Energy in Cells
The Need For Energy
•All living organisms must be able to obtain energy from the environment in
which they live.
•Plants and other green organisms are able to trap the light energy in
sunlight and store it in the bonds of certain molecules for later use.
•Other organisms cannot use sunlight directly.
•They eat green plants. In that way, they obtain the energy stored in plants.
The Source of Energy
•Every cell in a living organism must be provided with a supply of energy to
power movement, cell division, active transport, ect.
•The molecule used by cells for energy is Adenosine triphosphate (a.k.a.
“ATP”).
•Remember energy is stored when chemical bonds are formed; energy is
released when chemical bonds are broken.
•Cells get to the energy stored in ATP molecules by breaking the chemical
bond that holds the 3rd phosphate in place.
Where does ATP come from?
•During photosynthesis, plants store the energy from the sun by making
molecules of glucose and other simple sugars.
•In the photosynthesis reaction, carbon dioxide and water are united to form
glucose and release oxygen.
CO2 + H2O+ Sunlight  C6H12O6 + O2
•Individual cells in plants and animals must take the glucose molecules
provided to them and exchange them for smaller units of energy…ATP.
•This process is called “respiration.”
Photosynthesis (Trapping the Sun’s Energy)
•Photosynthesis happens in two phases:
–The light-dependent reactions convert light energy into chemical
energy.
–The molecules of ATP produced in the light-dependent reactions are then
used to fuel the light-independent reactions that produce simple sugars.
–These reactions occur in the plant’s chloroplasts.
Chloroplasts
•The cellular organelle where photosynthesis occurs.
•Contain chlorophyll.
•Chlorophyll absorbs most wavelengths of light except green.
Light-Dependent Reactions (sunlight is necessary)
•As sunlight strikes the chlorophyll molecules the energy in the light is
transferred to electrons.
•These highly energized, or excited, electrons are passed from chlorophyll to
an electron transport chain, a series of proteins embedded in the membrane.
•As the excited electrons are passed along from protein to protein the
electrons lose a little energy with each transfer.
•The energy released by the electrons is used for 2 purposes:
–To split apart H2O molecules.
–To form ATP from ADP & PO4.
•The oxygen from the split water molecules is released into the atmosphere.
Light Independent Reactions (the Calvin Cycle) (no sunlight is needed)
•CO2 from the atmosphere and H2 from split water molecules are used to
produce simple sugars and carbohydrates.
•See diagram on p.229.
Photosynthesis Summary
•Light Reactions:
–Sunlight used to split H2O & form ATP molecules.
•Calvin Cycle:
–Powered by ATP from light reactions.
–Sugars, (such as glucose) are formed.
Cellular Respiration
Converting Glucose to ATP
Purpose
•Chemical energy is stored in lipids, and carbohydrate molecules.
•These are broken down into glucose which can then be transported to
individual cells.
•However, glucose is much too large a package of energy to be useful to
cells.
•Glucose must be converted to ATP for use by the cell.
•This process is called cellular respiration.
Aerobic Respiration – involves the use of oxygen in the process of breaking
down glucose.
Anaerobic Respiration – glucose is broken down without oxygen.
Aerobic Respiration
•Requires oxygen.
•Involves 3 major reactions:
–Glycolysis
–Citric Acid Cycle (Kreb’s Cycle)
–Electron Transport Chain
Glycolysis
•Occurs in the cytoplasm.
•One 6-carbon glucose molecule is broken down into two 3-carbon pyruvic
acid molecules
•The reaction of glycolysis requires 2 ATP molecules in order to get started
(like a match that starts a fire).
•When the glucose molecule is split, 4 ATP molecules are immediately
produced.
•The cell is now 2 ATP’s ahead of where it started. (-2+4=2)
The Citric Acid Cycle
•The 2 pyruvic acid molecules then enter the mitochondrion.
•Each pyruvic acid molecule is further broken down and is cycled through a
series of chemical reactions (see p. 233).
•For every turn of the cycle, one molecule of ATP and two molecules of CO2
are produced (2ATP’s total produced from the two 3-carbon pyruvic acid
molecules)
•Two energized electrons are also left over from the reactions. These must
be removed from the cell.
The Electron Transport Chain
The energized electrons produced by the citric acid cycle enter the
membrane of the mitochondrion.
•The electrons are passed from protein to protein within the membrane
releasing energy with each transfer and producing ATP molecules.
•A total of 32 ATP’s are produced by the electron transport chain.
•At the end of the chain of protein molecules, an oxygen molecule, O2
absorbs the electron, becomes negatively charged, and bonds with four
positively charged hydrogen ions.
•What is produced by this union?
2 oxygen atoms + 4 hydrogen atoms = 2 water molecules
•The CO2 produced in the citric acid cycle and the H2O produced by the
electron transport chain are passed through the plasma membrane and into
the bloodstream.
•They are brought in the blood to the lungs where they are exhaled.
Aerobic Respiration ATP Summary
From one glucose molecule:
-2 ATP begins glycolysis
4 ATP produced by glycolysis
2 ATP produced by citric acid cycle
+32ATP produced by elec. trans. chain
36 total ATP molecules produced
ANAEROBIC RESPIRATION
Begins with glycolysis, and then continues down one of two possible
pathways:
Alcoholic Fermentation:
Occurs in Fungi and some bacteria
Pyruvate is converted to ethyl alcohol and carbon dioxide.
The ethyl alcohol is released into the environment and the carbon dioxide is
released into the atmosphere.
No additional ATP molecules are produced after glycolysis, therefore there is
only a net production of 2 ATP.
Lactic Acid Fermentation:
Occurs in muscle cells
Pyruvate is converted to lactic acid and carbon dioxide.
The lactic acid causes muscle soreness, the carbon dioxide is exhaled.
No additional ATP molecules are produced after glycolysis, therefore there is
only a net production of 2 ATP.