Plant Physiology: Structure and Function

• Unit 4 discusses plant physiology, focusing on the structure and function of plants

Perspectives on Biology

• Focuses on the two primary levels of biological organization
• One perspective looks at the organismic level and above, leading to ecology and related fields of study
• The other emphasizes cellular and molecular levels, which results in physiology and biochemistry

Physiological Processes in Flowering Plants

• Are described using the processes of photosynthesis, respiration, and plant growth
• These processes are detailed in molecular terms, with respect to cellular and organism-level activities
• Relationship of physiological processes to the environment is also considered

Melvin Calvin

• Melvin Calvin received his Ph.D. in Chemistry from the University of Minnesota and became a Professor of Chemistry at the University of California, Berkeley.
• Calvin and his co-workers explored using radioactivity for beneficial purposes: He studied photosynthetic reactions in green plants alongside J.A. Bassham.
• By labeling carbon dioxide with C¹⁴, Calvin studied how green plants produce sugar and other substances
• He found that plants transform light energy into chemical energy using pigment molecules and other substances
• Calvin received the Nobel Prize in 1961 for mapping carbon assimilation in photosynthesis
• Calvin's principles about photosynthesis are used in renewable energy and solar energy research

Chapter 11: Photosynthesis in Higher Plants

Introduction

• All animals depend on plants for food
• Green plants synthesize their food using photosynthesis and are called autotrophs
• Other organisms that rely on green plants for food are known as heterotrophs
• Green plants perform photosynthesis, turning light energy into organic compounds
• All life on Earth depends on sunlight for energy
• Photosynthesis provides food and releases oxygen into the atmosphere
• The chapter focuses on the structure of photosynthetic machinery and the light reactions that create chemical energy

What We Know

• Chlorophyll, light, and CO₂ are necessary for photosynthesis
• Photosynthesis occurs only in the green parts of leaves, where light is present

Early Experiments

• Joseph Priestley performed experiments that showed the role of air in the growth of green plants
• Priestley discovered oxygen in 1774
• He found that burning candles and animals "damaged" air in closed spaces, but plants could restore the air
• Jan Ingenhousz furthered Priestley's work by placing similar setups in the dark and sunlight, discovering that sunlight is essential for plants to purify the air
• Ingenhousz found that aquatic plants in sunlight produced bubbles, identifying them as oxygen and proved that only the green parts of plants release oxygen

Further Investigations

• Julius von Sachs found that plants produce glucose when they grow, which is stored as starch, locating this process to special bodies called chloroplasts
• T.W Engelmann split light with a prism and illuminated green algae (Cladophora) with aerobic bacteria
• The bacteria congregated in the blue and red light regions, identifying where oxygen evolved
• The understanding of photosynthesis key elements became known by the mid-19th century
• Plants use light energy to convert water and carbon dioxide into carbohydrates

Overall Process

• Represented through the formula: CO₂ + H₂O --(Light)--> [CH₂O] + O₂
• [CH₂O] symbolizes carbohydrate, like glucose
• Cornelius van Niel showed photosynthesis is a light-dependent reaction with hydrogen from an oxidizable compound reducing carbon dioxide into carbs
• Hydrogen donor in green plants is water, which leads to oxygen, while other organisms use H₂S and don't release oxygen
• Oxygen by green plants is from water, not carbon dioxide
• The correct equation for photosynthesis: 6CO₂ + 12H₂O --(Light)--> C6H12O6 + 6H₂O + 6O₂

Photosynthesis Location

• Green leaves and other green parts of the plant are the locations
• Mesophyll cells within leaves contain many chloroplasts
• Chloroplasts align on the cell walls to get the most light
• Within the chloroplast are the grana, stroma lamellae, and stroma
• These components contribute to the division of labor in the chloroplast
• The membrane system captures light and synthesizes ATP and NADPH
• Sugar synthesis takes place in the stroma through enzymatic actions
• Light reactions are light-driven, and dark reactions rely on the products of light reactions (ATP and NADPH)

Photosynthetic Pigments

• There are various shades of green in leaves
• Paper chromatography of leaf pigments shows the presence of four pigments
• Chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow), and carotenoids (yellow to yellow-orange)
• Pigments specifically absorb certain wavelengths of light
• Chlorophyll a mainly absorbs blue and red light

Light Absorption and Photosynthesis

• Regions where chlorophyll a absorbs the most light also have higher photosynthesis rates
• Chlorophyll a is the main pigment that is linked to photosynthesis
• Chlorophyll b, xanthophylls, and carotenoids are called accessory pigments, which absorb light and send the energy to chlorophyll a

Light Reaction

• Involves light absorption, water splitting, oxygen release, and the creation of ATP and NADPH
• Protein complexes are involved
• Pigments are arranged into light-harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II)
• LHCs contain hundreds of pigment molecules bound to proteins
• All photosystems have pigments that collect light (except one chlorophyll a molecule)
• Each chlorophyll a reaction center in PS I absorbs light best at 700 nm (P700), and in PS II absorbs best at 680 nm (P680)

Electron Transport

• Chlorophyll electron absorbs 680 nm of red light in photosystem II, causing the electrons to get excited
• The electrons are picked up by an electron acceptor and then passed to an electron transport system, which contains cytochromes
• Movement of electrons is downhill, measured by redox potential
• These electrons move to pigments within photosystem PS I
• Simultaneously, electrons in PS I get excited by 700 nm red light
• Electrons are transferred to an acceptor molecule with a greater redox potential
• These electrons transfer again, reducing NADP+ to NADPH + H+
• The Z scheme represents the complete transfer of electrons which starts from PS II, moves uphill to an acceptor, then through the electron transport chain to PS I, where electrons get excited again, and NADP+ id reduced to NADPH + H+
• The movement of all carriers create a redox potential

Splitting of Water

• The electrons moved from photosystem II get replaced with electrons from water splitting
• Water splits into H+, [O], and electrons, with oxygen as a net product of photosynthesis
• Formula is 2H₂O → 4H+ + O₂ + 4e¯
• The PS II and water splitting complex are located on the thylakoid membrane's inner side

Types of Phosphorylation

• Living organisms can obtain energy from oxidizable substances and store it as bond energy
• ATP carries this energy, and ATP is synthesized by cells
• Phosphorylation: Synthesis of ATP from ADP and inorganic phosphate in the presence of light
• Non-cyclic photo-phosphorylation: When both photosystems work in a series (PS II and then PS I) is non-cyclic photo-phosphorylation
• Cyclic photo-phosphorylation: When only PS I functions, electrons circulate and phosphorylation occurs due to cyclic electron flow

Chemiosmotic Hypothesis

• The chemiosmotic hypothesis puts forth the process that ATP is produced in the chloroplast
• ATP synthesis is connected to a proton gradient across a membrane
• Proton accumulation occurs toward the inside of the membrane, in the lumen
• Protons accumulate in the intermembrane space during respiration

Creating a Proton Gradient

• Water splitting on the inner side of the membrane causes protons to accumulate in the thylakoid lumen
• Electrons that move through photosystems cause protons to get distributed
• A primary electron acceptor that is located on the outer side of the membrane moves its electron to a hydrogen carrier
• A proton is released into the lumen when it passes to an electron carrier on the lumen side
• NADP reductase enzyme on the stroma side removes protons during the reduction of NADP+ to NADPH+ H+
• Within the chloroplast, protons accumulate in the lumen: Decreasing the proton number in the stroma leading to a pH gradient across the thylakoid membrane
• Breakdown of proton gradient results in the synthesis of ATP

ATP Synthase

• Requires the CF₀ channel of the ATP synthase
• The ATP synthase enzyme contains CF₀ (embedded in the thylakoid membrane, forms a transmembrane channel for facilitated diffusion) and CF₁ (protrudes on the membrane surface on the stroma)
• Breakdown of the gradient gives energy that makes the enzyme synthesize several molecules of ATP
• ATP is used in the biosynthetic reaction for CO₂ fixation and sugar synthesis

ATP and NADPH Use

• The products of light reaction are ATP, NADPH, and O₂: O₂ diffuses out, and ATP and NADPH are used for food synthesis
• The biosynthetic phase of photosynthesis depends on the products of the light reaction e.g.ATP and NADPH, CO2 and H2O
• After light ends, the process will continue for a while then stop, when light becomes available again, the process starts once more

Calvin Cycle

• Melvin Calvin discovered how CO₂ is combined with H₂O to produce sugars using radioactive ¹⁴C
• The first carbon dioxide fixation product was a 3-carbon organic acid known as 3-phosphoglyceric acid (PGA)
• Plants that have the C₃ pathway have PGA as the first product of CO₂ fixation
• Plants that have the C₄ pathway have oxaloacetic acid or OAA as their first product of CO₂ fixation
• The Calvin cycle occurs in all photosynthetic plants and includes carboxylation, reduction, and regeneration

Calvin Cycle Stages

• Carboxylation: It is the fixation of carbon dioxide into a stable organic that utilizes RuBP carboxylase to form two molecules of 3-PGA
• Reduction: A series of reactions forming glucose, using ATP for phosphorylation and NADPH for reduction
• Requires six molecules of CO₂ and six cycles to make one molecule of glucose
• Regeneration: RuBP is regenerated so cycle can go on which requires one ATP molecule

Energy Requirements in Calvin Cycle

• Three molecules of ATP and two molecules of NADPH are needed for every CO₂ molecule that enters the Calvin cycle
• Six cycles are used to create one glucose molecule
• Plants use the C3 pathway or the Calvin cycle as the main biosynthetic pathway and have C₁ oxaloacetic acid as the first CO₂ fixation product
• C4 plants have special leaf anatomy, are tolerant of higher temperatures, react to high light intensities, and do not use photorespiration with higher productivity

Kranz Anatomy

• Leaf anatomy which contains the bundle sheath cells
• Arrangement of cells where the bundle sheath cells may be in many layers which are composed of chloroplasts, thick walls with no intercellular gaps Photosynthesis in these plant types contain the primary (3-carbon molecule phosphoenol pyruvate(PEP)) co2 acceptor which is present in the mesophyll cells, and the enzyme (PEP carboxylase/PEPcase).
• Bundle sheath cells and mesophyll cells have the formation of 4-carbon compounds like malic or aspartic acid
• Broken down to release carbon dioxide and a 3-carbon molecule which is converted back to PEP
• The carbon pathway releases C3 or Calvin pathway and the bundle sheath cells contain a RuBisCO

Photorespiration

• Plants using Photorespiration create important differences
• RuBP combines with CO₂ to form molecules of 3PGA which is enhanced by RuBisCO to create a binding site
• The molecule has less affinity for CO₂ when there is more oxygen
• In C3 plants the CO₂ fixation is decreased and RuBP connects with oxygen leading to photorespiration
• The end cellular process makes carbon dioxides result in ATP utilization
• Photorespiration does not function in c4 plants instead the concentration of co2 is released at enzyme cite by c4 acid to make increase the inter cellular with ruBisCO

Factors Affecting Photosynthesis

• Understanding the determinants of photosynthesis is important
• Photosynthesis relies on multiple elements, both plant-related and external
• Plant factors are the quantity, size, age, and direction of leaves, mesophyll cells and chloroplasts, internal carbon dioxide levels, and chlorophyll quantity
• External factors include sunlight availability, temperature, carbon dioxide levels, and water
• The most limited resource determines rate
• According to Backman 2 factors happen when chemical reactions occur:
  • if a chemical process depends more then other factors where it is most restricted which can then determine it’s value quantity

Factors

• For optimum sunlight and limited carbon conditions, the plant will not photosynthesize id the temperature is lower in said regions
• Light conditions: the quantity intensity and time is required for photosynthesis
• There is a constant relationship with fixing carbon conditions at lower intensity, whereas at high intensity the levels do not showcase some intensity

Limiting Factors

• At full sunlight the saturation for light conditions will mostly occur with plants or other sources with limiting factors
• It is observed that carbon dioxide condition it is the most limited element as photosynthesis and the concentration is low/between .83 and .84
• Rates increase with carbon dioxide fixing conditions by roughly 8.5 which become to extreme over a certain amount of time

C3 and C4 Plants

• C3 and C4 respond well when using carbon dioxide conditions when not properly high these conditions are then not present at high light.
• Its important that C4 shows conditions at 368 and c3 does so properly with in large amounts
• The conditions of carbon being very limited is the results/ conditions of the c3.
• C2 responds to high Carbon conditions which makes good production and yields

Temperature

• Dark reactions control enzyme which controls heat. Photosynthesis responds under these conditions the plants are more appropriate where there good levels but a very small level for conditions
• The level of optimum for certain the photosynthesis depends highly on the adaptation from certain environments for tropical this will result in high levels

Water

• Conditions in water are one side affect in order to affect the effect is more higher towards the plant instead photosynthesis, reducing the stoma and leaves to wilt with water stress

Summary

• Green plants convert CO₂ into carbohydrates through photosynthesis, which occurs in chloroplasts with light and dark reactions
• Light is absorbed by pigments/chlorophylls in photosystems
• Water molecules divide, which results in releasing from the water molecule protons and transfer by PS II
• Calvin Cycle: The enzyme RuBisCO connects to a 5 carbon element an converts 2 molecule into three carbon PGA
• Both light is used where NADPH occurs, the other oxygenation responses in plants creates another element for photo-respiration C4 cycle