PHOTOSYNTHESISBIOLOGICAL SCIENCE 3A

Introduction6 CO2 + 6 H2O +light energy C6H12O6 + 6 O2Photosynthesis consists of two independent pathways called the light-dependent reaction (light reaction) and the light-independent reaction (dark reaction).

Structure of a ChloroplastThe light reactions take place in the thylakoid membraneThe dark reactions take place in the stroma

Structure of a Chloroplast

Electromagnetic Spectrum

Why are plants green?

PigmentsPigments are light-absorbing compounds.Pigments appear colored because they absorb light of certain wavelengths and reflect that of others.Chlorophyll a is the primary pigment in green plants that absorbs red and blue/violet light and reflects green light.

Structure of Chlorophyll

Accessory PigmentsChloroplasts also contain other pigments called accessory pigments.Accessory pigments trap wavelengths of light that cannot be absorbed by chlorophyll a and then transfer the energy to chlorophyll a molecules for use in photosynthesis. In this way, accessory pigments enable plants to use a greater amount of the suns energy than is available to chlorophyll alone.

EnergyEnergy is needed in the cells of all living things to form complex molecules from simple ones and to power the many metabolic activities of the cell.

Almost all energy comes originally from the sun in the form of heat and light. During photosynthesis, plants capture light energy and store it as chemical energy in complex molecules, such as glucose.

Animals must consume plants or other animals to obtain energy to drive the chemical reactions of their cells.

Chemical reactions are of two types:Reactions that release energy are called exergonic (downhill) reactions. For example, respiration releases energy during the breakdown of glucose. Reactions that use energy are called endergonic (uphill) reactions. For example, photosynthesis uses energy in the synthesis of glucose.

Organic molecules, such as glucose and proteins, have many energy containing bonds that can be broken apart in cells to release energy. Plants and animals have different ways of obtaining glucose. Green plants are autotrophs and make their own glucose during photosynthesis, whereas animals are heterotrophs and obtain all their organic compounds from food.

The facts about energyEnergy cannot be created or destroyed.Free energy is energy that is available to do work.When energy is transformed from one form into another, some energy is lost to the surroundings, usually in the form of heat energy, which cannot be used to do work in cellsExergonic reactions result in a net release of energy.Endergonic reactions require an input of energy in order to occur.All energy transformations obey the laws of thermodynamics:The total energy in the universe is constant.In the universe as a whole, the amount of free energy is declining.

ATPenergy from glucoseWhile plants and animals obtain glucose in different ways, organisms use energy in similar ways. For immediately usable energy, organisms use the chemical energy carried in the phosphate bonds of ATP (adenosine triphosphate). This is the major source of available energy for virtually all cellular functions. ATP is a molecule with a high energy terminal phosphate bond that is easily broken by hydrolysis to release a small packet of energy, which can be used to carry out cellular work.

The ultimate source of energy for virtually all living organisms is the radiant energy of sunlight.

Green plants trap light energy and transform it into chemical energy by the process of photosynthesis. Light energy is trapped by chlorophyll (chloro meaning green, phyll meaning leaf), a green pigment molecule, and is used to form ATP molecules.

These ATP molecules are then used to drive carbon dioxide fixation, which is the combination of and water to form glucose and oxygen gas.

Chlorophyll is found in plants, algae, purple and green bacteria, and cyanobacteria. In algae and the green parts of plants, such as the upper surface of a leaf, chlorophyll is in prominent green organelles called chloroplasts.

There may be several hundred chloroplasts in each cell. Chloroplasts are membranous structures about 6 m by 3 m, which are easily visible using a light microscope.

They consist of an outer membrane, many internal layers of membrane that form sacs or lamellae, and a fluid matrix.

Chlorophyll is located on the surface of these internal membranes. The enzymes needed to carry out photosynthesis are located in the matrix of the chloroplast.

PhotosynthesisPhotosynthesis is the process by which autotrophs use light energy to manufacture organic compounds from simpler inorganic compounds.Inorganic compounds are simple chemical compounds that do not usually contain carbon, such as water (H2O).Organic compounds are complex chemical compounds that contain carbon and are formed by living things during cellular processes such as photosynthesis; for example glucose, starch and protein.The process of photosynthesis can be summed up as a chemical reaction, as shown below.

6H2O + 6CO2 C6H12O6 + O2

Light Reactions: the energy in sunlight is trapped, O2 is released, and both ATP and NADPH + H+ (hydrogen-carrier molecule) are formedDark Reactions: the ATP and NADPH + H+ react with CO2 from the atmosphere and form glucoseThe entire process results in the transformation of light energy from the sun into energy stored in the bonds of the glucose molecule.

Although the overall equation of photosynthesis seems simple, it is a complex pathway with many steps.

These steps form two stages: the first stage are the light-dependent reactions, which take place on the inner membranes of the chloroplast. As suggested by their name, these reactions require light. Chlorophyll traps light energy and uses it to produce ATP and to split water into hydrogen ions and oxygen gas.

Light Dependent Stage

light energyWater hydrogen ions + oxygen gas + ATPchlorophyll

Light-dependent reactionsThese reactions take place on the internal membranes (grana) of the chloroplast and are similar in all plants.

Chlorophyll has a molecular structure that, when excited by light energy, initiates a series of steps that results in the formation of ATP molecules, at the same time splitting water molecules into hydrogen and oxygen. Oxygen gas is released as a by-product.

The light-dependent stage of photosynthesis can be summarised by the following equation: The ATP and carrier H+ produced are utilised during the next stage, the light-independent stage.Clearly, the full story for the light-dependent stage is not given in the above figure. For further details of what occurs in the light-dependent reactions, refer to next figure. Note that the carrier molecule for the ions produced during this stage is called NADP (nicotinamide adenine dinucleotide phosphate).

Simplified diagram showing a summary of the light-dependent reactions of photosynthesis. Light energy trapped by chlorophyll splits water molecules to form high-energy electrons (e) and protons (H+), with oxygen (O) as a waste product. The energy released from these electrons is used to drive the formation of ATP and loaded carrier molecules of NADPH (reduced nicotinamide adenine dinucleotide phosphate).

Even though the whole process of photosynthesis takes place in sunlight, the second stage of photosynthesis does not require light, so it is referred to the light-independent reactions (or sometimes as the dark reactions).

These reactions take place in the fluid matrix of the chloroplast. ATP made during the light-dependent stage provides the energy needed to combine carbon dioxide with hydrogen ions (also from the light-dependent stage) to form the energy-rich molecule glucose, and water.

Second StageATP + hydrogen ions + carbon dioxide glucose + water + ADP

The light-independent reactions involving carbon reduction occur inside the stroma of chloroplasts. The term light-independent means that the reaction is not dependent on light involvement in the way the previous stage is. However, it is dependent on the previous stage occurring.What happens?The synthesis part of photosynthesis involves the formation of sugar molecules from carbon dioxide. During this process, carbon atoms are moved from a highly oxidised state (as carbon dioxide: CO2) to a reduced state (as sugar: [C(H2O)]n).Carbon reduction requires a supply of carbon dioxide and hydrogen ions, and an input of energy. The carbon dioxide can come from the air surrounding the leaf or from cellular respiration reactions. The loaded carrier molecule supplies the necessary hydrogen ions. The energy required to drive these reactions comes from ATP and loaded carriers produced during the light-dependent stage.

Highly simplified representation of the light-independent stage of photosynthesis. Carbon dioxide enters stroma where it begins a series of reactions that use the hydrogen and ATP from the light-dependent stage of photosynthesis. Note the inputs and outputs of the light-independent stage.

Carrier H+ is the reducing agent and ATP is the source of energy for reducing carbon dioxide to organic compounds such as glucose and other sugars. The light-independent stage of photosynthesis can be represented by the equation.Plants do not build sugars simply by joining carbon dioxide molecules together. Sugar formation involves a cyclic set of reactions in which intermediate substances are formed.

Carbon reduction in C3 plantsIn many plants and in some photosynthetic bacteria, the carbon reduction reactions begin with the Calvin cycle. Because the product of this reaction contains three carbon atoms, plants that carry out this reaction are known as C3 plants. This reaction is referred to as carbon dioxide fixation because carbon is moved from being a free gas to being secured in an organic compound.

Each time the cycle proceeds, one carbon dioxide molecule enters the cycle and is fixed and reduced. To produce a 6-carbon compound that is released from the cycle, six turns of the cycle must take place. At the completion of each turn of the cycle, the starting compound is regenerated and so the cycle can proceed provided that CO2, ATP and NADPH are also available. The Calvin cycle in C3 plants occurs in mesophyll cells of nearly all trees and most shrubs and herbs.

Light-independent reactions in C3 plantsThese reactions take place in the fluid matrix within the chloroplast (stroma). Carbon dioxide diffuses into the leaf, into leaf cells and into the chloroplasts, where it is captured in a complex pathway.

This pathway is called the Calvin cycle, or C3 photosynthesis, because the first stable compound produced has three carbon atoms.

Eighteen ATP molecules, made during the light-dependent stage, are needed to provide the energy to combine six carbon dioxide molecules with twelve hydrogen molecules (also from the light-dependent stage) to form one molecule of glucose and re-form six molecules of water.

In C3 plants, up to 50% of the carbon dioxide captured in photosynthesis is released again before it can be converted to sugar! This is because under warm conditions, the enzyme that normally captures carbon dioxide in the C3 pathway reacts with oxygen instead. The efficiency of carbon dioxide capture in hot weather is reduced, so stomata must be open longer to obtain sufficient carbon dioxide, which increases water loss.

Light-independent reactions in C4 plants

To overcome the above problems with the C3 pathway, many species of plants native to hot climates, including maize, sorghum and sugarcane, have an additional process to capture carbon dioxide. This process is known as C4 photo synthesis because the first stable product produced (oxaloacetic acid) is a compound with four carbon atoms.

Eventually, this four-carbon compound is converted to a molecule of carbon dioxide and a three-carbon compound, which enters the Calvin cycle.

The enzyme used to capture carbon dioxide in C4 plants does not react with oxygen. This means that C4 plants are more efficient in warm conditions because they can capture more carbon dioxide in less time, and the stomata do not need to stay open for as long.

Their carbon-fixing pathway works well in warm environments with very high light intensity. C4 plants can produce two to three times as much sugar as a C3 plant on a hot, sunny day. However, on a milder day, a C3 plant is more efficient because its pathway uses less energy to capture the carbon dioxide.

SUMMARYPhotosynthesis can be divided into a light-dependent and a light-independent (carbon reduction) stage.The light-dependent stage occurs on the grana membranes inside chloroplasts.The light-dependent stage results in the formation of ATP and acceptor molecules loaded with H+.The carbon reduction stage occurs in the stroma of chloroplasts.The process of carbon reduction involves the Calvin cycle in which carbon dioxide is fixed.In C4 plants, preliminary steps precede the Calvin cycle.One product of the Calvin cycle is a 3-C compound (PGAL) which can react to form various sugars, including glucose, fructose and sucrose.Sucrose is the form in which carbohydrates are transported through the phloem.Starch is the storage carbohydrate in plant cells.

The rate of photosynthesisThe rate of photosynthesis is affected by the following factors.

TEMPERATUREThe rate of photosynthesis increases as the temperature increases. However, in most plants, the rate declines when the temperature reaches anoptimal. The rate of Photosynthesis increases with increasing temperature until around 20-40 degrees Celsius depending on the plant, after which it declines. Plants Adapted to hot temperatures operate in the upper range.

LIGHTThe rate of photosynthesis increases as light intensity increases. However, the rate will not increase beyond a certain level of light intensity.

Photosynthesis proceeds at different rates during the day. About 20% of the light that strikes a leaf is reflected; the remainder is absorbed and may, depending on Its wavelength, be captured photosynthetic pigments such as chlorophyll.However only 1% is actually captured and converted to chemical energy.

Carbon DioxideThe rate of photosynthesis increases as carbon dioxide levels increase. However, the rate will not increase beyond a certain concentration of carbon dioxide. For most plants, carbon dioxide from air dissolves in extracellular fluid before entering photosynthetic cells. There are local variations in carbon dioxide levels in air, in different habitats and at different times of the day.The slow increase in atmospheric levels of carbon dioxide as part of the greenhouse effect may actually benefit crop plants. Aquatic plants can also use hydrogen carbonate (carbonic acid), which forms when carbon dioxide dissolves in water.

The carbon dioxide released as a product of the plants cellular respiration can be used in photosynthesis, but this source usually provides only a relatively small fraction of the total carbon dioxide requirement.

At the high levels of light intensity in the middle of a sunny day, the rate of photosynthesis is greater than the rate of cellular respiration, so there is a net output of oxygen by plants. At low levels of light intensity, between late afternoon and early morning, the rate of photosynthesis is lower than the rate of cellular respiration, so there is a net output of carbon dioxide by plants.

The light intensity at which the rate of carbon dioxide produced by cellular respiration equals the rate that carbon dioxide is used in photosynthesis is the light compensation point. In other words, the compensation point is the light intensity at which the rate of respiration equals the rate of photosynthesis.

The degree to which the level of carbon dioxide affects the rate of photosynthesis is different for C3 and C4. C4 plants are more efficient than C3 plants at trapping carbon dioxide when it is warm, so they can survive if there are lower levels of carbon dioxide in the air.

Other factorsCertain other factors may affect the rate of photosynthesis under particular circumstances.

Although oxygen is not involved directly in photosynthesis, in C3 plants net carbon dioxide fixation is reduced in the presence of oxygen. Therefore, reducing oxygen in the controlled atmosphere of a greenhouse will result in a considerable increase in photosynthesis.

Water is required in photosynthesis, but only 1% of water passing up the xylem is used in photosynthesis. The rest is used in other chemical reactions, to hydrate cells or is lost in transpiration. If there is not enough water to hydrate the cells and keep them turgid (with high internal fluid pressure, the stomata close. This prevents carbon dioxide entering the leaves, therefore photosynthesis decreases.

Chlorophyll traps light energy, therefore the amount of chlorophyll in a leaf will limit the rate of photosynthesis. Chlorophyll contains the elements nitrogen and magnesium. If the soil is deficient in one or both of these elements, the plants cannot make sufficient chlorophyll and they appear yellow. Yellow leaves will have a lower rate of photosynthesis than green leaves rich in chlorophyll.

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