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Photosynthesis
CHAPTER 10
Sunlight as an Ultimate Energy Source
 All living things need energy
 Photosynthesis provides this energy
 Converts light energy into chemical energy
 Acquired by either autotrophic or heterotrophic
means
Autotrophs
 Live without
consuming anything
from other living things

Require water, soil
minerals, and CO2
 Producers of the
Heterotrophs
 Live on compounds
produced by other
organisms
 Consumers of the
biosphere

biosphere

Photoautotrophs
Use light as energy sources
 E.g. plants, algae, protists,
and bacteria

Eat living organisms for
energy

E.g. animals
 Decomposers of the
biosphere

Breaks down dead organic
matter

E.g. fungi
Anatomy of a Leaf
 Stomata allow gas
exchange
 Veins move water from
roots to leaves and
sugars from leaves to
roots
 Chloroplasts, the site
of photosynthesis,
Located in the
mesophyll or interior
leaf tissue

All green areas of plants,
concentrated in leaves
Chloroplasts
 Double membrane bound
organelle
 Fluid filled space called the
stroma
 Contains multiple
thylakoids, or
interconnected membranous
sacs


Stacked into grana
Chlorophyll pigment within
Gives plant characteristic
colors
 Captures energy for
photosynthesis

Equation of Photosynthesis
6CO2 + 6H2O + sunlight
C6H1206 + 6O2
What color line is showing reduction? oxidation?
Redox Revisited
 Cellular Respiration
 Energy from sugar as electrons from H to O2 = H2O
 Lose PE as fall to more electronegative oxygen
 Mitochondria use energy released
to make ATP
 Photosynthesis
 H20 split and electrons to CO2 = sugar (reduction)
 Gain PE as bond complexity increases
 Requires energy = endergonic

Light provides boost
Photosynthesis: An Overview
 Light reactions [photo part]
 Solar energy to chemical energy
 Light drives transfer of e -’s and H+



NADP+
NADPH (reduction or oxidation?)
Create ATP using chemiosmosis to power
photophosphorylation
NO sugar produced
 Calvin cycle (dark reaction) [synthesis part]
 CO2 incorporated into organic molecules, carbon fixation



Add e -’s from NADPH and ATP to reduce into carbohydrates
Makes sugar
Doesn’t need light directly
Photosynthesis
Understanding Sunlight
 Electromagnetic energy
 Exists as discrete packets of particles called photons
 All wavelengths make up an electromagnetic
spectrum


Wavelengths are distance between crests of waves and
inversely related to amount of energy
Visible light most important
to life
Detectable by human eye
 Violet end is shortest waves
 Red end is longest waves
 All combined = white light

Photosynthetic Pigments
 Light can be reflected,
transmitted, or absorbed
 Chloroplasts vary in
pigments

Chlorophyll a, b, and
carotenoids
Violet-blue and red light most
efficient for photosynthesis
 Carotenoids have role in
photoprotection
 In human eye too

Action spectrum
Excitation of Chlorophyll
 Absorption of light
elevates electrons of
pigments to higher
orbital ( PE)

Pigments absorb in
specific range
 Unstable in upper orbital
so ‘fall’ back quickly

Releases energy as heat
 White vs black cars or
clothing in the South
Photosystems
 Protein complex with a
reaction center surrounded
by light-harvesting
complexes


Chlorophyll a always bound
with reaction center molecules
Other pigments with lightharvesting complexes

Gather light from larger
surfaces
 Pigments absorb photons and
transfer to reaction center
complex
 Electrons transferred to
primary electron acceptor,
reducing it
 Two types, II and I
Light Reaction
 Occurs in the thylakoids
 Two Photosystems
 PS I absorbs at 700nm
 PS II at 680nm
 Two electron flow patterns
 Linear electron flow
 Cyclic
Linear Electron Flow
To Calvin cycle
Comparing Chemiosmosis
 Similarities


ETC in membranes pump
protons across as e-’s moved
to more EN carriers
ATP synthase utilizes [H+
gradient]
 Differences


M: e-’s from organics,
protons move out
P: e-’s from H2O, protons
move in
Calvin Cycle
 Anabolic reaction in the
stroma
 Products from light
reaction are reactants for
dark
 (3) CO2 molecules combine
to create (1) 3 carbon
sugars (glyceraldehyde 3phosphate, G3P)

Cycle must occur 3 times for
1 molecule to be made
 Broken into 3 steps
 Carbon fixation
 Reduction
 Regeneration of CO2
acceptor (RuBP)
CO2
3PG
RuBP
G3P
G3P
G3P
Carbon Fixation
 1 CO2 into stroma
 Attaches to ribulose bisphosphate (RuBP), a 5
carbon sugar
 Catalized by rubisco

Most abundant protein on Earth
 Forms unstable 6 carbon molecule
 Immediately to (2) 3-phosphoglycerate (3PG)
 2 for every 1 CO2 molecule
Reduction
 3PG gains a phosphate from ATP to create 1,3-
bisphosphoglycerate
 NADPH reduces 1,3-bisphosphoglycerate to G3P
 3 cycles (3 CO2’s) create 6 G3P
 Only 1 leaves (3 carbons out)
 Other 5 recycled (15 carbons remain)
Regeneration of CO2 Acceptor
 5 G3P are rearranged into 3 RuBP (5 carbons each)
 Cost 3 ATP
 Capable of accepting CO2 again
 Overall cost of cycle
 9 ATP
 6 NADPH
 3 CO2
 2 G3P to make sugars and other fuels
Review of Photosynthesis