Biology 20

Biology 20

Photosynthesis

2

Introduction

 Photosynthesis is…

 the process by which plants and some bacteria use chlorophyll, a green pigment, to trap sunlight energy.

 The energy is used to synthesize carbohydrates

Solar

Energy Chemical Bond

Energy of

Carbohydrates

Converted

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Respiration

 The process by which living things

convert the chemical energy in sugars into the energy used to fuel cellular

activities

Chemical Bond Energy of

Carbohydrates

ATP

ATP: Adenosine Triphosphate

the molecule which is the source of energy for most metabolic processes in living organisms

Ade nos ine Phosphate

Phosphate Phosphate

High Energy

Bond

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Electron Transfer

 The transfer of electrons is important in ATP production.

 Electron carriers

 strip a hydrogen proton and its electron from a number of organic compounds

 Respiration

 NAD

(becomes NADH)

• [Nicotinamide adenine dinucleotide =NAD⁺/NADH]

 Photosynthesis

 NADP

(becomes NADPH)

• [Nicotinamide adenine dinucleotide phosphate = NADP⁺/NADPH]

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Electron Transfer

 Oxidation - Loss of Electrons

 Reduction - Gain of Electrons

LEO the lion goes GER

OIL RIG

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General Equation

 There are two steps in photosynthesis

 Light Dependent Reaction

 Light Independent Reaction (AKA Dark Reaction, AKA Calvin Cycle, AKA Calvin-Benson Cycle, AKA

Clavin-Benson-Bassham [CBB] cycle)

6 CO + 6 H O

2 2 C H O + 6O

6 12 6 2

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General Outline

Light

Dependent Reaction

Light

Independent Reaction

Light

Energy Water

Oxygen

ATP

N A D P H

Carbon Dioxide

Sugar

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Chlorophyll

 A green chemical which traps sunlight energy

 Located in the

chloroplast

Chloroplast Parts

 Thylakoid

 Disk shaped

 Granum = stack of disks

 Grana = many stacks

 site of light dependent reaction

 Stroma

 gel surrounding the thylakoids

 site of light independent

reaction

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Thylakoid Location

Plant Cell

Chloroplast Thylakoid Stacks

Thylakoid

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Chloroplast Structure

Pigments

 White light from the Sun is composed of all colors

 red, orange, yellow, green, blue, indigo,

violet

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 Chlorophyll

 reflects green light

 absorbs blue and red ends of the spectrum

• chlorophyll a – absorbs violet-blue and orange-red

• chlorophyll b – absorbs blue and yellow

 Other pigments absorb light energy from other parts of the spectrum.

Carotenoids (orange) Xanthophyll (yellow)

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Action Spectra of Photosynthesis -

 red and blue light are the most effective colors for

photosynthesis

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Absorption Spectrum



a mixture of

chlorophyll a and chl orophyll b

in the range of

visible light as

measured by a

spectrophotometer

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More on Pigments

 Pigments are organized into clusters called PHOTOSYSTEMS

 The purpose of the pigments is to absorb light energy.

Q: What really happens when light is captured?

A: Electrons from chlorophyll move to

a HIGHER energy level.

Section 6.1 Questions

(a) How are the wavelength and energy of a photon related?

As the wavelength gets longer, the energy in a

photon decreases, and as the wavelength gets

shorter, the energy increases.

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(b) Which possess a higher energy value – red or green light?

Green light possesses a higher energy

value than red light because it has a shorter wavelength.

( c) How is the color of light related to its

energy? The colour of

light is determined by its

wavelength. A higher

wavelength has lower

energy. Red light is 750

nm, whereas violet light

is 380 nm

2. (a) What pigments are present in green leaves?

Green plants contain chlorophyll a, chlorophyll b, carotenoids, xanthophylls, and anthocyanins.

(b) Explain why yellow colored pigments are visible in autumn leaves but not in summer leaves.

Yellow-colored pigments are visible only in autumn leaves because the chlorophyll

pigments mask the yellow-colored pigments the rest of the year. Plants stop producing

chlorophyll in the autumn, so only the yellow, red, and brown colors are visible.

3. What do all photosynthetic organisms have in common?

All photosynthetic organisms contain the molecule chlorophyll to capture

electromagnetic radiation.

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4. A – thylakoid B – stroma

C – inner envelope membrane

D – outer envelope membrane

Light Dependent Reaction Preview

 Occurs on the thylakoid membrane.

 There are a few key points to understand:

 Light is absorbed by a photosystem (excites an electron)

 Electrons moves through ETS in a step by step fashion.

Lost energy is used to pump H

into the lumen (inside)

• These H

’s are used to produce ATP using the CF1 particle

 Electrons are delivered to another photosystem.

 Light again strikes this photosystem and electrons moves through another ETS

 The electrons join up with NADP

and H

to form NADPH

 ATP and NADPH go to the light independent reaction in

the stroma.

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Thylakoid Location

Plant Cell

Chloroplast Thylakoid Stacks

Thylakoid

Light Dependent Reaction

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Light Dependent Reaction

What happens to the H ⁺ in the Lumen??

 Eventually, H ⁺ in the lumen will get so

packed in that they will want to “leak” out.

 diffusion

 The only spot they can find to get out of the lumen is a special hollow molecule called a CF1 Particle.

This leads to

the production of ATP

H+

H+

H+

Light Dependent Reaction

CF1

Particle

Light Dependent Reaction

Phosphorylation

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Phosphorylation

The addition of phosphate to anything

For Example... ADP

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Wait a Minute...

 If light keeps on striking Photosystem 2, ejecting high energy electrons,

won’t the chlorophyll eventually run out of electrons?

 Yes.

 Except that the electrons are replaced by using some from …

H ₂ O

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Photolysis

 Photo = light

 lysis = break

 Photolysis = the breaking of a water molecule using light energy.

 Occurs in the lumen of the thylakoid.

Photolysis

HOH Lig ht Ph

oto ns 2 Electrons

2 Hydrogens 1 Oxygen

To Ph otosy stem 2 To CF1 Particle

Waits for another O to make O2,

then out of the plant

Thylakoid membrane

Light Dependent Reaction

Light Dependant

Reaction Review pg 6

Granum

Stroma

Site of Light dependent reaction

Site of Light Independent reaction

Light

Water

Oxygen

Carbon

Dioxide

Sugar

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A+B : Flow of Electrons C: Radiant Energy

D: Movement of H+

E: Formation of ATP by

chemiosmosis

Now...

The Light Independent Reaction

Biology20

Photosynthesis

Light Independent Reaction

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The Second Step

 Occurs in the stroma

 Does not require light

 Also called the Calvin Cycle

 Analyzed by a guy named Melvin Calvin

 This step uses CO

and H to form sugar!!

 CO

comes from the atmosphere

 H comes from NADPH of the Light Dependent Reaction

• Originally came from water

 Energy to drive this cycle is supplied by ATP from the

Light Dependent Reaction.

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Steps in the Calvin Cycle

1) Carbon dioxide is fixed by RuBP

 “Fixed” = to be incorporated into an organic molecule

 RuBP = Ribulose Biphosphate

• a 5 carbon sugar

• acts as a CO

acceptor

• RuBP + CO

= C

sugar

 This sugar is very unstable and immediately splits into two C

sugars called PGA

• PGA = phosphoglyceric acid

Step 1

R u B P + C O

2 Unstable C Sugar

6

PGA (C )

3

PGA (C ) 3

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Step 2

 PGA uses ATP as an energy

source to remove hydrogen from NADPH to form a new molecule called PGAL

 PGAL = phosphoglyceraldehyde

 PGAL = C

Step 2

PGA (C )

3 +

NADPH NADPH

NADPH H H H H H

ATP ATP ATP ATP ATP ATP

ATP ADP P ADP P

ADP P ADP P ADP ADP ADP ADP ADP ADP ADP

PGAL (C )

3 H

ATP

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Fates of PGAL

1) In 6 Calvin Cycles, 12 PGAL’s (C ₃ ) are formed.

 2 are used to produce C

H

O

 10 are used to recycle into RuBP

2) PGAL can be used as an energy source in cell respiration

3) PGAL can be modified to produce fat or protein.

Let’s Draw

Yeah it is great to draw isn’t it?

You have this diagram!

[6] PGA (C

) [6] PGA (C

)

Light Dependent

Reaction

ATP

NADPH E n e r g y

H

RuBP

[6] C

Sugars 6 CO

10 PGAL

2 PGAL Glucose

• Protein Formation

• Lipid Formation

• Cellular

Respiration

[12] PGAL (C

)

Now that you’re smarter...

 Chapter 6 Review

 Questions 1-8, 10-12 on page 200-201

 your photosynthesis review sheet

Review Answers

 Chapter 6 Review: (Pages 200–201)

 Part 1

 1. B

 2. B

 3. D

 4. A

 5. A

 6. D

 7. C

 8. B

 10. 4, 3, 1, 2

Review Answers

 Part 2

 11. (a) The shorter the wavelength, the greater the energy; therefore, a shorter wavelength has more energy.

 (b) Plants can use wavelengths from 380 to 480 nm and from 620 to 680 nm in photosynthesis.

 12. (a) Old textbooks often referred to the Calvin cycle as “the dark reaction” because it did not appear to require light to occur.

 (b) The term “dark reaction” is misleading because although the Calvin cycle does not

require light energy directly, it does require light to activate and produce certain Calvin cycle

enzymes.

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Review Q’s from notes

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