Photosynthesis
Part II

Step 2 - Light-Independent
Reactions........
Glucose

.........Also known as The Calvin cycle

From Air by way of Stomata
From Step 1 –
Light-dependent
Reactions

The Calvin Cycle
• Called a cycle because the starting material is regenerated each time the process occurs
• Enzymes join carbon dioxide molecules with molecules that are already in the cell - these molecules will regenerate each time
• The energy from ATP and NADPH are used in this process

3 X 5-C molecules ( 15 C)
+ 3 CO
2
3 x 5-C molecules
6 x 3-C molecules (18 C)
5 x 3-C molecules 1X 3-C molecule product/leaves the cycle
2 of these make 1 glucose

Sugar Production
 With each turn of the Calvin cycle a few energy-rich carbohydrates leave the cycle
 These molecules become the building blocks that the plant cell uses to make sugars, lipids, amino acids and other compounds
 The plant will use these compounds as the food and building materials it needs to grow

Summary of Light Independent
Reactions

Calvin cycle uses ATP , NADPH and hydrogen from light dependent reactions and carbon dioxide from the air to make sugars

Plant uses the sugar to meet its energy needs and to build molecules needed for growth

When other living things eat these plants they too will get the energy they need .

Factors that Affect Photosynthesis
• The process can speed up or slow down depending on several factors
• Temperature: enzymes work best between 15-
35 degrees Celsius
• Light : very bright light speeds up photosynthesis
• Water levels

Water Loss and Photosynthesis
• Water is lost through the stomata where carbon dioxide enters
• If its hot the plants will close the stomata to keep from loosing water but photosynthesis will slow down due to the lack of carbon dioxide
• Some plants that live in dry areas have special ways to save water and carry out photosynthesis

C4 Plants
• Have a special chemical pathway that gets carbon dioxide into the Calvin cycle
• It uses extra ATP but it allows the plant to carry out photosynthesis when its hot
• Examples: Corn, sugar cane

CAM Plants
• Save water by taking air into their leaves only at night
• At night carbon dioxide is used to make acids
• During the day these acids are turned back into carbon dioxide for photosynthesis
• Example: Pineapples trees, desert cacti