The Function of Plastids in the Photosynthesis Process and Their Structure and Types!

Function of plastids – We can know that the cell is the basic structural and functional unit of a living thing. As a structural unit, it means that living things are made up of cells. Organisms that are made up of a single cell are called single-celled organisms (unicellular = monocellular) and those that are made up of several cells are called multicellular.

The cell as a functional unit means that all important functions or vital activities (metabolism, reproduction, excitability, digestion, excretion, and other activities) in unicellular and multicellular organisms that take place in the body are carried out by cells. Cells are divided into two, namely prokaryotic cells and eukaryotic cells. An example of a eukaryotic cell is the plastids. Plastids are organelles present in all eukaryotic cells. The function of the plastids varies according to the type.

Plastids are very dynamic organelles and are capable of dividing, growing and differentiating into various forms. In young cells of higher plants, the plastids are usually colorless and are called leucoplasts or proplastids. In leaves, the plastids are green and are called chloroplasts, and in ripe fruit they are sometimes yellow or red, called chromoplasts. Plastids function for photosynthesis, and also for the synthesis of fatty acids and terpenes which are necessary for the growth of plant cells.

Definition of Plastids

Plastids are highly dynamic organelles capable of dividing, growing and differentiating into various shapes. In young cells of higher plants, the plastids are usually colorless and are called leukocytes or proplastids. In leaves, green plastids are called chloroplasts, in ripe fruit they are sometimes yellow or red called pigments. In cells that do not turn green, such as epidermal cells or hair stem cells (eg Rhoeo discolored cells), plastids remain colorless, which are called leukocytes (in the narrow sense).

Leucoplasts are also present in tissues that are not exposed to light. In tissues such as tubers, white blood cells form starch granules called trophoblasts. Statoliths are specialized fibroblasts enclosed in roots and in the nodes of some young plant stems, and are involved in gravity. Leucoplasts form oil or fat and are called elaioplas, for example in the epidermis and vanilla leaves.

Plastids function

  1. Photosynthesis. The function of this plastid is carried out by chloroplasts as a unit that contains many chlorophyll pigments to carry out photosynthesis.
  2. Discoloration. The function of these plastids is very closely related to the process of pollination and dispersal of seeds in plants. With the change in color, organisms such as insects will be interested in pollinating. Therefore, many types of chromoplast plastids are also found in flowers.
  3. Increase the storage of food reserves. The function of these plastids is played by chromoplasts and leucoplasts. The change of chloroplasts into chromoplasts results in an increase in the ability of tissues and cells to absorb water-soluble materials such as carbohydrates.
  4. Food storage. The function of these plastids is played by chloroplasts in small quantities and leucoplasts such as amyloplasts for storage of starch, elaioplas for lipids or fats and proteinoplasts for proteins.
  5. Production of amino acids and proteins. The function of these plastids is carried out by leucoplasts.
  6. The place where the light reactions occur which are important in the process of forming food. This function, of course, occurs primarily in the chloroplast.

Function of Plastids in Plant Reproduction

The function of the plastids is to play a role in storing food in the form of leukocytes and pigment cells, as well as changing the color of leaves, especially green which has a photosynthetic effect for plants.

Plastids are the main organelles found only in plants and algae. Plastids function in photosynthesis, as well as in the synthesis of fatty acids and terpenes necessary for plant cell growth.

Depending on their function and morphology, plastids are generally classified as chloroplasts, leukocytes (including rods and protoblasts) or chromatophores. Plastids are the derivatives of proplastids, which are formed in the meristem parts of plants.

Functions of Plastids in Algae Plants

In algae, the term leucoplast is used for all non-pigmented plastids. Its function is different from leukocytes in plants. Ethioplast, amyloplast and chromoplast are only found in plants and not in algae. Algal plastids can also differ from plant plastids which contain pyrenoids in algae.

The plastids themselves originate from cyanobacterial endogenous processes. In green algae and plants called chloroplasts, in red algae and their organelles. Plastids are distinguished by their pigmentation, but also by their superstructure.

Plastid Structure

Here are some plastid structures, consisting of:

1. Outer Membrane

2. Have a Flat Surface

This outer membrane serves to regulate the entry and exit of substances.

3. Space Between Membranes

Permeable to incoming substances.

4. Inner Membrane

Functions as a wrapping for chloroplast fluid called stroma.

5. Stroma

Serves as the site of the dark reaction.

6. Snow Tilakoid

The inner membrane is folded in pairs called lamellae. These lamellae periodically enlarge to form flat, membrane-enclosed vesicles called thylakoids. The stack of thylakoids is called the granum. Serves as a place to store photosynthetic pigments.

7. Thylakoid membrane

Serves to help in light reactions (there are enzymes)

8. Grain

These are stacks of thylakoids. As the light reaction occurs.

9. Tilakoid/Lamella

Inside the thylakoids are a collection of particles called quantosomes (quantosomes = chlorophyll sites). Serves as a liaison between grana.

10. Also

Is an organelle / basic material for the formation of photosynthetic products = glucose.

11. Ribosomes

Where protein synthesis occurs.

12. Plastid DNA

Regulates activities in the cell.

13. Plastoglobule

A kind of lipid.

Types of Plastids

1. Chloroplasts

Chloroplasts are the plastids that give leaves their green color, called chlorophyll. Chloroplasts are plastids that contain chlorophyll, carotenoids and other photosynthetic pigments.

The chloroplast shell consists of two membranes. Within the chloroplast there is another membrane system in the form of flattened sacs called thylakoids. Thylakoids are stacked to form structures called grana (plural particles). Thylakoids contain photosynthetic pigments, namely chlorophyll and carotenoids. The space between the grana is called the stroma.

Photosynthesis occurs in chloroplasts. Within the thylakoids, the pigment chlorophyll is responsible for capturing light energy, which is converted to chemical energy through a process called photoreactivity. The next reaction is the dark reaction, namely the formation of glucose. The dark reactions take place in the substrate using the chemical energy from the light reactions.

Chloroplasts are generally lenticular in shape, usually 4-6 µm in size. Within the chloroplast there is one green pigment or chlorophyll, and at least two yellow or red dyes, or groups of dyes (carotenoids): one or more carotenes (C40H56) and xanthophylls (C40H56O2). Chloroplasts have the function of photosynthesis and in most plants also have the function of forming starch from dissolved carbohydrates through photosynthesis, and breaking it down again.

Chloroplasts are green colored plastids. Chloroplasts develop in stem and leaf cells which contain green pigment, during photosynthesis, they absorb solar energy to convert carbon dioxide into sugar, which is a source of chemical energy and food for animals. Chloroplasts reproduce by dividing independently of the cell nucleus. This plastid functions to produce chlorophyll and serves as a place of photosynthesis. The chemical components of chloroplasts are proteins, phospholipids, green and yellow pigments, DNA and RNA.

Based on the picture above, it can be seen that in the stroma there are membrane structures called thylakoids. The stack of thylakoids is called the granum. The inside of the thylakoid is called the locus. The thylakoids that connect the grana are called frets. Inside the thylakoid membrane are the enzymes for the light reactions of photosynthesis, and this is where the chlorophyll is located. Thus, the function of the thylakoids is to allow the reactions of photosynthesis to occur. Meanwhile, in the stroma there are enzymes that are very important for reducing CO2 into carbohydrates. Thus, the function of the substrate is where the dark reactions of photosynthesis take place.

2. Chromoplast

The yellow, red or brick red color of the chromoplast is due to its carotenoid content. Chromoplasts usually originate from chloroplasts, but can also originate from proplastids. Synthesis and placement of carotenoid pigments such as carotenoids (in carrots, Daucus) or lycopene (in tomatoes, Lycopersicon) are important in melanocyte differentiation. The development of pigmentation is associated with changes, or even complete remodeling, of the thylakoids. During this process, lipid balls (bubbles) proliferate. In some chromoplasts, pigment is stored in a spherical form (yellow pepper, orange).

In other chromoplasts, the pigments aggregate into large numbers of protein filaments (chilli seeds). The third form of pigment is the crystalline form. In red tomatoes, lycopene grows as crystals bound to the thylakoid membrane. Some of the crystals become greatly elongated and the thylakoids elongate, while lycopene is formed. Carrot root carotenoid crystals form when the internal structure of the plastids breaks down and remains in contact with the lipoprotein shell.

Chromoplasts give color to various parts of the plant apparatus. However, not all plant colors are produced by pigments in the plastids, because in the vacuoles they can also be found as dyes. Different pigments in chromoplasts, for example

  1. Phycocyanin causes a blue color, for example in Cyanophyta.
  2. Phycoerythrin causes a red color, for example in Rhodophyta.
  3. Carotene gives rise to a golden color, for example, in carrots and Chrysophyta.
  4. Xanthophyll causes a yellow color, for example on old leaves.
  5. Phycocyanin causes a blonde color, for example in Phaeophyta.

3. Leukoplas

These white plastids function as food storage, including:

Amyloplast (for starch storage)

In some places, chloroplasts form large starch grains for food storage, as in the pseudobulb of an orchid. However, the largest food reserves are formed in the chloroplasts of roots, tubers, rhizomes and seeds. Starch or amylum can be shown easily because it is blue or black with iodine. When heated to 70˚C, the color disappears and turns blue again after cooling. This reaction is known as surface reaction.

The large element represents the layer that surrounds a central point, the hilum. The hilum may be in the middle of a starch grain or slightly on the edge. The cracks which are usually seen in a radial direction from the hilum appear to be caused by dehydration of starch granules. It is believed that the formation of this layer is due to the position of the molecules being denser at the beginning of layer formation and gradually becoming more brittle on the outside. This causes differences in the water content they contain.

Thus, the presence of layers is considered to be due to differences in water content in successive layers, while the density level causes differences in refractive index. In spirit, all of these layers are lost, perhaps because dehydration affects the concentration difference. In cereal starch, the appearance of layers depends on the daily rhythm. In potatoes, the periodic changes that cause coating originate from within (endogenous). In starch grains, the radially arranged molecules must exhibit crystalline properties. Therefore, if the starch is observed to have a pair of polarizers at the cross positions, the starch will appear shiny, except for the cut whose center coincides with the grain vein.

In seeds that are starting to germinate or tubers that are starting to develop, the starch granules start to erode from the outside and are gradually used up. In small starch grains, the hilum is located in the center of the surrounding layer. In larger grains, the hilum is often eccentric (not centered). If more than one starch granule is formed in a plastid, the granules will soon contact each other and form complex granules.

So the grains are known as wheat flour (Avena) and rice (Oryza sativa), semi-compound flour in potatoes, and simple flour grains as in irrut flour (Maranta). ). If the starch granules fill the cell to the edge, the edges will be slanted. The location of the seed, the shape and size of the seed, as well as the nature of the single or double seed help determine the starch-producing plant species in question.

Most plants only inherit plastids from their parents. Angiosperms usually inherit plastids from females while some gymnosperms inherit plastids from males. Algae also inherit plastids from their parents. Plastids in algae, the term leucoplast is used for all plastids that are not pigmented. Its function is different from leukocytes in plants. Ethioplast, amyloplast and chromoplast are only found in plants and not in algae. Algal plastids can also differ from plant plastids which contain pyrenoids in algae.


are plastids that store protein and are mainly found in plant seeds.


is a plastid that functions to store fats and oils that plants need, especially found in plant seeds.

4. Gerontoplas

Gerontoplasts are basically chloroplasts that are undergoing an aging process. Leaf chloroplasts that are starting to change into different organelles or are being reused.

Because leaves no longer use photosynthesis, as they do in the autumn months. Depending on their morphology and function, plastids have the ability to differentiate again.

The Role of Plastids in the Process of Photosynthesis

Plants with green leaves are autotrophs. Autotrophs mean they can cook or synthesize food directly. of inorganic compounds. Plants absorb carbon dioxide and water to produce the sugar and oxygen needed for food. The energy to carry out this process comes from photosynthesis. Consider the following equation for the reaction that produces glucose:

6H2O + 6CO2 + light → C6H12O6 (glucose) + 6O2

Glucose can be used to form other organic compounds such as cellulose and is also used as a fuel. This process takes place through cellular respiration which occurs in animals and plants. In general, the reaction that occurs in cellular respiration is the opposite of the equation above. During respiration, sugar (glucose) and other compounds react with oxygen to produce carbon dioxide, water, and chemical energy.

Plants absorb light because they contain a pigment called chlorophyll. This pigment gives plants their green color. Chlorophyll is found in organelles called chloroplasts. Chlorophyll absorbs light to be used in photosynthesis. Most of the photosynthetic energy is produced in the leaves, but it can also occur in the green organs of plants. Inside the leaf is a layer of cells called the mesoderm which contains half a million chloroplasts per square millimeter. Light passes through the epidermis, which is colorless and transparent, to the dermis, where most of photosynthesis takes place.

The leaf surface is usually covered with a cuticle of impermeable wax to prevent excessive absorption of sunlight or moisture evaporation. Photosynthetic reaction in plants, the main organ of photosynthesis is the leaf. But in general, all cells with chloroplasts are capable of this reaction. This organelle is where photosynthesis takes place, precisely in the buffer.

The products of photosynthesis (called photosynthesis) are usually delivered first to adjacent tissues. Basically, the chain reaction of photosynthesis can be divided into two main parts: the light reactions (because they need light) and the dark reactions (which don’t need light but do need carbon dioxide).