Question 7

7a) The carbon cycle invovles both the production and the fixation of carbon dioxide. Draw a labelled diagram to show the processes invovled in the carbon cycle. [5]

Reference [URL]: http://eo.ucar.edu/kids/green/cycles6.htm

7b) Explain the light-independent reactions of photosynthesis [8]

Light independent reactions occur in the stroma of the chloroplasts in a metabolic pathway called the calvin cycle.

Reference [URL]: http://www.bio.umass.edu/biology/conn.river/calvin.html



The cycle uses ATP and NADPH2 as an energy source. The first stage is carbon dioxide fixation. This is when carbon dioxide from the atmosphere joins with ribulose biphosphate (RuBp) this reaction is catayalsed by the enzyme Rubisco or RuBP carboxylase. It is the most abundant protein on earth. Then we get a 6 carbon compound which due to it’s instability then splits in half to form two molecules of phosphoglyerate (a  3 carbon molecule). Then this molecule is reduced in the second stage known as reduction using ATP and NADPH2 from the light dependent reaction to convert phosphoglycerate into glyceraldehyde 3 phosphate. This is the molecule that is a component of glucose and other sugars. The last phase is called the regeneration phase where ATP is used to convert some of the glyceraldehyde 3 phosphate back to RuBP thus completely the cycle. Note that for every 3 molecules of carbon dioxide (3 turns of the calvin cycle) one molecule of glyceraldehyde (G3P) is removed from the cycle. For each G3P produced the cycle uses 9 molecules of ATP and 6 molecules of NADPH2. One glucose molecule consists of two G3P molecules therefore 6 turns of the calvin cycle produced one molecule of glucose thus expending 18 molecules of ATP and 12 molecules of NADPH2.

Note that the light independent reaction although it doesn’t require light directly still needs light as the light dependent reactions provide the calvin cycle with the ATP and NADPH2.

7c) Outline the consequences of rising carbon dioxide concnetrations in the Earth's atmosphere. [5]

Gas produced by humans activity retain heat even more than the natural levels of greenhouse gases that retain some of the heat and trap it in the atmosphere. Gases such as carbon dioxide, methane and the oxides of nitrogen. Increased by increased activities such as burning fossil fuels and cattle ranching. A rise in human population resulting in the increase in these human activities and hence an increase in the concentration of these greenhouse gases. Since all 3 atmospheric gases have high potential for absorbing heat, these gases are responsible for intensifying the earth’s natural greenhouse effect.

Summary of impacts:

-increased photosynthetic rate

-extinction of certain species

-melting glaciers (land ice)

-a rise in sea level which would result in floods of coastal areas.

A case study for the impact would be human impact on artic ecosystems. Ecosystems are changing in other words intact ice which has the capacity to be the home for algae as they provide an area for the algae to attach. The surface gives the algae a place to attach and the transparency allows sunlight to pass through. As artic ice melt there is less and less surface for algae to stick to and there is a reduction in food for the next trophic level. As the algae is a producer which supports a lot of food chains and is one of the foundation for artic foodwebs.

Moreover, polar bears use ice for hunting. The technique is that they stand on the ice and swoop seals to eat when the seals jump. However, since there is less ice this technique becomes less productive and so there has been a reduction in successful catches resulting in the decrease in the number of polar bear cubs. Which the decrease in the polar bear population due to the rising carbon dioxide concentration has lead to changes in the population of other trophic levels as well such as the rise in the population of seals.

Also as temperature increased the habitat of organisms from ore temperature climate extend northwards. This is evident by mosquitoes and robins arriving in areas up north where they were never found before. New species arising is means that new pathogens may be introduced.

Also detritus which is frozen in the tundra will thaw and begin to decompose, releasing even more carbon dioxide gas and methane into the air.