A3. Photosynthesis and the Light-independent Reaction

Question 1

The Calvin cycle
The light-independent reaction is also called the Calvin cycle. It takes place in the ________ of the ______________. It makes a molecule called _________ ______________from carbon dioxide (CO2) and ___________bisphosphate (a 5-carbon compound). Triose phosphate can be used to make ________and other useful organic substances. There are a few steps in the cycle, and it needs ____and __ ions to keep it going. The reactions are linked in a cycle (see Figure 1), which means the starting compound, ribulose bisphosphate, is _______________.

Answer 1

The Calvin cycle
The light-independent reaction is also called the Calvin cycle. It takes place in the stroma of the chloroplasts. It makes a molecule called triose phosphate from carbon dioxide (CO2) and ribulose bisphosphate (a 5-carbon compound). Triose phosphate can be used to make glucose and other useful organic substances. There are a few steps in the cycle, and it needs ATP and H+ ions to keep it going. The reactions are linked in a cycle (see Figure 1), which means the starting compound, ribulose bisphosphate, is regenerated.

Question 2

The Calvin cycle - 1. Formation of ____________ _-______________

____enters the leaf through the stomata and diffuses into the stroma of the chloroplast. Here, it’s ______________with _____________ ________________(_____). This reaction is catalysed by the enzyme ____________. This gives an unstable 6-carbon compound, which quickly breaks down into ____molecules of a _-carbon compound called _____________ _-_____________(__).

Answer 2

CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. Here, it’s combined with ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme rubisco. This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP).

Question 3

The Calvin Cycle - 2. Formation of ________ _____________

The ______________of ___(from the ____-______________ ___________) provides _________to __________the _-carbon compound, __, to a different _-carbon compound called ____________ ____________(__). This reaction also requires __ ions, which come from _____________ ______(also from the _______-______________ ___________). Reduced NADP is recycled to ______. Some triose phosphate is then converted into useful organic compounds (e.g. glucose) and some continues in the Calvin cycle to ____________ ______.

Answer 3

The hydrolysis of ATP (from the light-dependent reaction) provides energy to reduce the 3-carbon compound, GP, to a different 3-carbon compound called triose phosphate (TP). This reaction also requires H+ ions, which come from reduced NADP (also from the light-dependent reaction). Reduced NADP is recycled to NADP. Some triose phosphate is then converted into useful organic compounds (e.g. glucose) and some continues in the Calvin cycle to regenerate RuBP.

Question 4

The Calvin Cycle - 3. Regeneration of ribulose bisphosphate
_____out of every six molecules of __produced in the cycle aren’t used to make useful organic compounds, but to ___________ _____. _____________ _____uses the rest of the ____produced by the _______-___________ __________.

Answer 4

Five out of every six molecules of TP produced in the cycle aren’t used to make useful organic compounds, but to regenerate RuBP. Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction.

Question 5

Tip: It’s really important that _____ is ______________. If it wasn’t then ____________ _-______________wouldn’t be formed, the Calvin cycle would stop and photosynthesis would be unable to continue.

Answer 5

Tip: It’s really important that RuBP is regenerated. If it wasn’t then glycerate 3-phosphate wouldn’t be formed, the Calvin cycle would stop and photosynthesis would be unable to continue.

Question 6

Exam Tip - If you’re asked in the exam to work out how many turns of the Calvin cycle are needed to produce a certain number of hexose sugars you need to remember that _____out of every six TP molecules are used to regenerate RuBP.

Answer 6

Exam Tip - If you’re asked in the exam to work out how many turns of the Calvin cycle are needed to produce a certain number of hexose sugars you need to remember that five out of every six TP molecules are used to regenerate RuBP.

Question 7

Figure 3: Six turns of the Calvin cycle.

Answer 7

Question 8

Carbohydrates, lipids and proteins

• Carbohydrates - hexose sugars are made from two __ molecules and larger carbohydrates are made by joining hexose sugars together in different ways.
• Lipids - these are made using glycerol, which is synthesised from __, and fatty acids, which are synthesised from __
• Amino acids - some amino acids are made from __.

Answer 8

• Carbohydrates - hexose sugars are made from two TP molecules and larger carbohydrates are made by joining hexose sugars together in different ways.
• Lipids - these are made using glycerol, which is synthesised from TP, and fatty acids, which are synthesised from GP
• Amino acids - some amino acids are made from GP

Question 9

Tip: The Calvin cycle can be summarised
as follows: Inputs …
Outputs…

Answer 9

Inputs - ATP, Reduced NADP, CO2
Outputs - Organic Substances, RUBP