Chapter 10 Photosynthesis
Photoautotrophs-organisms that use light as a source of energy to synthesize carbohydrates, lipids, proteins, and other organic substances. (examples: cyanobacteria, plants, algae)
Chemoautotrophs-organisms that use energy obtained by oxidizing inorganic substances such as sulfur or ammonia to produce organic compounds (example: certain prokaryotes such as Sulfolobus)
I. LEAF STRUCTURE
A. epidermis-1st line of protection against physical damage and pathogenic organisms. Covered by a waxy cuticle that prevents water loss.
B. stomata-(singular stoma) tiny pores in epidermis
C. guard cells-epidermal cells that control the opening and closing of stomata. Stomata allow gas exchange between the environment and photosynthetic cells inside the leaf. Guard cells regulate transpiration (the loss of water vapor from leaf by evaporation through stomata)
D. mesophyll-contains cells equipped with chloroplasts and specialized for photosynthesis
Palisade mesophyll (one of more layers of elongated cells on the upper part of the leaf
Spongy mesophyll (below the Palisade mesophyll with more loosely arranged cells with air
spaces between cells.
E. Veins- contain vascular tissue of the leaf.
1. xylem-water-conducting tissue
2. phloem- transports sucrose, other organic compounds.
II. CHLOROPLASTS
A. About 500,00 per square millimeter of leaf surface
B. Chlorophyll-green pigment located within chloroplasts
C. Found primarily in cells of mesophyll (30-40 chloroplasts in each mesophyll cell)
D. Structure of chloroplasts
1. intermembrane space
2. Thylakoids-chlorophyll is in the thylakoid membrane
3. Grana-stacks of thylakoids in a chloroplast
4. Stroma-viscous fluid outside the thylakoid
E. Thylakoids function in the steps of photosynthesis that convert light energy to chemical
energy.
F. Stroma-steps that use chemical energy to convert carbon dioxide to sugar occur in stroma.
PHOTOSYNTHESIS EQUATION
6CO2 + 12H2O + light energy ------>C6H12O6 + 6O2 +6 H2O
Water appears on both sides of the formula because 12 molecules of water are consumed and 6 molecules are newly formed during photosynthesis. The formula is usually simplified to show only the net consumption of water:
6CO2 + 6H2O + light energy ------>C6H12O6 + 6O2
III. CONTRAST OF RESPIRATION AND PHOTOSYNTHESIS
A. In respiration:
1. Energy is released from sugar when electrons associated with hydrogen are
transported by carriers to oxygen, forming water as a by-product.
2. Electrons lose potential energy during the process.
3. Mitochondria use the energy to synthesize ATP.
4. Respiration yields 686 kcal of free energy per mole of glucose oxidized to carbon.
B. In photosynthesis:
1. Water is split and electrons are transferred from the water to carbon dioxide,
reducing it to sugar.
2. Electrons, which cannot travel downhill in both directions, increase their potential
energy when moved from water to sugar.
3. 686 kcal of free energy are required to reduce carbon dioxide to glucose.
PHOTOSYNTHESIS IS AN ENDERGONIC REDOX PROCESS AND LIGHT PROVIDES THE ENERGY TO BOOST ELECTRONS FROM WATER TO THEIR HIGH LEVELS IN SUGAR.
IV. TWO STAGES OF PHOTOSYNTHESIS
A. Light dependent reactions-involve the conversion of light energy to chemical energy.
1. Light absorbed by chlorophyll drives a transfer of electrons from water to an acceptor, nicotinamide adenine dinucleotide phosphate (NADP+)
2. This coenzyme (NADP+) temporarily stores the energized electrons.
3. Water is split during this process and oxygen is given off as a by-product.
4. NADP+ (similar to the NAD+ in respiration) is reduced by solar energy to NADPH
by the addition of one pair of electrons along with a hydrogen nucleus (H+)
5. The light reactions also generate ATP by providing energy to add a phosphate group
to ADP during a process called photophosphorylation.
6. Light energy is initially converted to chemical energy in the form of two compounds:
NADPH which serves as a source of energized electrons and ATP which may be used by the cell.
B. The Calvin Cycle (Light Independent reactions)-incorporates carbon dioxide from the air into
organic material. This incorporation of carbon into organic compounds is known as carbon
fixation.
1. During the Calvin cycle fixed carbon is reduced to a carbohydrate by addition of
electrons.
2. Two necessary components for this process are an electron source and an energy
source.
3. These components are provided by NADPH and ATP both of which are produced in the light reactions.
4. NADPH provides the reducing power (energized electrons acquired in the light
reactions) and ATP provides the chemical energy.
5. The steps of the Calvin cycle are often referred to as the dark reactions as they do
not require light directly; however, the Calvin cycle in most plants occurs during
daylight for only then can enough NADPH and ATP be generated by the light
dependent reactions to power the reduction of carbon dioxide to a carbohydrate.
CHLOROPLASTS THUS USE LIGHT ENERGY TO MAKE SUGAR BY COORDINATING THESE TWO STAGES OF PHOTOSYNTHESIS.
· Light dependent reactions occur in the thylakoids of chloroplasts
· Calvin cycle (light independent reactions) occur in the stroma.
· As the NADP+ and ADP molecules contact the thylakoid membranes they pick up electrons
and phosphate respectively, and then transfer their high-energy cargo to the Calvin cycle.