AP Biology

Chapter 10: Photosynthesis - Life from Light

AP Biology

Energy needs of life All life needs a constant input of energy

Heterotrophs get their energy from “eating others” consumers of other organisms consume organic molecules

Autotrophs get their energy from “self” get their energy from sunlight use light energy to synthesize organic

molecules Chemoautotrophs

Harvest energy from oxidizing inorganic substances such as sulfur and ammonia

Unique to bacteria

AP Biology 2005-2006

How are they connected?

glucose + oxygen carbon + water + energydioxide

C6H12O6 6O2 6CO2 6H2O ATP+ + +

Heterotrophs

+ water + energy glucose + oxygencarbondioxide

6CO2 6H2O C6H12O6 6O2light

energy + ++

Autotrophsmaking energy & organic molecules from light energy

making energy & organic molecules from ingesting organic molecules

AP Biology

Energy cycle

Photosynthesis

Cellular Respiration

glucose O2H2OCO2

ATP

sun

The Great Circleof Life!

Where’s Mufasa?

AP Biology

What does it mean to be a plant Need to…

collect light energy transform it into chemical energy

store light energy in a stable form to be moved around the plant

& also saved for a rainy day need to get building block atoms from

the environment C,H,O,N,P,S

produce all organic molecules needed for growth carbohydrates, proteins, lipids, nucleic acids

AP Biology 2005-2006

Plant structure Obtaining raw materials

sunlight leaves = solar collectors

CO2

stomates = gas exchange regulation

Found under leaves

H2O uptake from roots

nutrients uptake from roots

AP Biology

Structure of the Leaf Mesophyll

Tissue forming the interior of the leaf; site of most chlorophyll

Stomata Microscopic pores in

the leaf; allows for gas exchange

AP Biology

Each Mesophyll Cell:

Has approx. 30-40 chloroplasts

Each chloroplast is equipped to carry out photosynthesis

AP Biology

Chloroplasts double membrane stroma thylakoid sacs grana stacks

Chlorophyll & ETC in thylakoid membrane H+ gradient built up

within thylakoid sac

Plant structure

H+ H+

H+

H+

H+H+

H+ H+

H+H+

H+

AP Biology

Structure of the Chloroplast Chlorophyll

Photosynthetic pigment found in the thylakoid

Thylakoid Membranous sacs filled

with fluid and chlorophyll – Site of the LIGHT REACTIONS

Granum = Stack of Thylakoids

Stroma Fluid portion of the

chloroplast; sight of the LIGHT INDEPENDENT REACTIONS (Calvin-Benson Cycle)

AP Biology

Chloroplasts split water molecules Evidence

Discovery that the O2 given off by plants comes from H2O not CO2

Before the 1930’s the hypothesis was that photosynthesis occurred in two steps:

1. CO2 C + O2

2. C+ H2O CH2O

AP Biology

Chloroplasts Split Water Molecules: Changing the Hypothesis Studying bacteria, C.B. van Neil challenged the

hypothesis:

H2S was used, not water

Proposed the following Rxn

CO2 + 2H2S CH2O +2S

Applied the same principle to plants

CO2 + 2H20 + light CH2O +O2

AP Biology 2005-2006

Pigments of photosynthesis

chlorophyll & accessory pigments “photosystem” embedded in thylakoid

membrane structure function

Why does this structure make sense?

AP Biology

Pigments of Photosynthesis Continued Leaves look green because

they absorb red and blue light, while transmitting and reflecting green light

Chlorophyll A Dominant pigment – absorbs

red/blue Chlorophyll B

Directs photons to chlorophyll A Carotenoids

Funnel energy from other wavelengths to Chlorophyll A (mostly orange/yellow)

AP Biology

Light: Absorption Spectra Photosynthesis performs work only with

absorbed wavelengths of light chlorophyll a — the dominant pigment —

absorbs best in red & blue wavelengths & least in green

other pigments with different structures have different absorption spectra

AP Biology

Photosynthesis overview Light reactions – Light Dependent Rxns

convert solar energy to chemical energy

ATP

Calvin cycle – Light Independent Rxns uses chemical

energy (NADPH & ATP) to reduce CO2 to build C6H12O6 (sugars)

AP Biology

Photosystems Photosystems

collections of chlorophyll molecules

2 photosystems in thylakoid membrane act as light-gathering “antenna complex” Photosystem II

chlorophyll a P680 = absorbs 680nm

wavelength red light Photosystem I

chlorophyll b P700 = absorbs 700nm

wavelength red light

AP Biology 2005-2006

Light reactions Similar to ETC in cellular respiration

membrane-bound proteins in organelle electron acceptors

NADP+ (Oxygen in cellular respiration) proton (H+)

gradient across inner membrane

ATP synthase enzyme

AP Biology

ETC of Photosynthesis ETC produces from light energy

ATP & NADPH NADPH (stored energy) goes to Calvin cycle

PS II absorbs light excited electron passes from chlorophyll to

“primary electron acceptor” at the REACTION CENTER. splits H2O (Photolysis!!) O2 released to atmosphere ATP is produced for later use

AP Biology 2005-2006

ETC of Photosynthesis Chloroplasts transform light

energy into chemical energy of ATP

use electron carrier NADPH

split H2O

AP Biology 2005-2006

2 Photosystems Light reactions

elevate electrons in 2 steps (PS II & PS I) PS II generates

energy as ATP PS I generates

reducing power as NADPH

This shows Noncyclic photophosphorylation.

AP Biology

ETC of Photosynthesis

AP Biology

ETC of Photosynthesis

AP Biology 2005-2006

Cyclic photophosphorylation If PS I can’t pass

electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH coordinates light

reactions to Calvin cycle

Calvin cycle uses more ATP than NADPH

AP Biology

Photosynthesis summary so far…Where did the energy come from?Where did the H2O come from?Where did the electrons come from?Where did the O2 come from?Where did the H+ come from?Where did the ATP come from?Where did the O2 go?What will the ATP be used for?What will the NADPH be used for?

AP Biology 2005-2006

From Light reactions to Calvin cycle Calvin cycle

Chloroplast stroma

Need products of light reactions to drive synthesis reactions ATP NADPH

AP Biology

From CO2 C6H12O6

CO2 has very little chemical energy fully oxidized

C6H12O6 contains a lot of chemical energy reduced endergonic

Reduction of CO2 C6H12O6 proceeds in many small uphill steps each catalyzed by specific enzyme using energy stored in ATP & NADPH

AP Biology

PGALto make glucose

6Cunstable

intermediate

1C CO2

Calvin cycle

5CRuBP

3C2xPGA

6 ADP

6 ATP

3C2x

3C x2PGAL

6 NADP

6 NADPH

3 ADP

3 ATPRubisco

1. Carbon fixation

2. Reduction

3. Regeneration

ribulose bisphosphate

ribulose bisphosphatecarboxylase

sucrosecellulose

etc.

AP Biology

Rubisco Enzyme which fixes carbon from

atmosphere ribulose bisphosphate carboxylase the most important enzyme in the world!

it makes life out of air! definitely the most abundant enzyme

AP Biology

Calvin cycle PGAL

end product of Calvin cycle energy rich sugar 3 carbon compound “C3 photosynthesis”

PGAL important intermediatePGAL glucose carbohydrates

lipids

amino acids

nucleic acids

AP Biology 2005-2006

Photosynthesis summary Light reactions

produced ATP produced NADPH consumed H2O produced O2 as byproduct

Calvin cycle consumed CO2

produced PGAL regenerated ADP regenerated NADP

AP Biology 2005-2006

Summary of photosynthesis

Where did the CO2 come from? Where did the CO2 go? Where did the H2O come from? Where did the H2O go? Where did the energy come from? What’s the energy used for? What will the C6H12O6 be used for? Where did the O2 come from? Where will the O2 go? What else is involved that is not listed in this

equation?

6CO2 6H2O C6H12O6 6O2light

energy + ++

AP Biology

Photosynthesis Drives Evolution Photosynthesis first evolved in

prokaryotic organisms Scientific evidence supports that

prokaryotic (bacterial) photosynthesis responsible for production of an oxygenated atmosphere

Prokaryotic photosynthetic pathways – foundation of eukaryotic photosynthesis

AP Biology

Plant Photosynthesis Adaptations Due to Weather Conditions C3

Most common type C4

Minimize the cost of photorespiration by incorporating CO2 into four carbon compounds in mesophyll cells

4 carbon compounds exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

CAM Open their stomata at night, incorporating CO2 into

organic acids During the day, the stomata close and the CO2 is

released from the organic acids for use in the Calvin cycle

AP Biology

C4 Plants

CO2

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Photosyntheticcells of C4 plantleaf

Stoma

Mesophyllcell

C4 leaf anatomy

PEP carboxylase

Oxaloacetate (4 C) PEP (3 C)

Malate (4 C)

ADP

ATP

Bundle-Sheathcell CO2

Pyruate (3 C)

CALVINCYCLE

Sugar

Vasculartissue

Figure 10.19

CO2

AP Biology

C4 and CAM Plants