Plastids: Definition, Types, Examples, Diagram, Function

What are Plastids?

Plastids are important cell organelles in plants and algae. These carry a range of functions, such as photosynthesis, pigment synthesis, and the storage of starches, oils, and proteins. Plastids contain their DNA and the system for synthesising proteins.

Plastids are essential for normal plant cell functioning. The process of energy production and metabolism is important for developing plants, and plastids play a consequential role in it. Plastids were first observed during the 19th century when scientists found the so-called photosynthetic organelle, the chloroplast.

Types of Plastids

There are different types of plastids, each of which performs different functions:

Chloroplasts

• The green pigment chlorophyll in plants is due to the presence of chloroplasts.

• These are double membrane cell organelles containing thylakoid and stroma.

• Chloroplasts are significant in the photosynthesis process.

Chromoplasts

• A pigment named carotenoid is present in the chloroplasts.

• The colour of different fruits and vegetables like tomatoes, bell peppers, and carrots is due to the presence of chromoplasts.

• These do not have the thylakoid structure like in chloroplasts.

• The chromoplasts function as the significant location to store and synthesise pigments.

Leucoplasts

• Leucoplasts do not have any pigment and are colourless organelles.

• Leucoplasts have many subtypes, namely amyloplast (stores and synthesises starch), elaioplast (stores fats and oils ), and proteinoplast (stores proteins).

• Their main function is to store essential substances used by the plants.

Gerontoplasts

• This type of plastid plays a significant role in the ageing and senescence of leaves.

• These are formed from chloroplasts during the breakdown process of photosynthesis.

Etioplasts

• The formation of etioplasts takes place in the dark.

• As the plant is exposed to light, etioplasts get converted into chromoplasts, and the process of photosynthesis starts.

Structure of Plastids

The plastid structure can be differentiated into three major components: the outer membrane, the inner membrane, and the thylakoid membrane.

• The outer membrane is smooth and permeable by small molecules as well as ions.

• The inner membrane is less permeable and has transport proteins regulating the passage of materials.

• The inside of a chloroplast consists of interconnected thylakoid membranes that are arranged in stacks called grana, where light-dependent reactions take place during photosynthesis.

• The stroma is a dense fluid inside plastids surrounding the thylakoid membranes containing enzymes, plastid DNA and ribosomes.

Different plastids have distinct structural differences related to specialisation:

Functions of Plastids

Plastids have different functions:

• Chloroplasts are the site of photosynthesis, they convert light energy to chemical energy which is stored as glucose.

• The photosynthetic machinery is set up in two ways: light-dependent reactions primarily occur in the thylakoid membranes.

• Chlorophyll, in the presence of light, catalyses the generation of ATP and NADPH and Calvin's cycle, which is a CO₂ fixation reaction, happens in the stroma.

• The plant can then utilise the glucose both for its daily activities requiring energy and as raw material for the synthesis of other organic molecules.

• Chromoplasts are the plastid type needed for synthesising and storing pigments like carotenoids and xanthophylls.

• These pigments result in the conspicuous colours of fruits, flowers, and other plant organs and all of these are necessary to attract the vectors that aid in pollination and seed dispersal.

• The location of chromoplasts in petals and fruits demonstrates their function in enhancing the efficiency of pollination and aiding in the dissemination of seeds by animals attracted to colourful displays.

• Leucoplasts are storage organelles mainly for starch, oils, and proteins.

• They play a leading role in the growth and development of plants, acting as reservoirs for important nutrients.

Plastid Biogenesis and Origin

The endosymbiotic theory accounts for the origin of plastids. This theory postulates that plastids arose from free-living cyanobacteria engulfed by ancestral eukaryotic cells. The genetic evidence is based on the presence of several genes, including plastid genes, indicating that plastid genomes share significant similarities with the genomes of cyanobacteria, indicating a common evolutionary ancestry.

Plastid development is marked by the differentiation of proplastids, the less developed and relatively undifferentiated plastids found in meristematic tissues. Various environmental factors, such as light, play an important role in their development into mature forms. For example, in the absence of light, proplastids can develop into etioplasts, a precursor to chloroplasts. These etioplasts finally form the chloroplasts.

Plastid Genome

The plastid DNA, also termed the plastome, is circular and compact, between 100 and 200 kb in length.

• Unlike nuclear DNA, plastid DNA is less gene-dense and lacks a helical chromosome structure.

• It most notably encodes proteins that mainly function in the photosynthesis process and gene expressions that include ribosomal proteins, RNA polymerases, and ATP synthase subunits.

• Plastid and nuclear genomes are coordinated closely to ensure the proper functioning of plastids.

• Indeed, the activity of these plastids has to be coordinated to maintain the efficiency and functioning of cellular activities.

Applications of Plastids

Plastids have applications various fields, especially in biotechnology and agriculture:

Biotechnology

In biotechnology, chloroplast genetic engineering is emerging as a potent tool. Scientists modify the chloroplast genome to express biopharmaceuticals like therapeutic proteins and vaccines, using the fact that chloroplasts express protein at higher levels. Since the yield is higher than in other systems and contamination by human pathogens is low, it reduces the cost and time needed for pharmaceutical production.

Agriculture

Plastids are instrumental in increasing the yield of nutrients in crops. These same chloroplasts can be engineered to develop plants with an improved ability to resist environmental stresses. More importantly, such improved crop varieties help to reduce the application of chemical pesticides and fertilisers, thus promoting sustainable agriculture.

Plastids NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Types of Plastids

• Functions of different Plastids

Practice Questions for NEET

Q1. Which of the following plastids store fat

1. Chromoplast

2. Elaioplast

3. Leucoplast

4. Amyloplast

Correct answer: 2) Elaioplast

Explanation:

Elaioplasts are specialized plastids that store fats and oils in plants. They belong to the category of leucoplasts, colourless plastids mainly accountable for storage functions. Elaioplasts have been specifically adapted to store lipids and bear oil body deposits called plastoglobuli, which appear as rounded fat droplets. These structures are vital for energy storage and have been found in many plant tissues, especially in seeds and fruits.

Hence, the correct answer is option 2) Elaioplast.

Q2. Which of the following plastids is responsible to impart colors other than green?

1. Chromoplast

2. Chloroplast

3. Leucoplast

4. None of the above

Correct answer: 1) Chromoplast

Explanation:

Chromoplast imparts different colours to the plant cell other than green. Chromoplasts impart different colours to the plant cell other than green. These pigments are responsible for the red, yellow, orange, and purple colours found in fruits, flowers, and some roots. The pigments, such as carotenoids and anthocyanins, help attract pollinators and seed dispersers, playing a key role in plant reproduction. Chromoplasts are formed from chloroplasts as the plant matures and undergoes colour changes, especially during fruit ripening.

Hence, the correct answer is option 1) Chomoplast.

Q3. Rubisco protein is present in:

1. Stroma

2. Grana

3. Thylakoid

4. None of the above

Correct answer: 1) Stroma

Explanation:

Stroma shows the presence of RUBISCO protein in it. Stroma shows the presence of RUBISCO protein in it, which plays a crucial role in the Calvin cycle of photosynthesis. It is the site where carbon fixation occurs, converting carbon dioxide into organic compounds. Additionally, the stroma contains enzymes, DNA, and ribosomes, facilitating various metabolic processes within the chloroplast.

Hence, the correct answer is option 1) Stroma.

Also Read:

• MCQs on Plastids and Different Types of Plastids

• Plasma Membrane

• Cell Wall