Photosynthesis
Autotrophs
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Plants and some other types of organisms that contain chlorophyll are able to use light energy from the sun to produce food.
Autotrophs include organisms that make their own food
Autotrophs can use the sun’s energy directly
Heterotrophs
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Heterotrophs are organisms that can NOT make their own food
Heterotrophs can NOT directly use the sun’s energy
Energy
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Energy Takes Many Forms such as light, heat, electrical, chemical, mechanical
Energy can be changed from one form to another
Energy can be stored in chemical bonds & then released later
ATP – Cellular Energy
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Adenosine Triphosphate
Contains two, high-energy phosphate bonds
Also contains the nitrogen base adenine & a ribose sugar
ADP
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Adenosine Diphosphate
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ATP releases energy, a free phosphate, & ADP when cells take energy from ATP
Sugar in ADP & ATP
Called ribose
Pentose sugar
Also found on RNA
Importance of ATP
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Principal Compound Used To Store Energy In Living Organisms
Releasing Energy From ATP
ATP is constantly being used and remade by cells
ATP provides all of the energy for cell activities
The high energy phosphate bonds can be BROKEN to release energy
The process of releasing ATP’s energy & reforming the molecule is called phosphorylation
Releasing Energy From ATP
Adding A Phosphate Group To ADP stores Energy in ATP
Removing A Phosphate Group From ATP Releases Energy & forms ADP
Cells Using Biochemical Energy
Cells Use ATP For:
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Active transport
Movement
Photosynthesis
Protein Synthesis
Cellular respiration
All other cellular reactions
More on ATP
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Cells Have Enough ATP To Last For A Few Seconds
ATP must constantly be made
ATP Transfers Energy Very Well
ATP Is NOT Good At Energy Storage
Glucose
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Glucose is a monosaccharide
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C6H12O6
One Molecule of glucose Stores 90 Times More Chemical Energy Than One Molecule of ATP
Photosynthesis
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Involves the Use Of light Energy to convert Water (H20) and Carbon Dioxide (CO2) into Oxygen (O2) and High Energy Carbohydrates (sugars,
e.g. Glucose) & Starches
Investigating Photosynthesis
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Many Scientists Have Contributed To Understanding Photosynthesis
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Early Research Focused On The Overall Process
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Later Researchers Investigated The Detailed Chemical Pathways
Pigments
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In addition to water, carbon dioxide, and light energy, photosynthesis requires Pigments
Chlorophyll is the primary light-absorbing pigment in autotrophs
Chlorophyll is found inside chloroplasts
Light and Pigments
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Energy From The Sun Enters Earth’s Biosphere As Photons
Photon = Light Energy Unit
Light Contains A Mixture Of Wavelengths
Different Wavelengths Have Different Colors
Different pigments absorb different wavelengths of light
Photons of light “excite” electrons in the plant’s pigments
Excited electrons carry the absorbed energy
Excited electrons move to HIGHER energy levels
Chlorophyll
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There are 2 main types of chlorophyll molecules:
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Chlorophyll a
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Chlorophyll b
A third type, chlorophyll c, is found in dinoflagellates
Chlorophyll a
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Found in all plants, algae, & cyanobacteria
Makes photosynthesis possible
Participates directly in the Light Reactions
Can accept energy from chlorophyll b
Chlorophyll b
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Chlorophyll b is an accessory pigment
Chlorophyll b acts indirectly in photosynthesis by transferring the light it absorbs to chlorophyll a
Like chlorophyll a, it absorbs red & blue light and REFLECTS GREEN
Inside A Chloroplast
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Structure of the Chloroplast
Double membrane organelle
Outer membrane smooth
Inner membrane forms stacks of connected sacs called thylakoids
Thylakoid stack is called the granun (grana-plural)
Gel-like material around grana called stroma
Function of the Stroma
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Light Independent reactions occur here
ATP used to make carbohydrates like glucose
Location of the Calvin Cycle
Thylakoid membranes
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Light Dependent reactions occur here
Photosystems are made up of clusters of chlorophyll molecules
Photosystems are embedded in the thylakoid membranes
The two photosystems are:
o Photosytem I
o Photosystem II
Energy Carriers
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Nicotinamide Adenine Dinucleotide Phosphate (NADP+)
NADP+ = Reduced Form
Picks Up 2 high-energy electrons and H+ from the Light Reaction to form NADPH
NADPH carries energy to be passed on to another molecule
Light Dependent Reactions
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Occurs across the thylakoid membranes
Uses light energy
Produce Oxygen from water
Convert ADP to ATP
Also convert NADP+ into the energy carrier NADPH
Photosystem I
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Discovered First
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Active in the final stage of the Light Dependent Reaction
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Made of 300 molecules of Chlorophyll
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Almost completely chlorophyll a
Photosystem II
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Discovered Second
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Active in the beginning stage Of the Light Dependent Reaction
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Contains about equal amounts of chlorophyll a and chlorophyll b
Photosynthesis Begins
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Photosystem II absorbs light energy
Electrons are energized and passed to the Electron Transport Chain
Lost electrons are replaced from the splitting of water into 2H+, free electrons, and Oxygen
2H+ pumped across thylakoid membrane
Photosystem I
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High-energy electrons are moved to Photosystem I through the Electron Transport Chain
Energy is used to transport H+ from stroma to inner thylakoid membrane
NADP+ converted to NADPH when it picks up 2 electrons & H+
Phosphorylation
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Enzyme in thylakoid membrane called ATP Synthase
As H+ ions passed through thylakoid membrane, enzyme binds them to ADP
Forms ATP for cellular
Light Reaction Summary
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Reactants:
o H2O
o Light Energy
Energy Products:
o ATP
o NADPH
Light Independent Reaction
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ATP & NADPH from light reactions used as energy
Atmospheric C02 is used to make sugars like glucose and fructose
Six-carbon Sugars made during the Calvin Cycle
Occurs in the stroma
The Calvin Cycle
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Two turns of the Calvin Cycle are required to make one molecule of glucose
3-CO2 molecules enter the cycle to form several intermediate compounds (PGA)
A 3-carbon molecule called Ribulose Biphosphate (RuBP) is used to regenerate the Calvin cycle
Factors Affecting the Rate of Photosynthesis
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Amount of available water
Temperature
Amount of available light energy
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
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One of the major problems facing terrestrial plants is dehydration.
At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis.
The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water.
On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis.
In most plants (C3 plants) initial fixation of CO2 occurs via rubisco and results in a three-carbon compound, 3-phosphoglycerate.
These plants include rice, wheat, and soybeans.
When their stomata are closed on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle.
At the same time, O2 levels rise as the light reaction converts light to chemical energy.
While rubisco normally accepts CO2, when the O2/CO2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O2 to RuBP.
When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration.
The two-carbon fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes.
Unlike normal respiration, this process produces no ATP, nor additional organic molecules.
Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle.
A hypothesis for the existence of photorespiraton (a inexact requirement for CO2 versus O2 by rubisco) is that it is evolutionary baggage.
When rubisco first evolved, the atmosphere had far less O2 and more CO2 than it does today.
The inability of the active site of rubisco to exclude O2 would have made little difference.
Today it does make a difference.
Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day.
Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.
The C4 plants fix CO2 first in a four-carbon compound.
Several thousand plants, including sugercane and corn, use this pathway.
In C4 plants, mesophyll cells incorporate CO2 into organic molecules.
The key enzyme, phosphoenolpyruvate carboxylase, adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetetate.
PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot, i.e. on hot, dry days when the stomata are
closed.
The mesophyll cells pump these four-carbon compounds into bundle-sheath cells.
The bundle-sheath cells strip a carbon, as CO2, from the four-carbon compound and return the three-carbon remainder to the mesophyll
cells.
The bundle-sheath cells then use rubisco to start the Calvin cycle with an abundant supply of CO 2.
In effect, the mesophyll cells pump CO2 into the bundle sheath cells, keeping CO2 levels high enough for rubisco to accept CO2 and not O2.
C4 photosynthesis minimizes photorespiration and enhances sugar production.
C4 plants thrive in hot regions with intense sunlight.
A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families.
These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day.
Temperatures are typically lower at night and humidity is higher.
During the night, these plants fix CO2 into a variety of organic acids in mesophyll cells.
During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids.
Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle.
In C4 plants, carbon fixation and the Calvin cycle are spatially separated.
In CAM plants, carbon fixation and the Calvin cycle are temporally separated.
Both eventually use the Calvin cycle to incorporate light energy into the production of sugar.