Mitochondria and Chloroplasts

All organisms need energy to perform vital processes and stay alive. That is why we need to eat, and organisms like plants gather energy from the sun to produce their food. How does the energy contained in the food we eat or in the sun get to every cell in an organism’s body? Fortunately, organelles called mitochondria and chloroplast do this job. Hence, they are considered the “powerhouses” of the cell. These organelles differ from other cell organelles in many ways, such as having their own DNA and ribosomes, suggesting a remarkably distinct origin.

The function of Mitochondria and Chloroplasts

Cells get energy from their environment, usually in the form of chemical energy from food molecules (like glucose) or solar energy. They then need to convert this energy into useful forms for everyday tasks. The function of mitochondria and chloroplasts is to transform the energy, from an energy source to ATP, for cellular use. They do this in different ways though, as we will discuss.

Fig. 1: Diagram of a mitochondrion and its components (left) and how they look under a microscope (right).

Mitochondria

Most eukaryotic cells (protist, plant, animal, and fungi cells) have hundreds of mitochondria (singular mitochondrion) dispersed in the cytosol. They can be elliptical or oval-shaped and have two bilayered membranes with an intermembrane space between them (Figure 1). The outer membrane surrounds the whole organelle and separates it from the cytoplasm. The inner membrane has numerous inward folds extending into the interior of the mitochondrion. The folds are called cristae and surround the interior space called the matrix. The matrix contains the mitochondrion’s own DNA and ribosomes.

Chloroplasts

Chloroplasts are found in the cells of plants and algae (photosynthetic protists) only. They perform photosynthesis, transferring energy from the sunlight into ATP, which is used to synthesize glucose. Chloroplasts belong to a group of organelles known as plastids that produce and store material in plants and algae.

Chloroplasts are lens-shaped and, like mitochondria, they have a double membrane and an intermembrane space (Figure 2). The inner membrane encloses the thylakoid membrane that forms numerous piles of interconnected fluid-filled membranous discs called thylakoids. Each pile of thylakoids is a granum (plural grana), and they are surrounded by a fluid called the stroma. The stroma contains the chloroplast’s own DNA and ribosomes.

Fig. 2: Diagram of a chloroplast and its components (DNA and ribosomes not shown), and how chloroplasts look inside the cells under a microscope (right).

Thylakoids contain several pigments (molecules that absorb visible light at specific waves) incorporated into their membrane. Chlorophyll is more abundant and the main pigment that captures the energy from sunlight. In photosynthesis, chloroplasts transfer energy from the sun into ATP which is used, along with carbon dioxide and water, to produce carbohydrates (mainly glucose), oxygen, and water (simplified description). ATP molecules are too unstable and must be used in the moment. Macromolecules are the best way to store and transport this energy to the rest of the plant.

In plants, chloroplasts are widely distributed but are more common and abundant in leaves and other green organs’ cells (like stems) where photosynthesis primarily occurs (chlorophyll is green, giving these organs their characteristic color). Organs that do not receive sunlight, like roots, do not have chloroplasts. Some cyanobacteria bacteria also perform photosynthesis, but they do not have chloroplasts. Their inner membrane (they are double-membrane bacteria) contains the chlorophyll molecules.

Similarities Between Chloroplasts and Mitochondria

There are similarities between chloroplasts and mitochondria that are related to their function, given that both organelles transform energy from one form to another. Other similarities are more related to the origin of these organelles (like having a double membrane and their own DNA and ribosomes, which we will discuss shortly). Some similarities between these organelles are:

• An increase in the surface area through folds (cristae in mitochondrial inner membrane) or interconnected sacs (thylakoid membrane in chloroplasts), optimizing the use of the interior space.

• Compartmentalization: The folds and sacs from the membrane also provide compartments inside the organelle. This allows separated environments for the execution of the different reactions needed for cellular respiration and photosynthesis. This is comparable to the compartmentalization given by membranes in eukaryotic cells.

• ATP synthesis: Both organelles synthesize ATP through chemiosmosis. As part of cellular respiration and photosynthesis, protons are transported across the membranes of chloroplasts and mitochondria. In brief, this transportation releases energy that drives the synthesis of ATP.

• Double membrane: They have the outer delimiting membrane and the inner membrane.

• DNA and ribosomes: They have a short DNA chain that codifies for a small number of proteins that their own ribosomes synthesize. However, most proteins for mitochondria and chloroplasts membranes are directed by the cell nucleus and synthesized by free ribosomes in the cytoplasm.

• Reproduction: They reproduce by themselves, independently of the cell cycle.

Differences Between Mitochondria and Chloroplasts

The ultimate purpose of both organelles is to provide cells with the required energy to function. However, they do so in different ways. The differences between mitochondria and chloroplasts are:

• The inner membrane in mitochondria folds inwards to the interior, while the inner membrane in chloroplasts does not. A different membrane forms the thylakoids in the interior of chloroplasts.

• Mitochondria break down carbohydrates (or lipids) to produce ATP through cellular respiration. Chloroplasts produce ATP from solar energy and store it in carbohydrates through photosynthesis.

Venn Diagram of Chloroplasts and Mitochondria

This Venn diagram of chloroplasts and mitochondria summarizes the similarities and differences we discussed in the previous sections: