Chapter 4
Cells and Energy
I.
Chemical Energy and ATP
A. Chemical energy used for most cell processes is carried by
ATP
1. Carbohydrates and lipids are the most important
energy sources in foods we eat
2. Adenosine triphosphate (ATP) carries the chemical
energy used by cells
3. ATP has three phosphate groups, the bond holding the
third phosphate group is unstable and easily broken.
Energy is released when this bond is broken and
adenosine diphosphate is formed
ATP  ADP + P + energy
B. Organisms break down carbon-based molecules to produce
ATP
1. The number of ATP molecules produced depends on
the type of molecule broken down
a. lipids yield the most ATP (triglycerides 146 ATPs)
b. carbohydrates yield the next most ATP (glucose 36
ATP)
c. proteins yield less ATP
C. Some organisms do not need sunlight and photosynthesis as
a source of energy
1. plants get energy directly from sunlight
(photosynthesis)
2. animals get their energy indirectly from sunlight from
the foods they eat
3. some organisms get their energy from chemicals such
as hydrogen sulfide (H2S) in the process of
chemosynthesis
II. Overview of Photosynthesis
A. Photosynthetic Organisms are Producers
1. Producers provide energy for themselves and other
organisms
2. Photosynthesis – a process that captures energy from
sunlight to make sugars that store chemical energy
3. Chlorophyll – molecule in chloroplasts that absorbs
some of the energy in visible light
B. Photosynthesis in plants occurs in chloroplast
1. Two main parts of chloroplast
a. grana – stacks of coined-shaped, membraneenclosed compartments called thylakoids
b. stroma – fluid that surrounds the grana inside a
chloroplast
2. Light-dependent reaction – captures energy from
sunlight
a. chlorophyll absorbs energy from sunlight. The
energy is transferred along the thylakoid
membrane. H2O molecules are broken down.
Oxygen molecules (O2) are released
b. Energy carried along the thylakoid membrane is
transferred to molecules that carry energy, such as
ATP
3. Light-independent reaction – uses energy from the
light-dependent reaction to make sugars
a. CO2 is added to a cycle of chemical reactions to
build larger molecules, using energy from the lightdependent reaction
b. A molecule of a simple sugar is formed, usually
glucose (C6H12O6)
4. Overall reaction for photosynthesis
6CO2 + 6H2O  C6H12O6 + 6O2
carbon dioxide
water
light, enzymes
glucose
oxygen
III. Photosynthesis in Detail
A. Light-Dependent Reaction
1. Photosystem II and Electron Transport Chain
a. Energy absorbed from sunlight – light-absorbing
molecules in the thylakoids absorb sunlight, energy
is transferred to electrons (e-), the high energy
electrons enter the electron transport chain
b. Water molecules split – enzymes break down water
molecules, hydrogen ions (H+) and electrons are
separated from each other, oxygen (O2) is released,
electrons from water replace electrons that entered
the electron transport chain
c. Hydrogen ions transported – Electrons move from
protein to protein in the electron transport chain,
their energy is used to pump H+ ions into the
thylakoid against the concentration gradient,
electrons move on to Photosystem I
2. Photosystem I
a. energy is absorbed from sunlight into the lightabsorbing molecules, electron are energized and
leave the molecules
b. NADPH is produced – energized electrons are
added to a molecule called NADP+ and a molecule
of NADPH is formed and moves on to the lightindependent reaction
c. Hydrogen ion diffusion – hydrogen ions flow
through a protein channel out of the thylakoid
membrane
d. ATP produced – the protein channel is part of a
complex enzyme called ATP synthase, as the ions
flow through the channel, ATP synthase makes
ATP
B. Calvin Cycle (light-independent reaction)
1. The Calvin Cycle takes place in the stroma
a. carbon dioxide is added – CO2 molecules are added
to five-carbon molecules already in the Calvin cycle
b. Three-carbon molecules are formed – ATP and
NADPH from the light-dependent reaction is used
by enzymes to split the six-carbon, three-carbon
molecules are formed and rearranged
c. Three-carbon molecules exit – most of the threecarbon molecules stay in the Calvin Cycle, after
two three-carbon molecules have left the cycle, they
are bonded together to build a six-carbon sugar
such as glucose
d. Three-carbon molecules are recycled – energy from
ATP is used to change the three-carbon molecules
back to five-carbon molecules to be used in the
Calvin Cycle
C. Functions of Photosynthesis
1. Captures the energy of light and makes chemical
energy (food)
2. Releases Oxygen to be used by consumers
IV. Overview of Cellular Respiration
A. Cellular Respiration makes ATP by breaking down sugars
1. Cellular Respiration is an aerobic process
2. Glycolysis – splits a glucose molecule into two threecarbon molecules and makes two molecules ATP
3. Glycolysis is an anaerobic process
4. Kreb Cycle – produces molecules that carry energy to
the second part of cellular respiration
B. Overall Reaction for cellular respiration
C6H12O6 + 6O2  6CO2 + 6H2O + energy
Glucose
Oxygen
Carbon Dioxide
water
V.
Cellular Respiration in Detail
A. Glycolysis is needed for cellular respiration
1. Two ATP molecules energize a glucose molecule and
split it into two three-carbon molecules
2. NADPH is formed from the electrons from the threecarbon molecules, pyruvate is made from the threecarbon molecules, 4 ATP molecules are made
3. A net of two ATP molecules are made in glycolysis
B. Krebs Cycle
1. Pyruvate broken down into a two-carbon molecule and
carbon dioxide, NADH is formed and moves to the
electron transport chain
2. Coenzyme A bonds to two carbon molecule and goes to
Kreb cycle
3. Citric Acid formed – two-carbon molecule is added to
a four-carbon molecule to form a six-carbon molecule
4. Citric Acid is broken down – an enzyme breaks the
six-carbon molecule down to a five-carbon molecule
5. Five-carbon molecule is broken down, a four-carbon
molecule, a molecule of NADH, and a molecule of ATP
are formed
6. Four-carbon molecule is rearranged
C. The electron transport chain
1. Electrons are removed from NADH and FADH2
2. Hydrogen ions transported along electron transport
chain
3. ATP produced, for each pair of electrons that pass
through the electron transport chain, 3 ATP molecules
are made
4. Water is formed
D. Cellular respiration can make 38 ATP molecules from 1
glucose molecule, but it used 2 ATP molecules for glycolysis
VI. Fermentation
A. Fermentation allows glycolysis to continue in the absence of
oxygen
1. Pyruvate and NADH from glycolysis enter the
fermentation process
2. Two molecules of NAD+ are recycled back to glycolysis
B. Products of fermentation
1. Lactic Acid – builds up in muscles during periods of
anaerobic exertion
2. Alcohol
3. Bread
4. Cheese, yogurt
5. Human digestion