What exactly IS photosynthesis? By Megan Whitley

Transcription

1 What exactly IS photosynthesis? By Megan Whitley

2 Formal Definition Photosynthesis can be defined as the physico-chemical process by which photosynthetic organisms use light energy to drive the synthesis of organic compounds.

3 What photosynthesis really is: Plants use sunlight to make food. In scientific terms, converting light energy into chemical bond energy. The plant is trying to make glucose, its food source. Summary Reaction: 6 CO2 + 6 H2O C6H12O6 + 6 O2

4 The Leaf The leaf is the primary photosynthetic organ of typical land plants. The leaf has an extensive surface area exposed to the sun in order to absorb the maximum amount of light (there are, of course, adaptive differences among plants) - There are several important features of leaves: - epidermis: found on upper and lower surface, contains guard cells that control the opening and closing of stomata, where diffusion of CO2 and O2 occurs

5 The Leaf, cont d - The major portion of the leaf is the mesophyll, which actually contains the photosynthetic cells, which are the parenchyma cells. - Each parenchyma cell has lots of chloroplasts. The chloroplast is the cellular organelle where photosynthesis akes place. - chloroplasts have double membranes - there are many thylakoids (membranous sacs) inside - a stack of thylakoids is a granum (sing. grana) - granums may be attached by a lamella - the grana are in liquid-filled compartments called the stroma

6 Let s clear that up a bit Chlorophyll molecules embedded in the thylakoid membranes give the leaf its green color

7 Pigments Photosynthetic pigments are embedded in the thylakoid membranes. chlorophyll a: primary photo pigment -absorbs blue and red wavelengths of light -must be present to carry out photosynthesis light chl a electrons boosted to high energy levels release of electrons starts photosynthesis accessory pigments: chlorophyll b, carotenes, xanthophylls -absorb different wavelengths of light

8 So how do these Pigments work? There are electrons in these pigments that absorb light energy. When light is absorbed, the electrons get boosted from their ground state to their excited state (e-!) through inductive resonance. When the electrons of chlorophyll a get excited (e-!), they actually get released from the molecule. This starts photosynthesis. This is why chl a is called the primary photo pigment The accessory pigments help to absorb more light energy and pass it along to chlorophyll a (through inductive resonance).

9 Now it starts to get tricky Let s remember: Summary Reaction: 6 CO2 + 6 H2O C6H12O6 + 6 O2 The total photosynthetic process is made up of two series of reactions: -light reactions (require light energy -dark reactions (require chemical energy)

10 Scary, huh? Let s break it down We ll start with the Light Reactions

11 Light Reactions These reactions take place in the thylakoid membrane and require light energy to take place. There are two photosystems, photosystem I (PSI) and photosystem II (PSII). In each of these photosystems, there is a reaction center made up of two chlorophyll a molecules. These reaction centers are called P680 (PSII) and P700 (PSI). There is an electron acceptor (EA) embedded in the thylakoid membrane. When the two chl a molecules in the P680 reaction center absorb light energy, each transfers a high energy electron (e-!) to an EA.

12 Light Reactions, cont d Meanwhile, in P680 (PSII), an enzyme is actively splitting water molecules (H2O). Splitting in the presence of light is called photolysis. The bond is broken between the O atom and the H atoms. <H2/O> The electrons go to the chl a molecules and the two H+ atoms are deposited in the thylakoid interior (lumen). The oxygen atoms combine to form O2. That first EA that receives the (e-!) from chl a transfers these electrons to two more electron acceptors. There is a loss of energy with each transfer.

13 Light Reactions, cont d Some of the lost energy has been spent pumping 2 H+ from the stroma into the lumen. Now, let s focus on PSI, moving down the chain. The chl a molecules in the P700 (PSI) reaction center absorb light energy and release (e-!) to another type of EA. This provides vacancies that are filled by the two electrons flowing out of PSII. The electrons that are released from P700 (PSI) reaction center go on to reduce a molecule of NADP, using 2 H+ from the lumen: NADP NADPH + (H+)

14 Light Reactions, cont d Let s visualize this: First, -two chl a molecules absorb light energy and transfer an (e-!) to an EA -water is split, producing oxygen molecules and H+ in the interior of the thylakoid -electrons are transported through a series of EAs (while protons are being pumped into the lumen)

15 Light Reactions, cont d Next, -these electrons (from PSII) enter PSI and are absorbed by the chl a molecules -the chl a molecules release (e-!) to another type of EA -the electrons released from PSI cause the reduction of NADP to NADPH and H+, consuming 2H+ from the lumen

16 Light Reactions, cont d The orderly flow of electrons is the key to the photosynthetic process. These are the results of this electron flow: -NADPH, which is used in glycolysis in dark reactions, is produced -a powerful electrochemical gradient is created by the build-up of H+ ions in the lumen ~ at the same time the stroma becomes negatively charged The system wishes to achieve equilibrium and therefore the H+ ions in the stroma flow through an enzyme complex, ATP Synthase. This transfer provides the energy for the reaction ADP+Pi ATP to occur. The light reaction produces NADPH and ATP and O2.

17 Light Reactions, cont d A few more visuals ATP Synthase and how it uses the electron pump to create ATP Summary of the light reactions:

18 Dark Reactions These reactions take place in the stroma and on the thylakoid surfaces facing the stroma. It is essential these reactions follow the light reactions because they require NADPH and ATP. Dark reactions don t have to take place in the dark, they can take place without the presence of light. They convert the chemical bond energy of NADPH and ATP into the chemical bond energy of glucose. The most common pattern of the dark reactions is the Calvin Cycle.

19 Dark Reactions, cont d The Calvin Cycle is just a cycle in which the same reactions are repeated over and over. There must be some starting material already present and continually regenerated in order for this to occur. Our starting material is a five carbon (5-C) sugar. The Calvin Cycle can be divided into three phases. In phase I, I carbon fixation occurs. Carbon dioxide molecules enter the leaf mesophyll through the stoma and go into the parenchyma cells. The CO2 molecule binds to the 5-C sugar aided by an enzyme. This makes a six carbon sugar that is highly unstable so it immediately splits into two three carbon molecules.

20 Dark Reactions, cont d In phase 2, 2 which is the reduction portion of the cycle, the 3-C molecules are phosphorylated by ATP and receive electrons from NADPH. This chemical reduction makes two glyceraldehyde 3- phosphate(g3p) molecules (3-C each). Some of these G3P molecules are the products of the cycle, but others stay in the cycle. In phase 3, 3 which is regeneration of the starting product, G3P molecules react to form molecules of the starting product. The cycle is ready to run again. Two molecules of G3P are bonded in an enzymatic reaction to make glucose, the end molecule (of photosynthesis) and food supply for the plant.

21 Dark Reactions, cont d A visual of the Calvin Cycle:

22 And there you have it