9.2 LESSON Photosynthesis Overview

Learning Objectives

▪ Explain the role of pigments in the process of

photosynthesis.

▪ Describe the role of electron carrier molecules.
▪ Identify the reactants and products of photosynthesis.

Chlorophyll and Chloroplasts

• Light energy from the sun must be captured for photosynthesis

to occur.

• Sunlight is “white” light—actually a mixture of different

wavelengths.

• Photosynthetic organisms capture energy from sunlight with

pigments—principally with chlorophyll.

Chloroplasts

Photosynthesis takes place inside organelles called chloroplasts.

Plant Cell

Chloroplast

Chloroplast Structure

In plants, photosynthesis takes place inside chloroplasts.

​liquid filling of chloroplast

stack of thylakoids

chlorophyll in the membrane traps sunlight

Electron Carriers

The high-energy electrons produced by chlorophyll are
highly reactive and require a special “carrier.”

Electron Carrier

• An electron carrier is a compound that can accept a pair

of high-energy electrons and transfer them, along with
most of their energy, to another molecule.

• NADPH can carry the high-energy electrons that were

produced by light absorption in chlorophyll to chemical
reactions elsewhere in the cell.

An Overview of Photosynthesis

Photosynthesis uses the energy of sunlight to convert water and
carbon dioxide (low-energy reactants) into high-energy sugars and
oxygen (products).

Carbon dioxide + Water → Sugars + Oxygen

light

Photosynthesis and Light

Photosynthesis involves two sets of reactions:
▪ Light-dependent reactions
▪ Light-independent reactions

Light-Dependent Reactions

Light-dependent reactions require the direct involvement of light
and light-absorbing pigments.

Water

Oxygen

Energy from sunlight is

used to make

NADPH

and
ATP

in light-dependent

reactions.

Light-Independent Reactions

Light-independent reactions use ATP and NADPH molecules
produced in the light-dependent reactions to produce high-energy
sugars from carbon dioxide

Sugars and
Other carbohydrates

Carbon Dioxide

Interdependence of Reactions

Light-dependent and light-independent reactions have an
interdependent relationship.