The Cell

Question 1

The ____________ occurs only in eukaryotic cells, and houses many different types of nucleic acids–the more famous ones being DNA and RNA.

Answer 1

Nucleus. A cell that contains a distinct nucleus is known as a eukaryotic cell, as opposed to a prokaryotic cell (no nucleus). RNA moves out into the cytoplasm ( what lies between the nucleus and the cell wall).

Question 2

The __________ is an area of the nucleus where ribosomes are constructed.

Answer 2

Nucleolus. There are usually two nucleoli per nucleus.

Question 3

The nuclear envelope is a double-membrane structure. Numerous pores occur in the envelope, allowing RNA and other chemicals to pass, but not allowing ______ to pass.

Answer 3

DNA. DNA is restricted to the nucleus.

Question 4

The nucleus of the average human cell is only six micrometers (6 ´ 10-6 meter) in diameter, yet it contains about 1.8 meters of DNA. This is distributed among 46 _____________, each consisting of a single DNA molecule about 40 millimeters (1 1/2 inches) long.

Answer 4

Chromosomes. It’s an extraordinary packaging problem. If you enlarge this by a million times, a DNA molecule would still be a thin string two millimeters thick, and the average chromosome would contain 40 kilometers (25 miles) of DNA. At this magnification, with a diameter of only six meters, the nucleus would contain 1,800 kilometers of DNA.

Question 5

DNA is not simply crammed into the nucleus; it is organized, by molecular interaction with specific nuclear proteins, into a precisely packaged structure, creating a dense, compact fiber called_____________.

Answer 5

Chromatin. An extreme example of the ordered folding and compaction that chromatin can undergo is seen during cell division, when the chromatin of each chromosome condenses and is divided between two daughter cells.

Question 6

The _________ maintains the shape of the cell as well as anchoring organelles, moving the cell and controlling internal movement of structures.

Answer 6

Cytoskeleton. The cytoskeleton consists of fibrous proteins that occur in the cytoplasm. The cytoplasm was defined earlier as the material between the plasma membrane (cell membrane) and the nuclear envelope.

Question 7

______________ are made up of globular proteins called tubulins. They play an important role in cell division (mitosis) and serve as a “temporary scaffolding” for other organelles.

Answer 7

Microtubules. They participate in the formation of spindle fibres during cell division (mitosis). They help give shape to many cells and are major components of cilia and flagella, the ‘limbs’ that enable cell movement.

Question 8

________________ are single-membrane organelles located within the cell–a space within a cell that is empty of cytoplasm, lined with a membrane, and filled with fluid.

Answer 8

Vacuoles. Many organisms will use vacuoles as storage areas. “Vesicles” are much smaller than vacuoles and function in transport within and to the outside of the cell.

Question 9

___________ are the sites of protein synthesis. It is inside them that the information carried in the genetic code is converted into protein molecules.

Answer 9

Ribosomes. Made up of 40% protein and 60% RNA, they are not membrane-bound and thus occur in both prokaryotes (no membrane bound organelles and no distinct membrane-bound nucleus) and eukaryotes (distinct, membrane-bound nucleus). Eukaryotic ribosomes are slightly larger than prokaryotic ones.

Question 10

Often ribosomes cluster on the ____________ reticulum (ER), in which case they resemble a series of factories adjoining a railroad line.

Answer 10

Endoplasmic. The ER usually constitutes more than half of the membrane of the cell and is continuous with the outer membrane of the nuclear envelope.

Question 11

___________ are the shipping containers within cells.

Answer 11

Vesicles. Vesicles are used to package liquids containing a wide variety of substances and carry these materials to other parts of the cell or to the outside of the cell. Vesicles also form around material (liquid or solid) that is brought into the cell.

Question 12

_______ __________ are flattened stacks of membrane-bound sacs. They function as a packaging plant, modifying vesicles from the ER (Endoplasmic Reticulum).

Answer 12

Golgi Complexes. The function of the Golgi apparatus (another name for golgi complex) is two-fold: First, modification of lipids and proteins; Second, storage and packaging of materials that will be exported from the cell.

Question 13

Lysosomes are relatively large vesicles formed by the Golgi. They contain hydrolytic ____________ that could destroy the cell were they not safely contained inside the vesicle.

Answer 13

Enzymes. Lysosomes are a special type of vesicle which contain these potent enzymes. These enzymes within the lysosomes are acid hydrolases, responsible for the cell’s digestion of macromolecules, old cell parts, and microorganisms.

Question 14

When members of two different species enter into a living relationship, the arrangement is known as ___________.

Answer 14

Symbiosis. These are classified into mutualism, commensalism, and parasitism. Both positive (beneficial) and negative (unfavorable to harmful) associations are therefore included, and the members are called symbionts.

Question 15

A long time ago (perhaps even a billion years), bacteria-like organisms are thought to have been incorporated into eukaryotic cells by some kind of symbiosis. Today, these organelles–the ___________– are known as the powerhouse of the cell.

Answer 15

Mitochondria. They contain their own DNA (termed mDNA) and function as the sites of energy release (following glycolysis in the cytoplasm) and respiration.

Remember: Mitochondria are the powerhouse of the cell.

Question 16

During the 1980s, Lynn Margulis proposed the theory of ____________ to explain the origin of mitochondria and chloroplasts from permanent resident prokaryotes.

Answer 16

Endosymbiosis. According to this idea, a larger prokaryote (or perhaps early eukaryote) engulfed or surrounded a smaller prokaryote some 1.5 billion to 700 million years ago. Instead of digesting the smaller organisms, the large one and the smaller one entered into a type of symbiosis known as mutualism, wherein both organisms benefit and neither is harmed.

Question 17

Chloroplasts are the sites of ______________ in eukaryotes. They contain many different types of accessory pigments, depending on the organism being observed.

Answer 17

Photosynthesis. The pigments are inside sacs called thylakoids which float in a fluid called stroma.

Obviously chloroplasts are only going to be found in autotrophs (organisms that create their own energy i.e. plants), as opposed to heterotrophs (organisms that obtain energy from another organism i.e. humans).

Question 18

Like mitochondria, chloroplasts have their own ______. This is why chloroplasts of Green Algae (Protista) and Plants (descendants of some Green Algae) are thought to have originated by endosymbiosis of a prokaryotic alga similar to living Prochloron (Prochlorobacteria).

Answer 18

DNA. The DNA of chloroplasts are termed cpDNA.

Question 19

Chloroplasts are one of the three types of _________. The other types are 1)Leukoplasts which store starch, and sometimes protein or oils, and 2) Chromoplasts which store pigments associated with the bright colors of flowers and/or fruits.

Answer 19

Plastids. Plastids are round, oval, or disk-shaped bodies which occur only in plants and photosynthetic eukaryotes.

Question 20

Internal movements of organelles within a cell are governed by __________ filaments. Internal movement is known as cytoplasmic streaming.

Answer 20

Actin. These filaments make an area in which organelles such as chloroplasts can move.

Question 21

External movement of cells is determined by special organelles for locomotion. __________ work as whips pulling or pushing the organism through the water.

Answer 21

Flagella. A shorter kind of whip is cilia. Cilia work like oars on a viking longship (Paramecium has 17,000 such oars covering its outer surface).

As you can see above, one of the cells uses whip-like flagella for movement, and the other one uses thousands of hair-like cilia to propel itself through the liquid.

Question 22

Both cilia and flagella have the characteristic __________ arrangement of microtubules.

Answer 22

9 + 2. See figure below.

Microtubules are straight, hollow cylinders which participate in a wide variety of cell activities–most of which involve motion. Cilia and flagella are constructed from microtubules, and it is important to remember that it is called a 9+2 arrangement.

Question 23

A third kind of _________ (external movement) device is Pseudopodia and is used by many cells, such as Amoeba, Chaos (Pelomyxa) and human leukocytes (white blood cells).

Answer 23

Motile. These are not structures like cilia and flagella; rather they are associated with actin near the moving edge of the cell, resulting in motility.

Question 24

The contents (both chemical and organelles) of the cell are termed protoplasm, and are further subdivided into ____________ and nucleoplasm (all of the material, plasma and DNA etc. within the nucleus).

Answer 24

Cytoplasm.

As you can see in the image above, everything inside the nucleus is nucleoplasm, and the area outside of the nucleus within the cell is the cytoplasm.

Question 25

A thin ____________, some .005 micrometers across, surrounds every living cell, delimiting the cell from the environment around it. It acts as a selective barrier for the cell.

Answer 25

Membrane. Enclosed by this plasma membrane are the cell’s constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acids, carbohydrates, and substances involved in cellular metabolism.

Question 26

Most current knowledge about the biochemical constituents of plasma membranes originates in studies of _____ _______ cells.

Answer 26

Red blood. They may be obtained easily in large amounts and have no typical membrane structure, other than the plasma membrane itself, to interfere with study of that structure.

Question 27

The cell membrane functions as a ______-________ barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell.

Answer 27

Semi-permeable. Semi-permeable means some things can go through this barrier, but it is selective about what it lets in and out of the cell. The development of a semi-permeable cell membrane was a major step in evolution.

Question 28

Imagine being in an elevator with a guy smoking a cigar. In physical terms, you smell the smoke because of the __________ of smoke molecules towards you, where their concentration is low.

Answer 28

Diffusion. Diffusion is a process resulting from random motion of molecules by which there is a net flow of matter from a region of high concentration to a region of low concentration.

In other words, there is a high concentration of molecules of cigar smoke around the smoker, and a low concentration of smoke around you. Through diffusion, the molecules move from the area of high concentration to the area of low concentration.

Below is a pic showing the diffusion of molecules within a liquid:

Question 29

Membranes consist largely of a ____________ bilayer, which is a double wall of phospholipid, cholesterol, and glycolipid molecules containing chains of fatty acids.

Answer 29

Lipid. The most common molecule in the model is the phospholipid, which has a polar (hydrophilic) head and two nonpolar (hydrophobic) tails.

Question 30

Phospholipids are aligned tail to tail so the nonpolar areas form a __________ region between the hydrophilic heads on the inner and outer surfaces of the membrane.

Answer 30

Hydrophobic. Remember, hydrophobic means not attracted to water, and hydrophilic means it is attracted to water.

The fatty-acid chains allow many small, fat-soluble molecules, such as oxygen, to permeate the membrane, but they repel large, water-soluble molecules, such as sugar, and electrically charged ions, such as calcium.

Below is a pic showing the components of the cell membrane.

Notice that the majority of the wall is made up of the phospholipids aligned tail to tail.

Question 31

Phospholipids are formed when a ___________ group (PO4-) replaces one of the three fatty acids normally found on a lipid.

Answer 31

Phosphate. The addition of this group makes a polar “head” and two nonpolar “tails”.

Question 32

Proteins are suspended in the inner layer, although the more hydrophilic areas of these proteins “stick out” into the cells interior as well as the outside of the cell. These integral proteins are sometimes known as ___________ proteins.

Answer 32

Gateway. Proteins also function in cellular recognition, as binding sites for substances to be brought into the cell, as through channels that will allow materials into the cell via a passive transport mechanism, and as gates that open and close to facilitate active transport of large molecules.

Notice in the pic below the integral proteins which are embedded in the cell membrane, but also stick out into the inside and outside of the cell:

Question 33

Multicellular organisms may have some mechanism to allow recognition of those cells that belong to the organism and those that are foreign. Many, but not all, animals have an __________ system that serves this sentry function.

Answer 33

Immune. When a cell does not display the chemical markers that identify it as belonging to the organism, an immune system response may be triggered. This is the basis for immunity, allergies, and autoimmune diseases.

Question 34

The outer surface of the membrane is rich in ____________, which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell.

Answer 34

Glycolipids. These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self.

Below is a simplified view of the cell membrane:

Question 35

Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by a type of ____________ known as osmosis.

Answer 35

Diffusion. It is a major method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. Gas exchange in the lungs operates by diffusion.

Question 36

Since a constant source of ________ ________ is inside the cell, the concentration gradient is constantly being replenished, thus the net flow is out of the cell due to diffusion.

Answer 36

Carbon Dioxide (CO2). In other words, the cell is constantly producing carbon dioxide, so the concentration of CO2 is higher on the inside of the cell, resulting in the net flow being CO2 diffusing out of the cell.

At the same time, metabolic processes in animals and plants usually require oxygen, which is in greater concentration outside the cell–thus the net flow of oxygen is into the cell.

Remember, in diffusion, the molecules move from the area of greater concentration to the area of lower concentration. This is how CO2 and oxygen are constantly flowing in and out of the cell.

Question 37

______________ is the diffusion of water across a semi-permeable membrane. In a cell, which has so many organelles and other large molecules, the water flow is generally into the cell.

Answer 37

Osmosis. For instance, there are more water molecules per unit of volume in a glass of fresh-water than there is in an equivalent volume of sea-water. The sea water has molecules of salt taking up a lot of the space that would be occupied by water.

Question 38

If Hypertonic solutions are those in which more solute is present, and Hypotonic solutions are those with less solute, Isotonic solutions have _______________ (iso-) concentrations of substances.

Answer 38

Equal. Of two solutions, the one with more solute is the hypertonic solution, and the one with less is the hypotonic solution. If the concentrations are equal, then they are isotonic.

There will still be equal amounts of water movement in and out of the cell, but the net flow is zero when the solutions on both the inside and outside of the cells are isotonic. One of the major functions of blood in animals is to maintain an isotonic internal environment.

Question 39

Paramecium and other single-celled freshwater organisms have difficulty since they are usually _______________ relative to their outside environment, and under the danger of bursting due to too much water.

Answer 39

Hypertonic. In other words, because they are hypertonic, water is constantly diffusing into the cell. To counteract this, a vacuole (called contractile) absorbs the water from the cytoplasm, and ejects it outside the cell. This is analogous to an airlock in spaceships.

Question 40

Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of _______, and facilitated diffusion.

Answer 40

Water. These are examples of passive transports. They are ways particles enter and leave the cell without any active “effort” on the part of the cell.

Question 41

Contractile vacuole mechanism is an example of ________ transport.

Answer 41

Active. This requires the cell to spend energy, usually in the form of ATP.

Question 42

Endocytosis: In this process the plasma membrane engulfs portions of the external medium, forms an almost complete sphere around it, and then draws the membrane-bounded vesicle into the cell. The opposite of endocytosis is ____________.

Answer 42

Exocytosis. Endocytosis is basically where the cell’s plasma membrane folds inward to bring substances into the cell.

Exocytosis is the exact opposite–substances contained in vesicles are discharged from the cell.

Question 43

The internal membranes ( around organelles, for instance) of eukaryotic cells differ both structurally and chemically from the plasma membrane. Like the plasma membrane, they are constructed of a ____________ bilayer into which are embedded, or bound, specific membrane proteins.

Answer 43

Phospholipid. The three major lipids forming the plasma membrane—phospholipids, cholesterol, and glycolipids—are also found in the internal membranes, but in different concentrations. Phospholipid is the primary lipid forming all cellular membranes.

Question 44

Cell membranes actively involved in metabolism (breakdown of substances into energy) contain a higher proportion of protein; thus, the membrane of the _________________, the most rapidly metabolizing organelle of the cell, contains as much as 75 percent protein.

Answer 44

Mitochondrion. The mitochondrion (plural mitochondria) is known as the ‘powerhouse’ of the cell. In contrast, the membrane of the Schwann cell, which forms an insulating sheath around many nerve cells, has as little as 20 percent protein.

Question 45

Like the plasma membrane, membranes of some organelles contain transport proteins, or ____________ , that allow chemical communication between organelles.

Answer 45

Permeases. In the lysosomal membrane, for example, they allow amino acids generated inside the lysosome to cross into the cytoplasm, where they can be used for the synthesis of new proteins.

Question 46

Although often perceived as an inactive product serving mainly mechanical and structural purposes, the plant cell ____________ actually has a multitude of functions upon which plant life depends.

Answer 46

Wall. Such functions include: (1) providing the living cell with mechanical protection and a chemically buffered environment; (2) providing a porous medium for the circulation of water, minerals, and other small nutrient molecules; (3) providing rigid building blocks from which stable structures of higher order, such as leaves and stems, can be produced; and (4) providing a storage site of regulatory molecules that sense the presence of pathogenic microbes and control the development of tissues.

Question 47

One primary difference between animal and plant cells is the existence of a _____ ______ surrounding plant cells.

Answer 47

Cell wall. Plant cells have a rigid cell wall in addition to the plasma membrane.

Question 48

Any cell or organism that possesses a clearly defined nucleus can be called a _______________.

Answer 48

Eukaryote. Obviously, this description excludes organisms like bacteria and blue-green algae. Prokaryotes lack a nuclear membrane and most of the components of eukaryotic cells.

Question 49

Prokaryotic cells have a much simpler design than is found in eukaryotic cells, most apparent in their lack of any intracellular ________________, a feature characteristically found in all eukaryotic cells.

Answer 49

Organelles. They are discrete membrane-enclosed structures floating in the cytoplasm and include the nucleus, the mitochondria, the lysosome, the endoplasmic reticulum and Golgi complex. Such organelles are lacking in prokaryotic cells.

Question 50

In most eukaryotic organisms, ____________ are much larger than they need to be to impart information for the synthesis of proteins.

Answer 50

Genes. Substantial portions of the RNA copy of the genetic information, or DNA are discarded, and the remaining messenger RNA (mRNA) is substantially modified before it is copied for protein synthesis. In contrast, bacterial (prokaryotic) mRNAs are exact copies of their gene and are not modified.

Question 51

In keeping with their simpler cell design, bacterial cells are usually much _________ than eukaryotic cells.

Answer 51

Smaller. The combination of small size, simple design, and broad metabolic capabilities allows bacteria to grow and divide very rapidly and to inhabit and flourish in almost any type of environment.

Question 52

__________, which are prokaryotes, have been divided into two major phylogenetic kingdoms, Eubacteria and Archaebacteria, based on such differences as chemistry and physiology.

Answer 52

Bacteria. All remaining living organisms are eukaryotes. Therefore prokaryotes are sometimes equated to bacteria.

Question 53

Prokaryotic cells are small (one to five micrometers in length) and contain only a single plasma _____________.

Answer 53

Membrane. Metabolic functions are often confined to different patches of the membrane rather than to areas in the body of the cell.

Question 54

Typical ___________ cells, by contrast, are much larger, the plasma membrane constituting only 10 percent or less of the total cellular membrane.

Answer 54

Eukaryotic. Metabolic functions in these cells are carried out in the organelles, compartments sequestered from the cell body, or cytoplasm, by internal membranes.

Question 55

Most bacterial cell membranes do not contain ___________ , which contributes to the fluidity and stability of all membranes.

Answer 55

Cholesterol. It is found in eukaryotic internal membranes at about 25 percent of the concentration in the plasma membrane.