Difference Between Chloroplast and Mitochondria

Main Difference – Chloroplast vs Mitochondria

Chloroplast and mitochondria are two organelles found in the cell. The chloroplast is a membrane-bound organelle found only in algae and plant cells. Mitochondria are found in fungi, plants and animal like eukaryotic cells. The main difference between chloroplast and mitochondria is their functions; chloroplasts are responsible for the production of sugars with the aid of sunlight in a process called photosynthesis whereas mitochondria are the powerhouses of the cell which break down sugar in order to capture energy in a process called cellular respiration.

This article looks at,

1. What is Chloroplast
      – Structure and Function
2. What is Mitochondria
      – Structure and Function
3. What is the difference between Chloroplast and Mitochondria

What is Chloroplast

Chloroplasts are a type of plastids found in algal and plant cells. They contain chlorophyll pigments in order to carry out photosynthesis. Chloroplast consists of their own DNA. The major function of chloroplast is the production of organic molecules, glucose from CO₂ and H₂O with the aid of sunlight.

Structure

Chloroplasts are identified as lens-shaped, green colour pigments in plants. They are 3-10 µm in diameter and their thickness is around 1-3 µm. Plant cells process 10-100 chloroplast per cell. Different shapes of the chloroplast can be found in algae. The algal cell contains a single chloroplast which can be a net, cup, or ribbon-like spiral in shape. 

Figure 1: Chloroplast structure in plants

Three membrane systems can be identified in a chloroplast. They are outer chloroplast membrane, inner chloroplast membrane and thylakoids.

Outer Chloroplast Membrane

The outer membrane of the chloroplast is semi-porous, allowing small molecules to diffuse easily. But large proteins are unable to diffuse. Therefore, proteins required by the chloroplast are transported from the cytoplasm by TOC complex in the outer membrane.

Inner Chloroplast Membrane

Inner chloroplast membrane maintains a constant environment in stroma by regulating the passage of substances. After proteins are passed through TOC complex, they are transported through TIC complex in the inner membrane. Stromules are the protrusions of the chloroplast membranes into the cytoplasm.

Chloroplast stroma is the fluid surrounded by two membranes of the chloroplast. Thylakoids, chloroplast DNA, ribosomes, starch granules and many proteins float around in the stroma. Ribosomes in the chloroplasts are 70S and responsible for the translation of proteins encoded by the chloroplast DNA. Chloroplast DNA is referred to as ctDNA or cpDNA. It is a single circular DNA located in the nucleoid in the chloroplast. The size of the chloroplast DNA is around 120-170 kb, containing 4-150 genes and inverted repeats. Chloroplast DNA is replicated through the double displacement unit (D-loop). Most of the chloroplast DNA transfers into the host genome by endosymbiotic gene transfer. A cleavable transit peptide is added to the N-terminus to the proteins translated in the cytoplasm as a targeting system for the chloroplast.  

Thylakoids

Thylakoid system is composed of thylakoids, which is a collection of highly dynamic, membranous sacks. Thylakoids consist of chlorophyll a, a blue-green pigment which is responsible for the light reaction in the photosynthesis. In addition to chlorophylls, two types of photosynthetic pigments can be present in plants: yellow-orange colour carotenoids and red colour phycobilins. Grana are the stacks formed by the arrangement of thylakoids together. Different grana are interconnected by the stromal thylakoids. Chloroplasts of C₄ plants and some algae consist of freely-floating chloroplasts.

Function

Chloroplasts can be found in leaves, cacti and stems of plants. A plant cell consisting of chlorophyll is referred to as chlorenchyma. Chloroplasts can change their orientation depending on the availability of sunlight. Chloroplasts are capable of producing glucose, by using CO₂ and H₂O with the aid of light energy in a process called photosynthesis. Photosynthesis proceeds through two steps: light reaction and the dark reaction.

Light Reaction

The light reaction occurs in the thylakoid membrane. During the light reaction, oxygen is produced by splitting of water. The light energy is also stored in NADPH and ATP by NADP⁺ reduction and photophosphorylation, respectively. Thus, the two energy carriers for the dark reaction are ATP and NADPH. A detailed diagram of the light reaction is shown in figure 2.

Figure 2: Light reaction

Dark Reaction

The dark reaction is also called the Calvin cycle. It occurs in the stroma of chloroplast. Calvin cycle proceeds through three phases: carbon fixation, reduction and ribulose regeneration. The end product of the Calvin cycle is glyceraldehyde-3-phosphate, which can be doubled to form glucose or fructose.

Figure 3: Calvin Cycle

 Chloroplasts are also capable of producing all amino acids and nitrogenous bases of the cell by themselves. This eliminates the requirement of exporting them from the cytosol. Chloroplasts also participate in the plant’s immune response for the defence against pathogens.

What are Mitochondria

A mitochondrion is a membrane-bound organelle found in all eukaryotic cells. The chemical energy source of the cell, which is the ATP, is generated in the mitochondria. Mitochondria also contain their own DNA inside the organelle.

Structure

A mitochondrion is a bean-like structure with 0.75 to 3 µm in its diameter. The number of mitochondria present in a particular cell depends on the cell type, tissue and organism. Five distinct components can be identified in the mitochondrial structure. The structure of a mitochondrion is shown in figure 4.

Figure 4: Mitochondrion

A mitochondrion consists of two membranes – inner and the outer membrane.

Outer Mitochondrial Membrane

The outer mitochondrial membrane contains a large number of integral membrane proteins called porins. Translocase is an outer membrane protein. Translocase-bound N-terminal signal sequence of large proteins allows the protein to enter into mitochondria. The association of mitochondrial outer membrane with endoplasmic reticulum forms a structure called MAM (mitochondria-associated ER-membrane). MAM allows the transport of lipids between mitochondria and the ER through calcium signalling.

Inner Mitochondrial Membrane

The inner mitochondrial membrane consists of more than 151 different protein types, functioning in many ways. It lacks porins; the type of translocase in the inner membrane is called as TIC complex. The intermembrane space is situated between inner and outer mitochondrial membranes.

The space enclosed by the two mitochondrial membranes is called the matrix. Mitochondrial DNA and ribosomes with numerous enzymes are suspended in the matrix. Mitochondrial DNA is a circular molecule. The size of the DNA is around 16 kb, encoding 37 genes. Mitochondria may contain 2-10 copies of its DNA in the organelle. The inner mitochondrial membrane forms folds in the matrix, which are called cristae. Cristae increase the surface area of the inner membrane.

Function

Mitochondria produce chemical energy in the form of ATP to use in cellular functions in the process called respiration. The reactions involved in the respiration are collectively called citric acid cycle or Krebs cycle. The citric acid cycle occurs in the inner membrane of mitochondria. It oxidises pyruvate and NADH produced in the cytosol from glucose with the aid of oxygen.

Figure 5: Citric Acid cycle

NADH and FADH₂ are the carriers of redox energy generated in the citric acid cycle. NADH and FADH₂ transfer their energy to O₂ by going through the electron transport chain. This process is called the oxidative phosphorylation. Protons released from the oxidative phosphorylation are used by ATP synthase to produce ATP from ADP. A diagram of electron transport chain is shown in figure 6. The produced ATPs pass through the membrane using porins. 

Figure 6: Electron transport chain

Functions of Mitochondrial Inner Membrane

• Performing the oxidative phosphorylation
• ATP synthesis
• Holding transport proteins to regulate the passage of substances
• Holding TIC complex to transport
• Involving in mitochondrial fission and fusion

Other Functions of Mitochondria

• Regulation of metabolism in the cell
• Synthesis of steroids
• Storage of calcium for signal transduction in the cell
• Membrane potential regulation
• Reactive oxygen species used in signalling
• Porphyrin synthesis in the heme synthesis pathway
• Hormonal signaling
• Regulation of apoptosis

Difference Between Chloroplast and Mitochondria

Type of Cell

Chloroplast: Chloroplasts are found in plant and algal cells.

Mitochondria: Mitochondria are found in all, aerobic eukaryotic cells.

Color

Chloroplast: Chloroplasts are green in colour.

Mitochondria: Mitochondria are usually colourless.

Shape

Chloroplast: Chloroplasts are disk-like in shape.

Mitochondria: Mitochondria are bean-like in shape.

Inner Membrane

Chloroplast: Foldings in inner membrane form stromules.

Mitochondria: Foldings in inner membrane form cristae.

Grana

Chloroplast: Thylakoids form stacks of disks which are called grana.

Mitochondria: Cristae do not form grana.

Compartments

Chloroplast: Two compartments can be identified: thylakoids and stroma.

Mitochondria: Two compartments can be found: cristae and the matrix.

Pigments

Chloroplast: Chlorophyll and carotenoids are present as photosynthetic pigments in the thylakoid membrane.

Mitochondria: No pigments can be found in mitochondria.

Energy Conversion

Chloroplast: Chloroplast stores solar energy in the chemical bonds of glucose.

Mitochondria: Mitochondria convert sugar into chemical energy which is ATP.

Raw Materials and End Products

Chloroplast: Chloroplasts use CO₂ and H₂O in order to build up glucose.

Mitochondria: Mitochondria break down glucose into CO₂ and H₂O.

Oxygen

Chloroplast: Chloroplasts liberate oxygen.

Mitochondria: Mitochondria consume oxygen.

Processes

Chloroplast: Photosynthesis and photorespiration occur in the chloroplast.

Mitochondria: Mitochondria are a site of electron transport chain, oxidative phosphorylation, beta oxidation and photorespiration.

Conclusion

Chloroplasts and mitochondria both are membrane-bound organelles which are involved in energy conversion. Chloroplast stores light energy in the chemical bonds of glucose in the process called as photosynthesis. Mitochondria convert the light energy stored in glucose into chemical energy, in the form of ATP which can be used in the cellular processes. This process is referred to as cellular respiration. Both of the organelles utilise CO₂ and O₂ in their processes. Both chloroplasts and mitochondria involve in cellular differentiation, signalling and cell death other than their main function. Also, they control the cell growth and cell cycle. Both organelles are considered as originated through endosymbiosis. They contain their own DNA. But, the main difference between chloroplasts and mitochondria is with their function in the cell.

Reference:
1. “Chloroplast”. Wikipedia, the free encyclopedia, 2017. Accessed 02 Feb 2017
2. “Mitochondrion”. Wikipedia, the free encyclopedia, 2017. Accessed 02 Feb 2017

Image Courtesy:
1. “Chloroplast structure” By Kelvinsong – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Thylakoid membrane 3” By Somepics – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “:Calvin-cycle4” By Mike Jones – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “Mitochondrion structure” By Kelvinsong; modified by Sowlos – Own work based on: Mitochondrion mini.svg, CC BY-SA 3.0) via Commons Wikimedia
5. “Citric acid cycle noi” By Narayanese (talk) – Modified version of Image:Citricacidcycle_ball2.png. (CC BY-SA 3.0) via Commons Wikipedia
6. “Electron transport chain” By T-Fork –  (Public Domain) via Commons Wikimedia