Photosynthesis

Organisms Depend Upon Photosynthesis

A. Organisms Depend Upon Photosynthesis

1. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate source of food for most life.

2. Photosynthesis transforms solar energy into chemical bond energy of carbohydrates.

3. Most food chains start with photosynthesizers.

Solar Radiation

Key Discoveries of Photosynthetic Process

Structure of Chloroplasts

Function of Chloroplasts

A. Solar Radiation

1. Solar radiation is described in terms of its energy content and its wavelength.

2. Photons are discrete packets of radiant energy that travel in waves.

3. The electromagnetic spectrum is the range of types of solar radiation based on wavelength.

a. Gamma rays have shortest wavelength.

b. Radio waves have longest wavelength.

c. Energy content of photons is inversely proportional to wavelength of particular type of radiation.

1. Short-wavelength ultraviolet radiation has photons of a higher energy content.

2. Long-wavelength infrared light has photons of lower energy content.

3. High-energy photons (e.g., those of ultraviolet radiation) are dangerous to cells because they can break down organic molecules by breaking chemical bonds.

4. Low-energy photons (e.g., those of infrared radiation) do not damage cells because they do not break chemical bonds but merely increase vibrational energy.

d. White light is made up of many different wavelengths; a prism separates them into a spectrum.

4. Only 42% of solar radiation that hits earth's atmosphere reaches surface; most is visible light.

a. Higher energy wavelengths are screened out by ozone layer in upper atmosphere.

b. Lower energy wavelengths are screened out by water vapor and CO2.

c. Consequently, both the organic molecules within organisms are processes, such as vision and photosynthesis, are adapted to radiation that is most prevalent in the environment.

5. Earth's Energy-Balance sheet

a. 42% of solar energy hitting atmosphere reaches earth surface; rest is reflected or heats atmosphere

b. Only 2% of 42% is eventually used by plants; rest becomes heat.

c. Of this plant-intercepted energy, only 0.1 to 1.6% is incorporated into plant tissue.

d. Of plant tissue, only 20% is eaten by herbivores; most of rest decays or is lost as heat.

e. Of herbivore tissues, only 30% is eaten by carnivores.

6. Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum.

a. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.

b. Both chlorophylls absorb violet, blue, and red wavelengths best.

c. Very little green light is absorbed; most is reflected back; this is why leaves appear green.

d. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions.

e. When chlorophyll in leaves breaks down in fall, the yellow-orange pigments show through.

7. Absorption and action spectrum

a. A spectrophotometer measures the amount of light that passes through a sample of pigments.

1. As different wavelengths are passed through, some are absorbed.

2. Graph of percent of light absorbed at each wavelength is absorption spectrum.

b. Action spectrum

1. Photosynthesis produces oxygen; production of oxygen is used to measure rate of photosynthesis.

2. Oxygen production and, therefore, photosynthetic activity is measured for plants under each specific wavelength; plotted on a graph, this produces an action spectrum.

3. Action spectrum resembles absorption spectrum; indicates chlorophylls contribute to photosynthesis.

B. Key Discoveries of Photosynthetic Process

1. The overall equation for photosynthesis is usually stated as carbon dioxide plus water forms carbohydrated plus oxygen.

2. In 1930 C.B. van Niel showed that oxygen given off by photosynthesis comes from water and not from carbon dioxide. The correct equation should then read: carbon dioxide plus water forms carbohydrate plus water plus oxygen.

C. Structure of Chloroplasts

1. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma; stroma contains enzyme-rich solution that reduces CO2, converting it to an organic compound.

2. Even more internal membranes within stroma form flattened sacs called thylakoids, which are sometimes organized into stacks called grana.

3. Spaces within all thylakoids are connected and form an inner compartment or thylakoid space.

4. Chlorophylls and other pigments involved in absorption of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy, energize electrons prior to reduction of CO2 in stroma.

D. Function of Chloroplasts

1. In 1905, F.F. Blackman proposed two sets of reactions for photosynthesis.

2. Light-dependent reactions cannot take place unless light is present.

a. Light-dependent reactions are the energy-capturing reactions.

b. Associated with light-absorbing molecules and electron transport systems of thylakoids.

c. They involve the splitting of water and the release of