Mastering Concepts
3.1
1. Why are cells, not atoms, the basic units of life?
A cell is the smallest unit of life because it is the smallest unit that can carry out all of the
functions of life. An atom is smaller than a cell, but an atom is a single particle that is a
component of all matter, living and nonliving.
2. How have microscopes advanced the study of cells?
Microscopes led to the discovery of cells, allow us to differentiate between prokaryotic
and eukaryotic cells, allow us to study the organelles inside a eukaryotic cell, and even let
us watch some basic cell functions as they occur.
3. What are the original components of the cell theory, and what parts of the theory came
later?
The two original components of cell theory are that all life is made of cells and that cells
are the fundamental unit of life. A third component, added soon after the original
formulation of the theory, is that all cells come from preexisting cells. More recently, cell
theory incorporated the ideas that all cells have the same basic chemical composition, use
energy, and contain DNA that is duplicated and passed on as each cell divides.
4. Rank the three main types of microscopes from lowest to highest potential
magnification.
From lowest to highest magnification, the order is light microscope, then scanning
electron microscope, then transmission electron microscope.
5. Which molecules and structures occur in all cells?
All cells have DNA, proteins, RNA, ribosomes, a watery cytoplasm, and a cell
membrane.
6. Describe adaptations that increase the ratio of surface area to volume in cells.
Adaptations that increase the surface area to volume ratio include elongated or flattened
cell shapes. An efficient transportation system, a water-filled vacuole, and improved
efficiency help cells sidestep surface area limitations.
3.2
1. How do prokaryotic cells differ from eukaryotic cells?
Prokaryotic cells lack a nucleus and other membrane-bounded organelles, have
chemically unique cell walls, and are typically much smaller than eukaryotic cells.
2. Compare and contrast bacteria and archaea.
Bacteria and archaea are small cells that lack nuclei and membrane-bounded organelles.
The two groups differ in the composition of the cell wall and the lipids in the cell
membrane.
3. What is the relationship between cells and organelles?
Eukaryotic cells contain specialized membrane-bounded organelles that carry out specific
functions.
4. How do organelles contribute to efficiency in eukaryotic cells?
Organelles concentrate the biochemicals needed for chemical reactions so that the
reactions proceed more rapidly. Also, because of organelles, a eukaryotic cell can
maintain high concentrations of each biochemical only in certain areas, not throughout
the cell.
3.3
1. Chemically, how is a phospholipid different from a triglyceride?
A phospholipid has two fatty acids and a phosphate group attached to a glycerol, whereas
a triglyceride has three fatty acids attached to a glycerol.
2. How does the chemical structure of phospholipids enable them to form a bilayer in
water?
The hydrophilic heads make contact with water while the hydrophobic tails avoid water.
When many phospholipids join, the hydrophilic heads and hydrophobic tails align,
forming a bilayer.
3. Where in the cell do phospholipid bilayers occur?
Phospholipid bilayers occur around organelles within the cell and form the plasma
membrane.
4. What are some functions of membrane proteins?
Transport proteins move substances into and out of cells; enzymes facilitate chemical
reactions; recognition proteins on cell surfaces allow the body to recognize its own cells;
adhesion proteins allow cells to stick together; receptor proteins bind molecules to the
outside of the cell and trigger chemical reactions inside the cell.
5. How does a cell membrane differ from a cell wall?
A cell membrane is a delicate phospholipid bilayer with selective permeability. All cells
have a membrane. A cell wall is a rigid structure that surrounds the cell membrane of
some cells.
3.4
1. How do organelles help eukaryotic cells compensate for a small ratio of surface area to
volume?
One way to compensate for a small ratio of surface area to volume is to use resources and
energy efficiently. Thanks to specialized organelles, a eukaryotic cell produces high
concentrations of biochemicals only in certain areas rather than producing them
throughout the cell.
2. Which parts of the cell interact to produce and secrete a complex substance such as
milk?
The nucleus, ribosomes, smooth ER, Golgi apparatus, vesicles, and cell membrane
interact in the production and secretion of milk. Mitochondria provide the energy
necessary to make the process happen.
3. What is the function of the nucleus and its contents?
The nucleus contains DNA (the molecule that contains the instructions for making
proteins) and the nucleolus (where ribosomes are assembled). mRNA molecules also are
produced in the nucleus, although they exit through nuclear pores before participating in
protein production.
4. Which organelles are the cell’s “recycling centers”?
Lysosomes, vacuoles, and peroxisomes are the cell’s recycling centers.
5. How are the functions of plastids essential to the life of a plant cell?
Chloroplasts are plastids that carry out photosynthesis. Other plastids store pigments or
food, or they help a plant to detect gravity.
6. Which organelle houses the reactions that extract chemical energy from nutrient
molecules?
Mitochondria house the reactions that extract chemical energy from nutrient molecules.
7. Which three organelles contain DNA?
The nucleus, chloroplasts, and mitochondria all contain DNA.
3.5
1. What are some functions of the cytoskeleton?
The cytoskeleton provides a structural framework for the cell, is a transportation system
within the cell, allows the cell to move, and connects cells to one another.
2. What are the main components of the cytoskeleton?
The major components of the cytoskeleton are proteins that assemble into
microfilaments, intermediate filaments, and microtubules.
3. Why are flagella and cilia important?
Unicellular organisms may use either flagella or cilia to move toward food or other
resources. In humans, sperm cells have flagella, and cilia sweep substances along the
respiratory tract and the female reproductive tract.
3.6
1. What are the three types of junctions that link cells in animals?
Tight junctions, anchoring junctions, and gap junctions link cells in animals.
2. What are the functions of a cell wall?
A cell wall imparts shape, regulates cell volume, and prevents a cell from bursting if it
takes in too much water. Cell walls also participate in plant cell differentiation.
3. What is the chemical composition of a plant cell wall?
Plant cell walls are made mostly of cellulose but also contain hemicellulose, pectin, and
many types of proteins.
4. What are plasmodesmata?
Plasmodesmata are tunnels through the cell wall; each plant cell can communicate with
its neighbors via the plasmodesmata.
3.7
1. How did the researchers determine that both magnetism and oxygen guided bacteria
movements?
The researchers devised an experiment in which bacteria in small glass tubes were free to
select their optimal O2 concentration while the researchers manipulated the magnetic
field. Wherever they were in the tube, the bacteria turned toward the magnetic field, but
they did not always swim in the same direction (as one might expect if the magnetic field
alone determined the direction of movement). Instead, they remained where the oxygen
concentration was optimal.
2. How do magnetosomes help bacteria save energy?
In their natural ocean habitat, Earth’s magnetic field lines are vertically oriented, so they
help the bacteria determine which direction is “up.” The optimal O2 concentration is in a
roughly horizontal layer of water. By swimming only up and down, not in random
directions, they can save energy as they search for the optimal O2 concentration in the
water column.
Write It Out
1. How does the formation of the cell theory illustrate the process of science?
Schleiden and Schwann integrated their own observations with those of many other
scientists as they developed their theory, which at first had two parts. Virchow added a
third component in 1855, and the theory has expanded further since that time, illustrating
that scientific theories are subject to change. The cell theory includes many testable
predictions and although it is potentially falsifiable, all observations to date have
supported the theory.
2. List the features that all cells share, then name three structures found in eukaryotic
cells but not in bacteria or archaea.
All cells have DNA, RNA, protein, ribosomes, cytoplasm, and a cell membrane.
Structures found only in eukaryotic cells include the nucleus and other membranebounded organelles, such as mitochondria, chloroplasts, and the endomembrane system.
3. If a eukaryotic cell is like a house, how is a prokaryotic cell like an efficiency (oneroom) apartment?
A eukaryotic cell is like a house with many different internal compartments and
specialized areas. A prokaryotic cell is more like an efficiency apartment because it is
smaller and has a simpler structure with fewer internal compartments. However, the
apartment still carries out the same basic functions as the house.
4. List three structural differences between plant and animal cells. Explain how each
structural difference reflects a functional difference between plants and animals.
Plant cells have chloroplasts; animal cells do not. Chloroplasts are the organelles where
photosynthesis occurs. Animals are heterotrophs and do not carry out photosynthesis.
A plant cell also has a central vacuole, a sac that occupies most of the cell’s volume;
animal cells do not. Turgor pressure exerted by the vacuole on the cell wall helps a plant
stay upright.
A plant cell has a cell wall, which provides rigidity and support. Animal cells lack cell
walls and typically rely on skeletal and muscular systems for movement and support.
5. Suppose you find a sample of cells at a crime scene. What criteria might you use to
determine if the cells are from prokaryotes, plants, or animals?
One approach would be to look at the cells with a microscope. If they lack a nucleus, they
are prokaryotic; if they contain a nucleus, they are eukaryotic. A eukaryotic cell with a
cell wall is likely to be a plant cell (although it could also come from a fungus or some
types of protists). A eukaryotic cell without a cell wall is likely to come from an animal
(although it could also belong to a protist).
6. Rank the following in order from smallest to largest: ant, prokaryotic cell, actin
molecule, microtubule, nitrogen atom. What type of microscope (if any) would you need
if you wanted to see each?
From smallest to largest, the order would be nitrogen atom, actin molecule, microtubule,
prokaryotic cell, ant. The nitrogen atom and individual actin molecule are too small to see
with any microscope. Microtubules and prokaryotic cells are best seen with an electron
microscope, although they are visible with a light microscope. An ant is visible with the
unaided eye.
7. Which cell in figure 3.31 has the highest ratio of surface area to volume? Explain your
answer
The prokaryotic cell has a higher ratio of surface area to volume.
8. What advantages does compartmentalization confer on a large cell?
Compartmentalization means that a cell must maintain high concentrations of each
biochemical only in certain organelles and not throughout the entire cell. Internal
compartments also increase the membrane surface area available for chemical reactions.
9. List the chemicals that make up cell membranes.
The chemicals in cell membranes are phospholipids, sterols, proteins, and sugars.
10. Emulsifiers are common food additives. A typical emulsifier molecule has a
hydrophilic end and a hydrophobic end. Draw a diagram explaining how an emulsifier
can enable oil to mix with water.
Answer will be visual. Emulsifier molecules should surround a droplet of oil, with the
hydrophilic end of the emulsifier facing the water and the hydrophobic end facing the oil.
11. Choose an organelle in a human cell, and imagine that a disease causes that organelle
to be faulty. How would the malfunctioning organelle affect the cell’s function?
Answers will vary; these are two examples. If mitochondria were affected, a cell would
be unable to generate energy. If the endoplasmic reticulum were to malfunction, many
other organelles (such as lysosomes and peroxisomes) would not function properly
12. One way to understand cell function is to compare the parts of a cell to the parts of a
factory. For example, the Golgi apparatus would be analogous to the factory’s shipping
department. How would the other cell parts fit into this analogy?
The nucleus would be the management office where product orders are issued, and the
ribosomes and smooth ER would be the sites of production. The forklifts would be
transport vesicles carrying raw materials and products within the factory, and the power
generators would be the mitochondria. The girders and steel beams that make up the
structure of the building would be the cytoskeleton.
13. Why does a muscle cell contain many mitochondria and a white blood cell (an
immune cell that engulfs bacteria) contain many lysosomes?
Muscle cells require a lot of energy to move parts of the body; mitochondria use aerobic
cellular respiration to extract energy from food. White blood cells have many lysosomes
because they engulf and dispose of debris and bacteria.
14. How does the cytoskeleton interact with other structures in eukaryotic cells?
Organelles can move within the cell along the cytoskeleton’s “tracks.” The cytoskeleton
also protects the cell’s organelles by helping maintain the cell’s shape and resisting
mechanical stress. Some components of the cytoskeleton tug on chromosomes as the cell
divides. The cytoskeleton also links adjacent cells together through the cell membrane.
15. How do plant cells form cell walls?
A plant cell secretes the components of its cell wall through its plasma membrane. The
primary wall is the first to be secreted; it is thin and flexible compared to the secondary
wall, which is thicker and tougher. The secondary wall is internal to the primary wall.
16. Describe how animal cells use junctions in different ways.
Tight junctions create an impermeable seal between adjacent cells. Anchoring junctions
act as “rivets” that secure cells in place. Gap junctions allow adjacent cells to exchange
cytoplasmic material.
Pull It Together
1. How might you connect the terms proteins and cytoskeleton?
“Proteins” could connect with the phrase “make up the” to “cytoskeleton.”
2. Add the three main components of the cytoskeleton to this map.
“Cytoskeleton” could connect with the phrase “has three main components, which are” to
three boxes titled “Microfilaments,” “Intermediate filaments,” and “Microtubules.”
3. In what ways are domains Bacteria and Archaea different?
Their cell walls and cell membranes are composed of different chemicals. They also
differ in certain key genetic sequences.
4. Add the eukaryotic kingdoms to this concept map.
“Domain Eukarya” could connect with the phrase “consists of” to four boxes titled
“Protists”, “Fungi”, “Plants”, and “Animals”.
5. Add chloroplast, lysosome, and vacuole to this concept map.
One possibility would be to connect “Organelles” with the phrase “include” to all three of
these terms. Another option would be to first add “Plants” and “Animals” as kingdoms
within domain Eukarya. Then, “Chloroplast” and “Vacuole” could be listed as organelles
found in the cells of plants, and “Lysosome” could be listed as an organelle found in the
cells of animals.
6. Which cell types have a cell wall?
The cells of most bacteria and archaea have cell walls. The cells of some eukaryotes
(notably plants, fungi, and some protists) also have cell walls, although those of animals
do not.