COLLEGE OF NATURAL SCIENCES

DEPARTMENT OF PLANT SCIENCES, MICROBIOLOGY & BIOTECHNOLOGY

PLANT PHYSIOLOGY

SERIES OF EVENTS THAT OCCUR IN A PLANT FROM LIGHT

ABSORPTION

GROUP 1

AWORI ANNA 23/U/07080/PS

AYELLA JOHNATHAN BONGOMIN 23/U/07171/PS
JJUKO LARRY ABRAHAM 23/U/08447/PS
NANYANGE CHRISTINE ANGELL 23/U/15342/PS
MWASE JOSHUA MARK 23/U/24242/PS
RUHANGARIYO EVELYN 23/U/17112/PS
NAMBALIRWA BENITAH 23/U/1104
KWEHAYO PRAISE 23/U/0657
Series Of Events That Occur in a Plant from Light Absorption

Photosynthesis is a fundamental process in plants that enables them to convert light

energy into chemical energy in the form of glucose. This process involves a series of events
that occur in plant cells, starting from the absorption of light to the synthesis of glucose.

The first step in photosynthesis is the absorption of light by chlorophyll molecules i.e. the
light dependent reactions, which are located in the chloroplasts of plant cells. Chlorophyll
is a pigment that is able to capture light energy and convert it into chemical energy. When
light strikes chlorophyll, it excites the electrons in the pigment, causing them to move to
a higher energy state.

Once the electrons in chlorophyll are excited, they are passed through a series of electron
carriers in the thylakoid membrane of the chloroplast. This process is known as the light-
dependent reactions, as it requires light to occur. As electrons are passed along the chain
of carriers, they release energy that is used to pump protons across the thylakoid
membrane, creating a proton gradient.

The protons that are pumped across the thylakoid membrane then flow back through an
enzyme called ATP synthase, which uses their energy to generate adenosine triphosphate
(ATP), the primary energy currency of cells. In addition to ATP, the light-dependent
reactions also produce another important molecule called nicotinamide adenine
dinucleotide phosphate (NADPH), which serves as a reducing agent in the subsequent
steps of photosynthesis.

The light dependent reactions and explained in two types of pathways i.e. cyclic
photophosphorylation and non-cyclic photophosphorylation.
Cyclic photophosphorylation is a process in photosynthesis that involves the generation
of ATP through the movement of electrons in a cyclic fashion. This process occurs in the
thylakoid membrane of chloroplasts and is essential for the production of energy that is
needed for various cellular processes. Cyclic photophosphorylation is an important
component of the overall photosynthetic process and plays a crucial role in the overall
efficiency of energy production in plants.

In cyclic photophosphorylation, light energy is absorbed by photosystem I (PSI) and is used

to excite electrons to a higher energy level. These energized electrons are then passed
along a series of carriers, ultimately returning back to PSI. As the electrons move through
the carriers, they generate a proton gradient across the thylakoid membrane, which drives
the production of ATP through the enzyme ATP synthase. This process is known as
photophosphorylation, as it involves the synthesis of ATP using light energy.

Unlike non-cyclic photophosphorylation, which involves both photosystem I and

photosystem II, cyclic photophosphorylation only involves photosystem I. This makes it a
simpler and more direct process compared to non-cyclic photophosphorylation. By
bypassing photosystem II, cyclic photophosphorylation allows for a more efficient and
rapid production of ATP, which is essential for meeting the energy demands of the cell.

One of the key benefits of cyclic photophosphorylation is its ability to regulate the
ATP/NADPH ratio in the cell. By producing ATP without the concurrent generation of
NADPH, cyclic photophosphorylation helps to balance the cellular energy requirements
and maintain homeostasis. This balance is crucial for the efficient functioning of various
metabolic pathways that require ATP as an energy source.
The end products of cyclic photophosphorylation are ATP and oxygen. ATP is used as an
energy source for various cellular processes, while oxygen is released as a byproduct of
the water-splitting reaction in photosystem II. This oxygen evolution is essential for
sustaining aerobic respiration in plants and other aerobic organisms. Overall, cyclic
photophosphorylation plays a crucial role in the global oxygen cycle and is vital for the
survival of aerobic life forms.

Cyclic photophosphorylation is also important for protecting the photosynthetic

machinery from damage caused by excessive light. In high light conditions, the rate of
electron flow through photosystem I can exceed the capacity of photosystem II, leading to
the over-reduction of electron carriers and the accumulation of reactive oxygen species.
By diverting electrons through cyclic photophosphorylation, plants can maintain a balance
in the electron transport chain and prevent oxidative damage to the photosystems.

Non-cyclic photophosphorylation is a process that occurs in the thylakoid membrane of

chloroplasts during photosynthesis. This process is essential for the conversion of light
energy into chemical energy in the form of ATP, which is used by the plant cell for various
metabolic processes. Non-cyclic photophosphorylation involves a series of complex
biochemical reactions that require the participation of several key components, including
photosystem I, photosystem II, electron carriers, and ATP synthase.

Non-cyclic photophosphorylation involves the two photosystems, which are large protein
complexes embedded in the thylakoid membrane. Photosystem II is responsible for the
initial absorption of light energy, which excites electrons in chlorophyll molecules and
initiates a cascade of reactions that ultimately lead to the production of ATP. Photosystem
I, on the other hand, plays a crucial role in the final stage of electron transfer, where
electrons are used to reduce NADP+ to NADPH, another important molecule in the
photosynthetic process.

The flow of electrons in non-cyclic photophosphorylation is facilitated by a series of

electron carriers embedded in the thylakoid membrane. These carriers, such as
plastoquinone, cytochrome b6f, and plastocyanin, shuttle electrons between the two
photosystems, creating a proton gradient across the membrane that drives the synthesis
of ATP by ATP synthase. This process, known as chemiosmosis, is a key mechanism by
which plants harness light energy to produce ATP for cellular energy metabolism.
One of the unique features of non-cyclic photophosphorylation is the production of both
ATP and NADPH, two crucial molecules needed for the Calvin cycle, the second stage of
photosynthesis. ATP provides the energy necessary for the synthesis of glucose from
carbon dioxide, while NADPH serves as a reducing agent to convert carbon dioxide into
carbohydrates. By coupling ATP and NADPH synthesis in non-cyclic photophosphorylation,
plants are able to efficiently convert light energy into chemical energy for carbon fixation.

In addition to its role in ATP and NADPH production, non-cyclic photophosphorylation also
plays a critical role in maintaining the redox balance of the cell. By transferring electrons
from water to NADP+, the plant cell is able to recycle electrons and prevent the
accumulation of reactive oxygen species, which can be harmful to cellular structures. This
process helps to ensure the smooth and efficient operation of the photosynthetic
machinery under varying environmental conditions.

The regulation of non-cyclic photophosphorylation is tightly controlled by a range of

factors, including light intensity, temperature, and the availability of nutrients. Light
intensity plays a key role in activating the photosystems and driving electron flow, while
temperature affects the overall rate of electron transfer and ATP synthesis. Nutrient
availability, particularly inorganic ions such as magnesium and manganese, is essential for
the proper functioning of the photosynthetic complexes and electron carriers.

With ATP and NADPH generated in the light-dependent reactions, the plant cells are now
able to proceed to the light-independent reactions, also known as the Calvin cycle. In this
series of reactions, carbon dioxide is taken up by the plant and converted into glucose
through a series of enzymatic steps. The first step in the Calvin cycle is the fixation of
carbon dioxide by the enzyme rubisco, which combines it with a five-carbon sugar
molecule to form a six-carbon compound.

This six-carbon compound is then converted into a three-carbon compound through a

series of reactions that require the energy of ATP and the reducing power of NADPH. This
three-carbon compound, known as 3-phosphoglycerate, is then transformed into glucose
through further enzymatic reactions. Glucose is a six-carbon sugar molecule that serves as
the primary source of energy for plant cells and is also used to synthesize other important
molecules such as starch and cellulose.

The light independent reactions occur differently in C3 and C4 plant anatomies and also in
CAM plants.
The C3 pathway, also known as the Calvin cycle, is one of the most important metabolic
pathways in plants. This pathway is responsible for fixing carbon dioxide from the
atmosphere and converting it into organic molecules that can be used for growth and
development. The C3 pathway is found in all types of plants, including both C3 and C4
plants, and plays a crucial role in photosynthesis.

The C3 pathway begins with the enzyme RuBisCO, which catalyzes the reaction between
carbon dioxide and ribulose-1,5-bisphosphate to form a 3-carbon compound known as 3-
phosphoglycerate. This compound is then converted into various other organic molecules
through a series of enzymatic reactions, ultimately producing glucose, the primary source
of energy for the plant.

One of the key features of the C3 pathway is its reliance on the enzyme RuBisCO, which is
not very efficient at fixing carbon dioxide. This leads to a phenomenon known as
photorespiration, where oxygen competes with carbon dioxide for binding to RuBisCO,
leading to the wasteful production of toxic byproducts such as glycolate. This inefficiency
in carbon fixation is a major limitation of the C3 pathway, especially in high temperature
and high light conditions.

Despite these limitations, the C3 pathway has several advantages over the C4 pathway,
such as requiring fewer enzymes and less energy to operate. Additionally, C3 plants are
not limited by the availability of carbon dioxide in the environment, as they can fix carbon
dioxide from the atmosphere more readily than C4 plants. This makes the C3 pathway
well-suited for cool and temperate climates where carbon dioxide availability is not a
limiting factor.
The C4 pathway is a unique photosynthetic mechanism utilized by certain plants to
maximize their efficiency in carbon fixation. This pathway involves a series of biochemical
reactions that occur in two distinct types of cells within the plant's leaves - mesophyll cells
and bundle sheath cells. The C4 pathway is particularly advantageous in hot, arid
environments where water availability is limited and photosynthesis can be hindered by
high temperatures.

In plants that utilize the C4 pathway, carbon dioxide is initially fixed into a four-carbon
compound known as oxaloacetate in the mesophyll cells. This process is catalyzed by the
enzyme phosphoenolpyruvate carboxylase (PEPCase). The oxaloacetate is then converted
into malate or aspartate before being transported into the bundle sheath cells where it is
decarboxylated to release carbon dioxide. This carbon dioxide is then fixed by the enzyme
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the Calvin cycle.

One of the main advantages of the C4 pathway is its ability to concentrate carbon dioxide
around Rubisco, thereby reducing the rate of photorespiration. Photorespiration is a
wasteful process in which oxygen competes with carbon dioxide for binding to Rubisco,
leading to a loss of fixed carbon and decreased efficiency in photosynthesis. By separating
the initial carbon fixation step from the Calvin cycle, C4 plants are able to minimize the
effects of photorespiration and maintain higher rates of photosynthesis.
Another benefit of the C4 pathway is its greater water use efficiency compared to C3
plants. In C3 plants, stomata must remain open for longer periods of time to allow for
sufficient carbon dioxide uptake, resulting in higher rates of water loss through
transpiration. C4 plants, on the other hand, can maintain optimal levels of carbon dioxide
within their leaves while minimizing water loss, making them better adapted to dry
conditions.

The evolution of the C4 pathway is thought to have arisen as a response to changing

environmental conditions, particularly the increasing levels of atmospheric oxygen that
occurred during the Cenozoic era. This rise in oxygen concentration favored the
development of mechanisms that could minimize the effects of photorespiration and
enhance carbon fixation efficiency. The C4 pathway provides a solution to these challenges
by compartmentalizing the different steps of photosynthesis and optimizing the use of
carbon dioxide.

Although the C4 pathway is more complex than the C3 pathway, it offers significant
advantages to plants living in hot, dry environments. C4 plants tend to be more efficient
in converting light energy into chemical energy, allowing them to thrive in conditions
where water and carbon dioxide may be limiting factors. This increased efficiency in
photosynthesis can lead to higher rates of growth and productivity, making C4 plants
valuable in agriculture and bioenergy production.
The CAM (Crassulacean acid metabolism) pathway is particularly advantageous for plants
living in hot and dry conditions because it allows them to minimize water loss while still
being able to carry out photosynthesis. By storing carbon dioxide as malic acid during the
night, plants can effectively use that stored carbon dioxide during the day without having
to continuously take in more through open stomata. This reduces the risk of dehydration
and allows these plants to thrive in environments where water availability is limited.

One of the key benefits of the CAM pathway is its ability to enhance photosynthetic
efficiency under stressful environmental conditions. By storing carbon dioxide during the
cooler, more humid nighttime hours and using it during the daytime, CAM plants can
continue to perform photosynthesis even when water is scarce. This adaptive mechanism
allows CAM plants to thrive in arid and semiarid regions where water availability is a
limiting factor for many other plant species.

Another advantage of the CAM pathway is its flexibility in responding to changing

environmental conditions. CAM plants have the ability to adjust their carbon uptake and
storage patterns based on factors such as light intensity, temperature, and water
availability. This dynamic response allows CAM plants to optimize their photosynthetic
efficiency and adapt to fluctuating environmental conditions, making them well-suited for
survival in unpredictable climates.

The CAM pathway also plays a crucial role in the conservation of energy and resources for
plants growing in arid environments. By storing carbon dioxide as malic acid during the
nighttime, CAM plants can reduce the energy cost associated with continuous stomatal
opening and closure. This energy-saving strategy allows CAM plants to allocate resources
more efficiently and ensures their survival in harsh and resource-limited habitats.

Furthermore, the CAM pathway has significant ecological implications for plant
communities in arid regions. CAM plants are often dominant species in these
environments, playing a vital role in maintaining ecosystem productivity and stability. By
efficiently utilizing water and resources, CAM plants contribute to the overall resilience
and biodiversity of arid ecosystems, making them essential components of these fragile
habitats.

In summary, the process of photosynthesis in plants involves a series of events that enable
them to convert light energy into chemical energy in the form of glucose. From the
absorption of light by chlorophyll to the synthesis of ATP and NADPH in the light-
dependent reactions, and finally to the fixation of carbon dioxide and the production of
glucose in the Calvin cycle, photosynthesis is a complex and intricate process that is
essential for the survival of plants and the sustenance of life on Earth.