Photosynthesis

Overview

• All energy on earth comes from the sun.

• We depend on:– Plants– Algae (underwater plants)– Cyanobacteria (photosynthetic bacteria)

• To provide this energy to us!

Overview

Leaves and Leaf Structure• The Raw Materials

of Photosynthesis Enter the Cells of the Leaf– Water and

Carbon Dioxide

• The Products of Photosynthesis Leave the Leaf– Sugar and Oxygen

Overview

• Carbon dioxide and water are taken in by plants• Plants absorb light energy and convert it to a

usable form.– ATP

• Energy is used to “fix” the carbon in carbon dioxide into sugar molecules– Chemical energy

• Sugar is converted to starch and stored for use by the plant, and by animals when they eat plants.

Nature of Light

Red and Blue Wavelengths most Important for Photosynthesis

Captured by Chloroplasts and Used to Initiate Photosynthesis Reactions

Photosynthetic Reactions

• Photos (light)

• Synthesis (to put together)

• Light Energy to Chemical Energy

• Life on Earth Depends on this Process

• Supplies Our Oxygen

Photosynthetic Reactions

Overall Equation

Photosynthetic Reactions• The Overall Equation for Photosynthesis Is

Deceptively Simple• In Fact, a Complex Set of Physical and

Chemical Reactions must Occur in a Coordinated Manner for the Synthesis of Carbohydrates

• To Produce a Sugar Molecule such as Sucrose, Plants Require nearly 30 Distinct Proteins that Work within a Complicated Membrane Structure

Organelles in a Plant Cell

Location of Photosynthesis?

Chloroplasts

Chloroplasts

• Have TWO membranes– A “bi-bilayer!”

• The inner membrane is called the thylakoid.

• The thylakoid is folded and looks like stacks of coins called granum (grana singular).

• The stroma is the space surrounding the granum

The Process of Photosynthesis does NOT Happen all at Once; rather it occurs in  TWO STAGES:

   

   

STAGE 1 - CALLED THE LIGHT REACTIONS OR LIGHT DEPENDENT REACTIONS. Energy is Captured from Sunlight.  Water is Split into Hydrogen Ions, Electrons, and Oxygen (O2).  The O2 Diffuses out of the Chloroplasts (Byproduct).  The Light Energy is Converted to Chemical Energy, which is Temporarily Stored in ATP and NADPH.

STAGE 2 - CALLED THE CALVIN CYCLE. The Chemical Energy Stored in ATP and NADPH powers the formation of Organic Compounds (Sugars), Using Carbon Dioxide, CO2.

How plants harness energy from sunlight…

Photosynthetic pigments

Chlorophyll a – the “main” photosynthetic pigment

Chlorophyll b

Accessory pigments..

Carotenoids

Chloroplasts

• Chlorophyll molecules are embedded in the thylakoid membrane

• Act like a light “antenna”

• These molecules can absorb sunlight energy.

Photosynthetic pigments are arranged as “photosystems”

2 Photosystems

Light reactions elevate electrons in 2 steps (PS II & PS I)

Photosystem II generates energy as ATP

Photosystem I generates reducing power as NADPH

Electron TransferThe energy changes of electrons as they flow through the light reactions are analogous to the cartoon.

As complicated as the scheme is, don’t lose track of its functions:

The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions.

Light (dependent) Reactions

• Happen ONLY in sunlight– Hence they depend

on light!

1. Light is absorbed by chlorophyll molecules

2. The energy generates molecules of ATP

Light Independent Reactions(formerly the “dark reactions”)

• Happen in sunlight, and in the dark.

– Hence “independent of light”

1. ATP generated by sunlight drives the Calvin Cycle.

2. Monosaccarides (eg. glucose) are manufactured in the cycle.

3. Monosaccarides are used to “build” polysaccharides (eg. Starch).

Light Independent Reactions• ‘Calvin Cycle’

• ‘Carbon Reactions Pathway’

• Do not Require Light Energy to Occur– Do Require

Energy Captured by Light Reactions

Photosynthesis.htm

Overall Reaction

What is the equation for photosynthesis?

carbon dioxide + water glucose + water + oxygen

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O