• Define terms photosynthesis.• The process of photosynthesis.

• Light-dependent photosynthesis.• Light-independence photosynthesis.

• Factors affecting the rate of photosynthesis.

OBJECTIVES:

By the end of the lesson, student should be able to :

• Define terms photosynthesis.• Understand the process of

photosynthesis.• Identify the stages of photosynthesis.• Understand some factors affecting

photosynthesis.

• The process of converting light energy to chemical energy and storing it in the bonds of sugar.

• This process occurs in plants and some algae (Kingdom Protista).

• Plants need only light energy, CO2, and H2O to make sugar.

• The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.

• A commonly used but slightly simplified equation for photosynthesis is:

carbon dioxide + water + light energy → glucose + oxygen

6 CO2(gas) + 12 H2O(liquid) + photons C6H12O6(aqueous) + 6 O2(gas)

• The thylakoid is the structural unit of photosynthesis.

• Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals.

• Only eukaryotes have chloroplasts with a surrounding membrane.

• Thylakoids are stacked like pancakes in stacks known collectively as grana.

• The areas between grana are referred to as stroma.

• While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments.

• The specialized membrane structures in which photosynthesis takes place.

• Internal membranes in the chloroplast where the light reaction chemicals are embedded.

• Collections of thylakoids form the grana.

• Photosynthesis occurs in two stages. • In the first phase, light-dependent reactions or

photosynthetic reactions (also called the Light reactions) capture the energy of light and use it to make high-energy molecules.

• During the second phase, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture carbon dioxide (CO2) and make the precursors of carbohydrates.

• In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state.

• In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH.

• Water is split in the process, releasing oxygen as a by-product of the reaction.

• The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

• In the Light Independent Process, carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form carbohydrates (general formula of carbohydrates is [CH2O]n).

• The incorporation of carbon dioxide into organic compounds is known as carbon fixation.

• The energy for this comes from the first phase of the photosynthetic process.

• Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes.

• Photosynthesis may simply be defined as the conversion of light energy into chemical energy by living organisms.

• It is affected by its surroundings and the rate of photosynthesis is affected by the concentration of carbon dioxide, the intensity of light, and the temperature.

• Each factor affects a different rate-limiting step.

• As light intensity increases, the photosynthetic rate increases until a point is reached where the rate begins to level off.

• At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions.

• As light intensity increases, more ATP and NADPH are created, thus increasing the photosynthetic rate.

• At high light intensity, photosynthetic rate levels out, not due to light intensity but due to other limiting factors, including competition between oxygen and carbon dioxide.

LOW CONC.

HIGH CONC.

• As carbon dioxide concentration increases, the rate of photosynthesis increases.

• At high concentrations, the rate of photosynthesis begins to level out due to factors not related to carbon dioxide concentration.

• One reason might be that some of the enzymes of photosynthesis are working at their maximum rate.

• In general, carbon dioxide is found in low concentration in the atmosphere, and so atmospheric carbon dioxide levels may be a major limiting factor on photosynthesis when at low levels.

LOW CONC.

HIGH CONC.

• As temperature increases above freezing, the rate of photosynthesis increases.

• This occurs because molecules are moving more quickly and there is a greater chance of a collision resulting in a chemical reaction.

• At some point, a temperature is reached that is an optimum temperature.

• The photosynthetic reaction rate is at its quickest rate at this point.

• Above that temperature, the enzymes begin to denature (as in RUBP carboxylase), slowing the rate of photosynthesis until a temperature is reached where photosynthesis does not occur at all.

LOW CONC.

HIGH CONC.