Plastids: Structure, Types, and Functions

Plant cells are immobile organisms and have many distinguishing structures than animal cells. The main difference is the absence of two membrane-bound components; plastids and vacuoles. 

Plastids are present in all living plant cells and some protozoans (Euglena). These are small bodies with double membranes, approximately spherical or disc-shaped, and 1 µm to 1 mm in diameter. Plastids may also appear elongated, lobed, or amoeboid in shape. These play the main role in food preparation and storage of prepared food in plants.

Plastids possess plastoglobuli (spherical lipid droplets that stores lipids according to the requirement), an internal membrane network of many discrete vesicles, and multiple copies of a small genome and 70s ribosomes. Bacteria, fungi, and animal cells lack plastids.

Types of Plastids

Plastids are derived from the Greek word “plastikas, meaning formed or molded,” and A.F.W Schimper used the term in 1885. The plastids are differentiated into three types based on their structure, pigments, and functions; chloroplasts, leucoplasts, and chromoplasts.

Chloroplasts

The chloroplasts (chloro-green, plast-living) are the plant cell’s most abundant and common plastid. These perform the photosynthetic activity, which carries the most significant biological importance, synthesizing carbohydrates for energy. The chloroplasts are evenly distributed in the cytoplasm of plant cells. But in some cells, these are concentrated around the nucleus or beneath the plasma membrane. These are motile and show passive and active movements.  

In higher plants, chloroplasts are biconvex or plano-convex. In contrast, in different plant cells, the chloroplasts have various shapes; filamentous, saucer-shaped, spheroid, ovoid, discoid, or slub-shaped. They have a colorless center and are vesicular. The size of chloroplasts varies from species to species. Generally, these measure 2-3 µm in thickness and 5-10 µm in diameter. Polyploid plant cells have larger chloroplasts compared to diploid plant cells. Likewise, plants growing in the shade have large chloroplast with more chlorophyll than the plants that grow in sunlight.   

The number of chloroplasts varies from cell to cell and species. The number depends on the cell’s physiological state but is usually constant for a particular plant cell. The algae have large single chloroplast, whereas higher plants have 20-40 chloroplasts. 

Chemical composition of Chloroplasts

Ultrastructure of Chloroplasts

The chloroplasts have three structural components; envelope, stroma, and thylakoids.

1. Envelope: The entire chloroplast is covered by an envelope made up of double membranes. The exchange of molecules between chloroplast and cytosol occurs across this envelope. Isolated membranes of the envelope lack chlorophyll and cytochromes but have yellow color due to the presence of carotenoids. They have 1-2 % of the total protein of chloroplast.  
2. Stroma: The matrix or stroma fills most of the chloroplasts and is a gel-like fluid that surrounds the thylakoids (grana). It has 50% of the proteins of the chloroplast, which are soluble. It also has ribosomes and DNA molecules. These are involved in the synthesis of some of the structural proteins of the chloroplast. CO₂ fixation and synthesis of sugars, starch, fatty acids, and some proteins occur in stroma.
3. Thylakoids: These are sac-like structures that consist of flattened and closed vesicles arranged as a membranous network. The outer surface of the thylakoids is in contact with the stroma and encloses the intrathylakoid space. The thylakoids may stack like the pile of coins that forms grana, or maybe unstacked, called intergranal or stromal thylakoids, that include a system of anastomosing tubules joining the grana thylakoids. 40-80 grana may be present in the matrix of a chloroplast. The number of thylakoids per granum varies from 1-50.  

Leucoplasts

The leucoplasts (leuco-white and plast-living) are colorless plastids found in embryonic and germ cells. They are also present in meristematic cells and the regions that do not receive light. These are located in cotyledons and primordium of the stem but eventually, change into chloroplasts. True leucoplasts occur in fully differentiated cells like epidermal and internal plant tissues and never becomes green or photosynthetic. These also lack thylakoids and ribosomes. 

They develop from the protoplasts, a small organelle in the meristematic tissue. The protoplast changes into plastids based on the requirements.  

The leucoplasts store food materials like carbohydrates, lipids, and proteins in the following manner: 

1. Amyloplasts: The amyloplasts are the leucoplasts that synthesize and store starch. The amyloplast occurs in some plant tissues like potato tuber. The outer membrane of amyloplasts encloses the stroma and contains one to eight starch granules. The starch granules are composed of concentric layers of starch.   
2. Elaioplasts: The elaioplasts store lipids and are present in the seeds of monocotyledons and dicotyledons. They also have sterol-rich sterinochloroplast. 
3. Proteinoplasts: Protein-storing leucoplasts are called proteinoplasts. It mainly occurs in seeds and has some thylakoids.

Chromoplasts

The chromoplasts (chroma-color and plast-living) are the colored plastids with carotenoids and other pigments. The chromoplasts impart color to some portions of plants, like flower petals (rose, daffodils, etc.), fruits (tomatoes), and some roots (carrots). 

The structure of chromoplasts varies widely; some can be round, ellipsoidal, or even needle-shaped. The carotenoids in the chromoplasts may be localized in droplets or crystalline structures.

They either originate from chloroplasts or leucoplasts. An example of their derivation from chloroplasts is in the green petals that initially turn into different colors. Likewise, the carrot roots initially are colorless due to the presence of leucoplasts and later turn into bright orange color. 

The function of chromoplasts is unclear, but in many cases, the color produced helps attract insects and birds for pollination or seed dispersal. They reduced the number of chloroplasts, so they do not play a role in photosynthesis. The carotenoid is responsible for providing color. The red color of ripe tomatoes results from red pigment lycopene, the blue-green algae or cyanobacteria containing various pigments like phycocyanin, phycoerythrin, and chlorophyll a. Chromoplasts are of two types; phaeoplast and rhodoplast.

1. Phalloplasty: The phaeoplast (phaeo means dark brown and plast meaning living) has the pigment fucoxanthin, which absorbs light. The diatoms, dinoflagellate, and brown algae have phaeoplast.
2. Rhodoplast: The rhodoplast consists of the pigment phycoerythrin that absorbs light. It is present in red algae.

Functions of Plastids

All plant cell has plastids and plays essential functions, which are as follows:

1. Chromoplasts’ presence in flowers help to attract insects and birds for pollination.
2. Amyloplasts help to synthesize and store starch for a more extended period.
3. Elaioplasts, a type of leucoplasts, help to store lipids in seeds.
4. Proteinoplasts, another type of leucoplasts, help to store proteins in seeds and thylakoids.
5. Chloroplasts help in synthesizing carbohydrates, the source of energy. The process by which carbohydrates are synthesized is known as photosynthesis.

References

• The Structure and Function of Plastids. (2006). https://doi.org/10.1007/978-1-4020-4061-0
• Verma, P., & Agarwal, V. (2004). Cell biology, genetics, molecular biology, evolution and ecology (24th ed., pp. 221-240). S. CHAND & COMPANY LTD.