Download Lesson 7 - Photosynthesis

Photosynthesis
Senior Biology
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Photosynthesis overview
Equation
Chloroplast & Pigments
Photosynthesis & Cellular
Respiration connection
• The Light Reaction
• The Light Independent
Reaction/Calvin Cycle
• Essential RuBisCO
The Photosynthesis Song
Photosynthesis
• Process whereby plants convert CO2, water and the
energy from the sun into sugar
• Occurs in the leaves of plants in organelles called
chloroplasts.
carbon dioxide  water  sugar  oxygen
light energy
Photosynthesis: An Overview
• Why?
– To make glucose
• How?
– Reduces CO2 into simple sugars like glucose
• Who?
– Any organism which contains chlorophyll (plants,
cyanobacteria, algae, etc.)
• Where?
– Prokaryotes (e.g. bacteria) – infolding of cell
membrane
– Eukaryotes (e.g. plants) – chloroplasts
Photosynthetic Equation
•
6 CO2 + 6 H2O + light energy--> C6H12O6 + 6O2
•
But not that simple
•
Involves 2 stages:
1) Making ATP and NADPH with energy from
light (Light Reaction)
2) Making glucose from carbon dioxide using
energy from ATP and NADPH (Light
Independent Reaction)
Photosynthesis
• Occurs in the Chloroplast
• Light interacts with the pigment chlorophyll which
is located on the thylakoid membranes
Chloroplast Structure
• Inner membrane called
the thylakoid
membrane.
• Thickened regions
called thylakoids. A
stack of thylakoids is
called a granum. (Plural
– grana)
• Stroma is a liquid
surrounding the
thylakoid.
Pigments
• Chlorophyll is the most important
photosynthetic pigment present in plants.
• Other pigments are also present in the leaf.
– Carotenoids
(orange / red)
– Xanthophylls
(yellow / brown)
Why are plants green?
Light Review
• Visible light comes in a spectrum from purple to red
• Different substances absorb different colours depending
on the structure.
• Colours that the substance doesn’t absorb, it reflects.
We see that reflection as the objects colour
Chlorophyll a & b
• Is a pigment that absorbs blue and red light but
reflects green. The light is reflected into our eyes
giving plants a green colour
High
absorption
Low
absorption/
High
reflection
How do plants capture light?
• Chlorophyll a and b
molecules absorb
photons (light
packets) with blueviolet and red
regions of the
spectrum. They
reflect the green
regions
Carotenoids (other accessory pigments)
• Other accessory pigments (called carotenoids)
also absorb light energy. They reflect photons
in the yellow-to-red range of the spectrum.
• Gives them a yellow-red colour
Why do some leaves turn colours
in the fall?
There are three main
pigments in plants;
Chlorophyll (reflect green),
Carotenoids (reflect
orange/red), and
Xanthophylls (reflect
yellow/brown).
As the most abundant
pigment, chlorophyll is what
gives leaves their green hue
in spring and summer.
Plant Pigment Absorption Spectrums
Why do some leaves turn colours
in the fall?
• During winter, there is not enough light or water for
photosynthesis. Therefore, the trees cut off the circulation of
water, nutrients and sugar to the leaves and live off the food they
stored during the summer.
• They begin to shut down their food-making factories. The green
chlorophyll pigment disappears from the leaves leaving the other
pigments behind. As the bright green fades away, we begin to see
yellow and orange colors. Small amounts of these colors have been
in the leaves all along we just can't see them in the summer,
because they are covered up by the green chlorophyll.
Cellular Respiration Recall
• The process by which sugar is converted
into carbon dioxide, water and energy
• The organism uses released energy for all
cellular activities
sugar  oxygen 
 carbon dioxide  water  energy
Photosynthesis and Cellular Respiration
Photosynthesis
carbon dioxide  water  sugar  oxygen
light energy
Notice Anything?
sugar  oxygen 
 carbon dioxide  water  energy
Cellular Respiration
Photosynthesis and Cellular Respiration
Photosynthesis
carbon dioxide  water  sugar  oxygen
light energy
sugar  oxygen 
 carbon dioxide  water  energy
Cellular Respiration
What is created in one reaction is used up in the other reaction!
Photosynthesis and Cellular Respiration
Photosynthesis
carbon dioxide  water  sugar  oxygen
light energy
sugar  oxygen 
 carbon dioxide  water  energy
Cellular Respiration
• What is created in one reaction is used up in the other
reaction!
• If one reaction were to stop, they both would come
to a halt.
Plants
• Plants need to go through both processes.
Why?
• Photosynthesis only makes glucose, it still
needs to be broken down for energy
• Since plants don’t eat, they break down some
of the glucose they made from photosynthesis
and create ATP through cellular respiration!
The Photosynthesis Song
Photosynthesis and Cellular
Respiration are Connected
Photosynthesis vs. Respiration video
Photosynthesis Respiration Song
Metabolism – Photosynthesis Day 2
Photosynthesis
The Light Reaction
and
The Calvin Cycle
Photosynthesis Overview Video
The Photosynthetic Equation
•
•
•
6 CO2 + 6 H2O + light energy--> C6H12O6 + 6O2
But not that simple
Involves 3 stages:
1) Capturing light energy
2) Making ATP and NADPH with energy from light
3) Making glucose from carbon dioxide using
energy from ATP and NADPH
Light Reaction
3 Stages
1. Photoexcitation -> absorption of a photon
2. ETC
3. Chemiosmosis
Photosynthesis
• Involves a series of reactions that are directly
energized by light.
• Require chlorophyll and occurs on the
thylakoid membrane in chloroplasts.
• Light energy is absorbed and eventually
transferred to carbohydrates in the last stage
of the process
– Light energy is in the form of a photon (packet of
solar energy)
1) Photoexcitation
• Light is absorbed by chlorophyll or accessory
pigments that are associated with proteins in
clusters.
– These clusters = photosystems
• Two Photosystems
1) Photosystem I - chlorophyll a- called P700
2) Photosystem II- chlorophyll a- called P680
2) ETC - Photosystems
• Plants use Photosystem I and II to make
NADPH and ATP
Photosystem II
• Is composed of proteins and P680 chlorophyll a molecules
(680 because it absorbs 680nm λ)
• Light strikes P680 and excites it (a)  P680* (it’s excited)
• The energized chlorophyll (P680*) then transfers a highenergy electron to acceptor A in the reaction centre (b).
Photosystem II
• The P680 Chlorophyll a molecule is now
oxidized (missing an electron)- how does it get
it back?
• Water is split into 2 protons,1/2 O2 and 2
electrons - these electrons replace the
missing one.
+
+
+
+
-
-
-
• After P680 oxidizes water (steals its electrons), it
becomes neutral again.
• The high-energy electron is transferred from the
reaction centre (A-) to the carrier molecule
plastoquinone (PQ) (c).
Photosystem II
• This process
releases both
oxygen gas and
protons into the
lumen
Linear (non-cyclic) electron transport
• Light hits photosystem II  electron excited  eventually
transferred from Plastoquinone (PQ)  Cytochrome
Complex  Photosystem I
• Electrons pass through components of ETC similar to
cellular respiration. As the electrons are transferred,
protons are pumped from Stroma into thylakoid lumen
creating a concentration gradient.
Our Electron
• By the time our poor electron reaches
Photosystem I, it’s tired and needs a boost
• It gets that from another photon strike at
Photosystem I which re-excites it.
Photosystem I
• The pigment P700 within Photosystem I is also hit by light
which re-excites our electron creating P700*.
• Photosystem I’s excited electron eventually is passed to
enzyme NADP+ reductase which uses H+ in the stroma to
reduce NADP+ to NADPH
– NADPH is used in the second phase of photosynthesis
3) Chemiosmosis
• Protons that accumulate in the thylakoid lumen
form an electrochemical gradient that drives ADP
and Pi into ATP (like the ETC)
– Recall that 3 H+ through an ATP synthase = 1 ATP
– This is called Photophosphorylation – we have created
ATP using energy gathered from sunlight.
Energy Changes within the Electron
The blue line represents the electron
Required Numbers
• 2 e- required to reduce 1 NADPH
• For every 4 e- transferred in the light reaction,
12 H+ are added to the thylakoid lumen  4 ATP
• 3H+ per 1 ATP
– 12/3 = 4
Why is it called non-cyclic?
• Has to do with the electrons….
• It’s a one-way street, electrons leave and
never return home.
The light reaction
Photosynthesis (Light Reactions)
The Light Independent Reaction –
The Calvin Cycle
• Does not require light, only the products from
the Light Dependent (LD) reaction
• In the stroma of the chloroplast, a series of 11
reactions uses NADPH (from the LD reaction) to
reduce CO2 into sugar.
• Reactions are cyclic and similar to the Krebs Cycle
3 stages in the Calvin Cycle
1. Carbon fixation
2. Reduction
3. Regeneration
Phase 1: Carbon Fixation
• CO2 is added to a 5 carbon
molecule of ribulose 1, 5bisphosphate (RuBP) which
splits into two 3 carbon
molecules- 3phosphoglycerate (PGA)
• Reaction occurs 3 times (3
CO2’s used) to make 6 PGAs
• Reaction is catalyzed by the
protein Ribulose 1,5bisphosphate carboxylase
oxygenase (RuBisCO)
Ribulose 1,5-bisphosphate carboxylase
oxygenase (RuBisCO)
• It is arguably the most important enzyme of the biosphere.
• By catalyzing CO2 fixation in all photoautotrophs, it provides the
source of organic carbon molecules for most of the world’s
organisms.
• It begins the conversion of about 100 billion tons of CO2 into
carbohydrates annually.
• There are so many RuBisCO molecules in chloroplasts that the
enzyme makes up 50 % or more of the total protein of plant
leaves.
• As such, rubisco is also the world’s most abundant protein,
estimated to total some 40 million tonnes worldwide
Phase 2: Reduction
• Each of the 6 PGA’s are
phosphorylated by ATP
– (6 ATP used from LD
reaction)
• 6 NADPH reduce the
molecules to 6
glyceraldehyde 3phosphate (G3P)
• 1 G3P molecule is sent
away to be made into
glucose
The lone G3P
• It takes three turns of the
cycle (3 CO2 inputs) to
create 6 G3P molecules
• 1 G3P molecules leaves to
be made into glucose
• Essentially it takes three
turns to create this one
usable G3P molecule
– Therefore, 3 CO2’s to make
1 G3P
The lone G3P
• The production of the 1
G3P is the goal of
photosynthesis
• To create it, the Calvin
cycle uses 6 reduced
NADPH and 9 ATP from
the LD reaction and 3
CO2 molecules
• We need 2 G3P to make
one glucose therefore …
Glucose
• The production of a single glucose molecule
requires the input of 6 CO2, 12 NADPH and 18 ATP
Phase 3: RuBP regeneration
• Remaining 5 molecules
of G3P are rearranged to
make 3 RuBP
• The cycle can now occur
again with the addition
of more CO2
Video 1
Video 2