Light Independent Reactions

The path of carbon in photosynthesis

The steps of the light-independent reactions

The experiments of Melvin Calvin’s team established the details of the path of carbon from carbon dioxide to glucose

Nobel Prize in 1961Occurs in the stroma (the fluid that

surrounds the thylakoids) They showed that …

The steps

The first product of the fixation of carbon dioxide is glycerate 3-phosphate (GP). This is the fixation step

The initial product is immediately reduced to the 3-carbon sugar phosphate, triose phosphate, using NADPH + H and ATP This is the reduction step 2 triose phosphate molecules can combine to

form hexose phosphate which can combine to form starch

The steps (continued)

Then the triose phosphate is further metabolized to produce carbohydrates (sugars, sugar phosphates, starch) and later lipids and amino acids This is the product synthesis step

Some of the triose phosphate is metabolized to produce the molecule that first reacts with carbon dioxide (the acceptor molecule This is the regeneration of acceptor step The reactions of this regeneration process are known as

the Calvin Cycle

Which intermediate is the actual acceptor molecule?

The acceptor molecule for carbon dioxide is a 5-carbon acceptor (ribulose biphosphate or RuBP)

When carbon dioxide has combined the 6-carbon product immediately splits into two 3-carbon GP molecules

The enzyme involved is called ribulose biophosphate carboxylase (Rubisco) Rubisco is the most common protein of green plant

leaves

Regeneration of RuBP

As RuBP is both consumed and produced, these reactions form a cycle Calvin Cycle

For the Calvin cycle to continue indefinitely, as much RuBP must be produced as consumed If 3 RuBP molecules are used, six triose phosphates are

produced Five of these are needed to regenerate the three RuBP

molecules, leaving just one triose phosphate for the conversion of photosynthesis products

To produce one molecule of glucose, 6 turns of the cycle are needed

The steps of carbon in photosynthesis

Summary of light independent reactions

Animations

http://www.sinauer.com/cooper/4e/animations0305.html

Glyceraldehyde 3 phosphate is the same as triose phosphate

Chloroplast structure and function

Table 9.4 identifies how the structure of the chloroplast facilitates function

Structure Function/Role

Double membrane

Containment, permeable to CO2, O2, ATP, sugars

Photosystems arranged on thylakoid

Huge surface area for maximum light absorption

Thylakoid spaces

Restricted regions for accumulation of protons and est. gradient

Fluid stroma with loosely arranged thylakoid membranes

Site of all the enzymes of fixation, reduction and regeneration of acceptor steps