Chapter 7: PHOTOSYNTHESIS - Los Angeles Mission … Chapter 7.pdf · Chapter 7: PHOTOSYNTHESIS 2....
Transcript of Chapter 7: PHOTOSYNTHESIS - Los Angeles Mission … Chapter 7.pdf · Chapter 7: PHOTOSYNTHESIS 2....
Chapter 7:PHOTOSYNTHESIS
2. The “Light” Reactions
1. Overview of Photosynthesis
3. The “Dark” Reactions
1. Overview of Photosynthesis
What is Photosynthesis?The process of converting light energy (kinetic)into energy stored in the covalent bonds of glucose molecules (potential).
• carried out by photoautotrophs• plants, phytoplankton, cyanobacteria (any
photosynthetic organism)
• the basis of almost all ecosystems• all “food energy” ultimately comes from the sun• source of all atmospheric oxygen (O2)
Lightenergy
PHOTOSYNTHESIS
6 CO2 6+ H2O
Carbon dioxide Water
C6H12O6 6+ O2
Glucose Oxygen gas
Photosynthesis vs Respirationessentially the
reverseof each other
Leaf Cross Section
Leaf
Mesophyll Cell
CO2 O2
Chloroplast
Chloroplast
Photosynthesis occurs in Chloroplasts
The Chloroplastouter membrane
inner membrane
granum
thylakoid
stroma
Photosynthesis consists of 2 sets of Reactions
The light-dependent or “Light” Reactions:• convert sunlightenergy intochemical energy(stored in ATP &NADPH)
“Dark” Reactions(Calvin cycle):
• use chemicalenergy fromlight reactionsto make glucose
Light
CO2H2OChloroplast
LIGHTREACTIONS(in thylakoids)
CALVINCYCLE
(in stroma)
NADP+
ADP+ P
ATP
NADPH
O Sugar
Electrons
2. Light-dependent(“Light”) Reactions
Light Reactions occur in Thylakoids
A variety of light-absorbing
pigments &electron transport
proteins areembedded within
the thylakoidmembrane
The Pigments absorb “Visible” Light
Chlorophyll a & b:• the major pigments (absorb red, blue…, reflect green)
Carotenoids (e.g., β-carotene)• accessory pigments (absorb green, blue, reflect red, yellow)
blue red
Absorption Range for each Pigment
Transmittedlight
Absorbedlight
Reflectedlight
Light
Chloroplast
Chlorophyll absorbs “non-green” light energy
• green lightpasses onthrough oris reflected,causing theleaves to appear green
1H2O ½ O2 + 2 H+ + 2 *e-
2e- transport chain(ETC) pumps H+
into thylakoid
PS I
PS II
3 ATP Synthaseuses H+ flow tomake ATP
4 2 e- toNADPHPS II
PS I
Light Energy absorbed by Pigments Fuels 4 General Steps of the “Light Reactions”:
1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II
H2O O2 + H+ + *e-sunlight
2) Energy released by a series of *e- transfers is used to generate H+ gradient• H+ accumulates inside the thylakoid membrane
• *e- ends up in NADPH (an electron carrier)
3) H+ gradient used to make ATP (chemiosmosis)
4) *e- “re-energized” in PS I, passed on to NADP+
e–
ATP
MillmakesATP
Phot
on
Phot
on
Photosystem II Photosystem I
NADPHe–
e–e–
e–
e–
e–
Analogy of Light reactions
Chloroplast
Stroma (low H+ concentration)
Light Light
NADP+ + H+ NADPH
H+
H+
H+
H+
ATPPADP +
Thylakoidmembrane
H2O 1
2O2 + 2 H+ H+
H+
H+ H+
H+
H+
H+ H+
H+
H+
Photosystem II Electrontransport chain
Photosystem I ATP synthase
Thylakoid space(high H+ concentration)
Summary of the“Light” Reactions
3. Light-independent(“Dark”) Reactions
The “Dark” ReactionsA series of reactions called the Calvin cycle that synthesize glucose from CO2 and H2O:
CO2 + H2O C6H12O6 (glucose)ATP, NADPH
• can occur in dark (doesn’t require light directly)• also occurs during daylight!
• takes place in the stroma of chloroplasts• outside the thylakoids
• uses energy stored in ATP and NADPH• produced by the light reactions
“Dark” Reactions
Don’tmemorize
this!!
Involves an anabolicpathway known as theCalvin cycle:
• endergonic reactions ofthis pathway are fueled byATP & NADPH from the“light” reactions
• resulting sugars can beused as a source ofenergy or to build otherorganic molecules
Calvincycle
Summary of Photosynthesis
stroma
Key Terms for Chapter 7
• chlorophyll, carotenoids
Relevant Review Questions: 1-6, 8-10, 12
• ATP synthase
• ATP, NADPH
• electron transport chain (ETC)
• photoautotroph
• chloroplast, thylakoid, stroma
• Light reactions, dark reactions, Calvin cycle