PHOTOSYNTHESIS
Using light to make food
PHOTOSYNTHESIS
• Autotrophs are the producers of the
biosphere
– Plants are autotrophs
• Producing their own food and sustaining themselves
without eating other organisms
• What are we?
• We are heterotrophs
– Plants, algae, and some bacteria are
photoautotrophs
•
Producers of food consumed by virtually all
organisms
Figure 7.1A–D
• PLANTS USE ENERGY FROM THE SUN TO
TRANSFORM CO2 AND WATER INTO
ENERGY RICH CARBOHYDRATES(glucose).
• Plants are AUTOTROPHS :make their
own food
• Plants supply all the energy (food) for life
on earth and the oxygen that most
organisms need
How old is photosynthesis?
How important is it?
• Evidence of photosynthesis exists in rocks
3.5 billion years old.
• Photosynthesis is the largest biochemical
process on earth.
Importance?
• Photosynthesis supplies OXYGEN to earth
atmosphere and FOOD to all organisms
Photosynthesis is the process by which certain
organisms use light energy to make sugar
and oxygen gas from carbon dioxide and
water.
Light
energy
6 CO2
+
Carbon dioxide
6
H2O
Water
C6H12O6
PHOTOSYNTHESIS
Glucose
+
6
O2
Oxygen gas
Where in the plant does photosynthesis
occur?
In the CHLOROPLAST. The leaf is a food
factory. The light trapping pigments are inside the
chloroplasts embedded in the thylakoid
membranes.
• STOMATA or STOMA are openings on the
leaf surface through which CO2 and O2 as well
as water vapor go in and out of leaves.
Where does it occur?
Photosynthesis occurs in chloroplasts
•
Occurs primarily in the leaves, in the chloroplasts, which
contain stroma, and stacks of thylakoids called grana
Mesophyll Cell
Leaf Cross Section
Leaf
LM 2,600 
Mesophyll
Chloroplast
Vein
Stoma
CO2
O2
TEM 9,750 
Chloroplast
Figure 7.2
Grana
Stroma
Outer
membrane
Inner
membrane
Stroma
Intermembrane
space
Granum
Thylakoid
Thylakoid
space
ENERGY-CONVERTING ORGANELLES
Chloroplasts convert solar energy to
chemical energy (in sugars).
. Found in plants and some protists. This is where
PHOTOSYNTHESIS takes place
Chloroplast
Stroma
Granum
Intermembrane
space
TEM 9,750
Inner and outer
membranes
Sites to check:
• http://www.mhhe.com/biosci/esp/2001_gbio/folder_str
ucture/ce/m6/s1/index.htm
• http://www.tvdsb.on.ca/westmin/science/Biology12/Me
tabolic%20Processes/stoma.htm
• http://www.tvdsb.on.ca/westmin/science/Biology12/Me
tabolic%20Processes/chloro.htm
• http://www.mhhe.com/biosci/esp/2001_gbio/folder_str
ucture/ce/m6/s3/index.htm
ENERGY-CONVERTING ORGANELLES
Chloroplasts convert solar energy to
chemical energy (in sugars).
. Found in plants and some protists. This is where
PHOTOSYNTHESIS takes place
Chloroplast
Stroma
Granum
Intermembrane
space
TEM 9,750
Inner and outer
membranes
Thylakoids
• Thylakoids are membranes that look like
green flattened sacs stacked upon each
other.
• Granum is a stack of these sacs
• The STROMA is the fluid between the
membranes.
Plants produce O2 gas by splitting water
– The O2 liberated by photosynthesis
• Is made from the oxygen in water
Experiment 1
6 CO2 + 12 H2O
C6H12O6 + 6 H2O + 6 O2
Not
labeled
Experiment 2
6 CO2
6 CO2
Reactants:
Products:
Figure 7.3A–C
C6H12O6 + 6 H2O + 6 O2
Labeled
+ 12 H2O
C6H12O6
12 H2O
6 H2O
6 O2
•Photosynthesis is a redox process, as is
cellular respiration
– In photosynthesis
•
H2O is oxidized and CO2 is reduced
Reduction
6 CO2

6 H2O
C6H12O6

6 O2

6 H2O
Oxidation
Oxidation
C6H12O6 
Figure 7.4A, B
6 O2
6 CO2
Reduction
PHOTONS AND PIGMENTS
• PHOTONS are packets of light energy
• Light: electromagnetic energy waves that travel in waves
of different lengths
• PIGMENTS are molecules that absorb light
energy (photons)
• CHLOROPHYLL is the main pigment in plants.
Absorbs the blue and red portions of the visible light
spectrum and scatters or reflects back the green wave
length. This is why plants look green.
• CAROTENOIDS reflect the reds and yellows
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO
CHEMICAL ENERGY
– wavelengths of visible light (red,blue), absorbed by pigments
drive the light reactions of photosynthesis
Increasing energy
10–5 nm
10–3 nm
Gamma
rays
1 nm
X-rays
103 nm
UV
1m
106 nm
Microwaves
Infrared
103 m
Radio
waves
Light
Reflected
light
Visible light
380 400
500
600
700
750
Wavelength (nm)
650
nm
Figure 7.6A, B
Chloroplast
Absorbed
light
Transmitted
light
• Overview: Photosynthesis occurs in two
stages linked by ATP and NADPH
– The complete process of photosynthesis
consists of two linked sets of reactions
•
•
The Light Dependent Reactions and
The Calvin Cycle (discovered by Calvin and
Benson)
Photosynthesis:
H2O + CO2
water
carbon
dioxide
  O2 + C6H12O6 + H2O
light
energy
oxygen
glucose
water vapor
STEPS OF PHOTOSYNTHESIS
• capture sun light
• use light energy to make ATP
• use ATP to make carbohydrate molecules
from CO2(carbon dioxide) and H2O
(water)
LIGHT DEPENDENT REACTIONS
• is the first stage of photosynthesis.
• It takes place in the thylakoid
membranes.
• Oxygen is released, Water molecules are
split. Oxygen gas diffuses out
• ATP and electrons are carried to the next
stage. Electrons are used to make ATP.
Light Dependent Reactions (In thylakoids of
chloroplasts)
• This is where light energy is converted to a form
that can be used by plants to build Carbon
compounds (sugars)
• 1.The energy of sunlight hits the
photosystems (cluster of pigment molecules)
and excites chlorophyll (pigment) electrons to a
higher energy state. Light energy is absorbed by the
photosystems
• 2. Water molecules split, oxygen is released.
• Light energy is converted to chemical energy
(ATP formation)
• NADPH electron carrier take electrons from
chlorophyll to the Calvin Cycle.
– The light reactions
•
Convert light energy to chemical energy and produce
O2
– The Calvin cycle assembles sugar molecules from
CO2
•
Using ATP and NADPH from the light reactions
H2O
CO2
Chloroplast
Light
NADP+
ADP
+P
CALVIN
CYCLE
(in stroma)
LIGHT
REACTIONS
(in thylakoids)
ATP
NADPH
Figure 7.5
O
Sugar
THE CALVIN CYCLE: CONVERTING CO2
TO SUGARS
•ATP and NADPH power sugar synthesis in the
Calvin cycle
– The Calvin cycle occurs in the chloroplast’s stroma
• Consists of carbon fixation, reduction, release of G3P,
(glyceraldehyde 3-phosphate)and regeneration of RuBP
Input
CO2
ATP
NADPH
CALVIN
CYCLE
Figure 7.10A
Output:
G3P
LIGHT INDEPENDENT REACTIONSCALVIN CYCLE
• (In stroma of chloroplasts ) This is a
cyclic pathway where the final products
are also the first reactants on the cycle
• The Calvin Cycle uses the energy in ATP
and NADPH to synthesize carbohydrates.
• Carbon enters the cycle as a molecule of
CO2
LIGHT INDEPENDENT REACTION or
CALVIN CYCLE
Takes place in the stroma of the chloroplasts.
• This is the “synthesis” part of
photosynthesis. Making food (glucose) and
trapping CO2 to incorporate carbon into
living things.
CALVIN CYCLE- making glucose
• The energy from ATP and NADPH+ go
into the bonds of a glucose molecule.
• Electrons ( hydrogens) from the carrier
molecule are put together with CO2 to
make glucose.
• An enzyme called RUBISCO fixes the
CO2 ( from the air) by bringing together the
CO2 and the sugar.
LIGHT INDEPENDENT REACTIONSCALVIN CYCLE
•
•
•
•
Fixes Carbon (from CO2) using the enzyme rubisco. This
enzyme catalizes the binding of carbon from CO2 to
RuBP ( ribulose bisphosphate). This is how all carbon
enters the world of life.
Put together sugars using ATP and NADPH as energy
sources.
The final product is a three carbon sugar called G3P
(glyceraldehyde 3-phosphate)
G3P is a sugar phosphate that can be modified to form
glucose
PHOTOSYNTHESIS REVIEWED
Review: Photosynthesis uses light energy to make food
molecules
H2O
CO2
Chloroplast
Light
NADP+
ADP
+P
RUBP
Photosystem II
Thylakoid
membranes
Electron
transport
chains
Photosystem I
CALVIN
CYCLE 3-PGA
(in stroma)
ATP
NADPH
Stroma
G3P
Cellular
respiration
Cellulose
O2
Figure 7.11
LIGHT REACTIONS
Sugars
CALVIN CYCLE
Starch
Other organic
compounds
C3 PLANTS
• Plants that use CO2 directly from the
air are called C3 plants. Soybeans, oats
wheat and rice are C3 plants.
• In hot dry weather these plants close their
stomata ( pores in underside of leaves) to
reduce water loss. Since no CO2 can
enter, the rate of photosynthesis is
reduced and your crop productivity is poor.
In C3 plants a drop in CO2 and rise in O2 when
stomata close on hot dry day divert the Calvin
cycle to photorespiration
( Rubisco can bind oxygen in place of CO2 as
CO2 becomes scarce. When Oxygen enters
the Calvin cycle instead of CO2 so it cannot
produce sugars )
C4 and CAM plants have special
adaptations that save water
CAM PLANTS
• CAM PLANTS
• In hot and very dry climates. (deserts)
• Examples: pineapple, cactus, orchids all the
“succulent plants such as Aloe and Jade
plants
• When stoma opens to get CO2 the water can
get out. Survival depends on water retention.
• Adaptation: Stoma is closed during the day
and open at night. Co2 is taken in at night
and banked until the next day when it is given
to the Calvin cycle to make carbohydrates.
• The CO2 taken in at night stays “banked” until
the sun’s energy comes in the next day.
– CAM plants open their stomata at night
• Making a four-carbon compound used as a CO2
source during the day
CO2
CO2
Night
4-C compound
CO2
CALVIN
CYCLE
3-C sugar
Figure 7.12 (right half)
CAM plant
Day
Pineapple
C4 PLANTS
• Have special adaptations to save water without shutting
down photosynthesis
• In hot climates.
• Examples: grasses , corn, sugarcane
• In hot climates the stoma closes to keep water in. It also
keeps CO2 out and oxygen builds up inside the leaves. Since
there is not enough CO2 the enzyme rubisco (that normally binds CO2)
binds oxygen and the plants would not grow well.
• Adaptation : It continues to make sugars with closed
stomata.
How? These plants use a different enzyme to bind CO2.This
enzyme fix carbon twice to produce a 4 carbon compound
which can then donate the C to the Calvin cycle
– C4 plants first fix CO2 into a four-carbon
compound
• That provides CO2 to the Calvin cycle
CO2
Mesophyll cell
4-C compound
CO2
CALVIN
CYCLE
Sugarcane
Bundle-sheath cell
Figure 7.12 (left half)
3-C sugar
C4 plant
ENVIRONMENTAL IMPACT OF
PHOTOSYNTHESIS
• Food. Photosynthesis is the source of all
the food that gives us energy ( from the sun)
• Oxygen production
• CO2 is the gas plants use to make sugars.
CO2 in the atmosphere retains heat from the
sun that would otherwise radiate back into
space
PHOTOSYNTHESIS, SOLAR RADIATION, AND
EARTH’S ATMOSPHERE CONNECTION
•Photosynthesis moderates global warming
– Greenhouses used to grow plants trap solar
radiation, raising the temperature inside
Figure 7.13A
Global warming and the Greenhouse
effect
• Global warming is the slow and steady rise in Earth
surface temperatures.
• Warming is caused by CO2 and other
greenhouse gases.
• It is called the green house effect because
CO2 traps heat and keeps it warm near the
earth surface.
• When this occurs in moderation it is a good thing,
otherwise the planet would be about 10 degrees
colder all the time. The trouble is that we get
overheating.
What are greenhouse gases?
• All the cars and industries that burn fossil fuel
produce so much CO2 that we now have 30%
more than ever before. This causes global
warming.
• Some greenhouse gases are carbon dioxide
CO2, methane CH4, water vapor and others
• http://www.net.org/globalwarming/sea_lev
el/
Web sites to check:
• http://www.fw.vt.edu/dendro/forestbiology/photos
ynthesis.swf
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter10/ani
mations.html
• http://www.sumanasinc.com/webcontent/anisam
ples/majorsbiology/harvestinglight.swf