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.
Have the ability to
metabolize
carbohydrates, lipids,
and proteins by
hydrolysis as sources
of free energy.
Autotrophs
Capture free energy
from physical sources
in the environment.
-
Chemosynthesis
can occur in the
absence of oxygen
Let’s look at Photosynthesis 1st
• 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)
Where in the plant does photosynthesis occur?
Outer Membrane
Inner Membrane
Stroma
Thylakoid
Chloroplasts are said to have been
bacteria that had a symbiotic
relationship with a eukaryotic cell after
the ancient eukaryotic cell engulfed it
CHLOROPLAST STRUCTURE &
FUNCTIONS
Chloroplast
Chloroplaststructure
structure
Function
Functionallowed
allowed
Extensive membrane
Thylakoids
surface area of the
thylakoids
Extensive membrane surface area 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 region similar
Stroma
to cytosol of the cell
Region similar to cytosol of the cell. Allows
an area to form the enzymes necessary for
the Calvin cycle to work
Double membrane on the outside
outside=
chloroplast envelope
Isolates the working parts & enzymes of the
chloroplast from the surrounding cytosol.
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.
Overall Process of Photosynthesis
How does the water get to the
chloroplast?
How does the CO2 get to the
chloroplast?
Houses most of
the chloroplasts
Let’s Look at some stomata
• Go on a hunt for leaves.
– Try to find 3 different leaves
– Make sure each type of leaf has a width
• Stay away from pine like leaves.
All arrows
are pointing
to stomata
Post Lab Questions
How do guard cells
open & close
stomata?
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, hotter regions, plants usually
have more of their stomata open at
night 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
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
Define transpiration
• Transpiration is the process by which moisture is carried through plants from
roots to the stomata, 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.
What 2 gases move in & out of the
leaf stomata?
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
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.
Why do leaves change
colors in the fall?
History of Photosynthesis
Originated in
prokaryotes.
Scientific evidence
supports that
prokaryotic
photosynthesis was
responsible for the
production of an
oxygenated
atmosphere.
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
•Chlorophylls absorb free energy from light,
boosting electrons to a higher energy level in
photosystems I & II
• Photosystem I
– Most efficient at absorbing wavelengths at 700nm
• Photosystem II
– Most efficient at absorbing wavelengths at 680nm
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
Photosystems I & II are connected by the
transfer of higher free energy electrons
through an electron transport chain (ETC).
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
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).
Electron Transport Chain (ETC)
Electrons are transferred between molecules in a sequence of
reactions as they pass through the ETC.
An
electrochemical
gradient of
hydrogen ions
(protons) is
established
across the
thylakoid
membrane
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 (aka Calvin
cycle).
Cyclic Photophosphorylation (Cyclic Electron Flow)
• Since the Calvin Cycle uses more ATP than NADPH sometimes
this type of electron flow is necessary.
• The accumulation of NADPH will trigger the shift from non
cyclic to cyclic photophosphorylation
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 CO2 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?
Calvin Cycle Song
Ribulose Biphosphate
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 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/ Calvin Cycle
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
It’s time for a simulation!!!!
• Summarize the simulation.
• What is limiting the Calvin cycle?
• What is produced in excess?
The amount of
ATP produced.
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.
Rubisco...friend with a bad habit
• Rubisco is so important to plants that it makes up 30%
or more of the soluble protein in a typical plant leaf.
• But rubisco also has a major flaw (bad habit): instead of
always using CO2, as a substrate, it sometimes picks up
O2instead.
What happens if O2 “hooks up”
with Rubisco?
• Photorespiration occurs.
– O2 binds to RuBP (which has a greater affinity to
oxygen) and enters the Calvin cycle.
– The oxygen splits carbon chains
– glucose is not produced by this process.
• ATP is consumed.
•Plants can lose as much as 50% of their fixed carbon
through photorespiration.
What determines which molecule is
"chosen"?
• Two key factors:
the relative concentrations of O2, & CO2
– the temperature.
• When a plant closes its stomata—for instance, to
reduce water loss by evaporation, O2 from
photosynthesis builds up inside the leaf.
–
• Photorespiration increases due to the higher ratio of O2
to CO2
• Rubisco has a higher affinity for O2, when temperatures
increase.
How can you explain the evolution
of photorespiration when this
process appears to be expensive
and counterproductive to the
survival of the plant?
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.
The bottom line is that hot, dry
conditions tend to cause more
photorespiration.
What about plants in areas that are
always like this?
Through the process of natural
selection….
Beneficial features that showed up
CAM plants such as pineapples &
cacti
• CAM (crassulacean acid metabolism): at night, they
open their stomata, allowing CO2 to diffuse into the
leaves.
– Helps plants conserve water.
– Take up CO2
– oxaloacetate by PEP carboxylase (the same step
used by C4)
– Store the organic acid in vacuoles until morning
– CO2 is taken out of the malic acid & sent to the
Calvin cycle.
CAM
plants
C4 plants & photosynthetic
adaptations
• The light-dependent reactions and the
Calvin cycle are physically separated.
– The light-dependent reactions occurring in
the mesophyll cells
– the Calvin cycle occurring in special cells
around the leaf veins. These cells are called
bundle-sheath cells
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
only an affinity towards
CO2
Makes oxaloacetate
a 4 C molecule
C4 plants
Because the mesophyll cells constantly pump CO2 into
neighboring bundle-sheath cells in the form of malate,
there’s always a high concentration CO2 relative to O2 right
around rubisco. This strategy minimizes photorespiration.
Summarize the difference between
C4 and CAM Plants
● C​4 plants minimize photorespiration by separating CO2,
fixation and the Calvin cycle in space, performing these
steps in different cell types.
● Crassulacean acid metabolism (CAM) plants
minimize photorespiration and save water by separating
these steps in time, between night and day.
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?
C4 plants use PEP to fix carbon, which has a much higher affinity to
carbon dioxide than rubisco. This allows C4 plants to keep their stomata
closed 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.
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.
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.
How would you determine if a
plant was a C3, C4, or CAM plant?
What about plants in hot
environments today?
Besides having a problem with
rubisco because of the hot
temperatures, what other problems
do they face?
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.