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BIOL1500

Assignment 4 Chapter 6: A Tour of the Cell

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 size to its real size. Resolution is a measure of the clarity of the image; it is the minimum distance two points can be separated and still be distinguished as two points.

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? The methods used to prepare the specimen kill the cells.

3.

Study the electron micrographs in your text. Describe the different types of images obtained from: scanning electron microscopy (SEM): 3-D component of the specimen image. transmission electron microscopy (TEM): This type of microscopy profiles a thin section of a specimen, resulting in various views of the cells prepared. 4.

Which two domains consist of prokaryotic cells? ___Bacteria and Archaea_______

5. On the figure of a prokaryotic cell, label each of these features and give its function or description. cell wall: Rigid outside the membrane plasma Membrane cytoplasm

structure plasma

membrane: enclosing the

bacterial chromosome: Carries genes in the form of DNA -1-

nucleoid: Region where the cell's DNA is located (not enclosed by a membrane) ribosomes: Interior of cell flagella: Locomotion organelles of some bacteria

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. 6.

Why are cells so small? Explain the relationship of surface area to volume. Cells are small because a high surface-to-volume ratio facilitates the exchange of materials between a cell and its environment. As a cell (or any other object) increases in size, its volume grows proportionally more than its surface area. (Area is proportional to a linear dimension cubed.) Thus, a smaller object has a greater ratio of surface area to volume.

7.

Exchange of materials across the plasma membrane requires a high surface-to-volume ratio. How do the microvilli of intestinal cells facilitate this? Microvilli are long, thin projections from the cell surface, which increase surface area without an appreciable increase in volume. A sufficiently high ratio of surface area to volume is especially important in cells that exchange a lot of materials with their surroundings, such as intestinal cells.

Describe the structure of the nucleus, chromosomes, and ribosomes, and explain their functions. 8.

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 encloses the nucleus, separating its contents from the cytoplasm. The nuclear envelope is a double membrane, meaning that there are two lipid bilayers. The nuclear lamina, a netlike array of protein filaments, connects the layers of the nuclear envelope. 9.

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 -2-

chromosomes as a cell prepares to divide. 10.

When are the nucleoli visible? What organelles are assembled here? Nucleoli are visible in a nondividing nucleus in cells active. Within the nucleoli, proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes.

11.What is the function of ribosomes? What are their two components? Ribosomes are the cellular components that carry out protein synthesis. Their two components are a large subunit and a small subunit. 12.

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

Location

Product

Free ribosomes

Suspended in cytosol

Produce protein that function within cytosol

Bound ribosomes

Attached to outside of endoplasmic reticulum of nuclear envelope

Produces proteins for insertion into membranes

Identify the parts of the endomembrane system and describe their roles in the cell. 13.

List all the structures of the endomembrane system.

Nuclear envelope Endoplasmic reticulum Golgi apparatus Lysosomes Peroxisomes Vesicles Vacuoles Plasma membrane

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14.

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 difference between smooth and rough ER.

The ER lumen is the cavity, or cisternal space. 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 travel to the Golgi apparatus and other destinations. Smooth ER is so named because its outer surface lacks ribosomes. Rough ER is studded with ribosomes on the outer surface of the membrane and thus appears rough through the electron microscope.

15.

List and describe three major functions of the smooth ER.

Its main functions are the synthesis of lipids, steroid hormones, the detoxification of harmful metabolic byproducts and the storage and metabolism of calcium ions within the cell. 16.

Why does alcohol abuse increase tolerance to other drugs such as barbiturates?

Barbiturates, alcohol, and many other drugs induce proliferation of smooth ER which are associated with detoxification enzymes, thus increasing rate of detoxification. The rough ER is studded with ribosomes.

17.

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?

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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.

18.

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. 19. 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

vesicland its contents after it arrives in the Golgi apparatus. The vesicle adds its membrane and contents to the cis face, it is enveloped by the golgi. the golgi then modifies the product as needed. Then a new vesicle is formed from the trans face and the vesicle leaves the golgi.

20.

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) macromolecules. The pH range inside a lysosome is acidic. 21.

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. The 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. Some of the human cells that carry out phagocytosis are macrophages, a type of white blood cell that helps defend the body by engulfing and destroying bacteria and other invaders. 22.

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

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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: 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. 23.

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.

Nuclear envelope is connected to rough ER, which is also continuous with smooth ER. 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.

Compare and contrast the structures and functions of mitochondria and chloroplasts. 24.

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.

25.

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. -6-

26.

Now sketch a chloroplast and label its outer membrane, inner membrane, inner membrane space, thylakoids, granum, and stroma. Notice that the mitochondrion has two membrane compartments, whereas the chloroplast has three compartments.

27.

What is the function of the mitochondria?

Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to generate ATP by extracting energy from sugars, fats, and other fuels. 28.

What is the function of the chloroplasts?

Chloroplasts are sites of photosynthesis. These organelles convert solar energy to chemical energy by absorbing sunlight and using it to drive synthesis of organic compounds such as sugars from carbon dioxide and water. 29.

Explain the important roles played by peroxisomes.

Peroxisomes contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen, thus producing hydrogen peroxide as a by-product. The H2O2 degradants quickly to oxygen and water. 30.

Use peroxisomes as an example to explain the value of compartmentalization in cells.

The cell's compartments provide different local environments that facilitate specific metabolic functions, incompatible processes can go on simultaneously inside the same cell. Different cell organelles perform different functions, many of which require specialized components for specific targets. Compartmentalization creates appropriate microenvironments for these diverse processes, allows damage limitation, minimizes non-specific interactions and consequently increased cellular efficiency.

Describe the subunits and structures of the three types of cytoskeletal fibers and list their functions. -7-

31.

What is cytoskeleton? There are three main types of fibers that make up the cytoskeleton. Name them The cytoskeleton is a network of fibers extending throughout the cytoplasm. Microtubules, Microfilaments, Intermediate Filaments

32.

What are three functions of the cytoskeleton? 1. Maintenance of cell shape 2. Mechanical support 3. Cell motility (movement) both of the cell as a whole and more limited movement of parts of the cell

33.

Microtubules are hollow rods made of a globular protein called tubulin. Each tubulin protein is a dimer made of two subunits. These are easily assembled and disassembled. Describe several functions of microtubules. 1. Maintenance of cell shape 2. Cell motility 3. Chromosome movement in cell division 4. Organelle movement 5. Involved in embryonic development. 6. Vesicle (such as glycoprotein) movement.

34.

Animal cells have a centrosome that contains a pair of centrioles. Plant cells do not have centrioles. What is believed to be the role of centrioles? Another name for centrosome is "microtubule-organizing center." The centrioles function as compression-resisting girders (beams) of the cytoskeleton.

35.

Compare and contrast cilia and flagella. For both, select a human cell that has this feature, and describe the role for that cell.

Cilia and flagella are both microtubule-containing extensions that project from some cells. Cilia and flagella share a common structure, each having a group of microtubules sheathed in an extension of the plasma membrane. Flagella and cilia differ in their beating patterns. A flagellum has an undulating motion that generates force in the same direction as the flagellum's axis, like the tail of a fish. In contrast, cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium's axis. 36.

How do motor proteins called dyneins cause movement of cilia? What is the role of ATP in this movement?

Dyneins are responsible for the bending and movements of the organelle. A dynein molecule performs a complex cycle of movements caused by changes in the shape of the protein, with ATP providing the energy for these changes. 37.

Microfilaments are solid, and they are built from a double chain of actin. Study Figure 6.26 in your text and explain three examples of movements that involve microfilaments. -8-

1. Myosin motors in muscle cell contraction: The "walking" of myosin projections (the so-called heads) drives the parallel myosin and actin filaments past each other so that the actin filaments approach each other in the middle. This shortens the muscle cell. Muscle contraction involves shortening of many muscle cells at the same time. 2. Amoeboid movement: Interaction of actin filaments with myosin causes contraction of the cell, pulling the cell's trailing end forward. 3. Cytoplasmic streaming in plant cells: A layer of cytoplasm cycles around the cell, moving over a carpet of parallel actin filaments. Myosin motors attached to organelles in the fluid cytosol may drive the streaming by interacting with the actin. 38.

What are the motor proteins that move the microfilaments? Myosin

39.

Intermediate filaments are bigger than microfilaments but smaller than microtubules. They are more permanent fixtures of cells. Give two functions of intermediate filaments. 1. Maintenance of cell shape (tension-bearing elements) 2. Anchorage of nucleus and certain other organelles 3. Formation of nuclear lamina

Compare and contrast (1) the extracellular components of plant and animal cells and (2) the cell junctions of plant and animal cells. 40.

What are three functions of the cell wall? 1. Protects the plant cell 2. Maintains its shape 3. Prevents excessive uptake of water

41.

What is the composition of the cell wall?

Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose synthase and secreted to the extracellular space, where they become embedded in a matrix of other polysaccharides and proteins. 42.

What is the relatively thin and flexible wall secreted first by a plant cell? Primary cell wall

43.

What is the middle lamella? Where is it found? What material is it made of?

The middle lamella is a thin layer of sticky polysaccharides called pectin’s, located between the primary -9-

walls of adjacent cells. 44.

Animal cells do not have cell walls, but they do have an extracellular matrix (ECM). On this figure, label the elements indicated, and give the role of each. Also label the extracellular fluid and cytoplasm.

45.

What are the intercellular junctions between plant cells? What can pass through them?

Plasmodesmata are intercellular junctions between plant cells that enable the transportation of materials between cells. A tight junction is a watertight seal between two adjacent animal cells, which prevents materials from leaking out of cells. 46.

Animal cells do not have plasmodesmata. This figure shows the three types of intercellular junctions seen in animal cells. Label each type and summarize its role.

. Tight junctions: Proteins hold adjacent cell membranes tightly together, creating an impermeable seal across a layer of epithelial. Desmosomes: Reinforced by intermediate filaments and rivet cells into strong sheets Gap junctions: Are cytoplasmic connections that allow for the exchange of ions and small molecules between cells through protein-lined pores.

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