photosynthesis
TRANSCRIPT
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Photosynthesis
The Sun is the main source of energy for life on earth
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•Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas
from carbon dioxide and water
AN OVERVIEW OF PHOTOSYNTHESIS
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During Photosynthesis…
• Organisms (plants) absorb light energy from the sun and store it in organic compounds.
• Energy is crucial to all life, without it work could not be done.
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An example of basic photosynthesis…
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Photosynthesis consists of two stages
• Light dependent reactions
• Take place in the chloroplasts
• Depends on sunlight for activation energy
• Responsible for the absorbing of light in Photosynthesis
• Water is split, giving off oxygen
• H2O O2 + ATP + NADPH2
• Non-light dependent (dark) reactions
• Takes place in the Stroma
• Includes the Calvin cycle
• Does not literally occur in the dark, but requires no light to occur
• Separated into three steps
• Carbon dioxide is split, providing carbon to make sugars
• ATP + NADPH2 + CO2 C6H12O6
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Chloroplasts
• The site of Light Dependent Reactions
• These organelles are surrounded by a double membrane and contain an
inner membrane separate into disk like sacs called thylakoids
• Thylakoid are arranged into granum, or neat stacks
• Each thylakoid contains the green pigment chlorophyll
• The light absorbing pigments are organized into photosystems, which transfer energy during the light reactions
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Pigments in chloroplasts
• Chloroplasts absorb all other color pigments, leaving green to be reflected resulting in a plant’s color.
• Chlorophyll a and b are
two are the 2 most
common types of chlorophyll
• Chlorophyll b absorbs colors or
light energy NOT absorbed by
chlorophyll a
Light Reflectedlight
Absorbedlight
Transmittedlight
Chloroplast
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The Calvin Cycle – Step one
• CO2 diffuses into the stroma from surrounding cytosol
• An enzyme combines a CO2 molecule with a five-carbon carbohydrate called RuBP
• The six-carbon molecule produced then splits immediately into a pair of three-carbon molecules known as PGA
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The Calvin Cycle – Step two
• Each PGA molecule receives a phosphate group from a molecule of ATP
• This compound then receives a proton from NADPH and releases a phosphate group producing PGAL
• These reactions produce ADP, NADP+, and phosphate which are used again in the Light Reactions
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The Calvin Cycle – Step three
• Most PGAL is converted back to RuBP to keep the Calvin cycle going
• Some PGAL leaves the Calvin Cycle and is used to make other organic compounds including amino acids, lipids, and carbohydrates
• PGAL serves as the starting material for the synthesis of glucose and fructose
• Each turn of the Calvin cycle fixes One CO2 molecule so it takes six
turns to make one molecule of glucose
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Photosystems & Electron Transport Chain
• Only 1 in 250 chlorophyll molecules (chlorophyll a) actually converts light energy into usable energy; these molecules are called reaction-center chlorophyll
• Antenna pigments- the other molecules (chlorophyll b, c, & d and carotenoids) tha absorb light energy and deliver it to the reaction-center molecule
• Photosystem- unit of several hundred antenna pigment molecules plus a reaction center
• There are 2 types of photosystems- photosystem I & photosystem 2
• Light is absorbed by the antenna pigments of photosystems II and I
• NADPH is used in the Calvin cycle
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Chemiosmosis • Synthesis of ATP (energy)
• Depends on the concentration gradient of protons ( H+) across the thylakoid membrane
• Protons (H+) are produced from the splitting of water in Photosystem II
• Concentration of Protons is HIGHER in the thylakoid than in the stroma
• Enzyme, ATP synthetase in the thylakoid membrane, makes ATP by adding a phosphate group to ADP
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Alternate Pathways• C3 Plants are the most common users of the Calvin Cycle
• Stomata are small openings on the underside of leaves used for gas exchange (O2 & CO2)
• Guard cells help to open and close the stomata
• Plants also lose H2O through stoma so they are closed during the hottest part of the day
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Review: Photosynthesis uses light energy to make food molecules
Chloroplast
Light
Stack ofthylakoids ADP
+ P
NADP
Stroma
Lightreactions
Calvincycle
Sugar used for
Cellular respiration Cellulose
Starch
Other organic compounds