Photosynthesis [B]

Hehehe, this song.



Photosynthesis

The Process that Feeds the Biosphere
Life on Earth is solar powered
Two kinds of major methods organisms get their energy
Autotrophs; self-feeders. They generate the energy from sunlight. Most plants and algae fall into this category.
Plants are photo-autotrophs; there are different kinds.
Referred to as the producers of the biosphere
Heterotrophs; energy from compounds other organisms create. Animals eating plants or other animals.
Some eat decomposed leaves, carcasses or feces
Referred to as the consumers of the biosphere

10.1 Photosynthesis converts light energy to the chemical energy of food
The ability of organisms to convert light to energy for the cell systems stem from basic cell structure. Photosynthetic enzymes are grouped together in a membrane, enabling sites for chemical reactions to carry out. Photosynthesis likely originated from a group of bacteria clumped together in a membrane infold that showed an ability to synthesize energy from light.
Chloroplasts: The Sites of Photosynthesis in Plants
Most green parts of plants have the ability of photosynthesis, but the majority of the chloroplasts lie in the leaves.
The color of the leaf is due to chlorophyll
Green pigment inside chloroplasts
Light energy absorbed by chlorophyll drives the photosynthesis
Chloroplast carry out the actual photosynthesis
Found mainly in cells of the mesophyll, located in the tissue in the interior of the leaf
Typical mesophyll cell has about 30-40 chloroplasts, each measuring about 2-4 by 2-7 micrometers
Envelope of two membranes encloses the stroma, the fluid within the chloroplast
Interconnected membrane sacks called thylakoids fill the inside, each in stacks called grana
Carbon dioxide enters and oxygen exits through microscopic pores called stomata (stoma)
Water absorbed by roots is delivered to the leaves through veins. These veins are also used to deliver sugar to other sites on the plant that can’t photosynthesize

Tracking Atoms Through Photosynthesis: Scientific Inquiry
Since 1800s been known that plants, through a chemical process, produce organic compounds and oxygen from carbon dioxide and water.
C_6 O_2+12H_2 O+Light Energy→C_6 H_12 O_6+6O_2++6H_2 O
C_6 H_12 O_6 (Glucose) is used as a method of simplification. In the real photosynthesis process, the sugar created is actually a three-carbon sugar that is used to create glucose.
The above equation can be simplified by removing repeating elements to
CO_2+H_2 O→[CH_2 O]+H_2 O+O_2
The brackets indicate that it’s not an actual sugar, but there to represent the sugar that will be created.

The Splitting of Water
The O_2 given off by plants is actually the oxygen from water, not carbon dioxide. Chloroplast splits water into hydrogen and oxygen. Before this discovery, it was largely believed that photosynthesis split carbon dioxide then added water to carbon.
In 1930s Van Neil changed this. He realized that there was at least one group of plants that took in CO_2, but didn’t release oxygen. Thus, he reasoned that something else must have been releasing the oxygen. His discoveries lead him to the conclusion that the mystery source was water. In the process, he compared the photosynthesis pattern of three photosynthetic organisms, finding that the ones who released oxygen all had water.

The Two stages of Photosynthesis: A Preview
Light reactions
Converts solar energy to chemical energy
Water is split
Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor capped the NADP+
Uses solar power to reduce NADP+ to NADPH
Generates ATP, called photophosphorylation
Produces no sugar
Calvin cycle
Named for Melvin Calvin, described its steps in 1940s with his colleagues
Incorporates CO_2 from air into organic molecules already in chloroplast
Called ‘carbon fixation’
Reduces fixed carbon to carbohydrates by addition of electrons.
Power provided by NADPH
To convert CO_2 to carbohydrate, ATP is required, which is also generated by the light reactions.
Metabolical steps sometimes referred to as ‘dark reactions’ because they happen without the presence of light

10.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH

The Nature of Sunlight
Light is also electromagnetic energy; electromagnetic radiation.
The distance between crests are ‘wavelength’
Range from more than a km to less than a nanometer
Entire range of radiation is the electromagnetic spectrum
From 380-750nm, the visible light spectrum
Drives photosynthesis
Light is composed of photons
Not tangible, but act like objects with their own fixed quantity of energy
Amount of energy inversely related to wavelength of light

Photosynthetic Pigments: The Light Receptors
Light can be reflected, transmitted, or absorbed when meeting matter. Things that absorb light are ‘pigments’. Different pigments absorb different wavelengths of light and reflect/transmit the rest. The colors that we see are the reflected wavelengths of light. A pigments ability to absorb light can be measured using a spectrometer. Absorption spectrum is the graph on which these results are put.

Excitation of Chlorophyll by Light
When chlorophyll absorbs light, the energy from that absorbed light has to go somewhere. Absorption of a photon boosts electrons to an orbital of higher energy. Thus, the electron needs to transfer the energy somewhere, releasing it as heat.

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes

Photosystem is a protein complex called ‘reaction-center complex’ that’s surrounded by light-harvesting complexes. Reaction-center complexes have chlorophyll a molecules. Light-harvesting complexes are composed of both chlorophyll a and b. Together, the light-harvesting complexes act like an antenna for the reaction-center complex.

10.3 The Calvin cycle uses ATP and NADPH to convert CO_2 to sugar
Calvin cycle is similar to citric acid cycle in that a starting material is made after molecules enter and exit the cycle. The Calvin cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the cycle in the form of CO2, and exits as sugar. The cycle uses ATP as an energy source, and NADPH as reducing power.
Major phases
Carbon fixation
Adds each CO2 molecule to a five carbon sugar named ribulose biphosphate
Enzyme that catalases this state is rubisco
The product of the reaction is a six-carbon so unstable it immediately splits in calf, forming two 3-phosphoglycerates
Reduction
Each molecule of the 3-phosphoglycerate gets another phosphate group from ATP. A pair of electrons from the NADPH reduces this 1,3-biphosphoglycerate into G3P which turns into the sugars the plant uses for energy
Regeneration of the CO2 acceptor (RuBP)
The carbon skeletons of five molecules of G3P are rearranged into three molecules of RuBP. Three more molecules of ATP are spent.
The cycle can now continue.
For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of 9 molecules of ATP and 6 of NADPH.