Lecture 04: Plant Physiology and Energetics: Suggested exam questions

Exam Questions

1. What are the two sets of reactions in photosynthesis? What happens in each reaction? What molecules transfer and store energy? What are their waste products?
The two sets of reaction in photosynthesis are light reactions and ‘light independent’ reactions.

Light Reactions- light energy is absorbed by chlorophyll molecules and is used indirectly to power the synthesis of ATP (adenosine triphosphate, a high energy compound). Simultaneously, water is split into oxygen gas and hydrogen atoms (electrons and protons). The electrons are accepted by NADP+ and H+ producing NADPH (Nicotinamide adenine dinucleotide phosphate, also a high energy compound).

Light Independent Reactions, also known as the Calvin-Benson cycle (are sometimes called the "dark reactions", though they can occur in light or dark)- Sugars are synthesized from CO2 and the hydrogen carried by NADPH. This process is powered by the ATP and NADPH produced in the light reactions. The sugars/carbohydrates produced in this reaction can then be used for longer-term energy storage.

2. What is the photosynthetically active region (PAR) of light? Explain its significance to photosynthesis.
For light energy to be used for photosynthesis, it must first be absorbed. A pigment is any substance that absorbs light. Chlorophyll, the pigment that makes leaves green, absorbs light in the violet and blue wavelengths and also in the red because it reflects green light, it appears green. Different pigments absorb light energy at different wavelengths. The absorption pattern of a pigment is known as the absorption spectrum. The absorption spectrum of chlorophyll is between 400 nm and 700 nm. This portion of the spectrum is known as the photosynthetically active region (PAR).

3. In which photosynthetic reaction do pigments play a role in capturing sunlight for photosynthesis?
the light reactions!

4. What are the three types of photosynthesis? How do they vary? How do they adapt plants to different environmental conditions?
C3, C4, CAM: In C3 photosynthesis carbon dioxide is reduced and the carbon dioxide acceptor is regenerated. C3 is the typical photosynthesis that most plants use. C3 is more efficient than C4 and CAM plants under cool and moist conditions and under normal light because it requires fewer enzymes and no specialized anatomy. C3 plants tend to have less water use efficiency, and grow where water is less limiting relative to C4 and CAM plants.

C4 and CAM photosynthesis are both adaptations to arid conditions that result in better water use efficiency.

C4 pathway- Includes C3 AND C4 pathway, but as a solution to photorespiration, the anatomy of the leaves in C4 plants establishes a spatial separation between C3 and C4 pathways. They tend to have a greater water use efficiency, and grow where water is more limiting relative to C3 plants. Most grasses are C4 plants.

CAM pathway (Crassulacean acid metabolism)- Use C3 and C4 pathways, but instead of a spatial separation between the two (as in C4 pathway), there is a temporal separation (separation in time) where their stomata close during the day, reducing water loss. They fix CO2 in the dark. This is advantageous in conditions of high light intensity and where water is limiting, as in the desert where most CAM plants live. In CAM, CO2 is fixed at night and then, during the daytime, the fixed CO2 is transferred to theCalvin cycle within the same cell. CAM plants have better water use efficiency than C3 and C4 plants under arid conditions because they open their stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). Characteristic of most succulent plants, like cacti.

ALL USE C3 pathway!

5. Why do different leaves on the same plant have different compensation points? What are their physiological differences specific to their locations?
Leaves on the same plant will have different compensation points (point where photosynthesis= respiration) depending on their location within the forest canopy. Sun leaves (from upper canopy) have a higher compensation point, and tend to have a smaller surface area and thicker cuticles than shade leaves.

6. Which two elements are considered "limiting"? What important role does each play?
Nitrogen and Phosphorus
Nitrogen: used to make amino acids (which link together to form proteins)
Phosphorus: used to make phospholipids (i.e. molecules in membranes), ATP, NADPH — the "batteries" of cells

7. What is Liebig's Law of the Minimum? What are some exceptions?
Liebig's Law states that whatever nutrient resource (N, P, Mg, Ca, etc.) is the least abundant in the organism’s environment, relative to the needs of the organism, will limit growth. The law has been expanded to encompass all "environmental factors".
Exceptions to Liebig's Law include:
— communities of species (vs. single-species populations)
— human populations because:

  • they are not homogeneous in resource requirements,
  • the environment varies enough in time and place to invoke different limiting factors at different times and places
  • because nonlinear human responses cannot be usefully approximated as linear,
  • nonlinear human responses cannot be usefully approximated as linear,
  • interactions among limiting factors (sometimes appearing as economic substitutions) are not small enough to neglect, and
  • the time interval over which the carrying capacity is to be estimated is not short enough to justify the neglect of adaptive economic, technological and cultural responses.

For further explanation of these effects, see How Many People Can the Earth Support by Joel E. Cohen, 1996. (Chapter 12).

8. Why measure dry mass when measuring plant (carbon) assimilation?
Because the amount of water stored in a plant can vary in time or space, and will affect the mass of a plant.

9. Why is soil texture important for the growth of plants? What are the different levels of water storage in soil, and why does it matter?
Ans: Soil texture determines both the capacity to store water and ability to drain that water. The size (surface area) of the particles determines how much water can be stored due to adhesion to the surface of a particle. Sandy soils, with their relatively large particles, are able to hold on to significantly less water than clayey soils. However, sandy soils are also able to drain water much better than clayey soils because of the larger spaces between particles, which is an important growth factor for plants requiring adequate drainage.
Ans: Field capacity and wilting point.

10. How would you expect a plant to optimize its carbon assimilation if it's in the shade? What about a plant in full sun?
A shade plant would be attempting to maximize exposure to sunlight. It can do this through large leaf structures, and contorted trunks and limbs that can grow towards sunlight ie phototropism. The large surface area of leaves would increase water loss, so this strategy would only be viable in wet climates. A sunlight plant would be concerned about temperature and water stress. It is more likely to utilize C4 or CAM photosynthesis. It may also have physical adaptations to reduce water loss such as waxy coatings on leaves, small leaf sizes, and hairs on the leaves.

11.a. What is the fertilization effect?
11.b. Why might it not be as powerful as many people think?
Ans a: The idea of the "fertilization effect" is that increased atmospheric concentrations of CO2, due to human activity, will enhance photosynthesis in many plants, thus stimulating increased carbon sequestration in forests. The mechanisms behind the effect: Increased CO2 concentrations mean plants do not have to open their stoma as often to get the carbon they need to grow, allowing them to conserve water.
Ans b: Increased CO2 may initially increase growth rate in some plants, in some areas. However, growth is still limited by many other factors, particularly nitrogen and phosphorous availability, temperature, soil conditions and water.

12. What are three types of environmental factors, related to plant phsyiology, that play a role in determining the habitability of the environment of a given plant species, according to the Theory of Tolerance?
Climatic factors are most important in determining the habitability of an environment. Soil factors (nutrients, texture, pH, etc) are next most important. Competition from other plants is the third most important factor. The Theory of Tolerance says that the range of habitat for a given species is the sum of tolerance limits for each of these environmental factors. The area in which the species is tolerant to ALL of these factors (not just some) is the area that is habitable by that species.

13. Describe the process that enables plants to draw water from the soil up to its leaves for photosynthesis. What role does soil texture play in this process?
Water enters the roots by osmosis and form a cohesive column in the xylem. This water column is pulled up the xylem to the leaves by the tension created by transpiration of water from the leaves. This explanation of the mechanism of water movement from roots to leaves is known as the cohesion-tension theory. Soil texture is an important factor in this process. The larger particles found in sandy soil have less surface area and therefore retain less water but also hold it less tightly. Conversely, the smaller particles found in clay and silt have more surface area and therefore are able to retain more water but also hold it much more tightly. The amount of water in the soil, together with the strength of the attractive forces holding water in the soil determine the ability of the tension created by the plant to draw water into its roots by osmosis. The point at which the soil tension outweighs the plant's tension is called the wilting point.

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