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Topics 3.8 + 8.2
Photosynthesis
IB Biology
Leaves • Green tissue in the interior
of the leaf (mesophyll) = contains chloroplasts
• CO2 enters the leaf and O2 exits through the stoma
• Stomata = bottom of leaf
Chloroplasts
• similar to mitochondria• outer and inner membrane• membranous sacs: THYLAKOIDS• thylakoids = concentrated in
stacks called GRANA• thylakoids: contain chlorophyll
(capture light)• filled with fluid: STROMA (where
sugars are made from CO2 )
Overview
Electromagnetic Spectrum
The 2 stages of Photosynthesis
• Light Dependent Reactions:– convert light energy to chemical
energy (ATP + NADPH) and produce oxygen gas as a waste product
• Light Independent Reactions (Calvin Cycle):
– make sugar using carbon dioxide and the energy-containing products of the light-reactions (ATP + NADPH)
Light Dependent Reaction
• Light can be..– Absorbed– Reflected– Transmitted
• Occurs in thylakoids
• Chlorophyll A and B = main photosynthetic pigments (also carotenes, anthocyanins)
Light Dependent Reaction• Light energy is used to split water. • H + is released = used by ATP Synthase to produce ATP• NADP + is reduced to NADPH+ • ATP and NADPH = used in light independent reactions• O2 = waste product
Photosystems• Photosystem: place in
thylakoid where light is harvested
• Photon is absorbed by a pigment molecule = electrons gain energy
• Excited electron is passed to a neighboring molecule - the primary acceptor
Photosystems I & II work together
• Both photosystems absorb light
• Electron Transport Chain = electrons go down an energy hill = lose energy at each step - this energy is stored in ATP or NADPH
• Electrons released from PSI is replaced by electrons coming from PSII
Chemiosmosis/Photophosphorylation
Role of Water
• The light energy is used to split water molecules (photolysis)
• H2O → H+ + O + e- The electrons allow
reduction of NADP+ / replace those leaving photosystem II
H + flow allows ATP production
The O2 is released and leaves the plant through the stoma
Mechanical Analogy for Making ATP
• As electrons skip down the ETC, ATP are generated.
Animations
http://www.youtube.com/watch?v=hj_WKgnL6MI
Light Independent Reaction (Calvin Cycle)
• Glucose phosphate is produced = stored as starch• CO2 , ATP and NADPH are used
Simple Calvin Cycle animations:
Summary
Light dependent
reaction
Light independent
reaction
plateau
At low CO2 concentration, rate is positively correlated with concentration
CO2 is a substrate in an enyme-catalysed light-dependent reaction.
At low light intensity, rate of photosynthesis is proportional to light intensity.
plateau
Increased temp. gives increased energy and increased rate of photosynthesis
Optimum temperature
Above the optimum temp., enzymes are denatured and rate drops steeply.
Light intensity
Rate
of
phot
osyn
thes
is
What are the conditions of each of these trials?
