PART TWO Cells
Overview of the Cell
Cells are the building blocks of all life, and nothing below the organizational level of a cell can be qualified as being alive (sorry, viruses, you don’t cut it). That doesn’t mean a single cell is a simple sort of structure—there’s a lot going on in each and every one of our little building blocks!
Cells are the building blocks of life.
The structure of the cell membrane is very important to its function.
When you say something is “alive,” what do you really mean? That it has eyes and can see? That it is able to move? That it eats food? The truth is, for something to be alive, it must be made of at least one cell. As our must know idea points out, the cell is the building block of life; something smaller than a cell (such as a virus) does not qualify as being “alive.” An organism may be composed of a single cell (unicellular), such as a bacterium, an amoeba, or a yeast cell. Just as bricks can be adhered together to form larger structures, so too can cells. A multicellular organism is one that is composed of more than one cell. Multicellularity allows cells to begin to specialize and form specific tissues, each with their own functions. The concept of specialization will be addressed further in Chapter 11 (Gene Expression and Differentiation). By understanding that a single cell is the building block for all types of life, you will be able to understand the processes shared by all living organisms on Earth.
All cells have an outer boundary called a cell membrane. This leads us directly into our second must know concept: the structure of the membrane is important to its function as a selective permeable barrier (meaning chooses what is able to pass through). The membrane consists of two layers of a special macromolecule called a phospholipid. This “phospholipid bilayer” (get it? It is two [bi-] layers of phospholipids!) is key to the evolution of cells because it is picky about what it lets pass through. The selective permeability of the cell membrane allows the cell to have an environment on the inside that is different from the environment surrounding the cell. The inside of the cell is the cytoplasm, and it contains dissolved substances such as ions, proteins, glucose molecules, and many other chemicals. The cell actively collects substances it wants (food! molecular building materials!) and exports wastes.
Membranes Are Composed of Lipids
The main component of a cell membrane is the phospholipid. Phospholipids are special because they are amphipathic, meaning the molecule has both hydrophilic (water loving) and hydrophobic (water avoiding) parts. If you look closely, you will notice that the phosphate group has some charges associated with it, yet those two long hydrocarbon tails are very nonpolar:
A single phospholipid molecule. Please note that “R” means the “rest” of the molecule (but for our purposes, it isn’t significant). The structure of the “tails” is also simplified by omitting the hydrogens.
Charged areas are polar, and since water is also polar, they “mix” well. The long carbon-carbon tails are very nonpolar, so they do whatever they can to hide from water.
A cell’s membrane is happy to be composed of two layers of phospholipids. Look at the image below and notice how the polar phosphate “head” groups face outward toward the water surrounding the cell and the water inside the cell. The hydrophobic “tails,” meanwhile, are tucked into the middle of the membrane, away from any chance of touching anything aqueous. This creates the semipermeable barrier around the cell. There are also embedded proteins that play an important role in transport across the cell membrane. Specifically, they help in either active transport (serving as tiny little pumps) or facilitated diffusion (tunnels through which molecules will passively flow). You’ll learn more about this in the cell transport chapter.
There’s a saying: “Like dissolves like.” This means that polar molecules dissolve in polar solvents (such as water), whereas nonpolar molecules dissolve in nonpolar solvents (such as oil). If you mix a polar and a nonpolar substance, they will not mix well at all! If you’ve ever poured oil into water, you’ve seen this firsthand.
Consider the adage “like dissolves like,” and you will see why phospholipids behave as they do in aqueous (water) solutions. As we learned in Chapter 1 (Chemistry), phospholipids are amphipathic in nature: the phosphate head groups are hydrophilic (love water), and the two fatty-acid tails are hydrophobic (hate water). That is why a cell membrane is also called a phospholipid bilayer: it is composed of two layers of phospholipids arranged so the polar head groups face the water and the tails are tucked in the middle of the membrane, safely away from water.
Generally speaking, what form of matter (solid, liquid, or gas) usually comes to mind if you think of really cold temperatures? Solid! In cold conditions, molecules tend to slow down and get closer together (generally speaking). When the temperature increases, so does the speed and distance between molecules, meaning a substance becomes a liquid. If you are a cell, and your very existence is dependent upon that cell membrane, neither of those options (solid as a rock or melting into liquid) sounds particularly pleasant.
Cells need to maintain their cell membrane at a nice, even, not-too-solid-not-too-loose level. The temperature at which a membrane solidifies is in part dependent on what types of phospholipids make up the membrane. Do you recall from the chemistry unit the differences between saturated and unsaturated fats? Saturated fats are solid at room temperature (butter), and unsaturated fats are liquid at room temperature (oils). Even though the type of lipid we’re currently talking about isn’t a fat, the saturated-versus-unsaturated thing still matters in phospholipids.
If a cell membrane is made up of phospholipids that are composed of a fair amount of unsaturated fatty acids, that will increase the membrane’s fluidity. As before, any double bonds in the unsaturated fatty acid tails will create a kink in the phospholipid, and they won’t be able to pack very tightly together, making it more fluid. On the flip side, if the fatty acids in the phospholipids are mostly saturated (without any weird bends and kinks), they will be able to pack very tightly together and the membrane will be more rigid. This relates directly to our must know concept that a cell membrane’s structure has a huge impact on its function (maintaining a nice even fluidity).
Cell membrane with only saturated phospholipids (rigid)
Cell membrane with some unsaturated phospholipids (fluid)
Author: MDougM. https://commons.wikimedia.org/wiki/File:Lipid_unsaturation_effect.svg
This is an important concept to consider if you are a cell that lives in very hot or very cold conditions. Archaebacteria are prokaryotic cells that inhabit some of Earth’s most extreme habitats. If a species of archaebacteria was living in a hot spring, it would endure temperatures near the boiling point. Clearly, this high of temperature would make the cell membrane very loose and flexible. To counteract this increase in fluidity, the cell membrane would be composed of mostly saturated phospholipids (to help stiffen it up a bit).
On the contrary, there are some species of fish that live in icy marine waters. Such brutally cold freezing temperatures make the fish’s cell membranes too stiff; they have evolved to have a higher concentration of unsaturated phospholipids to counteract the stiffness and help increase flexibility.
Finally, there are some clever species of fungi and bacteria that can actively increase the percentage of unsaturated fatty acids in their phospholipid bilayer when there is a decrease in environmental temperature.
Membranes Are Also Composed of Proteins
Proteins play an important role in cells, and different types of cells have different types of membrane proteins. Generally speaking, a membrane protein can either span the phospholipid bilayer or sit on its surface. A transmembrane protein (also called an integral protein) extends through the bilayer, and is often used in cell transport. Some proteins—peripheral proteins—instead sit on the surface of the membrane and play roles in things other than transport. Here are three examples, all of which we will talk about in more detail!
Transport A transmembrane protein that extends across the phospholipid bilayer can act like a tunnel. It creates a hydrophilic channel for things to pass through that would otherwise be blocked (or slowed down) by the phospholipid bilayer. For example, aquaporin proteins are membrane channels specifically to help water pass. Each aquaporin channel allows 3 billion water molecules to pass through, single file, every second! Some proteins take a more active approach and change shape to forcibly shuttle substances from one side to another. A protein pump that uses ATP in order to actively transport materials is a perfect example.
Enzymes Membrane-associated enzymes can either span the entire membrane or reside on only one side (either inside the cell or outside the cell). Enzymes that are stuck onto the membrane can form a cooperative little group that catalyzes a series of reactions in a metabolic pathway.
Signal Transduction When a chemical signal has a message for a particular cell, the cell “hears” the message because there are membrane proteins (receptors) that have a binding site for that particular signal. The receptor protein changes shape (while still remaining in the membrane), and essentially moves the message into the cell. The entire process of signal transduction will be covered in Chapter 8 (Cell Communication).
Cells can be divided into two large categories: prokaryotic cells and eukaryotic cells. Prokaryotic cells (bacteria) were the first cell type to evolve on Earth 3.5 billion years ago. They are simpler in structure and smaller in size. Prokaryotic cells lack organelles, which are specialized internal structures that perform specific functions (we will learn a lot more about organelles a bit later). Though all cells must contain DNA, prokaryotic cells do not house their DNA within a membrane-bound nucleus. Instead, bacterial DNA is condensed within a specific region called a nucleoid. Prokaryotic cells must undergo metabolic processes and create energy, so they have many enzymes needed to “run” their cellular reactions. Enzymes are made of proteins, and proteins are made by the bacterial ribosomes floating around inside. Some bacteria are motile (meaning they can move) and do so by using whip-like structures called flagella or smaller, hair-like projections called cilia. And, of course, a bacterial cell is surrounded by a cell membrane, plus an extra outside layer called a cell wall.
Though prokaryotic cells are simple cells, that does not mean they are at a disadvantage. On the contrary, their simplicity allows them to quickly reproduce. Furthermore, quick reproduction can introduce genetic mistakes (mutations), which create variation in a population. If you combine those two qualities—a quickly growing population of cells, many of which have newly introduced genetic mutations—you create perfect conditions for evolution to occur! This has enabled bacteria to colonize an amazing array of different environments on Earth. Some prokaryotes called archaea (for “archaic,” because they are closely related to the first cells to evolve on Earth 3.5 billion years ago) prefer what we consider “extreme” environments, such as deep-sea hydrothermal vents (where temperatures can exceed the boiling point!), and the Dead Sea (where nothing else can grow because of the high levels of salt). Healthy soil is rife with bacteria, and a single teaspoon can contain anywhere from 100 million to one billion bacteria! Bacteria are key in the food industry, such as fermentation of yogurt and cheese, and most importantly, bacteria keep us healthy. Bacteria provide us with antibiotics, and by colonizing our skin and digestive tract, good bacteria fill the ecological niches that would otherwise be taken over by pathogenic (disease-causing) microbes.
One major downside to their super-fast evolution, however, is their penchant to become resistant to the antibiotics that are supposed to stop their growth. You have most likely heard about the evolution of antibiotic-resistant bacteria. This occurs because a random mutation will enable a bacterial cell to survive in the presence of an antibiotic chemical that would otherwise kill it; this lone, lucky survivor quickly reproduces and creates offspring, all with that same, lucky, antibiotic-busting mutation. You can read more about the process of evolution in Chapter 15.
If prokaryotic cells are the small and simple types, eukaryotic cells are their larger and more complex cousins. The animals, fungi, protists, and plants are all composed of eukaryotic cells, and we’ll learn a ton more about eukaryotic cell structure in the next chapter.
Though it may seem straightforward to use the presence of cells as the defining characteristic of “life,” there is more to it. A living organism must also be able to reproduce and create progeny, and it must have a heritable genetic code (DNA). A living organism uses materials and undergoes metabolic processes, and it requires energy to survive. A living entity strives to maintain constant internal conditions (referred to as homeostasis) and responds to its environment. And finally, all living things evolve. To be alive is not only to be made of cells; “life” is an entire process.
What, then, about viruses? These little guys are composed of only nucleic acid (sometimes DNA, sometimes RNA) surrounded by a protein coat. There is no self-derived phospholipid bilayer, no cytoplasm, no organelles. Yet a virus appears to be an exception to the rule that to be considered alive, you must be composed of at least one cell. If you have ever come down with a bad cold, the seasonal flu, or something more exotic such as measles, you have definitely felt the wrath of those tiny little viral particles populating your body. A rhinovirus (the common cold) is inhaled, invades specific cells lining the respiratory tract, and once inside the cell, replicate themselves to such high numbers the cell essentially explodes. The newly released viruses then continue on commandeering adjacent cells until the immune system catches wind of the assault and shuts down the invasion. The virus does, indeed, reproduce and create more progeny viruses, but it cannot do so without first invading and taking over a cell—that important building block of all life.
1. The main structural component of the cell membrane is the [name the molecule].
2. How does archaea’s preference to live in extreme environments relate to its ancestry?
3. Label the following diagram (A—E) of a cell membrane with the following terms: hydrophilic phosphate group, phospholipid bilayer, protein, phospholipid, hydrophobic tails.
4. Why would transmembrane proteins (also called integral proteins) be useful in transport of things into and out of the cell?
5. Some cells have the ability to change the percentage of saturated versus unsaturated phospholipids in their cell membrane. Assume there is a bacterium whose membrane was composed of 50% saturated and 50% unsaturated phospholipids. If this cell suddenly was exposed to much colder temperatures, what might happen to the percentage composition of its cell membrane in order to try and maintain a “normal” fluidity?
6. Which cell type is smaller and lacks organelles?
7. Life is defined by a number of characteristics: list the eight characteristics of life covered in the chapter.
8. Choose the right term from each of the following pairs: A single phospholipid is amphipathic, meaning the phosphate head group portion of the molecule is polar/nonpolar and loves/hates water, whereas the two fatty acid chains are polar/nonpolar and loves/hates water.
9. Choose the correct term: If a cell lived in very hot conditions (such as a volcanic deep-sea vent), its phospholipids would be composed of more unsaturated/saturated fatty acids in order to prevent the cell membrane from becoming too fluid.
10. List three possible roles for cell membrane proteins, and provide a brief description of each.
11. For each of the following, indicate whether it is found in a bacterial cell. Write Y if it is found in prokaryotes, and N if it is not found in prokaryotes:
a. Cell membrane
e. Cell wall
g. Mitochondrion (an organelle)
12. What is an advantage to prokaryotic cells’ simplicity?
13. For the following multiple-choice question, choose the best answer. Which of the following is the smallest thing that could be considered alive?
a. An atom
b. A molecule
c. A virus
d. An amoeba
e. A tree
14. How is a cell able to maintain internal conditions different from its surroundings?