8.2 Photosynthesis

8.2.1. Chloroplast structure:

The chloroplast is a large complex double membrane organelle that performs the function of photosynthesis within plant cells and also contains a substance called chlorophyll that is essential for photosynthesis. The chlorophyll in addition also produces a sugar from the sun this sugar is made within the cell; this provides food for all the organelles.

Inter membrane:The membrane of the mitochondria that is the site of electron transport and chemiosmosis., where the electron transport chain occurs, Where the electron transport chain of cellular respiration occurs

Outer membrane:Forms a boundary between mitochondrion and cytoplasm; helps define the inner membrane space, smooth membrane in mitochondria

Inner membrane space:Fluid filled space between the inner and outer mitochondrial membranes, the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is nucleotide phosphorylation.

Stroma: Gel like substance. Sugars are made in the stroma by the enzymes form the Calvin cycle. Light independent reactions take place here.

Granum: A stacked portion of the thylakoid membrane. The Grana functions in the light reactions of photosynthesis

Thylakoid: The disk shaped organelle. The structure of the thylakoid is the site of the photosynthetic light reactions takes place. These are the reactions that initially excite the electrons so they fall down the electron transport chain and release ATP and continue on to release NADPH.

Lumen: this is the thylakoid membrane bound to the chloroplast compartment.

Lamella: linking a thylakoid within one granum to one in another. They are the sites of photosystem I. Simply put, lamellae may be considered as a pair of membranes containing chlorophyll. It is placed between the two primary cell walls of two plant cells and made up of intracellular matrix. 

8.2.2 STAGES OF PHOTOSYNTHESIS:

 Two stages:

8.2.3 LIGHT DEPENDENT REACTIONS: There are two types of photosystems, Photosystem II and Photosystem I. When a chlorophyll molecule absorbs light, the energy from this light raises an electron within the chlorophyll molecule to a higher energy state. The chlorophyll molecule is then said to be photo activated. Excited electron anywhere within the photosystem are then passed on from one chlorophyll molecule to the next until they reach a special chlorophyll molecule at the reaction center of the photosystem. This special chlorophyll molecule then passes on the excited electron to a chain of electron carriers. 

 Starts within Photosystem II. As this excited electron passes from one carrier to the next it releases energy. This energy is used to pump protons (hydrogen ions) across the thylakoid membrane and into the space within the thylakoids. Then this forms a gradient. Protons travel across the membrane down the concentration gradient, but to do so the must pass through ATP synthase. This is located in the thylakoid membrane. The energy used to move protons down the concentration gradient is used to synthesis ATP from ADP and inorganic phosphate. The synthesis of ATP is called non-cyclic photophosphorylation. Photosystem I then accepts the electrons from the chain of electron carriers. These electrons replace electrons previously lost from Photosystem I. Photosystem I then absorbs light and becomes photo activated. The electrons become excited again as they are raised to a higher energy state. These excited electrons then pass along a short chain of electron carriers and are eventually used to reduce NADP+ in the stroma.  NADP+ accepts two excited electrons from the chain of carriers and one H+ ion from the stroma to form NADPH. In addition to producing NADPH, the light dependent reactions also produce oxygen as a waste product.  

Summery:

·      Light energy is converted into chemical energy.

▪       Chlorophyll molecules are attached to the thylakoid membranes.

▪       They are often associated with accessory pigments and other proteins to form Photosystem.

▪       At the center of all photosystem are forms of chlorophyll a each of which is specialized to absorb a particular wavelength of light.

▪       Electrons within the chlorophyll absorb the energy from photons and this raises them to higher 'excited' states.

Excited electrons are more easily lost from the chlorophyll, which is a form of oxidation

8.2.5 LIGHT INDEPNDENT REACTONS: occurring in the stroma of the chloroplast and involve the conversion of carbon dioxide and other compounds into glucose. The light-independent reactions can be split into three stages; these are carbon fixation, the reduction reactions and finally the regeneration of ribulose bisphosphate. Collectively these stages are known as the Calvin Cycle. 

First stage: carbon fixation- The single carbon in carbon dioxide is first trapped by Ribulose bisphosphate (5C) to form a 2 molecules of Glycerate-3-phosphate (GP).  The product of the reactions is a 6-carbon intermediate, which is unstable and immediately splits in half to form two molecules of 3- phosphoglycerate

Second stage: reduction- each molecule of 3-phisphogycerate recives an additional phosphate group from ATP, becoming 1,3 bisphosphoglycerate. NADPH reduce the carboxyl group. Which store potential energy. G3P is a sugar- the same three-carbon sugar formed by glycolysis. For every three molecules of carbon dioxide there are six molecules of G3P. But only one molecule of this three- carbon sugar can be counted as a net gain of carbohydrate.  One molecule exits the cycle to be used in the plant.

Third stage: regeneration- In a complex series of reactions, the five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. For this to happen uses three more molecules of ATP. Now the RuBP is prepared to receive carbon dioxide again then the cycle happens again.

-       The regeneration of RuBP is essential for carbon fixation to continue. Five triose phosphate molecules will undergo a series of reactions requiring energy from ATP, to form three molecules of RuBP. RuBP is therefore consumed and produced during the light-independent reactions and therefore these reactions form a cycle, called the Calvin cycle.

8.2.4 PHOTOPHOSPHORYLATION

• As the electrons (released from chlorophyll) cycle through the electron transport chains located on the thylakoid membrane, they lose energy
• This free energy is used to pump H⁺ ions from the stroma into the thylakoid
• The build up of protons inside the thylakoid creates an electrochemical gradient (or proton motive force)
• The H⁺ ions return to the stroma via the transmembrane enzyme ATP synthase, which uses the potential energy from the proton motive force to convert ADP and an inorganic phosphate (Pi) into ATP
• This process is called chemiosmosis

8.2.6 STRUCTURE AND FUNCTION OF THE CHLOROPLAST 

(for more detail refer to 8.2.1)

-Function- To capture light energy, which is stored in ATP. This is all possible by the

A pigment called chlorophyll; chlorophyll absorbs the energy from sunlight and utilize this energy to synthesize food from carbon dioxide and water. Chloroplast is involved in the photosynthesis process of the plants.

8.2.7 ACTION SPECTRUM AND ABSORPTION SPECTRUM.

-Absorption spectrum: provides us clues to relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplast only if it is absorbed.  (Wavelengths are absorbed and to what extent.)

-The three curves show the wavelengths of light best absorbed by three types of pigments extracted from chloroplast. 

-Action spectrum: Shows the relative performance of different wavelengths. An action spectrum illuminates chloroplasts different colors of lights and then plotting wavelengths against some measure of photosynthetic rate such as carbon dioxide consumption or oxygen release.  (Wavelengths of light can actually be used to make photosynthesis work.)

-This graph plots the effectiveness of different wavelengths of light during photosynthesis. The peaks of the action spectrum are broader than the peaks in the absorption spectrum for chlorophyll and the valley is narrower and not as deep. 

8.2.8 LIMITING FACTORS ON THE RATE OF PHOTOSYNTHESIS

-Light: Gradually the rate falls of and at a certain light intensity the rate of photosynthesis stay constant.

-Temperature: The higher the temperature then the rate of photosynthesis goes up. But when temperatures above 40°C the rate slows down. This is because the enzymes involved in the chemical reactions of photosynthesis are temperature sensitive and destroyed at higher temperatures.

-Carbon dioxide: The rate of photosynthesis increases linearly with increasing carbon dioxide concentration. Gradually the rate falls at a certain carbon dioxide concentration the rate of photosynthesis stays constant.