Download Organelles at Work

S E C T I O N
2.2
Organelles at Work
E X P E C TAT I O N S
Describe how cell structures
work together to carry out
protein synthesis and
lysosomal digestion.
Illustrate and explain the
function of, and relationship
between, protein synthesis
and lysosomal digestion.
Figure 2.15 To make silk, a
spider’s spinnerets draw out a
fluid containing protein subunits.
How does this process resemble
the protein synthesis going on
inside the spider’s cells?
The processes of protein synthesis and lysosomal
digestion illustrate the dynamic nature of the way
that the cell’s organelles work together. The ability
of lysosomes to digest materials within the cell
depends on the correct production of enzymes
(proteins). Protein production relies on complex
interactions among organelles.
Protein Synthesis
Almost all cell processes require proteins, so the
production of many different kinds of proteins is a
fundamental activity of living cells. In fact, proteins
make up about half of the dry mass of cellular
material. Many proteins function as enzymes. An
important protein is the blood protein hemoglobin,
which carries oxygen and carbon dioxide. An
organism such as the spider in Figure 2.15 depends
on the production of proteins (in the form of the
silk threads of its web) to obtain food.
Cells produce proteins only in the amounts and
at the times required, and all cells use a similar
process for protein synthesis. Follow the numbered
steps in Figure 2.17 on the next page as you read
about how protein synthesis occurs in eukaryotes.
1. Transcription occurs in the nucleus. As you
learned in Chapter 1, DNA codes for cellular
proteins. It does this using a code of three
nucleotide bases (triplets) for each amino acid.
The cell transcribes (copies) the coded
information from a section of DNA called a
gene. To do this, the cell must unwind and
unzip that section of DNA and make an RNA
copy of it, as shown in Figure 2.16. During this
process of transcription, the information along
only one of the DNA strands is copied,
producing a strand of RNA nucleotides.
REWIND
To review protein structure, turn to Chapter 1, Section 1.1.
Figure 2.16 Many copies of RNA are being transcribed
simultaneously from a single strand of DNA (centre).
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MHR • Cellular Functions
1 DNA is transcribed
(copied) onto a
complementary
strand of RNA.
completed
polypeptide
7 Translation complete, a
ribosome releases the
finished polypeptide and
the mRNA.
RNA
DNA
mRNA
polypeptide
chain (forming)
2 mRNA takes the
coded message to
ribosomes in the
cytoplasm.
nuclear pore
3 mRNA attaches to a
ribosome, where its
coded message can be
translated.
representation of
three-nucleotide
code for one
amino acid
ribosome
peptide bond
4 A tRNA molecule transfers
a specific amino acid to the
ribosome.
amino
acid
tRNA molecule
(released)
5 The incoming tRNA attaches to the mRNA, which allows
the tRNA amino acid to bond to the polypeptide chain.
6 When the ribosome moves
along the mRNA, the tRNA
released can pick up another
amino acid.
Figure 2.17 The accuracy of transcription and translation relies on the pairing of
nucleotide bases in DNA and RNA. Enzymes direct both processes.
2. mRNA moves into the cytoplasm. The
completed RNA — called messenger RNA,
or mRNA — carries the coded message from
the DNA in the nucleus, to ribosomes in
the cytoplasm.
3. Translation starts when mRNA attaches to a
ribosome. The translation of the coded mRNA
message from a sequence of nucleotide bases
into a sequence of amino acids takes place on a
ribosome. A ribosome can translate any mRNA
strand. To begin the translation process, mRNA
attaches either to a cytoplasmic ribosome or to
a ribosome attached to the endoplasmic
reticulum (as shown in Figure 2.5 on page 48).
BIO
FACT
To confirm the path that mRNA takes during protein
synthesis, researchers stained its nucleic acids with
a fluorescing dye and then traced its progress with a
fluorescence microscope.
4. tRNA brings amino acids to the ribosome.
Dissolved amino acids in the cytoplasm must
be transferred to the ribosome for assembly into
a polypeptide. Another type of RNA called
transfer RNA, or tRNA, does this. Each tRNA
molecule recognizes only one amino acid. It
attaches this amino acid to one side of its
structure. On the other side of the tRNA
molecule, three nucleotide bases match one of
the triplet codes on the mRNA.
Organizing Life • MHR
57
DNA
enzyme
transcription
mRNA
Figure 2.18 Several ribosomes can move along the same
strand of mRNA at the same time, creating multiple copies
of the same polypeptide. This arrangement is called a
polyribosome.
5. An incoming tRNA positions its amino acid to
join the polypeptide chain. The tRNA carrying
the next amino acid in the mRNA code forms a
temporary bond with the mRNA at the
ribosome. This places the amino acid in the
correct position to form a peptide bond with
the amino acid on the tRNA molecule already
at the ribosome. In this way, about 15 amino
acids are added to the polypeptide chain
every second.
6. An outgoing tRNA releases its amino acid and
detaches from the mRNA. A ribosome has
room for only two tRNA molecules at a time.
When the ribosome moves along the mRNA to
the next coding unit, the outgoing tRNA
transfers the polypeptide chain to the other
tRNA. This frees up space for a new tRNA
molecule at the ribosome. The outgoing tRNA
molecule can now collect another amino acid.
7. Translation ends when a ribosome reaches a
“stop” instruction. When a ribosome reaches
the “stop” codes at the end of an mRNA strand,
it releases the completed polypeptide. Then the
ribosome separates into its subunits, which
detaches it from the mRNA. The polypeptide
must still complete its folding process before
becoming a finished protein.
PAUSE
RECORD
In Section 2.1, you learned how the endoplasmic reticulum
and Golgi apparatus prepare proteins for use by the cell
(or by other cells). Use this information and what you know
about protein synthesis to draw a simple flowchart that
shows how a protein destined for use in a lysosome is
produced and transported within the cell.
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MHR • Cellular Functions
ribosome
polypeptides
under
construction
translation
Figure 2.19 Without a nucleus, a prokaryotic cell can carry
out transcription and translation at the same time. Although
not pictured, tRNA molecules bring amino acids to
prokaryotic ribosomes to be added to a polypeptide.
Protein Synthesis in Prokaryotes
Although the processes of transcription and
translation in prokaryotes and eukaryotes follow
very similar basic steps, you can see from the
illustration in Figure 2.19 that protein synthesis in
prokaryotes is simpler. The prokaryotic ribosomes
(which are smaller than eukaryotic ones) lie close
to the DNA, so the beginning of an mRNA strand
can attach directly to a ribosome while the rest of
the strand is still being transcribed. This allows
prokaryotes to produce proteins very rapidly.
BIO
FACT
Like prokaryotic cells, mitochondria and chloroplasts of
eukaryotes use their own DNA and ribosomes to produce
some of the proteins they need. However, most of the
proteins needed by mitochondria and chloroplasts are coded
for on genes in the cell nucleus, synthesized on cytoplasmic
ribosomes, and then transported into the organelles.
Lysosomal Digestion
Only eukaryotic cells contain lysosomes. In plant
cells, the specialized vesicles that recycle cellular
material (the way that a lysosome does) are often
called vacuoles. Each lysosome (or vacuole)
contains over 40 different digestive enzymes. After
the lysosome leaves the Golgi apparatus, its
membrane actively pumps in hydrogen ions to
make its interior environment more acidic. This
acidic environment activates the lysosomal
enzymes.
When active, these enzymes can break apart
macromolecules in a step-by-step process. This
lysosomal digestion can occur in two ways:
by breaking down material ingested through
endocytosis and by recycling the cell’s own
organic material. Figure 2.21 illustrates the two
pathways of lysosomal digestion.
Endocytosis
After a cell engulfs material by endocytosis, a
lysosome fuses with the food vacuole formed to fill
the vacuole with digestive enzymes and break
down the captured material. You can follow the
steps involved in this process in Figure 2.21.
cell membrane
REWIND
Figure 2.20 You can see what happens to the cell
To review endocytosis, turn to Chapter 1, Section 1.4.
membrane of a didinium as it engulfs a paramecium.
endoplasmic
reticulum
nucleus
cytoplasm
food particles
or cells
Golgi apparatus
food vacuole
endocytosis
transport vesicle (containing
inactive lysosomal enzymes)
lysosomes
digestion
nutrients
assimilated
breakdown of
old organelle
cell membrane
old or
damaged
organelle
lysosome
Figure 2.21 Lysosomal digestion in an animal cell
Organizing Life • MHR
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Recycling
A lysosome fuses with and digests organelles no
longer useful to the cell. It then recycles their
components back to the cytoplasm. You can follow
the steps involved in this process in Figure 2.21.
FAST FORWARD
To learn more about Tay-Sachs disease, turn to Chapter 7,
Section 7.1.
Investigation
BIO
FACT
Errors in enzyme production can mean that a particular
lysosomal enzyme may not work. This interrupts the orderly
sequence of steps for breaking apart large molecules. As a
result, the cell accumulates useless molecule fragments.
Over long periods of time, the build-up of these fragments
inside cells can negatively affect the growth and development
of the individual. A number of serious genetic diseases such
as Tay-Sachs and Hunter syndrome are linked to faulty
lysosomal enzymes.
SKILL FOCUS
2 • A
Predicting
Exploring Organelle Function
Performing and recording
Lysosomal digestion can be difficult to demonstrate, but you can easily
observe the process of peroxisome enzymes breaking down molecules.
One of these, catalase, breaks hydrogen peroxide (H2O2 ) down to water
and oxygen. To function, lysosomal enzymes need an acidic
environment. Does catalase also need an acidic environment?
Pre-lab Questions
What is the pH value of an acid solution, a
neutral solution, and a dilute acid solution?
What function(s) do peroxisome enzymes serve?
Adding hydrogen peroxide to cut potatoes
causes foam to be produced. Why?
Problem
How can you demonstrate the effect of pH on the
catalase enzyme found in potato peroxisomes?
Prediction
Predict what pH condition will best support
enzyme function in peroxisomes.
CAUTION: Acids are corrosive; avoid contact with
skin and use water to flood spills. If you are
using a computer pH probe, ensure that the
probe remains stable so that it does not fall and
cause a spill.
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MHR • Cellular Functions
Analyzing and interpreting
Identifying variables
Materials
3 graduated cylinders
2 droppers
knife
dissecting probe
pH probe or indicator
paper
60 mL 3% hydrogen
peroxide
0.1 mol/L hydrochloric
acid solution
distilled water
potato
Procedure
1. Prepare 3 hydrogen peroxide solutions of
different pH as follows:
(a) Measure 50 mL of 3% hydrogen
peroxide into each of the 3 graduated
cylinders.
(b) Label the cylinders 1, 2, and 3.
(c) Use one dropper to add 10 drops of
0.5 mol/L hydrochloric acid solution
to cylinder 1 and 5 drops of 0.5 mol/L
hydrochloric acid solution to cylinder 2.
(d) Use the other dropper to add 5 drops of
distilled water to cylinder 2 and 10
drops of distilled water to cylinder 3.
substrate
products
active site
enzyme
enzyme-substrate complex
enzyme
Figure 2.22 The union of the enzyme with its substrate
(the molecule) weakens certain chemical bonds within the
substrate. This causes the substrate to break apart.
Enzyme Function
Enzymes make possible reactions that would
otherwise not proceed at temperatures low enough
for life to be possible. An enzyme is a protein that
functions as an organic catalyst. A catalyst helps a
particular reaction go forward without being used
up in the process. The cell makes a different
enzyme for each reaction it requires.
An enzymatic reaction may either combine
molecules to produce a new product or break a
molecule into smaller parts. Figure 2.22 illustrates
a model of the way an enzyme breaks apart a
molecule.
In Chapter 1, you learned that proteins have
three-dimensional shapes. An enzyme’s shape
allows its substrate(s) to attach at a spot called the
(e) Record the contents of each cylinder.
2. Check and record the pH of each cylinder. If
you are using a pH probe, keep a running
record of the pH.
3. Peel the potato, and cut it into 3 equal
cubes. Mince each cube (sample) to an equal
fineness.
4. Add one potato sample to each graduated
cylinder. Use the dissecting probe to push
all of the potato into each hydrogen
peroxide solution, rinsing the probe between
each use.
5. Observe the contents of each cylinder, and
record your observations once a minute for
5 min. Then test and record the final pH for
each cylinder. Use a chart to organize your
observations. Make a graph to compare the
reaction of each sample.
Post-lab Questions
1. In terms of their initial reactions, order the
hydrogen peroxide solutions from strongest
to weakest.
2. Which solution had the strongest final
reaction?
The catalase crystal inside a plant peroxisome allows the
organelle to break down hydrogen peroxide.
Conclude and Apply
3. How does pH affect the rate of the catalase
reaction?
4. In terms of pH, what kind of internal
environment would allow peroxisomes to
function most efficiently?
5. Identify the dependent and independent
variables in this experiment.
Exploring Further
6. What do you predict the natural pH of a
potato to be? How could you test this?
7. Conduct research to find out what other
factors can affect the rate of an enzyme
reaction.
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61
CONCEPT ORGANIZER
The Cellular Team in Action
Organelle Functions
(Chapter 2, Section 2.1)
Enzyme (Protein)
Synthesis and Shape
(Chapter 2, Section 2.2;
Chapter 1, Section 1.1)
Lysosomal Digestion of
Macromolecules in
Cellular Structures
Endocytosis
(Chapter 1, Section 1.2)
To understand how a lysosome carries out intracellular
digestion, you need to know how a lysosome functions,
how its enzymes are formed, and how it connects with the
materials it must break down. Which cellular structures are
involved in lysosmal digestion?
The process of forming a lysosome begins in the nucleus
with the transcription of mRNA for the lysosomal enzymes.
In the cytoplasm, ribosomes attached to the endoplasmic
reticulum translate this mRNA. The enzymes are transported
to the Golgi apparatus, which modifies the enzymes and
packages the correct ones together into a lysosome. The
three-dimensional shape of the enzymes allows them to do
their job. Some materials on which the lysosome works
enter the cell in vesicles (food vacuoles) formed by the cell
membrane through endocytosis. Once the lysosome has
digested the macromolecules, the lysosomal membrane
transports nutrients useful to the cell into the cytoplasm.
Mitochondria supply the energy required to drive many of
these processes.
Figure 2.23 What did happen to the paramecium captured by the didinium?
active site. Here, chemical bonds within a substrate
can be broken or bonds between substrates can be
formed. Then the enzyme releases the product(s)
and can start the process again.
SECTION
1.
2.
K/U Why do all cells need to perform protein
synthesis?
4.
K/U What is translation and where does it take
place?
5.
K/U Eukaryotic cells have a double membrane
surrounding their DNA, separating it from the
cytoplasm. Identify a reason for this.
8.
Make a flow chart for the lysosomal digestion of
a mitochondrion.
K/U
K/U
Write a scenario about the fate of a bacterium that has been
engulfed by a eukaryotic cell through phagocytosis.
determines the optimum pH for the catalase enzyme,
outlining the procedures you would use.
List the key steps involved in protein synthesis.
K/U What is transcription and where does it take
place?
7.
LINK
REVIEW
3.
6.
62
K/U
Wo rd
What is an enzyme and how does it function?
I Make a prediction about what the optimum pH
for catalase could be. Then design a lab that
MHR • Cellular Functions
9.
C A good analogy for protein synthesis is the
manufacturing process. Ribosomes are the factories
that produce proteins. Complete this analogy by
describing the role of DNA, mRNA, and tRNA. In a
small group, prepare a dramatic presentation of
this process.
UNIT INVESTIGATION PREP
Food coming from the stomach contains many molecules,
such as disaccharides, too large to diffuse or be
transported into cells lining the small intestine. How
could the intestinal cells break down these molecules?
Draw a diagram of this process. For the most economical
use of an intestinal cell’s resources, where could this
process take place?