Cells are the building blocks of life. Tiny as they may be, they have several jobs to do and contain several compartments in order to more efficiently perform the functions they must do to keep you alive. A basic knowledge of how and why cells compartmentalize can go a long way in understanding how cells work to keep plant and human life thriving on Earth.
Cell compartmentalization refers to the way organelles in eukaryotic cells live and work in separate areas within the cell in order to perform their specific functions more efficiently.
When people imagine a cell, they often picture an amorphous blend of all the water, proteins, carbohydrates and lipids that make it up. But cells function more like your body than most people realize. Your body contains separate components that do different jobs. Your legs help you to walk, for instance, and your kidneys work to filter waste, so your cells are made up of separate compartments that perform different jobs.
There are two types of cells: eukaryotic cells and prokaryotic cells. Most organisms are eukaryotes made up of eukaryote cells. Eukaryotic cells contain a membrane-bound nucleus, as well as membrane-bound organelles that each perform different functions within the cell. Those organelles live within different compartments inside the cell, so they can work in the microenvironment that suits them best.
Prokaryotic cells are unicellular, meaning they lack a nucleus, mitochondria and organelles bound by membranes. Examples of prokaryotic cells include bacteria such as E. coli. While these types of cells do have internal structures and are capable of making compartmentalized areas, they tend to do one job and don’t need to compartmentalize the way eukaryotic cells do.
Compartmentalization in eukaryotic cells is largely about efficiency. Separating the cell into different parts allows for the creation of specific microenvironments within a cell. That way, each organelle can have all the advantages it needs to perform to the best of its ability.
It’s similar to the way that a home needs different environments in different rooms. You want a comfortable bed and curtains that block the sun in your bedroom, for instance, and you need appliances and food to be able to cook a meal in your kitchen. Outfitting each room of your house with all the resources necessary to perform every household duty would be a waste of time, money and space. Cells compartmentalize their resources in the same way your do in your home, allowing each part of the cell to flourish in its own tiny environment.
In addition, several functions can be going on at once, also in the same way they do in a home. While you might be using your quiet basement to study, another family member could be using the garage to fix a car while someone else naps in the bedroom, all without disrupting each other. Since so many cellular reactions have to simultaneously happen to keep plant and animal life alive, it would be a serious inefficiency if each of your cells could not perform several jobs at once.
Therefore, your eukaryotic cells have evolved to become super efficient spaces where multiple activities happen, allowing plant and animal life to thrive.
Rachelle Dragani is a freelance writer based in Brooklyn with extensive experience covering the latest innovation and development in the world of science. Her pieces on topics including DNA sequencing, tissue engineering and stem cell advances have been featured in publications including BioTechniques: the International Journal of Life Science Methods, Popular Mechanics, Futurism and Gizmodo.
Proteins Can Move Between Compartments in Different Ways
All proteins begin being synthesized on ribosomes in the cytosol, except for the few that are synthesized on the ribosomes of mitochondria and plastids. Their subsequent fate depends on their amino acid sequence, which can contain sorting signals that direct their delivery to locations outside the cytosol. Most proteins do not have a sorting signal and consequently remain in the cytosol as permanent residents. Many others, however, have specific sorting signals that direct their transport from the cytosol into the nucleus, the ER, mitochondria, plastids, or peroxisomes; sorting signals can also direct the transport of proteins from the ER to other destinations in the cell.
To understand the general principles by which sorting signals operate, it is important to distinguish three fundamentally different ways by which proteins move from one compartment to another. These three mechanisms are described below, and their sites of action in the cell are outlined in . The first two mechanisms are detailed in this chapter, while the third (green arrows in ) is the subject of Chapter 13.
A simplified “roadmap” of protein traffic. Proteins can move from one compartment to another by gated transport (red), transmembrane transport (blue), or vesicular transport (green). The signals that direct a given proteins movement through (more…)
Vesicle budding and fusion during vesicular transport. Transport vesicles bud from one compartment (donor) and fuse with another (target) compartment. In the process, soluble components (red dots) are transferred from lumen to lumen. Note that membrane (more…)
Each of the three modes of protein transfer is usually guided by sorting signals in the transported protein that are recognized by complementary receptor proteins. If a large protein is to be imported into the nucleus, for example, it must possess a sorting signal that is recognized by receptor proteins that guide it through the nuclear pore complex. If a protein is to be transferred directly across a membrane, it must possess a sorting signal that is recognized by the translocator in the membrane to be crossed. Likewise, if a protein is to be loaded into a certain type of vesicle or retained in certain organelles, its sorting signal must be recognized by a complementary receptor in the appropriate membrane.
The Topological Relationships of Membrane-enclosed Organelles Can Be Interpreted in Terms of Their Evolutionary Origins
To understand the relationships between the compartments of the cell, it is helpful to consider how they might have evolved. The precursors of the first eucaryotic cells are thought to have been simple organisms that resembled bacteria, which generally have a plasma membrane but no internal membranes. The plasma membrane in such cells therefore provides all membrane-dependent functions, including the pumping of ions, ATP synthesis, protein secretion, and lipid synthesis. Typical present-day eucaryotic cells are 10–30 times larger in linear dimension and 1000–10,000 times greater in volume than a typical bacterium such as E. coli. The profusion of internal membranes can be seen in part as an adaptation to this increase in size: the eucaryotic cell has a much smaller ratio of surface area to volume, and its area of plasma membrane is presumably too small to sustain the many vital functions for which membranes are required. The extensive internal membrane systems of a eucaryotic cell alleviate this imbalance.
The evolution of internal membranes evidently accompanied the specialization of membrane function. Consider, for example, the generation of thylakoid vesicles in chloroplasts. These vesicles form during the development of chloroplasts from proplastids in the green leaves of plants. Proplastids are small precursor organelles that are present in all immature plant cells. They are surrounded by a double membrane and develop according to the needs of the differentiated cells: they develop into chloroplasts in leaf cells, for example, and into organelles that store starch, fat, or pigments in other cell types ( ). In the process of differentiating into chloroplasts, specialized membrane patches form and pinch off from the inner membrane of the proplastid. The vesicles that pinch off form a new specialized compartment, the thylakoid, that harbors all of the chloroplasts photosynthetic machinery ( ).
Development of plastids. (A) Proplastids are inherited with the cytoplasm of plant egg cells. As immature plant cells differentiate, the proplastids develop according to the needs of the specialized cell: they can become chloroplasts (in green leaf cells), (more…)
Other compartments in eucaryotic cells may have originated in a conceptually similar way ( ). Pinching off of specialized intracellular membrane structures from the plasma membrane, for example, would create organelles with an interior that is topologically equivalent to the exterior of the cell. We shall see that this topological relationship holds for all of the organelles involved in the secretory and endocytic pathways, including the ER, Golgi apparatus, endosomes, and lysosomes. We can therefore think of all of these organelles as members of the same family. As we discuss in detail in the next chapter, their interiors communicate extensively with one another and with the outside of the cell via transport vesicles that bud off from one organelle and fuse with another ( ).
Hypothetical schemes for the evolutionary origins of some membrane-enclosed organelles. The origins of mitochondria, chloroplasts, ER, and the cell nucleus can explain the topological relationships of these intra-cellular compartments in eucaryotic cells. (more…)
Topological relationships between compartments of the secretory and endocytic pathways in a eucaryotic cell. Topologically equivalent spaces are shown in red. In principle, cycles of membrane budding and fusion permit the lumen of any of these organelles (more…)
As described in Chapter 14, mitochondria and plastids differ from the other membrane-enclosed organelles in containing their own genomes. The nature of these genomes, and the close resemblance of the proteins in these organelles to those in some present-day bacteria, strongly suggest that mitochondria and plastids evolved from bacteria that were engulfed by other cells with which they initially lived in symbiosis (discussed in Chapters 1 and 14). According to the hypothetical scheme shown in , the inner membrane of mitochondria and plastids corresponds to the original plasma membrane of the bacterium, while the lumen of these organelles evolved from the bacterial cytosol. As might be expected from such an endocytic origin, these two organelles are surrounded by a double membrane, and they remain isolated from the extensive vesicular traffic that connects the interiors of most of the other membrane-enclosed organelles to each other and to the outside of the cell.
The evolutionary scheme described above groups the intracellular compartments in eucaryotic cells into four distinct families: (1) the nucleus and the cytosol, which communicate through nuclear pore complexes and are thus topologically continuous (although functionally distinct); (2) all organelles that function in the secretory and endocytic pathways—including the ER, Golgi apparatus, endosomes, lysosomes, the numerous classes of transport intermediates such as transport vesicles, and possibly peroxisomes; (3) the mitochondria; and (4) the plastids (in plants only).
Molecular Biology of the Cell. 4th edition.
In this introductory section we present a brief overview of the compartments of the cell and the relationships between them. In doing so, we organize the organelles conceptually into a small number of discrete families and discuss how proteins are directed to specific organelles and how they cross organelle membranes.
Do membranes compartmentalize organelles in the cell?
What is the significance of membrane compartmentalization in eukaryotes?