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).

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.

Mitochondria transfer energy from glucose or lipids into ATP (adenosine triphosphate, the main short-term energetic molecule of cells) through cellular respiration. Different chemical reactions of cellular respiration occur in the matrix and in the cristae. For cellular respiration (in a simplified description), mitochondria use glucose molecules and oxygen to produce ATP and, as by-products, carbon dioxide and water. Carbon dioxide is a waste product in eukaryotes; that is why we exhale it through breathing.

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.

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.

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.

• Mitochondria are present in most eukaryotic cells (from animals, plants, fungi, and protists), while only plants and algae have chloroplasts. This important difference explains the distinctive metabolic reactions each organelle performs. Photosynthetic organisms are autotrophs, meaning that they produce their food. That is why they have chloroplasts. On the other hand, heterotrophic organisms (like us) get their food by eating other organisms or absorbing food particles. But once they get their food, all organisms need mitochondria to break down these macromolecules for producing the ATP that their cells use.