Photosynthesis and
Cellular Respiration
Outline
I. Photosynthesis
A. Introduction
B. Reactions
II. Cellular Respiration
A. Introduction
B. Reactions
Photosynthesis


Method of converting sun energy into chemical
energy usable by cells
Autotrophs: self feeders, organisms capable of
making their own food
–
–
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

Photosynthesis takes place in specialized
structures inside plant cells called chloroplasts
–
Light absorbing pigment molecules e.g. chlorophyll
Overall Reaction
6CO2 + 6H2O → C6H12O6 + 6O2
carbon dioxide
water
light energy
sugar
oxygen

Carbohydrate made is glucose

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
Light-dependent Reactions


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


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




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)



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




So we see how energy enters food chains…
(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



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 cellular respiration
Overall Reaction:
C6H12O6 + 6O2 → 6CO2 + 6H2O
sugar
oxygen
energy
out
carbon dioxide
water
Where Does Cellular
Respiration Takes Place?

takes place in two
parts of the cell:
Glycolysis
occurs in the
Cytoplasm
Krebs Cycle & ETC
Take place in the
Mitochondria
Review of Mitochondria Structure




Smooth outer
Membrane
Folded inner
membrane
Folds called Cristae
Space inside cristae
called the Matrix
Cellular Respiration Overview


Breakdown of glucose begins in the cytoplasm:
the liquid matrix inside the cell
At this point life diverges into two forms and two
pathways
–
Anaerobic cellular respiration (fermentation)
–
Aerobic cellular respiration
Cellular Respiration Reactions

Glycolysis
–
Series of reactions which break 6-carbon glucose
molecule into two 3-carbon molecules “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
Glycolysis Summary
1. Takes place in the Cytoplasm
2. Anaerobic (Doesn’t Use Oxygen)
3. Requires input of 2 ATP
4. Glucose split into two molecules
of Pyruvate or Pyruvic Acid
5. Produces 2 NADH and 4 ATP
6. Pyruvate is oxidized to Acetyl CoA and
CO2 is removed
Anaerobic Cellular Respiration

Some organisms thrive in environments with little or no
oxygen
–



Marshes, bogs, gut of animals, sewage treatment ponds
No oxygen used = anaerobic “not 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)
Fermentation (2 forms)
Occurs when O2 NOT present (anaerobic)
1. Called Lactic Acid fermentation in muscle
cells (makes muscles tired)
2. Called Alcoholic fermentation in yeast
(produces ethanol and CO2)
Nets only 2 ATP
Aerobic Cellular Respiration

Oxygen required = aerobic

2 more sets of reactions which occur in a
specialized structure within the cell called the
mitochondria
–
–
1. Kreb’s Cycle
2. Electron Transport Chain
A Little Krebs Cycle History

Discovered by Hans
Krebs in 1937

He received the Nobel
Prize in physiology or
medicine in 1953 for
his discovery

Forced to leave
Germany prior to
WWII because he was
Jewish
Kreb’s Cycle

Completes the breakdown of glucose
–
–

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
Krebs Cycle Summary

Requires Oxygen (Aerobic)

Cyclical series of oxidation reactions that
give off CO2 and produce one ATP per cycle

Turns twice per glucose molecule

Produces two ATP

Takes place in matrix of mitochondria
Krebs Cycle Summary

Each turn of the Krebs Cycle also
produces 3NADH, 1FADH2, and 2CO2

Therefore, For each Glucose molecule, the
Krebs Cycle produces 6NADH, 2FADH2,
4CO2, and 2ATP
Kreb Cycle
ATP
NETS: 3NADH, 1ATP, 1FADH2, & 2CO2
Electron Transport Chain



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
Electron Transport Summary
34 ATP Produced
 H2O Produced
 Occurs Across Inner Mitochondrial
membrane
 Uses coenzymes NAD+ and FAD+ to
accept e- from glucose
 NADH = 3 ATP’s
 FADH2 = 2 ATP’s

Electron Transport Chain Animation
Glycolysi
s Diagram
Energy Tally


36 ATP for aerobic vs. 2 ATP for anaerobic
–
Glycolysis
2 ATP
–
Kreb’s
2 ATP
–
Electron Transport
32 ATP
36 ATP
Anaerobic organisms can’t be too energetic but
are important for global recycling of carbon