Photosynthesis

Energy, from light to sugar

Photosynthesis

Photosynthesis is the process by which the energy of a photon is captured and stored in the chemical bonds of a carbohydrate.

Light

‘All around us, moving through us, are countless photons, little packets of pure energy, each moving as fast as possible, and each riding a wave…’

Light

Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm)

In a scientific context, the word light is sometimes used to refer to the entire electromagnetic spectrum

Light is composed of an elementary particle called a photon

Photons•Photons are packets of energy, moving at 3x108 m/s.•As they travel they move in a waveform•The distance between two peaks is a called the wavelength•The number of wavelengths that pass in a given time period is the frequency

Electromagnetic Radiation

Higher frequency = short wavelength = higher energy photon

Pigment

A pigment is a material that changes the color of light it reflects as the result of selective color absorption (it absorbs specific

photons of certain wavelengths and reflects other photons)

Living organisms use pigment molecules transfer light energy to electrons

The main photosynthetic pigment found in plants is chlorophyll

Plant PigmentsPlants use a variety of pigments to capture a larger range of the light spectrum

Plant pigments

Absorption spectrumThe wavelengths of light absorbed by the molecular pigment chlorophyll

Action spectrumThe wavelengths of light absorbed by a plant

Absorption vs action spectrum

Engelmann’s experiment

Photosynthesis Overview

The Chloroplast

The chloroplast

Thylakoid

The protein complexes involved in the light dependent reaction are located in the thylakoid membrane

The Light Dependent Reaction

Light Dependent Reaction

1. Water is split, O2 is released • electrons pass to chlorophyll at reaction center• protons are released into thylakoid lumen

2. Photons strike photosystem II• resonance energy excites electrons

3. Excited electrons are passed through electron transport chain to photosystem I

• protons pumped into thylakoid lumen• ATP synthesized through chemiosmosis

4. Photons strike photosystem I5. Excited electrons are passed to NADP+

Photosystem

Redox and Resonance

Electrons can be donated – redox

Electrons can transfer energy to nearby electrons - resonance

Light dependent reaction

Photolysis– water molecule is lysed by enzyme

Photoactivation– e- are excited by photon energy

Photophosphorylation– e- are passed through electron transport chain– ATP generated by chemiosmosis

Photoreduction– e- are accepted by NADP+, it is reduced to NADPH

The Light Dependent Reaction

Non-cyclic Photophosphorylation

Electrons move linearly through reaction from lysis of water to reduction of NADP+

Cyclic Photophosphorylation

Electrons reach photosystem II and are returned to electron transport chain, producing additional ATP through chemiosmosis

Light dependent reaction

Reactants1. Water2. Photon3. ADP + Pi4. NADP+

Products5. Molecular oxygen - 02

6. ATP (from chemiosmosis)

7. NADPH

The Light independent reaction

The Light independent reaction

• Also known as the Calvin cycle• Occurs in the stroma of the

chloroplast• Combines CO2 with electrons from

NADPH and energy supplied by ATP• Ribulose biphospate (RuBp) is the initial

reactant and the final product• Rubisco adds CO2 to RuBp (carbon

fixation)• Each turn creates 1 G3P (3 carbons) that

exits the cycle• 2 G3Ps are then combined to make 1

glucose

The Light Independent Reaction:The Calvin Cycle

The Calvin Cycle

The light independent Reaction

Reactants1. RuBp

2. NADPH

3. ATP

4. CO2

Products1. RuBp

2. NADP+

3. ADP + Pi

4. G3P

Factors affecting rate of photosynthesis

The rate of photosynthesis can be measured directly by oxygen production, or indirectly by increase in biomass

Several variables will influence the rate of photosynthesis

1. Light intensity – amount of sunlight2. Atmospheric carbon dioxide concentration3. Temperature

Limiting Factor

The variable which determines the rate of photosynthesis is called the limiting factor

The limiting factor will vary between environments

Rates of photosyntesis

Factors affecting rate of photosynthesis

Can you give a reasoned explanation for the shape of these curves?

Review

Write a description of the light dependent reaction, include…– Photolysis, photoactivation, photophosphorylation,

and photoreduction

Explain how the light independent reaction (calvin cycle) uses the products of the light dependent reaction.

From memory draw the graphs showing the effects of the three factors that limit the rate of photosynthesis.