PHOTOSYNTHESISChapter 6

BIOCHEMICAL PATHWAY

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

Metabolism

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.

Photosynthesis in Overview• Process by which plants and other autotrophs

store the energy of sunlight into sugars.• Requires sunlight, water, and carbon dioxide.• Overall equation:

6 CO2 + 6 H20 C6H12O6 + 6 O2

• Occurs in the leaves of plants in organelles called chloroplasts.

Chloroplast Structure• 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.

Pigments

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

VISIBLE SPECTRUM

• 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

Photosynthetic Pigments

• Chlorophyll A • chlorophyll B• Accessory pigments:– Carotenoids

• Each pigment absorbs a particular wavelength of light in the visible spectrum

Pigments• 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.

Pigment Absorption

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.

Step I: Light Reactions

• Light-dependent reactions occur on the thylakoid membranes.

• Involves the splitting of water (photolysis) • 12 H2O + Energy 6 O2 + 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

• 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 ReactionsLight 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.

CALVIN CYCLE

or C3 pathwayOccurs in the stroma

The Calvin Cycle

STEP 1 CARBON FIXATIONCO2 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

Rubisco

RuBp CO2

RuBp

Rubisco

P G A P G A

P G A

Later becomes glucose!

CO2

CO2

O2

Photorespiration – these CO2’s did not get incorporated into glucose this time!

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

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