5.3: Mitochondria and Chloroplasts

Learning Objectives

• Explain the distinguishing characteristics of eukaryotic cells
• Describe internal and external structures of prokaryotic cells in terms of their physical structure, chemical structure, and function
• Identify and describe structures and organelles unique to eukaryotic cells
• Compare and contrast similar structures found in prokaryotic and eukaryotic cells

Mitochondria

The large, complex organelles in which aerobic cellular respiration occurs in eukaryotic cells are called mitochondria(Figure \(\PageIndex{15}\)). The term “mitochondrion” was first coined by German microbiologist Carl Benda in 1898 and was later connected with the process of respiration by Otto Warburg in 1913. Scientists during the 1960s discovered that mitochondria have their own genome and 70S ribosomes. The mitochondrial genome was found to be bacterial, when it was sequenced in 1976. These findings ultimately supported the endosymbiotic theory proposed by Lynn Margulis, which states that mitochondria originally arose through an endosymbiotic event in which a bacterium capable of aerobic cellular respiration was taken up by phagocytosis into a host cell and remained as a viable intracellular component.

Each mitochondrion has two lipid membranes. The outer membrane is a remnant of the original host cell’s membrane structures. The inner membrane was derived from the bacterial plasma membrane. The electron transport chain for aerobic respiration uses integral proteins embedded in the inner membrane. The mitochondrial matrix, corresponding to the location of the original bacterium’s cytoplasm, is the current location of many metabolic enzymes. It also contains mitochondrial DNA and 70S ribosomes. Invaginations of the inner membrane, called cristae, evolved to increase surface area for the location of biochemical reactions. The folding patterns of the cristae differ among various types of eukaryotic cells and are used to distinguish different eukaryotic organisms from each other.

Figure \(\PageIndex{15}\): Each mitochondrion is surrounded by two membranes, the inner of which is extensively folded into cristae and is the site of the intermembrane space. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, and metabolic enzymes. The transmission electron micrograph of a mitochondrion, on the right, shows both membranes, including cristae and the mitochondrial matrix. (credit “micrograph”: modification of work by Matthew Britton; scale-bar data from Matt Russell)

Query \(\PageIndex{1}\)

 

Chloroplasts

Plant cells and algal cells contain chloroplasts, the organelles in which photosynthesis occurs (Figure \(\PageIndex{16}\)). All chloroplasts have at least three membrane systems: the outer membrane, the inner membrane, and the thylakoid membrane system. Inside the outer and inner membranes is the chloroplast stroma, a gel-like fluid that makes up much of a chloroplast’s volume, and in which the thylakoid system floats. The thylakoid system is a highly dynamic collection of folded membrane sacs. It is where the green photosynthetic pigment chlorophyll is found and the light reactions of photosynthesis occur. In most plant chloroplasts, the thylakoids are arranged in stacks called grana (singular: granum), whereas in some algal chloroplasts, the thylakoids are free floating.

Figure \(\PageIndex{16}\): Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

Other organelles similar to mitochondria have arisen in other types of eukaryotes, but their roles differ. Hydrogenosomes are found in some anaerobic eukaryotes and serve as the location of anaerobic hydrogen production. Hydrogenosomes typically lack their own DNA and ribosomes. Kinetoplasts are a variation of the mitochondria found in some eukaryotic pathogens. In these organisms, each cell has a single, long, branched mitochondrion in which kinetoplast DNA, organized as multiple circular pieces of DNA, is found concentrated at one pole of the cell.

Mitochondria-Related Organelles in Protozoan Parasites

Many protozoans, including several protozoan parasites that cause infections in humans, can be identified by their unusual appearance. Distinguishing features may include complex cell morphologies, the presence of unique organelles, or the absence of common organelles. The protozoan parasites Giardia lamblia and Trichomonas vaginalis are two examples.

G. lamblia, a frequent cause of diarrhea in humans and many other animals, is an anaerobic parasite that possesses two nuclei and several flagella. Its Golgi apparatus and endoplasmic reticulum are greatly reduced, and it lacks mitochondria completely. However, it does have organelles known as mitosomes, double-membrane-bound organelles that appear to be severely reduced mitochondria. This has led scientists to believe that G. lamblia’s ancestors once possessed mitochondria that evolved to become mitosomes. T. vaginalis, which causes the sexually transmitted infection vaginitis, is another protozoan parasite that lacks conventional mitochondria. Instead, it possesses hydrogenosomes, mitochondrial-related, double-membrane-bound organelles that produce molecular hydrogen used in cellular metabolism. Scientists believe that hydrogenosomes, like mitosomes, also evolved from mitochondria.⁵

Key Concepts and Summary

• Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores.
• Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall.
• The nucleolus, located in the nucleus of eukaryotic cells, is the site of ribosomal synthesis and the first stages of ribosome assembly.
• Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum (membrane bound-ribosomes) and cytoplasm (free ribosomes). They contain 70s ribosomes in mitochondria and chloroplasts.
• Eukaryotic cells have evolved an endomembrane system, containing membrane-bound organelles involved in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus.
• The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds. The rough endoplasmic reticulum contains membrane-bound 80S ribosomes that synthesize proteins destined for the cell membrane
• The Golgi apparatus processes proteins and lipids, typically through the addition of sugar molecules, producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell communication.
• Lysosomes contain digestive enzymes that break down small particles ingested by endocytosis, large particles or cells ingested by phagocytosis, and damaged intracellular components.
• The cytoskeleton, composed of microfilaments, intermediate filaments, and microtubules, provides structural support in eukaryotic cells and serves as a network for transport of intracellular materials.
• Centrosomes are microtubule-organizing centers important in the formation of the mitotic spindle in mitosis.
• Mitochondria are the site of cellular respiration. They have two membranes: an outer membrane and an inner membrane with cristae. The mitochondrial matrix, within the inner membrane, contains the mitochondrial DNA, 70S ribosomes, and metabolic enzymes.
• The plasma membrane of eukaryotic cells is structurally similar to that found in prokaryotic cells, and membrane components move according to the fluid mosaic model. However, eukaryotic membranes contain sterols, which alter membrane fluidity, as well as glycoproteins and glycolipids, which help the cell recognize other cells and infectious particles.
• In addition to active transport and passive transport, eukaryotic cell membranes can take material into the cell via endocytosis, or expel matter from the cell via exocytosis.
• Cells of fungi, algae, plants, and some protists have a cell wall, whereas cells of animals and some protozoans have a sticky extracellular matrix that provides structural support and mediates cellular signaling.
• Eukaryotic flagella are structurally distinct from prokaryotic flagella but serve a similar purpose (locomotion). Ciliaare structurally similar to eukaryotic flagella, but shorter; they may be used for locomotion, feeding, or movement of extracellular particles.

Footnotes

1. 1 A.E. Barnhill, M.T. Brewer, S.A. Carlson. “Adverse Effects of Antimicrobials via Predictable or Idiosyncratic Inhibition of Host Mitochondrial Components.” Antimicrobial Agents and Chemotherapy 56 no. 8 (2012):4046–4051.
2. 2 Fuchs E, Cleveland DW. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
3. 3 E. Fuchs, D.W. Cleveland. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
4. 4 E. Fuchs, D.W. Cleveland. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
5. 5 N. Yarlett, J.H.P. Hackstein. “Hydrogenosomes: One Organelle, Multiple Origins.” BioScience 55 no. 8 (2005):657–658.
6. 6 M. Dudzick. “Protists.” OpenStax CNX. November 27, 2013. http://cnx.org/contents/f7048bb6-e46...ef291cf7049c@1