Question

1. Carbohydrates, oxygen, carbon dioxide, water, heat and light are all participants in the
processes of respiration and photosynthesis. Write an equation to show the substrates
and products of each process.

2. Photorespiration occurs in illuminated green leaves. Describe photorespiration in broad
terms, why it only happens in the light, and what cost it imposes on C3 plants.

3.Rising atmospheric CO2 levels are predicted to favour particular photosynthetic types
(recall C3, C4 & CAM photosynthesis) relative to others. Plants of which type are likely to
benefit from rising CO2 levels, and why?

4.Define the concept of plant ecological strategy

5. What do the C, S and R stand for in Grimes triangular C-S-R ecological strategy scheme?
What traits might characterise a species in the "S" corner of the scheme?

6. List three key ecological advantages of seeds.

Answer 1

1.

Aerobic respiration

Aerobic respiration requires oxygen. It happens in cells when glucose reacts with oxygen. Here are the word and symbol - higher only - equations:

glucose + oxygen    ?    carbon dioxide + water (+ energy)

C6H12O6 + 6O2    ?    6CO2 + 6H2O (+ energy)

Energy is shown in brackets in each equation because it is not a chemical substance.

Anaerobic respiration

Anaerobic respiration does not need oxygen. It happens when there is not enough oxygen for aerobic respiration. Here is the word equation:

glucose    ?    lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration.

Steps Involved

•             The first step involves glycolysis. Glycolysis takes place in the cell's cytoplasm and is an anaerobic process, that does not require oxygen. Glucose is broken down into two molecules of pyruvate in a 10-step process yielding 2 ATPs.

•             The next step involves the entry of pyruvate into the mitochondria that leads to the production of two molecules of acetyl-coenzyme A and 2 molecules of CO2.

•             The third step involves the Citric Acid Cycle (CAC). This is a 9-step reaction that takes place within the mitochondria. The reactions yield 2 ATPs and 4 CO2 molecules.

•             The last step involves the Electron Transport System or cytochrome system that takes place with the help of enzymes that are located in the inner mitochondrial membrane. This step yields the maximum number of ATPs, that is 32 ATP molecules, which makes the total energy produced to 36 ATPs.

These complex reactions lead to the production of 36 ATPs by utilizing one glucose molecule and six oxygen molecules.

In photosynthesis, carbon dioxide is used as a substrate base for producing glucose and oxygen with the help of light energy and water. Also, energy from the plants is utilized in fixing carbon dioxide and converting it to sugar. It is a complex process, but can be represented in a simplified reaction form. The following is the balanced photosynthesis equation, which will help you to understand this process:

6 CO2 + 6 H2O + Light energy ? C6H12O6 + 6 O2

Carbon dioxide + Water + Light energy ? Glucose + Oxygen

The reactants are carbon dioxide, water, and sunlight, which after completion of the reaction give rise to glucose and oxygen. The numerical value placed before the compounds indicates the number of molecules required for the process. For example; 6 CO2 stands for six molecules of carbon dioxide and 6 O2 means six molecules of oxygen. Thus, in this process, six molecules of carbon dioxide and six molecules of water react in presence of sunlight to produce one molecule of glucose and six molecules of oxygen.

The final product of this process, i.e., glucose is stored in the complex molecular structure of the plant cells. Oxygen produced as a byproduct is released to the surrounding for use in respiration by living organisms including humans and animals. Apart from oxygen supply, glucose synthesized by means of photosynthesis is source of energy for living organisms, and is circulated in the food chain. Thus, plants are crucial for balancing the atmospheric air composition, and supporting life in every step.

2.

Photosynthesis is a biochemical process of producing carbohydrates using light energy. The whole process is carried in two phases.

1.            Photochemical phase – In the photochemical phase, ATP and NADPH are produced

2.            Biosynthetic phase – In this phase, the final product glucose is formed.

Based on, how plants proceed in the biosynthetic phase, plants are further classified as C3 and C4plants. Another factor which differentiates a C4 plant from C3 is photorespiration.

Photorespiration

Photorespiration is a respiratory process in many higher plants. This is also known as the oxidative photosynthetic, or C2 photosynthesis or carbon cycle.

The first step of biosynthetic phase in C3 plants. The reaction in which carbon dioxide and water combine to give carbohydrates is termed as carbon fixation. Calvin cycle is the first step of carbon fixation where CO¬2 combines with Ribulose-1 and 5-bisphosphate (RuBP) to form two molecules of 3 carbon acid called 3-phosphoglycericacid (PGA). The reaction is catalyzed by the most abundant enzyme in the world called RuBisCO (RuBP carboxylase-oxygenase). RuBisCO is the enzyme that has an affinity for both CO2 and O2but has more affinity for CO2. Hence, the binding of CO2 and O2 are competitive in nature and the concentration of the molecules in the atmosphere determines the winner. This is the basis for photorespiration in plants.

Sometimes in C3 plants, RuBisCO binds to oxygen molecules and the reaction deviates from the regular metabolic pathway.The combination of RuBP and oxygen molecules leads to the formation one molecule of phosphoglycerate and phosphoglycolate. This pathway is called photorespiration. During photorespiration, no sugar or ATP molecules are synthesized, but just CO2 is released at the expense of ATP. And the whole process is futile.

However, C4 plants do not undergo photorespiration due to their special mechanism to increase the CO2 level for enzyme binding. During Hatch and Slack Pathway, the C4 acid, oxaloacetic acid (OAA) breaks down to release CO2. This ensures the high concentration of intercellular CO2.Thus, in C4 plants, RuBisCO is more active as carboxylase enzyme rather than as oxygenase. This is why C4 plants have better productivity.

3.

At present atmospheric levels of CO2, C4 plants are more efficient at photosynthesis than C3: in absolute conversion efficiency of light energy to stored chemical energy they are around 7% efficient, compared to 4% for C3. C4 plants typically use less water per weight of biomass produced, and can tolerate greater water and temperature stress than C3 plants. Accordingly, C4 crops are most often grown in tropical and equatorial regions.

The advantage that C4 plants have in terms of photosynthesis does not always translate into higher harvest yields, however, as only parts of the plant are edible. In terms of ground use, C3 crops can produce some of the highest amounts of edible calories and protein per acre: for example, potatoes and soybeans respectively.

C4 plants show a relatively small improvement in photosynthesis rate with increasing atmospheric CO2 above present levels; however, at increased levels of CO2 the leaf pores (stomata) of both C4 and C3 plants increasingly close up, which also reduces the amount of water lost by the plant (transpiration). Thus C3 and C4 plants significantly improve their water efficiency as CO2 levels increase.

C3 photosynthesis is less efficient than C4 partly because of an effect known as photo-respiration, which results in the loss (to the atmosphere or soil) of a substantial proportion of the carbon that has been extracted from the atmosphere by photosynthesis. C3 photo-respiration increases under heat stress and drought, which is a major factor behind the choice of C4 crops for hot dry climates. However, as CO2 levels increase, photo-respiration is suppressed, such that at double today’s levels of atmospheric CO2 the efficiencies of C3 plants (in photosynthesis rate and water use) are as good as or better than C4 plants. Moreover, at higher levels of CO2, C3 plants can maintain efficient photosynthesis rates at considerably higher temperatures than today’s conditions – their optimal temperatures for photosynthesis increase.

As CO2 concentrations increase, the photosynthetic efficiency gap between C3 and C4 plants rapidly closes, and at double today’s CO2 concentration (i.e. at 780 ppm instead of today’s 390 ppm), the photosynthesis rates are the same. Incidentally, the majority of the world’s most troublesome weeds use the C4 pathway, and so have a competitive advantage over C3 crops at current CO2 concentrations. At higher CO2 concentrations, competing for the same resources on the same patch (light, water, CO2, nutrients etc), C3 crops increasing out-compete the weeds.