5 Characteristics of chloroplast

The chloroplast is a very interesting photosynthetic organelle, since, based on the endosymbiotic theory, it is said that the origin of this organelle begins with the adaptation of an aerobic cyanobacteria, which was housed inside a non-photosynthetic proeukaryotic cell that contained mitochondria. After a while something surprising happened: the symbiotic cyanobacteria, which lived inside the cell, became a cytoplasmic organelle, which they called the chloroplast.

If you want to know more about what the chloroplast is, what its function and structure is , we invite you to read this article where you will learn the most important and essential data to understand some biochemical processes in which the chloroplast intervenes.

What is the chloroplast

The chloroplast is an organelle that we can only find in the cytoplasm of plant cells . We commonly recognize the chloroplast as a disc-shaped organelle. However, they do not have a specific shape, quantity or position .

For example, Spirogyra algae have a single spiral-shaped chloroplast, however, there are other shapes such as elliptical or biconvex. The size of the chloroplasts can vary between 5 to 10 mm in diameter and 2 to 4 mm in thickness. Therefore, to visualize a chloroplast we will need to use an optical microscope to help us amplify the image of a plant cell. Regarding the number of chloroplasts, in all those cells that carry out photosynthesis it is between 40 to 200 chloroplasts per cell.

On the other hand, chloroplasts will have DNA with genes that code for proteins . This is very important, since the proteins are going to be in charge of carrying out the functions in the chloroplasts. An interesting fact about chloroplast genes is inheritance from parents to children, because like mitochondria, genetic transmission occurs from a single parent to the child.

Characteristics of chloroplast

Chloroplasts are specialized organelles found in plant cells that are responsible for photosynthesis, the process by which plants convert sunlight into energy.

Here are some key characteristics of chloroplasts:

  1. Structure: Chloroplasts have a double membrane, with an outer membrane and an inner membrane. Within the chloroplast, there is a network of membrane sacs called thylakoids, which are stacked together to form grana. The grana are connected by stroma lamellae, which are thin sheets of membrane. The stroma is the space inside the chloroplast, surrounding the thylakoids.
  2. Chlorophyll Pigments: Chloroplasts contain green pigments called chlorophyll, which gives plants their characteristic green color. Chlorophyll molecules are embedded in the thylakoid membranes and are responsible for capturing light energy during photosynthesis. There are different types of chlorophyll, with chlorophyll a and chlorophyll b being the most common in plants.
  3. Photosynthesis: The primary function of chloroplasts is to carry out photosynthesis. During this process, chlorophyll absorbs light energy from the sun and converts it into chemical energy. This energy is then used to convert carbon dioxide and water into glucose (a type of sugar) and oxygen. The oxygen is released as a byproduct, while the glucose is used by the plant as a source of energy and building block for other molecules.
  4. DNA and Ribosomes: Chloroplasts have their own DNA and can produce some of their own proteins. This genetic material allows chloroplasts to replicate and divide independently of the cell. Chloroplasts also have ribosomes, which are involved in protein synthesis.
  5. Endosymbiotic Theory: The presence of DNA and the ability to divide independently have led scientists to propose the endosymbiotic theory. According to this theory, chloroplasts are thought to have originated from ancient bacteria that were engulfed by early eukaryotic cells. Over time, this symbiotic relationship evolved into the chloroplasts we see in plants today.

In summary, chloroplasts are organelles found in plant cells that contain chlorophyll pigments and are responsible for photosynthesis. They have a double membrane, thylakoid membranes, and a stroma. Chloroplasts have their own DNA and ribosomes, and their origin is believed to be a result of an ancient endosymbiotic relationship.

Chloroplast function

The chloroplast is essential in plant cells , because it is the site where photosynthesis will take place . Fundamentally, photosynthesis takes place in two separate successive stages. In the first stage, light-dependent, or photodependent, reactions occur in the thylakoid membranes; and in the second stage, non-directly light-dependent or photoindependent occurs in the stroma. Discover more about Photosynthesis: definition and phases .

Below we will mention other functions that the chloroplast performs:

  • Convert light energy into chemical energy .
  • The molecules present in the chloroplast participate in the set of photosynthesis reactions .
  • Chlorophyll, a pigment present in the chloroplast, absorbs the energy from the sun that is needed for photosynthesis.
  • Chloroplasts, through chlorophyll, give the characteristic green color to plant leaves .
  • They carry out various metabolic processes , such as amino acid metabolism, biosynthesis of lipids, fatty acids, biosynthesis of purines and pyrimidines, etc.
  • You may also be interested in Cellular organelles: what they are, functions and examples .

Chloroplast structure

The parts of the chloroplast are the outer membrane, the intermembranous space, the inner membrane, the stroma, the thylakoids and the granae. Below we will detail each one:


outer membrane

It is the outer envelope of the chloroplast. It has several porins, through which various solutes will pass. However, these channels may present certain selectivity towards some solutes. It is approximately 6 nm thick.

intermembranous space

It is the narrow space between the outer membrane and the inner membrane of the chloroplast.

internal membrane

It is the inner envelope of the chloroplast. It is approximately 6 nm thick. Unlike the outer membrane, in order for substances to pass through this membrane, they must be helped by specific transporters that are anchored in the membrane. Furthermore, a characteristic of this structure is its impermeability to ions and metabolites.


The stroma is the space inside the chloroplasts not occupied by the thylakoids. This space is relevant, because it has abundant enzymes responsible for all the reactions of the photoindependent phase of photosynthesis, such as rubisCO, DNApolymerase and RNApolymerase.

On the other hand, in the stroma there is DNA and enzymes that participate in the Calvin cycle, in the fixation of carbon dioxide in a process that is not directly dependent on light or photoindependent of photosynthesis.


The thylakoids are membranous sacs stacked on top of each other . These structures are surrounded by a membrane that delimits the intrathylakoid space, called lumen. The thylakoid membrane will have different substances, among which photosynthetic pigments, such as chlorophyll, stand out. It is worth mentioning that a series of thylakoids arranged in a pile collectively form a grana .

The thylakoid membrane contains many enzymes, proteins, substances necessary for photosynthesis, which we will mention below:

  • Proteins, 50%, can be classified into proteins associated with pigments, electron transport and ATP-synthetase.
  • Lipids by 38%
  • Pigments, such as carotenoids and chlorophylls, by 12%.

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