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Chapter 6: A Tour of the Cell

Chapter 6: A Tour of the Cell 6.1 6.2

Identify how common techniques of microscopy and biochemistry are used in cell biology. Compare and contrast (1) prokaryotic and eukaryotic cells and (2) animal and plant cells.

6.3

Describe the structure of the nucleus, chromosomes, and ribosomes, and explain their functions. Identify the parts of the endomembrane system and describe their roles in the cell.

6.4 6.5 6.6 6.7 6.8

Compare and contrast the structures and functions of mitochondria and chloroplasts. Describe the subunits and structures of the three types of cytoskeletal fibers and list their functions. Compare and contrast (1) the extracellular components of plant and animal cells and (2) the cell junctions of plant and animal cells. Explain how the parts of a cell work together to enhance cellular function.

In high school biology, you probably learned all the cell parts and what they do. In this course, change your focus to understanding the importance of specialized organelles, how they function in normal cells, and what may occur if their function is disrupted. Study Tip: Research clearly shows that when you make sketches, you remember details better. Follow the Study Tip suggestion, using an entire sheet of paper for an animal cell and another for a plant cell. Add structures and notes as you progress through each concept. The diagram of the plant cell on the opening page of this chapter gives a visual overview of how plant cells are organized. Note that plant cells have a large central vacuole and may contain both chloroplasts and mitochondria. Draw a plant cell labeled with the structures mentioned in Figure 6.1 in the space below.

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Chapter 6: A Tour of the Cell Concept 6.1 Biologists use microscopes and biochemistry to study cells LO 6.1: Identify how common techniques of microscopy and biochemistry are used in cell biology. 1.

The study of cells has been limited by their small size, and so they were not seen and described until 1665, when Robert Hooke first looked at dead cells from the bark of an oak tree. His contemporary, Antoni van Leeuwenhoek, crafted lenses and opened a new world with the improvements in optical aids. Magnification and resolving power limit what can be seen. Explain the difference. Magnification is the ratio of an object's image to the real size. Resolving power is the minimum distance two points can be separated and still be distinguished as two separate points (AKA clarity measurement).

2.

The development of electron microscopes has further opened our window on the cell and its organelles. What is considered a major disadvantage of the electron microscope? Electron microscopes can not view live cells because the method of preparation is lethal.

3.

Study the electron micrographs in your text. Describe the different types of images obtained from: scanning electron microscopy (SEM) Useful for studying topography of a specimen; The results are on a screen that appears 3-D; sample is coated with gold. transmission electron microscopy (TEM) Used to study internal structure of cells; Image displays pattern of transmitted electrons.

4.

In cell fractionation, whole cells are broken up in a blender, and this slurry is centrifuged several times. Each time, smaller and smaller cell parts are isolated. This will isolate different organelles and allow study of their biochemical activities. Which organelles are the smallest ones isolated in this procedure? Ribosomes.

Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions LO 6.2: Compare and contrast (1) prokaryotic and eukaryotic cells and (2) animal and plant cells. 5.

Which two domains consist of prokaryotic cells? Bacteria and Archaea

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Chapter 6: A Tour of the Cell 6.

7.

A major difference between prokaryotic and eukaryotic cells is the location of their DNA. Describe this difference. The DNA of eukaryotes is in a bound nucleus. The DNA present in prokaryotes is in an unbound region called the nucleoid. On the figure of a prokaryotic cell, label each of these features and give its function or description. cell wall

a rigid layer of polysaccharides lying outside the plasma membrane of the cells of plants, fungi, and bacteria. In the algae and higher plants it consists mainly of cellulose.

plasma membrane

Plasma membrane

a microscopic membrane of lipids and proteins which forms the external boundary of the cytoplasm of a cell or encloses a vacuole, and regulates the passage of molecules in and out of the cytoplasm.

ribosome

bacterial chromosome

Bacterial chromosomes and plasmids are double-stranded circular DNA molecules and are often referred to as covalently closed cell wall circular DNA, or cccDNA.

nucleoid

an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material.

flagella

bacterial chromosome nucleoid

ribosomes

a minute particle consisting of RNA and associated proteins found in large numbers in the cytoplasm of living cells. They bind messenger RNA and transfer RNA to synthesize polypeptides and proteins.

flagella a slender threadlike structure, especially a microscopic appendage that enables many protozoa, bacteria, spermatozoa, etc. to swim.

Surface area-to-volume ratio is an important concept as it determines the ability of a cell to exchange materials with the environment. It is important that you be able to both calculate this ratio and also understand its significance. 8.

Why are cells so small? Explain the relationship of surface area to volume. Metabolic requirements of the cell sets upper limit on the size of the cell. The surface of the cell is where all of the nutrients/gas/signal exchange happens between the cell and extracellular environment. So the bigger the surface area, t better. However, the surface area of a cell decreases as it increases in volume. So, cells stay the small size they are to retain the greatest surface area possible in relation to their volume.

9.

Exchange of materials across the plasma membrane requires a high surface-to-volume ratio. How do the microvilli of intestinal cells facilitate this? They increase the surface area without increasing the volume.

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Chapter 6: A Tour of the Cell 10.

Imagine an elongated cell (such as a nerve cell) that measures 125 × 1 × 1 arbitrary units (cell A). Predict how the surface-to-volume ratio would compare with a cell that is 5 × 5 × 5 (cell B) and then calculate the ratio for both cells. A small sketch will help you visualize this. (Calculations will be found at the end of this chapter.)

This cell would have the same volume as the cells in columns 2 and 3 but proportionally more surface area than that in column 2 and less than that in column 3. Thus, the surface-to-volume ratio should be greater than 1.2 but less than 6. To obtain the surface area, you would add the area of the six sides (the top, bottom, sides, and ends): 125 x 125 x 125 x 125 x 1 x 1 x 502. The surface-to-volume ratio equals 502 divided by a volume of 125, or 4.02. but the elongated cell A has a higher S-to-V ratio and therefore is better able to exchange materials with the environment.

11.

Spend some time with the calculations of surface area and volume on p. 98 and the Scientific Skills Exercise on p. 99 of Campbell Biology, 12e. The following two questions will ensure you understand the central concept. a. As the size of a cell increases, what happens to the surface area to volume ratio? As the cube size increases or the cell gets bigger, then the surface area to volume ratio - SA:V ratio decreases. When a cell grows, its volume increases at a greater rate than its surface area, therefore it's SA: V ratio decreases.

b. How would this affect traffic into and out of the cell? The surface area has decreased do the traffic will increase.

Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes LO 6.3: Describe the structure of the nucleus, chromosomes, and ribosomes, and explain their functions. 12.

Describe the nuclear envelope. How many layers does it consist of? What connects the layers? How do molecules such as mRNA pass through the envelope? The nuclear envelope surrounds the nucleus, separating it from the cytoplasm. The nuclear envelope has two lipid bilayers and the nuclear lamina connects the layers of nuclear envelope. Molecules such as mRNA pass through the nuclear envelope through nuclear pores.

13.

What are the nuclear lamina and nuclear matrix? What function do they perform?

Nuclear Lamina- a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. Nuclear matrix- a framework of fiber extending throughout the nuclear interior. Found within the nucleus are the chromosomes which are made of chromatin.

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Chapter 6: A Tour of the Cell 14.

Found within the nucleus are the chromosomes. They are made of chromatin. What are the two components of chromatin? When do the thin chromatin fibers condense to become distinct chromosomes? Chromatin is composed of proteins and DNA. Chromatin fibers condense to become distinct chromosomes as a cell prepares to divide.

15.

When are the nucleoli visible? What organelles are assembled here? - parts of the ribosome are put together here - not visible during cell division so visible in a nondividing nucleus

16.

What is the function of ribosomes? What are their two components? Ribosomes are the cellular components that carry out protein synthesis. Their two components, protein and RNA, are arranged as a large subunit atop a small subunit.

17.

Ribosomes in any type of organism are all the same, but we distinguish between two types of ribosomes based on where they are found and the destination of the protein product made. Complete this chart to demonstrate this concept. Type of Ribosome Free ribosomes

Bound ribosomes

Location suspended in the cytosol

attached to the outside of the nuclear envelope

Product Enzymes

packaging proteins within certain organelles

Concept 6.4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell LO 6.4: Identify the parts of the endomembrane system and describe their roles in the cell. 18.

List all the structures of the endomembrane system. (Use this list to update your Study Tip diagram) It includes the nuclear envelope, endoplasmic reticulum, golgi apparatus, lysosomes, various kinds of vacuoles, and plasma membrane.

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Chapter 6: A Tour of the Cell 19.

The endoplasmic reticulum (ER) makes up more than half the total membrane system in many eukaryotic cells. Use this sketch to label and explain the lumen, transport vesicles, and The ER lumen is the cavity, or cisternal spa the difference between smooth and rough ER.

Because the ER membrane is continuous within the nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen of the ER. Transport vesicles bud off from a region of the rough ER called transitional ER and trav to the Golgi apparatus and other destination Smooth ER is so named because its outer surface lacks ribosomes. Rough ER is studd with ribosomes on the outer surface of the membrane and thus appears rough through t electron microscope.

lumen

transport vesicles

20.

List and describe three major functions of the smooth ER. 1. Synthesis of lipids: Enzymes of the smooth ER are important in the synthesis of lipids, including oils, phospholipids and steroids. 2. Detoxification of drugs and poisons: Detoxification usually involves adding hydroxyl groups to drug molecules, mak them more soluble and easier to flush from the body. 3. Storage of calcium ions: In muscle cells, the smooth ER membrane pumps calcium ions from the cytosol into the ER lumen.

21.

Why does alcohol abuse increase tolerance to other drugs such as barbiturates? Barbiturates, alcohol, and many other drugs induce the proliferation of smooth ER and smooth ER is associated detoxification enzymes, thus increasing the rate of detoxification. This, in turn, increases the tolerance to drugs, meaning that higher doses are required to achieve a particular effect, such as sedation.

22.

The rough ER is studded with ribosomes. As proteins are synthesized, they are threaded into the lumen of the rough ER. Some of these proteins have carbohydrates attached to them in the ER to form glycoproteins. What does the ER then do with these secretory proteins? After secretory proteins are formed, the ER membrane keeps them separate from proteins that are produced by free ribosomes and that will remain in the cytosol. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER.

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Chapter 6: A Tour of the Cell 23.

What is another major function of the rough ER? The rough ER produces membrane proteins and phospholipids for the cell by adding them to its own membrane. The ER membrane expands, and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system.

24.

The transport vesicles formed from the rough ER fuse with the Golgi apparatus. Use this sketch to label the cisterna of the Golgi apparatus, and its cis and trans faces. Describe all that can happen to a transport vesicle and its contents after it arrives in the Golgi apparatus. cisterna The vesicle adds its membrane and contents to the cis face, it is enveloped by the Golgi. the Golgi th modifies the product as needed. The a new vesicle is formed from the tran face and the vesicle leaves the Golgi

Cis face

Trans Face

25.

What is a lysosome? What does it contain? What is the pH range inside a lysosome? A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest (hydrolyze) macromolecule The pH range inside a lysosome is acidic.

26.

One function of lysosomes is intracellular digestion of particles engulfed by phagocytosis. Describe this process of digestion. Which human cells carry out phagocytosis? Amoebas and many other protists eat by engulfing smaller organisms or food particles, a process called phagocytosis. T food vacuole formed in this way then fuses with a lysosome, whose enzymes digest the food. Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. Som of the human cells that carry out phagocytosis are macrophages, a type of white blood cell that helps defend the body b engulfing and destroying bacteria and other invaders.

27.

A second function of lysosomes is to recycle cellular components in a process called autophagy. Describe this process using Figure 6.13 in your text. During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane, and a lysosome fuses with the outer membrane of this vesicle. The lysosomal enzymes dismantle the enclosed material, and organic monomers are returned to the cytosol for reuse. With the help of the lysosomes, the cell community renews itse A human liver cell, for example, recycles half of its macromolecules each week.

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Chapter 6: A Tour of the Cell 28.

Explain what occurs in lysosomes to cause Tay-Sachs disease. In Tay-Sachs disease, a lipid-digesting enzyme is missing or inactive, and the brain becomes impaired by an accumulation of lipids in the cells. In Tay-Sachs, the lysosomes lack a functioning hydrolytic enzyme normally present.

29.

There are many types of vacuoles. Briefly describe each type of vacuole below. food vacuoles Are formed by phagocytosis.

contractile vacuoles Pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell.

central vacuoles in plants Develop by the coalescence (合并) of smaller vacuoles, contained in mature plant cells. Solution inside the central vacuole, called cell sap, is the plant cell's main repository of inorganic ions, including potassium and chloride. The cen vacuole plays a major role in the growth of plant cells, which enlarge as the vacuole absorbs water, enabling the cell to become larger with a minimal investment in new cytoplasm.

30.

Label and use this figure to explain how the elements of the endomembrane system function together to secrete a protein and to digest a cellular component.

Rough ER

Cis Face

Golgi Body

Transport Vesicle

Cisternae

Trans Face

Plasma Membrane

Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi apparatus. Golgi apparatus pinches off transport vesicles and other vesicles that give rise to lysosomes, other types of specialized vesicles, and vacuoles. Lysosome is available for fusion with another vesicle for digestion. Transport vesicle carries proteins to plasma membrane for secretion. Plasma membrane expands by fusion of vesicles; proteins are secreted from cell.

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Chapter 6: A Tour of the Cell Concept 6.5 Mitochondria and chloroplasts change energy from one form to another LO 6.5: Compare and contrast the structures and functions of mitochondria and chloroplasts. 31.

What is the endosymbiont theory? Summarize three lines of evidence that support the model of endosymbiosis.

The endosymbiont theory states that an early ancestor of eukaryotic cells engulfed an oxygen-using nonphotosynthetic prokaryotic cell, and over the course of evolution, the host cell and its endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts. Three lines of evidence that support the model of endosymbiosis: 1. Rather than being bound by a single membrane, like organelles of the endomembrane system, mitochondria and typical chloroplasts have two membranes surrounding them. 2. Like prokaryotes, mitochondria and chloroplasts contain ribosomes, as well as circular DNA molecules attached to their inner membranes. 3. Also consistent with their probable evolutionary origins as cells, mitochondria and chloroplasts are autonomous organelles that grow and reproduce within cells

32.

Mitochondria and chloroplasts are not considered part of the endomembrane system, although they are enclosed by membranes. Sketch a mitochondrion here and label its outer membrane, inner membrane, inner membrane space, cristae, matrix, and ribosomes.

33.

Now sketch a chloroplast and label its outer membrane, inner membrane, inner membr...