Chapter 6

Defined as--are series of chemical reactions occurring within a cell .
• In each pathway, a principal chemical is modified by chemical reactions .
• Because of the many chemicals that may be involved, pathways can be quite elaborate

Defined as--is a step-by-step modification of the initial molecule to shape it into another product. The result can be used in one of three ways:
• To be stored by the cell
• To be used immediately, as a metabolic product
• To initiate another metabolic pathway, called a flux generating step.

• Process by which plants and other autotrophs store the energy of sunlight into sugars.
• Requires sunlight, water, and carbon dioxide.
• Overall equation:
6 CO
2
+ 6 H
2
0  C
6
H
12
O
6
+ 6 O
2
• Occurs in the leaves of plants in organelles called chloroplasts.

• Inner membrane called the thylakoid membrane.
• Thickened regions called thylakoids . A stack of thylakoids is called a granum .
(Plural – grana)
• Stroma is a liquid surrounding the thylakoids.

• is a material that changes the color of reflected or transmitted light as the result of wavelength absorption.

• is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye .
• this range of wavelengths is called visible
light or simply light .
• Primary colors include R O Y G B I V

• Chlorophyll A
• chlorophyll B
• Accessory pigments:
– Carotenoids
• Each pigment absorbs a particular wavelength of light in the visible spectrum

• Chlorophyll A is the most important photosynthetic pigment.
• Other pigments called antenna or accessory pigments are also present in the leaf.
– Chlorophyll B
– Carotenoids (orange / red)
– Xanthophylls (yellow / brown)
• These pigments are embedded in the membranes of the chloroplast in groups called photosystems .

Photosynthesis: The Chemical Process
• Occurs in two main phases.
– Light reactions
– Dark reactions (aka – the Calvin Cycle)
• Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments.
• Dark reactions are the “synthesis” part of photosynthesis. Trapped energy from the sun is converted to the chemical energy of sugars.

• Light-dependent reactions occur on the thylakoid membranes.
• Involves the splitting of water (photolysis)
• 12 H
2
O + Energy  6 O
2
+ 24 H + + 24e -
• Requires light for the energy to split the water molecule
• Energy storage molecules are formed. (ATP and
NADPH)
• Oxygen gas is made as a waste product.

• Dark reactions (light-independent) occur in the stroma.
– Carbon dioxide is “fixed” into the sugar glucose.
– ATP and NADPH molecules created during the light reactions power the production of this glucose.

CHEMIOSMOSIS- the movement of H+ protons out of the membrane to make ATP

Step II: Dark Reactions
Light Independent
• Does not require light but can happen at any time of day.
• The energy from NADPH and ATP is used for carbon fixation
• 6 CO2 + 24 H+ + 24 e- ------> C6H12O6 + 6 H2O
• The chemical bonds present in glucose also contain a considerable amount of stored (potential) energy.

or C3 pathway
Occurs in the stroma

STEP 1 CARBON FIXATION
CO2 molecules enter and combine with RUBP (5 carbon) molecule to make a 6 carbon molecule– “will happen 3 times”

STEP 2 – New 6 carbon molecule quickly splits into 2 (3carbon molecules) (PGA)- energy is used to add phosphate and create PGAL

RuBp
Rubisco
CO
2
P G A P G A
Later becomes glucose!
RuBp
Rubisco
O
2
P G
Photorespiration – these CO
2
’s did not get incorporated into glucose this time!
A
CO
2
CO
2

STEP 3- Some parts of the PGAL molecule leave the cycle- some stay to make more RuBP

Step 3 (cont)- Used energy moleculescan return to electron transport chain