Chapter 8 Flashcards
Photosynthesis
- light energy captured, used to make carbohydrates
- CO2+H2O+light energy –> C6H12O6+o2
- CO2 is reduced
- H2O is oxidized
- energy from light drives this endergonic reaction (reactions are driven forward by coupling the reaction with an exergonic process that releases free energy)
Photosynthesis Powers the Biosphere
- life is largely driven by photosynthetic power of green plants
- cycle: cells use organic molecules molecules for energy and plants replenish those molecules using photosynthesis
- also produces oxygen
Autotrophs
- self feeding
- require only raw materials (water, minerals and a carbon source) to make their own food
- produce organic molecules from inorganic sources
Photoautotrophs
- green plants, algae, cyanobacteria
- need only water, minerals and CO2 + light
Heterotrophs
- “other” feeding
- require complex organic molecules from other organisms
- decomposers
Heterotrophs: decomposers
- live on “organic litter”
- decaying animal or plant matter
- carcasses, feces, other debris (plant or animal)
- everything that is not an autotroph, IS a heterotroph
Plants “fix” CO2
- plants use light energy to synthesize complex organic molecules that they use for structure and energy
- by “fixing” or converting CO2 gas into sugar molecules
- start with 1-C molecule and make 6-C molecules
Green tissue is photosynthetic
- photosynthesis occurs in the green parts of plants, leaves, stems etc.
- inside cells, inside the chloroplasts, in/on the thylakoid membranes
Chloroplasts
organelles that carry out photosynthesis
Chlorophyll
green pigment that captures energy
mesophyll cells
majority of photosynthesis occurs in leaves in the mesophyll cells
stomata
carbon dioxide enters and oxygen exists leaf
Chloroplast anatomy
- outer and inner membrane
- intermembrane space in the middle
- thylakoids are one
- stroma is the stack
Thylakoid membrane
- contains pigment molecules
- forms thylakoids
- encloses thylakoid lumen
Granum
stack of thylakoids
Stroma
fluid filled region between thylakoid membrane and inner membrane
Photosynthesis has 2 stages
- light reactions
2. dark reactions (calvin cycle)
Stage 1 of Photosynthesis
- light reactions occur in the thylakoid membranes
- only when light is present
- splits H2O to form ATP, NADPH and O2
Stage 2 of Photosynthesis
- in the stroma
- can take place with or without light
- takes ATP and NADPH from light reactions + CO2 to make sugar
light as a wave
- type of electromagnetic radiation
- travels in waves
- similar to what you see if you drop a rock into a pond
wavelength
- the distance between the tops of the wave
- the wavelengths can vary enormously (1nm-1km)
visible light
-has wavelength from 350nm-750nm
Light as a wave? or particle?
- but light also behaves as a particle
- called photons (“packets of energy”)
- each photon has a fixed quantity of energy
- the amount of energy in a photon is inversely proportional to its wavelength
- shorter wavelengths have more energy
- dangerous to biological tissues
Light receptors
- when light hits matter it can be…
1. reflected
2. absorbed
3. refracted - color of an object is dependent absorption and reflection
- chlorophyll absorbs red and blue light
- but it reflects green light
- so leaves look green
White and black receptors
- white- is total reflection, its everything being reflected and a combination of all the colors
- black is the total absorption, absence of color
its about electrons
- photosynthetic pigments absorb some light energy and reflect others
- absorption boosts electrons to higher energy levels
- wavelengths of light a pigment absorbs on amount of energy needed to boost an electron to a higher orbital
Excited state
- after an electron absorbs energy, it is an excited state and usually unstable
- releases energy as heat or light
- excited electrons in pigments can transferred to another moelcule or “captured”
absorption spectrum
-wavelengths that are absorbed by different pigments
action spectrum
rate of photosynthesis by whole plant at specific wavelengths
PS I and II
- protein complexes
- capture light energy
- transfer to other molecules to make energy intermediates
- in thylakoid membrane
- light excites pigment molecules in both PS I and II
PS II
- initial step in photosynthesis
- P680
- Oxidizes water, generating O2 and H+
- releases electron in ETC
- energy used to make H+ electrochemical gradient
PS I
- primary role = make NADPH
- P700
- addition of H+ to NADP+ contributes to H+ gradient by depleting H+ from the stroma
ATP synthesis in chloroplasts
- AKA photophorphorylation
- chemiosmotic mechanism
- driven by flow of H+ from thylakoid lumen into stroma via ATP synthase
ATP synthesis: H+ gradient is generated in 3 ways
- ^ H+ in thlyakoid lumen by splitting of water
- ^ H+ by ETC pumping H+ into lumen
- decrease H+ in stroma from formation of NADPH
Chemical Products: 1. Oxygen O2
- produced in thylakoid lumen by oxidation of H2O by PSII
- 2 electrons transferred to P680+ molecules
Chemical Products: 2. NADPH
- produced in the stroma from high-energy electrons that start in PSII and are boosted in PSI
- NADP+ + 2 electrons +H+ –> NADPH
Chemical Products: 3. ATP
-produced in stroma by ATP synthase using the H+ electrochemical gradient
Noncyclic
- electrons begin at PSII and eventually transfer to NADPH, a linear process
- produces both ATP and NADPH in equal amounts
Cyclic Photophosphorylation (cyclic electron flow)
- electron cycline releases energy to transport H+ into lumen driving ATP synthesis
- produces only ATP
- PSI electrons excited, release energy and eventually return to PSI
Calvin Cycle
-CO2 incorporated into carbohydrates precursor to other organic molecules and energy storage -requires massive energy inout -for every 6CO2, 18ATP and 12 NADPH used -Product- G3P -glucose is made later
3 phases of Calvin Cycle
- carbon fixation
- reduction and carbohydrate production
- regeneration of RuBP
Calvin Cycle: Phase 1: Carbon Fixation
- CO2 incorporated in RuBP by the enzyme rubisco
- 6 C intermediate splits into 2 3PG
Rubisco
- most abundant enzyme on the plant
- RuBP is the substrate
Calvin Cycle: Phase 2: Reduction
- reduction and carbohydrate production
- ATP is used to convert 3PG into 1,3-bisphosphoglycerate
- NADPH electrons reduce 1,3BPG to G3P
- 6 CO2 -> 12 G3P (2 glucose, 10 for regeneration of RuBP)
- use G3P to build glucose
Calvin Cycle: Phase3: Regeneration of RuBP
- 10 G3P converted into 6 RuBP using 6 ATP
Summary of the Calvin Cycle
- to make 1G3P takes 3CO2 + 9ATP +6 NADPH
- 2 G3P can join to make one glucose
- need BOTH light reactions and Calvin cycle to make sugar
why not just use the ATPs from the light cycle?
because plants need to store energy
variations in photosynthesis
- environmental conditions can influence Calvin Cycle
- light intensity
- temperature
- water availability
Stomata
- CO2 gas needs to enter
- O2 needs to escape
- if stomata closed then no gas exchange
- stomata also allows water to evaporation
C3 plants
- the stuff we’ve been talking about
- C3= 3 C molecules
- uses Rubisco as the primary means to collect and fix CO2
C4 plants
use a 4 C moelcule to selectively bind to CO2 and GIVE the CO2 to Rubisco
- use oxaloacetate (4C)
- evolved to minimize respiration
which is better C3 or C4?
- depends on environment
- in warm dry climates C4 plants conserve water and prevent CO2 loss
- in cooler climates, C3 plants use less energy to fix CO2
- 90% of plants are C3
CAM plants
- function is similar to C4
- but completely close stomata during the day (open at night)
- store C as malate
