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Energy: Photosynthesis and Cellular Respiration
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Energy is defined as the ability to do work, and everything requires energy
o Kinetic energy is the energy an object has due to motion
o Potential energy is the stored energy waiting to be used to do work
o Chemical energy is the potential energy stored in chemical bonds
▪ Forming bonds requires energy, breaking bonds releases energy
Metabolism is the sum of all chemical reactions in an organism
All energy ultimately originates from the sun
o Photosynthesis is when plants capture energy from the sun and store it in the
chemical bonds of sugars and other food molecules
o Cellular respiration is when organisms, including plants, release the energy stored
in the chemical bonds of the food chemicals they eat (or the sugar produced in
photosynthesis) and use it as fuel
Energy transformations
o Kinetic energy to potential energy
▪ Light energy form the sun --> energy transformed into heat --> chemical
energy stored in plants
o Potential energy to kinetic energy
▪ Chemical energy stored in muscles and liver --> energy transformed into
heat --> kinetic energy of forward motion
Chemical reactions absorb or release energy
o Endergonic reactions
▪ Require energy to proceed
▪ Builds complex molecules from simpler ones
o Exergonic reactions
▪ Releases energy
▪ Breaks apart large molecules
ATP is the cell's energy currency
o Made up of adenine, ribose, and three phosphate groups
o A molecule of ATP can release energy by breaking the covalent bond between the
last two phosphate groups
▪ At this point, ATP becomes ADP
• If you add energy, the covalent bond can be reformed to get ATP
Photosynthesis
o Plants, some bacteria, and some protists are considered producers
o Sunlight + Water + CO2 --> Oxygen + ATP + NADPH
o Pigments are molecules in the leaves that absorb light
▪ Different pigments absorb different wavelengths of light
• Pigments include Chlorophyll a, Chlorophyll b, and Carotenoids
▪ Pigments that are not absorbed are reflected, and this is what we see
o Photosynthesis is made up of two sets of reactions
▪ Light (dependent) Reactions
• Energy from the sun is stored and transformed into chemical energy
• Water is split to produce oxygen
• Produces ATP and NADPH (similar energy battery to ATP)

Energy: Photosynthesis and Cellular Respiration
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Light is absorbed by photosystem --> electrons are excited -->
electrons jump to electron acceptor molecules --> water is split and
produces oxygen to replace the electrons
• Electrons are passed through an electron transport chain, and energy
is released every time an electron is passed
• This energy is used to pump h+ across the chloroplast
• Results in a high H+ concentration on one side of the
membrane
• Gradient creates potential energy
• H+ rushes back over the membrane, and kinetic energy is
used to build ATP
• The electrons then pass to the second photosystem, where they
are then again energized by photon energy
• Electrons are added to NADP+ to create NADPH
▪ Dark Reactions/ Light Independent Reactions
• A cyclic reaction that does not require sunlight, and uses chemical
energy produced in light reactions to assemble CO2 into glucose
• ATP + NADPH + CO2 --> Sugar
• Plants synthesize glucose for energy, and store it as starch
• The uncharged leftover molecules are sent back to the light reactions
Cellular Respiration
o All organisms perform cellular respiration
▪ Harvests energy stored in organic molecules
o Generates ATP for cellular work
o Oxygen + Sugar --> CO2 + Water + ATP
o Includes three main parts
▪ Glycolysis
• Occurs in the cytoplasm and does not require oxygen
• A 6-C glucose molecule is split into two 3-C pyruvate molecules
• 2 ATP are generated
• Many unicellular organisms depend on this alone
▪ Krebs Cycle
• Occurs in the mitochondrion
• Carbons are converted into CO2, which is released into the air
• 2 ATP are generated (similar to amount on glycolysis)
▪ Electron Transport Chain
• Occurs in the mitochondrion
• Uses energy released by the fall of electrons to pump H ions across
the inner mitochondrial membrane
• Creates buildup of H+ on one side of the membrane
• Stores potential energy
• Used to make ~36 ATP
• Oxygen is the final electron acceptor
• If the electron isn't accepted, the chain fails to work

Energy: Photosynthesis and Cellular Respiration
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Cyanide blocks the ETC, removing the cell's energy

IMPORTANT
o Photosynthesis only takes place in plants
o Cellular respiration takes place in both animals and plants
What happens if there is no oxygen?
o Anaerobic respiration / Fermentation
▪ Animals
• Switch to anaerobic respiration in absence of oxygen
• Lactic acid builds up, causing burning in muscles
• Produces 2 ATP
▪ Yeast
• Switch to anaerobic respiration in absence of oxygen
• Production of ethanol
• Flavor of the alcohol depends on the source of the sugar



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