The functional unit of life is the cell. Every organism features at least one cell, and metabolism (the chemistry of life) occurs within cells. A cell is separated from its surroundings by a membrane.
Every cell features a cell membrane, which is also called the plasma membrane. The plasma membrane is a complex arrangement of several different types of molecules. The chief components are phospholipids. Each phospholipid molecule is an amphipathic molecule (polar at one end and non-polar at the other end). This explains why plasma membranes form. In the presence of water, phospholipids self-assemble into a bilayer, with the non-polar tails in each monolayer pointing toward the non-polar tails of the other monolayer, and the polar heads of each monolayer pointing toward the watery solution on its side of the membrane (the water interior of the cell for one monolayer, and the water exterior of the cell for the other monolayer). In addition to the phospholipid bilayer, a plasma membrane features various other macromolecules, including proteins, sterols, and polysaccharides. The plasma membrane is fundamental to life, so be sure to review its structure (and the structure of an individual phospholipid) by reading the beginning of this chapter.
A solution is a mixture consisting of a solvent and some number of solutes. The solvent is the part of the solution that dissolves the solutes, and the solutes are the parts that are dissolved by the solvent. An aqueous solution is one in which water is the solvent. A cell's plasma membrane is a barrier between intracellular fluid and extracellular fluid, both of which are aqueous solutions. The plasma membrane is selectively permeable, because certain particles pass through the membrane easily and other particles are blocked from passing through. Many solutes are effectively (though not perfectly) prevented from passing through the membrane, so we say that the membrane is impermeable to those solutes. Water, on the other hand, can pass through to a certain degree.
The mechanism by which water passes through a plasma membrane is called osmosis, which is a special case of diffusion (and it is therefore a passive process). The direction and rate of osmosis depends on the relative solute concentrations inside and outside the cell. Water always osmoses to where it is less watery (which means it always moves to the compartment that has a higher solute concentration). If the solute concentration of the extracellular fluid is higher than the solute concentration of the intracellular fluid, that means the extracellular fluid is less watery, so water will leave the cell by osmosis, and the cell volume will decrease. If the reverse is true (the gradient is reversed), then water will enter the cell by osmosis, and cell volume will increase. In each case, notice that water is moving toward the less watery compartment. Changes in osmotic gradients can have profoundly damaging effects on a cell, which is why organisms must regulate the osmotic conditions.
Watch this video to review your understanding of solutions and osmosis.
Although all cells share certain characteristics (for example, every cell has a plasma membrane), biologists recognize two fundamentally different categories of cells: prokaryotic and eukaryotic.
A prokaryotic cell is one that does not feature membrane-bounded organelles, whereas a eukaryotic cell is one that does feature membrane-bounded organelles. A membrane-bounded organelle is an organelle (tiny organ-like structure within a cell) that is enclosed by its own membrane, separate from the plasma membrane that encloses the entire cell.
Membrane-bounded organelles include such diverse structures as the nucleus, endoplasmic reticulum, lysosomes, mitochondria, chloroplasts, and others. Only eukaryotic cells feature these membrane bounded organelles, though a eukaryotic cell might feature only some (but not all) of them. For example, an animal cell (like one in a human body) features most of the membrane-bounded organelles, but it does not feature chloroplasts. A plant cell, on the other hand, typically includes the membrane-bounded organelles found in an animal cell, plus it also features chloroplasts. A bacterium, which is a prokaryotic cell, does not feature any of these membrane-bounded organelles. Ensure that you appreciate the differences between these major categories of cell types.
There are several organelles that you should recognize in this course:
Notice that most of these organelles are membrane-bounded, and they therefore appear only in eukaryotic cells, which are structurally more complex than prokaryotic cells from which they evolved. Review the structures of these important organelles by reading this section. Pay particular attention to Figure 1.
One difference between the various organelles is their shapes. However, the primary reason for classifying them as different types of organelles is that they perform different sets of functions, just as different organs in the body perform different sets of functions.
Those organelles that are membrane-bounded form sub-compartments, so their functions can be performed in isolation from the rest of the cellular contents. Before proceeding, be sure to refresh your memory of which functions are performed by which organelles. This video will help.
Signal molecules are examples of ligands, because they must bind to other molecules. Those molecules to which signals bind are called receptors. When a signal binds to a receptor, that binding causes changes in the cell. Those changes are the responses to the signal. Some signals are small and non-polar, so they are easily able to pass through a cell’s plasma membrane, and they therefore bind to internal receptors. Most signals, however, are too large or too polar to pass through the plasma membrane, so they must bind to receptors on the exterior surface of the cell. Though these signals don’t actually enter the cell, they still cause changes inside the cell. There are three primary mechanisms by which this takes place, and the difference lies in what kind of receptor receives these signals.
Be sure you understand the functional differences between these three classes of receptors, even though all three operate by binding to a signal molecule at the exterior surface. It helps to look at diagrams to make sense of the differences, so review the text and figures under the Types of Receptors section of this chapter.
Particles pass through biological membranes (including the plasma membrane) by various mechanisms, all of which can be lumped into two primary categories.
Transmembrane transport (that is, transport of a particle through a biological membrane) can be classified as either active or passive. The distinction between the two is the requirement for an external source of energy.
There are important subcategories of passive transport:
These transmembrane transport process are fundamental to life, because organisms must continuously exchange materials with their surroundings to stay alive. Review the categories and subcategories by watching this video and reading these subsections of Chapter 4.
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.