In this section, you will explore the following questions:
- How does the structure of the eukaryotic cell resemble as well as differ from the structure of the prokaryotic cell?
- What are structural differences between animal and plant cells?
- What are the functions of the major cell structures?
Connection for AP® Courses
Connection for AP® Courses
Eukaryotic cells possess many features that prokaryotic cells lack, including a nucleus with a double membrane that encloses DNA. In addition, eukaryotic cells tend to be larger and have a variety of membrane-bound organelles that perform specific, compartmentalized functions. Evidence supports the hypothesis that eukaryotic cells likely evolved from prokaryotic ancestors; for example, mitochondria and chloroplasts feature characteristics of independently-living prokaryotes. Eukaryotic cells come in all shapes, sizes, and types; (for example, animal cells, plant cells, and different types of cells in the body.) Hint—This a rare instance where you should create a list of organelles and their respective functions because later you will focus on how various organelles work together, similar to how your body’s organs work together to keep you healthy. Like prokaryotes, all eukaryotic cells have a plasma membrane, cytoplasm, ribosomes, and DNA. Many organelles are bound by membranes composed of phospholipid bilayers embedded with proteins to compartmentalize functions such as the storage of hydrolytic enzymes and the synthesis of proteins. The nucleus houses DNA, and the nucleolus within the nucleus is the site of ribosome assembly. Functional ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum where they perform protein synthesis. The Golgi apparatus receives, modifies, and packages small molecules like lipids and proteins for distribution. Mitochondria and chloroplasts participate in free energy capture and transfer through the processes of cellular respiration and photosynthesis, respectively. Peroxisomes oxidize fatty acids and amino acids, and they are equipped to break down hydrogen peroxide formed from these reactions without letting it into the cytoplasm where it can cause damage. Vesicles and vacuoles store substances, and in plant cells, the central vacuole stores pigments, salts, minerals, nutrients, proteins, and degradation enzymes, and helps maintain rigidity. In contrast, animal cells have centrosomes and lysosomes but lack cell walls.
Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 1, Big Idea 2, and Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.
|Big Idea 1||The process of evolution drives the diversity and unity of life.|
|Enduring Understanding 1.B||Organisms are linked by lines of descent from common ancestry.|
|Essential Knowledge||1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.|
|Science Practice||7.2 The student can connect concepts in and across domains to generalize or extrapolate in and/or across enduring understandings.|
|Learning Objective||1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms.|
|Big Idea 2||Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis.|
|Enduring Understanding 2.B||Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments.|
|Essential Knowledge||2.B.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.|
|Science Practice||6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.|
|Learning Objective||2.13 The student is able to explain how internal membranes and organelles contribute to cell functions.|
|Science Practice||1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.|
|Learning Objective||2.14 The student is able to use representations and models to describe differences in prokaryotic and eukaryotic cells.|
|Big Idea 4||Biological systems interact, and these systems and their interactions possess complex properties.|
|Enduring Understanding 4.A||Interactions within biological systems lead to complex properties.|
|Essential Knowledge||4.A.2 The structure and function of subcellular components, and their interactions, provide essential cellular processes.|
|Learning Objective||4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions.|
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:
- [APLO 1.15]
- [APLO 2.5]
- [APLO 2.25]
- [APLO 1.16]
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 4.8). 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. Because a eukaryotic cell’s nucleus is surrounded by a membrane, it is often said to have a true nucleus. 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. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.
If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?
- The structure of endoplasmic reticulum would not form.
- The function of lysosomes would be hindered, as hydrolases are formed by the nucleolus.
- The free ribosomes and the rough endoplasmic reticulum, which contains ribosomes, would not form.
- The Golgi apparatus will not be able to sort proteins properly.
The Plasma Membrane
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 4.9): A phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. 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.
The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus) (Figure 4.10). 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 4.8). 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.
Typically, the nucleus is the most prominent organelle in a cell (Figure 4.8). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail (Figure 4.11).
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 4.11). 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.
Chromatin and Chromosomes
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. Every eukaryotic species has a specific number of chromosomes in the nuclei of its body’s cells. 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 4.12); chromatin describes the material that makes up the chromosomes both when condensed and decondensed.
We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.
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 4.8). 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 4.13). Ribosomes receive their orders for protein synthesis from the nucleus, where the DNA is transcribed into messenger RNA (mRNA). 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.
Mitochondria (singular = mitochondrion) are often called the powerhouses or energy factories of a cell because they are responsible for making adenosine triphosphate (ATP)—the cell’s main energy-carrying molecule. 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 4.14) 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.
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. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.
Vesicles and Vacuoles
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: 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.
Animal Cells Versus Plant Cells
Animal Cells Versus Plant Cells
At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and, in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not.
The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles: two structures that lie perpendicular to each other (Figure 4.15). Each centriole is a cylinder of nine triplets of microtubules.
The centrosome—the organelle where all microtubules originate—replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division is not clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.
Animal cells have another set of organelles not found in plant cells: lysosomes. The lysosomes are the cell’s garbage disposal. In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.
The Cell Wall
If you examine Figure 4.8b, the diagram of a plant cell, you will see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (Figure 4.16)—a polysaccharide made up of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That is because you are tearing the rigid cell walls of the celery cells with your teeth.
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals: Plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.
Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 4.17). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.
The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.
We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.
Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = within) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.
Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.
Based on what you know about plant and animals cells, which of the following events is most likely to have occurred?
- A host cell that ingested aerobic bacteria gave rise to modern animals, while an ancestor of that cell that also ingested photoautotrophic bacteria gave rise to modern plants.
- A host cell that gave rise to modern plants ingested photoautotrophic bacteria only, while a host cell that gave rise to modern animals ingested aerobic bacteria only.
- A host cell that gave rise to modern plants ingested both aerobic and photoautotrophic bacteria, while a host cell that gave rise to modern animals ingested photoautotrophic bacteria only.
- A host cell that gave rise to modern plants and animals ingested both aerobic and photoautotrophic bacteria.
The Central Vacuole
Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 4.8b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That is because, as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.
The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.
Science Practice Connection for AP® Courses
- Construct a concept map or Venn diagram to describe the relationships that exist among the three domains of life (Archaea, Bacteria, and Eukarya) based on cellular features. Share your diagram with other students in the class for review and revision.
- Mystery Cell ID. Using a microscope, identify several types of cells, e.g., prokaryote/eukaryote, plant/animal, based on general features and justify your identification.
- Ten-Minute Debate. Working in small teams, create a visual representation to support the claim that eukaryotes evolved from symbiotic relationships among groups of prokaryotes.
Think About It
- If the nucleolus were not able to carry out its function, what other cellular organelles would be affected? Would a human liver cell that lacked endoplasmic reticulum be able to metabolize toxins?
- Antibiotics are medicines that are used to fight bacterial infections. These medicines kill prokaryotic cells without harming human cells. What part(s) of the bacterial cell do antibiotics target? Provide reasoning for your answer.