Chapter 2
Answers
1
QUICK-CHECK questions
Why are cells known as the basic building blocks of living organisms?
Cells are known as the basic building blocks of living organisms because all
organisms, including animals, plants, fungi, bacteria and archaeans, are composed of
one or more cells.
2
How might an examination of cells help diagnose disease?
Cancers are diseases that are characterised by abnormal growth of undifferentiated
mutated cells in healthy tissues. So, examination of the cellular makeup of tissues can
assist in the diagnosis of diseases such as cancers.
3
List three possible death signals a cell might receive to initiate apoptosis.
Signals for the initiation of programmed cell death or apoptosis can come either from
inside a cell or from outside the cell. They include:
 an internal signal that the cell has suffered damage
 an internal signal that the cell is infected with virus
 a signal from outside a cell that the cell is no longer required or useful, such as
T cells after an infection has been resolved or the cells between the digits of a fetus
 a signal from outside a cell that the cell is in excess of the number incorporated into
a developing tissue.
4
Make a labelled sketch of a typical plasma membrane.
One of many possible sketches is shown in figure 2.9 on page 39.
Other examples are available at the following websites:
http://home.earthlink.net/~dayvdanls/lecw4cells5.html
http://ocw.tufts.edu/Content/51/lecturenotes/551831/552089.
Important features to label include the bilayer of phospholipids and the inserted
(transmembrane) proteins and glycoproteins (proteins with carbohydrate chains
attached).
5
List the different ways in which materials cross plasma membranes. For each
way, indicate whether or not it is energy-requiring.
Materials may cross plasma membranes by:
 diffusion of dissolved material, which does not require energy
 active transport, which requires energy
 bulk transport or endocytosis which requires energy.
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Chapter 2: QUICK-CHECK answers
6
Many plant cells have secondary cell walls as well as primary. Of what
advantage is this to a plant?
The secondary cell wall in plant cells is a stronger structure than the primary cell wall.
It is thicker because the secondary cell wall contains several cellulose layers and the
cellulose fibrils in each layer are aligned in a different direction. This feature increases
the elastic strength of the secondary cell wall, gives support to the plant and provides
resistance to the force of gravity. In addition, in woody tissues, the cellulose in the
secondary cell wall is further strengthened by the addition of lignin, allowing plants,
such as trees, to grow to great heights.
(Note: In animals, structures such as internal bony skeletons and hard exoskeletons,
provide support but in a different manner.)
7
State whether the following are true or false and briefly explain your answer.
a
A nucleus from a plant cell would be expected to have a double nuclear
membrane.
True: Because plant cells are eukaryotic cells, they would be expected to have a
double nuclear membrane as this is a feature of eukaryotes.
b
Chromosomes are always visible in a eukaryotic cell.
False: Chromosomes are not visible in non-dividing cells and they become visible
only during the process of cell replication.
8
Suggest why the nucleus is sometimes called the ‘control centre’ of a cell.
The nucleus is sometimes called the ‘control centre’ of a cell because the nucleus
contains DNA, the genetic material that holds all the instructions for the life of the cell.
9
Of what advantage is a folded inner membrane in mitochondria?
Some of the reactions of cellular respiration involve components that form part of the
inner membrane of a mitochondrion. It is reasonable to suggest that, compared with a
non-folded structure, the folded inner membrane in a mitochondrion creates a larger
surface area for these components and so increases the amount of ATP produced.
10
What is the source of ribosomal RNA (rRNA)?
Ribosomal RNA (rRNA) is produced by the nucleolus of a cell. The nucleolus is the
region within the nucleus where the genes controlling production of ribosomal RNA are
concentrated.
11
Some ribosomes are free in cytosol; some are attached to endoplasmic
reticulum. What is the significance of this difference?
Ribosomes are site of protein synthesis. When ribosomes are attached to the
endoplasmic reticulum, the proteins that they produce enter the endoplasmic reticulum
for transport away from the cell. The proteins made by the ‘free’ ribosomes in the
cytoplasm are used within the cell.
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Chapter 2: QUICK-CHECK answers
12
Name three substances that would be produced at the surface of the ER of a cell
and transported for use outside the cell.
Substances that are produced on ribosomes at the surface of the endoplasmic
reticulum (ER) and transported for use outside the cell include:
 polypeptide/protein hormones, such as insulin
 digestive enzymes released from various glands/parts of the digestive systems,
such as amylase from the salivary glands and pepsinogen from the stomach cells
 immune proteins released as part of the body’s defence against infection, such as
antibodies from plasma cells (see figure 2.29 on page 57).
(Note: This question requires some lateral thinking and requires you to identify various
substances that are released from cells to carry out particular functions in the body.
You need to exclude substances that are produced by a cell but that are then retained
within the cell, such as the haemoglobin in red blood cells.)
13
Lysosomes are sometimes called ‘suicide bags’. Suggest why this name is
given.
Lysosomes are membrane-bound sacs that are filled with a fluid that contains
digestive enzymes. Lysosomes can take up and then digest or break down damaged
or unwanted material or worn-out organelles from within the cell or substances that are
taken up by the cell. Because lysosomes also play a role in programmed cell death,
they are sometimes called ‘suicide bags’.
(Note: Lysosomes could also be called the ‘garbage disposal’ system of a cell.)
14
How is the hydrogen peroxide produced in cellular metabolism detoxified?
Peroxisomes are cell organelles that contain various enzymes involved in reactions
that destroy toxic substances produced by the metabolic reactions in a cell. One of
these enzymes, catalase, catalyses the breakdown of harmful hydrogen peroxide to
the harmless products, water and oxygen.
15
What is the function of endosomes?
Endosomes are membrane-bound sacs formed when material is engulfed by the
plasma membrane when it enters a cell in a process known as endocytosis. The
function of endosomes is to transport the engulfed material to the lysosomes for
digestion.
16
What is the function of chlorophyll?
The green pigment, chlorophyll, captures the radiant energy of sunlight and converts it
to a form of chemical energy.
17
What are
a
thylakoids
A thylakoid is a membrane-bound sac inside a chloroplast.
b
grana
The grana are stacks of membrane-bound sacs within a chloroplast.
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Chapter 2: QUICK-CHECK answers
c
stroma?
The stroma comprises the fluid contents of a chloroplast.
18
Name the different structures that make up the cytoskeleton of a cell.
The cytoskeleton of a cell is made up of:
 microtubles
 microfilaments
 intermediate filaments.
(See figure 2.24.)
19
List the three types of connections possible between two animal cells and name
a characteristic of each.
The three types of connections that can occur between two animal cells are:
 occluding junctions, which prevent movement of material between cells
 communicating (gap) junctions, which permit the passage of small molecules and
electrical signals between cells
 anchoring junctions or desmosomes, which hold cells together strongly using
intermediate filaments.
(See figure 2.25.)
20
What are the connections between two plant cells called?
The connections between two adjacent plant cells are called plasmodesmata (see
figure 2.28).
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