Josh Ye
Ms. Buckley
AP Biology
12/3/2018
1. Distinguish between aerobic cellular respiration and photosynthesis in terms of
enthalpy (𝛥
𝛥H), entropy (𝛥
𝛥S), and Gibb’s free energy (𝛥
𝛥G).
Photosynthesis converts light energy into chemical energy stored in organic molecules. Aerobic
cellular respiration uses oxygen in the breakdown of glucose (or of other energy-rich organic
compounds) to yield carbon dioxide and water, in addition to releasing energy as ATP and heat.
Both are processes of ​metabolism​, which includes all the chemical reactions in an organism.
There are two classifications of reactions based on their free-energy changes. ​Exergonic
reactions​ proceed with a net release of free energy, while ​endergonic reactions​ absorb energy
from its surroundings, storing free energy in molecules. Since aerobic cellular respiration is an
exergonic reaction and releases energy by the breakdown of glucose, the Gibb’s free energy
change (𝛥G) will be negative. Photosynthesis, on the other hand, is an exergonic reaction,
storing energy in the form of chemical energy and absorbing photons to power this process.
Thus, the Gibb’s free energy change (𝛥G) will be positive.
Entropy (𝛥S), is a measure of disorder or randomness. One of the most disordered forms
of energy is heat. Thus, the release of heat into our surroundings is said to be be increasing the
entropy of the universe. As the cell breaks down glucose to power the production of ATP, heat is
produced as a result. Thus, the 𝛥S of aerobic cellular respiration must be positive. On the other
hand, the process of photosynthesis synthesizes a highly ordered carbohydrate (G3P) from more
disordered reactants, sunlight and gasses. Thus, photosynthesis reduced entropy and 𝛥S is
negative.
Since enthalpy (𝛥H) measures the total energy of a system, the 𝛥H in aerobic cellular
respiration must be negative because energy is used as glucose is broken down to power cellular
activities. In contrast, in photosynthesis 𝛥H must be positive because energy is being stored in
the form of chemical energy, thus increasing the total energy of the system
The process of aerobic cellular respiration involves the transfer of electrons between
molecules. When the glucose is oxidized, it transfers electrons to a lower energy state. This
releases energy,
2. Compare and contrast the electron transport chains of photosynthesis to those in cellular
respiration. Provide at least TWO similarities and THREE differences.
In photosynthesis, two different electron transport chains produce two different products,
ATP and NADPH for use in the Calvin Cycle. In cellular respiration, a single electron transport
chain produces ATP for use in other cellular processes. One similarity between the two processes
is the use of chemiosmosis to power the production of ATP. In both cellular respiration and
photosynthesis, electrons are pumped through a series of carriers that are increasingly
electronegative. The exergonic fall of electrons down the chains create a proton-motive force
which, through chemiosmosis, is used to generate ATP. In addition, the structure of the
mitochondrion is quite similar to that of the chloroplast. Both structures have a membrane that
separates areas of high H+ concentration and low H+ concentration.
The electron chains of these two processes have notable differences, however. Firstly, the
electrons that are used in the electron transport chain in cellular respiration are derived from
food, while the electrons that are used in photosynthesis are derived from water. In addition, in
photosynthesis, energy from photons is required to raise the electrons to an excited state. This is
accomplished with the use of two photoreceptor proteins, Photosystem I and Photosystem II. In
cellular respiration, however, there is no need to excite the electrons because they already are in
an excited state. Another difference is that the electron carrier molecules used in each of these
processes are different. In photosynthesis, the carrier molecule is NADPH, while in cellular
respiration the carrier molecule are NADP and FADH2. A third difference is that in
photosynthesis, the reduction of electron carriers occurs within the electron chain through the
enzyme NADP+ reductase. In cellular respiration, however, this reduction takes place through
glycolysis, pyruvate oxidation and the Citric Acid cycle.
3. Describe the similarities and differences between the Calvin cycle and the citric acid
(Krebs) cycle. Provide at least TWO similarities and THREE differences.
In plants, the calvin cycle is a process which plants use to synthesize sugar. This is
different from cellular respiration, where in the citric acid cycle NAD+ and FADH are reduced to
form NADH and FADH2, and ATP is produced. The citric acid cycle harvests the energy of
glucose, rather than producing it. In addition, the inputs are different. In cellular respiration,
Acetyl CoA combines with Oxaloacetate, while in the calvin cycle, the input, CO2, is combined
with Ribulose biphosphate. Finally, the calvin cycle only occurs in plants, while the citric acid
cycle occurs in the mitochondria of all cells. However, these processes are also in some sense
quite similar. Both are cyclical processes, where the last step of each cycle is combined with the
input to continue the process. In addition, neither process requires light to operate.
4. Biological molecules can be separated by using chromatography techniques. The
diagram above shows the separation of several spinach leaf pigments by paper
chromatography. Using the diagram above:
a. Explain how paper chromatography can be used to separate pigments based on
their chemical and physical properties.
Because pigments are molecules and different molecules have distinct chemical
properties, the solubilities of different pigments will be different, meaning that different
pigments will dissolve at different rates in a solvent. In addition, different pigments will
hydrogen bond with the cellulose of the paper at different strengths, also affecting the rate of
movement. If a pigment extract contains a mixture of several distinct pigments, paper
chromatography can be used to separate the pigments out into easily identifiable lines.
To do this, place the leaf pigment extract in a line towards the bottom of the paper. This
will be your start line. Then, immerse the paper in a small amount of solvent (acetone/petroleum
ether solution). As the solvent travels up the the paper, it will carry the different pigment
molecules with it at different rates. After the paper is removed from the solvent, if the extract is a
mixture, the lines formed by the different pigments should be clearly distinguishable.
b. Discuss the role of pigments both in capturing light energy and in converting it to
the chemical energy of ATP and NADPH.
Light behaves as if it consists of particles called photons. When a pigment molecule (in
photosystem II) absorbs energy from a photon with enough energy, one of its electrons jumps to
a higher energy orbital. The energy produced by this process is transferred between each of the l
molecules of the photosystem, and finally transferred to a special pair of chlorophyll a molecules
that transfer the excited electron the primary electron acceptor. Once at the primary electron
acceptor, the electron falls down the electron transport chain to generate ATP. Again, photons
excite the electrons, and they are once again transferred to the primary electron acceptor,
however this time of photosystem I. Once at this primary electron acceptor, the electron falls
down the second electron transport chain and reduces NADP+, via the enzyme NADP+
reductase, into NADPH.
c. Use the ruler shown above to determine the R​​f​​ value of xanthophyll. Show your
calculations.
R​f​ is given by the equation:
Rf =
distance of pigment f rom origin
distance of solvent f ront f rom origin
.
Since the distance of xanthophyll from the origin is 4 cm and the distance of the solvent line
from the origin is 8 cm,
Rf =
4
8
= 0.5 .