Chloroplast – Structure & Types

Introduction 

Energy flow in ecosystem 

The food chain and food web facilitate the movement of energy. Plants collect sunlight with the aid of chloroplasts during the process of energy flow in the ecosystem, and a portion of it is turned into chemical energy in the process of photosynthesis. When herbivores eat (primary consumers) the plants as food, this energy is stored in various organic products in the plants and passed on to the primary consumers in the food chain.  

The chemical energy contained in plant products is then converted into kinetic energy, and energy is degraded by heat conversion. The flow of energy in the ecosystem is one of the most important variables in the survival of such a large number of creatures. Solar energy is the principal source of energy for practically all species on Earth. It is amusing to learn that we only receive around half of the Sun’s effective radiation on Earth. When we say effective radiation, we mean radiation that plants can employ to perform photosynthesis. 

Chloroplast 

All green plants and algae include chloroplasts. They are the plant’s food producers. These are present in mesophyll cells found in plant leaves. They have a high concentration of chlorophyll, which absorbs sunlight. This cell organelle does not exist in mammalian cells. Chloroplasts, like mitochondria, have their own extra-nuclear DNA, and hence, are semiautonomous. They also create proteins and lipids that are necessary for the formation of the chloroplast membrane.  

Structure of chloroplast 

• All higher plants include chloroplasts. They are oval or biconvex in shape and are found within the mesophyll of the plant cell. Chloroplasts typically range in size from 4-6 m in diameter to 1-3 m in thickness. They are double-membrane organelles having outer, inner, and intermembrane space. The grana and stroma are two different areas found within a chloroplast. 
• Stroma is a homogeneous matrix that includes grana and is comparable to the cytoplasm in cells in that it contains all organelles. Stroma also contains enzymes, DNA, ribosomes, and other materials. Stromal lamellae link the stacks of thylakoid sacs or grana. 

• The chloroplast has two membranes, one inner and one outer, with an empty space in between. Grana, which are stacks of thylakoids, and stroma, which is the thick fluid inside the chloroplast, are found inside the chloroplast. 
• These thylakoids contain chlorophyll, which is required for photosynthesis in plants. The thylakoid space is the space that chlorophyll occupies. 

As a result, a chloroplast is made up of the following components: 

• Outer membrane (envelope): It is a semi-porous membrane that is permeable to tiny molecules and ions and allows for easy diffusion. Larger proteins are unable to get through the outer membrane. 
• Intermembrane Gap: This is a narrow intermembrane space between the chloroplast’s outer and inner membranes that is normally 10-20 nanometers wide. 

• Inner membrane: The chloroplast’s inner membrane establishes a barrier to the stroma. It controls the flow of materials into and out of the chloroplast. In addition to regulatory action, the inner chloroplast membrane synthesizes fatty acids, lipids, and carotenoids. 

• Stroma: Stroma is an alkaline, aqueous fluid that is protein-rich and is present within the inner membrane of the chloroplast. The stroma is the space outside of the thylakoid space. The stroma contains chloroplast DNA, chloroplast ribosomes, the thylakoid system, starch granules, and numerous proteins. 

 The Thylakoid System:  

• The stroma contains the thylakoid system. The thylakoid system is made up of membranous sacs known as thylakoids. The chlorophyll is present in the thylakoids and serves as the site for the light reactions of photosynthesis to occur. Thylakoids are organized into grana, which are stacked thylakoids. Each granum comprises between 10 and 20 thylakoids. 

Peripheral Reticulum: 

• Certain plants’ chloroplasts possess an extra set of membranous tubules termed peripheral reticulum, which starts from the envelope’s inner membrane. Tiny vesicles protrude from the chloroplast’s inner membrane and join together to create the tubules of the peripheral reticulum.  

Types of chloroplast 

Chloroplasts are the most well-known plastids that are responsible for photosynthesis. It is classified into three types: 

1. Chromoplasts – These organelles store and manufacture pigments in the plant. These can be found in flowering plants, fruits, and old leaves. 

2. Gerontoplasts – These are the chloroplasts that are ageing. 

3. Leucoplasts are organelles that are not colored. 

Chromoplasts: 

• Chromoplasts are plastids that are colored as a result of pigments synthesized and stored inside them. They can be found in ripe fruits, flowers, roots, and wilting leaves. Apart from chlorophyll, the color of various plant organs is connected with the existence of pigments. 
• The structural appearance of chromosomes changes under an electron microscope. They may be broadly classified into five types: (1) globular, (2) crystalline, (3) fibrillar,  
  (4) tubular, and (5) membranous.  

Gerontoplasts: 

• The gerontoplast develops in a formerly green plant tissue that is entering senescence. The gerontoplast grows from chloroplast during leaf senescence under genetic regulation.  
• The gerontoplast, like the chromoplast, has a decreased thylakoid system and many plastoglobuli. However, the gerontoplast can only convert to a chloroplast up to a certain extent. 
• When a cell reaches the ultimate phase of senescence, it dies, and the growth of gerontoplast from chloroplast becomes irreversible.  

Leucoplasts: 

• Leucoplasts are plastids that are colorless or non-pigmented. In contrast to chloroplasts, which contain chlorophyll (green pigment), and chromoplasts, which contain additional photosynthetic pigments, they lack photosynthetic pigments.  
• Leucoplasts are commonly found in non-photosynthetic plant tissues such as roots, seeds, and so on. They perform a variety of biosynthetic tasks. Storage is one of its key roles.  
• Leucoplasts are categorized as (1) amyloplasts (store starch), (2) elaioplasts (store fat), and (3) proteinoplasts or aleuroplasts (store proteins).