Which Of The Following Is Not One Of The Three Major Components Of Animal Cells?

Learning Objectives

By the end of this section, you will be able to:

Describe the construction of eukaryotic cells
Compare animal cells with plant cells
Land the part of the plasma membrane
Summarize the functions of the major cell organelles

Have you e'er heard the phrase "course follows part?" Information technology'southward 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 case, a skyscraper should exist built with several elevator banks; a hospital should be built so that its emergency room is easily accessible.

Our natural earth too utilizes the principle of form following role, peculiarly in cell biological science, and this volition become clear every bit nosotros explore eukaryotic cells ( Figure four.8). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-spring nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and iii) several, rod-shaped chromosomes. Because a eukaryotic jail cell's nucleus is surrounded past a membrane, information technology is frequently said to accept a "true nucleus." The word "organelle" means "little organ," and, every bit already mentioned, organelles take specialized cellular functions, just as the organs of your body take specialized functions.

At this signal, 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 jail cell. Before turning to organelles, allow'southward get-go examine two important components of the prison cell: the plasma membrane and the cytoplasm.

Art Connection

Part a: This illustration shows a typical eukaryotic animal cell, which is egg shaped. The fluid inside the cell is called the cytoplasm, and the cell is surrounded by a cell membrane. The nucleus takes up about one-half the width of the cell. Inside the nucleus is the chromatin, which is composed of DNA and associated proteins. A region of the chromatin is condensed into the nucleolus, a structure where ribosomes are synthesized. The nucleus is encased in a nuclear envelope, which is perforated by protein-lined pores that allow entry of material into the nucleus. The nucleus is surrounded by the rough and smooth endoplasmic reticulum, or ER. The smooth ER is the site of lipid synthesis. The rough ER has embedded ribosomes that give it a bumpy appearance. It synthesizes membrane and secretory proteins. In addition to the ER, many other organelles float inside the cytoplasm. These include the Golgi apparatus, which modifies proteins and lipids synthesized in the ER. The Golgi apparatus is made of layers of flat membranes. Mitochondria, which produce food for the cell, have an outer membrane and a highly folded inner membrane. Other, smaller organelles include peroxisomes that metabolize waste, lysosomes that digest food, and vacuoles. Ribosomes, responsible for protein synthesis, also float freely in the cytoplasm and are depicted as small dots. The last cellular component shown is the cytoskeleton, which has four different types of components: microfilaments, intermediate filaments, microtubules, and centrosomes. Microfilaments are fibrous proteins that line the cell membrane and make up the cellular cortex. Intermediate filaments are fibrous proteins that hold organelles in place. Microtubules form the mitotic spindle and maintain cell shape. Centrosomes are made of two tubular structures at right angles to one another. They form the microtubule-organizing center.

Part b: This illustration depicts a typical eukaryotic plant cell. The nucleus of a plant cell contains chromatin and a nucleolus, the same as an animal cell. Other structures that the plant cell has in common with the animal cell include rough and smooth endoplasmic reticulum, the Golgi apparatus, mitochondria, peroxisomes, and ribosomes. The fluid inside the plant cell is called the cytoplasm, just as it is in an animal cell. The plant cell has three of the four cytoskeletal components found in animal cells: microtubules, intermediate filaments, and microfilaments. Plant cells do not have centrosomes. Plant cells have four structures not found in animals cells: chloroplasts, plastids, a central vacuole, and a cell wall. Chloroplasts are responsible for photosynthesis; they have an outer membrane, an inner membrane, and stack of membranes inside the inner membrane. The central vacuole is a very large, fluid-filled structure that maintains pressure against the cell wall. Plastids store pigments. The cell wall is outside the cell membrane.

Effigy four.viii These figures evidence the major organelles and other cell components of (a) a typical brute cell and (b) a typical eukaryotic establish cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells. Plant cells do not take lysosomes or centrosomes.

If the nucleolus were not able to carry out its part, what other cellular organelles would be affected?

The Plasma Membrane

Like prokaryotes, eukaryotic cells take a plasma membrane ( Figure 4.9), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding surroundings. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing grouping. 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.

Figure 4.9 The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in information technology.

The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (atypical = 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 course post-obit function. People with celiac illness have an immune response to gluten, which is a protein constitute in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot blot nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free nutrition.

The left part of this figure is a transmission electron micrograph of microvilli, which appear as long, slender stalks extending from the plasma membrane. The right side illustrates cells containing microvilli. The microvilli cover the surface of the cell facing the interior of the small intestine.

Figure iv.10 Microvilli, shown here as they announced on cells lining the small intestine, increase the surface surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard)

The Cytoplasm

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-similar cytosol, the cytoskeleton, and various chemicals ( Figure iv.8). Even though the cytoplasm consists of lxx to lxxx pct water, information technology has a semi-solid consistency, which comes from the proteins inside it. However, proteins are not the only organic molecules constitute 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.

The Nucleus

Typically, the nucleus is the about prominent organelle in a cell ( Figure 4.8). The nucleus (plural = nuclei) houses the jail cell'southward DNA and directs the synthesis of ribosomes and proteins. Let's look at it in more particular ( Figure 4.11).

Figure four.11 The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. Information technology consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.

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 within 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 upwards of DNA, the hereditary fabric. You may think 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 viii. Chromosomes are just visible and distinguishable from one another when the cell is getting ready to split. When the cell is in the growth and maintenance phases of its life wheel, proteins are fastened to chromosomes, and they resemble an unwound, jumbled agglomeration of threads. These unwound protein-chromosome complexes are called chromatin ( Figure 4.12); chromatin describes the material that makes upwards the chromosomes both when condensed and decondensed.

Part a: In this illustration, DNA tightly coiled into two thick cylinders is shown in the upper right. A close-up shows how the DNA is coiled around proteins called histones.

Part b: This image shows paired chromosomes.

Figure four.12 (a) This paradigm shows various levels of the organization of chromatin (Dna and protein). (b) This epitome shows paired chromosomes. (credit b: modification of work by NIH; calibration-bar information from Matt Russell)

The Nucleolus

We already know that the nucleus directs the synthesis of ribosomes, simply how does it exercise 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 so transported out through the pores in the nuclear envelope to the cytoplasm.

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes announced either equally clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may exist 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 u.s.a. that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called big and small ( Effigy 4.13). Ribosomes receive their "orders" for poly peptide synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which interpret the lawmaking provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a poly peptide. Amino acids are the building blocks of proteins.

The ribosome consists of a small subunit and a large subunit, which is about three times as big as the small one. The large subunit sits on top of the small one. A chain of mRNA threads between the large and small subunits. A protein chain extends from the top of the large subunit.

Figure 4.13 Ribosomes are fabricated upwards of a big subunit (top) and a small-scale subunit (bottom). During poly peptide synthesis, ribosomes get together amino acids into proteins.

Because proteins synthesis is an essential part 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 incorporate many ribosomes. Thus, nosotros see another example of form following office.

Mitochondria

Mitochondria (singular = mitochondrion) are often called the "powerhouses" or "energy factories" of a prison cell considering they are responsible for making adenosine triphosphate (ATP), the cell'due south primary energy-carrying molecule. ATP represents the short-term stored energy of the prison cell. Cellular respiration is the procedure of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this procedure uses oxygen and produces carbon dioxide equally a waste matter. 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 accept a very high concentration of mitochondria that produce ATP. Your musculus cells need a lot of free energy to keep your trunk moving. When your cells don't get plenty oxygen, they do not brand a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the product 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 chosen cristae. The surface area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

This transmission electron micrograph of a mitochondrion shows an oval outer membrane and an inner membrane with many folds called cristae. Inside the inner membrane is a space called the mitochondrial matrix.

Figure 4.fourteen This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton; scale-bar information from Matt Russell)

Peroxisomes

Peroxisomes are small, circular organelles enclosed by single membranes. They conduct out oxidation reactions that break down fat acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H₂O_two, which would be damaging to cells; withal, when these reactions are confined to peroxisomes, enzymes safely pause downwards the H_twoO₂ into oxygen and water.) For example, alcohol is detoxified past peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-jump sacs that part 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 prison cell. Additionally, some agents such as enzymes within institute vacuoles pause downward macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components.

Animal Cells versus Plant Cells

At this point, y'all know that each eukaryotic prison cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some hitting differences betwixt animal and plant cells. While both beast and plant cells take microtubule organizing centers (MTOCs), beast cells also take centrioles associated with the MTOC: a complex chosen the centrosome. Fauna cells each have a centrosome and lysosomes, whereas plant cells do non. Plant cells take a jail cell wall, chloroplasts and other specialized plastids, and a big central vacuole, whereas animal cells practice not.

The Centrosome

The centrosome is a microtubule-organizing center institute near the nuclei of brute cells. It contains a pair of centrioles, 2 structures that lie perpendicular to each other ( Figure 4.xv). Each centriole is a cylinder of ix triplets of microtubules.

Effigy 4.15 The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated past the light-green lines) hold the microtubule triplets together.

The centrosome (the organelle where all microtubules originate) replicates itself before a prison cell divides, and the centrioles appear to accept some role in pulling the duplicated chromosomes to contrary ends of the dividing prison cell. Notwithstanding, the exact part of the centrioles in cell segmentation isn't clear, because cells that have had the centrosome removed can even so carve up, and institute cells, which lack centrosomes, are capable of jail cell division.

Lysosomes

Fauna cells have another ready of organelles not found in plant cells: lysosomes. The lysosomes are the cell'due south "garbage disposal." In institute cells, the digestive processes take place in vacuoles. Enzymes inside the lysosomes assist the breakup of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are agile at a much lower pH than that of the cytoplasm. Therefore, the pH inside lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could non occur at a low pH, so once more, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

The Jail cell Wall

If you lot examine Figure 4.eightb, the diagram of a plant cell, you will see a construction 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 prison cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic jail cell walls is peptidoglycan, the major organic molecule in the found prison cell wall is cellulose ( Figure 4.16), a polysaccharide made upwards of glucose units. Have y'all e'er noticed that when you lot bite into a raw vegetable, like celery, it crunches? That's because you are tearing the rigid cell walls of the celery cells with your teeth.

This illustration shows three glucose subunits that are attached together. Dashed lines at each end indicate that many more subunits make up an entire cellulose fiber. Each glucose subunit is a closed ring composed of carbon, hydrogen, and oxygen atoms.

Figure 4.16 Cellulose is a long chain of β-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure signal a series of many more glucose units. The size of the folio makes information technology impossible to portray an entire cellulose molecule.

Chloroplasts

Like the mitochondria, chloroplasts have their ain DNA and ribosomes, but chloroplasts have an entirely different role. Chloroplasts are constitute cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to brand glucose and oxygen. This is a major difference between plants and animals; plants (autotrophs) are able to make their own nutrient, like sugars, while animals (heterotrophs) must ingest their food.

Similar mitochondria, chloroplasts accept outer and inner membranes, just inside the space enclosed by a chloroplast's inner membrane is a set of interconnected and stacked fluid-filled membrane sacs chosen 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.

This illustration shows a chloroplast, which has an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Inside the inner membrane are flat, pancake-like structures called thylakoids. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoids is called the thylakoid space.

Figure four.17 The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid within the inner membrane, which is chosen the stroma. Chloroplasts also take their own genome, which is independent on a unmarried circular chromosome.

The chloroplasts contain a green paint called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Similar constitute cells, photosynthetic protists likewise accept chloroplasts. Some bacteria perform photosynthesis, simply their chlorophyll is non relegated to an organelle.

Evolution Connectedness

Evolution Connection

EndosymbiosisWe have mentioned that both mitochondria and chloroplasts contain Deoxyribonucleic acid and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis every bit the explanation.

Symbiosis is a relationship in which organisms from two split up 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 accept already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is benign for us because nosotros are unable to synthesize vitamin Grand. Information technology is likewise beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the surround of the big intestine.

Scientists take long noticed that bacteria, mitochondria, and chloroplasts are similar in size. Nosotros also know that bacteria take 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 non 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.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you await at Figure iv.8b, you will see that plant cells each have a large fundamental vacuole that occupies about of the area of the cell. The central vacuole plays a primal role in regulating the cell'due south concentration of water in changing environmental conditions. Have you lot ever noticed that if you forget to water a plant for a few days, information technology wilts? That's considering as the h2o concentration in the soil becomes lower than the water concentration in the constitute, water moves out of the central vacuoles and cytoplasm. Every bit the primal vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of constitute cells results in the wilted appearance of the institute.

The central vacuole also supports the expansion of the cell. When the central vacuole holds more than water, the prison cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.

Source: https://openstax.org/books/biology/pages/4-3-eukaryotic-cells

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