AGECO 121 EXAM 1 Worksheet Key

Why are pigments like carotenoids and phycoerythrin considered to be "accessory pigments" in photosynthesis, cooperating with chlorophyll to power electron transport?

They are able to collect colors of light that are not well absorbed by chlorophyll and funnel that energy into the "light reactions" and "electron transport". Essentially, they pass energy they absorb to chlorophyll, which drives electron transport and formation of O2, ATP, and NADPH in the light reactions.

Palisade mesophyll

primary light capture cell layer

Rubisco "grabs" CO2 and attaches it to a ________________ to produce ___, which is used to produce more sugar and all other carbon-containing molecules in the plant. Rubisco "fixes" ___ from the atmosphere to create more sugar. This is important because we are carbon-based life forms, and all carbon in our systems was originally captured by ___________ from the atmosphere.

5-carbon sugar (RUBP), PGA, CO2, Rubisco

high energy compound

ATP (adenosine triphosphate)- one of the very first energy-containing chemical products of photosynthesis

What is the difference between an absorption spectrum and an action spectrum?

An absorption spectrum is a measurement of how well a substance absorbs various wavelengths of light in the visible spectrum. An action spectrum is a measurement of how effectively various wavelengths of light in the visible spectrum trigger a light-dependent biological activity.

If you could genetically engineer a RUBISCO enzyme that only uses CO2 and never uses O2 (no photorespiration), to which type of plant (C3, C4, or CAM) would this give the greatest advantage and why?

C3 plants would benefit the most from the elimination of the RUBISCO oxygenase activity (photorespiration) because C3 plants do not have any special systems in place to prevent RUBISCO from "grabbing" O2 instead of CO2 and burning sugar. CAM plants and C4 plants protect RUBISCO from oxygen, so they have less of a problem with photorespiration.

Facultative hemiparasite

Can do photosynthesis and in fact can survive in the absence of the host, but it prefers to attach to a host when possible.

Using a device that can measure CO2 uptake by a plant, how could you determine whether a plant uses C3, C4, or CAM photosynthesis? What kind of experiment could you set up to test this, using the available equipment? (You may use the back of the sheet if needed.)

Measure CO2 uptake during the day (in the morning, preferably) and during the night. If CO2 uptake takes place at night, you've got a CAM plant. If CO2 uptake takes place during the day, it could be C3 or C4. To tell the difference, just measure CO2 uptake at various times of the day ranging from cool morning to hot afternoon. A C3 plant will experience a substantial reduction in CO2 uptake as the day gets hotter, but a C4 plant will have relatively stable CO2 uptake even as the temperatures get hot.

high energy and supplies H atoms for production of sugars.

NADPH (nicotinamide adenine dinucleotide hydrogen) - one of the very first energy-containing chemical products of photosynthesis

Re-write the following statement using scientific concepts: Photosynthesis transforms water to air, gas to solid, and sunshine into moonshine.

Photosynthesis breaks up water into O2 gas and hydrogen atoms through photolysis of water. Photosynthesis captures CO2 gas from the atmosphere and builds sugar (solid) with it. Photosynthesis converts the electromagnetic energy of sunlight into the chemical energy of sugar, which can be fermented to produce alcohol (moonshine).

Obligate Hemiparasite

Sometimes does photosynthesis but nevertheless is dependent on the host to complete its life cycle.

What are "stomata"? What is their most important role photosynthesis? (HINT: gas exchange)

Stomata are pores in the epidermis of leaves flanked by "guard cells" that regulate the aperture size. Their most important role in photosynthesis is to facilitate the entry of CO2 into the leaf to be used in photosynthesis, and to allow O2 produced by photosynthesis (photolysis) to escape the leaf into the atmosphere. That is what is mean by "gas exchange".

Provide four (4) answers to the question "where does oxygen gas (O2) come from?".

There are many possible answers: green plants; green leaves; chloroplasts; thylakoids; photosystem II; photolysis of water.

Consider air plants and orchids (see slide). These plants are often native to tropical areas with substantial rainfall. Yet they use CAM photosynthesis. Why might this be an advantage for these types of plants?

These plants are growing without contact with soil. The tree bark or chicken wire supports that they grow on do not hold moisture and dry out quickly after a rain event. Therefore, these plants have limited and sporadic access to water. These plants need to store water carefully, so they use the water-conserving CAM system of photosynthesis to minimize water use.

You discover that a plant response is activated by white light. How could you test experimentally whether this response could be controlled by the phytochrome photoreceptor?

This could be done in two ways. You could obtain an action spectrum for the response. If there is a single peak of activity in the red region of the action spectrum, this is consistent with the response being controlled by phytochrome. You could also test this by using red/far-red reversibility. Check if the activity is activated by red light and if immediate subsequent far-red light will reverse the red light activation effect. This is the hallmark of phytochrome photoreceptor activity.

Consider this statement: "CAM photosynthesis is more efficient than C3 photosynthesis". Why and under what circumstances is the statement correct?

This statement is correct under conditions where water is very scarce, often under hot desert conditions. CAM photosynthesis uses only about 10% of the water to do their photosynthesis compared to C3 plants.

Consider this statement: "CAM photosynthesis is more efficient than C3. Why or under what circumstances is the statement incorrect?

This statement is incorrect under circumstances where water is sufficiently available. Under these conditions, CAM plants waste a lot of material resources and energy storing CO2 in the form of malate or other 4-carbon acids. Furthermore, CAM plants only collect CO2 at night rather than during the day in real time like C3 plants do. This limits the overall productivity of CAM photosynthesis compared to C3 under conditions where water is not severely limited.

If CAM plants are so efficient with water use, why don't they take over everywhere? (They don't'.)

While highly efficient with water use, CAM photosynthesis is costly. Costs include energy needed to perform the initial C4 carbon capture reactions, storage space, and storage materials to hold the C4 acids (captured CO2). In addition, having the stomata only open at night means that the amount of CO2 that can be captured is limited compared to capturing CO2 while the sun shines.

If you see a plant growing and twining itself around another plant, how could you determine whether it is a parasite (drawing sustenance from the other plant) or just an epiphyte (growing on another plant but not drawing nutrition from that plant).

You need to look for haustoria, which are the connecting structures created by parasitic plants that they use to tap into the veins of the host plant. Using haustoria, the parasite connects its own veins to the veins of the target plant. By connecting to the host plant xylem and phloem, the parasite obtains water, minerals, and sugar from the host plant. If you can't find any haustoria connections, then the plant is most likely just an epiphyte, not a parasite.

Stomata

allow gas exchange (CO2 intake, O2 release, and H2O vapor loss)

Spongy mesophyll

allows gases to move around cells so that photosynthetic cells can absorb CO2 to produce sugar and release O2, which diffuses out of the stomata

carboxylase activity

builds sugar-hen RUBISCO "grabs" CO2 and binds it to sugar, building more sugar.

oxygenase activity,

burns sugar - activity is when RUBISCO "accidentally grabs" O2 and binds it to the sugar, which burns up the sugar partially. also called photorespiration

Veins/xylem

conduct water into leaf

Veins/phloem

conducts sugar out of leaf to rest of plant

Phycoerythrin is used as an "accessory pigment" by red algae to capture green light photons to perform photosynthesis. No land plants are known to use an accessory pigment like phytoerythrin to use green light. If some land plant species could evolve such a capacity, in what specific environment would it thrive better than "regular" plants, and why?

in the "understory" of the forest or just underneath other plants. If the plants above are capturing most of the red and blue light, the light that's available beneath is mostly the green and yellow wavelengths. Being able to use those wavelengths in the understory would be a huge advantage.

Holoparasites

never does any photosynthesis and is completely dependent on the host for water and energy and nutrition

Upper and lower epidermis

provide rigidity and reduce water loss

How might it be possible for short-day cocklebur plants to be blooming at the same time of year and same location as long-day spinach plants? Include a critical night length diagram as part of your answer.

For this to happen, the critical night length (CNL) requirements for both the short-day plant (SDP, cocklebur) and the long-day plant (LDP, spinach) must be satisfied. This can happen if the CNL of the SDP is shorter than the CNL of the LDP, and if the actual length of the night is be between these two values. That way, the cocklebur "thinks" that the days are short, since the length of the night exceeds its CNL, and so it blooms. Simultaneously, the spinach "thinks" that the days are long, since the length of the actual night is shorter than its CNL, so it blooms also. Remember to also include a CNL diagram!

What are some answers to the question, where does oxygen come from

Green photosynthetic plants and organisms, Chloroplasts, Thylakoid membranes, Photosystem II, Photolysis of water, Photosynthesis reactions

A tomato variety takes 50 days to bloom when grown from seed with 16 hours of daylight but takes 120 days to bloom with 8 hours of daylight. How can you determine experimentally whether this tomato is a "day-neutral" plant or a "long-day" plant? Keep in mind that a day-neutral plant might bloom sooner with more illumination due to faster growth overall.

If the tomato is indeed a long-day plant, then a night break provided in the middle of the dark period should trigger blooming. However, if it is day-neutral, then the night break won't trigger blooming.

How was the heavy oxygen isotope 18O used to show that the O2 released through photosynthesis comes from H2O? (HINT - write down the chemical reaction for photosynthesis and indicate where you could put the 18O to see where ends up.)

In the photosynthesis chemical reaction, the oxygen atoms of water were labeled with a heavy oxygen isotope 18O (the usual isotope is lighter, 16O). This allowed tracking of where the oxygen atoms ended up in the products of photosynthesis. All the 18O ended up in the oxygen gas released, none of it ended up in the carbohydrate. Indicate this also using the chemical formulae: CO2 + H218O + light energy => CH2O + 18O2.

In the classic van Helmont experiment in the 16th century, he grew a willow tree in a clay pot for 5 years, adding only rainwater. After the 5 years, the tree had gained 74 kg weight, but the soil decreased in weight by only 57g (i.e., 0.057 kg). His conclusion was that most of additional weight of tree was from water, not soil. He was partly right, but also partly wrong. Certainly, since plant cells are 90% water, most of the weight does come from water. But if you measure the "dry weight" of the tree (no water left), the weight gained is still far greater than the weight lost by the soil. Based on your knowledge of photosynthesis, answer the following. WHY WAS HE RIGHT? WHY WAS HE WRONG?

The answer is wrong in that it does not account for the carbon dioxide taken up by the plant and converted into sugars during the photosynthesis reaction shown below. This was partly right because water is split during photosynthesis. The plant holds on to the hydrogen atoms of the H2O and releases the oxygen as O2 gas. So yes, hydrogen atoms from the water split during photosynthesis get converted into sugar in the reaction:H2O + CO2 + light energy CH2O + O2 (gas)

How do we know that red algae effectively use green light to perform photosynthesis? (HINT: use the word "action spectrum" in your answer!). Why is this advantageous for them?

The photosynthesis action spectrum for red algae has a high point in the green part of the spectrum, unlike typical green plants, which whose photosynthesis action spectra have peaks in the blue and red parts of the spectrum. The ability to use green light effectively for photosynthesis is advantageous for red algae because they live under water and most of the light that reaches them is from the green part of the light spectrum and the blue and red wavelengths have been depleted by green photosynthetic organisms above them.

Use action and absorption spectra to argue that green cells beneath purple cells are getting plenty of light to power photosynthesis (use reverse as needed).

The purple cell layer absorbs mainly green photons, resulting in the purple color we see, which in this case is a mix of blue and red wavelengths, which are reflected or transmitted by the purple cells. In contrast, the green photosynthetic cells below absorb mainly red and blue photons and do not absorb green photons well. Therefore, the green cells are not missing out on much, since their "favorite" color photons are mostly passing through the purple cell layer for them to use.

A farmer has three large soybean fields and wants to harvest them at one-week intervals for convenience. The farmer plants the soybean seeds at 1-week intervals in the spring, figuring that their blooming and maturation will occur at 1-week intervals. However, to the farmer's chagrin, as the season progresses to August, all the soybeans bloom at the same time! They must then be harvested at the same time. Provide a photoperiodic explanation for this event.

The soybeans are probably responding to the photoperiodic conditions that trigger them to bloom at the same time of year, when their critical night length condition is satisfied. They are most likely short-day plants (SDP), and they are programmed to bloom after the nights exceed a certain length as the days shorten during August. Planting them at different times did not have an effect on when they bloomed, since they were responding the day length to make the blooming decision, not simply how long they had been growing.

The phytochrome photoreceptor is sensitive to both red light and far-red light, and it allows the plant to sense the relative abundance of red light compared to far-red light in the environment. In the shade of green plants, the chlorophyll in the leaves above is collecting most of the __________, while the __________ passes through easily. This shifts the balance of red and far-red light in the shade such that there is much more __________ than _____________ in the shade. The phytochrome photoreceptor of small plants growing in the shade senses this abundance of ______________, which shifts phytochrome more into its "OFF" position, triggering the shade avoidance response (earlier flowering, taller growth to compete for light, and even inhibiting the germination of some types of seeds in the shade). Phytochrome is a sensor of the relative abundance of red and far-red light.

red-light, far red light, far red light, red light, far red light