Chloroplasts: Functions, Structure and Process of Photosynthesis

The Chloroplasts Are cell organelles that are related to photosynthesis. They are specialized subunits in the cells of eukaryotic organisms such as plants and algae that perform the process of photosynthesis.

In this process, the pigment known as chlorophyll, captures the energy of the sun and converts it into energy, saving it in molecules specially designed for this purpose.

Chloroplast structure

In this article we will review its functions and structure, as well as its process of photosynthesis.

Structure of chloroplasts

The word chloroplast derives from the Greek word"chloros", which means"green". Chloroplasts are one of three types of plastids or plastids that exist in plant cells and algae. They are cellular organelles present only in plant cells.

They are similar to the mitochondria of animal cells, since both cellular organisms are responsible for generating metabolic energy, containing their own genetic systems and replicating by division.

The Eukaryotic cells As chloroplasts are organisms that evolved by endosymbiosis, that is, they have their own structure and a nucleus, separated from the external environment.

Like mitochondria, they have their own DNA, which is thought to correspond to that of a photosynthetic cyanobacteria, which was phagocytosed by a primary eukaryotic cell, giving rise to this organelle.

Despite their similarities with the mitochondria of animal cells, chloroplasts are larger and more complex than these. Not only are they responsible for the process of photosynthesis and power generation, but other fundamental processes in the cell.

Chloroplasts are responsible for the photosynthesis of converting CO 2 In carbohydrates. In addition to this, chloroplasts synthesize animo acids, lipid components of their own membranes and fatty acids.

In addition to this, chloroplasts are involved in the reduction of nitrites (NO 2 ) In ammonia (NH 3 ), A fundamental step in the incorporation of nitrogen into organic compounds.

Chloroplasts are the only plastids of eukaryotic cells that have different very important roles in plants. Their distribution is homogeneous in the cytoplasm of cells and some are usually concentrated around the nucleus or just below the plasma membrane. A typical plant cell has about 50 chloroplasts in it.

Chloroplasts are highly dynamic, move and circulate around and within plant cells and occasionally reproduce. This behavior is strongly influenced by environmental factors that affect the plant, such as the color and intensity of light.

The size of the chloroplasts is approximately 5 to 10 μm and are surrounded by a double membrane called chloroplast wrapping.

In addition to the inner and outer membranes, chloroplasts have a system of a third internal membrane, called the thylakoid membrane, formed from a network of flattened discs known as thylakoids, which are arranged in piles called grana.

Because of this structure of three membranes that have chloroplasts, the internal organization of these cells is more complex than that of the mitochondria. These membranes divide three different compartments that have chloroplasts:

  • The space between the two membranes of the chloroplast wrapping.
  • The stroma, which is inside the envelope but outside the thylakoid membrane.
  • The thalacoidal lumen.

However, both chloroplasts and mitochondria have a major function: the chemiosmotic generation of ATP, acronym for adenosine triphosphate, an organic component that provides energy to organisms for several different metabolic processes.

The chemiosmotic process is when plant cells produce ATP. In this case, the protons inside the cell reach the membrane and the ATP molecules are synthesized as a result.

Functions

The inner membrane of chloroplasts does not play a role in photosynthesis. The electron transport system in chloroplasts is located on the thylakoid membrane, and the protons are pumped through this membrane from the stroma to the thylakoid lumen.

It is the process of traversing the membrane that generates energy to filter the protons and that the energy created contains ATP. In the case of chloroplasts, ATP is produced in the first phase of photosynthesis and thus provides energy for the second stage of the process.

It is this electrochemical gradient that drives the synthesis of ATP as protons that return to the stroma. It is the thylakoid membrane that plays the main role in the process of generating metabolic energy in the plant cell.

After the start of the second phase of photosynthesis, this ATP provides the energy for the development of phosphoglycerides, glycerin molecules that are fundamental to convert them into organic compounds.

These organic compounds are combined in the form of a six-carbon glucose molecule. Plants then store glucose in different ways. Some plants collect carbon molecules, other plants store them in a kind of tubes and other plants convert this energy into fructose or fruit sugar.

The sugar we eat regularly, known as sucrose, is another product that results from the accumulation of glucose inside the plants.

In addition to this, when the process of photosynthesis is finished, ATP has other functions inside the plant cell, allows mobility, cell division and other biosynthetic reactions.

Chloroplasts and the process of photosynthesis

During photosynthesis, solar energy is converted into chemical energy. This energy is then stored in the form of glucose, ie sugar.

Carbon dioxide (CO 2 ), Water and sunlight are the elements that are involved in the process of producing glucose, water and oxygen known as photosynthesis. In total, photosynthesis occurs in two stages, related to the reaction of the plant cell to light and then the reaction in a stage without light.

The stage of reaction to light occurs when there is light present and the process begins in the grana of chloroplasts, where the pigment produced and used to convert light into chemical energy, known as chlorophyll a, is stored.

There are other pigments also involved in the process of photosynthesis such as chlorophyll b, carotene and xanthophyll, which is a yellow-colored pigment.

In this first stage, sunlight is converted into chemical energy, in the form of the ATP (adenosine triphosphate) molecule and the NADPH (nicotinamide adenine dinucleotide phosphate) molecule.

These two molecules, ATP and NADPH, are used in the dark stage of the process for the production of sugar. This stage is also called the carbon fixation stage or the Calvin's Cycle .

During the Calvin Cycle, reactions occur in the chloroplast stroma. This stroma contains enzymes that facilitate a series of reactions using ATP, NADPH and CO 2 For the production of sugar.

The sugar can then be stored in the form of carbohydrates, used during cellular respiration or in the production of cellulose by the plant.

Genetic material

Like mitochondria, chloroplasts have their own genetic system, their own DNA, which demonstrates their primary source of photosynthetic bacteria. The genomes of chloroplasts are similar to those of mitochondria, as they consist of circular DNA molecules that occur in many copies within each of the organelles.

Even the chloroplast genome is larger and more complex than that of the mitochondria, containing approximately 120 genes.

It is very interesting to consider that as both chloroplasts and mitochondria have thousands of copies of their DNA inside them, an animal or plant cell - has many mitochondria or chloroplasts to replicate their DNA. Thus, in an organism there may be millions of copies of DNA from a mitochondrion or a chloroplast.

When chloroplasts divide - including their genetic material - they are randomly distributed into"daughter"cells during mitosis or meiosis processes. When this cell division occurs, the organelles terminate in different cells.

In the case of non-nuclear DNA possessed by chloroplasts and also mitochondria, this can be inherited only from the father or the mother, not both. In humans, for example, children have DNA from their mother's mitochondria, not from the father.

The genome of chloroplasts has RNA (ribosomal ribonucleic acid) inside them, essential for the synthesis of proteins; In addition to other proteins that are related to the genetic expression of this DNA and functions related to photosynthesis.

The proteins involved in the photosynthesis process are more than 30, including the components of photosystems 1 and 2.

The Rubisco enzyme Is known for its role as CO catalyst 2 For its conversion into energy. This is the main component of the chloroplast stroma and is also the most abundant protein in the planet. This makes very interesting the fact that one of its subunits is present in the genome of chloroplasts.

Chloroplasts have more coded proteins than mitochondria, although 90% of their ribosomal proteins are still encoded by nuclear genes. These proteins must be classified in the correct compartment of chloroplasts, which is more complex because these organelles have three separate membranes.

The proteins are translocated to be imported into the chloroplasts, with a direct translocation between the two membranes of the chloroplast wrapping and then removed.

After other processes are transported from the internal membrane of the chloroplast to the stroma. Subsequently, the proteins are imported into the thylakoid lumen.

Location within the plant cell

Chloroplasts are not found everywhere in the plant, but only in green areas. In most plants, this occurs in the leaves, but in the cactus is found in the stems.

Chloroplasts are oriented in the best way relative to available light. In low light conditions, they will expand to the maximum on the sheet, trying to cover a larger surface to receive the maximum amount of light possible to absorb.

On the other hand, in case of too much light, they will line up in vertical columns along the cell walls or in lateral form to look for production and to avoid that the light arrives to them of very direct form and can damage them. This reduces the exposure and also protects them from photooxidative damage.

The cell walls in plants are made of cellulose, which is also the most abundant macromolecule on the planet. Cellulose fibers are long, linear polymers composed of hundreds of glucose molecules.

This ability to distribute chloroplasts so they can shelter one after another or expand is the reason why terrestrial plants evolved and have many small chloroplasts instead of few and larger ones.

In large plants, this movement of chloroplasts occurs by phototropines, that is, light photoresists that react to blue light. In other plants such as those with flowers, algae and others, the movement of chloroplasts is also influenced by red light.

Other Functions

In addition to being fundamental in the process of photosynthesis, there are other functions of chloroplasts within plants. These are:

Plant Immunity

Plants do not have specialized cells of immunity, all cells of the plant organism are participants in the immune response.

However, chloroplasts, together with the nucleus, cell membrane and endoplasmic reticulum are primordial in defense against pathogens. This is why aggressive agents usually attack chloroplasts before another plant structure.

The immune system of plants responds in two ways: the first is a hypersensitive response, which auto-infects cells and schedules cell death; And acquired systemic resistance, where infected cells send warning signals to the rest of the plant of the presence of the pathogen.

Chloroplasts respond to both mechanisms, deliberately damaging their photosynthetic system, causing a kind of reactive oxygen, which in turn triggers the hypersensitive response. This reactive oxygen also removes any pathogens into the cell.

In the case of lower levels of reactive oxygen, the second phase of acquired systemic resistance begins, which triggers the production of defense molecules in the rest of the plant.

References

  1. Biology. Animal and Plant cells. Retrieved from biology.tutorvista.com.
  2. Chloroplast and Other Plastids. Retrieved from ncbi.nlm.nih.gov.
  3. What role does ATP play in photosynthesis? Retrieved from reference.com.
  4. Inheritance of mitochondrial and chloroplast DNA. Retrieved from khanacademy.org.
  5. Retrieved from biology.about.com.
  6. Plant Cells, Chloroplasts, and Cell Walls. Retrieved from nature.com.


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