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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). 56 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. 58 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 59 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. 60 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. Organizing Life • MHR 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?