes two, three, and four.