Honors Biology - Unit 5: Check & Challenges

Pg. 113

1. How is photosynthesis essential to life on Earth?

Photosynthesis is essential to life on Earth by allowing photoautotrophs to use sunlight energy to turn carbon dioxide into sugars for fuel. The fuel helps living organisms survive and grow. We depend on photosynthesis for the production of cotton, wood, grains, vegetables, fruits, etc. that us humans need to survive. In fact, petroleum and coal are important sources of fuel for us, and they are ancient products of photosynthesis (they formed from the dead bodies of plants and animals millions of years ago).

2. Why are photosynthetic organisms called producers in an ecosystem?

Photosynthetic organisms are called producers in an ecosystem because the photoautotrophs produce their own food, and therefore become food sources for other organisms.

3. How is light used in photosynthesis?

Light is used in photosynthesis by being absorbed, converted into chemical energy, then stored in the form of sugars. In light reactions, the thylakoids’ pigment molecules absorb light and convert it into chemical energy carried by short-lived, energy-rich molecules. The energy then forms 3-carbon sugars from the carbon dioxide taken in during the Calvin cycle. So, light energy is vital in the process of photosynthesis.

4. What are the products of the light reactions, and how are they used?

The products of the light reactions are ATP, NADPH, and oxygen gas (chemical energy). The energy of ATP and NADPH are used in the Calvin cycle to convert carbon dioxide into stable, easily transported sugars, which provide energy for building new cells. When a water molecule is broken apart, the oxygen was released.

5. Why does the Calvin cycle not operate at night?

The Calvin cycle cannot operate at night because it can’t do so in the dark; there needs to be light to activate rubisco (incorporates carbon dioxide into the Calvin cycle) and other enzymes. Additionally, sufficient carbon dioxide is unavailable when the stomata are closed.

6. What are the products of the Calvin cycle, and how are they used?

The products of the Calvin cycle are six molecules of PGAL, or three-carbon sugars. Five molecules of PGAL are required to regenerate RuBP, the starting material. The sixth one is available for the organism to use for maintenance and growth. PGAL, or sugar-phosphates, are often removed from the Calvin cycle for use in other cellular functions. Plants can synthesize them into other compounds (like lipids & amino acids [later proteins]), convert them into sucralose, etc. PGAL and other sugar-phosphates are basically food for the plant.

Pg. 122:

1. Why is metabolic activity most accurately expressed as a rate?

Metabolic activity is most accurately expressed as a rate because it makes things easier to measure the amount of carbon dioxide used in the process as a rate. You can measure the environmental changes on a biological process or the biological process itself.

2. How does increasing light intensity affect the rate of photosynthesis?

Increasing light intensity affects the rate of photosynthesis. Before light reaches the intensity of full sunlight, the rate of photosynthesis levels off. However, once light intensity starts increasing, the rate of photosynthesis increases along with it. Then, it reaches a maximum point known as the light saturation point, then levels off into a maximum rate. In very bright light, the rate of photosynthesis may decline (photoinhibition) because extra energy may pass to oxygen molecules, which may form hydroxyl ions with water, which can damage chloroplasts.

3. Describe the process of photorespiration. What is its effect on the rate of photosynthesis?

 Photorespiration occurs when oxygen replaces carbon dioxide in a reaction; only one molecule of PGA and one molecule of the 2-carbon acid glycolate. Glycolate is transported out of the chloroplast and partially broken down into carbon dioxide. This results in the organism losing fixed carbon atoms instead of gaining them. Increasing oxygen concentration slows the rate of photosynthesis. Relatively high levels of oxygen are favored conditions for photosynthesis.

4. Describe the location and function of the two systems of carbon dioxide fixation in C4 plants.

The location of the first system of carbon dioxide fixation in C4 is the surrounding of each vein in the leaves (a layer of tightly packed cells), called the bundle sheath, and the mesophyll cells that surround the bundle sheath. Carbon dioxide is combined with a 3-carbon acid, the resulting 4-carbon acid is rearranged and then transported to the bundle-sheath cells, where carbon dioxide is released from the 4-carbon acid and refixed by rubisco which forms PGA through the Calvin cycle. Carbon dioxide is thus delivered effectively to the bundle-sheath cells, favoring photosynthesis and inhibits photorespiration. The second system is located in desert plants like cacti; it is called CAM, or crassulacean acid metabolism. CAM plants open their stomata at night, incorporating carbon dioxide into organic acids, just as C4 plants do in the daylight. During hot, dry desert days, stomata close to conserve water. Enzymes break down organic acids, releasing carbon dioxide that will enter the Calvin cycle. CAM plants can survive intense heat, but they usually grow very slowly; this system is not very efficient. O2 does not inhibit photosynthesis in C4 plants; they can still function. Conversely, O2 can inhibit photosynthesis in C3 plants.

5. Which reaction in photosynthesis is inhibited by oxygen? Construct a graph that compares the effect of oxygen on C3 and C4 photosynthesis.

The reaction in photosynthesis that is inhibited by oxygen is the Calvin cycle, or C3 plants.

6. Explain the following statement: The C4 pathway separates carbon fixation from the Calvin cycle in space, and the CAM pathway separates them in time.

This statement basically means that C4 plants use different sections of the cell to perform different functions that will keep O2 out of the active site of Rubisco, and the CAM plants are able to keep O2 out of the active site by collecting CO2 at a different time as the Calvin cycle.

Pg. 125:

1. What part of the light reactions of photosynthesis is similar to the oxidation of minerals by chemoautotrophs?

The part of light reactions of photosynthesis that is similar to the oxidation of minerals by chemoautotrophs is the splitting of H2O into H+, O2, and e- near Photosystem II. When the molecule is split, water is oxidized because it loses an electron; the lost electrons are added to the Electron Transport Chain.

2. Why are chemoautotrophs rare among familiar organisms?

Chemoautotrophs are rare among familiar organisms because many of them must oxidize large quantities of inorganic material to obtain enough energy in order to survive. The process also does not provide as much energy as photosynthesis or heterotrophy, thus explaining why it is harder for a chemoautotroph to survive.

3. How do chemoautotrophs obtain organic nutrients?

Chemoautotrophs obtain organic nutrients by fixing carbon dioxide, usually with the Calvin cycle. Similar to photoautotrophs, chemoautotrophs have electron transport systems; they attract electrons from various substances to generate ATP and reduce NADPH or NADH. The more reduced the electron source becomes, the more energy is released when it is oxidized, allowing the chemoautotroph to grow faster than others who rely on partly oxidized electron donors.

4. Some deep-earth bacteria consume petroleum or natural gas. Are these organisms chemoautotrophs?

These organisms are indeed chemoautotrophs because there is not enough sunlight for them to conduct photosynthesis, yet they still survive by obtaining energy from oxidizing substances like petroleum and natural gas.

5. Some bacteria reduce metal ions or other inorganic substances. Is this behavior a clue that these organisms are chemoautotrophs? Explain.

This behavior of the bacteria is a clue that these organisms are chemoautotrophs. Chemoautotrophs obtain energy by oxidizing inorganic substances (also known as electron sources), such as iron, sulfur, and various other minerals. They then use this energy to form sugars from carbon dioxide. These bacteria are chemoautotrophs because they live in an area/environment where photosynthesis is not possible.