Transcript
Photosynthesis
6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O
Synthesizes energy-rich organic molecules (glucose) from energy-poor molecules (CO2, H2O).
Uses CO2 as carbon source & light energy as energy source.
Directly or indirectly supplies energy to most living organisms.
Fig. 10.4
The major “Play-as”
Photosynthesis occurs in chloroplasts in eukaryotic organisms:
1. Light dependent reactions occur in the thylakoid membranes of the grana and yield ATP and NADPH(obtained by reducing NADP with H2O). O2 is a wasteproduct
2. The Calvin cycle occurs in the space between grana called stroma
Fig. 10.6
Photo = lightsynthesis = putting together
= production of sugar (glucose = C6H12O6) using E from solar radiation (photons), CO2 and H2O
Photons = fixed quantity of light E
Utilized by most plants, some bacteria, some protists
Nutritional categories
1. Autotrophs – require no organic nutrients. All can “fix” or reduce CO2 into glucose via the Calvin Cycle:
6CO2 + 12 NADPH + 18 ATP
C6H12O6 + 12 NADP + 18 ADP + 18 P
They can then synthesize all other organic constituentsfrom this glucose
Autotrophs
A) Photoautotrophs – use light energy to generate boththe ATP and NADPH for the Calvin cycle.(photosynthetic organisms)
B) Chemoautotrophs – cannot use light. Use respirationsof inorganic substrates like reduced sulfur compounds,nitrogen compounds, and iron to generate the ATP and NADPH for the Calvin cycle (all chemosynthetic organismsare bacteria)
2. Heterotrophs – require at least 1 organic nutrient. Heterotrophs are dependent upon autotrophs, usually the photosynthetic organisms, for a source of fixed carbon(ie carbohydrates) and other nutrients
Photoautotrophs are considered “producers” in an ecosystem. Heterotrophs are considered as “consumers”
All life is therefore either directly or indirectly dependentupon the energy of the sun
In eukaryotes, takes place in chloroplasts
Plants - chloroplasts in leaves, other green parts
Contain chlorophyll= green pigmentcaptures/absorbs light E
Electromagnetic Spectrum
Nm = nanometer = 10-9 m0.0000000001 m
Based on WavelengthShort wavelength = high E Long wavelength = low E
Fig. 10.7
Electromagnetic Spectrum
Range = wavelengths of less than 1 nm (gamma rays) to wavelengths of more than 1 km (radio waves)
gamma = high Eradio = low E
Visible LightDrives photosynthesis= light detectable by human eye380-750 nm
Ranges from violet redROY G BIV backwardsred, orange, yellow, green, blue, indigo, violet
Blue and red most important in photosynthesis
Why?Colors (wavelengths) absorbed by chlorophyll
Why is chlorophyll green?
Light can be:reflected, transmitted, absorbed
reflected - “bounces” off of pigment= color that you see
transmitted - goes through pigment
absorbed - captured by pigmentdon’t see
Fig. 10.7+8
Different pigments absorb different wavelengths of light
pigment = substance that absorb visible light
Wavelengths absorbed, disappear
black = all wavelengths absorbed
white = all wavelengths reflected/transmitted
Photosynthetic Pigments in Plants
Chlorophyll a
Chlorophyll b= yellow-green absorbs slightly different wavelength
Carotenoids= yellow & orangePhycocyanins= blue and purple
Chlorophyll a = primary pigment
Chlorophyll b, carotenoids and phycocyanins = accessory pigments
Expand range of wavelengths available for photosynthesis
Fig. 10.10
Absorption and actionspectra for photosynthesis
Chloroplast Structure
Lens-shaped
surrounded by double membrane
divided into 3 compartments by system of membranes
Chloroplast Structure1. Intermembrane space= space between the 2 outer membranes
2. Thylakoid spacethylakoids = flattened membranous sacs inside
chloroplastStacks of thylakoids = grana
chlorophyll embedded within thylakoid membrane
membrane separates thylakoid space = area inside of thylakoids from stroma
3. Stroma = thick fluid outside/surrounding thylakoids
Fig. 10.11
Photosynthesis- light (kinetic) E chemical (potential) E
E stored in bonds of glucose moleculesBreaking bonds releases E
Photosynthetic Process
2 stages1. Light Dependent Reactions - require sunlight
2. Light-Independent Reactions (Calvin Cycle) - don’t require sunlight
1. Light Reactions
convert light energy to chemical energy
energy stored in bonds of ATP & NADPH - adenosine triphosphate
- Nicotinamide adenine dinucleotide phosphate
Light energy harvesting occurs via photosytems in thylakoid membranes:
ADP + P + NADP + H2O + light energy
ATP + NADPH + O2
Involves 2 photosytems interconnected by an electron transport chain
Light Rxns.Require sunlight = light dependent rxns.
Occur in thylakoid membranes of chloroplasts
Thylakoid membranes contain photosystems= light harvesting units
consist of reaction center, antenna molecules, and e- acceptors
Example of a photosystemFig. 10.13a
Excitation of an isolated chlorophyl molecule
Fig. 10.12
Reaction Center = single, specialized chlorophyll a molecule + primary e- acceptor
Antenna molecules = all other photosynthetic pigment molecules
(chlorophyll b, carotenoids, phycocyanins)
e- acceptor molecules = molecules that accept electrons from “excited” chlorophyll molecules
How photosystems work
1. Antenna molecules absorb photons
2. Pass energy from molecule to molecule until rxn. center reached
3. Chlorophyll a molecule in rxn center donates excited e- to primary e- acceptor
Photosystems
Chlorophyll a molecule at rxn. center loses e- to primary e- acceptor
= electron transfer
e- excited
boosted to higher energy state
Transfer of e- from chlorophyll a to primary acceptor = redox rxn.
= reduction/oxidation rxn.
Reduction = gain of e- = more negative chgOxidation = loss of e- = more positive chg
primary acceptor gains e- (reduced)chlorophyll a loses e- (oxidized)= first step of light rxns.
Thylakoid membrane contains 2 types of photosystems
- photosystem I
- photosystem II
each has characteristic rxn. center
systems cooperate
Photosystem Irxn. center absorbs light having wavelength of
700 nm= P700
Photosystem IIabsorbs light having wavelength of 680 nm= P680
2 systems cooperate to generate ATP & NADPH
** = PRIMARY FUNCTION OF LIGHT REACTIONS **
Non-cyclic electron flow generates ATP and NADPH
Fig. 10.14
Non-cyclic e- flow
“mechanical analogy”
Fig. 10.15
Chemiosmosis of ATP
Fig. 10.18
Chemiosmosis in mitochondria and chloroplasts:
10.17
Light Independent Reactions
CO2 fixation via the Calvin Cycle (recall from previousNotes)
6CO2 + 12 NADPH + 18 ATP
C6H12O6 + 12 NADP + 18 ADP + 18 P
See Fig. 10.17 for normal C3 pathway:
CalvinCycle
Fig. 10.19
Calvin Cycle
6 CO2’s yield1 glucose
Occurs inStroma
Catalyzed byRubisco(RuBP)
- CO2 enter the plant leaf openings called stomata -these openings are surrounded by guard cells which when flaccid close the opening
-During dry conditions, stoma are thus closed and CO2becomes limiting
-The Rubisco enzyme then reacts with O2 rather than CO2 and photorespiration occurs instead of CO2 fixation
Photorespiration:
Photorespiration:
- Photorespiration wastes fixed carbon by convertingribulose biphosphate into only 1 glyceraldehyde phosphate plus 1 glycolic acid (CH2OHCOOH)
-this glycolic acid is removed from the cycle and is Wasted
-plants evolved photosynthetic pathways to preventthis wasteful process
C4 Plants and the C4 Photosynthetic Pathway:
-Occur/originated in tropics, Mediterranean
-Adapted to hot/arid environment
-Adaptations save water, prevent photorespiration
Initial enzyme = PEP (phosphoenolpryuvate), fixes C into 4 C molecule (oxaloacetate)
Rubisco not involved initially - eliminates photorespiration
C stored in 4 C molecule in mesophyll cells
Calvin cycle occurs in nearby cells= bundle sheath cells
Mesophyll cells shuttle C to bundle sheath cells
Allows photosynthesis to occur even if stomates closed
Examples: corn, sugarcane, Bermuda grass
C4 anatomy and pathway:Fig. 10.20
C4 can handle heat, drought, high light
C3 more efficient if water is available and under lowlight conditions
CAM Plants= Crassulacean Acid Metabolism - Cacti, pineapples, succulents
Open stomates at night- lets in CO2, minimizes water loss
CO2 incorporated into organic acids (stored)
Used in light rxns during day while stomates closed
Comparison of C4 and CAM
Fig. 10.21
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