PhotoSynthesis
Content
What is photosynthesis?
The main goal of photosynthesis is to convert light energy from the sun to chemical energy which can be used by the organism.
The formula for photosynthesis is:
6CO2 + 6H20 + photons --> C6H12O6 + 6O2
The formula for photosynthesis is:
6CO2 + 6H20 + photons --> C6H12O6 + 6O2
Where does photosynthesis occur around the globe?
![Picture](/uploads/6/0/8/4/60840857/8510711.png?439)
The image to the left shows the distribution of photosynthetic organisms around the globe. Contrary to what most students think, the majority of the globes photosynthesis is occurring in the oceans by photosynthetic bacteria.
Photosynthesis by terrestrial plants only represents a small portion of the globes photosynthetic production.
McClain et al (1998)
Why is photosynthesis important?
All living organisms on planet Earth owe their existence to photosynthetic organisms. Without photosynthesis, all life on Earth would cease to exist since these organisms produce oxygen require by all animal life. Before photosynthetic organisms existed, the Earth atmosphere was harsh and could not sustain life. The video below titled 'The History of Earth' does a great job of showcasing the history of life on Earth. The video discusses the first photosynthetic organisms to live on the ocean floors called cyanobacteria which formed rock-like colonies called stromatolites. Later in the video, the Cambrian explosion is discussed and the movement of life into terrestrial environments. Although this video is over an hour long, you can choose to show brief excerpts of the video using TubeChop.
Science for the Win (2012)
The Leaf Cross-Section
![Picture](/uploads/6/0/8/4/60840857/6290781.png?523)
There are four main components that make up a leaf:
- Epidermis
- Protects plant from water loss and the invasion of unwanted elements
- Palisade Mesophyll
- Site for photosynthesis
- Contain chloroplasts
- Spongy Mesophyll
- Site for photosynthesis
- Contain chloroplasts
- Guard Cells
- Allow gas and water exchange between the leaf's interior and the outside world
The Chloroplast
![Picture](/uploads/6/0/8/4/60840857/1480314.png?596)
Chloroplasts are found within the mesophyll cells of the leaf. There are three main structures within the chloroplast cell:
- Thylakoids
- Site of light-dependent reactions of photosynthesis
- Membranes contain chlorophyll
- Granum
- Stack of membranes made up of thylakoids
- Stroma
- Fluid inside the chloroplast cell
- Site of light-independent reactions of photosynthesis
Why are Plants Green?
Light is a form of electromagnetic radiation. Together, the different types of electromagnetic radiation forms the electromagnetic spectrum (seen below). Each type of electromagnetic radiation has a specific wavelength and the wavelength is directly related to the amount of energy. The portion of the electromagnetic spectrum that we can see is called the visual portion. This is the portion of the spectrum that can be seen by the human eye and ranges from 400nm to 700nm. This visible portion of the electromagnetic spectrum contain many different colours, however visible light appears white to us since it is made up of all of the colours. According to the particle theory, light travels in particles called photons.
OpenStax (2015)
![Picture](/uploads/6/0/8/4/60840857/8045737.png?406)
Although photosynthetic organisms are exposed to the entire visible portion of the electromagnetic spectrum, they do not use all wavelengths equally.
Plants contain pigments that absorb specific wavelengths of light. The common pigments found in plants are chlorophyll a, chlorophyll b, and beta-carotene. The absorption spectra for these pigments are seen in the image to the right.
Chlorophyll a and chlorophyll b are the main pigments used in plant photosynthesis. As seen in the image to the right, these pigments mainly absorb photons in the blue and red regions as indicated by their peaks.
Green wavelengths, however, are not absorbed and are instead reflected. This reflected green light is what makes plants appear green to us!
OpenStax (2015)
![Picture](/uploads/6/0/8/4/60840857/4302376.png?584)
Chlorophyll a plays a major role in the light-dependent reactions of photosynthesis. It contains a hydrophobic tail and a porphyrin ring head.
The hydrophobic tail inserts into the thylakoid membrane.
The porphyrin ring is the portion of chlorophyll a that absorbs photons used for photosynthesis.
The hydrophobic tail inserts into the thylakoid membrane.
The porphyrin ring is the portion of chlorophyll a that absorbs photons used for photosynthesis.
OpenStax (2015)
![Picture](/uploads/6/0/8/4/60840857/1263845.png?672)
When a photon of light is absorbed by chlorophyll a, the energy bumps an electron up to a higher-energy state (excited state).
When an electron is excited, it gains potential energy which is used in the light-dependent reactions of photosynthesis.
OpenStax (2015)
The Stages of Photosynthesis
Light-Dependent Reactions
The light-dependent reactions of photosynthesis are the first stage of the process. These reactions occur in the thylakoid membrane of the chloroplast cell. The light-dependent reactions are composed of two main stages:
PSII takes electrons from water to replace electrons sent down the electron transport chain. Oxygen is released, and protons are freed into the thylakoid lumen.
PSI receives electrons from the Electron Transport Chain. Photons are once again absorbed by chlorophyll, exciting the electrons once again. The electrons are accepted by NADP+ to produce NADPH.
Protons are pumped by ATP synthase from the thylakoid lumen into the stroma to produce ATP.
NADPH, ATP, and oxygen are the products of the light-dependent reactions.
- Photosystem II (PSII or P680)
- Photosystem I (PSI or P700)
PSII takes electrons from water to replace electrons sent down the electron transport chain. Oxygen is released, and protons are freed into the thylakoid lumen.
PSI receives electrons from the Electron Transport Chain. Photons are once again absorbed by chlorophyll, exciting the electrons once again. The electrons are accepted by NADP+ to produce NADPH.
Protons are pumped by ATP synthase from the thylakoid lumen into the stroma to produce ATP.
NADPH, ATP, and oxygen are the products of the light-dependent reactions.
OpenStax (2015)
Light-Independent Reactions (Calvin Cycle)
The light-independent reactions are the second stage in the process of photosynthesis. These reactions occur in the stroma of the chloroplast. In this stage of photosynthesis, carbon atoms from carbon dioxide are incorporated into organic molecules and used to build 3-carbon sugars. This entire process is run by ATP. The Calvin cycle is composed of three main stages:
In the next stage of the Calvin cycle, ATP and NADPH from the light reactions are used to convert 3-PGA into a 3-carbon sugar called G3P.
In the final stage of the Calvin cycle, some of the G3P molecules are used to make glucose and some are recycled to regenerate the 5-carbon acceptor molecule, RuBP.
For each turn of the Calvin cycle, three carbon dioxide molecules are required to produce one molecule of glucose. This is so one molecule of G3P can be made into glucose while the remaining five molecules of G3P can be recycled to produce three more RuBP molecules.
- Carbon fixation
- Reduction
- Regeneration
In the next stage of the Calvin cycle, ATP and NADPH from the light reactions are used to convert 3-PGA into a 3-carbon sugar called G3P.
In the final stage of the Calvin cycle, some of the G3P molecules are used to make glucose and some are recycled to regenerate the 5-carbon acceptor molecule, RuBP.
For each turn of the Calvin cycle, three carbon dioxide molecules are required to produce one molecule of glucose. This is so one molecule of G3P can be made into glucose while the remaining five molecules of G3P can be recycled to produce three more RuBP molecules.
OpenStax (2015)
Summary Video
Pearson Marketing (2011)