Advance Montessori Education Center of Isabela, Inc.

Maligaya, Tumauini, Isabela

Email: amecimontessori@gmail.com

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General Biology 1
Learner’s Module 4, 5 & 6 H
November 1-19, 2021
Name: _____________________________________________ S
Grade and Section: ___________________________________ S.Y. 2021-2022

Learning Competencies

At the end of this module, you should be able to:

The learner…
I. explains the importance of chlorophyll and other pigments (STEM_BIO11/12-IIa-j-3);
and
II. describes the patterns of electron flow through light reaction events (STEM_BIO11/12-
IIa-j-4).

Overview
This module focuses on respiration and photosynthetic process as reactions that
complements each other to enable life to survive. It will enhance your understanding of major
features and events involved such as important steps in Calvin cycle, glycolysis, and Krebs cycle.
At the end of this module, you will be able to have a deeper understanding on the importance of
photosynthesis and cellular respiration to all forms of living things.

Learning Content

Photosynthesis
Autotrophic organisms use the pigment chlorophyll to
harvest solar energy to produce the stored energy as
chemical bonds of ATP and carbohydrates. In eukaryotes,
chlorophyll is associated with thylakoid membranes of the
chloroplast (Figure1). The number of chloroplasts varies per
plant but generally, a leaf the size of your palm will have
billions of chloroplasts. Photosynthesis in eukaryotes
involves three essential processes:

1. Energy absorption from sunlight via pigments during light- dependent reaction.
2. Reactivation of reaction centers. Figure 1: Chloroplast visible in the cells of
3. Carbohydrate production by carbon fixation during dark reaction. Plagiomnium affine

Energy Absorption and Reactivation

Pigments that trap sunlight are located in the chloroplasts of eukaryotic organisms. The site
where conversion of the trapped light energy occurs is the thylakoid membrane. This is the third

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innermost membrane system of the chloroplast (Figure 2). It is folded and its continuous system
occurs as stromal and grannal lamellae.
1. Stromal lamellae are sacs (Cisternae) that run across the membrane.
2. Grannal lamellae appear as stack of sacs.

Inside the thylakoid membrane are also some protein complexes

a. Light is harvesting complexes interspersed with photosystem I and l
b. Electron transport complexes
c. ATP is synthesizing complexes that compose the photosynthetic electron transport chain

Figure 2: Diagram of chloroplast showing

the thylakoid membrane on the stroma.

The absorption of a photon by a chlorophyll molecule results in an “excited state" wherein

the electrons move to a higher energy level. Since molecules inevitably interact with their neighboring
molecule, this results in the transfer of energy from the excited molecule to the neighboring molecule,
therefore resulting in a cascading transfer of energy from one to another. This is among the most
efficient ways of energy transfer.

The conversion of light energy to chemical energy occurs in the photosystems embedded in the
thylakoid membrane, that is, photosystem ll (PSII) and photosystem I (PSI). The two complexes
differ on the basis of what they oxidize (i.e., the source of the low-energy electron supply) and what
they reduce (the place to which they deliver their energized electrons) (Open Stax 2015). Antenna
proteins are present in both photosystems. Chlorophyll molecules are bound around these antenna
proteins surrounding the reaction center. A light-harvesting complex is also present to pass the
energy from the sun to the reaction center. The light-harvesting complex contains the pigments such
as 300-400 chlorophyll a and b molecules, carotenoids, and other pigments. The energy is transferred
from chlorophyll to chlorophyll until eventually (after about a millionth of a second) it is delivered to
the reaction center. Up to this point, only energy has been transferred between molecules, not
electrons (OpenStax 2015). The unique orientation of the pigments in these complexes permits the
excitation of dozens of pigment molecules which go toward a specialized point in the photosystem
complex. This specialized point in the chlorophyll molecule is known as the reaction center. The
reaction center contains specialized molecule (P680 in PSII and P700 in PSI) which unlike other
molecules, releases the excited electrons rather than simply transfer the energy. The released electron
is taken by an electron acceptor which creates a charge gradient across the thylakoid membrane.

Since the P680 and P700 are changed to a net positive charge, an electron donor acts on these
PSI and PSII to return to its original state. The donor in the PSII is neutralized by electron removal
from H2O which produces O2, and four protons for every four electrons displaced in the reaction
center. The water-splitting complex in the PSII catalyzes the reaction that splits the water into
electrons, protons, and oxygen. Once the reaction center of the P680 transforms into an excited state,
the photon energy will be absorbed resulting in an electron with high-energy state. Electrons are then
transferred from the excited state P680 to pheophytin and then to plastoquinone. Other than
receiving the high-energy electrons, plastoquinone picks up two protons along the way resulting in its
transformation to plastoquinol. This transformed molecule is released as a mobile electron carrier.
Plastiquinol served as a receiving molecule at the stromal end of PSII to pass the electron to the
cytochrome b6f complex. This is where the PSll connects to PSI via the transmembrane proton pump,
cytochrome bóf complex. Electrons are removed from plastoquinol, thus reverting back to
plastoquinone. These electrons are then transferred to plastocyanin, a water soluble electron carrier.

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 The PSI itself is capable of absorbing photons which permit the movement of electron on the
stromal side of the membrane. Ferredoxin, a small intermediate electron acceptor protein, receives the
electron from PSI then passes it to NAD to form NADPH. A concentration gradient forms as the
electrons accumulate at the lumen of the thylakoid membrane. This leads to the formation of ATP
using the protein complex ATP synthase that catalyzes ATP synthesis. The utilization of this enzyme
permits the protons to cross against the concentration gradient. For every three protons, one ATP
molecule is synthesized.

The water-PSII-PSI to NADP electron flow is one-way; thus it is called non-cyclic

photophosporylation. If the excited electrons in PSI become transported by ferredoxin and
cytochrome b/6 complex back to PSI, a different pathway of electron flow occurs. This process
involves the pumping of protons across the membrane, leading to the synthesis of ATP. The process is
called cycic photophosphorylation. This process causes the variation of ATP and NADPH produced
via this electron chain since this pathway does not use NADP at the end of the pathway.

Learning Activity!

Activity 1│
Directions: Differentiate aerobic respiration, anaerobic respiration and photosynthesis.
Write your answers in the column of the table given below.
Anaerobic
Aerobic respiration Photosynthesis
respiration

Occurs in

Reactants are

Products are

Processes

Activity 2│Directions: Read and answer the questions below. Write your answer in the space
provided after each question.
1. Why are chlorophyll and other pigments important in photosynthesis?
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2. Summarize the pattern/steps of electron flow during light-dependent reaction using your own
understanding.
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3. If energy is never destroyed, why do we “run out of energy” in a race?

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Activity 3 │Directions: Describe the process occurs in photosynthetic eukaryotes. How do

prokaryotes perform photosynthesis? Create a simple flowchart in the box.

Activity 4 │Directions: Draw diagram that demonstrates the process of photosynthesis. Use short
bond paper to draw the process.

NOTE: For further clarification/s about the lesson, you can contact the teacher at 0975-758-5226.

REFERENCES: Rea, MA. D., Dequillo, MZ. M., Chua, JL. C., General Biology 1

Prepared by: Checked by: Recommending Approval:

Rowena L. Gatan Alex G. Soriano JUDELYN A.PARUNGAO

Science Teacher Subject Coordinator Head, SHS Dep’t.

Approved by: Noted by:

JERIC T. VALDEZ NELIA Z-ANGULUAN, PhD

School Principal School Director

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