ENERGY FLOW
Photosynthesis and Respiration
CONDITIONS FOR LIFE
• Life exists on this planet because there is a constant
recycling of atoms and a continual input of energy from
the sun.
• Atoms are recycled in nutrient cycles (C, O, N, H ….). For
example you may contain C atoms that were once breathed
out of a dinosaur.
• Energy cannot be recycled and is continually captured by
photosynthetic organisms and passed up food chains.
• Two major processes that cycle atoms and transfer energy
within cells are photosynthesis and cellular respiration.
• Cellular respiration occurs in all cells but only plants are
capable of respiration and photosynthesis.
PHOTOSYNTHESIS
• Autotrophs are organisms that can produce their own food
usually by photosynthesis as shown in the equation below:
• 6 CO2 + 6 H2O + light energy ---> C6H12O6 + 6 O2
• photosynthesis occurs in the chloroplasts of plant cells.
CELLULAR RESPIRATION
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The process that uses oxygen and glucose to produce usable energy in
the cell is cellular respiration:
C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + 36 ATP
Heterotrophs are organisms that rely entirely on respiration for their
energy needs.
Energy released from glucose is repackaged into smaller, more useful
molecules called ATP (adenosine triphosphate).
34 of the 36 ATP released per glucose occurs in the mitochondria of
the cell.
• A close look at the 2 equations shows that the reactions are
closely intertwined.
• Recycling of carbon, oxygen, and hydrogen occurs in
ecosystems as well
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Cellular respiration consists of a series of reactions that break down
glucose in small steps.
Energy from some of these steps is captured and stored as ATP which
is the cell’s energy currency.
By coupling energy releasing reactions (exergonic) from cellular
respiration to energy absorbing reactions (endergonic) in the
conversion of ADP + P --> ATP, energy can be transferred from the
bonds of glucose to the more useful ATP molecules.
ATP is a high energy molecule because of the energy stored in the
bond between the 2nd and 3rd phosphate.
• Notice in the diagram below that energy stored in ATP at
one place in the cell (ie. Mitochondria) can be released
elsewhere in the cell when ATP is converted to the lower
energy form, ADP (ie. Active transport)
• Think of ATP & ADP like a rechargeable battery which
continually reused in the cell.
• Cellular respiration in cells occurs in 3 main steps:
1 . Glycolysis occurs in the cytoplasm and breaks glucose down partially
into pyruvic acid which can move into the mitochondrion for further
breakdown.
2 . Kreb’s Cycle occurs in the matrix of the mitochondrion and finishes
the break up of glucose into carbon dioxide.
3 . Electron Transport System (ETS) occurs on the inner membrane of
the mitochondrion and uses high energy electrons from glucose to
produce most of the ATP. Oxygen is the final e- acceptor which also
bonds with protons to form water.
STEP BY STEP NRG RELEASE
TYPES OF RESPIRATION
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Aerobic respiration takes place in the presence of oxygen with the
complete oxidation of glucose into carbon dioxide and water and the
formation of 36 ATP molecules per glucose molecule.
Since O2 is the final electron acceptor, more energy can be extracted
from these electrons as they pass through the ETS.
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Anaerobic respiration takes place in the absence of oxygen. Two
types are lactic acid fermentation and alcoholic fermentation.
Lactic acid fermentation occurs in your muscle cells when not
enough oxygen is present to meet the muscle’s energy needs.
Extra glucose is partially broken down to lactic acid which releases
some extra energy for the muscles (2 ATP / glucose).
Equation: Glucose ----> lactic acid + 2 ATP
lactic acid build-up in the muscles is responsible for the burning
sensation felt during and after strenuous exercise.
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Alcoholic fermentation occurs in the cytoplasm of yeast cells.
This process also only yields 2 ATP / glucose molecule but produces
ethanol and carbon dioxide instead of lactic acid.
Although it is only 2% of the energy from glucose, it is enough to
allow yeast cells to survive in anaerobic environments.
Equation: Glucose -----> ethanol + CO2 + 2 ATP
Yeast is used to ferment wines and beers and it’s CO2 bubbles cause
bread dough to rise (fortunately ethanol is baked off in the oven).
Ethanol still contains 93% of the energy from glucose and could be a
valuable fuel for industry and transportation.
REVIEW
• What is the equation for cellular
respiration?
• C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + 36 ATP
• Which has more energy, ATP or ADP?
• ATP ---- energy stored between P’s
• Is cellular respiration endergonic or
exergonic?
• Exergonic --- energy is released
REVIEW
• Where in the cell does fermentation occur?
• Cytoplasm (off-shoot of glycolysis)
• What are the products of fermentation in
your cells?
• Lactic acid & ATP
• Compare the energy release in aerobic Vs
anaerobic respiration
• Aerobic = 36 ATP/ glucose molecule
• Anaerobic = 2 ATP/ glucose molecule