Organelles

Essential Question

How do organelles work together to keep a cell alive?

A eukaryotic cell is not a bag of fluid. It is divided into compartments called organelles, each with a job. In this lesson, you’ll learn what the main organelles do and follow a protein from the nucleus’s instructions to the outside of the cell. By the end, you should be able to explain how organelles depend on one another.

The Nucleus and Ribosomes

• The nucleus holds the DNA
• Pores in the nuclear envelope let mRNA leave
• The nucleolus builds parts of ribosomes
• Ribosomes build proteins

The nucleus holds the cell’s DNA inside a double membrane called the nuclear envelope. DNA cannot leave the nucleus, so the cell copies the instructions for one protein into a molecule of messenger RNA (mRNA), which leaves through a nuclear pore. In the cytoplasm, a ribosome reads the mRNA and builds the protein. The nucleolus inside the nucleus builds parts of ribosomes. Ribosomes are not surrounded by a membrane. Some float free in the cytoplasm, and others are attached to the rough ER.

Making and Moving Proteins

• Rough ER: ribosomes build proteins into it
• Smooth ER makes lipids
• Golgi apparatus modifies, sorts, and packages
• Vesicles carry cargo between parts

Proteins that a cell will release or put in its membrane are made on ribosomes attached to the rough endoplasmic reticulum (rough ER), a network of flattened sacs. The new protein enters the ER, which helps it fold, and then a vesicle (a small membrane sac) carries it to the Golgi apparatus. The Golgi modifies the protein, sorts it, and packages it into vesicles. A vesicle then moves to the cell membrane, fuses with it, and releases the protein outside the cell. The smooth ER has no ribosomes and makes lipids.

Energy Organelles

• Mitochondria release usable energy as ATP
• Folded inner membrane gives more surface
• Chloroplasts carry out photosynthesis
• Chloroplasts are in plant and algae cells

Mitochondria are found in most eukaryotic cells. They use food molecules such as glucose, and oxygen, to make ATP in a process called cellular respiration. The folds of the inner membrane give it a large surface area for this work. Chloroplasts are found in plant cells and in algae. They capture light energy and use it to make glucose in photosynthesis, and their stacked membrane disks hold the light-capturing molecules. You will study both processes in Unit 8.

Storage, Breakdown, and Support

• Lysosomes break down wastes and old parts
• Vacuoles store water and other materials
• The cytoskeleton gives shape and helps parts move
• Vesicles move materials around the cell

Lysosomes are sacs of digestive enzymes that break down wastes, worn-out organelles, and food particles. A vacuole stores water, nutrients, and wastes, and a plant cell’s large central vacuole can take up most of the cell. The cytoskeleton is a network of protein fibers that gives the cell its shape and helps organelles and vesicles move. Keeping digestive enzymes inside lysosomes protects the rest of the cell from them.

Organelles Work Together

• Mitochondria make ATP for the whole cell
• ATP powers protein building and vesicle movement
• A protein passes through several organelles
• If one organelle fails, the others are affected

No organelle works alone. The nucleus supplies instructions, ribosomes build the protein, the ER and Golgi fold, modify, and package it, and vesicles deliver it. Meanwhile, mitochondria supply the ATP that powers many of these steps, which is why the same ATP you studied in Unit 5 appears again here. Dividing the cell into compartments lets many different reactions go on at once without interfering with each other.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Trace a Secreted Protein

A cell in the pancreas makes a digestive enzyme (a protein) and releases it outside the cell. Use the model to list, in order, the structures the protein passes through, from the ribosome to the outside of the cell. Give the job of each step.

Show the solution

1. 1. Ribosome on the rough ER: builds the protein into the ER, where it folds.
2. 2. Transport vesicle: carries the protein from the ER to the Golgi apparatus.
3. 3. Golgi apparatus: modifies, sorts, and packages the protein.
4. 4. Secretory vesicle: carries the packaged protein to the cell membrane.
5. 5. Cell membrane: the vesicle fuses with it and releases the protein outside the cell.
6. Answer: Ribosome on rough ER, vesicle, Golgi apparatus, vesicle, then the cell membrane and out of the cell.

Example 2: Follow the Label

Scientists gave a cell radioactive amino acids for a short time, then measured where the labeled protein was over the next 2 hours (sample data). Which organelle does the protein reach first? Write a claim, evidence, and reasoning about the path it takes.

Time (min)	Rough ER (%)	Golgi (%)	Vesicles (%)	Outside (%)
3	85	10	5	0
20	25	60	15	0
40	8	30	62	0
120	3	5	22	70

Show the solution

1. Claim: the protein goes from the rough ER to the Golgi apparatus to vesicles and then out of the cell.
2. Evidence: the label peaks in the rough ER at 3 min (85%), in the Golgi at 20 min (60%), in vesicles at 40 min (62%), and outside the cell at 120 min (70%).
3. Reasoning: each location peaks later than the one before it, so the protein passes through them in that order. This matches the secretion pathway in the model.
4. Answer: The rough ER is first. The protein then moves to the Golgi, vesicles, and outside the cell.

Example 3: A Mitochondria Poison

A poison stops the mitochondria in a cell. Use the model to predict what will happen to the cell’s ability to build and move proteins.

Show the solution

1. Mitochondria: they make most of the cell’s ATP, so ATP will run low.
2. Effects: building proteins on ribosomes and moving vesicles along the cytoskeleton both use ATP, so both will slow down.
3. Result: fewer proteins will be made and delivered, and the cell will eventually die.
4. Answer: Protein building and vesicle movement slow or stop because ATP runs out.

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Practice

Match organelles to their jobs, label the path of a secreted protein, read data that follow a protein through the cell, and predict what happens when organelles fail. Try it on your own first, then check your work with the answer key.

• Organelles Practice Worksheet (PDF)
• Organelles Practice Answer Key (PDF)

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Lesson Slides

• 6.3 Organelles Slides (PDF)

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Big Ideas

• The nucleus holds DNA; ribosomes build proteins
• Rough ER, Golgi apparatus, and vesicles make and deliver proteins
• Mitochondria release usable energy as ATP; chloroplasts capture light energy
• Lysosomes, vacuoles, and the cytoskeleton store, break down, and support
• Organelles depend on one another

Back to the essential question: the nucleus sends out instructions, ribosomes build proteins, the rough ER and Golgi apparatus fold, modify, and package them, and vesicles deliver them. Mitochondria supply the ATP that powers these steps. Because each organelle depends on the others, a problem in one can stop the whole cell. Next, you’ll see how cells in a multicellular organism become specialized.

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