PHOTOSYNTHESISLife’s ultimate reaction

Photosynthesis

The definition: A process in which light energy, water and carbon dioxide are used to create glucose for energy.

Photo= light Synthesis= to make

All of life stems from this reaction!

Photosynthesis

The Chemical Equation

6 CO2 6 H2O C6H12O6 6 O2

light energy

Carbondioxide

+ water

Name: ______________________________________ Date: _____________________ #______

+ glucose + oxygen

Photosynthesis

Photosynthesis takes place in the chloroplasts. Only the cells of the leaf have chloroplasts.

In eukaryotic cells, the chloroplasts contain specialized light-absorbing pigments.

Chloroplast

Pigments

Chlorophyll A (green) is the most important photosynthetic pigment.

Other pigments called antenna or accessory pigments are also present in the leaf. Chlorophyll B (also green) Carotenoids (orange / red) Xanthophylls (yellow / brown)

Pigments- Photosystems

The pigments are embedded in the membranes of the chloroplast in groups called photosystems.

PHOTOSYSTEM II PHOTOSYSTEM I

*Note* Photosystem I was discovered before Photosystem II, that explains why the order is switched.

Chlorophyll

400 500 600 700

Chlorophyll absorbs most of the energy for the plant.

There are two categories of chlorophyll: Chlorophyll a and Chlorophyll b.

Recall the electromagnetic spectrum. Visible light ranges from wavelengths of 400 nm to ~750 nm.

Which wavelengths- or colors- of light does chlorophyll mostly absorb?

Factors that Affect Photosynthesis

Sunlight- the amount of sunlight available and the color of the light (plants do not use green light- they reflect it).

Temperature- As temperature increases, photosynthesis will increase but only up to a certain point (higher temperature mean higher rate of transpiration [water loss]).

Availability of water and carbon dioxide.

Structure of the Chloroplast

The chloroplast is the organelle in which photosynthesis takes place (animal cells do not have chloroplasts).

It is a double-membrane organelle. It is green because it contains chlorophyll.

Label the Chloroplast

1 granum= one stack of thylakoids (disc-shaped structures)

Grana= multiple granum

*Lumen= space inside the thylakoids

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 Reactions- Overview

Light reactions occur in the thylakoid of the chloroplast.

Light and water are required for this process.

Energy storage molecules are formed. (ATP and NADPH)

Oxygen gas is made as a waste product.

Light Reactions- Specific Steps

Step one: pigments absorb light energy

Step two: pigment electrons gain the energy and jump an energy level (described as “excited electrons”).

Electron symbol = e-

Light Reactions- Specific Steps

Step three: Excited electrons enter an electron transport chain (a series of proteins).

As the electrons are passed from protein to protein, chemical energy is generated and stored in ATP and NADPH (energy storage molecules or energy “carriers”).

Light Reactions- Photolysis

Aside from the chain of reactions that occur when pigments absorb light energy, there is another occurrence going on “behind the scenes” during the light reactions. This occurrence is known as photolysis.

Photo=lightLysis= to break or split

Light Reactions- Photolysis

During photolysis, water molecules are split apart into hydrogen ions, electrons, and oxygen.

Important because the electrons are used to replace excited electrons so that the thylakoids remain stable.

Oxygen gas is released as waste into the atmosphere.

Hydrogen is used to convert NADP+ into NADPH

Dark Reactions- Overview

Dark reactions (light-independent) occur in the stroma. Carbon dioxide is “fixed”

into the sugar glucose in a process known as carbon fixation

ATP and NADPH molecules created during the light reactions power the production of glucose.

Dark Reactions- The Calvin Cycle

During the Calvin cycle, carbon dioxide from the atmosphere enters a series of reactions and is used to create glucose, as well as other carbon-based compounds.

NADPH and ATP produced in the earlier light reactions are important for this series of reactions.

One glucose molecule is produced per two cycles.

Leftover compounds are recycled. The process repeats.

Steps of the Calvin Cycle

1. Carbon fixation: One carbon atom from CO2 is added to a 5-carbon sugar called ribulose biphosphate (RuBP).

2. PGA Formation: The unstable six-carbon sugar formed in step 1 is then split into 2 molecules of phosphoglyceric acid (PGA).

3. Use of ATP and NADPH: A series of reactions involving ATP and NADPH from the light reactions converts a molecule of PGA into PGAL (phosphoglycer-aldehyde), another 3-carbon compound.

4. Glucose production: After several rounds of the Calvin cycle, two molecules of PGAL leave the cycle to form glucose.

5. ATP and PGAL replenish RuBP: Some PGAL molecules are recycled and reform the 5-carbon RuBP with the help of ATP. Each 5-carbon RuBP is ready to begin the cycle again.

Unstable

CO2 C-C-C-C-C-C

(unstable intermediate)

C-C-CC-C-C(PGA)

C-C-CC-C-C(PGAL)

C-C-C(PGAL)

C-C-C-C-C-C(glucose)

ATP

ADP + P

NADPH

NADP +

ATP

ADP + P

C-C-C-C-C(RuBP)

CO2

RecapLight reactions: Sunlight and water are absorbed by the chloroplast and used in the thylakoids.

Sunlight is used for energy; water is broken down into hydrogen and oxygen. O2 comes out and is released into the atmosphere (lucky for us!)

ATP and NADPH are created, they carry energy. The energy is used during the Calvin Cycle.

Dark reactions: ATP, NADPH and carbon dioxide are used to create glucose. Glucose is important because it is the main energy source for all living things.

Copyrights

Copyright © 2012 Vanessa Jason (Biology Roots www.biology-roots.com)

All rights reserved by author. Permission to copy for single classroom

use only. Electronic distribution limited to single

classroom use only. Not for public display.

“Who dares to teach must never cease to learn.” ~John Cotton Dana