c3 and c4 pathways

[ofgreenstar]

I'm in a human bone class and we were discussing c3 and c4 pathways in relation to determining diet in ancient people's. What is the actual process taking place? Why do they have distinctive qualities?

[gonelli]

are you refering to c3 and c4 pathways that are seen in different types of plants used during photosynthesis to produce sugars?

[ofgreenstar]

yes

[gonelli]

Well, plants that use a c3 pathway produce a 3-carbon sugar as their first stable molecule (from carbon dioxide). These kind of plants like to have a mild temperature environment. They have their stomata open for most of the day to capture CO2, but due to be open so long they lose some H2O from evaporation.

 

Plants that use the c4 pathway produce a 4-carbon sugar as the first stable molecule. These plants like hot temperatures. They have their stomata open for shorter periods during the day to stop the lose of off too much H2O because of the hot temperatures. The c4 process is much better at capturing CO2 from the air, so the plant can suvive with less "stomata open" time.

 

That's a fairly basic overview, the chemicals and enzymes involved do have names, but i don't know them off the top of head. If someone else knows them please post.

[ofgreenstar]

How do you tell if a plant is C3 or C4? Are there plants that can do both?

[Genecks]

Here's some information on plant anatomy:

 

http://en.wikipedia.org/wiki/C4_plants#C4_Leaf_Anatomy

[Paralith]

photosynthesis consists of two types of reactions. the light reactions are those that convert sunlight into chemical energy. the dark reactions are those that take that chemical energy and use it to fix atmospheric carbon into organic molecules. one of the primary enzymes used in the dark reactions is rubisco. the problem with rubisco is that it not only binds to CO2, but also to oxygen. If it binds to oxygen, no carbon fixation happens, the energy gain from the light reactions is essentially wasted.

 

open stomata bring in CO2, but let out water. You need to keep bringing in CO2, because if you don't the concentration will lower, and rubisco will start binding with oxygen more and more. But in hot and arid temperatures, you lose precious water too fast by keeping your stomata open all the time.

 

C4 plants adjust for this problem by conducting the light reactions and the dark reactions in separate areas. At night they have their stomata open, and whatever CO2 they absorb, they fix to organic acids - store it, in otherwords. And by having your stomata open at night, when it's cooler, you lose less water. During the day, stomata close, and light reactions go on throughout the leaf. The stored carbon dioxide moves into the bundle sheath, a rather thick, tightly sealed "tube" usually down the center of the leaf. This makes the CO2 concentration in the bundle sheath far higher than the oxygen concentration, so the dark reactinos can go on with little worry of oxygen binding to rubisco. Most grasses are C4.

 

I find wikipedia doesn't always have the clearest explanations, especially if you're not already familiar with the topic.

[Genecks]

To answer this question in a more specific manner, I suppose people would eat certain plants, which could have been C3, C4, or CAM. However, I believe this would depend on the natural habitat, ecosystem, and so forth. Sugar cane is a type of plant that people could eat, albeit it wasn't as sweet to some people. Also, some people would eat pineapples and other fruits, because they were available and well adapted to the environment. Maize is a C4 plant that people would often eat.

 

Regardless, however, sweet plants, such as corn, tended to create cavities in ancient persons because of the sugar content.