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Converts light energy to a usable form: [blank]
photosynthesis
What does respiration do and what does it facilitate?
Respiration releases stored energy from glucose and is broken into individual carbon dioxide molecules, development and reproduction.
[blank] is the sum of all interrelated biochemical processes in living organisms.
metabolism
Animals rely on green plants for oxygen, food, shelter, and other products.
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Which of the following processes are regulated by enzymes? (Select all that apply)
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What does anabolism do?
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Which of the following processes breaks chemical bonds?
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Photosynthesis involves the respiration cycle that transfers
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What is the difference between oxidation and reduction?
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[blank] is lost during oxidation and gained during reduction
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What is the final acceptor of an electron?
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Where is respiration initiated and completed?
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Aerobic respiration can be completed without oxygen.
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The cellular respiration equation is...
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What is a key difference between anaerobic respiration and fermentation?
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Which of the following are types of fermentation?
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Which of the following statements about glycolysis is true?
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What is the primary function of chlorophyll in photosynthesis?
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Which factor can limit the rate of photosynthesis?
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What happens during photorespiration in plants?
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How do plants primarily obtain carbon dioxide for photosynthesis?
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What is a consequence of too high light intensity on photosynthesis?
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What is the primary role of chlorophyll b in photosynthesis?
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Where do the light-dependent reactions of photosynthesis occur?
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During the light-independent reactions, what molecule combines with carbon dioxide to help form sugars?
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Which statement accurately describes the process of photosynthesis?
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What type of pigment absorbs blue or red light in certain algae and cyanobacteria?
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What is the primary purpose of chlorophyll when it absorbs light?
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What happens to the electrons lost from P680 in Photosystem II?
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Which reaction occurs immediately after light photons are absorbed by P680?
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What is the role of pheophytin in Photosystem II?
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Which statement correctly describes the relationship between photosystem I and photosystem II?
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How is ATP formed in the electron transport system of photosynthesis?
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What is the result of the electron transport chain involving cytochromes and plastocyanin?
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What defines fluorescence as it relates to chlorophyll?
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What kind of molecules primarily accept electrons during the light-dependent reactions?
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What key product results from the water-splitting reaction in photosystem II?
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Study Notes
Photosynthesis
- Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen
- Photosynthesis takes place in chloroplasts and other green parts of plants
- The chemical equation for photosynthesis is: 6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O
- Photosynthesis is essential for life on Earth as it provides the energy and oxygen necessary for all living organisms
Carbon Dioxide
- Carbon dioxide reaches chloroplasts in mesophyll cells by diffusing through stomata into the leaf interior
- Human activities like burning fossil fuels and deforestation are adding more carbon dioxide to the atmosphere than is being removed
- Increased carbon dioxide in the atmosphere can lead to global increases in temperature
- Increased carbon dioxide can enhance photosynthesis
Water
- Less than 1% of all water absorbed by plants is used in photosynthesis, most is transpired or incorporated into plant materials
- Water is the source of electrons in photosynthesis and oxygen is released as a byproduct
- When water is in short supply or light intensities are too high, plants close their stomata to reduce the supply of carbon dioxide available for photosynthesis
Light
- About 40% of the radiant energy received on Earth is in the form of visible light
- Violet to blue and red-orange to red wavelengths of light are used most extensively in photosynthesis
- Green light is reflected by plants
Optimal Rates and Limiting Factors
- Plants vary considerably in light intensities needed for optimal photosynthetic rates
- Temperature and amount of carbon dioxide can also be limiting factors for photosynthesis
### Effects of Changing Light and Temperature
- If light and temperatures are too high, the ratio of carbon dioxide to oxygen inside leaves may change
- Photorespiration occurs when the ratio of carbon dioxide to oxygen inside leaves changes, which uses oxygen and releases carbon dioxide
- Photorespiration can help some plants survive under adverse conditions
- If light intensity is too high, photooxidation occurs, which results in the destruction of chlorophyll
Chlorophyll
- Chlorophyll is a green pigment found in plants that absorbs light energy for photosynthesis
- There are several types of chlorophyll molecules, but chlorophyll a (blue-green) and chlorophyll b (yellow-green) are the most prevalent
- Chlorophyll b transfers energy from light to chlorophyll a, which makes it possible for photosynthesis to occur over a broader spectrum of light
- The magnesium end of a chlorophyll molecule captures light energy
- The lipid tail of a chlorophyll molecule anchors it into the thylakoid membrane
Photosynthetic Pigments
- Photosynthetic pigments other than chlorophyll include carotenoids (yellow and orange), phycobilins (blue or red), and other types of chlorophyll
- About 250-400 pigment molecules are grouped in a light-harvesting complex = photosynthetic unit
- Two types of photosynthetic units work together in light-dependent reactions
### The Two Phases of Photosynthesis
- Light-dependent reactions: occur in the thylakoid membranes of chloroplasts and produce ATP and NADPH
- Light-independent reactions: occur in the stroma of chloroplasts and use ATP and NADPH to make sugars
Light-Dependent Reactions
- Water molecules are split apart, releasing electrons and hydrogen ions, and oxygen gas is released
- Electrons pass along an electron transport system
- ATP is produced
- NADP is reduced, forming NADPH (used in light-independent reactions)
Light-Independent Reactions
- ATP and NADPH are used to form sugars
- The Calvin cycle: carbon dioxide combines with RuBP (ribulose bisphosphate), and the combined molecules are converted to sugars (glucose).
A Closer Look at Photosynthesis
- 1772: Joseph Priestley noted that photosynthesis “restored” oxygen
- 1779: Jan Ingen-Housz showed that air is only restored when the green parts of plants received sunlight
- 1782: Jean Senebier discovered that photosynthesis requires carbon dioxide
- 1796: Ingen-Housz showed that carbon is a plant nutrient
- 1804: Theodore de Saussure showed that water is required for photosynthesis
Light-Dependent Reactions Reexamined
- Each pigment has its own distinctive pattern of light absorption called the pigment’s absorption spectrum
- Shorter wavelengths of light carry higher amounts of energy
- Chlorophylls absorb light in the violet to blue and red wavelengths.
When Pigments Absorb Light
- When a pigment absorbs light, the energy levels of some of the pigment’s electrons are elevated
- These electrons are said to be in an excited state
- Energy from an excited electron is released when it drops back to its ground state
- Fluorescence: energy is immediately released as light
- Phosphorescence: energy is emitted as light after a delay
- Energy may otherwise be converted to heat.
- In photosynthesis, energy is stored in chemical bonds.
The Light-Dependent Reactions Reexamined
- Two types of photosynthetic units: photosystem I and photosystem II
- Events of photosystem II come before those of photosystem I
- Both can produce ATP
- Only organisms with both photosystem I and photosystem II can produce NADPH and oxygen as a consequence of electron flow.
Photosystems I and II
-
Photosystem I = chlorophyll a, small amount of chlorophyll b, carotenoid pigment, and P700
- P700 = reaction-center molecule - Only one that actually can use light energy
- Remaining pigments = antenna pigments
- Gather and pass light energy to the reaction center
- Iron-sulfur proteins - Primary electron acceptors, first to receive electrons from P700
-
Photosystem II = chlorophyll a, B-carotene, small amounts of chlorophyll b, and reaction-center molecule: P680
- Pheophytin (Pheo) - Primary electron acceptor
The Light-Dependent Reactions of Photosynthesis
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Photolysis - Water-splitting, occurs in Photosystem II
- Light photons absorbed by P680, which boosts electrons to a higher energy level.
- Electrons are passed to the acceptor molecule, pheophytin, then to PQ (plastoquinone), then along the electron transport system to photosystem I.
- Electrons extracted from water replace electrons lost by P680.
- One molecule of oxygen, 4 protons, and 4 electrons are produced from two water molecules.
Electron Flow and Photophosphorylation
- The electron transport system consists of cytochromes, other electron transfer molecules, and plastocyanin.
- Photons move across the thylakoid membrane by chemiosmosis.
- Phosphorylation - ATP is formed from ADP
Photosystem I
- Light is absorbed by P700, which boosts electrons to a higher energy level.
- Electrons are passed to the iron-sulfur acceptor molecule, Fd (ferredoxin), then to FAD (flavin adenine dinucleotide).
- NADP is reduced to NADPH.
- Electrons removed from P700 are replaced by electrons from Photosystem II
Chemiosmosis
- Net accumulation of protons in the thylakoid lumen occurs from the splitting of water molecules and electron transport.
- The proton gradient gives ATPase (special proteins in the thylakoid membrane) the potential to move protons from the lumen to the stroma.
- The movement of protons across the membrane = source of energy for the synthesis of ATP
Calvin Cycle
- Six molecules of CO2 combine with six molecules of RuBP (ribulose 1,5-bisphosphate) with the aid of rubisco.
- This eventually results in twelve 3-carbon molecules of 3PGA (3-phosphoglyceric acid).
- NADPH and ATP supply energy and electrons that reduce 3PGA to GA3P (glyceraldehyde 3-phosphate).
- Ten of the twelve GA3P molecules are restructured, using 6 ATP, into six 5-carbon RuBP molecules.
- Net gain of 2 GA3P, which can be converted to carbohydrates or used to make lipids and amino acids.
### Photorespiration
- Photorespiration - Competes with carbon-fixing role of photosynthesis
- Rubisco fixes oxygen instead of carbon dioxide.
CAM and C4 Plants
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CAM photosynthesis - Similar to C4 photosynthesis in that 4-carbon compounds are produced during light-independent reactions, however:
- Converted back to CO2 during the day for use in the Calvin cycle (stomata closed)
- Allows plants to function well under limited water supply, as well as high light intensity.
Other Significant Processes that Occur in Chloroplast
- Reduction of sulfate to sulfide
- Sulfides are used to make amino-acids
- Nitrates converted to ammonia
- Ammonia is used to make amino-acids, for eg-glutamine which is stored in roots and specialized stems
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Description
Test your knowledge on the process of photosynthesis and how plants convert light energy into chemical energy. This quiz covers key concepts and terminology related to this fundamental biological process.
