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Cellular Respiration &
Photosynthesis
AP Biology
Photosynthesis
• Photosynthesis is the process of converting
energy from sunlight into energy in chemical bonds:
• Autotrophs are able to make their own food
(glucose)
– Ex: Plants, mosses, some protists, kelp, algae,
cyanobacteria
• The process of photosynthesis occurs in the
chloroplast an organelle in autotrophs.
• Chloroplasts have
– sac structures called thylakoids. (A stack of thylakoids
is called a granum (grana))
– fluid where sugars are made called stroma
• The light reactions of photosynthesis occur on the
thylakoid membranes- the dark reactions occur in the
stroma.
Light
• Light is unique in that it acts as a wave, but also acts as a
particle (called a photon).
• Plants utilize blue and red light, but reflect green light (which
is why they appear green to us)
Pigments
• A pigment molecule is able to absorb energy from light within a narrow
range of wavelengths. The process of photosynthesis begins with lightabsorbing pigments in plant cells.
• Because of this, plants use a variety of pigments to absorb different
wavelengths. These include:
Chlorophyll a- absorbs violet and
red (reflects green and yellow)
Chlorophyll b- absorbs violet and
red (reflects green and blue)
The carotenoids- reflect red,
orange and yellow *accessory
pigment
Xanthophylls- reflect yellow,
brown, blue and purple*accessory
pigment
Pigments in action
• When light is absorbed into one of these pigments the
energy from the light is incorporated into electrons within
the atoms of that pigment molecule.
• These energized/excited electrons are unstable and
almost immediately re-emit the absorbed energy.
• This energy bounces from one pigment molecule to
another.
• The process ends when the energy is absorbed by one of
two special chlorophyll a molecules
• Together with other pigments, these pigments form
clusters called photosystems. Photosystem I and
Photosystem II.
A Photosystem
https://www.youtube.com/watch?v=mYbMPwmwx88
Photosynthesis
• Photosynthesis is divided into two stages:
1. The light reactions- occur on the thylakoid membranes. Convert solar
energy into chemical energy. The light reactions are divided into two
processes- cyclic electron flow and noncyclic electron flow.
2. Calvin Cycle- occurs in the stroma and produces sugar.
https://www.youtu
be.com/watch?v=
wJDlxp17rY4
Light Reaction- Noncyclic Photophosphorylation
• Photophosphorylation- the process of making ATP from ADP
and inorganic phosphate (Pi) using energy from light.
– Each photosystem is directly next to an electron transport
chain located within the thylakoid membrane
Light Reaction- Noncyclic Photophosphorylation
Noncyclic electron flow begins with photosystem II and follows these steps:
1. Photosystem II- electrons trapped are energized by light.
2. Primary electron acceptor- two energized electrons are passed to a
molecule called a primary electron acceptor.
3. Electron transport chain- those electrons are passed from one protein to
another in a chain.
4. Phosphorylation- as the electrons move “down” the chain they lose
energy. The energy lost by the electrons is used to phosphorylate (add a
phosphate to) ATP molecules
5. Photosystem I- The electron transport chain terminates with photosystem
I. The electrons are again energized by sunlight and passed to another
primary electron acceptor NADP to make NADPH.
Light Reaction- Noncyclic Photophosphorylation
6. NADPH- the electrons pass through a short electron
transport chain. At the end of the chain, the electrons
combine with NADP+ and H+ to form NADPH. NADPH is a
coenzyme. Since the electrons have a considerable amount
of energy left, NADPH is an energy-rich molecule.
7. Photolysis- the electrons that originated in photosystem II
have now been incorporated in NADPH. The loss of these
two electrons from photosystem II is replaced when water is
split into two electrons (2 H+ and 1/2 02) This is why water is
needed and oxygen is produced from photosynthesis.
In summary, photophosphorylation take the energy in light and
electrons in water to make the energy rich molecules ATP
and NADPH:
Water + ADP + phosphate + NADP + light--> ATP + NADPH +
oxygen
Light Reaction- Noncyclic Photophosphorylation
Cyclic photophosphorylation
• Only 1 photosystem (P700) allows
excited electron to travel back to PS1
• Only ATP is produced
• Photolysis is absent; oxygen is not
evolved
• Predominant in bacteria
Light Reaction- Cyclic Photophosphorylation
• A second photophosphorylation
sequence occurs (in green plants
also) when the electrons
energized in photosystem I are
“recycled” due to excess NADPH
• Energized electrons join with
protein carriers and generate ATP
as they pass through another
electron transport chain. The
electrons then return to
photosystem I (in a circle).
• This process is necessary
because the calvin-benson cycle
requires more ATP than NADPH.
Light Independent ReactionCalvin Benson Cycle
Uses NADPH and ATP to convert carbon dioxide into sugar.
Occurs in the stroma of the chloroplasts.
Carbon fixation- CO2 is attached to the protein RuBP- this
reaction is catalyzed by the enzyme rubisco. This molecule
and others cycle through 5 more times each time adding
more CO2 until glucose is complete.
– H comes from NADPH
– ATP is required
– Glucose is stored as starch (and later converted to sucrose and
distributed to stems leaves and roots)
Light Independent ReactionCalvin Benson Cycle
Calvin Cycle
• In summary, the calvin benson cycle takes carbon
dioxide from the atmosphere and energy from ATP
and NADPH to create a glucose molecule. The energy
in ATP and NADPH was captured from the sun
through photophosphorylation (cyclic and noncyclic):
• 6CO2 + 18 ATP + 12 NADPH --> 18 ADP + 12 NADP
+ 1 glucose
• Each step of the calvin cycle is catalyzed by a specific
enzyme.
Chemiosmotic Theory
• Chemiosmotic theory describes the process by which ADP is
phosphorylated to ATP:
– 1. H+ ions (protons) accumulate inside the thylakoids. The
H+ ions come from photolysis during the light reactions.
– 2. An electrical gradient is created across the thylakoid
membrane as H+ is concentrated in the thylakoid space.
– 3. As protons pass through the enzyme ATP synthase
(embedded in the thylakoid membrane) the enzyme uses the
energy to phosphorylate ATP from ADP.
Chemiosmotic Theory
Types of Photosynthesis
• The plants that
photosynthesize in the
methods we’ve
discussed so far are
called C3 plants. In
these plants, on hot
and dry days their
stomata close to
prevent water loss.
– Oxygen builds up.
– CO2 levels decrease.
– This causes rubisco to
bind to oxygen instead
of carbon dioxide
Photorespiration
• Rubisco- the most abundant protein/enzyme on Earth.
• In addition to being able to fix carbon dioxide, it can
also fix oxygen if the oxygen levels are too high in a
plant.
• This leads to O2 and RuBP reacting to produce waste
products which are broken down by peroxisomes.
• This is called photorespiration.
C4 Plants
• C4 plants- have an
alternative method of
carbon fixation evolved to
prevent excess water loss
in hot, arid climates.
• C4 plants use PEP
carboxylase, which has a
higher affinity for CO2, to fix
CO2 into a 4 carbon
compound (oxaloacetic
acid- OAA) and have a
unique leaf anatomy.
C4 Plants
• C4 plants have two types of
photosynthetic cells.
• 1. Bundle Sheath cells: are where the
Calvin cycle occurs.
• 2. Mesophyll cells-contain chloroplasts:
where carbon dioxide is fixed. The bound
CO2 then moves into Bundle sheath cells
through plasmodesmata. This keeps CO2
levels high in Bundle sheath cells so that
rubisco accepts it even when stomata are
closed.
• Examples: tropic grasses
CAM Plants
• CAM- have an alternative method of carbon fixation
have evolved to prevent excess water loss in hot, arid
climates.
• Crassulacean acid metabolism (CAM- named for the
type of plant in which it was discovered) has a
pathway similar to C4 plants by fixing CO2 using PEP
carboxylase and storing it into a 4 carbon sugar.
They are unique because they
reverse the opening of their
stomata. They open at night and
close them during the day (a shift
in the diurnal behavior).
Examples: Succulents and many
cacti
Classwork
• Generate a 3 circle Venn diagram to
compare and contrast C3, C4 and CAM
plants.
Cellular Respiration
Ch 6 Bat book
Ch 7 Babboon book
• With your group of 3 and Bat book
– Person 1: read 6.1 p90 and answer Q: How is your
breathing related to your cellular respiration?
– Person 2: read 6.2 p90-91 and answer Q: Why are
sweating and other body-cooling mechanisms
necessary during vigorous exercise?
– Person 3: read 6.3 p91 and answer Q: Walking at 3
mph, how far would you have to travel to “burn off”
a slice of pizza (475 kcal)? How long would it take?
Cellular Respiration
• In class activity: Biology Flower text
• Group 1: read Overview and Figure 9.1
and summarize (responsible for recording)
• Group 2: read 9.1- Catabolic Pathways
and Production of ATP and summarize
• Group 3: read 9.1- Redox Reactions- the
Principle of Redox and summarize
• Group 4: read 9.1- Redox ReactionsOxidation of Organic Fuel and summarize
• Group 5x2: read 9.1- Redox ReactionsStepwise Energy Harvest and summarize
Cellular Respiration
Ch 6 Bat book
Ch 7 Babboon book
• Respiration extracts stored energy from glucose to form ATP
(from ADP and Pi) in a series of steps:
– Chemical Equation:
C6H12O6 + 6 O2  6 H2O + 6 CO2 + energy
glucose
+
oxygen 
water
+
carbon dioxide + (ATP)
• These steps couple energy-releasing (exergonic) chemical
reactions to energy-storing (endergonic) ones. As hydrogen
changes places as H+ and e- energy is transferred.
Hydrogen Transfer and Carriers
• Oxidation-Reduction reaction (redox rxn)Movement of electrons from one molecule to
another.
– Oxidation/oxidized: the loss of electrons
– Reduction/reduced: the addition of electrons
In cellular respiration
glucose loses e- while
LEO the lion says GER
oxygen gains e(all transported by H)
– Examples of e- carriers
• NAD+ and NADH (nicotinamide adenine dinucleotide)
• Cellular respiration occurs in the
mitochondria. The mitochondria
has two membranes. The inner
membrane is highly folded- the
folds are called christae. The inside
of the mitochondria is called the
matrix and the space between the
two membranes is called the
intermembrane space.
Respiration in the presence of oxygen is called
aerobic respiration. It’s divided into three steps:
1. Glycolosis
2. Krebs Cycle
3. Oxidative Phosphorylation/Electron
Transport Chain
Cellular Respiration
Glycolysis
• Glycolysis is breaking down glucose into the molecule
pyruvate. Nine intermediate products are formed and
enzymes catalyze each step.
• Glucose--> 2 pyruvate molecules
• Glucose--> (net) 2 ATP + 2 NADH molecules
• Releases < 25% of the energy in glucose- rest is stored in
pyruvate molecules
• No CO2 is released / No O2 is required
• Takes place in the cytosol of the cell- outside the
mitochondria
• VIDEO
http://www.youtube.com/watch?v=3GTj
QTqUuOw
• Go to page 97 Bat book and look at
steps 1-9 to understand where ATP,
NADH and Pyruvate/Pyruvic acid are
generated.
• Summarize your findings in a
picture/words/poetry/3-D figure.
Prep Step
• If oxygen is present- pyruvate enters the
mitochondrion and is converted to Acetyl
CoA by coenzyme A.
– Pyruvate is oxidized (NAD+ = NADH)
– A carbon atom is released as CO2
– Coenzyme A joins with the remaining 2 carbon
atoms = Acetyl CoA
• Now ready for the Krebs Cycle
The Krebs Cycle/ Citric Acid Cycle
• Takes place in the mitochondrial matrix.
• Krebs cycle has 8 steps, each catalyzed by a specific enzyme.
Each acetylCoA molecule that enters produces 3 molecules of
NADH. Another molecule is also reduced (gains electrons)
FADH2. Both of these molecules will donate their electrons in
the 3rd step. Carbon dioxide (that you will exhale) is produced
as a biproduct of this step.
Oxidative Phosphorylation/ Electron Transport Chain
• Oxidative Phosphorylation is the process of extracting ATP form NADH
and FADH2.
• Occurs on the inner membrane of the mitochondria.
• Electrons from NADH and FADH2 pass along an electron transport
chain from one protein to another, losing energy at each step
• The last electron acceptor is oxygen. When it accepts the two electron,
with two H+, it forms water.
Overview of Cellular Respiration
• Glycolysis- makes 2 ATP
• Krebs Cycle- makes 2 ATP
• Electron Transport Chain- makes approx. 34 ATP
• Total- 36 ATP made for each glucose molecule
Anaerobic Respiration
• If oxygen’s not present there is no electron acceptor
to accept the electrons at the end of the electron
transport chain. If this occurs, NADH accumulates.
This causes the krebs cycle and glycolosis to both
stop. If this happens, the cells soon dies as no ATP
is made.
• Anaerobic respiration is a method cells use to
escape this fate. The pathways in plants and
animals, alcoholic and lactate fermentation,
respectively, are slightly different but the objective is
the same: to replenish NAD+ so that glycolysis can
proceed again.
• Anaerobic respiration occurs in the cytoplasm.
Anaerobic Respiration
• Alcoholic Fermentation- Occurs in plants, fungi
(such as yeast) and bacteria. Produces alcohol
as a biproduct. This process is used to
commercially produce alcoholic beverages.
• Lactate Fermentation- occurs in animals.
Pyruvate is converted to lactate (or lactic acid).
This molecule causes muscle cramps after
strenuous exercise.
Types of Phosphorylation
• Substrate-level phosphorylation- direct
phosphorylation of ADP to ATP from a
coupled reaction.
– Ex: glycolysis generates enough energy to add a
P to ADP
• Oxidative phosphorylation-ATP is generated
from the oxidation of NADH and FADH2, the
ETC and ATP synthase.
• Photophosphorylation- sunlight powered
photolysis, ETC and ATP synthase