Chapter 10 - Photosynthesis Flashcards
Types of organims who use energy
Autotrophs: use energy from the sun to make organic molecules
- All plants are photoautotrophs as light is their main source of energy
Heterotrophs: obtain organic food molecules by eating other organisms or substances derived from them
- Most are known as decomposers
Stomata
micro pores which are surrounded by guard cells in the surface of leaves allowing gaseous exchange
Chlorophyll
green pigment located within the thylakoid disks of chloroplasts which absorb light energy and convert to chemical energy
Photosynthesis
the process transforming sunlight energy into chemical energy in the form of 3 carbon molecules which are transformed into sugar
3 stages:
- light dependent
- calvin cycle
Stages of Photosynthesis: light dependent
first stage which occurs in the thylakoid membrane and converts solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process
- Water is split, providing electrons and protons and giving of O2
- Light absorbed drives a transfer of electrons and hydrogen ions to an acceptor converting NADP+ to NADPH
- generates ATP through photophosphorylation
Stage of Photosynthesis: Calvin Cycle
the second stage involving fixation of atmospherically carbon dioxide and reduction of the fixed carbon into carbohydrates occurring in the stroma
- 3 cycles bring in 3 Co2 molecules to produce 1x G3P molecule(6 cycles in total)
- CO2 from air is incorporated through carbon fixation
- Co2 reduced to 3-PGA by addition of electrons, oxidizing NADPH back to NADP+
- ATP converts CO2 to G3P
Photophosphorylation
the process of generating ATP by means of chemiosmosis(PMF) across the thylakoid membrane
Photons
not tangible objects but waves of light which behavior like objects in that they have fixed quantities of energy
- Amount of energy is inversely related to wavelength; the shorter it is, the greater the energy
Pigment
substances that absorb visible light
- If a pigment is illuminated with green light it is because it’s the colour most reflected or transmitted by the pigment
Electrons being Excited
- When molecules absorb a photon of light, the molecules electrons is elevated to an orbital where it has more potential energy
- The only photon absorbed is those with energy exactly equal to the energy difference between ground state and excited state
- Once excited, an electron cannot remain there for long as it is unstable and drop back down to ground state, releasing excess energy as heat
Photosystem
a light capturing unit in the thylakoid membrane which consists of a reaction centred complex surrounded by several light harvesting complexes
- Different types which absorb light at different wavelengths
- When a photon is absorbed, energy is transferred from pigment molecule to pigment molecule within the complex until it reaches the reaction center complex - the light energy is used to boost one of their electrons to a higher level and is transferred to an electron acceptor; reducing it
Photosystem I: 700nm
Photosystem II: 680nm
Components of Photosystem
Reaction-center complex: an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
Light Harvesting Complexes: consist of various pigment molecules bound to proteins enabling the photosystem to harvest light over a larger surface area
Types of Electron Flow
Linear Electron Flow: the route of electrons during the light reaction involving light stimulating both photosystems in order to produce ATP, NADPH and O2
Cyclic electron flow: an alternative route of electron flow which only uses PS I and produces ATP but not O2 or NADPH
- Generates a surplus of ATP required to satisfy the higher demands of the calvin cycle
Steps in Linear Flow
- Photon of light strikes one of the pigment molecule in PSII, boosting an electron to a higher energy level stimulating a chain reaction until it reaches RCC where its transferred to Primary Electron acceptor
- An enzyme catalyzes the splitting of water and electrons replace ones lost in PSII, H move into thylakoid space and O combines to form O2 and is released
- photoexcited electron passes from PSII to PS I through ETC which carries out redox reactions that release free energy used to pump protons into the thylakoid space creating PMF
- Light energy has been transferred to PS I RCC resulting in Photoexcited electrons passed down a series of redox reactions from primary electron acceptor of PS I to a secondary ETC
- this reduces NADP+ into NADPH in the stroma
- build up of H+ in thylakoid space then creates PMF to go down ATP synthase
Calvin Cycle
- Takes place in the stroma
- Is anabolic, building carbs from smaller molecules and consuming energy
- The cycle spends 9 ATP and consumes 6 NADPH for a net synthesis of 1x G3P molecule(3 cycles/1 turn)
3 phases:
- carbon fixation
- reduction
- Regeneration
Phases of Calvin Cycle: Carbon Fixation
- CO2 from the atmosphere is incorporated one molecule at a time, attaching it to RuBP(5 carbon sugar) catalyzed by rubisco
- This produces a 6 carbon intermediate that is energetically unstable, resulting in it splitting in half to form 6x 3-PGA
Phases of Carbon Cycle: Reduction
- The 6 x3-PGA molecule receives an additional phosphate from ATP hydrolysis
- Then a pair of electrons reduces a carboxyl group of 3-PGA to become G3P, allowing more potential energy storage, which is combined with the other carbon atoms to form 6xG3P in total\
- 1x of this G3P can leave the cycle while the rest is required to complete the cycle
Phases of Calvin Cycle: Regeneration
Carbon skeletons of the 5xG3P use ATP to be rearranged through complex reactions back into 3 molecules of RuBP
C3 Plants
a plant that uses the calvin cycle in mesophyll cells for the initial steps including incorporating Co2 to form a 3-PGA
- All agricultural plants
- stages of photosynthesis are not seperated by time or location and
Reasons for Different Mechanisms of Carbon Fixation
- Photorespiration: a pathway consuming oxygen and ATP which releases Co2 and decreases photosynthetic output due to oxygen binding to rubisco rather than Co2
- as stomata pores are main avenues of evaporative loss of water they close when its hot to limit water loss, resulting in less Co2 levels coming in
○ This results in higher concentrations of O2 which rubisco is more likely to bind too
C4 Plants
a plant which separates the Calvin cycle from the mesophyll cell and is proceeded by reactions incorporating Co2 into a 4 carbon compound with an end product supplying Co2 for the calvin cycle
- Uses ATP generated from the cyclic electron flow as they only have PS I
- seperates calvin cycle carbon fixation(mesophyll) from rest of cycle(bundle sheath cell)
Process of C4 Plant Photosynthesis
- Pep carboxylase has a higher affinity for Co2 than Rubisco and adds Co2 to PEP to create Oxaloacetate(4 carbon) in the mesophyll cells
- Oxaloacetate is then exported to bundle sheath cells through plasmodesmata
- Within the bundle sheath cell it is released by an enzyme before it is fixed by rubisco and enters the Calvin cycle
- Simultaneously, pyruvate is regenerated which travels back to the mesophyll cells where ATP is used to convert it back to PEP
CAM
open up their stomata at night and close during the day, which helps conserve water during the day when its hottest, but they still take up CO2 at night
- Known as Crassulacean acid metabolism(CAM) - Processes separated by time not by space
Process of CAM Photosynthesis
- Mesophyll cells store the organic acids(similar to C4) which are made at night when CO2 is brought in
When light reactions the next day supply ATP and NADPH, the Co2 is released from the organic acids to be incorporated into the Calvin cycle
