Bio: Ch6 flashcards
6.1: Overview of Photosynthesis:
Photosynthesis - the prosses of changing solar energy into chemical energy that can be used by the plants or used by the animal eating the plant.
producers or the organisms that carry out photosynthesis are responsible for most chemical energy sources and fuel most life on earth.
Consists of two reactions, one where water makes the enzymes needed for the second reaction where CO2 is reduced.
Stomata - openings in plant leaves that collect CO2
Mesophyll cells - cells in plants that have the chloroplasts for photosynthesis.
Chloroplasts - a duoble mebrain sorounds the stroma which is full of thylakoids where the photosynthesis happens.
stroma - the fluid in the chloroplast
thylakoids - the mebrain system in the stroma that forms flattened button like sacks that has the green pigment to catch the solar energy and power the photosynthesis.
grana - stacks of thylakoids
thylakoid space - the space formed by all the connected thylakoids
Formula - CO2 from the air and water from the ground to make carbohydrates and oxygen
Solar energy + H2O + CO2 = C6H12O6 + O2
Reduction Reaction - when molecule gains electrons and hydrogen ions
Oxidation Reaction - when a molecule losses hydrogen ions
Redox Reaction - when one molecule gains an electron and the other losses an electron
6.2
Learning Outcomes
Upon completion of this section, you should be able to:
Describe the function of photosynthetic pigments.
Explain the flow of electrons in the light reactions of photosynthesis.
Explain how ATP and NADPH are generated in the light reactions.
Overview of Solar Energy
Definition: Solar energy, also known as radiant energy, can be described in terms of its wavelength and energy content in the electromagnetic spectrum.
Electromagnetic Spectrum: The types of solar energy range from gamma rays (shortest wavelength) to radio waves (longest wavelength).
Visible Light: This specific region occupies only a small portion of the entire electromagnetic spectrum.
Wavelengths of Visible Light:
Ranges from violet (shortest wavelength, highest energy) to red (longest wavelength, lowest energy).
Demonstration: When visible light is passed through a prism, different wavelengths appear as colors of the rainbow.
Atmospheric Interference: Less than half of the solar radiation that hits Earth's atmosphere reaches its surface, primarily due to absorption by the ozone layer (which screens higher energy wavelengths) and water vapor (which screens lower energy wavelengths).
Photosynthetic Pigments
Types of Photosynthetic Pigments:
Chlorophylls: Includes chlorophyll a and b, which primarily absorb violet, blue, and red wavelengths better than others, reflecting green light which makes leaves appear green.
Carotenoids: These appear yellow or orange because they absorb light in the violet-blue-green range but not in the yellow-orange range.
Seasonal Changes: The presence of carotenoids becomes noticeable in the fall when chlorophyll degrades.
Function of Photosynthetic Pigments
Antennae for Solar Energy: Photosynthetic pigments act as an antenna for capturing solar energy, transferring energy until concentrated in a specific pair of chlorophyll a molecules known as the reaction center.
Light Reactions Overview
Comparison to Solar Panels: Similar to how a solar panel captures the sun’s energy, the light reactions convert solar energy into a hydrogen ion gradient, leading to the production of ATP and NADPH.
Hydrogen Ion Gradient: Essential for the synthesis of ATP molecules.
Photosystems in Light Reactions
Photosystems: There are two key photosystems in the light reactions: Photosystem I (PS I) and Photosystem II (PS II).
Named for the order of discovery, not the order of their role in the light reactions.
Process Flow:
PS II: Absorbs photons and splits water (photolysis) to release oxygen, generating energized electrons that move to an electron acceptor.
Photolysis: Water is split into oxygen and hydrogen ions; it releases oxygen and provides electrons to replace those lost by chlorophyll in PS II.
The electron transport chain receives energized electrons from the reaction center in PS II, establishing an energy gradient that aids in ATP production.
PS I: Absorbs solar energy that energizes electrons which are captured by a different electron acceptor, leading to NADPH production.
Electron Transport Chain
Functionality: Situated within the thylakoid membranes to promote efficient electron transfer.
ATP Synthase: Enzyme complex that synthesizes ATP using the energy from hydrogen ion flow down their concentration gradient.
ATP and NADPH Production
ATP Production:
Hydrogen ions channel down their concentration gradient through ATP synthase, causing ATP to form from ADP and inorganic phosphate (ADP + P
ightarrow ATP).
NADPH Production:
NADP+ acts as a coenzyme that accepts electrons and a hydrogen ion to become NADPH.
NADPH functions as an electron carrier in the synthesis of carbohydrates during the Calvin Cycle.
Connection of Concepts
The light reactions capture solar energy to produce ATP and NADPH, necessary for synthesizing carbohydrates, indicating the foundational role of light reactions in photosynthesis.
Energy Absorption in Photosynthesis
Global Impact: It is estimated that approximately 130 terawatts of energy from photosynthesis is captured globally each year, which corresponds to 1.3 trillion 100-watt light bulbs or over seven times human energy consumption.
Key Questions
Explain why plants have more than one photosynthetic pigment.
Summarize how the light reactions capture solar energy and identify their outputs.
Describe the path of electrons from water to NADPH.
Explain the necessity of water in the light reactions.
6.3 The Calvin Cycle Reactions—Making Sugars
Learning Outcomes
Objective: Upon completion of this section, students should be able to:
Summarize the purpose of the Calvin cycle in photosynthesis.
Describe how ATP and NADPH are utilized in the manufacture of carbohydrates by the Calvin cycle.
Summarize how the output of the Calvin cycle is used to make other carbohydrates.
Overview of the Calvin Cycle
Location: The Calvin cycle reactions occur in the stroma of chloroplasts.
Process: The cycle consists of three main steps:
Fixation of carbon dioxide (CO₂)
Reduction of CO₂
Regeneration of RuBP (ribulose-1,5-bisphosphate)
Outputs: The cycle produces molecules of G3P (glyceraldehyde 3-phosphate), which are utilized by plants to synthesize glucose and other organic molecules.
Energy Requirement: The cycle is energy-intensive, driven by ATP and NADPH generated from the light-dependent reactions of photosynthesis.
Steps of the Calvin Cycle
1. Fixation of Carbon Dioxide
Process:
Carbon dioxide from the atmosphere is attached to RuBP, a 5-carbon molecule.
Enzyme: The enzyme that catalyzes this reaction is RuBP carboxylase (commonly known as rubisco).
Outcome: This results in the formation of a 6-carbon intermediate that quickly splits into two 3-carbon molecules.
2. Reduction of Carbon Dioxide
Process:
The reduction stage utilizes NADPH and ATP produced from the light reactions.
Roles:
NADPH provides the electrons needed for the reduction.
ATP supplies the energy necessary for the reactions.
Outcome: Carbon dioxide is ultimately reduced to form G3P (glyceraldehyde 3-phosphate), a carbohydrate precursor.
3. Regeneration of RuBP
Cyclic Nature: The Calvin cycle is cyclic, meaning some of the products return to initiate the cycle again.
Process:
In the final stage, ATP from the light reactions is used to regenerate RuBP so the cycle can continue.
Detail:
For every three turns of the cycle, five molecules of G3P are utilized to reform three molecules of RuBP.
This regeneration is critical to carbon fixation and allows for the synthesis of glucose and other carbohydrates.
Net Gain Calculation:
It takes three turns of the Calvin cycle to achieve a net gain of one G3P molecule, as it requires two G3P molecules to synthesize one glucose molecule.
Environmental Context
Carbon Offset:
It is estimated that there are over 3 trillion trees on the planet, yet they are only capable of absorbing a fraction of the approximately 40 billion tons of CO₂ released annually by human activities.
Each tree absorbs roughly 48 pounds of CO₂ per year, equating to about 1 ton of CO₂ over the average lifespan of a hardwood tree (approximately 40 years).
To offset an individual's carbon footprint, it is estimated that each person would need to plant around 700 trees per year.
Projects, like the Trillion Tree Campaign, aim to address this challenge by promoting global reforestation efforts.
Utilization of G3P
Biochemical Capabilities of Plants
Diversity of Products:
Compared to animal cells, plants and algae have extensive biochemical mechanisms to utilize G3P.
Fatty Acids and Glycerol:
From G3P, plants synthesize fatty acids and glycerol, which combine to form plant oils (e.g., corn oil, sunflower oil, olive oil).
Amino Acids Formation:
When nitrogen is added to the hydrocarbon skeleton derived from G3P, amino acids are synthesized.
Glucose Metabolism:
Glucose Phosphate:
Among the organic compounds resulting from G3P metabolism is glucose phosphate.
Glucose is primarily metabolized by plants and animals to generate ATP for energy needs.
Carbohydrate Transport and Storage:
Glucose phosphate can connect with fructose (after phosphate removal) to form sucrose, facilitating carbohydrate transport within plants.
It also serves as the precursor for the synthesis of starch and cellulose.
Starch:
The storage form of glucose, accumulated in chloroplasts and amyloplasts found in roots.
Cellulose:
Provides structural integrity to plant cell walls and acts as dietary fiber in human diets, which cannot be digested by humans.
Connecting the Concepts
The Calvin cycle effectively uses the products from the light reactions (ATP and NADPH) to reduce carbon compounds and produce G3P, a crucial building block for the synthesis of carbohydrates.
Check Your Progress
Briefly summarize the role of the Calvin cycle in photosynthesis.
The Calvin cycle's primary role is to convert atmospheric CO₂ into glucose through carbon fixation, reduction, and regeneration processes.
Identify the source of the NADPH and ATP used in the Calvin cycle.
NADPH and ATP are sourced from the light-dependent reactions of photosynthesis.
Explain the importance of G3P to plants.
G3P is crucial as it serves not only as a precursor for glucose but also for fatty acids, glycerol, amino acids, sucrose, starch, and cellulose, demonstrating the versatility of plant cellular metabolism.
Variations in Photosynthesis
Learning Outcomes
Upon completion of this section, you should be able to:
Define photosynthesis and explain why some plants must use this type of photosynthesis.
Describe both the advantages and the disadvantages of photosynthesis over photosynthesis.
Compare and contrast the leaf structure of C4 plants with that of C3 plants.
Explain CAM photosynthesis and describe the conditions under which plants can use it.
Photosynthesis Overview
Photosynthesis is a vital metabolic process in which plants convert light energy into chemical energy.
Plants are metabolically adapted to their environments:
Evergreen trees in cold, windy climates: possess small, narrow leaves that resemble needles, reducing water loss.
Evergreen trees in warm, wet climates: have large, flat leaves to maximize sunlight capture.
C3 Photosynthesis
Most plants utilize C3 photosynthesis (the term refers to the three-carbon molecule produced during carbon fixation).
In C3 photosynthesis:
The first detectable molecule after carbon fixation is a 3-carbon molecule (thus the name C3).
Leaf structure: in a C3 leaf, mesophyll cells are arranged in parallel rows and contain well-formed chloroplasts, where the Calvin cycle occurs.
Stomata and water loss: In hot, dry conditions:
Stomata close to prevent water loss, which also stops carbon dioxide from entering the leaf.
This creates a buildup of oxygen (a by-product of photosynthesis), which leads to inefficiencies due to photorespiration.
Photorespiration: Occurs when oxygen competes with carbon dioxide for the active site of rubisco (the first enzyme of the Calvin cycle), reducing photosynthesis efficiency.
C4 Photosynthesis
Some plants have evolved to utilize C4 photosynthesis to adapt to hot, dry conditions.
Characteristics of C4 photosynthesis:
The first detectable molecule following carbon fixation is a 4-carbon molecule.
C4 plants avoid the competition of oxygen with rubisco.
Anatomy differences in C4 plants:
Chloroplasts are located in both mesophyll cells and bundle sheath cells surrounding leaf veins.
Mesophyll cells are arranged concentrically around bundle sheath cells, which protect them from oxygen buildup.
Calvin Cycle in C4 Plants
In C4 plants:
The Calvin cycle occurs ONLY in bundle sheath cells rather than in mesophyll cells, preventing direct exposure to air.
Carbon dioxide is first fixed in the mesophyll cells using a PEP carboxylase enzyme to form a 4-carbon molecule, which is then modified and sent to bundle sheath cells.
Advantages of C4 over C3 Plants
C4 plants can maintain yields even when stomata are closed during hot, dry conditions, unlike C3 plants.
Net photosynthetic rates for C4 plants (e.g., sugarcane, corn, Bermuda grass) can be 2 to 3 times higher than for C3 plants (e.g., wheat, rice, oats).
C4 plants have ecological advantages in moderate weather conditions, dominating during hot, dry summers where C3 plants might struggle.
CAM Photosynthesis
CAM photosynthesis (Crassulacean-Acid Metabolism) involves partitioning in time rather than space.
Key features of CAM photosynthesis:
Predominantly found in water-storing succulent plants like cacti with adaptations for arid environments.
During the night, CAM plants fix carbon dioxide into 4-carbon molecules, which are stored in vacuoles within mesophyll cells.
During the day, stored acids release carbon dioxide to the Calvin cycle when NADPH and ATP are available from the light reactions.
Benefits of CAM:
Stomata are open only at night, significantly reducing water loss and maintaining photosynthesis under stressful conditions.
Photosynthesis in CAM plants is lower due to limited carbon dioxide fixation at night, but the adaptation allows them to survive in extreme aridity.
Evolutionary Trends
C4 plants evolved to thrive in conditions with high light, temperature, and low rainfall.
Overall, it is estimated that 4% of plant species are C4, yet they account for over 20% of global plant biomass growth.
Notably, many invasive weed species are C4 plants.
CAM photosynthesis is widely distributed across more than 30 families of flowering plants, including cacti and orchids, and even found in some non-flowering plants like ferns.
Connecting the Concepts
Variations in photosynthetic pathways enable plants (producers) to adapt to diverse environments, maximizing their survival and ecological success in various climatic conditions.