Transcript Photosynthesis

Organisms Capture & Store Free
Energy for Use in Biological
Processes
Photosynthesis & Cellular Respiration
Anabolic pathway
Catabolic pathway
Heterotrophs
-Capture
free energy present in carbon compounds
produced by other organisms.
Autotrophs
-Capture free energy from physical
sources in the environment.
Photosynthesis
• Plants & other photosynthetic organisms
produce foods that begin food chains.
• The sun is a constant energy source.
– Must be converted into a chemical energy in order
to be useful to all non-photosynthetic organisms.
• Most common chemical energy is glucose
which is also the most common fuel organisms
use for cellular respiration (more on that later)
How does the CO2 get to the
chloroplast?
LET’S COUNT
STOMATA!!!!
Post Lab Questions
Define transpiration
• Transpiration is the process by which moisture is carried through plants from
roots to small pores on the underside of leaves, where it changes to vapor
and is released to the atmosphere.
• evaporation of water from plant leaves.
•
Transpiration also includes a process called guttation, which is the loss of
water in liquid form from the uninjured leaf or stem of the plant, principally
through water stomata.
How do guard cells
open & close stomata?
Why does the density of stomata
differ among plants?
It depends on the environmental conditions, such as:
• Amount of sunlight
• Amount of atmospheric carbon dioxide
concentrations
• Amount of humidity in the environment
At what time of day might more
stomata be closed & justify your
answer.
• Usually plants open most of their
stomata during the day.
• In drier regions, plants usually have
more of their stomata open in order to
reduce loss of water vapor.
Why does the lower epidermis usually have
more stomata than the upper epidermis?
• Many plants are adapted to an environment where the upper surface is
exposed to strong sunlight and higher temperatures and/or where water
is more limited compared to a watery environment.
=more stomata on the bottom than the top
What about other plants?
• Underwater plants are in 100 percent humidity; transpiration does not
occur. So there is no need for water vapor.
= zero stomata
• Plants adapted to an environment where only the upper side of the
leaf is exposed to air; thus, only one surface can exchange water
vapor with the environment.
= lots of stomata on the upper side of the leaves
What does a larger number of leaf
stomata indicate about the
growing climate of that plant?
A large number of stoma indicate that there is an excess
rate of transpiration from the leaves which is an indication
that the plant is in excess water supply
Outer Membrane
Inner Membrane
Stroma
Thylakoid
Where do plants get the resources
they need to make their own food?
What does each resource offer the plant?
Sun  is the energy source used to
drive anabolic/endergonic synthesis of
glucose.
Air  provides the carbon necessary for glucose production.
Soil  water and trace elements come from here.
What are the by-products of photosynthesis?
Oxygen and water
If plants, bacteria & other
autotrophs did not make glucose
from air & sunlight, , how would
the earth’s heterotrophs be
affected?
They would all die once everything on
earth had been eaten, since only
autotrophs can make food.
What pigments do leaves have?
chlorophyll a, chlorophyll b, carotene,
an xanthophyll
LET’S CHECK THEM OUT!!!
ACTION AND ABSORPTION
SPECTRA OF PHOTOSYNTHESIS
Various pigments in photosynthesis absorb photons of light from specific
wavelengths of the visible spectrum.
Which leaves are able to absorb shorter
wavelengths of light? Longer?
short long
SHORTER
LONGER
Overall Process of Photosynthesis
There are two major stages
The Light-Dependent Reaction
The Light-Independent Reaction
(Calvin Cycle)
Write a one-sentence summary,
describe what happens in each
of these phases.
Light Dependent Reactions
• The photon energy of sunlight is captured and
converted to molecules that can be used to
power the second phase; specifically NADPH
& ATP.
Light Independent Reactions
(Calvin Cycle)
• The molecules from the light dependent
reactions are used to build carbon chains from
carbon dioxide
Let’s start with the Light
Reactions
How does a satellite dish bring more TV
stations & better reception to your TV?
The larger the parabola, the more
signals it can gather & bounce on to a
single focus point before it sends the
signal to your TV.
How are the pigments like a satellite dish?
Accessory pigments in the thylakoid membranes train the
collected energy onto a focal point so that the sum total of
its strength is used to excite the electrons on chlorophyll a.
Which pigments are at the focal point?
Which pigments are accessory pigments
surrounding the chlorophyll a?
What are these central chlorophyll a
molecules called?
Photosystems – PS I and PS II
Chlorophyll a
Chlorophyll b & carotenoids
Modern-day plants have 2
photosystems
• Photosystem I
– Most efficient at absorbing wavelengths at 700nm
• Photosystem II
– Most efficient at absorbing wavelengths at 680nm
• These 2 photosytems work together to bring
about a non-cyclic electron transfer.
Light Dependent Reaction
• Occurs in the thylakoids or grana of chloroplast.
• Light is absorbed in the pigments (chlorophylls and
carotenoids) which are organized on the membranes of
the thylakoids.
• The regions of organization are called photosystems
which include:
• Chlorophyll a molecules
• Accessory pigments
• A protein matrix
• The reaction centre is the portion
of the photosystem that contains:
• A pair of cholorophyll molecules
• A matrix of protein
• A primary electron acceptor
If an atom’s electrons are energized, then they can get
so excited they will leave the orbital & jump off the
atom/molecule in a state of high energy
At what point does the electron have the greatest potential energy?
When the electron is in its excited state
Photosystem II
Collects photons from the surrounding pigments embedded in
the thylakoid membranes & uses that gathered energy to
excite two electrons on a chlorophyll a molecule in the reaction
center (also embedded in the thylakoid membrane).
Chlorophyll a now has two excited, high energy electrons
in PS II & needs to capture their kinesthetic energy to
convert it to ATP or NADPH (the 2 energy molecules
needed to fuel the Calvin cycle).
When would be the best
time to capture the
electron’s energy?
PHOTOSYSTEM II
These electrons are captured by the primary
acceptor of the reaction center.
Chlorophyll a is a strong oxidizing agent
when it has lost its electron. What will the
chlorophyll a molecule do now that it is
missing an electron from its orbital?
Water is split by an enzyme to produce
electrons, hydrogen ions, and an oxygen atom.
This process is driven by light energy & is
called photolysis.
The electrons are supplied one by one to the
chlorophyll a molecules of the reaction center.
The leftover oxygen will find another broken water molecule
& become O2 gas (a by-product of photosynthesis).
The excited electrons pass from the primary acceptor down an
electron transport chain (ETC) losing energy at each exchange.
The energy lost from the electrons moving down the ETC drives
chemiosmosis to bring about phosphorylation of ADP to produce ATP
Movement of ions down their electrochemical gradient through a
selectively permeable membrane.
The Calvin cycle needs 18ATP molecules and 12
NADPH molecules for every molecule of glucose
produced.
NADPH is an energy storage/shuttle molecule.
We have just discussed how ATP is generated.
How do you think NADPH is generated?
• PS I captures light energy (nearly the same manner PS II captured
light to generate ATP) & generates an NADPH molecule.
• Chlorophyll a molecule from PS I replaces its missing electrons
with the electrons that came from the electron transport chain
following PS II.
Photophosphorylation
High-energy electrons derived from light
activation of chlorophyll molecules
-no carbon fuel source necessary
-Final electron acceptor is NADPH
• NADPH is not made from a chemiosmotic gradient in the
thylakoids, but instead the electron pair is given to NADP+ directly
to be used in the form of NADPH.
What does the Light Dependent
Reactions Do Overall?
• The production of:
– NADPH (Nicotinamide Adenine Dinucleotide Phosphate Hydrogen)
– ATP (Adenosine Tri-Phosphate)
• Oxygen is given off as a waste product
(lucky for us  ).
NADPH & ATP supply the chemical energy for
the light independent reactions.
Now for the Light Independent
Reactions
AKA Calvin Cycle
Occurs within the stroma of the
chloroplast
Light Independent Reactions
AKA: Calvin Cycle
• This reaction uses the ATP and NADPH produced by
the light dependent reaction.
• We are synthesizing sugar in this reaction.
What are the starting molecules
and the ending molecules?
The process begins with carbon dioxide
binding to ribulose bisphosphate
After three turns of the Calvin cycle, half
a glucose molecule, called G3P, is
produced.
How much energy is used to fuel
this anabolic process?
Sunlight, 9 ATP molecules and 6 NADPH
molecules are used to make one molecule of G3P
What Happens in the Calvin Cycle?
• Ribulose bisphosphate (RbBP), a 5 carbon compound, binds to
an incoming CO2 molecule in a process called carbon fixation.
– RuBP carboxylase (rubisco) catalyzes this fixation.
• An unstable 6 carbon compound is made & then breaks down
to two 3 carbon compounds known as glycerate-3-phosphate.
• These molecules are acted upon by ATP & NADPH (remember
these guys?) to form 2 more compounds called glyceraldehyde
3 phosphate (triose phosphate).
• These molecules may then go in one of two directions.
– Leave the cycle to become sugars
– Continue in the cycle to help reproduce the originating compound RuBP
with the help of ATP
2 of these are made
The overall equation of the Calvin cycle
(black circles represent carbon atoms)
Products of one turn of the Calvin Cycle
• 2 glyceraldehyde-3-phosphate (G3P) molecules, 3
ADP, and 2 NADP+.
– (ADP and NADP+ are not really "products." They are regenerated
and later used again in the light-dependent reactions).
– Each G3P molecule is composed of 3 carbons. In order for the
Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must
be regenerated.
• So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose.
Therefore, there is only 1 net carbon produced to play with for each turn.
– To create 1 surplus G3P requires 3 carbons, and therefore 3 turns
of the Calvin cycle.
– To make one glucose molecule (which can be created from 2 G3P
molecules) would require 6 turns of the Calvin cycle.
Each reaction in this multi-step
process is catalyzed by a reactantspecific enzyme. The 1st enzyme
performs a critical step of
capturing CO2 & “fixing” it so that
it’s committed to entering the
Calvin cycle.
Name this 1st enzyme:
Rubisco (aka RuBP carboxylase) is the enzyme
that binds carbon to ribulose biphosphate.
How does the Calvin cycle
regenerate the starting molecule
ribulose bisphosphate (RuBP)?
The cycle uses a series of reactions and 3
molecules of ATP to regenerate RuBP.
Light-dependent
Light-independent
Occurs in the thylakoids
Occurs in the stroma
Uses light energy to form ATP & NADPH
Uses ATP & NADPH to form
glyceraldehyde 3 phosphate (triose
phosphate).
Splits water (photolysis) to provide
replacement electrons and H+ , & to
release oxygen
Returns ADP, inorganic phosphate &
NADP to the light-dependent reaction.
Includes ETC & photosystems I & II
Involves the Calvin cycle
CHLOROPLAST STRUCTURE &
FUNCTIONS
Chloroplast
Chloroplaststructure
structure
Function
Functionallowed
allowed
Extensive
Extensivemembrane
membranesurface
surfacearea
areaofofthe
the
thylakoids
thylakoids
Allows greater absorption of light by
photosystems
Small
Smallspace
space(lumen)
(lumen)within
withinthe
thethylakoids
thylakoids
Allows faster accumulation of protons to
create a concentration gradient
Stroma
Stromaregion
regionsimilar
similarto
tocytosol
cytosolofofthe
thecell
cell
Allows an area form the enzymes
necessary for the Calvin cycle to work
Double
Doublemembrane
membraneon
onthe
theoutside
outside
Isolates the working pars & enzymes of the
chloroplast from the surrounding cytosol.
It’s time for a simulation!!!!
• Summarize the simulation.
• What is limiting the Calvin cycle?
The amount of
ATP produced.
• What is produced in excess? NADPH
• How can the stroma accumulate more ATP?
By running through the 1st electron transport
chain of the light reactions more often, rather than
running through both electron transport chains an
equal number of times.
Cyclic Photophosphorylation (Cyclic Electron Flow)
• Another way that light dependent reactions produce ATP
– Light is not a limiting factor
– An accumulation of NADPH in the chloroplast
• The additional ATP’s are sent to the Calvin Cycle so it can
proceed more rapidly
What happens if there is a lack of
available CO2?
• Photorespiration occurs.
– O2 binds to RuBP (which has a greater affinity to O2
than CO2) and enters the Calvin cycle.
– The oxygen splits carbon chains but neither ATP nor
glucose are produced by this process.
• ATP is actually consumed.
– Plants can lose as much as 50% of their fixed carbon
through photorespiration.
– It is most likely to occur on hot, dry days when the
stomata close to conserve water and the
concentration of CO2 inside the leaf drops.
How can you explain the evolution
of photorespiration when this
process appears to be expensive and
counterproductive to the survival of
the plant?
Work in a small group to develop a
supporting hypothesis to this
question.
Hypothesis currently favored by
the scientific community:
• It is assumed that rubisco’s flaw is due to the
fact that during the evolution of this ancient
process the amount of oxygen was either
nonexistent or very low.
What type of challenges do you
think this plant might face in its
native habitat?
dehydration
What do plants lose when
their stomata are open,
collecting CO2 ?
water
What part of photosynthesis would
stop if water were unavailable?
• Chlorophyll a would not have an electron
donor, so ATP would not be made and the
Calvin cycle, in turn, would stop.
How has this plant evolved
to conserve water?
• It has a thicker, waxier cuticle
• It has leaves modified to be spines so that
its surface to volume ratio is reduced.
• Many cacti have clear hairs on their surfaces
to reflect sunlight and make an insulated
layer of humidity around the plant.
• Cacti are able to expand greatly when it
rains in order to store water for times of
drought.
CAM plants
CAM plants & photosynthetic
adaptations
• CAM (crassulacean acid metabolism): a temporal
separation of the light (occurring only at light) and
dark reactions (occur anytime it’s convenient).
• Open their stomata at night and keep them closed
during the day.
– Helps plants conserve water.
– Take up CO2 & makes malic acid
– Store the organic acid in vacuoles until morning
– CO2 is taken out of the malic acid & sent to the Calvin
cycle.
C4 plants
C4 plants & photosynthetic
adaptations
C4: a spatial separation of light and dark reactions.
Use 2
different cells
Fixes carbon with the
help of PEP
carboxylase which has
a higher affinity
towards CO2
Makes oxaloacetate
Determine the answers to the
following questions.
• Why are C4 and CAM photosynthesis considered
to be coping mechanisms used by plants living in
arid climates?
• Describe 3 specific differences in the processes of
C4 and CAM compared to the processes that
occur in C3 photosynthesis.
• Do you think C4 and CAM plants photorespirate?
Support your opinion with a scientific argument.
Why are C4 and CAM
photosynthesis considered to be
coping mechanisms used by
plants living in arid climates?
Why are C4 and CAM photosynthesis considered to be
coping mechanisms used by plants living in arid
climates?
C4 plants use PEP to fix carbon, which has a much higher affinity to
carbon dioxide than rubisco, and creates oxyaloacetate to store CO2.
This allows C4 plants to keep their stomata open or partially closed
without losing the ability to fix carbon.
CAM plants keep their stomata closed during the day to
minimize water loss when the sun is hottest.
Describe 3 specific differences in
the processes of C4 and CAM
compared to the processes that
occur in C3 photosynthesis.
Describe 3 specific differences in the processes of C4
and CAM compared to the processes that occur in C3
photosynthesis.
C4 plants use PEP rather than rubisco to fix carbon.
C4 plants have a spatial separation of carbon fixation & the Calvin cycle.
C4 plants use 2 distinct types of mesophyll cells- mesophyll cells for carbon
fixation and bundle sheath cells for the Calvin cycle.
C4 plants store carbon as oxaloacetate.
CAM plants store carbon as an organic acid until it is needed by the Calvin
cycle.
CAM plants have a temporal separation of carbon fixation and the Calvin
cycle.
CAM plants open their stomata during the night and close them during the
day.
Do you think C4 and CAM plants
photorespirate? Support your
opinion with a scientific
argument.
Do you think C4 and CAM plants photorespirate?
Support your opinion with a scientific argument.
C4 plants are less likely to photorespirate because photorespiration takes
place when rubisco is in the presence of higher concentrations of oxygen &
low concentrations of carbon dioxide. In C4 plants the Calvin Cycle occurs
in bundle-sheath cells where the carbon dioxide levels are kept high.
CAM plants are unlikely to lose much of their energy to photorespiration
because these plants maintain a high level of carbon dioxide by fixing
adequate amounts of carbon in organic acids during the night.
Because we see C4 plants and CAM plants dominating arid environments
where photorespiration would normally be very high, it can be assumed that
these plants have more successfully adapted to this particular type of
environmental stress.
Comparing Chemiosmosis in
Respiration vs Photosynthesis
Respiration Chemiosmosis
Photosynthesis Chemiosmosis
Involves an ETC embedded in the membranes
of the cristae
Involves ETC embedded in the membranes
of the thylakoids
Energy is released when electrons are
exchanged from one carrier to another
Energy is released when electrons are
exchanged from one carrier to another
Released energy is used to actively pump
hydrogen ions into the intermembrane space
Released energy is used to actively pump
hydrogen ions into the thylakoid space
Hydrogen ions come from the matrix
Hydrogen ions come from the stroma
Hydrogen ions diffuse back into the matrix
through the channels of ATP synthase
Hydrogen ions diffuse back into the stroma
through the channels of ATP synthase
ATP synthase catalyses the oxidative
phosphorylation of ADP to ATP
ATP synthase catalyses the
photophosphorylation of ADP to form ATP