Photosynthesis: Light Reaction and Carbon Fixation Flashcards
Define photosynthesis
The process by which light energy is converted to chemical energy (bonds) in sugars. Sugar is constructed by CO2 and H2O, while oxygen is released as a byproduct.
What’s the relationship between light reactions and carbon fixations?
Light reactions - convert light energy to chemical energy as ATP and NADPH. It occurs in the thylakoid
Carbon-fixation reactions - uses the ATP and NADPH and CO2 to produce carbohydrates. Occurs in the stroma
Describe light harvesting reactions
Light energy is captured in light harvesting complexes and transferred to reaction centers. When a pigment molecule absorbs a photon, the excited state is unstable and the energy is quickly released. The energy is then absorbed by other pigment molecules and passed to chlorophyll a in a reaction center. In the reaction center, the light energy is converted to chemical energy: excited chlorophyll a molecule (Chl) gives up an electron to an acceptor. A redox reaction: chlorophyll gets oxidized to Chl; the acceptor molecule gets reduced. The electron acceptor is the 1st in a chain of carriers in the thylakoid membrane, which starts the transport chain of photosynthesis
Chloroplasts
Specialized organelles that carry out photosynthesis. Thylakoid membranes contain green pigments called chlorophylls. Fluid-filled space around the thylakoids is called the stroma. Light dependent reactions make ATP and NADPH
Cyclic electron transport
Uses 1 photosystem: Photosystem I (PSI), while noncyclic uses 2 - Photosystem I and II (PSI) and (PSII). Photon hits pigment in PSII and electron is energized. Energy is passed inward from pigment to pigment until it reaches the reaction center. Reaction center chlorophyll’s electron is boosted to a higher level
Autotrophs and heterotrophs
Autotrophs carry out photosynthesis, like plants and algae, while heterotrophs consume other organisms
Describe the process of photosynthesis
Part 1:
- Step 1: Light dependent: CO2 enters and O2 and H2O leaves through the pores of the leaf called the stomata
- Step 2: Light dependent: light hits the membrane of the thylakoid, splitting the H2O and O2
- Step 3: Light dependent: electrons move to enzymes
- Step 4: Light dependent: Sunlight hits the second pigment molecule allowing the enzymes to convert ADP to ATP and NADP+ gets converted to NADPH
Describe the process of photosynthesis
Part 2:
- Step 5: Light independent: The ATP and NADPH is used by the Calvin cycle as a power source for converting carbon dioxide from the atmosphere into simple sugar glucose.
- Step 6: Light independent: The Calvin cycle converts 3CO2 molecules from the atmosphere to glucose
Calvin cycle steps
Carbon fixation, reduction, and regeneration
- Carbon fixation - 5CO2 + 2 RuBP + 1CO2 = 3 PGA
- Reduction - 3PGA gets phosphate from ATP. These carbon containing molecules are reduced by NADPH and becomes G3P
- Regeneration - some G3P is used to make glucose, others are recycled to
regenerate the 5-carbon RuBP, which is necessary for carbon fixation
Key enzyme in the Calvin cycle
RuBP oxygenase-carboxylase (rubisco), is a key enzyme in the Calvin cycle. Usually uses CO2 as a substrate to fix carbon, but other times it’s O2, leading to lost carbon photorespiration
C3, C4, and CAM difference
The Calvin cycle produces a three-carbon compound from C3 photosynthesis, whereas C4 photosynthesis produces an intermediate 4C compound that splits into a 3C compound for the Calvin cycle. Plants that use CAM photosynthesis collect sunlight during the day and fix CO2 molecules at night, and then malate is stored in vacuoles
Examples of C4
C4: sugarcane, corn, crabgrass
C4 pathway
CO2 -> oxaloacetate -> malate -> CO2 or pyruvate. If pyruvate -> ATP + Pi -> AMP + Pii -> PEP. If CO2 -> the Calvin cycle
Examples of CAM
CAM: jade, cacti, orchids, pineapples
