# Structural Biochemistry/Cell Organelles/Chloroplast
## Introduction

Algal cells contain chloroplasts, membrane enclosed organelles of photosynthesis that evolved from a cyanobacterium. In Earth's biosphere, algae plus bacterial phototrophs feed all marine and aquatic ecosystems, producing the majority of oxygen and biomass available for Earth's consumers. Besides, the "green plants" include primary endosymbiont algae descended from a common ancestor containing a chloroplast. The chloroplasts of primary endosymbionts are enclosed by two membranes. They are the inner membrane and the outer membrane. The inner membrane is from the ancestral phototroph's cell membrane and the outer membrane is from the host cell membrane as it enclosed its prey. The chloroplast of the green algae and red algae have diverged from their common ancestor that use the pigments absorbing different ranges of the light spectrum.
Chloroplasts are organelles responsible for photosynthesis or the conversion of CO2 to glucose in plant cells, some protists, and algae. There are three types of membranes in chloroplasts:
* The smooth outer membrane that is permeable to molecules
* The smooth inner membrane contains membrane proteins that selectively allow access to small molecules and proteins
* The thylakoid membrane system
The fluid inside the chloroplast and surrounding the thylakoid system is known as the stroma.

## Thylakoid

Thylakoid membranes envelop a system of interconnected vesicles. Stacks of these thylakoid membranes are known as grana. The main function of the thylakoid membranes is to house the following protein assemblies:
* Photosystem I
* Photosystem II
* Cytochrome b and f
* ATP synthase
All of these carry out photosynthesis as a whole.
The thylakoid membranes also contain chlorophyll, which gives plants its green color. The function of the chlorophyll is to capture the light necessary for photosynthesis.
In the thylakoid membrane and compartment, the light reactions take place in the thylakoid membrane and the thylakoid compartment and are concerned with the initial conversion of light energy into chemical energy that stored in ATP and NADPH.
Besides, the ATP and NADPH feed into the Calvin cycle in the Chloroplast stroma where the ATP provides energy for the molecular rearrangments and also the electrons carried by the NADPH are transferred to the organic molecules involved in the Calvin cycle.

## Inhibition of photosynthesis

There are many herbicides on the market that advertise for their ability to kill weeds. In order to kill the weeds, a disruption of photosystem I or photosystem II is required. The basic idea is that the electrons are activated once they received sunlight. The activated electrons then moves from the chlorophyll into a series of cytochromes creating what is called an electron transport chain. These electrons that are transferred along the electron transport chain are later used for carbon fixing, hence resulting in photosynthesis. An interruption in photosystem II is a result of the inhibitors stopping the flow of electrons, whereas an interruption in photosystem I is initiated by an inhibitor that creates change in energy through electron diversion at the terminal photosystem. Both types of interruptions can stop the process of photosynthesis.
Some inhibitors of photosystem II are diuron and atrazine. Diuron blocks the active site of plastoquinone in photosystem II, and its blockage disrupts the steady flow of electrons to the plastoquinone. As a result, sunlight cannot be converted to energy, and ultimately leading to the death of weeds. Atrazine works in similar ways as diuron. An example of photosystem I inhibitor is paraquat. Paraquat converts the electrons that it receives from photosystem I to radicals. The radical then interacts with oxygen to form reactive oxygen substances. These substances, in turn, interact with the membrane lipids, which can cause damage to the membrane because the double bonds of the membrane lipids are altered.

## Stroma

The stroma is the fluid contents of the chloroplast, it contains the enzymes of the chloroplast such as RUBISCO, which is responsible for the dark reactions. The stroma is also home to the DNA of the chloroplasts that helps it to carry out its functions, some ribosomes of its own, and RNA. Due to the chloroplasts’ individual DNA, it has been hypothesized that the chloroplasts were once free living bacteria that became involved in symbiotic relationship with eukaryotes and eventually became permanently incorporated into the cell’s structure.
The Dark reaction, mainly the Calvin Cycle that take place in the substance surrounding the thylakoids. Carbon from the carbon dioxide is brought into the cycle as a source of carbon in the Calvin cycle.

## Purpose

Chloroplasts produces starches and sugars. In order to do so, the plants extract energy from the sun. The energy extraction from the sun is called photosynthesis. With the energy extracted, the chlorophyll in the plants can combine carbon dioxide and water together. Both the animals a plants uses chloroplast for energy and food. Animals also uses it for oxygen to breath.
* Molecular Form: 6 CO2 + 6 H2O --> sugar(C6H2O6) + 02

## Reference

* http://www.biology4kids.com/files/cell_chloroplast.html
* Berg, Jeremy "Biochemistry", Chapter 27 the Integration of Metabolism. pp 584. Seventh edition. Freeman and Company, 2010.
* http://www.ehow.com/about_6540515_herbicide-effects-photosynthesis.html
* http://www.ehow.com/list_5985010_effects-diuron-photosynthesis-rates_.html
Slonczewski, Joan L. Microbiology "An Evolving Science." Second Edition.