Photosynthesis and Cellular
Respiration
Trapping the Sun’s Energy
• The process by which plants capture energy
from the sun to build carbohydrates through
chemical pathways is called photosynthesis
– Solar energy converts water and carbon dioxide into
chemical energy stored in simple sugars
• The simple sugar that photosynthesis produces
is glucose which the plant uses to store energy.
• The equation that represents photosynthesis is:
6CO2 + 6H2O  C6H12O6 + 6O2
Solar
energy
Phases of Photosynthesis
• Photosynthesis requires energy from the sun,
but the sun is not available 24 hours a day.
• Photosynthesis must occur in two phases
– Light-dependent Reactions (light reactions)
• Convert light energy into chemical energy (ATP and
NADPH)
– Light-independent Reactions (dark reactions)
• Uses the ATP and NADPH from the light-dependent
reactions to build glucose
6CO2 + 6H2O  C6H12O6 + 6O2
Solar
energy
The Role of Chloroplasts and Pigments
• Photosynthesis takes place in
the chloroplasts
• Light-dependent reactions
take place in the membranes
of the thylakoid disks
(contain chlorophyll)
• Light-independent reactions
take place in the stroma
(thick fluid that forms the
framework of a chloroplast)
The Role of Chloroplasts and Pigments
• The thylakoid membranes contain the pigments that
can absorb certain wavelengths of sunlight.
• The most common pigment in the
chloroplasts is chlorophyll.
• Chlorophyll a and b absorb most wavelengths of light
except for green. Green is reflected making the plants
appear green.
• In the fall, plants reabsorb chlorophyll leaving
other pigments that reflect other wavelengths
of light – making the leaves appear red, yellow,
or orange.
Light-dependent Reactions
• Sunlight strikes the chlorophyll molecules in the
thylakoid membrane.
• Light energy is transferred to electrons
• The electrons become highly energized and are passed
down an Electron Transport Chain
Light-dependent Reactions
• The Electron Transport Chain is a series of proteins in
the thylakoid membrane
• As the electrons are transferred from one protein to
another, some energy is released which
– helps join ADP and Phosphate to form ATP
– Pump hydrogen ions into the center of the thylakoid disk to
join H+ and NADP+ forming NADPH (electron carrier)
– ATP and NADPH will be used during the light-independent
reactions
Light-dependent Reactions
• The electrons excited by the light energy that passed
down the electron transport chain and left with
NADPH need to be replaced so the reaction can
happen again.
• To replace those electrons, a water molecule is split
(photolysis), sending electrons back to the chlorophyll
and releasing Oxygen and Hydrogen ions into the
atmosphere – this supplies the oxygen that we breathe
Photolysis 
Light-dependent Reactions
Solar Energy absorbed
by chloroplasts
Oxygen released
NADPH released
ATP Released
Products of Light Reactions
(ATP and NADPH) fuel the
dark reactions
Light-independent Reactions
• The second phase of photosynthesis does not
require light and is called the Calvin Cycle.
• The Calvin Cycle occurs in the stroma of the
chloroplast.
• The Calvin Cycle uses the ATP and NADPH
that was built during the light-dependent
reactions
The Calvin Cycle
Uses Carbon Dioxide
from the air
Uses another ATP
to replenish RuBP
Builds a glucose molecule
Uses ATP and
NADPH from
light reactions
Photosynthesis Equation
Light Energy
6CO2 + 6H2O
Carbon Dioxide
from the air –
Used in the
Calvin Cycle
during the Dark
Reactions
chlorophyll
C6H12O6 + 6O2
Glucose Made in
the Calvin Cycle
during the Dark
Reactions
Water Split during
Photolysis in the
Light Reactions
Oxygen Released
during Photolysis
in the Light
Reactions
Plants can use this glucose molecule for energy during Cellular Respiration. Plants can
also convert this glucose molecule into other organic compounds such as proteins and
fats/lipids or other carbohydrates like starch and cellulose
3 Factors that Affect Photosynthesis
• Carbon Dioxide (CO2)
– Without CO2, the plant would not have one of the
raw materials needed in the photosynthesis equation
– CO2 is used in the first step of the Calvin Cycle
• Temperature
– The temperature must be in the appropriate range
for the plant in order for photosynthesis to properly
occur
3 Factors that Affect Photosynthesis
• Intensity of Light
– If the intensity of light is lower, the available energy
for photosynthesis is lower.
• In a greenhouse, if the light source is further away,
intensity is lower and less photosynthesis can occur
• If light is not available at all, the light-dependent reactions
cannot occur (nor can they provide the materials used in
the light-independent reactions)
IMPORTANT!!!!
• Why are photosynthesis and cellular respiration
often described as being complementary reactions?
– Photosynthesis
• Carbon Dioxide + Water are taken into the plant
• Glucose + Oxygen are produced
– Cellular Respiration
• Glucose + Oxygen are used for cellular respiration
• Cellular respiration releases Carbon Dioxide + Water
• Therefore, products of photosynthesis are the
reactants for cellular respiration AND the products
of cellular respiration are the reactants for
photosynthesis
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Cellular Respiration
• Cellular Respiration: Process by which mitochondria
break down food molecules to produce ATP in plants
and animals
Nutrients + oxygen  water + ATP + CO2
• Changes organic chemical energy (glucose) into
inorganic chemical energy (ATP)
• There are three stages of Cellular Respiration
– Glycolysis
• Anaerobic – does not require oxygen
– Citric Acid (Krebs) Cycle
• Aerobic – does require oxygen
– Electron Transport Chain
• Aerobic – does require oxygen
Glycolysis
• Glycolysis: Breaks down glucose into two molecules of
pyruvic acid (a colorless acid formed as an important intermediate in
metabolism or fermentation)
• This reaction uses enzymes and takes place in the
cytoplasm of the cell
(anaerobic reaction)
• Produces
– 2 pyruvic acid molecules (used in the next step of Cellular
Respiration)
– 2 ATP molecules (energy the cell can use)
– 2 NADH (electron carrier)
Into the Mitochondria…
• Before the next step of Cellular Respiration can
occur, the pyruvic acid molecules must go into
the mitochondria
• The two oxygen-dependent (aerobic) reactions
are the Citric Acid Cycle (or Krebs Cycle) and
the electron transport chain
Pyruvic acid  CO2 + water + ATP
Citric Acid/Krebs Cycle (see page 138 in your book)
CO2 is
released
Pyruvate from
Glycolysis
fuels the cycle
CO2 is
released
NADH
and
FADH2 is
released
ATP is
released
Electron Transport Chain
• Electron Transport Chain uses the electron carriers
(NADH and FADH2) to pass electrons down the
protein chain and slowly release energy that is used to
form ATP and water molecules
• Electron Transport Chain transfers the most energy
Cellular Respiration
ATP
Glucose
Pyruvic
Acid
NADH
and FADH
Oxygen
Glycolysis
Citric Acid Cycle
Pyruvic
Acid
CO2
NADH
and FADH
Water
Electron Transport Chain
ATP
ATP
Cellular Respiration Equation
C6H12O6 + 6O2
Glucose made in
photosynthesis
by plants or
consumed by
animals
Used in
Glycolysis
Oxygen from the
atmosphere
Used in Electron
Transport Chain
6CO2 + 6H2O + energy
Carbon
Dioxide –
waste
product of
the Citric
Acid Cycle
Water –
released
from
Electron
Transport
Chain
ATP released from
Glycolysis, Citric
Acid Cycle, and
Electron Transport
Chain
Between 34-36 ATP can be made with this process.
This ATP can be used by the cells for cellular metabolism.
Fermentation
• When oxygen is not available anaerobic
respiration, fermentation, can follow glycolysis
in order to continue to produce energy.
• This is not as efficient as aerobic respiration and
produces far fewer ATP’s
• Two types of fermentation:
– Lactic acid Fermentation
– Alcoholic Fermentation
Lactic acid Fermentation
• Lactic acid fermentation occurs in muscle cells
during strenuous exercise when a lot of energy
is required and oxygen is scarce (oxygen debt).
Glucose  pyruvic acid  lactic acid + ATP
• The lactic acid is transferred from the muscle
cells to the liver where it will be converted
back into pyruvic acid
• The build up of lactic acid in the
muscles is what causes them to
be fatigued and sore.
Alcoholic Fermentation
• Yeast and some bacteria cells are capable of
alcoholic fermentation during which glucose is
broken down to release CO2 and ethyl alcohol
Glucose  pyruvic acid  alcohol + CO2 + ATP
• The bubbles formed by the CO2 make
bread rise
• The alcohol released
turns grape juice into wine
Photosynthesis vs. Cellular Respiration
Photosynthesis
Stores Energy as glucose
Cellular Respiration


Releases Energy in glucose
Occurs in Living Cells
Uses an Electron Transport
Chain
Occurs in Plant Cells






Occurs in Animal Cells
Releases Oxygen


Releases Carbon Dioxide
Creates Energy

Neither!
Aerobic Respiration
• Process by which cells use oxygen to break
down organic molecules, with the release of
energy that can be used for biological work
• A sequence of 30+ chemical reactions;
• Breaks down fuel molecules – most common,
glucose
• Ultimately, it releases energy, carbon dioxide, and water
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