Photosynthesis

Objectives

3.8.1 – State that photosynthesis involves the conversion of light energy

into chemical energy.

3.8.2 – State that light from the sun is composed of a range of wavelengths (colors).

3.8.3 – State that chlorophyll is the main photosyn- thetic pigment.

3.8.4 – Outline the differences in absorption of red, blue, and green light by chlorophyll.

Metabolism reviewPlants and other autotrophs are the

producers of the biosphere.Photoautotrophs use light as the energy source.Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia.

Uniquely bacterial.

Heterotrophs get organic compounds from other organisms.

These organisms are consumers and decomposers.Almost all heterotrophs depend on photoauto- trophs for food and for oxygen, a byproduct of photosynthesis.

Metabolism reviewPhotosynthesis and respiration recycle

compounds required for all life on Earth.The energy for this comes from the sun.

Animals need plants, and vice versa.

Definition of photosynthesisPhotosynthesis is the conversion of light

energy to chemical energy that is stored in glucose or other organic compounds.Occurs in

plants algae

cyanobacteria which began creating our

oxygen-rich atmosphere 2 billion years ago as a waste product.

Definition of photosynthesisDuring photosynthesis water and carbon

dioxide are converted into glucose and oxygen.

Sunlight provides the energy for the reaction.The chemical equation (you must remember) :

6CO2 + 6H2O → C6H12O6 + 6O2

light energy

Energy of sunlightSunlight is composed of a range of

wavelengths.Distance between crests

of waves is called the wavelength.

Wavelengths of electro-magnetic radiation

range from <1 nm (x rays & gamma rays)

to >1 km (radio waves).

We see a smallfraction of thesun’s energy.

Energy of sunlightThe range of electromagnetic radiation is

the electromagnetic spectrum.The most important segment for life is between

380 - 750 nm: visible light.

Energy of sunlightAlthough a wave, many of light’s properties

are those of a discrete particle, the photon, with fixed quantities of energy.

Energy in a photon is inversely related to its wavelength.The atmosphere screens out most wavelengths, letting only visible light pass in sig. quantity.

Consider UV light: →a smaller wavelength produces more energy than visible light, caus-ing skin cancer.

Light absorption by chlorophyllA spectrophotometer measures the ability of

pigments to absorb various wavelengths of light.

Green light isnot absorbed

Blue (& red) lightis absorbed.

Light absorption by chlorophyllAn absorption spectrum plots a pigment's

light absorption versus wavelength.Photosynthesis performs work with the wave- lengths of light that are absorbed.

Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.

There are several pigments in leaves.

Carotenoids are theyellow/red pigments.

Pigments in leaves

Chlorophyll captures sunlightWhen light meets matter, it may be

reflected, transmitted, or absorbed.Chlorophyll is a pigment that absorbs red &

blue light, whiletransmitting and

reflecting green light.

Chlorophyll captures sunlightChlorophyll is the main photosynthetic

pigment.It contains magnesium. It captures solar energy.

Sunlight converted to chemical energyLight energy is used to produce ATP (adenosine triphosphate).

The energy in the electron from chlorophyll (knock- ed off by a photon from the sun) is used to add a PO4

- group to ADP.

When energy is needed by a cell, the PO4-

group is removed, and the energy is released.

Release energy → ← Insert energy

Photosynthesis

Chloroplast

ObjectivesC.4.1 – Draw & label a diagram showing the structure of a chloroplast as seen in electron micrographs.3.8.5 – State that light energy is used to produce ATP and to split water molecules (photolysis) to form oxygen and hydrogen. 3.8.6 – State that ATP and hydrogen (derived from the photolysis of water) are used to fix CO2 to make organic molecules.3.8.7 – Explain that the rate of photosynthesis can be measured directly by production of oxygen or uptake of CO2, or indirectly by an increase in biomass.3.8.8 – Outline effects of temperature, light intensity, & CO2 concentration on rate of photosynthesis

Structure of a chloroplastIn electron micrographs the stacks of thylakoids

(called grana) are obvious.

Structure of a chloroplastReminder: the theory of endosymbiosis says that

chloro-plasts are derived from what were once free-living bacteria.

Both have similar DNA and ribosomes, and similar size.Chloroplasts have a

double membrane.Stroma is cytoplasmic.

Chloroplast structure vs. functionChloroplasts have:

1) Large surface area of thylakoid membrane for light absorption and enzymatic energy capture.

2) Compartmentalization within thylakoids for accumulation of protons.

3) Space for enzymes of the Calvin cycle in the stroma.

Photosynthetic reactionsPhotosynthesis consists of light-dependent and

light-independent reactions.

6CO2 + 6H2O → C6H12O6 + 6O2

light energy

The light-dependent reactionsLight energy is used to produce ATP and to split

water molecules (photolysis) to form oxygen and hydrogen.

The light-dependent reactions:Light is required.

1st, electrons & hydrogen are moved from

water to NADP+ (nicotinamide adenine dinucleotide

phosphate) → NADPH (carries the energy-rich electrons to the Calvin cycle).2nd, ATP is produced by photophosphorylation.

The light-dependent reactionsLight energy is used to produce ATP (adenosine triphosphate).

The energy traveled from the sun, to the plant, to be eaten by the animal.Solar energy is used to add a PO4

- group to ADP.When energy is needed by a cell, the PO4

- group is removed, and the energy is released.

Release energy → ← Insert energy

The light-dependent reactionsWhen chlorophyll absorbs a photon, one of its

electrons is elevated to an orbital with more potential energy.

An electron from Mg in the porphyrin ring is

excited.

The light-dependent reactionsNormally the excited electron would

immediately drop down to a lower energy state and give up its energy as fluorescence.

However, the energy is captured and slowly released to make ATP.

Fluorescence

Both photosystems harvest light (see photons).

Photosynthesis fixes CO2

ATP and hydrogen (from the photolysis of water) are used to fix, or bind, CO2 to make organic molecules.

The light-independent reactions:Occur with or

without sunlight.Use the energy in

the ATP and theNADPH from the

light reactions.Combine six CO2 molecules to make a 6-carbon sugar (glucose).

24H+ + 6CO2 → C6H12O6 + 6H20 (partial equation)

(NADPH is oxidized to NADP+; CO2 is reduced)

The rate of photosynthesisThe rate of photosynthesis can be measured

directly by the production of oxygen or uptake of CO2, or indirectly by an increase in biomass.

Conversion of light energy into plant biomass

More sunlight should produce more biomass.

Weigh plant material

The rate of photosynthesisThe rate of photosynthesis can be measured

directly by the production of oxygen or uptake of CO2, or indirectly by an increase in biomass.

Variables affect photosynthesisTemperature, light intensity, and CO2

concen-tration affect the rate of photosynthesis.

Why does photos-synthesis decrease as the temperature rises beyond a certain point?

Variables affect photosynthesisTemperature, light intensity, and CO2

concen-tration affect the rate of photosynthesis.

Chlorophyll (an enzyme) becomes saturated with CO2 (the substrate) at some point.

Variables affect photosynthesisTemperature, light intensity, and CO2

concen-tration affect the rate of photosynthesis.

There are only so many chlorophyll molecules in a leaf; at some point all are being used.

What is happening here?