Photosynthesis
Chapter 6
Section 1: Light Reactions
• All organisms use energy to carry out the functions
of life
• Organisms are classified by how they get energy
– Autotrophs: use energy from sunlight
• Photosynthesis: convert light energy into chemical energy in
the form of organic compounds (carbohydrates)
– Heterotrophs: get energy from food rather than
sunlight
Light Reactions Cont.
• Almost all organisms ultimately depend upon
autotrophs to obtain the energy necessary to
carry out the processes of life
Overview of Photosynthesis
Overview of Photosynthesis
• Photosynthesis produces organic compounds
from carbon dioxide and water
• Oxygen and some organic compounds are
used by the cell during cellular respiration
• During Cellular respiration carbon dioxide and
water are produced
Photosynthesis and Cellular Respiration
The reactants of photosynthesis are the products of
cellular respiration
Photosynthesis
• Can be broken down into two stages:
1. Light reactions: light energy is converted to
chemical energy, which is temporarily stored
in ATP and the energy carrier molecule
NADPH
2. Calvin cycle: organic compounds are formed
using carbon dioxide and chemical energy
stored in ATP and NADPH
Capturing Light Energy
• Light reactions require light
• Light is absorbed in chloroplasts
• Each chloroplast contains
– Inner and outer membranes
– Thylakoids: membranes arranged in flattened sacs
– Grana: stacks of thylakoids
– Stroma: fluid inside inner membrane
The Chloroplast
Light and Pigments
• Light from the sun appears white, but it is
actually made up of a variety of colors
• Light can be broken down into the visible
spectrum
Light and Pigments
• Light can be reflected, transmitted, or
absorbed by an object
• Pigments: compounds that absorb light
• Most pigments absorb some colors more than
others
• Light that is reflected or transmitted has not
been absorbed
– Green leaves: all colors absorbed besides green
Chloroplast Pigments
• Most important pigments are chlorophylls
• Several types of chlorophylls
– Chlorophyll a: absorbs red light
– Chlorophyll b: absorbs blue light
– Note: neither absorbs green light well
Chloroplast Pigments
• Mainly chlorophyll a in light reactions
– Chlorophyll b and carotenoids serve as accessory
pigments
– Accessory pigments allow more light to be
captured
Light Energy  Chemical Energy
• Light is absorbed and transformed into
chemical energy
• Chemical energy is temporarily stored in ATP
and NADPH
• Photosystems: clusters/groups of carotenoid
and chlorophyll pigments
– There are two photosystems
• Photosystem I and Photosystem II
Photosystem I and II
• Photosystems have similar pigments, different
roles in th light reactions
Light Reactions
1. Light energy forces electrons to enter a higher
energy level (excites them) in photosystem II
2. Electrons leave chlorophyll a molecules are
accepted by the primary electron acceptor
3.Primary electron acceptor donates electrons to
the electron transport chain (within the thylakoid
membrane)
-moving from molecule to molecule they lose
energy
Light Reactions
4. Light is absorbed by photosystem I and II.
Electrons from chlorophyll molecules in
photosystem II replace electrons that leave
chlorophyll molecules in photosytem I
-this needs to happen in order for
photosynthesis to continue!
Light Reactions
5. Primary electron acceptor of photosystem I
donates electrons to a different ETC
This ETC brings electrons to the thylakoid
membrane
Electrons combine with a proton and NADP+
This creates NADPH from NADP+
Light Reactions
Photosystems
Replacing Electrons In LR
• Electrons in PS II replace electrons in PS I
• Replacement electrons for PS II come from
water
• Water is split into protons, electrons and
oxygen
2H2O  4H+ + 4e- + O2
Photosynthesis Video
• https://www.youtube.com/watch?v=joZ1EsA5
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Making ATP in LR
• Chemiosmosis: synthesizes ATP during light
reactions
– Relies on concentration gradient of protons across
thylakoid membrane
– Concentration of protons is greater inside
thylakoid than in the stroma (outside thylakoid)
– Potential energy from gradient harnessed by ATP
synthase
https://www.youtube.com/watch?v=3y1dO4n
NaKY
Making ATP Continued
• ATP Synthase makes ATP by
adding a phosphate group to
ADP
• Energy for this reaction comes
from movement of protons
across thylakoid membrane
• Potential energy converted to
chemical energy
Chapter 6 Sect. 2 The Calvin Cycle
• The Calvin Cycle
– Series of enzyme-assisted chemical reactions that
produces a 3 carbon sugar
– Carbon dioxide molecules are “fixed” into organic
compounds (sugars) in a process known as carbon
fixation
– 3 Carbon dioxide molecules are needed to make
each organic compound
The Calvin Cycle
The Calvin Cycle
The Calvin Cycle
1. CO2 diffuses into the stroma from the
surrounding cytosol
An enzyme binds each CO2 with a 5 carbon
molecule—ribulose bisphosphate (RuBP)
The resulting 6 carbon molecule is very unstable
and immediately splits into two 3 carbon
molecules—3-phosphoglycerate (3-PGA)
The Calvin Cycle
2. 3-PGA is converted into glyceraldehyde 3phosphate (G3P) in a two part process
A. 3-PGA receives a phosphate croup from ATP
B. Then compound receives a proton (H+) from
NADPH and releases a phosphate group
– *The ADP and NADP+ are then used in the Light Rxns
The Calvin Cycle
3. One of the G3P molecules leaves the Calvin
Cycle and is used to make carbohydrates where
energy will be stored
4. Remaining G3P molecules are converted back
into RuBP through addition of phosphate groups
from ATP. The RuBP molecules enter the Calvin
Cycle again
The Calvin Cycle
The Calvin Cycle
• The most common pathway for carbon
fixation
• Plant species that fix carbon exclusively with
the Calvin Cycle are called C3 plants because
of the 3 carbon compound initially formed in
this cycle
Alternative Pathways
• Plants that live in hot and dry conditions have
evolved to fix carbon via alternative pathways
• CO2 enters leaves through stomata
• Plants living in hot and dry areas can lose a lot
of water through these stomata
• Water loss can be reduced through closing or
partially opening stomata
Alternative Pathways
• C4 Pathway
– Fixes carbon into four-carbon compounds
– Plants that use this pathway are called C4 plants
– C4 plants have partially closed stomata during the
hot parts of the day
– Carbon compounds can still be produced with
limited CO2