chapter xi photosynthesis -...

DMA: Chapter 11 Hartmann's Plant Science, 4th edition

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CHAPTER XI PHOTOSYNTHESIS


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The nature of light

• The sun's energy travels through space to the earth as electromagnetic radiation waves at the speed of light, about 300,000 Km/s.

• Light is transmitted to the earth in discrete bundles of energy called photons.


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• The wavelengths of this radiation vary from very long radio waves of more than 1 kilometer to very short cosmic rays of less than 10-4nm.

• Visible light is that portion of the electromagnetic spectrum between 400 and 760 nm; however, plants respond to the somewhat wider spectrum of about 300 to 800 nm.


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• The number of waves passing a given point in space per second is called its frequency.

• Light traveling in shorter wavelengths at higher frequencies is more energetic than light at longer wavelengths and lower frequencies.


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• The rate of photosynthesis can be determined by measuring the rate at which carbon dioxide is absorbed by a leaf.

• Rate of total photosynthesis = rate of net photosynthesis + rate of respirationin light.


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• Only about 5 % of the radiant energy provided by the sun is used directly in photosynthesis (Fig. 11-3).


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Carbon atom• Carbon has six protons, six neutrons, and six electrons

(atomic weight of 12).

• No other element has an ideal combination of sharabilityof electrons plus stability of the bonds formed.1. The two inner electrons (K shell) cannot be shared, but the four

outer electrons (L shell) can be shared. Thus, carbon can form up to four bonds:

2. The strength of the C-C bond is 82.6 kcal/mole (an estimate of how much energy is needed to break the bond).


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Oxidation/Reduction

• A compound that is reduced is a good electron donor relative to a compound that is oxidized.

• Carbon is at the center of this process in living organisms because it is so versatile in sharing electrons.

• One way to look at photosynthesis is to say that it is the process of converting:

Oxidized carbon (CO2) → Reduced carbon [carbohydrate or C(H2O)].


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PHOTOSYNTHESIS: LIGHT ENERGY → REDUCING POWER

• Photosynthesis is the process of conversion of

Light energy → Chemical energy

• The simplest chemical reaction that can be written to represent photosynthesis is:


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• Photosynthesis occurs in chloroplasts (Fig. 11-1). The chloroplast contains a complex array of membranes called thylakoids, and many enzymatic proteins are imbedded in the thylakoid membranes (Fig. 11-2).


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• The essential function of photosystem I and photosystem II in the chloroplast is to utilize light energy to obtain electrons from water:

• Note that the oxygen produced in photosynthesis comes from water, not carbon dioxide.

• The photosystems use light energy to raise the electronsto a very high energetic state so that they can be used to start the cascade of reduction reactions.

• The protons (H+ generated in the water-splitting reaction also set up a proton (H+) gradient across the thylakoidmembranes, and this gradient is used to drive ATP synthesis.


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• Multiple photons are received by multiple chlorophyll molecules and shunted to a reaction center where the electrons from water are energized (Fig. 11-4). The analogy of chlorophyll as an antenna molecule is appropriate.


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The light-requiring reactions of photosynthesis result in

• The conversion of water to O2.

• The generation of NADPH and the energy-rich compound adenosine triphosphate(ATP).


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PHOTOSYNTHESIS: CARBON DIOXIDE → CARBOHYDRATE• The first step in the generation of carbohydrate is the

reaction of CO2 with the compound ribulose 1,5 bisphosphate. (called Carbon Fixation step)

• The reaction is catalyzed by the enzyme ribulosebisphosphate carboxylase, usually referred to as RUBISCO.

• This reaction produce two molecules of 3-phosphoglycerate.– Phosphoglycerate + ATP→ 1,3-diphosphoglycerate + ADP– 1,3-diphosphoglycerate + NADPH → glyceraldehyde 3-

phosphate + NADP + inorganic phosphate


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PHOTOSYNTHESIS: CARBON DIOXIDE → CARBOHYDRATE• Next is the regeneration of the beginning substrate,

ribulose 1,5-bisphosphate. It is known as the photosynthetic carbon reduction cycle, or Calvin cycle.

• A totally new molecule of glyceraldehyde 3-phosphate can be spun off with every three revolutions of the cycle.

• The new glyceraldehyde 3-phosphate molecule is combined with another 3-carbon phosphorylated sugar to give fructose-1, 6-bisphosphate

• In general, dead-end metabolites (that do not participate in many other reactions) are used for long-distance transport. Example: sucrose


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FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS

1. Light quality (wavelength).2. Light intensity (the amount of incident light

energy absorbed by the leaf).3. Carbon dioxide concentration.4. Heat.5. Water availability.6. Plant development and source-sink

relationships.


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Light Quality

• Chloroplasts contain pigments (chlorophyll a, chlorophyll b, and some carotenoids) that absorb mostly the redand blue portions of the visible wavelengths spectrum and reflect the green portion.

• Blue and red wavelengths are more photosynthetically active than green.


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Light Intensity

The effect varies with different plants:• Sun-loving plants: Some species require high

light intensities to grow well (corn, potatoes, sugarcane).

• Shade-loving plants: Some plant species that do not grow well in high light intensities (dense shade of the forest floor, useful as house ornamentals).

• Intermediate: Other plant species grow well in moderately intense light.


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Light intensity distinctly affects the size and shape of leaves:

• The leaves of a given plant species grow thinner and larger in area at the low light intensity than leaves grown at the high light intensity.

• The leaves of plants grown in high light intensities tend to be darker green than those grown in low light intensities.


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Light compensation point• Light compensation point

is a light intensity at which photosynthesis and respiration rates are equal and net gas exchange is zero (CO2 is still exchanged) (Fig. 11-6).

• The plant is said to be light saturated when further increases in light intensity increase photosynthesis little or not at all.


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• The light intensity at which saturation occurs increases as the CO2 concentration surrounding the plant rises (Fig. 11-7).

• At very high light intensities, the rate at which CO2 is available to the plant could limit the photosynthetic rate.


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Carbon Dioxide

• The carbon dioxide compensation point is the carbon dioxide concentration at equilibrium such that the amount of carbon dioxide evolved in respiration exactly equals the amount consumed in photosynthesis.


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• Increasing the CO2 concentration in a closed system, such as a sealed greenhouse to about 0.10 % approximately doubles the photosynthetic rate of certain crops

– Applications of organic matter in the form of crop residues or green manure crops to the soil tends to increase CO2 levels in the atmosphere above the soil.

– In greenhouses, the use of horizontal airflow fans (HAFs) increases airflow around plants and exposes the leaves to a constant supply of CO2. The use of HAFs may eliminate the need for supplemental CO2 in most greenhouses.


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HeatIf light is not limiting, the rate of photosynthetic

activity approximately doubles for each 10°Cincrease in temperature for many plant species in the temperate climates.

The effect of temperature varies with species:• Plants adapted to tropical conditions require a

higher temperature for maximum photosynthesis than those adapted to colder regions.

• Excessively high temperatures reduce the photosynthetic rate of some plants not accustomed to such high temperatures by causing the stomata of the leaves to close.


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Water• Under conditions of drought (low soil moisture and

hot, drying winds), plants often lose water through transpiration faster than their roots can absorb it. This causes:– The stomata to close and the leaves to wilt temporarily. – When this occurs, the exchange of CO2 and O2 is

restricted, – Resulting in a dramatic drop in photosynthesis.

• Excessive soil moisture sometimes creates an anaerobic condition (lack of oxygen) around the roots. This causes:– Reducing root respiration and mineral uptake and

transport of water and minerals to the leaves– Thus indirectly depressing photosynthesis in the leaves.


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Plant development and the Source-Sink relationship• When the leaf reaches full expansion, it is often called a

source leaf because the carbohydrate synthesized in that leaf is in excess of local requirements and is exported to other parts of the plant that are actively growing. These sites of active growth and metabolism are often called sink tissues. (Roots and reproductive organs)

• In many plant species, the photosynthesis rates in all the mature leaves begin to decline drastically when the plants flower and envelop fruits and seeds. (Proteins in the leaves, and specially photosynthetic enzyme proteins in the chloroplasts, are degraded to amino acids as the leaves begin the process called senescence. The amino acids in the leaves are transported to the developing seeds).


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Different Photosynthetic Mechanisms

• Plants that evolved in more arid regions have adaptations that help conserve water but do not inhibit photosynthesis during dry periods.

• C4• CAM: crassulacean acid metabolism


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The C4 plants:• The C4 mechanism is found in tropical grasses

such as corn, sorghum. These plants have a special anatomy that concentrates CO2 in special areas of the leaf that allow CO2 to be fixed more efficiently.

• They are called C4 plants because CO2 entering the leaf is temporarily attached to a three-carbon organic acid making a four-carbon organic acid.

• The acid is shuttled to the special areas and the CO2 is released at the areas where it can be used efficiently.


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The CAM plants

• The CAM plants are water-storing desert plants such as succulents and cacti.

• As with C4, the CAM process also attaches CO2 to a three-carbon organic acid, but this time it happens at nightwhen the stomata can open without extreme water loss.


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RESPIRATION• The building and maintenance of cells and tissues

require not only carbohydrates, but also proteins, lipids, nucleic acids, and so forth to accomplish the thousands of metabolic tasks required for growth and reproduction.

• Respiration is the conversion of carbohydrate into energy-rich ATP:

C6H12O6 + 6O2 + 36ADP + 36 PO43- → 6CO2 + 6H2O + 36 ATP3-

• So, respiration is the reverse of photosynthesiswith respect to carbon dioxide ↔ carbohydrate.


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Steps of respiration:

1. Glycolysis: the conversion of carbohydrate to a three-carbon organic acid named pyruvic acid

• C6H12O6 + 2 ADP + 2 PO43- + 2 NAD+ → 2 C3H4O3 + 2ATP + 2NADH + H2O


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2. Tricarboxylic acid (Krebs) cycle:

• Before entry into the cycle, the pyruvate is first converted to acetyl coenzyme A, with the loss of one CO2 and the production of another NADH (Fig. 11-8).


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• The reduced carbon enters the cycle as an acetyl group and is degraded to two more CO2 molecules coupled to

1. The generation of three more NADH molecules.

2. A new type of electron acceptor—flavin adenine dinucleotide (FAD)—is also involved here.

3. Also, one ATP is gained with each turn of the cycle.

• Remember: for every glucose that is processed through glycolysis, two pyruvate molecules are formed.


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3. Electron transport system.

• One way to describe this process is to imagine that the electrons in NADH are passed down a cascade of stairs, with energy being released at each step. During this process,

– One NADH yields three ATPs while – One FADH2 yields two ATPs,

• Note that molecular oxygen is the final electron acceptor and that H2O is the final product.


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• Enzymes for glycolysis are located in the cell cytoplasm. The remaining two steps are localized in mitochondria. Thus, pyruvate—the product of glycolysis—must be transported into the mitochondrion (Fig. 11-10).

36Grand total

3042428TCA cycle + electron transport

6-4*22Glycolysis

From FADH2From NADHDirect

Total ATP Production

ATP SynthesisNADH Produ

ced

Step

* Six are produced but two must be subtracted to support the transport of NADH into the mitochondria

ATP is the energy currency of living organisms.

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