Chapter 17 Reactions Photosynthesis

Photosynthesis is the process by which most of the energy in the biosphere

is produced. Most of the carbon dioxide and the production of oxygen in

the atmosphere results from photosynthesis. The overall reaction of

photosynthesis was 6CO2 + 6H2O --> C6H12O6 + 6O2 in the presence of light.

Photosynthesis consists of a set of light and dark reactions. In the

light reactions, the energy from sunlight is utilized in the photchemical

reduction of H2O so that oxidizing agents NADP+ is reduced to NADPH. Dark

reactions occurs all the time and not just the dark. NADPH + ATP produced

in light reactions lead to the reductive synthesis of carbohydrate from

CO2 and H2O.

The site of photosynthesis is the chloroplast, which is under the surface

of the leaf. Chloroplasts are semi-autonomous with its own DNA for

someproteins and ribosomes. The light reactions occur within or on the

thylakoid membrane and the dark reactions occur within the stroma. The

light reactions consist of two steps, photosystem I and photosystem

II. Photosystem II oxidizes water and Photosystem I reduces

NADP+. Together these two systems create a pH gradient across the

chloroplast membrane which drives ATP production. The dark reactions

consist of the Calvin cycle which is divided into two stages. The first

stage uses CO2 and ribulose-1,5-bisphosphate wich can be cleaved and

reduced to form trioses which lead to hexoses. The second stage uses

these trioses and hexoses to regenrate RuBP.

Paragraph on Ch. 17

Photosynthesis is the other half of energy-carbon cycle. The reverse of

carbohydrate oxidation is done by plants, algae, etc using energy from

sunlight. Photosynthesis is the major path through which carbon reenters

the biosphere as well as the principal means for carbon fixation. The

proces as a while is divided into light and dark reactions...the dark

reactions can be done without light, but that is not a requirement. The

light reactions use the energy from the sun to break up water, extract

electrons as reductive potential, producing O2, and inducing a proton

gradiant (reminiscent of citric acid cycle). This proton gradient drives

ATP formation. The dark reactions reduce CO2 to carbohydrates. These

reactions take place in chloroplasts. Photons are absorbed by antenae

molecules and the energy is transported to reaction centers where it gets

sent to either photosystem I or II. These two systems work together to

carry out light reactions. Photosystem II oxidizes water whereas I reduces

NADP+.

The dark reactions are essentially the Calvin Cycle. The Calvin cycle is

also divided into two stages. First, CO2 is fixed into carbohydrate form,

and secondly the acceptor molecule is regenerated (RuBP).

In times of low CO2 and high O2 plants can engage in potorespiration. This

path as a whole is inefficient and thus some plants take part in the C4

cycle. The C4 cycle can take place more readily than the calvin cycle in

conditions of high oxygen concentration.

This chapter was all about photosynthesis and how it ties into the citric

acid cycle and the rest of metabolism. This was a big connecting piece

for me because now that we have learned how glucose is digested and broken

down and used to make ATP we finally learn how glucose is made from the

discarded products of the citric acid cycle and phosphorylation. This

chapter brought together many of the ideas we learned in physics, organic

chemistry and biology, and I found this useful after having taken all of

these courses because things are finally being tied together. The part of

this chapter that I found most interesting is the Dark Reactions of

photosynthesis. I remember hearing that they existed and that the

organisms were found in the deep sea, but I still find it fascinating that

photosynthesis can occur without sunlight. I would really like to hear

more about this phenomenon and learn how people are able to study this

process. The rest of the chapter was about the Calvin cycle, which

described dark reactions a bit, and photorespiration in plants. Yay

plants!

I remember photosynthesis being one of my favorite topics in my high

school AP biology class. I think that it is quite amazing that this

process is responsible for providing for life of all things, not just

plants. Photosynthesis is the process of converting carbon dioxide and

water to oxygen and sugars in plants and higher bacteria. This process

uses light energy, specifically visible light, in performing these

reactions. Photosynthesis has two different series of reactions, the light

and dark reactions. The light reaction uses sunlight energy to produce

NADPH and ATP, releasing oxygen. The dark reaction uses NADPH and ATP

produced to fix carbon dioxide as sugars. The dark reaction does not take

place in the dark, as its name suggests. In fact, both reactions work well

in sunlight and take place in organelles called chloroplasts. Chloroplasts

mirror mitochondria in much of its structure and function. Light

gathering/absorbing pigments in the chloroplasts are called chromophores,

and in total cover the wide spectrum of visible light. In the thylakoid

membranes of the chloroplast are light harvesting complexes and reaction

centers. These provide the photons necessary to drive the processes in

Photosystems I and II in the light reaction. These systems are linked in a

series, with photosystem II splitting water molecules to produce electrons

and oxygen, and photosystem I using those electrons to generate

NADPH. Both of these systems create a proton gradient similar to that of

the mitochondria, by pumping protons from the stroma into the lumen. When

the protons fall back through the CF0-CF1 complex, ATPs are made. Cyclic

electron flow occurs when NADPH is in excess, but ATP still should be

produced. The dark reaction, or Calvin cycle, first fixes CO2 by adding it

to ribulose-1,5-bisphosphate. Hexoses are formed and RuBP is regenerated

as the CO2 acceptor. Overall, the efficiency of photosynthesis is about

35%. Dark reactions can be regulated by the amount of light availble to

the organism, so sugars won't be produced in excess amounts when there is

no light to stimulate the light reactions. Under low CO2 conditions plants

also undergo a process that's called photorespiration, where O2 is

consumed. Some tropical plants use what is called the C4 cycle to have

more efficient because it is less sensitive to high oxygen levels.

This chapter looks at photosynthesis in plants. It begins by describing a

chloroplast and how it is possible to derive energy from sunlight. It

also explains that there are two portions to photosynthesis, the light

reactions, and the dark reactions which do not require photon's to be

absorbed to work. The chapter explains that light is absorbed by the

light absorbing chromophores in the chloroplast. These compounds absorb

mainly visible light, which excites their electrons. These electrons and

this energy then bump around inside the chloroplast until they are traped

in a reaction center, which is possible because the energy of its excited

state is lower than that of the other molecules. These electrons then

react with electron acceptors in reactions called Hill reactions, which

are driven by the energy from the absorbed photons. The book notes that

the ultimate source of these electrons is water molecules. The light

reactions of photosynethese are devided into two parts. The first part

discussed by the book is Photosystem II, which is responsible for

splitting water, this results in oxygen being released outside of the

chloroplast. This part of the light reaction also generates a proton

gradiaent, which makes ATP synthesis possible. In Photosystem 1 NADPH is

generated which is the ultimate electron acceptor. I didn't understand

this part of the reaction very well and I had trouble following the

pathway the electrons take. The book then presetns an alternative light

reaction whichis called cyclic electron flow, which is used when NADP+ is

scarce. The book then describes photosynthesis in bacteria. The dark

reactions of photosynthese are also called the Calvin cycle, named after

the descoveror. The dark reactions do not require light themselves,

however, the energy and products generated by the light reactions are

required,such as ATP and NADPH, s o the dark reactions are stimulated by

light

absorbtion. These reactions result in the incorperation of CO2 into

carbohydrates. In the first stage fof the carbon cycle sugars are made,

and in the seccont the electron acceptor ribulose-1,5-bisphosphate is

regenerated. The chapter ends by discussing the C4 cycle, which is an

alternative pathway carried out by plants subjected to hight levels of

light and heat. It helps compartmentalize the reactions of photosynthesis

so that dangerous species like O2- are not created. It works by converting

the C02 which enters the cell into oxaloacetate, which then travels to a

bundle sheath cell where the calvin cycle occures. The book notes that

this process is fairly inefficient.