Chloroplast Structure and Function in detail with Labelled Diagram
The leaves of the trees are colored in different shades of green. This color is imparted due to the presence of various pigments present in the cells of the leaves. The 3 types of pigments present in plants are chlorophyll, carotenoids, and anthocyanins. Chlorophyll imparts the green color to plants.
Plastids are membrane-bound cytoplasmic organelles that can be found in the cells of plants and algae. They have their own circular double-stranded DNA molecule. They take part in storing and synthesizing organic compounds. Plastids are actually cyanobacteria that live as endosymbionts inside the cells of eukaryotes. Plastids can be classified into the following types- chloroplast, chromoplast, etioplast, leucoplast, amyloplast, elaioplast, and proteinoplast.
Etioplast contains crystalline pro-lamellar bodies, which is a precursor of chlorophyll. Chromoplast creates colored pigments and stores them. Leucoplast takes part in fatty acid and amino acid synthesis. Amyloplast stores carbohydrate sugars, elaioplast stores fats, and lipids and proteinoplast contain sites for enzyme activity. Chloroplast contains chlorophyll and takes part in photosynthesis.
The word chloroplast or chloroplastid has a Greek origin. Chloros means green, and plastos means molded. They can be found in plant cells (Leaf anatomy) and some protists. They were first observed by Leeuwenhoek and N. Grew.
Chloroplasts Structure & Characteristics
Chloroplasts in green plants are oval or elliptical in shape. They have a diameter of 5 to 10 micrometers and a thickness of 2 to 4 micrometers.
Chloroplasts are the 2nd largest cell organelles in plant cells. In Spirogyra, the chloroplasts are Ribbon shaped and spirally coiled running from one end of the cell to the other. In cells of Chlamydomonas, ulothrix, and other algae, a single chloroplast is present in the cell. The number of chloroplasts in each cell of plant varies; for example, a chlorenchyma cell of a leaf may have about 40 chloroplasts in the cytoplasm.
The main components of a chloroplast are protein(50 to 60%), lipids, pigments like carotenoids and chlorophyll, a small amount of RNA and DNA, traces of Vitamin E and K and minerals like iron, magnesium, manganese, etc.
A chloroplast is covered by a membranous envelope, inside which matrix and thylakoids are present. In green algae pyrenoids and stigma are also present.The membrane of chloroplast consists of an outer and an inner membrane.A perichloroplastidial space is present between the two membranes.The outer membrane is attached to the endoplasmic reticulum and thus communicates with the ER system. The outer membrane has porin like channels and is permeable to various solutes. The inner membrane is selectively permeable and transports only some substances. The thylakoids are formed by the infolds of the inner membrane.These infold become free in a mature chloroplast.
The Matrix or stroma is present within the membranous envelope. Different components like DNA, mRNA, tRNA, granules, etc are present in this stroma.The ribosomes present are of 70S type. Chloroplast DNA is known as cpDNA. Ribosomes in the chloroplast take part in the synthesis of photosynthetic pigments, degradation of starch, replication of DNA, lipid metabolism, Calvin cycle, etc. Ribulose bisphosphate carboxylase(RuBisCO), the most abundant plant protein is present in the matrix and takes part in the Calvin cycle of photosynthesis.
Plastoglobuli are lipid globules present in stroma, containing vitamin E and K, quinones etc.
Thylakoids are flattened membranous sacs inside which components for light-dependent reactions are present.They are the structural and functional elements of the chloroplast.The intrathylakoid space is called loculus. The thylakoid membrane contains chlorophyll (a and b in plants), cytochromes, carotenoids, ATP synthase, etc.
Stacks of thylakoids are known as grana. These grana are interconnected with stromal lamellae.
Origin of chloroplast
Chloroplasts are semi-autonomous. An offspring receives its chloroplasts or proplastids from its mother.Plastids divide from pre-existing plastids.
In algae and lower plants, chloroplasts develop when pre-existing chloroplasts divide.
Chloroplasts develop from proplastids in higher plants. Proplastids multiply by division, then each proplastid grows in size, and its vesicles fuse to form lamellae. If kept in the dark environment, the proplastid develops into an elioplast with prolamellar bodies. When it receives radiation, the prolamellar body develops into thylakoids, and photosynthetic pigments are synthesized and get associated with the thylakoid membrane.
Chloroplasts are considered to be autotrophic prokaryotic endosymbionts. Chloroplast contains its own DNA called ctDNA(circular and naked). The replication of ctDNA is independent of the replication of nuclear DNA. Chloroplast contains its own RNA and ribosomes, which synthesize some of the proteins of the chloroplast. The development of photosynthetic pigments inside chloroplast is most under the control of the chloroplast’s genetic system. However it is not completely autonomous.
The main function of to perform photosynthesis which is the synthesis of organic compounds from inorganic materials and sunlight.
The chloroplast is responsible for converting the energy from the sun into chemical energy, which stores into bonds that can be broken when needed to provide energy for different processes in the plant.
The green-colored chloroplast is transformed into a colored chromoplast, which is orange or yellow color. This occurs in flowers or fruits and helps in attracting insects to perform pollination.
When performing photosynthesis, chloroplast absorbs CO2 and releases oxygen. This absorption decreases CO2 levels, which are otherwise increased by combustion and burning of fossil fuels.Higher levels of CO2 can be harmful to health.
The photosynthetic process in chloroplasts releases oxygen, which is essential in most organisms to carry out aerobic respiration.
Chloroplast participates in the storage of many organic compounds like starch, Vitamin E and K, fat, etc.
The chloroplast is photosensitive and shows phototactic movements, thus increasing or decreasing the amount of sunlight it absorbs.
Chloroplasts also help in the synthesis of amino acids and fatty acids.
Role in Photosynthesis
The thylakoid membrane in the chloroplast has protein complexes like photosystem I, photosystem II, and ATP synthase and chlorophyll.The light reactions occur inside the thylakoid. When sunlight falls on the thylakoid membrane, the photosystems containing chlorophyll get excited, thus giving up electrons that enter the electron transport chain, a series of reactions that results in the phosphorylation of adenosine diphosphate (ADP) to the energy-rich storage compound ATP and production of reducing NADPH. ATP and NADPH take part in the dark reactions which occur in stroma containing enzyme RuBisCO which catalyzes carbon fixation, thus helping to produce glucose.