A Learning Objective merges required content with one or more of the seven Science Practices. The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP exam.
These questions address the following standards:. Have you ever heard the phrase form follows function? It is a philosophy practiced in many industries. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should be built with several elevator banks; a hospital should be built so that its emergency room is easily accessible.
Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells Figure 4. Unlike prokaryotic cells, eukaryotic cells have: 1 a membrane-bound nucleus; 2 numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and 3 several rod-shaped chromosomes. The word organelle means little organ , and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions.
At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. If the nucleolus were not able to carry out its function, what other cellular organelles would be affected? Like prokaryotes, eukaryotic cells have a plasma membrane Figure 4.
A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell.
Wastes such as carbon dioxide and ammonia also leave the cell by passing through the plasma membrane. Such cells are typically found lining the small intestine—the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea.
Patients suffering from celiac disease must follow a gluten-free diet. The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope a structure to be discussed shortly. It is made up of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals Figure 4.
Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
Eukaryotic cells are larger than prokaryotic cells and have a “true” nucleus, Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and . Before discussing the functions of organelles within a eukaryotic cell, let us first . They are short, hair-like structures that are used to move entire cells (such as.
Typically, the nucleus is the most prominent organelle in a cell Figure 4. The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus Figure 4.
Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.
To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA—the hereditary material. You may remember that, in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide.
When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin Figure 4. We already know that the nucleus directs the synthesis of ribosomes, but how does it do this?
Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters polyribosomes or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope Figure 4.
Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small Figure 4. The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.
Because protein synthesis is an essential function of all cells including enzymes, hormones, antibodies, pigments, structural components, and surface receptors , ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes, and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.
ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found 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 you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.
In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP.
Your muscle cells need a lot of energy to keep your body moving. When your cells do not get enough oxygen, they do not make a lot of ATP.
Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid. Mitochondria are oval-shaped, double membrane organelles Figure 4. 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. 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, H 2 O 2 , which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H 2 O 2 into oxygen and water.
Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. 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: The membranes of vesicles 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 membrane of a vacuole does not fuse with the membranes of other cellular components.