1. Photosynthesis: An Overview
The net overall equation for
photosynthesis is:
Photosynthesis occurs in 2 “stages”:
1. The Light Reactions (or Light-Dependent
Reactions)
2. The Calvin Cycle (or Calvin-Benson Cycle
or Dark Reactions or Light-Independent
Reactions)
1
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
2. Photosynthesis: An Overview
To follow the energy in photosynthesis,
2
light
light ATP
NADPH
Light
Reactions
thylakoids
Calvin
Cycle
stroma
Organic
compounds
(carbs)
3. Phase 2: The Calvin Cycle
In the Calvin Cycle, chemical energy (from the
light reactions) and CO2 (from the atmosphere)
are used to produce organic compounds (like
glucose).
The Calvin Cycle occurs in the stroma of
chloroplasts.
3
4. Phase 2: The Calvin Cycle
The Calvin Cycle involves the process of
carbon fixation.
• This is the process of assimilating carbon from a
non-organic compound (ie. CO2) and
incorporating it into an organic compound (ie.
carbohydrates).
4
CARBON FIXATION
5. Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
3 molecules of CO2 (from the atmosphere)
are joined to 3 molecules of RuBP (a 5-carbon
sugar) by Rubisco (an enzyme also known as
RuBP carboxylase)
5
C C
C C
C
C C
C C
C
C C
C C
C
C
C
C
3 carbon dioxide
molecules
3 RuBP molecules
Rubisco
This forms 3
molecules
which each
have 6 carbons
(for a total of 18
carbons!)
6. Phase 2: The Calvin Cycle
Step 2: Reduction
The three 6-carbon molecules (very unstable)
split in half, forming six 3-carbon molecules.
These molecules are then reduced by gaining
electrons from NADPH.
ATP is required for this molecular
rearranging
6
C C
C C
C C
C C
C C
C C
C C
C C
C C
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
NADPH
NADP+
ATP ADP P
Where did the NADPH and
ATP come from to do this?
7. Phase 2: The Calvin Cycle
There are now six 3-carbon molecules, which
are known as G3P or PGAL.
Since the Calvin Cycle started with 15 carbons
(three 5-carbon molecules) and there are now
18 carbons, we have a net gain of 3 carbons.
7
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
Where did these 3 extra
carbons come from?
• One of these “extra” 3-
carbon G3P/PGAL
molecules will exit the
cycle and be used to form
½ a glucose molecule.
8. Phase 2: The Calvin Cycle
Once the Calvin Cycle “turns” twice (well,
actually 6 times), those 2 molecules of G3P (a
3-carbon carbohydrate) will combine to form
1 molecule of glucose (a 6-carbon
carbohydrate molecule) OR another organic
compound.
8
C
C C
G3P
(from 3 turns of
the Calvin Cycle)
C C
C
G3P
(from 3 turns of
the Calvin Cycle)
C
C C C C
C
glucose
9. Phase 2: The Calvin Cycle
Step 3: Regeneration of RuBP
Since this is the Calvin Cycle, we must end
up back at the beginning.
The remaining 5 G3P molecules (3-carbons
each!) get rearranged (using ATP) to form 3
RuBP molecules (5-carbons each).
9
C C
C
C C
C
C C
C
C C C
C
C C
5 G3P molecules
Total: 15 carbons
3 RuBP molecules
Total: 15 carbons
Where does the ATP
come from to do this?
ATP
ADP
P
10. Phase 2: The Calvin Cycle
ORGANIC
COMPOUND
NADPH
NADP+
ATP
ADP
P
RuBP
CO2
12. Phase 2: The Calvin Cycle
Quick recap:
In the Calvin Cycle, energy and electrons from the
Light Reactions (in the form of ATP and NADPH)
and carbon dioxide from the atmosphere are used to
produce organic compounds.
The Calvin Cycle occurs in the stroma inside the
chloroplasts (inside the cells…).
Carbon dioxide, ATP, and NADPH are required
(reactants).
Organic compounds (G3P) are produced
(products).
12
13. Photosynthesis: A Recap
So, as a broad overview of photosynthesis,
• The Light Reactions (Phase 1) capture the
energy in sunlight and convert it to chemical
energy in the form of ATP and NADPH
through the use of photosystems, electron
transport chains, and chemiosmosis.
• The Calvin Cycle (Phase 2) uses the energy
transformed by the light reactions along with
carbon dioxide to produce organic compounds.
13
14. Photosynthesis: A Recap
14
Based on this equation,
how could the rate of
photosynthesis be
measured?
The photosynthetic equation:
light
Excites
electrons
during the
light
reactions
6 H2O
Split during the
light reactions
to replace
electrons lost
from
Photosystem II
6 CO2
Provides the carbon to
produce organic
compounds during the
Calvin Cycle
Produced as a
byproduct of the
splitting of
water during the
light reactions
6 O2 C6H12O6
The organic compound
ultimately produced
during the Calvin Cycle
15. Environmental Factors & Photosynthesis
The rate (or speed) of photosynthesis can
vary, based on environmental conditions.
• Light intensity
• Temperature
• Oxygen concentration
15
16. Environmental Factors & Photosynthesis
Light intensity
• As light intensity increases, so too does the rate of
photosynthesis.
16
• This occurs due to increased
excitation of electrons in the
photosystems.
• However, the photosystems
will eventually become
saturated.
• Above this limiting level, no
further increase in
photosynthetic rate will occur.
light
saturation
point
17. Environmental Factors & Photosynthesis
Temperature
• The effect of temperature on the rate of
photosynthesis is linked to the action of enzymes.
• As the temperature increases up to a certain
point, the rate of photosynthesis increases.
17
• Molecules are moving faster &
colliding with enzymes more
frequently, facilitating chemical
reactions.
• However, at temperatures
higher than this point, the rate
of photosynthesis decreases.
• Enzymes are denatured.
18. Environmental Factors & Photosynthesis
Oxygen concentration
• As the concentration of oxygen increases, the
rate of photosynthesis decreases.
• This occurs due to the phenomenon of
photorespiration.
18
19. Photorespiration
Photorespiration occurs when Rubisco (RuBP
carboxylase) joins oxygen to RuBP in the first
step of the Calvin Cycle rather than carbon
dioxide.
• Whichever compound (O2 or CO2) is present in higher
concentration will be joined by Rubisco to RuBP.
• Photorespiration prevents the synthesis of glucose AND
utilizes the plant’s ATP.
19
More CO2
More O2
Rubisco joins
CO2 to RuBP
Rubisco joins
O2 to RuBP
Photosynthesis
occurs; glucose is
produced
Photorespiration
occurs; glucose is
NOT produced
20. Photorespiration
Photorespiration is primarily a problem for
plants under water stress.
• When plants are under water stress, their stomata
close to prevent water loss through transpiration.
• However, this also limits gas exchange.
O2 is still being produced (through the light reactions).
20
• Thus, the concentration of O2
is increasing.
• CO2 is not entering the leaf since
the stomata are closed.
• Thus, as the CO2 is being used
up (in the Calvin Cycle) and not
replenished, the concentration
of CO2 is decreasing.
21.
22. Photorespiration
As the concentration of O2 increases and the
concentration of CO2 decreases (due to the
closure of the stomata to prevent excessive water
loss), photorespiration is favored over
photosynthesis.
Some plant species that live in hot, dry climates
(where photorespiration is an especially big
problem) have developed mechanisms through
natural selection to prevent photorespiration.
• C4 plants
• CAM plants
22
23. C3 Plants
C3 plants, which are “normal” plants,
perform the light reactions and the Calvin
Cycle in the mesophyll cells of the leaves.
23
• The bundle sheath cells of
C3 plants do not contain
chloroplasts
palisade mesophyll
spongy mesophyll
bundle sheath cells
24. C3 Photosynthesis/Plants
Called C3 because the CO2 is first
incorporated into a 3-carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in
photosynthesis, is also the enzyme
involved in the uptake of CO2.
Photosynthesis takes place throughout
the leaf.
Adaptive Value: more efficient than
C4 and CAM plants under cool and
moist conditions and under normal light
because requires less machinery (fewer
enzymes and no specialized anatomy)..
Most plants are C3.
25. C4 and CAM Plants
C4 plants and CAM plants modify the process
of C3 photosynthesis to prevent
photorespiration.
Overview:
• C4 plants perform the Calvin Cycle in a different
location within the leaf than C3 plants.
• CAM plants obtain CO2 at a different time than
C3 plants.
• Both C4 and CAM plants separate the initial
fixing of CO2 (carbon fixation) from the using of
CO2 in the Calvin Cycle.
25
26. Leaf Anatomy C3 vs C4
The C4 plants have two rings of cells surrounding their vascular bundles.
The inner ring is called the Bundle Sheath cell which contains starch rich
chloroplasts that do not have grana that is in the outer layer of mesophyll
cells. This particular anatomy is called the kranz anatomy. The function of
it is to provide an area where the CO2 can be concentrated around the
Rubsico, and by doing this photoresperation is reduced.
27. C4 Plants: Preventing Photorespiration
Plants that use C4 photosynthesis include corn,
sugar cane, and sorghum.
In this process, CO2 is transferred from the
mesophyll cells into the bundle-sheath cells,
which are impermeable to CO2.
27
• This increases the concentration of
CO2.
• Thus, the Calvin Cycle is favored
over photorespiration.
• The bundle-sheath cells of C4
plants do contain chloroplasts.
28. C4 Plants: Preventing Photorespiration
C4 plants use the Hatch-Slack
pathway prior to the Calvin
Cycle:
• PEP carboxylase adds carbon dioxide
to PEP, a 3-carbon compound, in the
mesophyll cells.
This produces a 4-carbon
compound (which is why it’s
known as C4 photosynthesis).
• This 4-carbon molecule then moves
into the bundle-sheath cells via
plasmodesmata.
28
• In the bundle sheath cells, the CO2 is released and the
Calvin Cycle begins.
29. C4 Photosynthesis/Plants
Called C4 because the CO2 is first incorporated into a 4-carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme
allows CO2 to be taken into the plant very quickly, and then it "delivers" the
CO2 directly to RUBISCO for photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz
Anatomy)
Adaptive Value:
Photosynthesizes faster than C3 plants under high light intensity and high
temperatures because the CO2 is delivered directly to RUBISCO, not allowing it
to grab oxygen and undergo photorespiration.
Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster
and so does not need to keep stomata open as much (less water lost by
transpiration) for the same amount of CO2 gain for photosynthesis.
C4 plants include several thousand species in at least 19 plant families. Example:
fourwing saltbush pictured here, corn, and many of our summer annual plants.
30. CAM Plants: Preventing Photorespiration
Plants that use CAM photosynthesis include
succulent plants (like cacti) and pineapples.
In CAM (crassulacean acid metabolism)
photosynthesis, plants open their stomata at
night to obtain CO2 and release O2.
• This prevents them from drying out by keeping
their stomata closed during the hottest & driest
part of the day.
30
31. When the stomata are opened at night, the CO2 is
converted to an organic acid (via the C4 pathway) and
stored overnight.
During the day – when light is present to drive the
Light Reactions to power the Calvin Cycle – carbon
dioxide is released from the organic acid and used in
the Calvin Cycle to produce organic compounds.
Remember:
31
• Even though the CO2 is
taken in at night, the Calvin
Cycle cannot occur because
the Light Reactions can’t
occur in the dark!
CAM Plants: Preventing Photorespiration
33. CAM Photosynthesis/Plants
Called CAM after the plant family in which it was first found (Crassulaceae) and
because the CO2 is stored in the form of an acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower) and are usually
closed during the day. The CO2 is converted to an acid and stored during the night.
During the day, the acid is broken down and the CO2 is released to RUBISCO for
photosynthesis
Adaptive Value:
Better Water Use Efficiency than C3 plants under arid conditions due to
opening stomata at night when transpiration rates are lower (no sunlight, lower
temperatures, lower wind speeds, etc.).
May CAM-idle. When conditions are extremely arid, CAM plants can just
leave their stomata closed night and day. Oxygen given off in photosynthesis is
used for respiration and CO2given off in respiration is used for photosynthesis.
This is a little like a perpetual energy machine, but there are costs associated
with running the machinery for respiration and photosynthesis so the plant
cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry
spells, and it allows the plant to recover very quickly when water is available
again (unlike plants that drop their leaves and twigs and go dormant during dry
spells).
CAM plants include many succulents such as cactuses and agaves and also some
orchids and bromeliads
34. Avoiding Photorespiration
Both C4 and CAM plants – which are primarily found in hot,
dry climates – have evolutionary adaptations which help
prevent photorespiration.
C4 plants perform the Calvin Cycle in the bundle-
sheath cells.
CAM plants open their stomata at night and store the CO2 until
morning.
34
Editor's Notes
Photosynthesis is an endergonic reaction because it requires an input of energy to occur; that energy comes in the form of light.
This is a review from the previous PowerPoint, to activate students’ prior knowledge.
It cannot be overemphasized that energy is not MADE, but is instead transformed through the processes of photosynthesis and cellular respiration. Each of the “boxes” (light/ATP & NADPH/organic compounds) contains energy, but in a different form. Photosynthesis is a process that converts energy from an “un-usable form” (light) into a “usable form” (organic compounds), and requires an intermediate step (ATP/NADPH).
This is a review from the previous PowerPoint, to activate students’ prior knowledge.
- Only the carbons are shown in this diagram for clarity, though oxygen and hydrogen are also present. Also, the carbon atoms shown in red and those shown in black are identical, but are color-coded to show where they come from (red are CO2 from the atmosphere, black are the carbons in RuBP).
- Emphasize to students that the NADPH and ATP required to perform these reactions as part of the Calvin Cycle were produced during the light reactions.
Emphasize to students that these 3 extra carbons came from the carbon dioxide, which was obtained through the stomata from the atmosphere; these 3 additional carbons are denoted in red.
Remember to emphasize that this is the Calvin Cycle; we end up where we began. So, since we started with 15 carbons, we will also return to 15 carbons.
- We say that the Calvin Cycle turns twice to make one molecule of glucose (6-carbons), but really it turns 6 times; each entering carbon dioxide molecule represents one “turn” of the cycle, and 6 carbon dioxide molecules must be incorporated into organic compounds in order for one 6-carbon glucose molecule to be produced. This PowerPoint (along with many textbooks) shows 3 carbon dioxide molecules entering together for clarity (it’s hard to show 1/3 of a G3P molecule as the product of each turn of the cycle).
- Glucose is what we usually think of as being the major product of photosynthesis; however, G3P (also known as PGAL) is the real product, and though it is often used to make glucose it can also be used as a carbon skeleton to form other organic molecules.
Emphasize to students that the other G3P molecule has left the cycle and was used to form glucose (or other organic compounds).
The ATP required to do this rearranging comes from the ATP generated during the light reactions.
- This simple schematic diagram gives a basic overview of what occurs during the Calvin Cycle. Carbon dioxide enters the cycle from the atmosphere and is joined to RuBP by Rubisco. NADPH and ATP are used to “turn” the cycle, and organic compounds (such as G3P/PGAL) are produced.
- Though the AP curriculum framework very clearly states that students do not need to memorize the steps in the Calvin cycle, the structure of the molecules and the names of the enzymes involved (except for ATP synthase), some students may find this diagram helpful in understanding the cyclical nature of the Calvin cycle.
Emphasize to students the importance of understanding how and when each component of the photosynthetic equation is used; this is much more valuable (and less intimidating!) than simply having them memorize the equation!
Most realistically, the rate of photosynthesis could be measured by using the:
Decrease in environmental CO2 (in a closed system)
Increase in environmental O2 (in a closed system)
Increase in glucose (perhaps measured using radioactive carbon)
- Emphasize to students what it means to be saturated – as “full” as an item can be, or at its full capacity. A sponge is a good example to illustrate saturation; if a sponge is fully saturated with water, it can be left in a bucket of water overnight and will not gain any more water. In the same way, electrons in photosystems can be excited more often as light intensity increases, but eventually a “maximum” rate of excitation will be achieved; increasing light intensity beyond this point of light saturation will not yield an increase in photosynthetic rate. Be certain students don’t confused “stopped increasing the rate” with “ceases”!
-Ask students what type of “situation” is pictured here. They should answer an “optimum” situation is represented by this graph.
-This is an excellent opportunity to review the structure & function of enzymes as 3-D proteins with three or four levels of structure (primary, secondary, tertiary, quaternary) that are subject to external stresses such as temperature extremes. Emphasize to students the increased rate of molecular motion as temperatures increase, as well as the process of denaturation on protein structure and the resultant loss of molecular function.
- Emphasize to students the enzymes involved in photosynthesis, even though their names and specific functions do not need to be memorized. NADP+ reductase, Rubisco, and ATP synthase are all examples of enzymes involved in the process of photosynthesis.
- Photorespiration is a negative process for photosynthetic organisms.
Again, emphasize to students that photorespiration is unfavorable for photosynthetic organisms. It consumes ATP and does not produce glucose; the strong selective pressure against photorespiration has favored the proliferation of adaptations that increase the evolutionary fitness of those organisms who possess these adaptations.
This is another opportunity to stress how evolutionary adaptations come to exist. A mutation occurs, which may increase or decrease an organism’s chance of survival. If the mutation allows the organism that possesses it to reproduce more than other members of his/her/its population, the mutation will be favored through natural selection and will become more common in the population as organisms that possess the favorable mutation (adaptation) survive and reproduce at higher rates than members of the population which do not possess this adaptation.
Both C4 and CAM plants fix CO2 with an enzyme other than Rubisco (both use PEP carboxylase) so they are able to fix CO2 in spite of the relatively high concentrations of O2. Then they use that CO2 separately in a normal Calvin cycle.
- Remember, Rubisco will join whichever compound is present in highest concentration (O2 or CO2) to RuBP; by shuttling CO2 into the bundle-sheath cells from which CO2 cannot escape, the concentration of CO2 is increased, which leads to the joining of CO2 to RuBP and the resultant production of organic compounds through the Calvin Cycle.
- The Hatch-Slack pathway is described on this slide, and in the accompanying diagram. The movement of carbon dioxide into the bundle sheath cells from the mesophyll cells through its binding to PEP by PEP carboxylase facilitates the “stockpiling” of CO2 in the bundle-sheath cells and favors the Calvin Cycle over photorespiration.
- Emphasize to students that the Calvin Cycle is not performed at night by CAM plants (or any other!). It is impossible for the Calvin Cycle to occur while it is dark because ATP and NADPH (from the Light Reactions) are required to run the Calvin Cycle. Instead, CAM plants store their CO2 as part of malic acid overnight until it can be released and used when the Light Reactions start again during the lighted hours.
- An excellent overview of the process of CAM photosynthesis
- Again, stress the location difference of the Calvin Cycle between C3 and C4 plants, and the temporal difference of the uptake of CO2 between C3 and CAM plants. Both mechanisms are adaptations that promote adequate CO2 levels to promote the Calvin Cycle over photorespiration while preventing desiccation.
