Chloroplasts

Chloroplasts Definition, functions and Characteristics

Animals and human beings feed on smaller organisms or plants to survive. We have various techniques, from cooking it to eating it raw.

Have you ever thought about how plants survive?

What do they feed on? How do they do it? They cannot move or hunt for their prey. So how do they survive?
Read on to find out about one of the basic and most important components of the plant cell.

Definition




They are organelles that are found in plant cells and eukaryotic algae. They can be called the 'cook' of the plant because they use sunlight, carbon dioxide and water to create food and energy for the plant. The word chloroplast comes from the Greek word khloros, meaning “green”, and plates, meaning “formed”. These unique structures are one of a kind, without this plant would never be able to store energy, hence our main source of oxygen is lost. They are considered to be organelles that are basically the small structures in plants that perform specific functions. Chloroplasts main function is Photosynthesis.

The evolution of Chloroplast

The evolution of Chloroplasts can be understood much better through the explanation of the Endosymbiotic Theory. The theory says that chloroplasts have been embedded into the eukaryotic cell the same way as mitochondria were subdued into all eukaryotic cells: by first existing as free-living cyanobacteria that had a symbiotic relationship with a cell, along with creating energy for the cell in return for a place to live, and slowly evolving into a form that could no longer exist separately from the cell.
Chloroplasts contain their own, separate DNA that is biconcave, like that of a bacterial cell, and inherited from the mother plant alga. New chloroplasts are mostly formed through a process called binary fission, or splitting, which is how bacteria generally reproduce. This is similar to the way mitochondria reproduces. Other theories suggest that chloroplasts have evolved from cyanobacteria, and mitochondria evolved from aerobic bacteria. The structure of chloroplasts is very similar to that of cyanobacteria; both have two membranes, circular DNA, thylakoids, and ribosomes.

Characteristics




They are either round, oval, or disk-shaped body that is involved in the synthesis and storage of food and energy. Chloroplasts are divided into two types, chlorophyll a and chlorophyll b.

One of the primary functions of those pigments is to take in light energy. In most plants, chloroplasts occur in all green tissues, though they are mostly concentrated in the parenchyma cells of the leaf mesophyll.

They are roughly 1–2 μm (1 μm = 0.001 mm) in thickness and 5–7 μm in diameter.

Structure




They have a planoconvex or biconvex lens-like structure. It is enclosed by a double-layered envelope. The Osmophilic globules are called Plastoglobuli. It also contains the Chloroplast DNA. The stroma takes up most of the volume of the chloroplast. They are vesicular and have a colorless center. The size of the chloroplast varies from species to species and is constant for a given size of the cell.

These cell organelles can be found in the mesophyll in leaves. The chloroplast is enveloped by an inner and outer membrane with an intermediate space between. The chloroplast contains smaller parts inside called Thylakoids. These thylakoids are arranged like stacks of coins which are called grana. There may be 40-80 grana in the matrix of the chloroplast. There is an average of 35 per mesophyll cell. The space that the chlorophyll takes is called the thylakoid space. It contains the pigment that gives the plant its green color- Chlorophyll.

Function

The chloroplast is the site for photosynthesis. Food for the plant is prepared in the form of sugars. Sunlight, water and carbon dioxide are taken to produce oxygen and sugar. These light reactions take place on the membranes of the thylakoids. It absorbs light energy and converts it into biological energy.

The most important function of Chloroplasts is to provide food by the process of photosynthesis. Chloroplasts produce, along with the nucleus, the ER and the cell membrane; the chloroplast also plays an integral part in Pathogen Defense.
Photolysis of water also takes place here. NADPH2 molecules and water are the products of this reaction.

Calvin Cycle

The light reaction takes place in the Thylakoid Membranes, and dark reaction also known as the Calvin cycle takes place in the stroma. ATP is produced by photophosphorylation. NADPH2 and ATP are termed as the assimilatory powers of photosynthesis.
In order to make one G3P molecule that can exit the cycle and go towards making glucose, three turns of the Calvin cycle are needed. Sixty-six molecules of glyceraldehyde-3-phosphate (G3P) are formed. From that, 11 G3P molecules exit the cycle and go towards making glucose.

55 G3P molecules are recycled, regenerating 33 RuBP acceptor molecules. 99 ATP is converted to 99 ADP (66 during the fixation step, 33 during the regeneration step). NADPH is converted to NADP during the reduction step. A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle or 66 \text {CO}_2CO

Location

Chloroplasts are situated in the cells of the leaves of green plants and the cells of eukaryotic algae.
All green plants have a unique cell organelle called plastid, and chloroplasts are a kind of plastic. The chlorophyll in these chloroplasts is probed by sunlight which results in the conversion of solar energy into usable chemical form. And, this is how sunlight enters our living ecosystem.

As mentioned earlier, the chloroplast is enveloped by a membrane. Chloroplasts have three membranes. The outer membrane is semipermeable and gives way to small molecules. The inner membrane is a slightly less porous layer, and the last and final membrane is called the thylakoid membrane which appears as a series of flattened disks piled on top of each other. Chlorophyll is mostly found in these thylakoids.

Role in Photosynthesis

         6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
The above reactions capture sunlight through chlorophyll and carotenoids to form adenosine triphosphate (ATP) which is the energy currency of the cell along with nicotinamide adenine dinucleotide phosphate (NADPH), which carries charged electrons. This follows the second stage which consists of the light-independent reactions, popularly known as the Calvin cycle. In the Calvin cycle, the electrons that are carried by NADPH convert inorganic carbon dioxide into organic matter in the form of carbohydrates. This process is called CO2 fixation. The remaining carbohydrates and other organic molecules which are not used at that particular time can be stored and used later for different mechanisms of the plant.

What happens to Plants that do not have Chloroplast?

Chloroplasts are an essential element for the health and survival of plants and photosynthetic algae. Chloroplasts can be compared to Solar Panels. They take light energy and convert it into use for that powers multiple functions. So, now what happens to plants that do not have chlorophyll? There is one such exception- Rafflesia, this is a parasitic plant which obtains nutrients from other plants, specifically, Tetrastigma vines. Because Rafflesia gets all of its energy from parasitizing the other plant, it does not need its chloroplast. Hence, the plant has gotten so used to this mechanism, that it has lost the genes that code the development of the chloroplast. It is known to be the only genus belonging to plants on land that lack chloroplasts. 



Some Facts about Chloroplast

1. Simple cells have very few Chloroplasts, whereas complex plants can contain hundreds of them.
2. The most abundant protein in Chloroplast is called Rubisco. It is found in copious amounts.
3. Animals and humans do not need Chloroplasts, because we get our energy from eating and digesting food.
4. Chlorophyll A is the most common type, showcasing the color green, and chlorophyll C is a golden brown color.
5. They also fight diseases as part of the cells immune system.
6. They make amino acids for the cell.
7. Ten percent of a chloroplasts protein is encoded by its own DNA.
8. When they degrade, they turn into chromoplast which is a colorful pigment which in turn is the reason for fruits to change color when they ripen.