Chapter 6: A Tour of the Cell

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

Microtubules are hollow rods made of a globular protein called tubulin. Each tubulin protein is a dimer made of two subunits. These are dynamically assembled and disassembled. What are four 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.

What are the three roles 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

What is a lysosome? What do they 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.

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.

Explain the important role 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 degradates quickly to oxygen and water.

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

Plasmodesmata are the intercellular junctions between plant cells. Cytosol passes through the plasmodesmata and joins the internal chemical environments of adjacent cells.

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

Primary cell wall

Contractile vacuoles

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

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.

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.

Food vacuoles

Are formed by phagocytosis.

Microfilaments are solid, and they are built from a double chain of actin. Study Figure 6.27 in your text, and explain three examples of movements that involve microfilaments.

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.

What are three functions of the cell wall?

1. Protects the plant cell 2. Maintains its shape 3. Prevents excessive uptake of water

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, making 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.

Scanning electron microscopy (SEM):

3-D component of the specimen image.

One function of lysosomes is intracellular digestion of particles engulfed by phagocytosis. Describe this process of digestion. What 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.

What is an endosymbiont?

An endosymbiont is a cell living within another cell.

Animal cells have a centrosome that contains a pair of centrioles. Plant cells do not have centrioles. What is another name for centrosomes? 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.

Recall the relationship of structure to function. Why is the inner membrane of the mitochondria highly folded? What role do all the individual thylakoid membranes serve? (Notice that you will have the same answer for both questions.)

As highly folded surfaces, the cristae give the inner mitochondrial membrane a large surface area, thus enhancing the productivity of cellular respiration. As in mitochondria, thylakoid membranes serve to increase the surface area and thus the function of the chloroplasts.

Bound ribosome

Attached to the outside of endoplasmic reticulum of nuclear envelope. Produce proteins for insertion into membranes.

Which two domains consist of prokaryotic cells?

Bacteria and Archaea

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.

Bacterial chromosome

Carries genes in the form of DNA

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.

Compare and contrast cilia and flagella.

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.

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 central 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.

A second function of lysosomes is to recycle cellular components in a process called autophagy. Describe this process.

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 the organic monomers are returned to the cytosol for reuse. With the help of the lysosomes, the cell community renews itself. A human liver cell, for example, recycles half of its macromolecules each week.

How do motor proteins called dyneins cause movement of cilia? What is the role of ATP in this movement? This figure might help you explain. See page of your text for the labeled figure.

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.

What happens in Tay-Sachs disease? Explain the role of the lysosomes in Tay-Sachs.

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.

A major difference between prokaryotic and eukaryotic cells is the location of their DNA. Describe this difference.

In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a double membrane. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane enclosed, called a nucleoid.

Cytoplasm

Interior of cell

Flagella

Locomotion organelles of some bacteria

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 an oak tree. His contemporary, Anton van Leeuwenhoek, crafted lenses and with the improvements in optical aids, a new world was opened. 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.

Plasma membrane

Membrane enclosing the cytoplasm

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.

There are three main types of fibers that make up the cytoskeleton. Name them.

Microtubules, Microfilaments, Intermediate Filaments

What are microvilli? How do these structures relate to the function of intestinal cells?

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.

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.

What are the motor proteins that move the microfilaments in muscle?

Myosin

List all the structures of the endomembrane system.

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

Use this figure to explain how the elements of the endomembrane system function together to secrete a protein and to digest a cellular component. Label as you explain. See page 108 in your text for the labeled figure.

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.

When are the nucleoli visible? What 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.

Nucleoid

Region where the cell's DNA is located (not enclosed by a membrane)

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

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.

Cell wall

Rigid structure outside the plasma membrane

Free ribosome

Suspended in cytosol. Produce protein that function within the cytosol.

The endoplasmic reticulum (ER) makes up more than half the total membrane system in many eukaryotic cells. Use this sketch to explain the lumen, transport vesicles, and the difference between smooth and rough ER. See page 104 of your text for the labeled figure.

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.

What is the cytoskeleton?

The cytoskeleton is a network of fibers extending throughout the cytoplasm.

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

The development of electron microscopes has further opened our window on the cell and its organelles. What is considered a major disadvantage of electron microscopes?

The methods used to prepare the specimen kill the cells.

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 pectins, located between the primary walls of adjacent cells.

Describe the nuclear envelope. How many layers is it? What connects the layers?

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.

What is the nuclear lamina? Nuclear matrix?

The nuclear lamina is the netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. The nuclear matrix is a framework of protein fibers extending throughout the nuclear interior. The nuclear matrix and nuclear lamina may help organize the genetic material so it functions efficiently.

Besides packaging secretory proteins into transport vesicles, 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.

Explain the deposition of a secondary cell wall.

The secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support.

Describe the organization of microtubules in a centriole. Make a sketch here that shows this arrangement in cross section. See page 114 of your text for the labeled figure.

The two centrioles are at right angles to each other, and each is made up of nine sets of three microtubules.

Transmission electron microscopy (TEM)

This type of microscopy profiles a thin section of a specimen, resulting in various views of the cells prepared.