Mitochondria
Mitochondria (singular mitochondrion) (Figure 2.5) are oval-shaped, double membrane organelles that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.
Scientists often call mitochondria “powerhouses” or “energy factories” of both plant and animal cells because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the cell’s short-term stored energy. Cellular respiration is the process of making ATP using the chemical energy in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that we exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct. It is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. The muscle cells need considerable energy to keep the body moving. When the cells do not get enough oxygen, they do not make much ATP. Instead, producing lactic acid accompanies the small amount of ATP they make in the absence of oxygen.
Figure 2.5: A mitochondrion seen through an electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane (credit: modification of work by Matthew Britton; scale-bar data from Matt Russell).
Peroxisomes
Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water. For example, peroxisomes in liver cells detoxify alcohol.
Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes that play a role in metabolism, pathogene defense, and stress response, to mention a few.
Vesicles and vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. Vesicle membranes can fuse with either the plasma membrane, or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The vacuole’s membrane does not fuse with the membranes of other cellular components.
Endomembrane system
The endomembrane system (endo “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport membrane lipids and proteins. The endomembrane system includes: the nuclear envelope, lysosomes, vesicles, the endoplasmic reticulum, Golgi apparatus, and the plasma membrane.
Although not technically within the cell, the plasma membrane is included in the endomembrane system because it interacts with the other endomembranous organelles.
The endomembrane system does not include either mitochondria or chloroplast membranes.
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate areas of the ER: the rough ER and the smooth ER, respectively. The ER tubules’ hollow portion is called the lumen or cisternal space. The ER‘s membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.
- Rough ER
Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope. Ribosomes transfer their newly synthesized proteins into the RER‘s lumen where they undergo structural modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular membranes – the ER or the ER‘s of other organelles’ membranes. The proteins can also secrete from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane. Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, it would be correct to assume that the RER is abundant in cells that secrete proteins. This is the case with liver cells, for example. - Smooth ER
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions. In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cells’ coordinated contractions.
Golgi Apparatus
The vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes.
The Golgi apparatus is called the cis face. The opposite side is the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids then tag with phosphate groups or other small molecules in order to travel to their proper destinations. Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi’s trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell. Cells that engage in a great deal of secretory activity (such as salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an abundance of Golgi.
In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.
Lysosomes
In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are part of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in macrophages, a group of white blood cells which are part of the body’s immune system. In a process that scientists call phagocytosis or endocytosis, a section of the macrophage’s plasma membrane invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen.
Cytoskeleton
If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton. There are three types of protein fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules.
Microfilaments
Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They have a diameter of about 7 nm and are comprised of two globular protein intertwined strands, which are called actin. For this reason, microfilaments are also called actin filaments. Microfilaments function in cellular movement. ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin. This enables actin to engage in cellular events requiring motion, such as eukaryotic cell division and cytoplasmic streaming (the cell cytoplasm’s circular movement in plant cells). Actin and myosin are plentiful in muscle cells. When the actin and myosin filaments slide past each other, the muscles contract.
Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (the body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.
Intermediate filaments
Several strands of fibrous proteins that are wound together comprise intermediate filaments. Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules. Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. They create a supportive scaffolding inside the cell. The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. An example is the keratin, the fibrous protein that strengthens the human hair, nails, and skin’s epidermis.
Microtubules
As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls. With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly. Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes.
Flagella and cilia
The flagella (singular flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example: sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella.
However, when cilia (singular cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward the nostrils).
Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array”. This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center.
References:
- Fowler, Samantha, et al. Concepts of Biology. OpenStax College, Rice University, 2013. Download for free at: https://openstax.org/details/books/concepts-biology.
- Clark, Mary, Jung Choi, and Matthew Douglas. Biology 2E. 2018. Access for free at https://openstax.org/details/books/biology-2e.