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Photosynthesis - light reaction (redirected from Photosynthesis)

Page history last edited by Charles Forstbauer 14 years, 4 months ago

Totaled and closed 11/12 Mr F

 

Totaled 11/09 Mr F 

Totaled 11/06 Mr F

OK, it happened again.  After you make your edits - check the page! If it is messed up go to the page history and restore it to before your edits. Mr F.   PK & Lauren your stuff is gone. (you should look at the page 1st. If it doesn't look right check it out before you add stuff)

 

Totaled 11/03 Mr F

 

http://hawaii.hawaii.edu/laurab/generalbotany/images/photosynthesis.gif

 

The exergonic-A chemical reactions that releases energy. light-dependent reactions of photosynthesis convert light energy into chemical energy, producing ATP and NADPH. These reactions occur in the thylakoids of the chloroplasts. The products of the light-dependent reactions, ATP and NADPH, are both required for the endergoniclight-A chemical reactions that requires energy.independent reactions.

 

 

Adenosine-Triphosphate (ATP) is a multifunctional nucleotide that plays an important role in cell biology as a coenzyme, that is, the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism. It is an energy source produced during photosynthesis and cellular respiration and consumed by many enzymes and a multitude of cellular processes, including biosynthetic reactions, motility, and cell division. ATP is made from adenosine diphosphate (ADP) or adenosine monophosphate (AMP) and its use in metabolism converts it back into these precursors. ATP is therefore continuously recycled in organisms, with the human body turning over its own weight in ATP each day.

 

Nicotinamide Adenine Dinucleotide Phosphate (NADPH)

IN PLANTS- In chloroplasts, NADP is reduced by ferredoxin-NADP+ reductase in the last step of the electron chain of the light reactions of photosynthesis. The NADPH produced is then used as reducing power for the biosynthetic reactions in the Calvin cycle of photosynthesis.

IN ANIMALS- The oxidative phase of the pentose phosphate pathway is the major source of NADPH in cells, producing approximately 60% of the NADPH required.

 

 

 

Each antenna complex is able to trap light and transfer energy to a complex of chlorophyll molecules and proteins called the reaction center.As photons are absorbed by chlorophyll and accessory pigments, that energy is eventually transfered to the reaction center where, when absorbed by an excitable electron, moves it to a higher energy level. Here the electron may be accepted by an electron acceptor molecule of an electron transport chain where the light energy is converted to chemical energy by chemiosmosis-The production of ATP utilizing the energy released when hydrogen ions flow through an ATP synthase complex.

 

Thylakoids are specialized cell membrane structures in which photosynthesis takes place. They are made up of huge packs of chlorophyll molecules called 'photosystems'. These groups of chlorophyll molecules are where the photosynthesis occurs in the thylakoid. Prokaryotes have only one type of photosystem: Photosystem I. Eukaryotes have two types of photosystems: Photosystem I and II. Inside these photosystems the suns energy is converted by the photoreceptive pigment chlorophyll into the bonds of glucose.  

 

This visual may help you understand the ideas discussed in the following explanation of the light dependent reaction:

http://student.ccbcmd.edu/~gkaiser/biotutorials/photosyn/images/u4fg44.jpg

     Photosynthesis begins when pigments in photosystem II absorb light.  Energy from the light is absorbed by electrons, which increases their energy level, these are now sent on to the electric transport chain.  The electrons will not run out because the thylakoid membrane contains a system that provides new electrons to chlorophyll to replace the ones it has lost.  Enzymes break up water into 2 H+ ions and an oxygen atom.  They replace chlorophyll's lost electrons.  The oxygen is released as a waste product.  Then, energy from electrons moving through the electron transport system is used to transport H+ ions from the stroma to the inner thylakoid.  Pigments in photosystem I use energy from light to reenergrize the electrons, which are then picked up by NADP+ which also picks up H+, therefore becoming NADPH.  The inside of the thylakoid membrane will then become positively charged and the outside negatively charged, which will provide the energy to make ATP.  Since H+ ions cannot cross the membrane directly, ATP synthase (enzyme) helps out by allowing them to spin through, since the protein rotates like a turbine.  During the rotation, the enzyme binds ADP and a phosphate group together to produce ATP. 

 

Factors Affecting Photosynthesis

  • Because water is such an important component of photosynthesis, a shortage of water can slow or even stop a reaction
  • Plants that live in dry conditions have waxy coatings on their leaves that reduce water loss
  • Temperature is also a factor that affects photosynthesis. Photosynthesis is dependent on temperature.  As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops.
  • Photosynthesis depends on enzymes which funtion best between 0C and 35C
  • As we learned in the previous unit, if temperatures change then the enzymes loose their shape which means they loose their function
  • At very low temperatures photosynthesis can stop entirely
  • The intensity of light also affects the rate of photosynthesis
  • Increasing light intensity increases the rate of phototsynthesis
  • After the light intensity reaches a certain level, the plant reaches its maximum rate of photosynthesis
  • This level changes from plant to plant
  • The wavelength of light being radiated onto a plant affects the rate of photosynthesis. For example, green light slows the rate of photosynthesis, as the wavelength of green light is reflected (not absorbed) by the plant. Blue light is the most effective wavelength of light to use as it is the most absorbed wavelength of light
  • Carbon dioxide concentration affects the rate at which photosynthesis occurs, as it increases the rate at which sugars are produced in a light-independent reaction as the concentration itself increases
  • Whichever factor is in the most limited supply, or the limiting factor, affects the productivity of the reaction more than the amount of the other reactants in the reaction
  • The concentration of Chlorophyll also affects the rate of photosynthesis, as the rate of light being absorbed would change if the amount of chlorophyll present changed
  • Pollution also affects photosynthesis to some degree, as soot in the air can block stomata, and reduce the transparency of leaves
  • A limited amount of water in a reaction would also limit the quantity of carbon dioxide present in the reaction, limiting the rate of photosynthesis

 

The importance of Photosynthesis Video:

 

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          This video is a very good video.  It shows what reaction takes place in a cell very well.  The video shows down to every molecule.  It starts with light hitting the cell.  After that it shows the reactions that take place step by step.  It even shows the hydrogen molecules and what role they play and how they do it.  This is a very informative video and is informational.

 

This video: http://www.youtube.com/watch?v=HrMka8vj73U provides a good explanation of the light dependant reaction of photosynthesis.  Specifically, it helps with the understanding of what exactly happens involving light energy and chlorophyll at the beginning of the reaction.

 

Here is another great video animation about the process of a light dependent reaction.  It's very informative and the animations are great!

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The light-dependent reactions take place on the thylakoid membrane inside a chloroplast. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes which catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem I (PSI), Photosystem II (PSII), Cytochrome b6f complex and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.

 The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. When a special chlorophyll molecule of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis various electron donors are used.

 

A very important stage that happens during light dependent reaction is the spliting of the water molecules.  This is important because the electrons lost during photosystem 2 trapt molecule.  This step is critical because it creates hydrogen ions for the gradient. 

 

The following link shows a great animation on the four stages of the light reaction during photosynthesis. If you are having trouble trying to imagine how the light reaction works, this really helps vizualize it.

http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html

 

Light dependent reactions ALWAYS require the direct energy of light to make energy carrier molecules that are used in the second phase of photosynthesis - light independent reactions. Here is an easy to read diagram that demonstrates the light dependent reaction that takes place in every plant.

 

 

 

Stages of Photosynthesis:

Photosynthesis is a 2 stage process:

1. The firist process is the Light Dependent Process (Light Reactions)

     - it requires direct energy of light to make energy carrier molecules that are used in the second process

     - light reactions occur in the grana

2. Light independent Process (or dark reaction)

     - occurs when the products of the light reaction are used to form C-C covalent bonds of carbohydrates

     - Dark Reactions take place in the stroma of the chloroplasts

     - The Dark Reactions can occur in the dark, if the energy carriers from the light process are present

 

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This video has a very good discussion of photosynthesis. It is students from an AP class so it is easy to understand and also very well done. (the fisrt 50 seconds of the video is this video game thing which isnt useful so just skip to over that to get to the actual dicussion.) 

 

 

 

This a great 3-D diagram of the chloroplast and its parts. The 3-D provides a clearer way of seeing the inside of the organelle. It is helpful because you can clearly see how the thylakoids are the individual pieces that form the granum.

 

detailed pictorial presentation of photosynthesisI really like this diagram, it shows both the light and dark reactions. I got it from this really helpful website: tutorvista.com The site gives good info on photosynthesis. The site is great if anyone is confused and needs help with light/dark reactions.

 

To view a detailed animation on light reactions, visit http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html . The animation shows four steps to light reactions, broken down into "Electron flow to NADP+", "Splitting Water", "Pumping protons (H+)", and "ATP Production". You can see what exactly is happening in the thylakoids of chloroplasts during light reactions. With each animation, there is a short description to the right explaining that particular step.

 

*This is a video podcast discussing photosynthesis.  This really helped me understand everything that I was confused about because the guy that does the podcast has an arrow pointer that he uses to show the direction of the reactants and products.  He also discusses both the light dependent reaction and the light independent reaction.  This also has some of the visuals Mr.F used in class. 

 

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 This is a straight forward equation for the light reactions. It shows the reactants, products and where the reaction occurs. Very Useful!!

 

 

Non-Cyclic and Cyclic Energy Transfer

 

Non-cyclic energy transfer:

Once light is absorbed by pigments in the chloroplast, its energy is transferred to one of two types of reaction centers, Photosystem-II (PS-II) or Photosystem-I (PS-I).

 

In non-cyclic energy transfer, light absorbed by PS-II splits a water molecule, producing oxygen and exciting chlorophyll to a higher energy level. Then, the excitation energy passes through a series of special electron carriers. Each electron carrier in the series is slightly lower in energy than the previous one. During electron transfer, the excitation energy is harnessed to synthesize ATP. This part of photosynthesis is referred to as non-cyclic photophosphorylation, where "photo-" refers to the light requirement and "-phosphorylation" refers to addition of a phosphate to ADP (adenosine diphosphate) to make ATP.

 

Finally, one of the electron carriers of PS-II transfers electrons to PS-I. When chlorophyll transfers its excitation energy to PS-I, it is excited to higher energy levels. PS-I harnesses this excitation energy to make NADPH, analogous to the way PS-II harnessed excitation energy to make ATP.

 

In the 1950s, the botanist Robert Emerson (1903-1959) demonstrated that the rate of photosynthesis was much higher under simultaneous illumination by shorter wavelength red light (near 680 nm) and long wavelength red light (near 700 nm). We now know this is because PS-II absorbs shorter wavelength red light (680 nm) whereas PS-I absorbs long wavelength red light (700 nm) and both must be photoactivated to make the ATP and NADPH needed by the dark reactions.

 

Cyclic energy transfer:

ATP can also be made by a special series of light reactions referred to as cyclic photophosphorylation. This also occurs in the thylakoid membranes of the chloroplast. In cyclic photophosphorylation, the excitation energy from PS-I is transferred to a special electron carrier and this energy is harnessed to make ATP.

 

The relative rates of cyclic and non-cyclic photophosphorylation determine the ratio of ATP and NADPH which become available for the dark reactions. Photosynthetic plant cells regulate cyclic and non-cyclic energy transfer by phosphorylating (adding a phosphate) to the pigment-protein complexes associated with PS-I and PS-II.

 

 

 

Location of light reactions:

 

In higher plants and algae, the light reactions occur on the thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts which are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. A stack of thylakoids is referred to as a granum.

 

The light reactions of higher plants require photosynthetic pigments, chlorophyll-a, chlorophyll-b, and various types of carotenoids. These pigments are associated with special proteins which are embedded in the thylakoid membranes. Chlorophyll-a and chlorophyll-b strongly absorb light in the red and blue regions of the spectrum. Most carotenoids strongly absorb blue light.Thus, plant leaves are green simply because their photosynthetic pigments absorb blue and red light but not green light.

 

 

This diagram explains what occurs during a Light Dependent Reaction

 

http://www.wiziq.com/tutorial/6486-Light-and-Dark-reactions-in-photosynthesis video completely explained to me the similarities and differences is light and dark reactions in a quick summary 

http://www.cst.cmich.edu/users/baile1re/bio101fall/enzphoto/photoanima.htm: this link shows a great animation of the light dependent part of the reaction, it shows the steps the electrons take to become a NADPH energy carrier. Watch!

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