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CHAPTER 8
Photosynthesis
http://www.botany.com/index.16.htm
Obtaining Energy
• The sun is the direct or indirect source of
energy for most living things.
• Autotrophs —organisms that can make their
own food
• Heterotrophs —organisms that can not make
food. They obtain energy from eating food.
http://image.wistatutor.com/content/environment/food-chain-system.jpeg
Photosynthesis
• Photosynthesis is the
process used by autotrophs
to convert light energy
from sunlight into chemical
energy in the form of
organic compounds.
• Involves a complex series of
chemical reactions known as a
biochemical pathway.
• Product of one reaction is
consumed in the next
reaction
http://www.vtaide.com/png/images/photosyn.jpg
Overview
• Photosynthesis is often summarized in the following
equation:
Light energy
6CO2 + 6H2O
C6H12O6 + 6O2
The Reactants are carbon dioxide and water
The Products are glucose and oxygen
The Stages of Photosynthesis
• There are two stages to the
process
• Light Reactions —light energy is
converted to chemical energy, which is
temporarily stored in ATP and the
energy carrier molecule NADPH
• Dark Reactions (Calvin Cycle)—
organic compounds are formed using
CO2 and the chemical energy stored in
ATP and NADPH
http://bioweb.uwlax.edu/bio203/s2009/schroeer_paul/i
mages/484pxSimple_photosynthesis_overview_svg.png
The Light Reactions
• Require light to happen
• Take place in the chloroplasts
• Chloroplasts contain pigments that absorb
sunlight.
• Pigment —a compound that absorbs light
http://www.quranandscience.com/images/stories/chloroplasts2.jpg
The Structure of a Chloroplast
• Surrounded by an outer and inner membrane
• Thylakoids —membrane system arranged as
flattened sacs. (from the Greek meaning “pocket”)
• Grana (pl.) Granum (singular)—stacks of
thylakoid membrane sacs
• Stroma —solution that surrounds the grana
http://www.scool.co.uk/assets/learn_its/alevel/biology/cel
ls-and-organelles/organelles/chloroplastb.gif
• Thylakoids contain the pigments known as
Chlorophylls .
• Chlorophylls —absorb colors other than green.
Therefore, green is reflected and is visible.
• Two types:
• Chlorophyll a and Chlorophyll b
• Chlorophyll a —directly involved in the light reactions
• Chlorophyll b —accessory pigment that assists in
photosynthesis
• Carotenoids —accessory pigments responsible for
fall colors and also assist in photosynthesis
Converting Light Energy to Chemical Energy
• Chlorophylls and carotenoids are grouped in
clusters embedded in proteins in the thylakoid
membrane.
• These clusters are called photosystems
• Two photosystems exist, each with its own job
to do:
• Photosystem I and Photosystem II
• Plants have both photosystems. Prokaryotic
autotrophs only have photosystem II. It is only
numbered as II because it was the second one
discovered. However, it probably evolved 1st.
I
II
The Calvin Cycle
• Named for Melvin Calvin
• Most common pathway for carbon fixation
• Carbon fixation —changing CO2 into organic
compounds (carbohydrates)
• It is the second set of reactions in photosynthesis
and does not require light.
• It uses the energy that was stored in ATP and
NADPH during the light reactions to produce organic
compounds in the form of sugars.
• The Calvin Cycle occurs in the stroma of the
chloroplasts and requires CO2
The Calvin Cycle
3 ADP
3 CO2
3 ATP
3 RuBP
6 PGA
6 ATP
6 ADP
glucose
5 G3P
1 G3P
6 G3P
6 NADPH
starch
6NADP+
http://bioap.wikispaces.com/file/view/Carbon_Fixation.gif/120055293/Carbon_Fixation.gif
6P
• Plant species that fix carbon using the Calvin
Cycle only are known as C3 plants because of
the three-carbon compound that is initially
formed in the process. They include most
plants.
http://stjoseph.iaswcd.org/23rd%20Annual%20Tree%20Sale.htm
Alternative Pathways
• Plants living in hot, dry climates have
trouble using the Calvin Cycle to fix
carbon.
• This is because they must partially close
their stomata to conserve water.
• This allows less CO₂ to enter and an
excess of O₂ to build up, both of which
inhibit the Calvin Cycle
• Two alternate pathways have evolved for
these plants—both allow the plants to
conserve water.
• They are the C4 pathway and the CAM
pathway
The C4 Pathway
• C4 plants include: corn, sugar
cane and crab grass
• Cells called mesophyll cells in
C4 plants use an enzyme to fix
CO2 into a four carbon
compound
• This compound travels to other
cells where CO2 can be
released and enter the Calvin
Cycle
• These plants lose about ½ as
much water as C3 plants when
producing the same amount of
carbohydrates.
The CAM Pathway
• CAM plants include:
cactuses, pineapples, and
jade plants.
• These plants open their
stomata at night and close
them during the day (opposite
of most plants).
• CO2 absorbed at night can
enter the Calvin Cycle during
the day, allowing the stomata
to stay closed and conserve
water.
• These plants lose less water
than any other plants
CHAPTER 7
Cellular Respiration
Mighty Mitochondria
http://www.ageofautism.com/2008/04/dr-blaylock-on.html
Cellular Respiration
• Cellular Respiration —the process by which cells
get
energy from carbohydrates; oxygen combines with
glucose to form water and carbon dioxide
C6H12O6 + 6O2
6CO2 + 6H2O + energy (ATP)
• The equation is a simple summary of a very complex
•
•
•
•
process.
The overall purpose is to convert food into energy by
breaking down organic fuel molecules.
When oxygen is present during this process it is called
aerobic respiration ( which is the most efficient).
If no oxygen is present it is called anaerobic respiration
(which is much less efficient).
Both types (aerobic and anaerobic) start with a process
called glycolysis.
Glycolysis
• Glycolysis —first stage of cellular respiration.
“glucose splitting”
• Occurs in the cytosol
• No oxygen is needed
• Glucose molecules are broken down into two
3-carbon
molecules of pyruvic acid
• Pyruvic acid is then used in the Krebs Cycle (which is the second
• Glycolysis means
stage of aerobic respiration)
enzymes are needed
• 2 molecules of ATP are produced
• 2 molecules of NADH (an electron carrier molecule) are produced
• Specific
G3P
G3P
http://science.halleyhosting.com/sci/ibbio/cellenergy/resp/respirnotes/glycolysis2.htm
Summary of Glycolysis
• Basically:
• One glucose (6C) is broken into two molecules of pyruvic acid (3C)
• If oxygen is available, the pyruvic acid will move into the
mitochondria and aerobic respiration will begin.
• 4 ATP molecules are produced. Two are used to break apart the next
glucose molecule and keep glycolysis going.
• This leaves a net yield of 2 ATP molecules for use by the cell.
• Two NAD+ are converted into 2 NADH and 2H+. These go to Electron
Transport.
Efficiency of Glycolysis
• Measured in kilocalories (kcal)
• One kilocalorie equals 1,000 calories (cal)
• Complete oxidation of glucose releases 686 kcal
• Production of ATP absorbs 7 kcal
• 2ATP are produced from every glucose molecule broken down by
glycolysis
• The efficiency is therefore calculated by the following formula:
Efficiency of
glycolysis
=
=
Energy required to make ATP
Energy released by oxidation of glucose
2 x 7 kcal
686 kcal
x
100%
= 2%
Aerobic Respiration
• In most cells, the pyruvic acid produced in glycolysis
enters the pathway of aerobic respiration.
• This pathway produces nearly 20 times as much ATP as
is produced by glycolysis alone and is therefore the most
efficient.
• Oxygen must be available for this to happen.
• There are two major stages: The Krebs Cycle and the
Electron Transport Chain
Intermediate Step
• Aerobic Respiration takes
place in the mitochondria of
the cell.
• Before the Krebs Cycle can
begin, each of the two pyruvic
acid molecules must be
converted.
• The pyruvic acid enters the
mitochondrial matrix (space
inside the inner membrane of
the mitochondria)
• It reacts with a molecule called
coenzyme A to form Acetyl
Coenzyme A (acetyl CoA)
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
The Krebs Cycle
• The Krebs Cycle (named for Hans Krebs) is a biochemical
pathway that breaks down acetyl CoA.
• Two turns of the Krebs Cycle produce:
• 2 ATP molecules
• 4 CO2 molecules
• 6 NADH molecule
• 2 FADH2 molecules
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
Review of the Gylcolysis and the Krebs
Cycle
• In Glycolysis, one glucose molecule produces two
pyruvic acid molecules, which can then form two
molecules of Acetyl CoA.
• Both of the Acetyl CoA molecules enter the Krebs Cycle
creating two turns of the cycle.
• This produces 6 NADH, 2 FADH2, 2 ATP and 4 CO2
molecules (waste product that diffuses out of the cell).
• The 6 NADH and 2 FADH2 molecules drive the next stage
of aerobic respiration—the Electron Transport Chain.
Electron Transport Chain
• The Electron Transport Chain, linked with chemiosmosis
makes up the second stage of aerobic respiration.
• Electrons are transferred from one molecule to another by several
electron carrying molecules located in the membrane of the
mitochondria.
• All steps occur in the
cristae (inner membrane)
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
Efficiency of Cellular Respiration
• Through Aerobic Cellular Respiration, a maximum of 38
ATP molecules can be produced from one glucose
molecule.
• 2 from Glycolysis
• 2 from Krebs cycle
• 32-34 from the Electron Transport Chain
• To see how we get 38, follow
along….
• 2 ATPs directly from
glycolysis
• 2ATPs directly from Krebs
cycle
• Each NADH can generate
3ATPs from electron
transport (30 total)
• Each FADH2 can generate
2ATPs from electron
transport (4 total)
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
• The actual number of ATP molecules generated through
Aerobic Respiration varies from cell to cell. (36-38)
• Most eukaryotic cells produce only 36 molecules per
glucose molecule because the active transport of NADH
through a cell membrane uses up some ATP.
• When 38 ATP molecules are generated the efficiency is
calculated as follows:
Efficiency of
Cellular Respiration
=
=
Energy required to make ATP
Energy released by oxidation of glucose
38 x 7 kcal
686 kcal
x 100%
=
.
39%
This is 20 times more efficient than glycolysis alone !!
Anaerobic Respiration
• If no oxygen is present, the Krebs Cycle and Electron
•
•
•
•
•
Transport Chain are not utilized.
The cell must have a way to keep glycolysis going.
Glycolysis would stop without a cellular process that
recycles NAD+ and NADH.
Without such a process, glycolysis would quickly use up
all the NAD+ in the cell.
Glycolysis and ATP production would stop and the cell
would die.
Fermentation to the rescue 
Fermentation
• Fermentation is the chemical pathway that recycles NAD+
in the absence of oxygen. It keeps glycolysis going. No
additional ATP is made. Therefore, you still have the 2%
efficiency rate for energy release.
• Two types of fermentation:
• Lactic Acid Fermentation
• Alcoholic Fermentation
Lactic Acid Fermentation
• Pyruvic acid is converted by a specific enzyme into lactic acid.
• Two hydrogen atoms from NADH and H+ are transferred to
pyruvic acid to form the lactic acid molecule.
• NADH is oxidized to NAD+ and reused to keep glycolysis going.
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
• Lactic acid fermentation occurs in
foods such as yogurt and cheese
as well as certain animal cells.
• Occurs mostly in muscle cells
during hard exercise.
• Muscle cells use up oxygen too fast
and switch from aerobic to anaerobic
respiration.
• Lactic acid builds up reducing the cells
ability to contract. This causes fatigue,
pain and cramps.
http://www.burnthefatchallenge.com/wp/wpcontent/uploads/2011/01/treadmill-300x300.gif
Slow down!!! Allow the lactic
acid time to diffuse back into the
blood stream and to the liver
where it is converted back into
pyruvic acid.
Alcoholic Fermentation
• Converts pyruvic acid to carbon dioxide and ethyl alcohol.
• NAD+ is recycled in the same manner as before.
http://www.methuen.k12.ma.us/mnmelan/Respiration%20L2.htm
• Bakers use the alcoholic
fermentation of yeast to make
bread.
• CO2 is produced and trapped
in the dough, causing it to
rise.
• When the dough is baked,
yeast cells die and the
alcohol evaporates.
You can’t get drunk from eating
bread !!!
Click to reveal
COMPARING PHOTOSYNTHESIS AND CELLULAR RESPIRATION
PHOTOSYNTHESIS
RESPIRATION
FUNCTION
Production of Glucose
Oxidation of Glucose
LOCATION
chloroplasts
mitochondria
REACTANTS
6CO2 + 6H2O
C6H12O6 + 6O2
PRODUCTS
C6H12O6 + 6O2
6CO2 + 6H2O
light
EQUATION
6CO2 + 6H2O
C6H12O6 + 6O2
C6H12O6 + 6O2
6CO2 + 6H2O +ATP
Photosynthesis and Cellular Respiration Cycle