Download 578 18.2 Photosynthesis and Respiration - District 196 e

Section 18.2 Photosynthesis and Respiration
18.2 Photosynthesis and Respiration
Plants use
sunlight to
make sugars
and starches
In the previous section we established that the energy living
things need comes from food.
The sugar molecule,
glucose, is the most important energy yielding molecule in
living systems. Here we will look at how the glucose
molecule is formed in a process called photosynthesis.
Plants harness sunlight and use it’s energy to convert water
and carbon dioxide molecules into sugars and starches
during the process of photosynthesis. Photosynthesis is the
ultimate first step in this natural pathway that uses carbon
atoms from carbon dioxide as building blocks to eventually
make complex organic molecules. Plants use the energy
they harness to grow and transport nutrients. Photosynthesis also makes food energy
available to animals that eat plants. Animals that consume plants take the energy
harnessed in the molecules and use it to carry out life processes.
The overall equation
for photosynthesis is:
This equation may look familiar if you remember it from biology or from the chemical
reactions chapter 10, when you were learning about endothermic reactions.
Carbon dioxide
and water are
the building
blocks
Plants, algae and some bacteria are capable of using sunlight to make glucose. Plants use
water and carbon dioxide gas as the building blocks for sugar molecules. These sugar
molecules are turned into starch for more efficient storage. Lucky for us oxygen is also
produced! Plants get the carbon dioxide that they need from the air or from water
depending on where they grow. Plants need a certain amount of sun and water to flourish.
Many of us who have raised plants have seen this first hand.
Chlorophyll
Energy is required to make sugars from stable molecules like CO2 and H2O. So how does
the plant use the energy of sunlight to make larger six carbon molecules? The answer to
this lies in a pigment molecule called chlorophyll. Chlorophyll is a large molecule
capable of absorbing sunlight in the blue, violet and red wavelengths of the visible
spectrum. Chlorophyll transfers this absorbed light energy to electrons within its
molecular structure, and these electrons become higher in energy. These high energy
electrons make the process of photosynthesis work by indirectly providing the energy to
break chemical bonds and reform new chemical bonds.
photosynthesis - the process where plants and algae, capture sunlight and use it’s
energy to convert water and carbon dioxide into glucose (sugar) and oxygen.
chlorophyll - pigment molecule that absorbs sunlight and uses it to excite electrons
in it’s molecular structure, thereby storing energy to aid in photosynthesis.
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Overview of photosynthesis (part 1)
How does a plant put together carbon dioxide and water molecules to form glucose and
oxygen? The first part of this process involves water molecules being split apart using the
energy absorbed by sunlight. These are often referred to as the “light dependent”
reactions of photosynthesis, because they depend upon sunlight. Chlorophyll molecules
absorb light which allows the water molecules to be split apart. Each water molecule is
split apart into 2 electrons, 2H+ ions, and 1 oxygen atom. As electrons continue to be
removed from water, oxygen is released into the air. The reaction looks like this:
2H2O(l) → 4H+(aq) + O2(g) + 4 electrons.
These four electrons are passed onto special “carrier molecules” that are
capable of transferring the energy obtained while loosing very little of it. It
takes very special molecules to do this. Nicotinamide Adenine Dinucleotide
Phosphate (NADP+), is such a molecule. It is able to pass these high energy
electrons through a series of oxidation and reduction reactions. One NADP+
molecule is capable of holding two high energy electrons and a hydrogen ion
to form NADPH. The formation of NADPH is one way sunlight can be stored
in a chemical form. The energy from sunlight is stored in the chemical bonds
of a “carrier molecule” like NADPH. NADPH can then be oxidized to once
again form of NADP+.
ATP molecule
Another very important molecule that stores
energy is formed during these light dependent
reactions. This molecule is adenosine
triphosphate or ATP. ATP also “holds” and
carries energy in it’s bonds. Specifically in the
bonds between each phosphate group, look
carefully at the picture on the right. The
phosphate groups are broken off by the
addition of water molecules, and the energy
released is used to power chemical reactions that require energy.
ATP & NADPH
carry energy to
chemical
reactions
The important piece to understand about ATP and NADPH is that they carry energy from
spontaneous reactions to non-spontaneous reactions. These molecules that are formed
provide the energy for the next step of photosynthesis.
NADPH- molecule that carries two high energy electrons and stores sunlight as
chemical energy.
ATP- adenosine triphosphate. A molecule that carries chemical energy from
spontaneous chemical reactions to non spontaneous reactions.
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Section 18.2 Photosynthesis and Respiration
Overview of photosynthesis (part 2)
CO2 is added to
carbon
molecules to
make glucose
In the second phase of photosynthesis the Calvin cycle, sunlight is not required and
these reactions are sometimes referred to as the “dark reactions” in contrast to the light
reactions.
During the Calvin cycle, carbon dioxide is added to other carbon
molecules and the six carbon molecule of glucose is formed
However, the steps of this process require energy and are non spontaneous. They could
not occur with out the NADPH and the ATP molecules formed in the first part! The
energy harnessed in these molecules helps to put together the six carbon sugar of glucose.
NADPH and
ATP provide the
energy for
these reactions
The way nature works in simple yet interconnected ways is amazing.
Here you can see that the light dependent reactions take in water and light and release
ATP and NADPH molecules that feed into the Calvin cycle. The Calvin cycle takes in
carbon dioxide and the high energy molecules to make the sugar glucose. The Calvin
cycle recycles the high energy molecules back to the light dependent reactions. Together
the light and dark reactions use six carbon dioxide molecules to make a single six carbon
molecule of glucose.
Plants work continuously removing CO2 from our atmosphere and making
energy rich sugar molecules
Plants use glucose for their own energy needs and to make more complex molecules like
cellulose and starch. These more complex molecules are used for structure and growth.
Calvin cycle - makes energy rich sugars from CO2 by using the high energy
molecules ATP and NADPH formed in the light dependent reactions.
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Cellular respiration
Cellular
respiration
provides
energy for life
processes
We now need to understand how we
use food molecules to give us energy.
How is the six carbon sugar, glucose,
metabolized and utilized by our
bodies? We know that chemicals
store energy in their bonds, but how
do living systems obtain this energy
and use it for life processes? The
glucose molecule gives us energy
through the process of cellular
respiration. During cellular respiration, glucose and other food molecules are broken
apart in the presence of oxygen and energy is released.
The overall chemical equation for cellular respiration is :
6O2(g) + C6H12O6 −> 6CO2(g) + 6H2O(l) + Energy
Oxygen is a powerful electron acceptor in this process. Without oxygen cellular
respiration cannot take place. This equation is greatly over-simplified, because it makes
the process appear to happen all at once. Similar to other chemical reactions it actually
occurs in a series of steps.
Slow release of
energy from
glucose allows
it to be
captured
Cellular respiration involves three steps. The three step process helps to
control the slow release of energy from glucose
If the release of energy from the bonds of the glucose molecule is not slow, too much of
the energy will be lost as heat and therefore unavailable to use for other life processes.
There must be a way to capture some of the energy released when the chemical bonds in
glucose are broken. Each of the three steps in cellular respiration captures energy. The
energy captured by forming ATP molecules provides the energy for transporting nutrients
across cell membranes, powering muscle contractions during movement, and
maintaining body temperature. Many of the bodies chemical reactions require energy and
the ATP molecule provides this energy.
cellular respiration - breaks down glucose and other food molecules in the
presence of oxygen and releases energy that is used to carry out life processes.
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Section 18.2 Photosynthesis and Respiration
Respiration: Glycolysis
The first step in cellular respiration is glycolysis (gly-KOHL-ih-sis). Glycolysis is
carried out in the cytoplasm of plant and animal cells. During glycolysis one molecule of
glucose is broken in half yielding two 3 carbon molecules.
Some energy is
required to
split the
glucose
molecule.
You’ll notice in the first step to split the glucose molecule apart 2ATP’s are required, but
then in the second step 4 ATP’s are produced. A little energy needs to be added at the
start of the reaction, but more energy is obtained in the overall process. This is similar to
charging the battery on your cell phone. You have to plug it in and use energy to charge
your phone when the battery is low, but once charged you get many more hours of power
back. The amount of energy you get back from the charged battery represents a larger
return. Glycolysis also gives a larger return!
In the second step you will see that 2 NADH, nicotinamide
adenine dinucleotide molecules are formed. Like NADPH in
photosynthesis NADH is a high-energy electron carrier
molecule. NADH is made of two nucleotides, adenine and
nicotinamide, which are connected through their phosphate
groups. In it’s oxidized form, after it loses the 2 electrons it
carries, NADH becomes NAD+. Each NAD+ molecule is able
to accept two high-energy electrons, becoming reduced to
NADH once again. NADH acts as a strong reducing agent!
In cell respiration, NADH sends the electrons to a pathway,
where they are used to make ATP molecules.
Overall energy
yield of
glycolysis is
small
The overall energy yield of glycolysis is small, however it can produce energy quickly,
and without oxygen. When oxygen is present, a second step called the Krebs cycle is able
to proceed. The 3 carbon molecules of purveyed acid are passed on to the Krebs cycle,
where they release more energy.
glycolysis - (gly-KOHL-ih-sis) first step in cellular respiration. During glycolysis one
molecule of glucose is broken in half yielding two 3 carbon molecules (of pyruvic
acid).
NADH - nicotinamide adenine dinucleotide; is a molecule capable of carrying highenergy electrons and transferring them to another pathway.
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Respiration: The Krebs cycle and electron transport
The Krebs cycle
removes
energy while
breaking
chemical
bonds
Glycolysis only uses a small portion of the energy available
in the glucose molecule, about 10%. The majority of the
energy still remains locked inside the pyruvic acid molecules.
These 3 carbon molecules enter the Krebs cycle. The
Krebs cycle is a sequence of chemical reactions that remove
energy while breaking pyruvic acid down to carbon dioxide.
During the steps of the Krebs cycle the three carbons in pyruvic acid are lost as single
carbons in the form of carbon dioxide. The breakdown of pyruvic acid yields several
more ATP molecules for the cell to use. The rest of the energy is yielded in the form of
high energy electrons. NADH and other carrier molecules bring these high-energy
electrons to the electron transport chain. The electron transport chain uses these high
energy electrons from the Krebs cycle to make more ATP molecules. To accomplish this
the electron transport chain uses special proteins called cytochromes to transfer the
electrons, which we will discuss in the next section about proteins. In general, each pair
of electrons that moves along the chain has enough energy to form 3 ATP molecules.
Oxygen is the
final electron
acceptor
At the end of the electron transport chain, hydrogen ions, H+ and electrons combine with
oxygen to form water. Oxygen serves as the final electron acceptor. Oxygen is necessary
to remove the low-energy electrons, and hydrogen ions which are considered waste
molecules.
4H+ + 4e- + O2 → 2H2O(l)
Overall the process of cellular respiration uses roughly 38% of the energy in the glucose
molecule. The other 62% is lost as heat.
Krebs cycle - a series of energy extracting chemical reactions that break pyruvic
acid down into CO2(g), during cellular respiration.
electron transport chain - uses high-energy electrons from the Krebs cycle to
make ATP molecules.
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