Photosynthesis

The Sun is the main source of energy for life on earth

•Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas

from carbon dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

During Photosynthesis…

• Organisms (plants) absorb light energy from the sun and store it in organic compounds.

• Energy is crucial to all life, without it work could not be done.

An example of basic photosynthesis…

Photosynthesis consists of two stages

• Light dependent reactions

• Take place in the chloroplasts

• Depends on sunlight for activation energy

• Responsible for the absorbing of light in Photosynthesis

• Water is split, giving off oxygen

• H2O   O2 + ATP + NADPH2

• Non-light dependent (dark) reactions

• Takes place in the Stroma

• Includes the Calvin cycle

• Does not literally occur in the dark, but requires no light to occur

• Separated into three steps

• Carbon dioxide is split, providing carbon to make sugars

•  ATP + NADPH2 + CO2   C6H12O6

Chloroplasts

• The site of Light Dependent Reactions

• These organelles are surrounded by a double membrane and contain an

inner membrane separate into disk like sacs called thylakoids

• Thylakoid are arranged into granum, or neat stacks

• Each thylakoid contains the green pigment chlorophyll

• The light absorbing pigments are organized into photosystems, which transfer energy during the light reactions

Pigments in chloroplasts

• Chloroplasts absorb all other color pigments, leaving green to be reflected resulting in a plant’s color.

• Chlorophyll a and b are

two are the 2 most

common types of chlorophyll

• Chlorophyll b absorbs colors or

light energy NOT absorbed by

chlorophyll a

Light Reflectedlight

Absorbedlight

Transmittedlight

Chloroplast

The Calvin Cycle – Step one

• CO2 diffuses into the stroma from surrounding cytosol

• An enzyme combines a CO2 molecule with a five-carbon carbohydrate called RuBP

• The six-carbon molecule produced then splits immediately into a pair of three-carbon molecules known as PGA

The Calvin Cycle – Step two

• Each PGA molecule receives a phosphate group from a molecule of ATP

• This compound then receives a proton from NADPH and releases a phosphate group producing PGAL

• These reactions produce ADP, NADP+, and phosphate which are used again in the Light Reactions

The Calvin Cycle – Step three

• Most PGAL is converted back to RuBP to keep the Calvin cycle going

• Some PGAL leaves the Calvin Cycle and is used to make other organic compounds including amino acids, lipids, and carbohydrates

• PGAL serves as the starting material for the synthesis of glucose and fructose

• Each turn of the Calvin cycle fixes One CO2 molecule so it takes six

turns to make one molecule of glucose

Photosystems & Electron Transport Chain

• Only 1 in 250 chlorophyll molecules (chlorophyll a) actually converts light energy into usable energy; these molecules are called reaction-center chlorophyll

• Antenna pigments- the other molecules (chlorophyll b, c, & d and carotenoids) tha absorb light energy and deliver it to the reaction-center molecule

• Photosystem- unit of several hundred antenna pigment molecules plus a reaction center

• There are 2 types of photosystems- photosystem I & photosystem 2

• Light is absorbed by the antenna pigments of photosystems II and I

• NADPH is used in the Calvin cycle

Chemiosmosis • Synthesis of ATP (energy)

• Depends on the concentration gradient of protons ( H+) across the thylakoid membrane

• Protons (H+) are produced from the splitting of water in Photosystem II

• Concentration of Protons is HIGHER in the thylakoid than in the stroma

• Enzyme, ATP synthetase in the thylakoid membrane, makes ATP by adding a phosphate group to ADP

Alternate Pathways•  C3 Plants are the most common users of the Calvin Cycle

• Stomata are small openings on the underside of leaves used for gas exchange (O2 & CO2)

• Guard cells help to open and close the stomata

• Plants also lose H2O through stoma so they are closed during the hottest part of the day

Review: Photosynthesis uses light energy to make food molecules

Chloroplast

Light

Stack ofthylakoids ADP

+ P

NADP

Stroma

Lightreactions

Calvincycle

Sugar used for

Cellular respiration Cellulose

Starch

Other organic compounds