Transcript Chapter 5

Chapter 9
Photosynthesis and Cellular
Respiration
Energy and Living Things
• Energy from the sun enters living systems when
plants, algae, and certain bacteria absorb
sunlight. Some of the energy in sunlight is
captured and used to make organic compounds
that store energy and serves as food for
organisms.
• Metabolism involves either using energy to build
molecules or breaking down molecules in which
energy is stored. Photosynthesis is the
process by which light energy is converted to
chemical energy by autotrophs (organisms that
use energy from the sun or inorganic substances
to make organic compounds.
Breaking Down Food for Energy
• Organisms that must get energy from food
instead of directly from sunlight or inorganic
substances are called heterotrophs.
• Cellular respiration is a metabolic process
similar to burning fuel. It releases much of the
energy in food to make ATP. This ATP provides
cells with the energy they need to carry out the
activities of life.
• In cells, chemical energy stored in food
molecules is released gradually in a series of
enzyme-assisted chemical reactions.
• When cells break down food molecules, some of
the energy in the molecules is released as heat.
Much of the remaining energy is stored in
molecules of ATP. ATP delivers energy wherever
energy is needed in a cell.
ATP Stores and Releases Energy
• ATP (adenosine triphosphate) is a nucleotide
with two extra energy-storing phosphate groups.
The removal of a phosphate group from ATP
produces adenosine diphosphate, or ADP. Cells
use the energy released by this reaction to
power metabolism.
ATP -> ADP + P + energy
5.2, Photosynthesis
•
Plants, algae, and some bacteria capture about 1%
of the energy in the sunlight that reaches Earth and
convert it to chemical energy through the process of
photosynthesis.
Overview:
1. Energy is captured from the sun
2. Light energy is converted to chemical energy, which
is temporarily stored in ATP and NADPH.
3. The chemical energy stored in ATP and NADPH
powers the formation of organic compounds, using
carbon dioxide, CO2
• Photosynthesis occurs in the chloroplasts of plant
cells and algae and in the cell membrane of certain
bacteria.
3CO2 + H2O ->light-> C3H6O3 + 3O2
Stage One, Light Energy Absorbed
• 1st and 2nd stages of photosynthesis are
sometimes called the “light reactions”.
Without the absorption of light, these
reactions would not take place.
• Light is a form of radiation, energy in the
forms of waves that travel through space.
When the sun shines on you, your body is
bombarded by many kinds of radiation
from the sun. However, you can see only
radiation known as visible light.
Pigments Absorb Different
Wavelengths of Light
• Pigments (light absorbing) absorb only certain
wavelengths and reflect all others. Chlorophyll,
the primary pigment involved in photosynthesis,
absorbs mostly blue and red light and reflects
green and yellow. This makes many plants,
especially their leaves, look green.
• Pigments that produce yellow and orange fall
leaf colors as well as on many fruits/veggies are
called carotenoids. Carotenoids absorb
wavelengths of light differently from those
absorbed by chlorophyll, so using both pigments
enables plants to absorb more light energy
during photosynthesis.
Production of Oxygen
• Pigments involved in plant photosynthesis are
located in the chloroplasts of leaf cells. Clusters of
pigments are embedded in the membranes of diskshaped structures called thylakoids.
• When light strikes a thylakoid, energy is transferred
to electrons in chlorophyll and other pigments. This
energy transfer causes the electrons to jump to a
higher energy level. Electrons with extra energy are
said to be “excited”.
• Excited electrons jump from chlorophyll molecules to
other nearby molecules in the thylakoid membrane,
where the electrons are used to power the 2nd stage
of photosynthesis. The excited electrons that leave
chlorophyll molecules must be replaced by other
electrons. Plants get these replacements from water
molecules. Water molecules are split by an enzyme
inside the thylakoid.
Stage Two, Light Energy to
Chemical Energy
• Excited electrons that leave chlorophyll
molecules are used to produce new
molecules that temporarily store chemical
energy (like ATP). The electron is passed
through a series of molecules along the
thylakoid membrane. The series of
molecules through which excited electrons
are passed along a thylakoid membrane
are called electron transport chains.
Action of Electron Transport Chains
• Electron transport chain contains a protein that
acts as a membrane pump. Excited electrons
lose some of their energy as they each pass
through this protein. The energy lost by the
electrons is used to pump hydrogen ions, H+,
into the thylakoid.
• As H+ pass through the channel portion of the
protein, the protein catalyzes a reaction in which
a phosphate group is added to a molecule of
ADP, making ATP. This creates the ATP needed
to complete the 3rd stage of photosynthesis.
• While one electron transport chain provides
energy used to make ATP, a second electron
transport chain provides energy used to make
NADPH. NADPH is an electron carrier that
provides the high energy electrons needed to
make carbon-hydrogen bonds in the 3rd stage.
• Pigment molecules in the thylakoids of
chloroplasts absorb light energy. Electrons in
the pigments are excited by light and move
through electron transport chains in the thylakoid
membranes. These electrons are replaced by
electrons from water molecules. The H+
accumulates to provide energy to make ATP and
NADPH and the O2 is a byproduct.
Stage Three, Energy is Stored
• Carbon atoms from carbon dioxide in the
atmosphere are used to make organic
compounds, in which chemical energy is stored.
The transfer of carbon dioxide to organic
compounds is called carbon dioxide fixation.
• The reactions that “fix” carbon dioxide are
sometimes called “dark reactions” or lightindependent reactions. (This does NOT occur
only at night. It means it doesn’t need light).
The Calvin Cycle
The most common method of carbon dioxide fixation is the
Calvin cycle. The Calvin cycle is a series of enzymeassisted chemical reactions that produces a 3-carbon
sugar (3 CO2s to run one cycle).
•
In carbon dioxide fixation, each molecule of CO2 is
added to a 5-carbon compound by an enzyme.
•
The resulting 6-carbon compound splits into two 3carbon compounds. Phosphate groups from ATP and
electrons from NADPH are added to the 3-carbon
compounds, forming 3-carbon sugars.
•
One of the resulting 3-carbon sugars is used to make
organic compounds in which energy is stored for later
use.
•
The other 3-carbon sugars are used to regenerate the
initial 5-carbon compound, thereby completing the
cycle.
5.3,Cellular Respiration
• Like in most organisms, your cells transfer
the energy in organic compounds,
especially glucose, to ATP through a
process called cellular respiration.
• Oxygen makes the production of ATP
more efficient, although some ATP is made
without oxygen. Metabolic processes that
require oxygen are called aerobic.
Metabolic processes that do not require
oxygen are called anaerobic (means
“without air”).
C6H1206 +6O2 -> 6CO2 + 6H2O + ATP
•
•
Glucose is converted to pyruvate
creating a small amount of ATP and
NADPH.
Aerobic respiration (produces the most
ATP) occurs in the mitochondria of
eukaryotic cells and in the cell membrane
of prokaryotic cells. When oxygen is not
present, pyruvate is converted to either
lactate or ethanol and CO2.
Stage One, Glucose is Broken by
Glycolysis
• In the 1st stage of cellular
respiration, glucose is
broken down in the
cytoplasm, during a process
called glycolysis.
Glycolysis is an enzymeassisted anaerobic process
that breaks down one 6carbon molecule of glucose
to two 3-carbon pyruvates.
• Glycolysis uses two ATP
molecules but produces four
ATP molecules, yielding a
net gain of two ATP
molecules.
Stage Two, More ATP is Made
• When oxygen is present, pyruvate produced during
glycolysis enters a mitochondrion and is converted to a
2-carbon compound. This produces a carbon dioxide
molecule, one NADH molecule, and a 2-carbon acetyl
group. The acetyl group attaches to a coenzyme A
(CoA) forming the compound acetyl-CoA.
• Acetyl-CoA enters a series of enzyme-assisted reactions
called the Krebs cycle.
•When the Krebs cycle is
completed, the 4-carbon
compound that began the cycle
has been recycled, and acetylCoA can enter the cycle again.
•This is also known as the
Citric Acid Cycle.
Electron Transport Chain
• In aerobic respiration, electrons donated by
NADH and FADH2 pass through an electron
transport chain.
• In eukaryotes, the electron transport chain
occurs in the inner membranes of mitochondria.
The energy of these electrons is used to pump
hydrogen ions out of the inner mitochondrial
compartment.
Fermentation Follows Glycolysis in the
Absence of Oxygen
• When oxygen is not present, NAD+ is recycled another
way. Under anaerobic conditions, electrons carried by
NADH are transferred to pyruvate produced during
glycolysis.
• The recycling of NAD+ using an organic hydrogen
acceptor is called fermentation. Two important types of
fermentation are lactic acid fermentation and alcoholic
fermentation.
Lactic Acid Fermentation
•A 3-carbon pyruvate is converted to a three carbon lactate
through lactic acid fermentation.
•Fermentation enables glycolysis to continue producing ATP
in muscles as long as the glucose supply lasts. Blood
removes excess lactate from muscles. Lactate can build up
in muscle cells if it is not removed quickly enough,
sometimes causing muscle soreness.
Alcoholic Fermentation
• 3-carbon pyruvate is broken down to ethanol, a 2-carbon
compound, through alcoholic fermentation. (Carbon
dioxide is released).
• First, pyruvate is converted to a 2-carbon compound,
releasing carbon dioxide. Second, electrons are
transferred from a molecule of NADH to the 2-carbon
compound, producing ethanol. (NAD+ is recycled)
• Alcoholic fermentation by yeast, a fungus, has been
used in the preparation of many foods and beverages.
Wine and beer contain ethanol made during alcoholic
fermentation by yeast. Bread dough rising and the
carbonation of some alcoholic beverages is actually
caused by the releasing of CO2.
• *Just for Fun* Ethanol is actually toxic to yeast. Ethanol
kills yeast at a concentration of about 12%. Thus,
naturally fermented wine contains about 12% ethanol
Comparing Anaerobic Processes
with Aerobic Respiration
• The total amount of ATP that a cell is able to
harvest from each glucose molecule that enters
glycolysis depends on the presence of absence
of oxygen.
• There is a total of 38 ATP made from cellular
respiration, none are produced in
fermentation.
• When oxygen is present, respiration occurs.
When oxygen is not present, fermentation
occurs instead.
• Most of a cells ATP is made during aerobic
respiration.
End of Chapter 5
• Know your key terms.
• Know the steps to photosynthesis (lightdependent and light-independent)
• Know the steps in aerobic respiration
(Krebs cycle)
• Know what happens if O2 is not present.
• Be able to describe ATP