Chloroplasts Function, Structure and Diagram in Plant Cell

What are Chloroplasts?

Chloroplasts are the organelles in the cell of the plant that converts light energy through photosynthesis into stable chemical energy. They are found within the mesophyll of the cell of all higher plants and as they carry out their function, they sustain plant life. Plants are the basis of life on planet earth and as primary producers, chloroplast plays a crucial role in plants.

The chloroplasts contains chlorophyll that gives green plants their colour
The chloroplasts contain the green pigment molecule (chlorophyll) that gives green plants their color

Chloroplasts Definition

In biology, chloroplast can be defined as a plant cell organelle that contains green pigment. It is seen in cells of plants and other eukaryotes that are photosynthetic. This organelle is semi-autonomous like the mitochondria and possesses its own DNA. Hence, it does not rely solely on the genes in the nucleus. The DNA of the chloroplast is called the chloroplast DNA or cpDNA and certain proteins are produced from it.

Chloroplast location

In plants, these organelles are particularly concentrated in the internal cell layers of the leaf. Chloroplasts are found in parenchyma cells of the leaf mesophyll and in the cells of all green tissues of algae and plants. They are also found in photosynthetic tissues that do not appear green e.g the red leaves of some plants, brown blades of giant kelp, etc.

Chloroplast as a type of plastid

Plant cells have double-membraned organelles referred to as plastids which are not present in the animal cell. These plastids are of three types and play a major role in the manufacturing and storage of food in the plant cell. However, certain plastids are capable of converting from one form to another like in the case of fruit ripening. In ripening fruits, the chloroplasts transform into another form of plastid (chromoplasts). There are 3 types of plastids.

Types of plastid

  1. Chromoplasts
  2. Leucoplasts
  3. Chloroplasts
Chromoplasts

These are plastids are colored due to the pigment produced and stored inside them. They are found in all flowers, roots, senescent leaves, and fruits. Chromoplast is responsible for the distinctive color of the plant organs mentioned which is associated with the presence of pigment aside from chlorophyll.

They vary in structural appearance and may be grouped into five types:

  1. Globular
  2. Crystalline
  3. Fibrillar
  4. Tubular
  5. Membranous

However, when the structure of some chromoplast is complicated, it can be harder to classify them. For instance, tomatoes have both a crystalline and membranous appearance.

Leucoplasts

These are the colorless and non-pigmented plastids in the plant cell that are used mainly for the storage of starch, proteins, and lipids. They lack photosynthetic pigments and are found in non-photosynthetic plant tissues such as seeds and roots. Thus, their key function is in biosynthesis and storage.

Leucoplasts according to what they store can be classified into 3 types:

  1. Amyloplasts
  2. Proteinoplasts
  3. Elaioplasts

Amyloplasts are the leucoplasts that store starch while proteinoplasts store proteins. Then, the elaioplasts are the leucoplasts that store fat. However, there is a special type of leucoplasts known as tannosome that is crucial in the synthesis and production of tannins and polyphenols. Some leucoplasts aside from storage are involved in synthesizing fatty acids, tetrapyrrole compounds (e.g heme), and some amino acids.

Chloroplasts

These are the green-colored plastids that contain the green-colored pigments in the plant cell known as chlorophyll. Therefore, chloroplasts are distinct from chromoplasts because they are not associated with the production and storage of pigment but are rather involved in photosynthesis.

Origin of chloroplasts

There is a hypothesis by scientists that small free-living prokaryotes were engulfed millions of years ago by larger prokaryotes. These small prokaryotes were engulfed and not consumed probably because they could resist the digestive enzymes of the engulfing organism. Based on DNA evidence, eukaryotic organisms that later became plants probably added the pathway of photosynthesis by acquiring a photosynthetic bacterium as an endosymbiont.

As this hypothesis suggests, the two organisms over time formed a symbiotic relationship. The larger organism provided the smaller organism with adequate nutrients while the smaller organism made ATP molecules available for the larger one. Eventually, the smaller organism developed into the chloroplast while the larger organisms developed into the eukaryotic cell. There are a lot of prokaryotic traits that chloroplasts have and their DNA is circular like the prokaryotes. Also, their reproductive methods involve binary fission, and their ribosomes are like those of the prokaryotes.

Therefore, the chloroplast in plant cell come through endosymbiosis that involved a eukaryotic mitochondrial cell. The endosymbiotic cyanobacterium evolved over the years in structure and function, retaining its DNA and ability to reproduce via binary fission. However, it gave up its autonomy by transferring some of its genes to the nuclear genome.

What do chloroplasts do?

The function of the chloroplasts in the cell is basically associated with photosynthesis. These organelles carry out photosynthesis and play a major role in providing the site for light and dark reactions. Water, inorganic sources, and light energy through chloroplasts are converted into food (glucose).

Hence, they are crucial for organisms that are photosynthetic and produce their own food. Chloroplast is also a crucial site for the production of oxygen being that oxygen is one of the byproducts of photosynthesis which is released from the cell to the environment. Oxygen itself is important for various physiological and biochemical processes in animals.

Chloroplast in Photosynthesis

One of the major characteristics of plants is their photosynthetic ability wherein they manufacture food by converting light energy from the sun into chemical energy. This process of photosynthesis happens in almost all plant species and the chloroplast is the specialized organelle that carries out this process. In fact, all the green plant parts contain chloroplasts including unripened fruit and stems. However, the majority of photosynthetic activities take place in the leaves of most plants. The chloroplast density on a leaf surface is estimated to be about one and a half million per square millimeter.

Green plants can make their own food through a process of photosynthesis, that makes use of a green pigment called chlorophyll. A pigment has a certain color and depending on its color can absorb light at different wavelengths. In nature, there are various types of pigment but chlorophyll is unique because of its ability to enable plants to absorb the energy needed for tissue building. This chlorophyll is located in the chloroplasts of the plant cell and this is actually where photosynthesis takes place.

Importance of chloroplast in photosynthesis

The role of chloroplast in photosynthesis is very significant as the chloroplast in the plant cell each has a light-harvesting system that contains chlorophyll. This chlorophyll absorbs light in the blue and red electromagnetic spectrum and reflects the green portion of the spectrum. The chlorophyll in chloroplast gives plants their green color. This is because it does not absorb the green wavelengths of white light rather the particular light wavelength is reflected from the plant, making it appear green. Every food web in the ecosystem, from terrestrial to marine ecosystem begins with photosynthesis. Hence, chlorophyll and chloroplasts can be said to be a foundation for all life on earth.

The microscopic floating plants known as phytoplankton that form the basis of the food web in marine habitats contain chlorophyll. This is why high phytoplankton concentrations in water can make the water body look green. The main function of the chlorophyll in a plant is to absorb energy from sunlight which is transferred to 2 kinds of energy-storing molecules (sugar and starch). Plants through the process of photosynthesis use the stored energy to convert water and carbon dioxide absorbed from the air into glucose (sugar). They make use of glucose as well as the nutrients absorbed from the soil to form new leaves and other parts of the plant. This photosynthetic process produces oxygen, which plants release into the air.

Chloroplasts in the process of photosynthesis

In the process of photosynthesis, light energy called photons travels and is absorbed in its energy form by chlorophyll molecules in the thylakoid discs of the chloroplast. The chlorophyll then emits electrons that were gotten from water as they absorb the photons. As the electrons move, it triggers the propelling of hydrogen ions across the membrane that surrounds the thylakoid stack. This, therefore, initiates the formation of an electrochemical gradient that drives the production of ATP (adenosine triphosphate) by the stroma in the chloroplast. ATP is the cell’s chemical energy that powers the metabolic activities of the cell.

The light-independent reactions of photosynthesis in the stroma that involves carbon fixation occurs and low-energy carbon dioxide is transformed into a high-energy compound like glucose. Two organelles in plant cells function in the production of energy; chloroplasts and mitochondria. Chloroplast creates energy through the process of photosynthesis while mitochondria generate energy through respiration. The chloroplasts and mitochondria are distinct compared to other organelles. This is because they have their own DNA and can reproduce independently of the cell in which they are found.

Furthermore, the presence of chloroplast in eukaryotes indicates its ability to make its own food. Plants possess numerous chloroplast that spread throughout their cytoplasm. However, plants aren’t the only organisms that possess chloroplast, the eukaryotic algae do too e.g green algae. Also, there are some bacteria that are photosynthetic such as cyanobacteria. Unlike plants, their chlorophyll is not found in the chloroplast but is located in the thylakoid membrane of the bacterial cell.

Chloroplasts Structure

The chloroplast structure is oval or biconvex. This organelle possesses its own DNA and does not rely solely on the genes in the nucleus. The DNA of the chloroplast produces certain proteins. Chloroplast structure varies in size with a diameter ranging between 4-6 µm and thickness between 1-3 µm. The structure of chloroplast is made up of membranes, thylakoid system, stroma, grana, and chlorophyll.

Inner membrane

The inner membrane is one of the double membrane systems that make up the chloroplast structure. This membrane encloses a fluid-filled region referred to as the stroma which contains enzymes involved in the light reactions of photosynthesis. The inner membrane, therefore, separates the intermembrane space and the stroma.

As one of the envelope membranes of the chloroplast, the inner membrane is a lipid bilayer with a thickness that ranges between 6-8nm. In spinach chloroplasts, the lipid composition of the inner membrane is said to be 16% phospholipids, 5% sulfolipids, and 79% galactolipids. This membrane of the chloroplast is specialized highly with transport proteins e.g carbohydrate is transported across this membrane by a triose phosphate translocator.

Outer membrane

The outer membrane is one of the lipid bilayers that makes up the envelope membrane of the chloroplast. It is between 6-8 nm thick and is permeable to most metabolites and ions. This membrane has a lipid composition that contains 48% phospholipids, 7% sulfolipids, and 46% galactolipids.

Intermembrane Space

The envelope membranes are separated by a gap. This gap between inner and outer membranes is called the intermembrane space and it ranges between 10-20 nm.

Thylakoid system

The thylakoid system is made up of membranous sacs called thylakoids. Thylakoids are disc-shaped structures that provide a site for the light reactions of photosynthesis. Its role is to harvest and collect photons from light sources (sunlight). The stack of thylakoids is called granum (plural- grana) which resembles a stack of coins and each granum consist of about 10-20 thylakoids.

This thylakoid membrane has an antenna complex embedded in it that consists of proteins and light-absorbing pigments such as carotenoids and chlorophyll. Therefore, chlorophyll (green colored pigments) are found in the thylakoid membranes and the thylakoids are the site where the light-dependent reactions of photosynthesis occur. These reactions take place when the pigment chlorophyll in the thylakoid membranes traps energy from the sun to begin the breakdown of water molecules.

A central aqueous region referred to as the thylakoid lumen is enveloped by the thylakoid membrane. There is a matrix that contains dissolved enzymes, copies of the chloroplast genome, and starch granules which fill up the space between the inner membrane and the thylakoid membrane. This matrix is the stroma.

Grana

The grana are one of the distinct regions inside the chloroplast. They are made up of stacks of disc-shaped thylakoids and each stack is called a granum. This structure contributes to the large surface area to volume ratio of the chloroplast. Grana are connected by inter-granal thylakoids.

The inter-granal thylakoids that connect the grana are extensions that run from one granum through the stroma and then into a nearby granum. These extensions can also be called stroma thylakoids or lamellae. However, the different protein compositions that the stroma thylakoids and Grana thylakoids have, distinguish them from each other.

A granum is the functional unit of the chloroplast which consists of chlorophyll pigments. The grana are sites for the conversion of light energy into chemical energy. Thus, it functions in the light reactions of photosynthesis. The chloroplast structure can be made up of 10 -100 grana.

Stroma

The stroma is one of the distinct regions that make up the structure of the chloroplast. It is the homogenous matrix of the chloroplast that contains enzymes, ions, and molecules. The stroma is the thick fluid in between the grana that serve as a site for the dark reactions of photosynthesis. These dark reactions of photosynthesis are the light-independent process of sugar formation.

Furthermore, stroma contains grana and is a colorless aqueous, alkaline, protein-rich fluid that is present within the inner membrane of the chloroplast. It is similar to the cytoplasm in cells that embeds all the organelles. Also, the stroma contains DNA, ribosomes, various enzymes, and other substances. This jelly-like matrix of the chloroplasts contains enzymes involved in the dark reaction of photosynthesis and the function of the stroma lamellae is to connect the stacks of thylakoids sacs.

Stroma lamellae

The stroma lamellae is a structure of the chloroplast that connects the stacks of thylakoids sacs. It is an extension that connects the grana and runs from one granum through the stroma and then into a neighboring granum.

Chlorophyll

This is an important component contained in the structure of the chloroplast. Chlorophyll is a photosynthetic green pigment that aids the process of photosynthesis. It is located within the thylakoid membrane of the chloroplast and is distinguished from other types of plastids by its green color. The green color of the chlorophyll is a result of the presence of two pigments: chlorophyll a and chlorophyll b.

In nature, there are various types of pigment but chlorophyll is unique because of its ability to enable plants to absorb the energy needed for tissue building. It is because of this green pigment called chlorophyll that green plants can make their own food through the process of photosynthesis. A pigment is a molecule that possesses a particular color and has the ability to absorb light at different wavelengths depending on the color.

Therefore, the chlorophyll in chloroplast gives plants their green color. This is because it does not absorb the green wavelengths of white light rather the particular light wavelength is reflected from the plant, making it appear green. Every food web in the ecosystem from terrestrial to marine ecosystem begins with photosynthesis. Hence, chlorophyll can be said to be a foundation for all life on earth.

Chloroplasts diagram

Diagram of a Chloroplast
Diagram of a Chloroplast
Photo Credit: https://micro.magnet.fsu.edu

Above is the chloroplast diagram showing its structure and parts.

The Functions of Chloroplast in Plant cell

  • Chloroplasts function in the absorption of light energy and the conversion of this light energy into chemical energy.
  • The chlorophyll in the chloroplast traps solar energy that is used for the synthesis of food in all green plants.
  • One of the major functions of chloroplast in plant cells is the synthesis of food through the process of photosynthesis.
  • Through the photolysis of water, the chloroplasts function in the production of molecular oxygen and NADPH (Nicotinamide adenine dinucleotide phosphate). NADPH is a crucial electron donor in organisms.
  • The chloroplast in plant cell obtains carbon dioxide from the air and use it to form sugar and carbon during the dark reaction of photosynthesis or Calvin cycle.
  • Chloroplasts function in the production of ATP (Adenosine triphosphate) via the process of photosynthesis.

Frequently Asked Questions

Do animal cells have chloroplasts?

There is absolutely no chloroplast in an animal cell. These organelles are the food producers of the cell and are only found in some protists like algae and plant cells. There is a significant absence of the chloroplast in an animal cell and this is another defining characteristic that helps identify plants from animals. Also, the presence of a cell wall in plants is another distinguishing characteristic of plants from animals.

Animal cells lack chloroplast because the chloroplast is the organelle involved in the process of photosynthesis which animals do not carry out. The chloroplasts convert light energy into sugar which is used by the cell and this entire process depends on the green pigment molecule (chlorophyll).

What is the function of the chloroplast?

The function of a chloroplast is the absorption of light energy and conversion of light energy into chemical energy. It contains chlorophyll that traps the solar energy that is used by green plants for the synthesis of food. Also, through the photolysis of water, the chloroplasts function in the production of molecular oxygen and NADPH (Nicotinamide adenine dinucleotide phosphate) which is a crucial electron donor in organisms. Moreso, the chloroplast also produces ATP (Adenosine triphosphate) via the process of photosynthesis.

Where are chloroplast found?

In plants, these organelles are particularly concentrated in the internal cell layers of the leaf. Chloroplasts are found in parenchyma cells of the leaf mesophyll and in the cells of all green tissues of algae and plants. Also, there are some photosynthetic tissues that do not appear green but possess chloroplasts. The red leaves of some plants and brown blades of giant kelp are typical examples of photosynthetic plant tissues that do not appear green.

What is the structure and function of chloroplast?

The chloroplast structure is oval or biconvex. Chloroplast structure varies in size with a diameter ranging between 4-6 µm and thickness between 1-3 µm. The structure of chloroplast is made up of membranes, thylakoid system, stroma, grana, and chlorophyll. This organelle possesses its own DNA and does not rely solely on the genes in the nucleus. The DNA of the chloroplast produces certain proteins.

The function of these organelles in the plant cell is to convert light energy through photosynthesis into stable chemical energy. They absorb light energy and convert it into chemical energy. The chloroplasts contain chlorophyll that traps solar energy which is used by green plants for the synthesis of food.

Do chloroplasts have DNA?

Yes, chloroplasts have DNA. The chloroplasts, as well as mitochondria, are different from other organelles as they have small circular chromosomes called extranuclear DNA. The Chloroplast DNA possesses genes that are involved with the aspects of photosynthesis and other chloroplast activities.

There is a hypothesis that both chloroplasts and mitochondria are descended from free-living cyanobacteria. This is why they have DNA that is distinct from the rest of the cell. The DNA of the chloroplast is called the chloroplast DNA or cpDNA and certain proteins are produced from it.

Why are chloroplasts green?

Chloroplast in plant cells is green because they contain the green-colored pigments in the plant cell known as chlorophyll. They are distinct from other plastids. This is because they are not associated with the production and storage of pigment but are rather involved in photosynthesis.

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