CELL ENERGETICS
Photosynthesis and Cellular Respiration
Photosynthesis
• All energy for Earth is from the sun.
• Photoautotrophs are organisms that can
convert light energy to chemical energy.
• This process is called photosynthesis and
can be summarized in the following
equation:
light energy
6H2O + 6CO2
6O2+ C6H12O6
Photosynthesis
• Photosynthesis takes place in the
chloroplasts of plant cells and consists of 2
sets of reactions.
• Light dependent reactions take place in the
thylakoid and use water and light energy.
Oxygen is a product of these reactions.
• Light independent reactions take place in
the stroma and use carbon dioxide.
Carbohydrates are a product of this reaction.
Light Dependent Reactions
• Sunlight strikes chlorophyll molecules.
• The energy of sunlight splits water molecules
(photolysis).
• Oxygen atoms are released.
• Hydrogen atoms are stripped of their
electrons, which are passed to the electron
carrier NADP+ forming NADPH and H+.
• The concentration gradient of hydrogen is
used to drive an ATP synthase, making ATP.
Light Independent Reactions
• Carbon dioxide, ATP, and the electrons
carried by NADP+ are used to build sugar.
• The molecule RuBP rearranges carbon
dioxide to form an intermediate molecule
called PGA .
• PGA is converted to PGAL, using ATP and
the NADP+.
• Some PGAL is used to make glucose and
some is used to regenerate RuBP to continue
the process.
Photosynthesis
CO2
Water
ADP + Pi +
NADP+
RuBP PGAL
ATP + NADPH +
H+
Light Dependent
Reactions
O2
Light Independent
Reactions
Carbohydrates
Cellular Respiration
• Neither autotrophs nor heterotrophs can
utilize the energy of the carbohydrate as it is.
Both types of organisms must convert the
carbohydrate to ATP, the energy currency of
the cell, in order to carry out metabolic
activity.
• This process is called cellular respiration and
can be summarized in the following equation:
C6H12O6 + 6O2
+ ATP
6H2O + 6CO2
Cellular Respiration
• Cellular respiration takes place in the cytoplasm
and the mitochondria of eukaryotic cells and
consists of 3 sets of reactions.
• Glycolysis occurs in the cytoplasm and breaks
down glucose to pyruvate, producing 2 molecules
of ATP.
• The Kreb’s Cycle occurs in the mitochondrial
matrix and breaks down pyruvate, producing
CO2 and 2 molecules of ATP.
• Oxidative phosphorylation occurs in the
mitochondrial matrix, uses oxygen, and produces
32 molecules of ATP.
Glycolysis
• Glycolysis breaks down glucose, a 6-carbon
sugar, into 2 molecules of pyruvate, a 3carbon intermediate.
• When glucose is broken down, some of the
hydrogen atoms are stripped of their
electrons, which are passed to the electron
carrier NAD+, forming NADH and H+.
• An initial investment of 2 ATP are used to
reconfigure the glucose atoms, but 4 ATP are
produced in the process for a net total of 2
ATP.
Kreb’s Cycle
• Pyruvate is converted to Acetyl-Coenzyme A
in the cytoplasm of the cell.
• Acetyl-CoA moves into the mitochondrial
matrix, and through a series of reactions,
carbon and oxygen molecules are stripped,
releasing CO2, and electrons are passed to
NAD+ and FAD, forming NADH and FADH2
and many H+.
• These reactions generate 2 molecules of
ATP.
Oxidative Phosphorylation
• The only substances that move into the
mitochondrial membrane are the electron
carriers, NADH and FADH2, and the hydrogen
protons.
• The hydrogen electrons are passed through a
series of protein complexes and ultimately to
oxygen, which accepts them.
• Just like in the light dependent reactions of
photosynthesis, the hydrogen protons develop a
steep concentration gradient, which drives the
production of ATP by an ATP synthase. 32
molecules of ATP are produced.
Cellular Respiration
Electrons carried
via NADH
Electrons carried via
NADH and FADH2
OXIDATIVE
PHOSPHORYLATION
KREB’S
CYCLE
GLYCOLYSIS
Glucose  Pyruvate
ATP
ATP, CO2
ATP, H2O
O2
Fermentation
• If oxygen is present, aerobic cellular
respiration is done.
• In the absence of oxygen, anaerobic
respiration takes place.
– Glycolysis occurs.
– Following glycolysis, the pyruvate is converted to
either lactic acid or ethanol. This is fermentation.
– Fermentation involves very few chemical
reactions (compared to cellular respiration) but
only produces 2 molecules of ATP.