Photosynthesis

“Life on Earth is solar-powered”

- Campbell, Biology

Where we begin….• Autotrophs -

a.k.a. producers - organisms which can make their own food

• Producers turn light energy into chemical energy using pigments.

Pigments - substances that absorb visible light

• Chlorophyll - green pigment; the main pigment found in most producers– Two types: a and b– absorbs red and violet

light; reflects greens and yellows

– located in the chloroplasts of plants and algae

• Carotenoids - accessory pigments, yellow and orange– absorbs other

colors of light and transfers the energy to chlorophyll

Chlorophyll

Remember… CHLOROPLASTS?

• All green parts of a plant have chloroplasts, but the leaves are the major sites of photosynthesis.

History of Chloroplasts

• Double membrane bound organelle• Contains the pigment chlorophyll• Has its own DNA and can replicate

independently! • Thought to once have been a free-

living organism that became a part of a larger cell and eventually the two cells became interdependent.

Structure of a Chloroplast

• Granum - (pl. grana) whole stack of membranes (pancakes)

• Thylakoid - individual membrane of a granum

• Stroma – fluid filled space between the grana

Review of Chloroplast Structure• Chlorophyll -

found in the membranes of the thylakoids; traps light for photosynthesis

Anatomy of a leaf

• The leaf is the major site of photosynthesis

• One leaf has tens of thousands of cells and each cell contains 40 to 50 chloroplasts.

Relationship:

Electromagnetic Spectrum• Light is measured in wavelengths.

Visible light = 400 to 700 nm. • ROY G. BIV• Chlorophyll best absorbs blue & red. It

appears green b/c it reflects green light.• Photons - energy of light

• What does the dip in the graphs between 500 -600 nm tell you?

Figure 7.3a

Overview

What is photosynthesis?

• To use the energy from the sun, cells must trap light energy and store it in a form that can be used by cell organelles.

• Photosynthesis is the process plants, algae and some bacteria use to trap the sun’s energy & build a carbohydrate, called glucose, that stores energy.

General Equation

This is a coupled reaction…called a redox reaction.

This is also an endergonic reaction.

Requirements

• Light Energy (Sun)• Chlorophyll a,b and accessory

pigments (these absorb wavelengths of light) and accessory pigments

• Raw materials (CO2 and H2O )• Enzyme: NADP+ (Little yellow bus

with a P!)

Figure 7.4

Stages of Photosynthesis• Light Reactions

(occurs in thylakoid)

• Calvin Cycle or Dark Reaction (occurs in stroma)

Light ReactionsOverview: 1. Light energy (photons) is absorbed in the THYLAKOID2. Water is split apart: 2H+ + 1/2O2 + 2e-

3. ATP and NADPH are formed4. Reactants: chlorophyll a & b, accessory pigments, light energy, water, ADP, Pi and NADP+

– CO2 is NOT involved

Details of Light Reactions 1. Light energy is absorbed by chlorophyll a in the

reaction center of the 2 photosystems. (The energy can come from the accessory pigments!)

2. Magnesium electrons in chlorophyll become “excited” (high energy) and leave the molecule, moved by a series of electron acceptors

3. The electrons lost by the chlorophyll in P680 are replaced by electrons from water as it is split apart by an enzyme (Z). H2O → H++2e-+1/2O2

4. The electrons “lost” from P680 go down the ETC until they reach P700. On the way down the ETC, the energy is used to pump a H+ across the thylakoid membrane. The electrons replace the electrons “lost” by the chlorophyll a in P700.

5. The electrons lost by P700 join H+ and NADP+ to make NADPH

Light Reactions6. The ETC has added 3 H+ ions inside the thylakoid

space to create the concentration gradient needed for the phosphoralation of ADP to ATP.

7. 3 H+ will pass through the ATP synthase to allow for the photophosphorylation of ADP to ATP.

8. To have enough high E molecules (ATP and NADPH), this entire process must occur 12 times producing: 12 NADPH and 12 ATP (and used 12 H2O)

9. This is still not enough ATP! Photosystem I will generate an additional 6 ATP (using an alternative pathway) for a total of 18 ATP!

10. Now we have enough energy to make a glucose!

ETC in photosynthesis

Products of Light Reactions• 6 O2 are released into the air via stomata

(simple diffusion!). What molecule were they a part of originally?• 12 NADPH and 18 ATP: go to the stroma for the Calvin Cycle.

Question: How did the plant get the water in the first place?

Let’s Watch…• http://highered.mcgraw-hill.com/sit

es/0072437316/student_view0/chapter10/animations.html#

• http://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/OXYGENIC.HTML

• http://www.web.virginia.edu/gg_demo/movies/figure18_12b.html

Calvin Cycle or Dark Reaction (or Light Independent Reaction)Overview• Reactants:

• ATP and NADPH from Light Reactions• 6 CO2

• 6 RuBP• Products: Glucose (C6H12O6) and 6 RuBP

(and NADP+ and ADP)• Takes place in the stroma

Steps of the Calvin Cycle1. CO2 Fixation:

CO2 combines with RuBP, a 5 carbon compound.2. The resulting 6C compound is unstable and

immediately splits into 2-3C compounds. 3. A series of steps take place which ultimately

replenish the RuBP, using 3 ATP and 2 NADPH. 4. The cycle occurs 3 times in order to produce a

molecule of PGAL, a 3C compound. It takes 9 ATP and 6 NADPH to make one PGAL (a.k.a. G3P).

5. Two PGAL will be joined outside of the Calvin Cycle to make one glucose molecule.

*Ultimately, it takes 18 ATP and 12 NADPH to make one molecule of glucose.

To produce one glucose: 6CO2 + 6RuBP + 12 NADPH + 18 ATP 2 PGAL (1 Glucose)

Question: What happens to the NADP+ and ADP?

Calvin Cycle Con’t.• Formation of glucose: PGAL + PGAL Glucose

C3H6O3 + C3H6O3 C6H12O6

Recall: PGAL is a 3- Carbon molecule and is also known as G3P.

Calvin Cycle

Figure 7.8

Importance of Photosynthesis• Forms glucose which is necessary

for cellular respiration• Forms the source of oxygen we

breathe• The G3P (a.k.a. PGAL) is very

versatile and also forms other very important biological molecules.

Figure 7.9

Factors that Affect Photosynthesis

1. Amount of water2. Temperatures3. Light Intensity4. Amount of CO2

Summary of Photosynthesis

Can you fill this out?

Check your answers…

Water

Oxygen

Carbon dioxide

Carbohydrate

ADP + P

NADP+

NADPH

ATP

Good website…• http://www.cix.co.uk/~argus/Drea

mbio/photosynthesis/photosynthsis%20animation.htm

Picture it happening...• CO2 is taken in

and O2 is released.• H2O is absorbed

by the roots• Glucose (sugar ) is

made and used or stored by the plant.

History with important information...

• At first it was believed that the O2 given off by plants was derived from CO2. Later scientists discovered that the O2 comes from the splitting of H2O (known as hydrolysis).

• The CO2 is used for the glucose as well as the H from water. The O from water is given off as a by-product.

• From what molecule does the O2 you breath originally come?

Gas Exchange• Gas exchange, CO2

enters and O2 exits, through specialized structures known as stomata of a leaf.

• stoma; plural = stomata; Greek for mouth (hole)

• flanked by two guard cells which regulate the opening

Photosynthetic Variations

Location, location, location…Adaptations to the environment

Variations• Based on how carbon is “trapped”

and the molecule that is formed when CO2 is incorporated.

C3 Plants• C3 plants- ex. roses, oak trees, peas,

sunflowers, beans, peanuts• C3 plants must capture their CO2 while

the sun shines. They open their stomata to get the CO2 during the day . As a result, the heat and wind may dry them out and they are more likely to die.

• Advantageous in moderate weather.

Figure 7.10a

Pg 126

Photosynthetic Variations con’t.

• C4 plants - ex. Bermuda grass, crabgrass, corn, sugar cane

• C4 plants open their stomata at night and capture CO2 when the air is cooler so they won’t dry out. The next day (with the pores closed) they use the light to incorporate the CO2. C4 plants capture more CO2 than C3 plants.

Figure 7.10b

Photosynthetic Variations cont.

• CAM plants - Crassulacean Acid Metabolism

• To inhibit loss of water during intense sun (like in the desert), CAM plants take up CO2 during the night. The CO2 is stored in the vacuoles and used in the daytime for photosynthesis.

• Ex. Cacti, aloe, other desert plants

Figure 7.11