photosynthesis
Post on 05-Aug-2015
41 Views
Preview:
TRANSCRIPT
Photosynthesis
• The process by which plants, algae, and some microorganisms harness solar energy and convert it into chemical energy.
• Endergonic reaction• Redox reaction• Only done by autotrophs
• Glucose used for: fuel own plant respiration (50%), growth, make other important compounds (amino acids, cellulose, starch, sucrose)
6CO2 + 6H2O C6H12O6 + 6O2
Photosynthesis/Respiration Cycle
Cellularrespiration
CO2 and H2O
ATP (available for cellular tasks)
Heat energy
Light
• Light is the source of energy for photosynthesis– Made of photons—packets of kinetic energy– Part of electromagnetic spectrum– 3 types from the sun get to the earth• Ultraviolet • Visible• Infrared
Pigments
• Pigment—Substance that absorbs light energy• Several types of pigments:– Chlorophyll a—most abundant, green pigment,
absorb blue/red, reflect green– Accessory Pigments:• Chlorophyll b—absorb blue/red, reflect green• Carotenes—absorb blue, reflect orange/red• Xanthophylls—absorb purple/blue/ green, reflect
yellow
PigmentsPigment Color Organisms
Major PigmentChlorophyll a
green (or yellow) plants, algae, bacteria
Accessory PigmentChlorophyll b
yellow plants, algae
Carotenoids (xanthophylls and carotenes)
orange, red, yellow plants, algae, bacteria, archaea
Chloroplasts
• Mainly found in cells in the LEAF– Lots of surface area to absorb light– Has abundant water– Main site of gas exchange• Exchange occurs through stomata surrounded by guard
cells
– Mainly located in mesophyll
Chloroplasts
• Stroma—inner fluid with DNA, ribosomes, fluid• Grana—Stacks of thylakoid• Thylakoid—Disks, membranes with photosynthetic
pigments• Photosystem—in thylakoid membrane– Chlorophyll a (approx. 300 molecules)
• Reaction Center
– Accessory pigments (approx. 50 molecules)• Antenna pigment to funnel light to reaction center
– Proteins
Photosynthesis Overview
• Happens in 2 stages– Light Reactions—convert solar energy into
chemical energy• Occurs in thylakoid membrane
– Carbon Reactions—use ATP and NADPH to reduce CO2 to glucose• Occurs in the stroma
The Light Reactions
• Photosystem II– Pigment molecules absorb light and transfer to reaction center
(chlorophyll a)– Water is split into 2H+ and ½ O2
– Water donates 2 electrons– Energy “excites” 2 electrons to a higher energy orbital– Chlorophyll a ejects “excited” electrons to first electron
transport chain (ETC)– ETC makes a proton gradient from stroma into the thylakoid
space– ATP synthase uses proton gradient to make ATP (chemiosmotic
phosphorylation)• Used in carbon reactions
The Light Reactions
• Photosystem I – Pigment molecules absorb light and transfer to
reaction center (chlorophyll a)– 2 electrons come from first ETC– Energy “excites” 2 electrons to a higher energy
orbital– Chlorophyll a ejects “excited” electrons to first
electron transport chain (ETC)– Electrons are passed to NADP+ to reduce it to
NADPH (used in carbon reactions)
Making ATP: PhotophosphorylationH+ gradient: as electrons moved within membrane, H+ is pumped into the thylakoid space
Making ATP: Photophosphorylation
ATP produced: ATP synthase allows H+ to go down its concentration gradient, generates ATP
The Carbon Reactions
• Also known as: Calvin Cycle, “Dark reactions”• Occurs in the stroma• Uses ATP and NADPH to make glucose from CO2
• Calvin Cycle:– Step 1: Carbon fixation—incorporation of CO2 into an
organic molecule• CO2 combines with RuBP, using enzyme called rubisco
– Step 2: PGAL Synthesis– Step3: PGAL makes glucose– Step 4: Regeneration of RuBP
C3 Plants
• Calvin Cycle = C3 Pathway
• All plants use Calvin Cycle, but some plants ONLY use C3 pathway– 95% of plants are this way
• Inefficient—lose some energy to heat– 30% on the best sunny day– In Photorespiration rubisco uses O2 instead of CO2 as a substrate
– Stomates open, O2 diffuses out, CO2 is used
– Hot dry climates, stomates cannot stay open—lost water, O2
builds up, photorespiration takes over
C4 Plants
• C4—adaptation to help minimize photorespiration (1% of plants)
• C4 Plants—Separate light reactions and Calvin Cycle into different cells– Light reactions and carbon fixation—mesophyll– CO2 combines with 3 carbon molecule to make 4 carbon—
C4
– C4—(malate) moves to bundle sheath cells, rest of Calvin Cycle
• Bundle sheath cells NOT exposed to O2
C3 and C4 Plant AnatomyC4 plantC3 plant
Vein
Stoma
Mesophyll cell
Bundle-sheath cell
Mesophyll cell
Stoma
Vein
Bundle-sheath cell
CAM Plants
• Occurs in desert plants (3–4% of plants)• Only open stomates at night to fix CO2, then
fix again during the day using Calvin Cycle– Store night time CO2 as malate in vacuoles
– Stomates open, malate to chloroplast, release CO2, used in Calvin Cycle
• Happens in same cells
top related