Module 2 - Plant Physiology (Photosynthesis) Flashcards
(38 cards)
Why should we care about photosynthesis?
Arguably one of the most important processes on Earth - provides food, habitats, fossil fuel & wood fuel energy, regulates the atmosphere (increased O2 and decreased CO2)
What is photosynthesis?
The conversion of light energy into chemical energy - the capture of light energy, and combination of CO2 and H2O to create sugars and O2 (by-product of H2O being split).
What is the basic chemical formula of photosynthesis?
- Carbon dioxide + Water + light energy –> carbohydrates + 2 oxygen OR
CO2 + 2H2O (electron donor) + light –> [CH2O] (carbs) + 2O2 (oxidised electron donor) + H2O (water)
What do the light dependent reactions capture and create?
- Capture the energy of light to use it to make NADPH (H+ carrier)
- Make ATP (energy storing molecule)
What does the light independent (dark) reaction convert and create? And where does it occur?
- Converts NADPH and ATP to NADP+ and ADP
- In the stroma of the chloroplast
Describe the major steps in the “Z” scheme (light reaction) and its main purpose
- Chlorophyll and other pigment absorb photons
- Electrons are created by splitting water into H+ and O2 (photolysis)
- Electrons are moved through PSII to create ATP after a series of electron donations
- PSI absorbs photons and takes electrons further through the electron transport chain and electron donations to create NADPH
- Main purpose of light reactions is to create ATP and NADPH that are used in dark reactions
What does the light independent reaction start off with? What are the basics of this reaction? What are the products used for?
- Starts off with the NADPH created in the light reactions
- Enzyme RuBisCo transforms CO2 from seawater/atmosphere to create sugars
- Sugars created will eventually be used for metabolic fuel in cellular respiration or creation of new tissues
During the first step of the light-independent reaction, what does RuBisCO do?
The enzyme RuBisCO welds one carbon atom from a CO2 molecule with the RuBP chain (five carbons) to build an initial six-carbon sequence.
Why is RuBisCO not efficient at its job? And why is photorespiration costly? How have certain plants overcome this problem?
It evolved during a time with low atmospheric O2 concentrations, therefore carboxylase activity competes with oxygenase activity - high oxygen concentration leads to the switch from photosynthesis to photorespiration. C4 and CAM plants have overcome this by having CCMs, and either separating carbon fixation spatially or temporally.
What do CO2-concentrating mechanisms do?
Concentrate CO2 at the site of RuBisCO
What are abundant and limiting in the terrestrial and marine/freshwater environment?
Terrestrial: CO2 abundant (diffuses rapidly), water can be limiting
Marine/Freshwater: Water abundant, CO2 limiting (diffuses slowly)
What is the C3 plants’ process of photosynthesis?
RuBisCO welds CO2 to RuBP to create 2, three carbon phosphoglycerates (PGAs)
- Non-CCM photosynthesis
Why is the C3 process inefficient in arid environments?
- Loss water through stomata, although closing them to stop water loss prevents CO2 uptake, and photorespiration occurs (adding oxygen instead of CO2 to RuBP)
What is the process of C4 plants? And why is it more efficient in arid environments?
- Creates a 4-carbon molecule in mesophyll
- Concentrates CO2 at RuBisCO in the mesophyll. Carbon fixation occurs in the spatially isolated bundle sheath cells
- Costs more ATP than C3, but conserves more water and limits photorespiration
What is the process of CAM plants? And how are they particularly adapted for extremely arid environments?
- Seperates process temporally: stomata open at night, CO2 reacts with PEP to form malate
- Malate is stored in vacuoles until daytime when stomata close, where it is then decarboxylated and CO2 is released into Calvin cycle.
What environments do C3 plants thrive in? And why might this be?
- Where sunlight is moderate, temperatures are moderate, and water is plentiful
- They evolved when atmospheric CO2 levels were high, and therefore fall into photorespiration and a loss of carbon and nitrogen in warm environments
What do C4 plants use less of compared to C3 plants? And what advantage does this give them?
- Nitrogen, as C4 plants largely use PEPC instead, which needs much less nitrogen compared to RuBisCO
- Therefore C4 have an advantage in places where nitrogen is limited
What are the two main ways that algae uptake carbon dioxide? And why is one way not considered to be a true CCM? When is this strategy inefficient?
1) Via energized direct uptake (true CCM) through antiports and/or symports –> Requires less energy than CO2 diffusion
2) Catalyzed external conversion of HCO3- into CO2 to enhance diffusion (not a true CCM) - the enzyme carbonic anhydrase externally catalyzes the conversion of HCO3- to CO2
- Does not concentrate/elevate CO2 levels at site of Rubisco, although costs less energy than energized uptake.
- Not a good strategy when pH is high/CO2 levels are low
How might having a CCM or a suite of different CCM components be advantageous in algal species?
- CCMs overcome the low CO2 concentration and slow diffusivity in water
- Having a CCM can be more or less efficient in certain environments, and could be advantageous over neighboring species
What conditions are terrestrial plant CCMs favored?
- Low water
- High temperature
- Low nitrogen
What do algal CCMs require and what conditions are they favored in?
- CCM use requires high light and high nutrients
- CCM use favored at higher temps and lower CO2 concentrations
Non-CCM algal species tend to increase as:
- Light decreases
- Nutrients decrease
- CO2 increases
- Temperature decreases
What is one of the most limiting nutrients for terrestrial plants? And how do they take this up?
Nitrogen!
- Obtained from soil via plant roots (Nitrite > Nitrate > Ammonium), and can be increased through symbiosis with nitrogen-fixing microorganisms within nodules
What other important nutrients are required by plants? And how do they take these up?
- Include potassium, magnesium, calcium, copper, iron, manganese, cobalt, sodium, zinc
- Most of the sulfur in plants is from sulfate absorbed via a H+ -SO42- symporter from soil
- HPO42- in the soil is absorbed by roots via a H+ -HPO42- symporter (usually for ATP creation)
