Photosynthesis

Introduction to Photosynthesis

Definition: Photosynthesis is a biochemical process by which green

plants, algae, and some bacteria convert light energy into chemical
energy, stored in the form of glucose or other organic compounds. This
process is fundamental for life on Earth as it provides the primary source
of energy for nearly all living organisms.

Importance:
o Oxygen Production: Photosynthesis is responsible for producing the
oxygen in our atmosphere.
o Energy Source: It forms the base of the food chain, providing energy
for plants (producers) and all organisms that consume them
(consumers).
o Carbon Sequestration: Photosynthesis plays a crucial role in the
carbon cycle, helping to regulate atmospheric carbon dioxide levels.

• Reactants: Carbon dioxide (CO₂), water (H₂O), and lightenergy.

• Products: Glucose (C₆H₁₂O₆) and oxygen (O₂).

Two Main Stages:

• Light-Dependent Reactions (Photophosphorylation): Occur
in the thylakoid membranes of the chloroplasts. Require light
to produce ATP and NADPH.
• Water is split to provide electrons and protons, releasing
oxygen as a byproduct.
• Light-Independent Reactions (Calvin Cycle): Occur
in the stroma of the chloroplasts.
 • Do not require light directly but use ATP and NADPH from the light-
dependent reactions to fix carbon dioxide into glucose.

The Chloroplast: The Photosynthesis Organelle

• Structure:
• Thylakoids: Flattened membrane-bound sacs where the light-
dependent reactions occur. Thylakoids are stacked into structures
called grana.
• Stroma: The fluid-filled space surrounding the grana, where the
Calvin cycle occurs.
• Chlorophyll: The main pigment involved in photosynthesis, located
within the thylakoid membranes. Chlorophyll absorbs light most
efficiently in the blue and redwavelengths.
• Function:
• Chloroplasts are specialized organelles in plant cells that capture
light energy and convert it into chemical energy.

Mechanism of Photosynthesis
1. Light-Dependent Reactions:
2. Photons from sunlight strike the chlorophyll molecules in the
thylakoid membranes.
3. Excitation of Electrons: Absorbed light energy excites electrons in
chlorophyll, which are then transferred to the primary electron
acceptor in the photosystem II (PSII).
4. Water Splitting: PSII uses light energy to split water molecules into
oxygen, protons, and electrons (Photolysis).
5. Electron Transport Chain (ETC): The excited electrons travel
through the ETC from PSII to photosystem I (PSI). As they move
down the chain, their energy is used to pump protons into the
thylakoid lumen, creating a proton gradient.
6. ATP Synthesis: Protons flow back into the stroma through ATP
synthase due to the gradient, driving the production of ATP from
ADP and inorganic phosphate.
 7. NADPH Formation: Electrons are re-energized in PSI and are
used to reduce NADP+ to NADPH.
Light-Independent Reactions (Calvin Cycle)

Carbon Fixation: CO₂ is attached to a five-carbon sugar, ribulose

bisphosphate (RuBP), by the enzyme ribulose-1,5-bisphosphate
carboxylase/oxygenase (RuBisCO), forming an unstable six- carbon
compound that immediately splits into two molecules of 3-
phosphoglycerate (3-PGA).
Reduction Phase: ATP and NADPH produced in the light- dependent
reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate
(G3P), a three-carbon sugar.
Regeneration of RuBP: Some G3P molecules are used to regenerate
RuBP, enabling the cycle to continue.
Glucose Formation: Two G3P molecules can be combined to form glucose
or other carbohydrates.

Factors Affecting Photosynthesis

1. Light Intensity: Higher light intensity increases the rate of

photosynthesis up to a point,
2. Carbon Dioxide Concentration: Increasing CO₂ concentration
boosts the rate of photosynthesis until the plant reaches a saturation
point.
3. Temperature: Photosynthesis has an optimal temperature range.
Too low or too high temperatures can inhibit enzyme activity,
affecting the rate of photosynthesis.
4. Water Availability: Adequate water is essential for photosynthesis;
a lack of water can slow down or stop the process.
5. Chlorophyll Concentration: The amount of chlorophyll can affect
the plant’s ability to capture light energy.

Types of Photosynthesis
• C3 Photosynthesis:
• The most common form, occurring in most plants.
• The first stable product is a three-carbon compound, 3-PGA.
• C4 Photosynthesis:
• Found in plants like maize and sugarcane.
• Involves the formation of a four-carbon compound, oxaloacetate, as
the first stable product. This adaptation helps in reducing
photorespiration, increasing efficiency in hot, dry environments.
• CAM Photosynthesis:
• Occurs in succulents like cacti.
 • Stomata open at night to minimize water loss, storing CO₂ as malate
to be used in the Calvin cycle during the day.

Significance of Photosynthesis
• Ecological Impact: Photosynthesis is the foundation of the food
web, supporting life by providing energy and organic matter.
• Environmental Role: Helps in maintaining atmospheric oxygen
levels and reducing carbon dioxide, mitigating climate change.
• Economic Importance: Supports agriculture, forestry, and all
industries dependent on plant products.

Conclusion
• Photosynthesis is a complex, vital process that sustains life on Earth
by converting solar energy into chemical energy, providing food,
oxygen, and regulating the atmosphere's carbon balance.
Understanding its mechanisms and factors influencing it is crucial
for advancing agricultural productivity, environmental conservation,
and addressing global challenges like climate change.

Introduction to Chlorophyll
• Definition: Chlorophyll is a group of green pigments found in the
chloroplasts of plants, algae, and cyanobacteria. It plays a crucial
role in photosynthesis by absorbing light energy, which is then used
to convert carbon dioxide and water into glucose and oxygen.
• Importance:
• Primary Pigment in Photosynthesis: Chlorophyll is essential for
capturing light energy from the sun, making it the primary pigment
in the photosynthesis process.
 • Gives Plants Their Green Color: The green appearance of plants is
due to the presence of chlorophyll, which reflects green wavelengths
of light.
• Foundation of the Food Chain: By enabling photosynthesis,
chlorophyll indirectly supports life on Earth by providing the energy
that sustains the food chain.

Types of Chlorophyll
• Chlorophyll a:
• Primary Pigment: Found in all photosynthetic organisms, it is the
main pigment involved in photosynthesis.
• Absorption Spectrum: Absorbs light primarily in the blue- violet
and red regions
• Function: Acts as the primary electron donor in the electron
transport chain of photosynthesis.
• Chlorophyll b:
• Accessory Pigment: Found in plants and green algae, chlorophyll b
assists chlorophyll a in capturing light energy.
• Absorption Spectrum: Absorbs light primarily in the blue and red-
orange regions of the spectrum
• Function: Broadens the range of light that a plant can use for energy
by transferring the energy it captures to chlorophyll a.

Structure of Chlorophyll
• General Structure:
o Porphyrin Ring: The core structure of chlorophyll is a porphyrin ring,
a large, stable ring made up of four smaller pyrrole rings connected by
carbon atoms. This ring structure is essential for absorbing light.

o Magnesium Ion (Mg²⁺): At the center of the porphyrin ring, a

magnesium ion is coordinated. This magnesium ion is critical for
chlorophyll's ability to absorb light.

o Phytol Tail: Chlorophyll molecules have a long, hydrophobic tail

known as the phytol tail. This tail anchors the chlorophyll molecule
within the lipid membranes of the thylakoids in the chloroplast.

o Functional Groups: Different types of chlorophyll have slightly

different functional groups attached to the porphyrin ring, which alters
their light absorption properties.

Functions of Chlorophyll
• Light Absorption:

o Primary Function: Chlorophyll’s main function is to absorb

light energy from the sun. The energy absorbed excites
electrons within the chlorophyll molecule,
 making them ready for transfer through the photosynthetic
electron transport chain.
o Absorption Spectrum: Chlorophyll a absorbs light in the blue-
violet and red regions, while chlorophyll b extends the range of
light absorption by capturing light in the blue and red-orange
regions.
• Energy Transfer:
o Electron Excitation: When chlorophyll absorbs light, its
electrons are excited to a higher energy state. These high-
energy electrons are then transferred to a series of molecules
in the electron transport chain.
o Primary Electron Donor: Chlorophyll a acts as the primary
electron donor in the reaction center of photosystems, passing
excited electrons to the next carrier in the electron transport
chain.
• Photosystem Formation:
o Role in Photosystems: Chlorophyll molecules are organized
into complexes called photosystems (Photosystem I and
Photosystem II). These complexes are embedded in the
thylakoid membranes and are essential for the light-dependent
reactions ofphotosynthesis.
o Reaction Centers: Within each photosystem, chlorophyll
molecules surround a specialized chlorophyll a molecule in the
reaction center, which plays a crucial role in converting light
energy into chemical energy.
• Oxygen Production:
o Water Splitting: In Photosystem II, chlorophyll helps drive the
splitting of water molecules (photolysis), leading to the release
of oxygen as a byproduct. This process is essential for
sustaining life on Earth.

Importance of Chlorophyll
• Crucial for Photosynthesis: Without chlorophyll, the process
of photosynthesis could not occur. As the main
 pigment, it captures the energy needed to drive the reactions that
produce glucose and oxygen.
• Supports Life on Earth: Chlorophyll’s role in photosynthesis
underpins the survival of nearly all living organisms. It is the basis
for the production of food and oxygen, supporting life on Earth.
• Carbon Dioxide Reduction: By enabling photosynthesis,
chlorophyll plays a vital role in reducing atmospheric carbon dioxide
levels, helping to mitigate the effects of climate change.
• Ecological Impact: Chlorophyll’s presence in plants ensures the
continuation of the food chain and supports ecosystems worldwide.
Plants, through photosynthesis, provide food for herbivores, which
in turn support carnivores and omnivores.
• Economic Importance: Chlorophyll in plants supports agriculture,
which is a critical industry globally. The production of crops, fruits,
and vegetables depends on the efficiency of photosynthesis.

Conclusion
• Chlorophyll is an indispensable pigment that drives the process of
photosynthesis by absorbing light energy and converting it into
chemical energy. Its various types (a, b, c, d, and f) allow organisms
to capture light across a broad spectrum, making photosynthesis
more efficient. The structure of chlorophyll, with its porphyrin ring
and magnesium ion, is perfectly adapted to this role. Its functions not
only support plant life but also sustain all life on Earth by
producing food and oxygen. Understanding chlorophyll is key to
understanding the fundamentals of biology, ecology, and even global
climate systems.
Plant Growth Substances / Plant Hormones or Phytohormones
Plant growth substances, also known as plant hormones or phytohormones, are
organic compounds that regulate various physiological processes in plants. They play
essential roles in growth, development, and responses to environmental stimuli. These
substances can be naturally occurring within the plant or applied externally to
influence growth and development.
Major Types of Plant Growth Substances
Auxins
• Function: Promote cell elongation, root formation, and differentiation. They are
involved in the regulation of fruit development and apical dominance.
• Examples: Indole-3-acetic acid (IAA) is the most common natural auxin.
• Effects: Auxins stimulate phototropism (growth toward light) and gravitropism
(growth in response to gravity).
Gibberellins
• Function: Stimulate stem elongation, seed germination, and flowering. They are
involved in breaking dormancy and promoting fruit development.
• Examples: Gibberellic acid (GA3) is a well-known gibberellin used in agricultural
practices.
• Effects: Gibberellins can induce seedless fruit development and enhance growth
in certain plant varieties.
Cytokinins
• Function: Promote cell division (cytokinesis) and shoot formation. They also
delay leaf senescence and influence nutrient mobilization.
• Examples: Zeatin and kinetin are commonly studied cytokinins.
• Effects: Cytokinins can promote lateral bud growth and enhance root
development when combined with auxins.
Abscisic Acid (ABA)
• Function: Inhibits growth and promotes seed dormancy. It plays a crucial role
in plant responses to stress, particularly drought.
• Effects: ABA induces stomatal closure to reduce water loss and helps plants
cope with unfavorable conditions.
• Ethylene
• Function: A gaseous hormone involved in fruit ripening, leaf abscission, and
flower senescence.
• Examples: Ethylene is produced by the plant itself and can be applied
externally (e.g., ethephon).
• Effects: Ethylene promotes ripening processes, such as color change and softening
of fruits, and is used commercially to accelerate ripening.
Brassinosteroids
• Function: Promote cell elongation, division, and differentiation. They enhance
resistance to stress and influence flowering.
• Examples: Brassinolide is a well-known brassinosteroid.
• Effects: They help improve crop yield and stress tolerance.
Interaction of Plant Growth Substances
• Balance and Interaction: The effects of plant hormones are not isolated;
they often work together or against each other to regulate growth. For
example, auxins and cytokinins interact to determine shoot and root
development.
• Environmental Influence: Plant growth substances can be influenced by
environmental factors like light, temperature, and water availability, impacting
their concentrations and effects on growth.
Applications of Plant Growth Substances
• Agriculture: Hormones like auxins and gibberellins are used to enhance crop
yields, promote rooting in cuttings, and induce flowering.
• Horticulture: Ethylene is used to control fruit ripening and improve the
quality of harvested fruits.
• Research: Understanding the role of plant hormones in growth and development
helps in developing new agricultural practices and improving plant breeding
techniques.
Plant growth substances are vital for regulating a wide range of physiological
processes in plants. Understanding these hormones and their interactions can enhance
agricultural practices, improve crop production, and aid in the management of plant
responses to environmental stresses.
Plant Nutrients: Classification and Role in Plants
Plants require a variety of nutrients for growth, development, and physiological
functions. These nutrients are absorbed primarily from the soil and are classified based
on their quantity requirements and function in plant systems.
Classification of Plant Nutrients: Plant nutrients are broadly classified into two
categories: Macronutrients are essential elements that plants need in large quantities
for growth and development. They play fundamental roles in various physiological
and biochemical processes such as photosynthesis, energy transfer, protein
synthesis, and structural development. Macronutrients are divided into primary and
secondary categories:
A. Primary Macronutrients
• Nitrogen (N): Key component of amino acids, proteins, chlorophyll, and nucleic
acids. It promotes vegetative growth and green leaf development.
• Phosphorus (P): Vital for energy transfer (ATP), DNA and RNA
synthesis, root development, and flowering/fruiting.
• Potassium (K): Regulates water balance, enzyme activation, photosynthesis,
and overall plant stress resistance.
B. Secondary Macronutrients: Secondary macronutrients are essential elements
that plants require in moderate quantities, more than micronutrients but less than the
primary macronutrients (nitrogen, phosphorus, and potassium). These nutrients play
important roles in plant growth, development, and physiological processes. The
secondary macronutrients include:
• Calcium (Ca): Essential for cell wall structure, membrane stability, root
and shoot development, and enzyme function.
• Magnesium (Mg): Central component of the chlorophyll molecule and
necessary for photosynthesis and enzyme activation.
• Sulfur (S): Important for protein formation, as it is a part of amino acids and
vitamins. Also plays a role in plant defense mechanisms.
2. Deficiency Symptoms of Macronutrients
• Nitrogen Deficiency: Causes yellowing (chlorosis) of older leaves, stunted
growth, and pale overall appearance due to reduced chlorophyll production.
• Phosphorus Deficiency: Leads to dark green or purplish leaves, poor root
development, delayed maturity, and reduced flowering and fruiting.
• Potassium Deficiency: Shows as yellowing or browning along leaf edges (leaf
margin scorch), weak stems, and reduced resistance to stress.
• Calcium Deficiency: Causes distorted, curled, or necrotic (dead) young leaves,
poor root growth, and blossom end rot in fruits.
• Magnesium Deficiency: Results in interveinal chlorosis (yellowing between leaf
veins) on older leaves, affecting photosynthesis.
• Sulfur Deficiency: Similar to nitrogen deficiency but affects young leaves first,
causing uniform yellowing and stunted growth.
Macronutrients are crucial for plant growth and development, influencing critical
functions such as energy production, structural integrity, and stress response.
Deficiencies in these nutrients manifest as specific symptoms, affecting leaf color,
growth patterns, and overall plant health. Proper nutrient management is essential for
preventing these deficiencies and ensuring optimal crop productivity.
B. Micronutrients
Micronutrients are essential elements required by plants in small quantities (trace
amounts) but are critical for their growth, development, and metabolic processes.
These nutrients support enzyme function, photosynthesis, hormone regulation, and
overall plant health.
Functions of Micronutrients
• Iron (Fe): Essential for chlorophyll synthesis and acts as a cofactor in electron
transport during photosynthesis and respiration.
• Manganese (Mn): Plays a role in photosynthesis, nitrogen metabolism, and
activating several enzymes.
• Zinc (Zn): Important for enzyme activity, protein synthesis, and hormone
production (e.g., auxin).
• Copper (Cu): Involved in photosynthesis, respiration, and lignin synthesis,
which strengthens cell walls.
• Boron (B): Crucial for cell wall formation, reproductive growth, and sugar
transport within the plant.
• Molybdenum (Mo): Required for nitrogen fixation and nitrate reduction,
making it vital for nitrogen metabolism.
 • Chlorine (Cl): Involved in osmotic regulation, ionic balance, and
photosynthesis, especially in maintaining the opening and closing of
stomata.
Deficiency Symptoms of Micronutrients
• Iron Deficiency: Causes interveinal chlorosis (yellowing) in young leaves,
as iron is immobile and cannot be moved from older leaves.
• Manganese Deficiency: Leads to interveinal chlorosis similar to iron deficiency
but often accompanied by necrotic (dead) spots on leaves.
• Zinc Deficiency: Results in stunted growth, smaller leaves, and interveinal
chlorosis, especially in younger leaves.
• Copper Deficiency: Causes wilting, chlorosis of young leaves, and twisted or
misshapen leaves due to its role in photosynthesis and lignin production.
• Boron Deficiency: Leads to brittle leaves, poor root and flower development, and
distorted growth in young leaves and stems.
• Molybdenum Deficiency: Shows up as yellowing or chlorosis on older leaves
and poor nitrogen assimilation, affecting overall plant growth.
• Chlorine Deficiency: Rare, but may cause wilting, chlorosis, and reduced growth
due to its role in stomatal function and water balance.
Micronutrients, although needed in small amounts, are critical for plant health and
function. Deficiencies in these nutrients can lead to specific symptoms that impact
growth, photosynthesis, and overall productivity. Proper nutrient management is
essential for preventing these deficiencies and ensuring optimal plant development.
Seeds and Seed Germination and Dormancy
Seeds are the reproductive units of flowering plants (angiosperms) and gymnosperms.
They consist of an embryo, a food supply, and a protective seed coat. Seeds are crucial
for the survival and propagation of plant species, allowing them to reproduce and
disperse in various environments.
Structure of Seeds
• Embryo: The young plant that develops from the fertilized ovule, consisting of
the radicle (root), plumule (shoot), and cotyledons (seed leaves).
• Seed Coat: A protective outer layer that safeguards the embryo and food
supply from environmental hazards.
• Endosperm: A nutrient-rich tissue that provides energy and sustenance to the
developing embryo during germination (in some seeds) or the cotyledons serve
this purpose.
Seed Germination
Seed germination is the process by which a seed develops into a new plant. This
process is influenced by various environmental factors and internal conditions.
Stages of Germination:
1. Imbibition:
• The seed absorbs water, causing it to swell and break the seed coat.
• Activation of metabolic processes begins as enzymes are activated.
2. Metabolic Activation:
• The seed begins to respire and convert stored food (starch, proteins, and fats)
into usable energy (ATP).
• Enzymatic activity increases, facilitating the mobilization of nutrients.
3. Radicle Emergence:
• The radicle (embryonic root) is the first part to emerge from the seed, anchoring
the plant and absorbing water and nutrients from the soil.
4. Plumule Emergence:
• The plumule (embryonic shoot) follows, growing upward toward the light and
developing into the stem and leaves.
Factors Affecting Germination:
• Water: Essential for imbibition and metabolic processes.
• Oxygen: Required for respiration; insufficient oxygen can hinder germination.
• Temperature: Different seeds have optimal temperature ranges for germination.
• Light: Some seeds require light to germinate, while others may germinate in
darkness.
3. Seed Dormancy
Seed dormancy is a survival mechanism that prevents seeds from germinating until
conditions are favorable for growth. Dormancy allows seeds to withstand unfavorable
environmental conditions, such as drought, extreme temperatures, or lack of
nutrients.
Types of Seed Dormancy:
1. Physiological Dormancy:
• The seed exhibits internal physiological conditions that inhibit germination,
often due to immature embryos or insufficient hormone levels (e.g.,
gibberellins).
2. Morphological Dormancy:
• Seeds have immature embryos at the time of dispersal and require a
period of development before germination can occur.
3. Physical Dormancy:
• The seed coat is impermeable to water or gases, preventing germination until
the coat is altered (e.g., by abrasion, heat, or chemicals).
4. Environmental Dormancy:
• Seeds require specific environmental conditions (e.g., light, temperature, or
moisture) to germinate. This type of dormancy ensures seeds germinate at the
right time.
Seeds are essential reproductive structures that enable plants to propagate and survive
in various environments. Seed germination is a complex process influenced by factors
like water, oxygen, temperature, and light. Conversely, seed dormancy is a crucial
survival strategy that ensures seeds remain inactive until conditions are favorable for
germination, enhancing their chances of successful establishment and growth.
 PLANT PHYSIOLOGY is the study of the functions and processes that
occur within plants.

Here are some basic terminologies in plant

physiology

1. Photosynthesis: The process by which plants convert light energy into

chemical energy, producing glucose and oxygen from carbon dioxide and
water. It primarily occurs in the chloroplasts of plant cells.

2. Respiration: The process by which plants break down glucose to produce

energy (ATP), carbon dioxide, and water. This occurs in the mitochondria of
plant cells and is vital for growth and maintenance.

3. Transpiration: The loss of water vapor from plant leaves through small
openings called stomata. This process helps in cooling the plant and
maintaining nutrient flow from the roots.

4. Stomata: Tiny openings usually found on the underside of leaves that allow for
gas exchange, including the release of oxygen and the uptake of carbon
dioxide. Stomata also play a key role in transpiration.

5. Chlorophyll: The green pigment found in chloroplasts that absorbs light

energy used in photosynthesis. Chlorophyll is essential for the conversion of
solar energy into chemical energy.

6. Xylem: The tissue in plants responsible for transporting water and dissolved
nutrients from the roots to the rest of the plant. Xylem also provides structural
support.

7. Phloem: The tissue that transports the products of photosynthesis (mainly

sugars) from the leaves to other parts of the plant where they are used or
stored.

8. Turgor Pressure: The pressure exerted by water inside the cell against
the cell wall. It is crucial for maintaining the plant's rigidity and structural
integrity.
9. Phototropism: The growth response of a plant to light direction. Plants
typically grow towards the light source, a behavior influenced by the hormone
auxin.

10. Hormones: Chemical substances produced in plants that regulate growth,

development, and responses to environmental stimuli. Key plant hormones
include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

11. Osmosis: The movement of water across a semipermeable membrane from

a region of lower solute concentration to a region of higher solute
concentration. This process is essential for water uptake in plant roots.

12. Nutrient Uptake: The process by which plants absorb essential nutrients
(like nitrogen, phosphorus, potassium, etc.) from the soil through their
roots.

13. Photoperiodism: The physiological reaction of plants to the length of

day or night. This influences flowering, seed germination, and other
developmental processes.

14. Germination: The process by which a seed develops into a new plant. It
involves the activation of metabolic pathways that lead to the growth of the
embryo into a seedling.

15. Plasmolysis: The process in which plant cells lose water in a hypertonic
solution, causing the plasma membrane to pull away from the cell wall. This
can lead to wilting and cell death if not reversed.

16. Tropism: The directional growth of a plant in response to an external

stimulus. Types of tropism include:
 1. Gravitropism (or Geotropism): Growth in response to gravity; roots
typically show positive gravitropism (growing downward), while shoots
exhibit negative gravitropism (growing upward).

2. Thigmotropism: Growth in response to touch or physical contact,

commonly observed in climbing plants like vines.

17. Phytochrome: A photoreceptor pigment in plants that detects light and

influences processes such as seed germination, flowering, and shade
avoidance. Phytochromes are sensitive to red and far-red light.

18. Abscission: The process by which plants shed leaves, flowers, or fruits. This
is often regulated by plant hormones like ethylene and is a response to
environmental cues or stress.

19. Endodermis: A layer of cells surrounding the vascular bundles (xylem and
phloem) in roots.

The endodermis regulates the flow of water and nutrients into the plant’s
vascular system.

20. Mesophyll: The inner tissue of a leaf, containing chloroplasts, where

photosynthesis primarily takes place. It consists of two layers:

1. Palisade Mesophyll: The upper layer with tightly packed cells rich in
chloroplasts.

2. Spongy Mesophyll: The lower layer with loosely arranged cells that
facilitate gas exchange.

21. Guard Cells: Specialized cells that surround stomata and control their
opening and closing. By regulating the size of the stomatal pores, guard cells
help control water loss and gas exchange.

22. Plasmodesmata: Microscopic channels that traverse the cell walls of plant
cells, allowing for the transport of materials and communication between
adjacent cells.
23. CAM Photosynthesis : A water-conserving photosynthetic process used by
some plants, especially in arid environments. CAM plants open their stomata
at night to minimize water loss and store carbon dioxide for use during the
day.

24. Translocation: The movement of organic compounds (such as sugars)

from sources (like leaves) to sinks (like roots or fruits) through the
phloem.

25. Photoinhibition: A reduction in the photosynthetic capacity of a plant due

to excessive light, which can damage the photosynthetic apparatus,
particularly the chloroplasts.

26. Etiolation: The process by which a plant grows in partial or complete absence
of light, resulting in pale, elongated stems and underdeveloped leaves. This is a
survival strategy to reach light.

27. Hydrotropism: Growth or movement of a plant's roots towards moisture. It

helps the plant to maximize water absorption from the soil.

28. Apoplast and Symplast: Pathways for the movement of water and solutes in
plants:

1. Apoplast: The network of cell walls and intercellular spaces through

which water and solutes move.

2. Symplast: The network of cytoplasm of plant cells, interconnected by

plasmodesmata, through which water and solutes can pass.

34. Calvin Cycle: A series of biochemical reactions that take place in the stroma
of chloroplasts during photosynthesis. It’s also known as the light-
independent or dark reactions and is where carbon fixation occurs,
resulting in the production of glucose.
35. Guttation: The exudation of water droplets from the edges of leaves,
typically occurring at night when soil moisture is high, and transpiration is
low. It is caused by root pressure pushing water up through the plant.

36. Photoperiod: The length of day and night, which influences the timing of
flowering and other developmental processes in plants. Plants can be
categorized as short-day, long-day, or day-neutral based on their
photoperiodic response.

37. Circadian Rhythms: Biological processes in plants that follow a roughly

24-hour cycle, responding primarily to light and darkness in their
environment. These rhythms affect various functions such as leaf
movements, opening and closing of stomata, and flower opening.

38. C3 Plants: Plants that use the Calvin cycle for carbon fixation, where the
first product formed is a three-carbon compound (3-phosphoglycerate).
Most temperate crops, such as wheat and rice, are C3 plants.

39. C4 Plants: Plants that have an additional set of reactions to fix carbon
dioxide, forming a four-carbon compound as the first stable product. This
adaptation allows them to photosynthesize more efficiently in hot, dry
environments.

40. CAM Plants Plants that open their stomata at night to fix CO₂ into organic
acids, which are then used during the day for photosynthesis. This
adaptation helps minimize water loss in arid conditions.

41. Symbiosis: A close and often long-term interaction between two different
biological species. In plant physiology, this term is often used to describe
beneficial relationships, such as those between plants and nitrogen-fixing
bacteria or mycorrhizal fungi.
42. Nitrogen Fixation: The process by which certain bacteria, often in
symbiosis with plants (such as legumes), convert atmospheric nitrogen
(N₂) into ammonia (NH₃), which plants can use to synthesize amino acids
and other nitrogen-containing compounds.

43. Photomorphogenesis: The development of a plant in response to light.

This includes processes like seed germination, stem elongation, leaf
expansion, and flowering, which are regulated by light quality, intensity,
and duration.

44. Source and Sink: Terms used to describe the movement of nutrients and
carbohydrates in plants. The "source" refers to the part of the plant where
these substances are produced or stored (like leaves), and the "sink" refers
to where they are needed or stored (like roots, fruits, or seeds).

45. Lenticels: Pores found on the stems and roots of woody plants that
allow gas exchange between the internal tissues and the atmosphere,
facilitating respiration.

46. Gibberellins: A group of plant hormones that promote growth and

influence various developmental processes, including seed germination,
stem elongation, and flowering.
 TRANSPIRATION
Plant Leaf Structure and Functions

External Structure
LEAF APEX is the tip or the farthest point of the leaf, located opposite the
petiole or leaf base. It plays a key role in directing water away from the leaf
surface to prevent damage and water logging..
LAMINA (LEAF BLADE): The flat, broad part of the leaf that is the main
site for photosynthesis.
PETIOLE: The stalk that connects the leaf blade to the stem, allowing the
transport of nutrients and water between the leaf and the plant.
MIDRIB: The central vein that runs through the leaf blade, providing
structural support and containing vascular tissues for nutrient and water
transport.
VEINS: The network of veins within the lamina is part of the vascular
system that helps in the transportation of water, nutrients, and food.
STIPULE is a small, leaf-like or scale-like structure located at the base of
the leaf petiole where it attaches to the stem. Stipules protect the young
leaf or bud as it develop, stipules contribute to photosynthesis
Internal Structure
The internal structure of a leaf is organized into different tissue layers, each
performing specific functions:
EPIDERMIS:
• The outermost layer of cells that covers both the upper (adaxial) and
lower (abaxial) surfaces of the leaf.
• CUTICLE: A waxy, waterproof layer on the epidermis that reduces water
loss and provides protection.
• STOMATA: Tiny pores found primarily on the lower epidermis. They
are surrounded by guard cells that control the opening and closing of
the stomata, regulating gas exchange and water loss.
MESOPHYLL
• PALISADE MESOPHYLL: Located just beneath the upper epidermis, this
layer consists of tightly packed cells rich in chloroplasts, the site of most
photosynthesis.
• SPONGY MESOPHYLL: Below the palisade layer, the cells here are
loosely packed, creating spaces that allow gases like carbon dioxide and
oxygen to circulate. The spongy mesophyll is also involved in
photosynthesis but to a lesser extent.
• VASCULAR BUNDLE:
Composed of xylem and phloem, these tissues are responsible for the
transport of water, minerals, and nutrients.
• XYLEM: Transports water and dissolved minerals from the roots to the
leaves.
• PHLOEM: Transports the products of photosynthesis (mainly
sugars) from the leaves to other parts of the plant.
LEAF FUNCTIONS
LEAVES where plants convert light energy into chemical energy in the
form of glucose. This process occurs in the chloroplasts, which are
abundant in the mesophyll layer.
GAS EXCHANGE The leaf allows for the exchange of gases—mainly carbon
dioxide (CO₂), oxygen (O₂), and water vapor—through the stomata. During
photosynthesis, CO₂ enters the leaf, and O₂, a byproduct, exits. This gas
exchange is essential for photosynthesis and respiration.
TRANSPIRATION is the process of water vapor loss from the leaf through
the stomata. STORAGE Some leaves are modified to store food, water, and
nutrients. For example, in succulent plants like aloe vera, leaves are thick
and fleshy to store water, helping the plant survive in arid environments.
DEFENSE Leaves can also be modified to protect plants from herbivores or
environmental stresses. Spines: As in cacti, where leaves are reduced to
spines to minimize water loss and deter herbivores.
THICK CUTICLE: Some plants have a thicker cuticle to reduce water loss
and protect against pathogens.
Adaptations of Leaves

Different plants adapt their leaves based on their environment:

• Xerophytes (plants in dry environments): Have leaves with thick

cuticles, reduced surface area, or sunken stomata to minimize water
loss.
• Hydrophytes (aquatic plants): Have leaves with large air spaces for
buoyancy and stomata only on the upper surface since the lower
surface is submerged.
• Mesophytes (plants in moderate environments): Have a well-balanced
leaf structure with no extreme adaptations, as they live in
environments with adequate water supply.
TRANSPIRATION is the process by which plants lose water in the form of
water vapor from aerial parts, mainly through the stomata on the leaves. It is a
vital physiological process that plays a key role in water regulation, nutrient
transport, and temperature control in plants. MECHANISM OF
TRANSPIRATION
Transpiration primarily occurs through stomata, though water loss can also
happen through the cuticle (cuticular transpiration) and lenticels (lenticellular
transpiration).
STOMATAL TRANSPIRATION (PRIMARY MECHANISM)
• Water Uptake: Water is absorbed from the soil through the roots by
the process of osmosis. It enters the root hairs, moves into the root
cortex, and then reaches the xylem vessels.
• Ascent of Sap: The water, along with dissolved minerals, is transported
upward through the xylem to the leaves. This upward movement of
water, called the ascent of sap, is driven by transpiration pull, root
pressure, and capillary action.
• Evaporation of Water from Mesophyll Cells: Inside the leaf, water
moves from the xylem into the mesophyll cells (spongy and palisade
parenchyma) and then evaporates into the air spaces between the cells.
• Diffusion of Water Vapor through Stomata: The water vapor from the
air spaces diffuses through the stomatal pores to the outside
environment. This is driven by the water vapor concentration
gradient between the leaf's internal air spaces and the external
atmosphere.
• Guard Cells Regulation: The opening and closing of stomata are
controlled by guard cells, which respond to various environmental
factors like light, humidity, and CO₂ concentration. When guard cells
are turgid (due to water influx), stomata open, allowing transpiration;
when flaccid, stomata close, reducing water loss.
CUTICULAR TRANSPIRATION
• Occurs through the cuticle, a waxy layer covering the epidermis of
leaves and stems. It accounts for a smaller percentage of water loss,
especially in plants with thick cuticles.
LENTICELLULAR TRANSPIRATION
• Takes place through small openings in the bark called lenticels in
woody plants. This is a minor contributor to the overall water loss.
PROCESS OF TRANSPIRATION
Water Absorption
• Root Hairs: Water is absorbed from the soil into root hairs by
osmosis, where the soil water potential is higher than that inside the
plant cells.
Movement through Root Cortex
• Water travels across the root cortex by three pathways:
• Apoplastic Pathway: Water moves through the cell walls without entering
the cells.
• Symplastic Pathway: Water moves through the cytoplasm and
plasmodesmata connecting cells.
• Transmembrane Pathway: Water crosses cell membranes into and out of
cells.
Transport through Xylem
• Water reaches the xylem vessels, where it is pulled upward to the
leaves via cohesion (water molecules sticking together) and
adhesion (water molecules sticking to the walls of xylem vessels).
The transpiration pull due to water
 evaporating from the leaves creates a negative pressure that helps draw
water upward against gravity.
Water Loss through Stomata
• Once the water reaches the leaves, it evaporates from the surface of
mesophyll cells and exits through the stomatal pores as water vapor.
Stomatal Opening and Closing
• Opening: Stomata open when guard cells are turgid, which happens
when they absorb water through osmosis due to the active uptake of
potassium ions (K⁺). This process is influenced by factors such as light
and CO₂ concentration.
• Closing: Stomata close when guard cells lose turgidity, usually during
water stress, darkness, or high CO₂ levels.
PARTS INVOLVED IN TRANSPIRATION
1. Roots: Absorb water from the soil.
2. Xylem: Conducts water from the roots to the leaves.
3. Mesophyll Cells: Provide the surface area for water to evaporate inside
the leaf.
4. Stomata and Guard Cells: Regulate water loss by opening and
closing the stomatal pores.
5. Cuticle: Though a protective barrier, water can diffuse through
the cuticle in small amounts.
6. Lenticels: Present on stems and bark, allowing a small amount of
water vapor to escape.
Types of Transpiration
• Stomatal Transpiration: Responsible for about 90% of water loss.
• Cuticular Transpiration: Water loss through the cuticle, accounting
for about 5- 10% of total water loss.
• Lenticellular Transpiration: Minor water loss through lenticels in woody
plants.
Factors Affecting Transpiration
Environmental Factors:
• Light: Increases transpiration by stimulating stomatal opening.
• Temperature: Higher temperatures increase the rate of
evaporation and transpiration.
• Humidity: Low humidity leads to faster transpiration due to a steeper
gradient for water vapor diffusion.
• Wind: Wind removes the humid air layer around the leaf, increasing
transpiration.
• Soil Water Availability: When soil moisture is low, transpiration
decreases as the plant closes stomata to conserve water.
• Internal Factors:
• Leaf Surface Area: Larger leaves have more stomata and a
higher rate of transpiration.
• Number of Stomata: More stomata lead to greater water loss.
• Thickness of Cuticle: Thicker cuticles reduce water loss.
• Leaf Orientation: Leaves that are more upright lose less water.
Importance of Transpiration
Water Transport and Nutrient Uptake:
• Transpiration creates a transpiration pull, which helps in the upward
movement of water from the roots to the leaves through the xylem.
Along with water, essential nutrients dissolved in water are also
transported to different parts of the plant.
Cooling Effect:
• Evaporative cooling during transpiration helps regulate the
temperature of the plant, especially in hot environments, protecting
the plant from overheating.
Maintaining Water Potential:
• By losing water, transpiration maintains the plant’s water
potential gradient, which aids in the continuous uptake of water
from the soil, ensuring the plant remains hydrated.
Gas Exchange:
• Transpiration keeps the stomata open, which is necessary for the
exchange of gases (CO₂ for photosynthesis and O₂ as a byproduct).
Turgor Pressure Maintenance:
• Transpiration helps maintain turgor pressure in cells, which is
essential for plant structure and function, particularly in non-woody
parts of plants like leaves and flowers.
Nutrient Distribution:
• Transpiration drives the distribution of minerals and nutrients
throughout the plant, enabling proper growth and development.
Negative Effects of Excessive Transpiration
• Water Loss: If transpiration exceeds water uptake, it can lead to
wilting, drought stress, and even death.
• Dehydration: In arid conditions, excessive transpiration may
deplete water reserves, resulting in stunted growth or plant
failure.
Adaptations to Reduce Transpiration
• Smaller or Needle-like Leaves: Reduce surface area to minimize
water loss (e.g., conifers).
• Thick Cuticle: A waxy cuticle helps to reduce water loss through
cuticular transpiration.
• Stomatal Location: Some plants have stomata located in deep
pits (sunken stomata) to reduce transpiration (e.g., desert plants).
• Leaf Hair: Hair on leaves traps moisture and reduces airflow,
reducing water loss (e.g., xerophytes).
• Closing Stomata: During hot, dry conditions, stomata remain closed
during the day to conserve water (e.g., CAM plants).
In summary, transpiration is crucial for plant physiology, providing
water transport, nutrient movement, temperature regulation, and
facilitating gas exchange, but plants must also balance it to avoid
excessive water loss.
Water Uptake and Transport in Crop Plants
Water is essential for the growth and development of crop plants, and its uptake and
transport involve various plant parts and tissues. This process is crucial for
physiological activities, nutrient transport, and maintaining plant structure.
Parts and Tissues Involved in Water Transport
• Roots: The primary site for water uptake from the soil. Root hairs increase the
surface area for absorption, and the epidermis is the outermost layer that facilitates
water entry.
• Xylem: The vascular tissue responsible for long-distance water transport from
roots to leaves. It consists of vessel elements and tracheids, which form a
network of tubes.
• Stem: Acts as a conduit for water movement from the roots to the leaves. It
contains xylem vessels that conduct water upward.
• Leaves: Water transported to leaves is crucial for photosynthesis and transpiration.
Stomata (small pores) in the leaf surface regulate water loss and gas exchange.
Functions of Water Transport
• Photosynthesis: Water is a raw material for photosynthesis, and its transport to
leaves is essential for food production in plants.
• Nutrient Transport: Water dissolves minerals and nutrients in the soil and
carries them from the roots to different parts of the plant.
• Turgor Pressure: Water maintains turgor pressure in plant cells, providing
structure and rigidity to stems and leaves, and is essential for growth and cell
elongation.
• Transpiration: Water is lost through stomata in leaves during transpiration,
which helps regulate plant temperature and create a pull that drives water
movement upward.
Mechanism of Water Transport in Plants
Water transport in plants is a vital process that involves the uptake of water from the
soil, its movement through plant tissues, and its eventual evaporation into the
atmosphere. This process, essential for plant survival and growth, is primarily driven
by transpiration and relies on the properties of water and the plant's vascular system.
Water Uptake from the Soil
• Root Hair Zone: Water is absorbed mainly through root hairs, which are
extensions of the root epidermis. These root hairs increase the surface area for
absorption, allowing efficient water uptake.
• Osmosis: Water moves from the soil (where water potential is high) into the root
cells (where water potential is lower) through osmosis. This process occurs
because of the semi- permeable nature of the cell membranes.
Pathways in Roots
• Apoplast Pathway: Water moves through the cell walls and intercellular spaces
without crossing any membranes.
• Symplast Pathway: Water enters the cytoplasm of root cells and travels from
cell to cell through plasmodesmata (cytoplasmic connections between cells).
• Transmembrane Pathway: Water crosses cell membranes repeatedly, moving in
and out of cells as it progresses through the root tissue.
Movement of Water to the Xylem
• Endodermis and the Casparian Strip: The water reaches the endodermis, a layer
of cells with a waxy band called the Casparian strip. This strip forces water to
move into the symplast pathway, ensuring it passes through the plasma
membranes, which act as selective barriers. This regulates the entry of water and
dissolved minerals into the vascular system.
• Entry into Xylem Vessels: Water and dissolved nutrients pass through the
endodermal cells and enter the xylem vessels. The xylem is composed of hollow,
tube-like structures (vessel elements and tracheids) that facilitate long-distance
water transport.
Water Transport through the Xylem
• Cohesion-Tension Theory: The upward movement of water in the xylem is
explained by the cohesion-tension theory, which involves several key concepts:
• Transpiration: Water evaporates from the mesophyll cells of leaves and exits
through stomata. This creates a negative pressure (suction) in the leaf.
• Cohesion: Water molecules are polar, which means they are attracted to
each other (cohesion). This allows them to form a continuous column of
water in the xylem.
• Adhesion: Water molecules also adhere to the walls of xylem vessels. This
adhesion helps counteract gravity and stabilize the water column as it moves
upward.
• Tension: The evaporative pull from transpiration creates tension in the xylem,
which pulls the continuous water column upward from the roots to the leaves.
Water Transport to Leaves and Transpiration
• Water Movement to Mesophyll Cells: Once water reaches the leaves, it moves
into the mesophyll cells, where it plays a crucial role in photosynthesis, as well as
maintaining cell turgor (pressure that keeps the cells rigid).
• Evaporation and Transpiration: Water evaporates from the cell surfaces in the
mesophyll and diffuses out through the stomata, a process called transpiration.
Transpiration not only facilitates water transport but also cools the plant and helps
maintain a continuous flow of water from roots to leaves.
Regulation of Water Transport
• Stomatal Control: The opening and closing of stomata regulate transpiration rates.
Stomata open when there is sufficient light and water availability, allowing gas
exchange (CO₂ uptake) and water loss. They close during drought or high
temperatures to conserve water.
• Root Pressure: At night or in low transpiration conditions, roots can generate
positive pressure (root pressure) by actively transporting ions into the xylem,
drawing water into the roots via osmosis. Root pressure helps push water up the
xylem, but it is insufficient for long- distance transport and mostly contributes to
phenomena like guttation (exudation of water droplets on leaf edges).
Role of Capillary Action
• Xylem Structure: The narrow diameter of xylem vessels enhances capillary
action, where water rises due to the adhesion of water molecules to the walls and
cohesion among water molecules. This aids in lifting water, especially in small
plants.
Factors Influencing Water Transport
• Environmental Factors: Light, humidity, temperature, and wind affect the
rate of transpiration. For instance, higher temperatures increase evaporation,
speeding up transpiration.
• Soil Moisture Availability: Adequate water in the soil is essential for
continuous water uptake. When soil moisture is low, water uptake and transport
are reduced, affecting plant growth and function.
• Plant Structure: The structure and development of the root system and xylem
network influence the efficiency of water uptake and transport.
This mechanism supports vital physiological functions such as nutrient transport,
photosynthesis, and temperature regulation, ensuring the growth and survival of the
plant.