Chapter 10
Describe the structure of chloroplasts
A chloroplast has two membranes surrounding a dense fluid called the stroma. Suspended in the stroma is a third membrane system made up of sacs called thylakoids. In the thylakoid membranes is chlorophyll, the green pigment that gives leaves their color
What is a photosystem composed of?
A photosystem is composed of a reaction center complex surrounded by several light harvesting complexes
How does ATP form exactly?
ATP forms in the stroma, where H+ ions cross the ATP synthase complexes after leaving the thylakoid membrane
How can absorption be measured and plotted?
Absorption can be measured with a spectrophotometer and an absorption spectrum graph can be plotted to see absorption versus wavelength
What does the action spectrum for photosynthesis do?
Action spectrum for photosynthesis profiles the relative effectiveness of different wavelengths of radiation in driving the process
What are alternate modes of carbon fixation?
Alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle even in hot, dry climates. The two most important are C4 Photosynthesis and crassulacean acid metabolism (CAM)
What are autotrophs and how do they produce their organic molecules?
Autotrophs are "self-feeders" and produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment
How are bundle sheath cells and mesophyll cells arranged?
Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf. Between the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells
What are C3 plants?
C3 plants are those that use rubisco to make a three-carbon compound 3-phosphoglycerate
Describe C4 Photosynthesis.
C4 Photosynthesis: PEP carboxylase adds CO2 to phosphoenolpyruvate (PEP), forming the four carbon product oxaloacetate. PEP carboxylase can bind CO2 with more affinity making it more efficient than rubisco. After the CO2 is fixed in the mesophyll cells, the four carbon products are exported to bundle-sheath cells through plasmodesmata. Within the bundle-sheath cells, the four-carbon compounds release CO2, which is re-fixed into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate to PEP, which can accept addition of another CO2, allowing the reaction cycle to continue. To generate this extra ATP, bundle-sheath cells carry out cyclic electron flowhnh
Is C3 or C4 photosynthesis more efficient?
C4 photosynthesis is considered more efficient than C3 photosynthesis because it uses less water and resources
What is the difference is separation of steps between C4 and CAM>?
C4 plants undergo spatial separation of steps while CAM undergo temporal separation of steps
What are CAM plants?
CAM plants open their stomata during the night and close them during the day. This helps to store water but prevents CO2 from entering the leaves.
Describe the CAM photosynthesis process.
CAM: The mesophyll cells of CAM plants store the organic acids they make during thee night in their vacuoles until morning, when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the calvin cycle, CO2 is released from the organic acids makde the night before to become incorporated into sugar in the chloroplasts
Describe Carbon Fixation
Carbon Fixation: attaches each CO2 molecule to a five carbon sugar called ribulose biphosphate (RuBP) with the enzyme rubisco. The product is a six-carbon intermediate that immediately splits in half due to instability, forming two molecules of 3 phosphoglycerate (for each CO2 fixed)
What are carotenoids?
Carotenoids are hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light.
What do carotenoids do?
Carotenoids broaden the spectrum of absorbance, but their more important role is photoprotection where they absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen forming reactive oxidative molecules dangerous to the cell
Where are chloroplasts found and what is that structure?
Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf
What are chloroplasts?
Chloroplasts: a plastid that contains chlorophyll and in which photosynthesis takes place.
What does cyclic flow produce and not produce?
Cyclic flow does not produce NADPH and does not release oxygen, but it does generate ATP
What does a light harvesting complex consist of and what does that allow it to do?
Each light-harvesting complex consists of various pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) bound to proteins and together they make for a greater surface area and larger portion of the spectrum and act as the antenna for the reaction center complex. When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light harvesting complex until it is passed to the pair of chlorophyll a molecules in the reaction center complex.
What does each photosystem have? How are they different in PI and PII?
Each photosystem has a characteristic reaction-center complex. The reaction-center chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum), and the reaction-center complex of photosystem I is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far red part of the spectrum)
Why does each pigment have a unique absorption spectrum?
Each pigment has a unique absorption spectrum because the only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state and this difference varies from one kind of molecule to another
What is the electromagnetic spectrum?
Electromagnetic spectrum is the entire range of radiation
What is usually given off when excited electrons fall back down to the ground state
High energy states are unstable, so once excited electrons fall back down to the ground state, heat is given off.
What are the two distinct types of photosynthetic cells in C4 plants?
In C4 plants, there are two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells
In isolation, what happens when high energy electrons of some pigments (including chlorophyll) drop down?
In isolation, some pigments, including chlorophyll give photons off causing an afterglow called fluorescence
What are photosystems?
In the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems
Why does isolated chlorophyll fluoresce?
Isolated chlorophyll fluoresces because there is no electron acceptor, so electrons of photoexcited chlorophyll drop right back to the ground state
What is function of chlorophyll?
Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast for the process of photosynthesis
What is linear electron flow?
Linear electron flow is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane occuring during the light reactions of photosynthesis
Describe linear electron flow.
Linear electron flow: 1. A photon of light strikes one of the pigment molecules in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. As this electron falls back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. The process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state. 2. This electron is transferred from the excited P680 to the primary electron acceptor. We can refer to the resulting form of P680, missing the negative charge of an electron, as P680+. 3. An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. (P680+ is the strongest biological oxidiz- ing agent known; its electron "hole" must be filled. This greatly facilitates the transfer of electrons from the split water molecule.) The H+ are released into the thyla- koid space (interior of the thylakoid). The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2. 4. Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plasto- cyanin (Pc). Each component carries out redox reactions as electrons flow down the electron transport chain, re- leasing free energy that is used to pump protons (H+) into the thylakoid space, contributing to a proton gradient across the thylakoid membrane. 5. The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis, to be discussed shortly. 6. Meanwhile, light energy has been transferred via light- harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chloro- phyll a molecules located there. The photoexcited electron is then transferred to PS I's primary electron acceptor, creating an electron "hole" in the P700—which we now can call P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS II. 7. Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.) 8. The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. Electrons in NADPH are at a higher energy level than they are in water (where they started), so they are more readily available for the reactions of the Calvin cycle. This process also removes an H+ from the stroma.
What is the alternate path photoexcited electrons can take and describe it.
Photoexcited electrons can take an alternate path called cyclic electron flow, which uses photosystem I, but not photosystem II and is basically when electrons cycle back from Fd to the cytochrome complex, then via a Pc to a P700 chlorophyll in the PS1 reaction center complex
What are photons?
Photons are discrete particles that light consists of
What is photorespiration?
Photorespiration: As CO2 becomes scarce within the air spaces of the leaf, typically on hot days where the stomata closes, and O2 builds up, rubsico adds O2 to the Calvin cycle. The product splits, and a two carbon compound leaves the chloroplast. Peroxisomes and mitochondria within the plant cell rearrange and split this compound, releasing CO2. The CO2 can produce no sugar however
What is the photosynthesis equation?
Photosynthesis equation: 6 CO2 + 12 H2O + Light Energy --> 3-carbon sugar (can be used to make glucose) + 6 O2 + 6 H2O
In addition to oxygen, what is produced by the splitting of water and how does this play a role in the calvin cycle?
Photosynthesis reverses (compared to cellular respiration) the direction of electron flow where water is split, and its electrons are transferred along with hydrogen ions (H+) from the water to carbon dioxide, reducing it to sugar
What is photosynthesis generally?
Photosynthesis: the process by which green plants and some other organisms use sunlight to synthesize G3P used to make sugar from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct.
What are pigments and why do we see leaves as green?
Pigments are substances that absorb visible light and we see leaves as green because chlorophyll (a pigment) absorbs violet-blue and red light while transmitting and reflecting green light
What kind of autotrophs are plants and why?
Plants are photoautotrophs which use light as a source of energy to synthesize organic substances
Describe Reduction
Reduction: an ATP gives a phosphate to each 3-phosphoglycerate forming 1,3 bisphosphoglycerate. After, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate's carboxyl group to the aldehyde group of G3P, which also loses a phosphate group in the process, becoming G3P. For every 3 molecules of CO2, there are six molecules of G3P formed, but only one can be counted as a net gain because the others are required to complete the cycle
Describe Regeneration of the CO2 acceptor RuBP.
Regeneration of the CO2 acceptor (RuBP): The cycle spends three more molecules of ATP in order to rearrange five molecules of G3P to three molecules of RuBP.
What effects do CO2 levels have on C3 plants?
Rising CO2 levels should benefit C3 plants by lowering the amount of photorespiration that occurs and at the same time, rising temperatures have the opposite effect, increasing photorespiration.
Describe the calvin cycle.
The Calvin Cycle begins with carbon fixation: the incorporation of CO2 from the air into organic compounds. The electrons from the NADPH reduce the CO2 into a carbohydrate. The calvin cycle also requires chemical energy from ATP.
What type of reaction is the calvin cycle and why?
The Calvin Cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the calvin cycle in the form of CO2 and leaves in the form of sugar
What are the phases of the calvin cycle?
The Calvin Cycle takes place in three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor
Which reactant oxidizes and which reactant reduces?
The H2O oxidizes and the CO2 reduces
Where reactant does the O2 come from?
The O2 given off by plants is derived from H2O and not from CO2
What is the calvin cycle also known as and why? Why does it usually only happen in daylight?
The calvin cycle is also known as the light-independent reactions because it doesn't require light directly. Although, the process usually only happens in daylight because it needs the outputs of the light-dependent reacts in order to function.
What is the carbohydrate produced in the calvin cycle?
The carbohydrate produced by the calvin cycle is a three carbon sugar called glyceraldehyde 3 phosphate (G3P)
Describe the light reactions process.
The light reactions are the steps that convert solar energy to chemical energy where water is split, providing a source of electrons and protons (H+) and giving off O2 as a by product. The light absorbed by chlorophyll drives a transfer of electrons and hydrogen ions from water to an acceptor called NADP+ where they are stored temporarily. A pair of electrons along with an H+ reduce NADP+ to NADPH. The light also generates ATP from ADP by powering the addition of a phosphate group (AKA photophosphorylation).
Where do the two process occur respectively? What about the outputs of the light-dependent reactions (inputs of the calvin cycle)?
The light-dependent reactions occur in the thylakoids while the calvin cycle takes place in the stroma (NADP+ and ADP picks up electrons and the phosphate respectively in the stroma before entering the calvin cycle)
What does the pair of chlorophyll a molecules in the reaction center complex use the energy from light to do?
The pair of chlorophyll a molecules in the reaction center complex uses the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to the primary electron acceptor
What is a primary electron acceptor?
The primary electron acceptor is a molecule capable of accepting electrons and becoming reduced
What is a reaction -center complex
The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
What is the function of a stomata in photosynthesis?
The stomata is where carbon dioxide enters the leaf, and oxygen exits
What are the two stages of photosynthesis?
The two stages of photosynthesis are known as the light reactions and the Calvin cycle
What are the two types of photosystems that light reactions in the thylakoid membrane include? What order does light pass through?
The two types of photosystems that cooperate in the light reactions of photosynthesis in the thylakoid membrane include photosystem II and photosystem I and light passes through in that respective order
What are three types of pigments in chloroplasts?
Three types of pigments in chloroplasts: chlorophyll a is the key light-capturing pigment that participates directly in the light reaction, chlorophyll b is an accessory pigment, and carotenoids are a separate group of accessory pigments
How many times does the calvin cycle need to take place to synthesize G3P
To synthesize G3P, the calvin cycle must take place three times fixing three molecules of CO2
What is visible light?
Visible light is the radiation that can be detected as various colors by the human eye and that drives photosynthesis
What is wavelength?
Wavelength: the distance between the crests of electromagnetic waves
Why is the action spectrum for photosynthesis much broader than the absorption spectrum of chlorophyll a?
the action spectrum for photosynthesis is much broader than the absorption spectrum of chlorophyll a partly because accessory pigments with different absorption spectra are also present in chloroplasts, including chlorophyll b and carotenoids, which broaden the spectrum of colors (because of slightly different structure) that can be used for photosynthesis. Chlorophyll a appears blue green and chlorophyll b olive green under visible light.