Humans need the energy to carry out life processes. We are dependent on plants for food. Chloroplast is the site of photosynthesis in the plant cell. Chloroplast captures light energy and makes sugar by the process of photosynthesis. When we consume food produced by plants, mitochondria generate energy in the form of ATP using nutrients and oxygen through a process known as cellular respiration. Mitochondria are called the powerhouse of the cell. Mitochondria and chloroplast are double-membrane organelles found in eukaryotic cells. Both organelles contain DNA and ribosomes and can carry out replication independent of the nucleus.
Mitochondria are oval-shaped double membranous organelles found suspended in the cytosol of the eukaryotic cells. The number of mitochondria varies depending upon the physiological activity of the cell. Muscle cells require high energy and have multiple mitochondria. But in red blood cells, mitochondria are absent in order to facilitate the transportation of oxygen efficiently.
Structure − It has an average diameter of 0.5μm and a length of 1.0-4.0µm. The mitochondria consist of the following parts −
Outer mitochondrial membrane − It has porins which facilitate the movement of small molecules, ions, and proteins.
Intermembrane space − This refers to the space between the outer and inner mitochondrial membranes.
Inner mitochondrial membrane − Highly selective due to the absence of porins. The movement of molecules occurs via membrane transporter proteins.
Cristae − Infolds of inner mitochondrial membrane, which increases surface area.
Matrix − Located within the inner mitochondrial membrane. Contains enzymes for ATP production.
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Function − Mitochondria are the site of aerobic respiration. It produces energy in the form of ATP in the presence of oxygen. The formation of iron-sulfur clusters takes place here.
Chloroplasts are disc-shaped double membrane-bound organelle found in the plant cells and various photosynthetic algae. Most of the chloroplasts of the green plants are situated in the mesophyll cells of the green leaves. The number of chloroplasts varies from 1 in green algae to 40 per cell in plants. Chloroplasts contain chlorophyll a and b pigments and absorb solar energy. The shape of the chloroplasts varies widely. Cup-shaped chloroplasts can be seen in Chlamydomonas, green algae whereas in higher plants it is spherical or ovoid shaped.
Structure − Chloroplasts are around 4 to 6 µm in size. They consist of the following regions- the grana and stroma.
Grana − Comprise of disc-shaped structures called lamellae or thylakoids. IT houses chlorophyll pigments (functional units of chloroplasts).
Stroma − Similar to the cytoplasm of the cell containing grana, DNA, enzymes, ribosomes and various other substances.
The chloroplast consists of the following parts.
Outer membrane − Permeable and semi-porous membrane allowing the movement of ions and small molecules. Does not allow proteins to pass through.
Intermembrane space − Situated between the outer and inner membranes.
Inner membrane − Highly selective to the movement of materials.
Stroma − A protein-rich alkaline aqueous fluid inside the inner membrane consisting of DNA, starch and chloroplast ribosomes, proteins, and a thylakoid system.
Thylakoid − Made of multiple thylakoids, consisting of chlorophyll, the sites of light reaction.
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Function − Chloroplasts trap light energy and convert it to chemical energy through the process of photosynthesis. Chloroplasts can also synthesize fatty acids, amino acids etc.
Lynn Margulis proposed the endosymbiotic theory about the origin of mitochondria and chloroplast in 1970. The theory is based on the similarities of chloroplast and mitochondria with the prokaryotic cells. These organelles have their own DNA and ribosome for protein synthesis.
Mitochondria are believed to have evolved from aerobic prokaryotes after their ingestion into proto eukaryotes through phagocytosis. After evolution, it became modern-day mitochondria.
Chloroplasts are thought to be evolved from photosynthetic prokaryotes. These prokaryotic organisms were ingested by the proto-eukaryotic cells and after evolution established a symbiotic relationship and later on became the present-day chloroplast.
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Some of the evidence that supports the endosymbiotic theory is presented below −
Both mitochondria and chloroplast are capable of self-replication and can divide by binary fission.
Both the organelles are double membranous and the compositions are much like bacteria.
DNA, ribosomes and enzymes and transport mechanisms are similar to that found in bacteria.
| Parameters | Mitochondria | Chloroplast |
|---|---|---|
| Occurrence | Aerobic plants and animals | In plants and green algae |
| Shape and Size | Bean shaped and smaller in size | Disc shaped and complex in structure, larger than mitochondria |
| Colour and Pigment | Colourless, no pigment present | Green in colour, contains photosynthetic pigments |
| Inner membrane | Folded into cristae | Forms thylakoid |
| Function | Cellular respiration | Photosynthesis |
| Energy | Consume oxygen and provide energy to the cells | Releases oxygen and store energy |
Mitochondria and chloroplast are membrane-bound organelles found in eukaryotic cells. Mitochondria can break the sugar and generate energy in the form of ATP whereas chloroplast produces sugar in plants by capturing solar energy in a process known as photosynthesis. These organelles are semi-autonomous organelles which contains DNA and ribosomes. Mitochondria have evolved from aerobic bacteria whereas chloroplasts have evolved from photosynthetic bacteria.
Q1. Suppose a chloroplast is taken out of the cell and light energy is provided. Will it be able to do photosynthesis?
Ans. Chloroplast is a semi-autonomous organelle having its own DNA and ribosome. So, when chloroplast is taken out of the cell and provided with light energy it will still be able to perform photosynthesis. But it should be kept in an isotonic solution and provided with raw materials for photosynthesis.
Q2. What differentiates mitochondria from other cell organelles?
Ans. Mitochondria, the "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome. They also divide independently of the cell cycle by simple fission.
Q3. What problem can a cell face, if its mitochondria do not function properly?
Ans. If the mitochondria are defective, they cannot produce sufficient ATP to power normal cellular processes of the cell so all processes requiring energy will be hampered. For example, muscles may become weaker and get tired faster.
Q4. Unicellular algae P and Q of the same species were taken and experimentally chloroplasts were removed from P. After some time, they both were kept in bright sunlight for a few hours and then an iodine test was performed on them. Which of the following will be the possible observation of the experiment?
Ans. In the presence of sunlight, algae photosynthesise and produce starch. As cell P lacks chloroplast, it will not be able to photosynthesize and make starch while cell Q will make it. Iodine turns starch into a blue-black colour. Cell Q can make starch by photosynthesis, thus it will turn blue-black with iodine solution
Q5. Bacterial cells do not contain mitochondria. So, how does respiration take place in bacteria?
Ans. Bacteria contain a special structure called mesosomes that helps in cellular respiration. It is the infolding of the plasma membrane. Mesosomes are analogous to mitochondria.
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