Photosynthesis
Light-dependent Reactions
video
• http://www.youtube.com/watch?v=hj_WKgnL
6MI&feature=related
Overview
• Photosynthesis transforms the radiant energy
from the sun into the chemical energy of highenergy compounds
• Much of the glucose produced by plants is
used to make cellulose, other sugars and
starches as well as other essential cellular
components such as amino acids
• Other organisms use the substances made by
plants for their own use
Overview
• The process of photosynthesis can be
summarized by the following equation:
6CO2(g) + 6H2O(l) + energy
C6H12O6(s) + 6O2(g)
• There are more than 100 reactions that lead
to the end products
Photo (light) Synthesis (the reactions
that produce the carbohydrate)
• There are two distinct sets of reactions:
– The light–dependent reactions
– The light–independent reactions
• In the light-dependent reactions light energy
is trapped to generate ATP and NADPH (similar
to NADH)
• The light-independent reactions use the
energy of ATP and the reducing power of
NADPH to make glucose
Cross-section of a dicot leaf
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html
Chloroplasts
• Photosynthesis occurs in the 40 – 200
chloroplasts that are contained in each
photosynthetic cell
• Thylakoids are interconnected disks that are
formed by a membrane system within the
chloroplast
• Thylakoids are stacked to form grana (singular,
granum)
Chloroplasts
• Molecules that absorb the sun’s energy are
embedded in the thylakoid membranes
• Surrounding the grana is a fluid filled interior
called the stroma
• The stroma contains the enzymes that
catalyse the conversion of CO2 into
carbohydrates
Chloroplasts
Absorption of Light energy
• When any matter absorbs light energy, the
light is absorbed in packets of energy called
photons
• Photons carry specific amounts of energy
• Each wavelength of light is associated with
photons of one distinct amount of energy
• Longer-wavelength photons have smaller
amounts of energy
Light wavelengths
http://www.elixa.com/light/healing.htm
Light absorption
• The wavelength of the photon and thus the
colour of the light, that an atom or molecule
absorbs is determined by the energy levels of
the electrons in the atom or molecule.
• An atom or molecule can absorb a photon
only if they have an amount of energy exactly
equal to the difference between two energy
levels
http://www.energygroove.net/atoms.php
Photosynthetic pigments
• Pigments are compounds that absorb certain
wavelengths
• Chlorophyll is the main photosynthetic
pigment
• Chloroplasts contain two types: chlorophyll a
and chlorophyll b
• They absorb photons from different
wavelengths
http://ghadvisor.blogspot.com/2011/03/plasma-emitting-diode-lights-for.html
Photosystems capture energy
• In the thylakoid, pigments are arranged into
clusters called photosystems.
• Each photosystem has about 250-400 pigment
molecules
• Energy absorbed by a pigment is passed to a
neighbouring pigment until it reaches the reaction
centre (chlorophyll a)
• All the surrounding pigment molecules that gather
the light energy are called the antenna complex
Image from http://kvhs.nbed.nb.ca/gallant/biology/photosystem.jpg
Photosystems
• In green plants and algae, there are 2
photosystems (I and II) named in order of
their discovery
• Photosystem I: P700 is the chlorophyll a that
is associated with its reaction centre
• Photosystem II: P680 is the chlorophyll a that
is associated with its reaction centre.
• The names are based on the wavelength of
light that these molecules absorb
• There are thousands of photosystems in the
thylakoid membranes of just one chloroplast
How does photosynthetic pigment
bind light energy?
• Pigment molecules have conjugated double bonds,
this means that every second bond is a double bond
• The electrons in these bonds are easily
"detachable", so when a light photon excites them,
the energy level of one of the electrons in the bond
increases
• The energy is quickly given off as heat, light,
phosphorescence or it can be transferred to another
pigment molecule.
Image from http://www.lycocard.com/images/main/chem_structure.gif
Light-dependent Reactions
Image from http://www.biology.arizona.edu/biochemistry/problem_sets/photosynthesis_1/graphics/z-scheme.GIF
Light-dependent Reactions
Step 1
• The P680 molecule in the reaction centre of
photosystem II absorbs a photon, exciting an
electron
• The excited electron is picked up by an
electron carrier leaving a “hole” in the P680
molecule
• The P680+ pulls an electron from a water
molecule
Light-dependent Reactions
Step 2
• The energized electrons are passed from the
electron acceptor along an electron transport
system
• With each transfer, a small amount of energy is
released and used by the b6f complex to pump
hydrogen ions from the stroma into the thylakoid
space
• This, along with the water splitting, creates a
hydrogen ion gradient
Light-dependent Reactions
Step 3
• While steps 1 and 2 are happening, light
energy is absorbed by photosystem I and is
transferred to the reactions centre P700
molecule
• Excited electrons are passed to an electron
acceptor
• Lost electrons are replaced by those that have
reached the end of the transport system
Light-dependent Reactions
Step 4
• Electrons are used by enzyme NADP reductase
to reduce NADP+ to NADPH
• The reducing power of NADPH will be used in
the light-independent reactions
Making ATP
• ATP is formed using the energy of
chemiosmosis
• The H+ ion gradient formed by the b6-f
complex of the electron transport chain is
used by the ATP synthase molecule to make
ATP from ADP and Pi
• This is photophosphorylation
Cyclic and noncyclic
Photophosphorylation
• The production of ATP by the passing of
electrons through the Z–scheme is often
called noncyclic photophosphorylation
• The flow of electrons is unidirectional
• The passage of one electron pair produces
one NADPH and slightly more than one ATP
• Light-independent reactions require two
NADPH and three ATP
Cyclic and noncyclic
Photophosphorylation
• Cyclic photophosphorylation produces more
ATP
• Excited electrons leave photosystem I and are
passed to a electron acceptor, then to b6-f
complex and back to photosystem I
• ATP is formed by chemiosmosis as the proton
gradient is generated
• No NADPH is made
Cyclic Photophosphorylation
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/etcyc.html
http://bioweb.wku.edu/courses/Biol120/images/Photophosphryl.htm
animations
• http://www.stolaf.edu/people/giannini/flashanim
at/metabolism/photosynthesis.swf
• http://dendro.cnre.vt.edu/forestbiology/photosy
nthesis.swf
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::5
35::/sites/dl/free/0072437316/120072/bio12.swf
::Cyclic and Noncyclic Photophosphorylation
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapt
er10/animations.html#
• The end of part 1
• The light-independent reactions are next