Photosynthesis and
Cellular Respiration
Outline
I. Photosynthesis
A. Introduction
B. Reactions
II. Cellular Respiration
A. Introduction
B. Reactions
Photosynthesis
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Method of converting sun energy into chemical
energy usable by cells
Autotrophs: self feeders, organisms capable of
making their own food
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Photoautotrophs: use sun energy e.g. plants
photosynthesis-makes organic compounds (glucose)
from light
Chemoautotrophs: use chemical energy e.g.
bacteria that use sulfide or methane
chemosynthesis-makes organic compounds from
chemical energy contained in sulfide or methane
Photosynthesis
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Photosynthesis takes place in specialized
structures inside plant cells called chloroplasts
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Light absorbing pigment molecules e.g. chlorophyll
Overall Reaction
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6CO2 + 12 H2O + light
energy → C6H12O6 + 6O2+ 6H2O
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Carbohydrate made is glucose
Water appears on both sides because 12 H2O molecules
are required and 6 new H2O molecules are made
Water is split as a source of electrons from hydrogen
atoms releasing O2 as a byproduct
Electrons increase potential energy when moved from
water to sugar therefore energy is required
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Light-dependent Reactions
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Overview: light energy is absorbed by
chlorophyll molecules-this light energy excites
electrons and boosts them to higher energy
levels. They are trapped by electron acceptor
molecules that are poised at the start of a
neighboring transport system. The electrons
“fall” to a lower energy state, releasing energy
that is harnessed to make ATP
Energy Shuttling
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Recall ATP: cellular energy-nucleotide based
molecule with 3 phosphate groups bonded to it,
when removing the third phosphate group, lots of
energy liberated= superb molecule for
shuttling energy around within cells.
Other energy shuttles-coenzymes (nucleotide
based molecules): move electrons and protons
around within the cell
NADP+, NADPH
NAD+, NADP
FAD, FADH2
Light-dependent Reactions
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Photosystem: light capturing unit, contains chlorophyll,
the light capturing pigment
Electron transport system: sequence of electron
carrier molecules that shuttle electrons, energy released
to make ATP
Electrons in chlorophyll must be replaced so that cycle
may continue-these electrons come from water
molecules, Oxygen is liberated from the light reactions
Light reactions yield ATP and NADPH used to fuel the
reactions of the Calvin cycle (light independent or dark
reactions)
Calvin Cycle (light independent or
“dark” reactions)
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ATP and NADPH generated in light reactions
used to fuel the reactions which take CO2 and
break it apart, then reassemble the carbons into
glucose.
Called carbon fixation: taking carbon from an
inorganic molecule (atmospheric CO2) and
making an organic molecule out of it (glucose)
Simplified version of how carbon and energy
enter the food chain
Harvesting Chemical Energy
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So we see how energy enters food chains (via
autotrophs) we can look at how organisms use
that energy to fuel their bodies.
Plants and animals both use products of
photosynthesis (glucose) for metabolic fuel
Heterotrophs: must take in energy from outside
sources, cannot make their own e.g. animals
When we take in glucose (or other carbs),
proteins, and fats-these foods don’t come to
us the way our cells can use them
Cellular Respiration Overview
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Transformation of chemical energy in food into
chemical energy cells can use: ATP
These reactions proceed the same way in plants
and animals. Process is called cellular
respiration
Overall Reaction:
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C6H12O6 + 6O2 → 6CO2 + 6H2O
Cellular Respiration Overview
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Breakdown of glucose begins in the cytoplasm:
the liquid matrix inside the cell
At this point life diverges into two forms and two
pathways
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Anaerobic cellular respiration (aka fermentation)
Aerobic cellular respiration
C.R. Reactions
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Glycolysis
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Series of reactions which break the 6-carbon glucose
molecule down into two 3-carbon molecules called
pyruvate
Process is an ancient one-all organisms from simple
bacteria to humans perform it the same way
Yields 2 ATP molecules for every one glucose
molecule broken down
Yields 2 NADH per glucose molecule
Anaerobic Cellular Respiration
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Some organisms thrive in environments with little or no
oxygen
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Marshes, bogs, gut of animals, sewage treatment ponds
No oxygen used= ‘an’aerobic
Results in no more ATP, final steps in these pathways
serve ONLY to regenerate NAD+ so it can return to pick
up more electrons and hydrogens in glycolysis.
End products such as ethanol and CO2 (single cell fungi
(yeast) in beer/bread) or lactic acid (muscle cells)
Aerobic Cellular Respiration
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Oxygen required=aerobic
2 more sets of reactions which occur in a
specialized structure within the cell called the
mitochondria
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1. Kreb’s Cycle
2. Electron Transport Chain
Kreb’s Cycle
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Completes the breakdown of glucose
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Takes the pyruvate (3-carbons) and breaks it down,
the carbon and oxygen atoms end up in CO2 and H2O
Hydrogens and electrons are stripped and loaded onto
NAD+ and FAD to produce NADH and FADH2
Production of only 2 more ATP but loads up
the coenzymes with H+ and electrons which
move to the 3rd stage
Electron Transport Chain
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Electron carriers loaded with electrons and
protons from the Kreb’s cycle move to this chainlike a series of steps (staircase).
As electrons drop down stairs, energy released
to form a total of 32 ATP
Oxygen waits at bottom of staircase, picks up
electrons and protons and in doing so becomes
water
Energy Tally
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36 ATP for aerobic vs. 2 ATP for anaerobic
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Glycolysis
2 ATP
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Kreb’s
2 ATP
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Electron Transport
32 ATP
36 ATP
Anaerobic organisms can’t be too energetic but
are important for global recycling of carbon