General Bio 1 Ch. 4

Centrosomes are microtubule-organizing centers.

Centrosomes help assemble the nuclear division apparatus of animal cells (figure 4.20).

Chloroplasts use light to generate ATP and sugars.

Chloroplasts capture light energy via thylakoid membranes arranged in stacks called grana, and use it to synthesize glucose (figure 4.17).

Mitochondria and Chloroplasts: Cellular Generators

Mitochondria and chloroplasts have a double-membrane structure, contain their own DNA, and can divide independently.

Learning Outcomes Review 4.5

Mitochondria and chloroplasts have similar structures, with an outer membrane and an extensive inner membrane compartment. Both mitochondria and chloroplasts have their own DNA, but both also depend on nuclear genes for some functions. Mitochondria and chloroplasts are both involved in energy conversion: Mitochondria metabolize sugar to produce ATP, whereas chloroplasts harness light energy to produce ATP and synthesize sugars. Endosymbiosis theory proposes that both mitochondria and chloroplasts arose as prokaryotic cells were engulfed by a eukaryotic precursor.

The cytoskeleton helps move materials within cells.

Molecular motors move vesicles along microtubules, like a train on a railroad track. Kinesin and dynein are two motor proteins.

Microbodies are a diverse category of?

Organelles

Would you expect cells in different organs in complex animals to have the same structure?

Part of what gives different organs their unique identities are the specialized cell types found in each. That does not mean that there will not be some cell types common to all (epidermal cells for example) but organs tend to have specialized cell types.

Bacterial cell walls consist of peptidoglycan.

Peptidoglycan is composed of carbohydrate cross-linked with short peptides.

Plant cell walls provide protection and support.

Plants have cell walls composed of cellulose fibers. The middle lamella, between cell walls, holds adjacent cells together.

Would finding life on Mars change our view of cell theory?

The statement about all cells coming from preexisting cells might need to be modified. It would really depend on whether these Martian life-forms had a similar molecular/cellular basis as that of terrestrial life.

The smooth ER has multiple roles.

The smooth endoplasmic reticulum (SER) lacks ribosomes; it is involved in carbohydrate and lipid synthesis and detoxification.

Plants use vacuoles for?

storage and water balance.

Three types of fibers compose the cytoskeleton.

Actin filaments, or microfilaments, are long, thin polymers involved in cellular movement. Microtubules are hollow structures that move materials within a cell. Intermediate filaments serve a wide variety of functions.

All cells share many structural features.

All cells have centrally located DNA, a semifluid cytoplasm, and an enclosing plasma membrane.

Cell theory is the unifying foundation of cell biology.

All organisms are composed of one or more cells. Cells arise only by division of preexisting cells.

Learning Outcomes Review 4.1

All organisms are single cells or aggregates of cells, and all cells arise from preexisting cells. Cell size is limited primarily by the efficiency of diffusion across the plasma membrane. As a cell becomes larger, its volume increases more quickly than its surface area. Past a certain point, diffusion cannot support the cell's needs. All cells are bounded by a plasma membrane and filled with cytoplasm. The genetic material is found in the central portion of the cell; and in eukaryotic cells, it is contained in a membrane-bounded nucleus.

Archaea lack peptidoglycan.

Archaeal cell walls do not contain peptidoglycan, and they have unique plasma membranes.

What features do bacteria and archaea share?

Bacteria and archaea both tend to be single cells that lack a membrane-bounded nucleus, and lack extensive internal endomembrane systems. They both have a cell wall, although the composition is different. They do not undergo mitosis, although the proteins involved in DNA replication and cell division are not similar.

Mitochondria and chloroplasts both generate ATP. What structural features do they share?

Both mitochondria and chloroplasts have highly infolded inner membranes, where many of the reactions take place leading to the production of ATP. They also have an inner chamber with soluble material containing enzymes.

Some cells crawl.

Cell crawling occurs as actin polymerization forces the cell membrane forward, while myosin pulls the cell body forward.

How do cell junctions help to form tissues?

Cell junctions help to put together cells into higher level structures that are organized and joined in different ways. Different kinds of junctions can be used for different functional purposes.

Cell connections mediate cell-to-cell adhesion.

Cell junctions include tight junctions, adhesive junctions, and communicating junctions. In animals, gap junctions allow the passage of small molecules between cells. In plants, plasmodesmata penetrate the cell wall and connect cells.

Learning Outcomes Review 4.7

Cell movement involves proteins. These can either be internal in the case of crawling cells that use actin and myosin, or external in the case of cells powered by cilia or flagella. Eukaryotic cilia and flagella are different from prokaryotic flagella because they are composed of bundles of microtubules in a 9 + 2 array. They undulate rather than rotate. Plant cells have a cellulose-based cell wall. Animal cells lack a cell wall. In animal cells, the cytoskeleton is linked to a web of glycoproteins called the extracellular matrix.

Cell size is limited.

Cell size is constrained by the diffusion distance. As cell size increases, diffusion becomes inefficient.

The passageways of the human trachea (the path of air flow into and out of the lungs) are known to be lined with ciliated cells. What function could these cilia perform?

Ciliated cells in the trachea help to remove par-ticulate matter from the respiratory tract so that it can be expelled or swallowed and processed in the digestive tract.

Eukaryotic Cells (figures 4.6 and 4.7)

Eukaryotic cells have a membrane-bounded nucleus, an endomembrane system, and many different organelles.

Flagella and cilia aid movement.

Eukaryotic flagella have a 9 + 2 structure and arise from a basal body. Cilia are shorter and more numerous than flagella.

Surface proteins give cells identity.

Glycolipids and MHC proteins on cell surfaces help distinguish self from nonself.

Animal cells secrete an extracellular matrix.

Glycoproteins are the main component of the extracellular matrix (ECM) of animal cells.

Use the information provided in table 4.3 to develop a set of predictions about the properties of mitochondria and chloroplasts if these organelles were once free-living prokaryotic cells. How do your predictions match with the evidence for endosymbiosis?

If these organelles were free-living bacteria, they would have the features found in bacteria. Mitochondria and chloroplasts both have DNA but no nucleus, and they lack the complex organelles found in eukaryotes. At first glance, the cristae may seem to be an internal membrane system, but they are actually infoldings of the inner membrane. If endosymbiosis occurred, this would be the plasma membrane of the endosymbiont, and the outer membrane would be the plasma membrane of the engulfing cell. Another test would be to compare DNA in these organelles with current bacteria. This has actually shown similarities that make us confident of the identity of the endosymbionts.

Learning Outcomes Review 4.3

In contrast to prokaryotic cells, eukaryotic cells exhibit compartmentalization. Eukaryotic cells contain an endomembrane system and organelles that carry out specialized functions. The nucleus, composed of a double membrane connected to the endomembrane system, contains the cell's genetic information. Material moves between the nucleus and cytoplasm through nuclear pores. Ribosomes translate mRNA, which is transcribed from DNA in the nucleus, into polypeptides that make up proteins. Ribosomes are a universal organelle found in all known cells.

What advantage does the cytoskeleton give to large eukaryotic cells?

It provides structure and support for larger cells, especially in animal cells that lack a cell wall.

Lysosomes contain digestive enzymes.

Lysosomes break down macromolecules and recycle the components of old organelles (figure 4.13).

Microscopes allow visualization of cells and components.

Magnification gives better resolution than is possible with the naked eye. Staining with chemicals enhances contrast of structures.

What cellular roles are performed by microtubules and microfilaments and not intermediate filaments?

Microtubules and microfilaments are both involved in cell motility and in movement of substance around cells. Intermediate filaments do not have this dynamic role, but are more structural.

Learning Outcomes Review 4.2

Prokaryotes are small cells that lack complex interior organization. The two domains of prokaryotes are archaea and bacteria. The cell wall of bacteria is composed of peptidoglycan, which is not found in archaea. Archaea have cell walls made from a variety of polysaccharides and peptides, as well as membranes containing unusual lipids. Some bacteria move using a rotating flagellum.

Prokaryotic cells have relatively simple organization.

Prokaryotic cells contain DNA and ribosomes, but they lack a nucleus, an internal membrane system, and membrane-bounded organelles. A rigid cell wall surrounds the plasma membrane.

Some prokaryotes move by means of rotating flagella.

Prokaryotic flagella rotate because of proton transfer across the plasma membrane.

Ribosomes are the cell's protein synthesis machinery.

Ribosomes translate mRNA to produce polypeptides. They are found in all cell types.

What modifications would you include if you wanted to make a cell as large as possible?

Stretch, dent, convolute, fold, add more than one nucleus, anything which would increase the amount of diffusion between the cytoplasm and the external environment.

The Golgi apparatus sorts and packages proteins.

The Golgi apparatus receives vesicles from the ER, modifies and packages macromolecules, and transports them (figure 4.11).

The Cytoskeleton

The cytoskeleton consists of crisscrossed protein fibers that support the shape of the cell and anchor organelles (figure 4.19).

The Endomembrane System (figure 4.10).

The endoplasmic reticulum (ER) creates channels and passages within the cytoplasm.

Learning Outcomes Review 4.4

The endoplasmic reticulum (ER) is an extensive system of folded membranes that spatially organize the cell's biosynthetic activities. Smooth ER (SER) is the site of lipid and membrane synthesis and is used to store Ca2+. Rough ER (RER) is covered with ribosomes and is a site of protein synthesis. Proteins from the RER are transported by vesicles to the Golgi apparatus where they are modified, packaged, and distributed to their final location. Lysosomes are vesicles that contain digestive enzymes used to degrade materials such as invaders or worn-out components. Peroxisomes carry out oxidative metabolism that generates peroxides. Vacuoles are membrane-bounded structures with roles ranging from storage to cell growth in plants. They are also found in some fungi and protists.

Mitochondria and chloroplasts arose by endosymbiosis.

The endosymbiont theory proposes that mitochondria and chloroplasts were once prokaryotes engulfed by another cell.

Learning Outcomes Review 4.8

The evolution of multicellularity required the acquisition of cell adhesion molecules to connect cells together. Cell connections fall into three basic categories: (1) adhesive junctions provide strength and flexibility; (2) tight, or septate, junctions help to make sheets of cells that form watertight seals; and (3) communicating junctions, including gap junctions in animals and plasmodesmata in plants, allow passage of some materials between cells. Cells in multicellular organisms have distinct identity and connections. Cell identity is conferred by surface glycoproteins, which include the MHC proteins that are important in the immune system.

Mitochondria metabolize sugar to generate ATP.

The inner membrane of mitochondria is extensively folded into layers called cristae. Proteins on the surface and in the inner membrane carry out metabolism to produce ATP (figure 4.16).

Many proteins in mitochondria and chloroplasts are encoded by nuclear genes. In light of the endosymbiont hypothesis, how might this come about?

The nuclear genes that encode organellar proteins moved from the organelle to the nucleus. There is evidence for a lot of "horizontal gene transfer" across domains; this is an example of how that can occur.

The nucleus acts as the information center.

The nucleus is surrounded by an envelope of two phospholipid bilayers; the outer layer is contiguous with the ER. Pores allow exchange of small molecules. The nucleolus is a region of the nucleoplasm where rRNA is transcribed and ribosomes are assembled.In most prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, numerous chromosomes are present.

The protist Giardia intestinalis is the organism associated with water-borne diarrheal diseases. Giardia is an unusual eukaryote because it seems to lack mitochondria. Provide two possible evolutionary scenarios for this in the context of the endosymbiotic theory.

The origins of mitochondria and chloroplasts are hypothesized to be the result of a bit of cellular "indigestion" in which aerobic or photosyn-thetic prokaryotes were engulfed but not digested by the larger ancestor eukaryote. Given this information, there are two possible scenarios for the origin of Giardia. In the first scenario, the ancestor of Giardia split off from the eukaryotic lineage after the evolution of the nucleus but before the acquisition of mitochondria. In the second scenario, the ancestor of Giardia split off after the acquisition of mitochondria and subsequently lost the mitochondria. At present, neither of these two scenarios can be rejected. The first case was long thought to be the best explanation, but recently it has been challenged by evidence for the second case.

In evolutionary theory, homologous traits are those with a similar structure and function derived from a common ancestor. Analogous traits represent adaptations to a similar environment, but from distantly related organisms. Consider the structure and function of the flagella found on eukaryotic and prokaryotic cells. Are the flagella an example of a homologous or analogous trait? Defend your answer.

The prokaryotic and eukaryotic flagella are examples of an analogous trait. Both flagella function to propel the cell through its environment by converting chemical energy into mechanical force. The key difference is in the structure of the flagella. The bacterial flagellum is composed of a single protein emerging from a basal body anchored within the cell's plasma membrane and using the potential energy of a proton gradient to cause a rotary movement. In contrast, the flagellum of the eukaryote is composed of many different proteins assembled into a complex axoneme structure that uses ATP energy to cause an undulating motion.

The rough ER is a site of protein synthesis.

The rough ER (RER), studded with ribosomes, synthesizes and modifies proteins and manufactures membranes.

Learning Outcomes Review 4.6

The three principal fibers of the cytoskeleton are actin filaments (microfilaments), microtubules, and intermediate filaments. These fibers interact to modulate cell shape and permit cell movement. They also act to move materials within the cytoplasm. Material is also moved in large cells using vesicles and molecular motors. The motor proteins move vesicles along tracks of microtubules.

How do ribosomes on the RER differ from cytoplasmic ribosomes?

They don't!

The smooth endoplasmic reticulum is the site of synthesis of the phospholipids that make up all the membranes of a cell—especially the plasma membrane. Use the diagram of an animal cell (figure 4.6) to trace a pathway that would carry a phospholipid molecule from the SER to the plasma membrane. What endomembrane compartments would the phospholipids travel through? How can a phospholipid molecule move between membrane compartments?

Your diagram should start at the SER and then move to the RER, Golgi apparatus, and finally to the plasma membrane. Small transport vesicles are the mechanism that would carry a phospholipid molecule between two membrane compartments. Transport vesicles are small "membrane bubbles" composed of a phospholipid bilayer.

Which of the following are differences between bacteria and archaea? a. The molecular architecture of their cell walls b. The type of ribosomes found in each c. Archaea have an internal membrane system that bacteria lack. d. Both a and b are correct.

a. The molecular architecture of their cell walls

Chloroplasts and mitochondria have many common features because both: a. are present in plant cells. b. arose by endosymbiosis. c. function to oxidize glucose. d. function to produce glucose.

a. are present in plant cells.

Eukaryotic cells are composed of three types of cytoskeletal filaments. How are these three filaments similar? a. They contribute to the shape of the cell. b. They are all made of the same type of protein. c. They are all the same size and shape. d. They are all equally dynamic and flexible.

b. They are all made of the same type of protein.

All eukaryotic cells possess each of the following except: a. mitochondria. b. cell wall. c. cytoskeleton. d. nucleus.

b. cell wall.

Plasmodesmata in plants and gap junctions in animals are functionally similar in that: a. each is used to anchor layers of cells. b. they form channels between cells that allow diffusion of small molecules. c. they form tight junctions between cells.d. they are anchored to the extracellular matrix.

b. they form channels between cells that allow diffusion of small molecules.

Different motor proteins like kinesin and myosin are similar in that they can: a. interact with microtubules. b. use energy from ATP to produce movement. c. interact with actin. d. do both a and b.

b. use energy from ATP to produce movement.

Adherens junctions, which contain cadherin, are found in all animals. Given this, which of the following predictions is most likely? a. Cadherins would not be found in the ancestor to all animals. b. Cadherins would be found in prokaryotes. c. Cadherins would be found in the ancestor to all animals. d. Cadherins would be found in vertebrates but not invertebrates.

c. Cadherins would be found in the ancestor to all animals.

Eukaryotic cells are more complex than prokaryotic cells. Which of the following are found only in a eukaryotic cell? a. Cell wall b. Plasma membrane c. Endoplasmic reticulum d. Ribosomes

c. Endoplasmic reticulum

The protein sorting pathway involves the following organelles/compartments in order: a. SER, RER, transport vesicle, Golgi. b. RER, lysosome, Golgi. c. RER, transport vesicle, Golgi, final destination. d. Golgi, transport vesicle, RER, final destination. 6

c. RER, transport vesicle, Golgi, final destination.

The cytoskeleton includes: a. microtubules made of actin filaments. b. microfilaments made of tubulin. c. intermediate filaments made of twisted fibers of vimentin and keratin. d. smooth endoplasmic reticulum.

c. intermediate filaments made of twisted fibers of vimentin and keratin.

The most important factor that limits the size of a cell is the: a. quantity of proteins and organelles a cell can make. b. rate of diffusion of small molecules. c. surface area-to-volume ratio of the cell. d. amount of DNA in the cell.

c. surface area-to-volume ratio of the cell.

Which of the following statements is NOT part of the cell theory? a. All organisms are composed of one or more cells. b. Cells come from other cells by division. c. Cells are the smallest living things. d. Eukaryotic cells have evolved from prokaryotic cells.

d. Eukaryotic cells have evolved from prokaryotic cells.

All cells have all of the following except: a. plasma membrane. b. genetic material. c. cytoplasm. d. cell wall.

d. cell wall.

The smooth endoplasmic reticulum is: a. involved in protein synthesis .b. a site of protein glycosylation. c. used to store a variety of ions. d. the site of lipid and membrane synthesis.

d. the site of lipid and membrane synthesis.