Ch 8: Photosynthesis Flashcards
Which of the following statements is not true of most cellular redox reactions?
a. The reactant that is reduced gains electrons.
b. The electron acceptor is reduced, acting as an oxidizing agent.
c. The electron acceptor is reduced, acting as a reducing agent.
d. The reactant that is oxidized loses electrons, acting as a reducing agent.
c. The electron acceptor is reduced, acting as a reducing agent.
What happens along an electron transport chain?
a. Electrons store energy that can be used to break down sugar molecules.
b. Electrons are passed from molecule to molecule, gaining potential energy at each step.
c. Protons are pumped through ATP synthase, making ATP.
d. Electrons are passed from a reducing agent to an oxidizing agent, releasing free energy at each step.
d. Electrons are passed from a reducing agent to an oxidizing agent, releasing free energy at each step.
Which of the following is a true statement?
a. Anabolic pathways build molecules, require energy, and are endergonic.
b. Anabolic pathways break down molecules, require energy, and are endergonic.
c. Catabolic pathways break down molecules, require energy, and are exergonic.
d. Catabolic pathways build molecules, release energy, and are exergonic.
a. Anabolic pathways build molecules, require energy, and are endergonic.
What is the etymology of the word ‘autotroph’?
auto means “self”, and trophos means “feeder”
Autotrophs are the ultimate sources of ________ compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the _________ of the biosphere.
organic
producers
Almost all plants are autotrophs; the only nutrients they require are….
….water and minerals from the soil, and carbon dioxide from the air.
Heterotrophs (hetero- means other) are unable to make their own food; they live on compounds produced by other organisms, and thus are thought of as the biosphere’s _________.
consumers
Some heterotrophs decompose and feed on the remains of other organisms and organic litter such as feces and fallen leaves; these types of heterotrophs are known as __________. Most _____ and many types of prokaryotes get their nourishment this way.
decomposers
fungi
Almost all heterotrophs, including humans, are completely dependent on photoautotrophs for ___ as well as ______.
food
oxygen
All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the ______ are the major sites of photosynthesis in most plants. Chloroplasts are found mainly in the cells of the _________, the tissue and the interior of the leaf.
leaves
mesophyll
How does carbon dioxide enter the leaf? (This is also how oxygen exits.)
By microscopic pores called stomata.
A typical mesophyll cell has about __–__ chloroplasts. Each one has an envelope of 2 membranes (just like mitochondria), surrounding a dense fluid called the ______. Suspended within the stroma is a third membrane system, made up of sacks called __________, which segregates the stroma from the _________ space inside these sacs.
30-40
stroma
thylakoids
thylakoid
What is the chemical reaction for photosynthesis?
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
(equation can be simplified by showing only the NET consumption of water:
6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2)
One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from ___ and not from ___. The chloroplast splits water into hydrogen and oxygen. Before this discovery, the prevailing hypothesis was that photosynthesis split carbon dioxide (CO2→C+O2) and then added water to the carbon (C+H2O→[CH2O]). That hypothesis predicted that the O2 released during photosynthesis came from CO2. This idea was challenged in the 1930s by C. B. van Niel, of Stanford University.
H2O
CO2
How did van Niel’s hypothesis get validated?
Nearly 20 years later, scientists confirmed it by using oxygen-18 (18O), a heavy isotope, as a tracer to follow the path of oxygen atoms during photosynthesis. The experiments showed that the O2 from plants was labeled with 18O only if water was the source of the tracer (experiment 1). If the 18O was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2).
Both photosynthesis and cellular respiration involve multiple _____ _________.
redox reactions
Photosynthesis essentially reverses the direction of electron flow when compared with cellular respiration. Water is split, and its electrons are transferred along with hydrogen ions from the water to carbon dioxide, reducing it to _____.
sugar
Because electrons increase in potential energy as they move from water to sugar, this process….
….requires energy; it is endergonic. This energy boost is provided by sunlight.
The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the _____ ________ (the photo part of photosynthesis) and the ______cycle (the synthesis part)
Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (____________ _______ ____________ ________), where they are temporarily stored. (The electron acceptor NADP+ is first cousin to NAD+, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP+ molecule.)
nicotinamide adenine dinucleotide phosphate
The light reactions use solar energy to reduce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using ____________ to power the addition of a phosphate group to ADP.
chemiosmosis
Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells. Notice that the light reactions produce no _____; that happens in the second stage of photosynthesis, the Calvin cycle.
sugar
Thus, it is the Calvin cycle that makes sugar, but only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the ____ _________, or light-independent reactions, because none of the steps requires light directly.
dark reactions
Where do the light reactions happen, and where does the Calvin cycle happen?
in the thylakoids of the chloroplast
in the stroma
The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a _________ (for gamma rays) to more than a _________ (for radio waves).
nanometer
kilometer
The amount of energy is __________ related to the wavelength of the light: The shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light
inversely
When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as ________.
pigments
The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green light. This is why leaves appear green.
Explain chemoautotrophs vs photoautotrophs.
Chemoautotrophs derive energy from the consumption of inorganic molecules.
Photoautotrophs derive energy by absorbing light energy.
What kinds of organisms are photosynthetic?
plants, multicellular alga, unicellular eukaryotes, cyanobacteria, and purple sulfur bacteria
What’s the most important observation you can make about this equation?
photosynthesis is essential the reverse of cellular respiration
The key function of each of the two photosystems is to absorb light and convert the energy of the absorbed light into redox energy, which drives electron transport.
- In PS II (the first photosystem in the sequence), ____ is oxidized (which in turn oxidizes _____), and the PS II primary electron acceptor is reduced (which in turn reduces the electron transport chain between the photosystems).
- In PS I, the PS I primary electron acceptor is reduced (which in turn reduces other compounds that ultimately reduce NADP+ to NADPH), and P700 is oxidized (which in turn oxidizes the electron transport chain between the photosystems).
P680
water
In both PS II and PS I, light energy is used to drive a redox reaction that would not otherwise occur. In each photosystem, this redox reaction moves an electron from the special chlorophyll pair (P680 in PS II and P700 in PS I) to that photosystem’s primary electron acceptor. The result in each case is a reductant (the reduced primary electron acceptor) and an oxidant (P680+ in PS II and P700+ in PS I) that are able to power the rest of the electron transfer reactions without further energy input.
Photosynthetic electron transport contributes to the formation of a proton (H+) gradient across the thylakoid membrane in two places.
- In PS II, the oxidation of water releases protons into the thylakoid space.
- Electron transport between PS II and the cytochrome complex (through Pq) pumps protons from the stroma into the thylakoid space.
The resulting proton gradient is used by the ATP synthase complex to convert ADP to ATP in the stroma.
Which wavelengths of light drive photosynthesis?
We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light (Figure 8.7). Violet-blue and red lights have wavelengths of 420-480ish, and 640-680ish.
Compare and contrast chemiosmosis in chloroplasts and mitochondria.
Where is the electron transport chain and where does ATP get pumped?
In mitochondria, it’s in the inner mitochondrial membrane, and ATP is pumped into the matrix.
In chloroplasts, it’s in the thylakoid membrane, and ATP is pumped back into the stroma.
What are the 3 stages of the Calvin cycle?
- carbon fixation
- reduction
- regeneration of the CO2 acceptor (Ribulose bisphosphate)
