Chapter 3 - Photosynthesis:The Details

Photosynthesis• Using the sun to make useful forms of energy• Sunlight plays a much larger role in our

sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems

How does this occur

• Various forms of radiation surround us, from the sun and other sources.

• Some are visible and some are invisible.

Wave model of light

• Electromagnetic radiation travels at 300000000m/s

• Frequencies of visible radiation (light) are perceived as different colours

We can remember the visible spectrum with ROYGBIV!

• Frequencies of visible radiation (light) are perceived as different colours.

•Highest frequency, smallest wavelength = violet

•Lowest frequency, largest wavelength = red

•All frequencies and wavelengths = white

Properties of Light

• http://wps.prenhall.com/esm_krogh_biology_3/0,8750,1135943-,00.html

• http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html

Photon model of light• Light travels in energy

packets called photons• Photons travel at

300000000m/s• The amount of energy in a

photon depends on the frequency of the light. The higher the frequency of light, the more energy the photon carries

• Light can transmitted reflected or absorbed

• Air, water ….mirror…plants

How Does a Plant Capture Light?

Light can be • transmitted (light passes through an object.

• Reflected (light bounces off object)

• Absorbed (light goes into object)

How Does a Plant Capture Light?

• Plants have chlorophyll PIGMENTS (molecules that can absorb specific wavelengths of light)

Plant leaves appear green. Therefore, what colours must the chlorophyll pigments absorb? reflect?

GREENEverything but Green

http://www.johnkyrk.com/photosynthesis.html

Chlorophyll Pigments• There are 2 types

of cholorophyll

• Yellow - green

• Blue – green

• Found in the highly folded plant organelle

Chlorophyll Absorption Whatever chlorophyll a does

not absorb, chlorophyll b will absorb – they are good complements to each other.

• As you can see, the region between 500-600 nm is not absorbed very well, thus plants appear green.

• As the amount of chlorophyll in autumn decays, the green colour fades and is replaced with oranges and reds of carotenoids (other plant pigments).

• Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. http://wps.prenhall.com/esm_krogh_biology_3/0,8750,1135943-,00.html

Photosynthesis in plants• Light energy is used to transform carbon

dioxide and water to energy rich food molecules composed of glucose monomers

• There are 2 stages in this process

Photosynthesis

Divided into two steps:

1. The Light Reactions• Noncyclic electron flow

2. The Calvin Cycle• Cyclic electron flow

The Light Reactions

Divided into three steps:

1. Photoexcitation

2. Electron Transport Chain

3. Chemiosmosis

The Light Reactions

Photosystems are embedded in the thylakoid membrane.

They contain chlorophyll and accessory pigments that are associated with proteins.

A photosystem consists of an antenna complex and a reaction centre.

The Light Reactions

The antenna complex absorbs a photon and transfers energy to the reaction centre.

The reaction centre contains chlorophyll a, whose electrons absorb energy and begin photosynthesis.

The Light Reactions

Photosystem II (P680) Two photons strike photosystem II and

excite 2 electrons from chlorophyll P680. The excited electrons are captured by a

primary electron acceptor and are then transferred to plastoquinone (PQ) and the ETC.

The Light Reactions

Photosystem II (P680) In the ETC, the 2 electrons pass through a

proton pump (Q cycle). The Q cycle transports 4 protons from the

stroma into the thylakoid lumen to create a proton gradient.

The Light Reactions

Photosystem II (P680) The electrochemical gradient drives the

photophosphorylation of ADP to ATP. 1 ATP forms for every 4 protons that pass

through ATPase from the thylakoid lumen into the stroma.

The Light Reactions

Photosystem II (P680) A Z protein splits water into 2 protons, 2

electrons and 1 oxygen atom.The electrons replace those lost from

chlorophyll P680.The protons remain in the thylakoid space to

add to the proton gradient.Oxygen leaves as a byproduct.

The Light Reactions

Photosystem I (P700) Two photons strike photosystem I and excite 2

electrons from chlorophyll P700 (replaced by electrons from P680).

These electrons pass through another ETC. The enzyme NADP reductase uses the 2

electrons and a proton from the stroma to reduce 1 NADP+ to 1 NADPH.

The Calvin Cycle Occurs in the stroma of chloroplasts. Cyclical reactions similar to the Krebs

Cycle.

Divided into three phases:

1. Carbon Fixation

2. Reduction Reactions

3. Regeneration of RuBP

The Calvin Cycle

Phase 1: Carbon Fixation 3 CO2 are added to RuBP to form 3

unstable 6-carbon intermediates. The intermediates split into six 3-carbon

molecules called PGA. These reactions are catalyzed by rubisco.

The Calvin Cycle

Phase 2: Reduction Reactions 6 PGAs are phosphorylated by 6 ATPs to

form 6 molecules of 1, 3-BPG. 6 NADPH molecules reduce the six 1,3-

BPG to 6 G3P or PGAL. One molecule of G3P exits the cycle as a

final product.

The Calvin Cycle

Phase 3: Regeneration of RuBP 3 ATP are used to rearrange the

remaining 5 G3P into 3 molecules of RuBP.

The cycle continues with the RuBP fixing more CO2.

To Produce One G3P…

3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O 9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP