Download Light Independent Reactions notes 3

Light Independent Reactions
• Does
require light
• Also known as the
• Occur in the
of the chloroplast
• Glucose is
which requires:
a. energy in the form of
and
(there has to be enough)
b. H since each glucose molecule has 12 H atoms
The Calvin-Benson Cycle
• The Calvin cycle
molecules enter and leave the cycle
its starting material after
• CO2 enters the cycle and leaves as
• The cycle spends the energy of ATP and the reducing power of electrons
carried by NADPH to make the sugar
• The actual sugar product of the Calvin cycle is not glucose, but a three-carbon
sugar,
• Each turn of the Calvin cycle
.
• For the net synthesis of
cycle must take place
molecule, the
times, fixing
.
• To make
glucose molecules would require
cycles and the fixation of
molecules.
• The Calvin cycle has three phases.
Light Independent
• Occurs in 3 stages:
• Step 1:
• RuBP (
joins with CO2 (catalyzed by
) to form an
6
carbon molecule which splits to become
(phosphoglyceric
acid)
)
Light Independent
• Step 2:
• The 3 carbon compounds are activated by
and then
by
• The molecule now become
•2
step 3
(given energy)
(given more energy)
molecules known as
molecules move on to make glucose,
go to
Light Independent
• Step 3:
• Remaining PGAL will be used to make
• ATP will help break and
bonds to make the
the chemical
Calvin Benson Cycle – more details
8
9
Let’s Put It All Together…
Photosynthesis Stores Energy in Organic
Compounds Review
• Photosynthesis consists of two separate sets of chemical reactions: lightdependent and light-independent reactions.
light-dependent
reactions
NADPH
ATP
chemiosmosis
light-independent
reactions
Photosynthesis Stores Energy in Organic Compounds Review
• Chlorophylls a and b and the carotenoids are photosynthetic pigments that
absorb light.
Photosynthesis Stores Energy in Organic Compounds Review
• Light energy trapped by a pigment
molecule excites electrons.
• When an electron in photosystem
II is excited, it is transferred to and
then passed along an electron
transport system.
Photosynthesis Stores Energy in Organic Compounds Review
• Energy released during electron transport is used to force hydrogen ions across the
thylakoid membrane and create a concentration gradient.
• Energy from the concentration gradient is used to generate ATP from ADP and phosphate
by means of chemiosmosis. As hydrogen ions move down their concentration gradient,
they drive the reaction that generates ATP.
Photosynthesis Stores Energy in Organic Compounds Review
• An electron from water replaces the
electron that was lost from
photosystem II. The oxygen from the
water molecule is converted to
molecular oxygen.
• When an electron from photosystem I is
excited, it is eventually used to reduce
NADP+ to NADPH.
Photosynthesis Stores Energy in Organic Compounds Review
• The series of reactions that synthesize carbohydrates is the CalvinBenson cycle, which occurs in the stroma.
• In this cycle, carbon dioxide combines with RuBP to form a sixcarbon compound that immediately
splits into two three-carbon compounds.
Photosynthesis Stores Energy in Organic Compounds Review
• ATP and NADPH from the light-dependent reactions
provide energy and reducing power to form PGAL from the
newly formed three-carbon compounds.
• Six cycles produce 12 PGAL molecules, 10 of which
regenerate RuBP and 2 of which are used to make glucose.