Skills you’ll gain in this topic:
- Describe structures and functions of organelles like the nucleus, mitochondria, and ribosomes.
- Explain how different organelles contribute to cell health and function.
- Compare prokaryotic and eukaryotic cell structures.
- Analyze the impact of organelle dysfunction on cell processes.
- Relate organelle organization to cellular activities like energy production and protein synthesis.
Subcomponents
A cell has subcomponents, or organelles that perform different jobs! Even a tiny cell has a lot going on, so it needs different organelles to do different jobs. Let's take a look at these components
Plasma Membrane
Arguably the most important component! The plasma membrane is made up of a phospholipid bilayer. This means that the membrane is made up of two lipid layers. Phospholipids also have two special properties. The head is hydrophilic, which means it likes water. The tail, on the other hand, is hydrophobic, meaning it doesn't like water. The membrane is then set up in a way so that the head is pointing towards the inside and outside of the cell (thus touching water) while the tail is nested between the sandwich of the heads.
The membrane is also known as the fluid-mosaic model, which means that the membrane is very flexible. It's also made up of different proteins along the membrane, and these proteins, which you'll learn in future units, act as transporters for things that can't go through the hydrophobic tail section.
Nucleus
The nucleus is the largest component of the cell. It's the "brain" of the cell and directs everything the cell does. It's also in charge of reproduction because it contains all the genetic information (DNA). The nucleolus is where the ribosomes are assembled within the DNA.
The nuclear envelope is a double membrane that surrounds the nucleus and is an integral part of the endomembrane system. It contains nuclear pores that regulate the transport of materials between the nucleus and cytoplasm. As part of the endomembrane system, the nuclear envelope is continuous with the endoplasmic reticulum.
Ribosomes
Ribosomes are made of primarily ribosomal RNA (rRNA). They are the site of translation and are responsible for making all of the proteins for the cell. Importantly, ribosomes are found in all forms of life—prokaryotes and eukaryotes alike—reflecting the common ancestry of all organisms. There are 2 kinds found in different locations.
Free ribosomes are in the cytosol and make proteins that stay in the cell for various functions. Bound ribosomes are attached to the rough endoplasmic reticulum and mainly make proteins for export. Ribosomes synthesize proteins according to mRNA sequences that they receive during the process of translation.
Endoplasmic Reticulum
The endoplasmic reticulum (ER), made up of two parts, serves to make other products that the cell needs, provides mechanical support to help cells maintain their shape, and plays a crucial role in intracellular transport. The smooth ER has many functions. It performs synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and stores calcium ions.
The rough ER, is called rough because it has ribosomes attached to its surface, making it "rough". It secretes proteins made by bound ribosomes and allows for compartmentalization of the cell, keeping protein synthesis and modification separate from other cellular processes. Proteins then are moved to the transitional ER, where they are wrapped in a transport vesicle to head to the Golgi apparatus.
The Endomembrane System
The endomembrane system is a group of membranes and organelles that work together to modify, package, and transport proteins, lipids, and polysaccharides. The key components of the endomembrane system include:
- Nuclear envelope - continuous with the ER, regulates transport in and out of the nucleus
- Endoplasmic reticulum (ER) - synthesizes and modifies proteins and lipids
- Golgi apparatus - further modifies, packages, and sorts molecules
- Lysosomes - digest cellular materials
- Transport vesicles - shuttle materials between endomembrane components
- Plasma membrane - the final destination for many products
Transport vesicles are crucial for moving materials between these compartments, maintaining the flow of molecules through the endomembrane system.
The Golgi Apparatus
The Golgi apparatus, shown above, modifies, stores, and sends proteins that come from the rough ER. The Golgi plays a crucial role in correctly folding newly synthesized proteins and other cellular products. Things like glycoproteins are modified in the Golgi. There are 2 sides on the Golgi apparatus, cis and trans face. Vesicles enter the Golgi apparatus via the cis face and depart via the trans face. It is in the Golgi that proteins are packaged and distributed to desired locations. These proteins are packaged in little sacs called vesicles, which are pinched off from the Golgi. The Golgi is also involved in the production of lysosomes.
Illustrative Example: Glycosylation
- Glycosylation is a critical modification that occurs in the Golgi, where sugars are added to proteins and lipids
- These chemical modifications are essential for proper protein function, stability, and signaling
- This process highlights the Golgi's role in preparing molecules for their specific cellular functions.
Mitochondria
Mitochondria have a double membrane, which is a phospholipid bilayer. This double membrane structure provides separate compartments for different metabolic reactions—the intermembrane space between the membranes and the matrix inside the inner membrane. The outer membrane is smooth, while the inner has many folds, called cristae. These folds help to increase the surface area available for the Electron Transport Chain. The inside of the inner membrane is called the mitochondrial matrix, which is the site of the Krebs Cycle. The Mitochondria creates ATP for the cell to use via cellular respiration. Because the mitochondria also have their own circular DNA, most biologists think the mitochondria was its own organism until it was swallowed by eukaryotes (us!). The structure of the mitochondria is important to know because it comes in handy with cellular respiration. The mitochondrial contains an outer membrane and inner membrane, making up its double membrane. The inner membrane consists of folds called cristae, which is where most of the ATP production happens. The fold increases surface area and thus efficiency. Inside the inner membrane is the matrix.
Lysosomes
Lysosomes hydrolyze most foods, amino acids, and other molecules. The inside of lysosome is extremely acidic. Lysosomes can digest foods by using phagocytosis or engulfing nutrients to digest them. The hydrolytic enzymes inside of the lysosome work to break down anything that comes into contact with it. The lysosome is also used to recycle and digest old or damaged parts of the cell. Think of it as the trash can of the cell! It's also in charge of apoptosis, which is programmed cell death. Essentially, the lysosome bursts, causing the acid to kill the cell.
Vacuoles
Vacuoles are large vesicles which store many different things, such as food or water. Many unicellular eukaryotes have contractile vacuoles to pump water out of the cell.
In plant cells, the large central vacuole stores water and ions, and plays a crucial role in maintaining turgor pressure—the force that keeps plant cells rigid and helps support the plant structure. When the vacuole is full of water, it pushes against the cell wall, keeping the plant upright.
Animal cells also have vacuoles, but they are much smaller and more numerous than in plant cells. These smaller vacuoles store various cellular materials and help with transport and storage functions.
Chloroplast
The chloroplast is the site of photosynthesis and is found in plants and photosynthetic algae. These organelles have a double membrane and contain green pigments called chlorophyll that allow for the absorption of photons. The chloroplast is made up of the stroma, or liquid filling of the chloroplast, and the thylakoids, flat sacs of membranes that allow for the absorption of light. The chloroplast converts light energy into chemical energy through photosynthesis.
Centrioles
Centrioles are small, cylindrical components of the cell and are mostly active during cell division. You'll learn more about the role of centrioles in the mitosis unit, but basically, it pulls apart chromosomes by producing microtubules.
Plant Cells vs. Animal Cells
Plant cells have something called cell walls, which is made out of cellulose. It's mostly a protective outer layer other than the membrane. Animal cells don't have this. In contrast, animal cells have centrioles, while plant cells do not.
When thinking about the differences of plant cells and animal cells, also try to think about prokaryotic cells. On the AP exam, there will be questions that can only be answered if you can correctly identify if the cell is an animal, plant, or prokaryotic cell. That's why key identifiers are important to memorize.
| Prokaryote 🧫 | Plant Cell 🌼 | Animal Cell 🐄 | |
|---|---|---|---|
| Cell wall | ✅ | ✅ | ❌ |
| Plasma membrane | ✅ | ✅ | ✅ |
| Nucleus | ❌ | ✅ | ✅ |
| Centrioles | ❌ | ❌ | ✅ |
| Ribosomes | ✅ | ✅ | ✅ |
Also keep in mind, a prokaryotic cell does not have any subcomponents like a lysosome or golgi body. Instead it only has a flagella, which acts like a tail to help movement. Other than that, a prokaryotic cell is pretty much an empty capsule with DNA and ribosomes.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| adenosine triphosphate | The primary energy currency of cells that powers cellular functions. |
| aerobic cellular respiration | The metabolic pathway that uses oxygen as the terminal electron acceptor to generate ATP from biological macromolecules. |
| chemical modification | Changes made to proteins in the Golgi that affect their function or cellular location. |
| chloroplasts | Specialized organelles found in plants and photosynthetic algae that contain a double membrane and serve as the location for photosynthesis. |
| double membrane | Two layers of membrane found in mitochondria and chloroplasts that create separate compartments for different cellular processes. |
| endomembrane system | A group of membrane-bound organelles and subcellular components that work together to modify, package, and transport polysaccharides, lipids, and proteins within cells. |
| endoplasmic reticulum (ER) | A membrane-bound organelle that provides mechanical support, maintains cell shape, and plays a role in intracellular transport. |
| glycosylation | A chemical modification of proteins that takes place within the Golgi and determines protein function or targeting. |
| Golgi complex | A membrane-bound organelle consisting of flattened membrane sacs that folds and chemically modifies newly synthesized proteins and packages them for trafficking. |
| hydrolytic enzyme | Enzymes found in lysosomes that break down and digest cellular materials. |
| intracellular transport | The movement of materials within a cell, facilitated by organelles like the endoplasmic reticulum. |
| lipid | Hydrophobic or amphipathic biological molecules composed primarily of carbon, hydrogen, and oxygen that store energy and form cell membranes. |
| lipid synthesis | The production of lipids, a function carried out by smooth endoplasmic reticulum. |
| lysosomes | Membrane-enclosed sacs that contain hydrolytic enzymes for digesting material and play a role in programmed cell death. |
| mitochondria | Membrane-bound organelles in eukaryotic cells that are the primary site of aerobic cellular respiration and ATP synthesis. |
| nuclear envelope | A membrane-bound component of the endomembrane system that surrounds the nucleus. |
| organelle | Membrane-bound or non-membrane-bound structures within eukaryotic cells that perform specific cellular functions. |
| photosynthesis | The series of reactions that use carbon dioxide, water, and light energy to produce carbohydrates and oxygen, allowing organisms to capture and store energy from the sun. |
| plasma membrane | The selectively permeable membrane that surrounds the cell, composed of phospholipids, proteins, and other molecules that regulate what enters and exits the cell. |
| polysaccharides | Complex carbohydrates formed by linking many monosaccharide monomers together through covalent bonds. |
| programmed cell death | Programmed cell death, a controlled process in which a cell actively participates in its own destruction. |
| protein | Macromolecules composed of amino acids linked together, containing carbon, hydrogen, oxygen, nitrogen, and often sulfur, that perform diverse functions in cells. |
| protein synthesis | The process by which ribosomes build proteins according to mRNA sequences. |
| ribosomes | Non-membrane subcellular structures composed of ribosomal RNA and protein that synthesize proteins according to messenger RNA sequences. |
| rough endoplasmic reticulum | Endoplasmic reticulum with attached ribosomes on its cytoplasmic surface; site of synthesis for proteins destined for secretion or membrane insertion. |
| smooth endoplasmic reticulum | Endoplasmic reticulum that functions in the detoxification of cells and lipid synthesis. |
| subcellular component | Structures within a cell that perform specific functions, including both membrane-bound organelles and non-membrane structures. |
| transport vesicle | Membrane-bound structures that are part of the endomembrane system and transport materials between organelles. |
| turgor pressure | The pressure maintained in plant cells by a large vacuole through nutrient and water storage. |
| vacuole | Membrane-bound sacs that store cellular materials and play various roles in plant and animal cells. |
Frequently Asked Questions
What is the endomembrane system and why is it important?
The endomembrane system is a group of membrane-bound organelles that work together to modify, package, and transport proteins, lipids, and polysaccharides inside the cell. Key parts are the nuclear envelope, rough ER (with ribosomes for protein synthesis and compartmentalization), smooth ER (lipid synthesis and detox), Golgi apparatus (folding, chemical modification like glycosylation, and packaging), transport vesicles, lysosomes (digestive enzymes and apoptosis), vacuoles (storage/turgor in plants), and the plasma membrane (export/import)—all listed in the CED (EK 2.1.A.2–A.8). It’s important because it creates compartments for different steps (increasing efficiency and control), ensures proteins are correctly folded/modified and sent to the right place, and supports intracellular transport and cell shape (EK 2.1.A.3–A.4). This is a core LO for Topic 2.1 (LO 2.1.A). For a clear study summary, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9). For broader review and practice, check Unit 2 (https://library.fiveable.me/ap-biology/unit-2) and Fiveable’s practice problems (https://library.fiveable.me/practice/ap-biology).
How do ribosomes actually make proteins from mRNA?
Ribosomes turn an mRNA sequence into a protein by reading codons (three-nucleotide words) and stitching amino acids together—that process is called translation (matches EK 2.1.A.1). A ribosome has two subunits (rRNA + protein). It binds the mRNA at the start codon (AUG). tRNAs with complementary anticodons bring specific amino acids to the ribosome’s A site; the ribosome catalyzes peptide bonds, moves the mRNA one codon, shifts the tRNA from the A to the P site, and ejects empty tRNAs at the E site. Translation continues until a stop codon signals release. Proteins made on membrane-bound ribosomes enter the rough ER for folding and processing, then go to the Golgi for modification/packaging (EKs 2.1.A.2–4). For AP review, this maps to LO 2.1.A; see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and practice problems (https://library.fiveable.me/practice/ap-biology).
What's the difference between rough ER and smooth ER?
Rough ER and smooth ER are parts of the same endoplasmic reticulum but have different structures and jobs (LO 2.1.A; EK 2.1.A.3). Rough ER has membrane-bound ribosomes on its surface, so it helps compartmentalize the cell and is a major site of protein synthesis and initial protein folding for secreted or membrane-bound proteins. Smooth ER lacks ribosomes; its functions include lipid synthesis (phospholipids, steroids in some cells), detoxification of harmful molecules, and roles in intracellular transport and helping maintain cell shape. Both contribute to the endomembrane system that modifies, packages, and ships proteins and lipids to the Golgi (EK 2.1.A.2, 2.1.A.4). For quick review, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice problems (https://library.fiveable.me/practice/ap-biology).
I'm confused about how the Golgi complex works - can someone explain it simply?
Think of the Golgi complex as the cell’s post office in the endomembrane system. Proteins made on rough ER (by ribosomes) arrive in transport vesicles and fuse with the Golgi’s cis face (the “receiving” side). As they move through the flattened sacs (cis → medial → trans), enzymes fold and chemically modify them (e.g., glycosylation), which can determine function or where the protein will go. At the trans face the Golgi sorts and packages finished proteins into vesicles that bud off and are sent to the plasma membrane, other organelles, or secreted. Key AP terms: Golgi complex (membrane-bound flattened sacs), glycosylation, packaging, transport vesicles, endomembrane system (EK 2.1.A.2, EK 2.1.A.4). For a quick review tied to the CED, check the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and practice questions (https://library.fiveable.me/practice/ap-biology).
Why do mitochondria have two membranes instead of just one?
Mitochondria have two membranes because that structure creates separate compartments needed for aerobic respiration (EK 2.1.A.5). The outer membrane is smooth and defines the organelle; the inner membrane is highly folded (cristae) to increase surface area for electron transport chains and ATP synthase. Separating the matrix (inside the inner membrane) from the intermembrane space lets the electron transport chain pump protons into the intermembrane space, building an electrochemical gradient. ATP synthase then uses that gradient across the inner membrane to make ATP—so two membranes let different metabolic steps (Krebs in the matrix, ETC and chemiosmosis on/across the inner membrane) be compartmentalized and efficient. For more on organelle structure and AP-style connections, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and practice questions (https://library.fiveable.me/practice/ap-biology).
What happens inside lysosomes and why don't they digest the whole cell?
Lysosomes are membrane-bound sacs full of hydrolytic enzymes that break down macromolecules, worn organelles, and pathogens—part of the endomembrane system that cells use to recycle materials and help trigger apoptosis (EK 2.1.A.6). They don’t digest the whole cell because of three safety features: (1) a single limiting membrane keeps enzymes separated from the cytoplasm; (2) the interior is very acidic (optimal for those enzymes) while the cytosol isn’t, so enzymes are less active if they leak; (3) many lysosomal enzymes are delivered as inactive precursors and only activated inside lysosomes after Golgi packaging. Together these structural/functional features let lysosomes do targeted digestion without destroying the cell. For extra review on organelle roles in the endomembrane system, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and more unit resources (https://library.fiveable.me/ap-biology/unit-2).
How are plant cell vacuoles different from animal cell vacuoles?
Plant and animal vacuoles are both membrane-bound sacs (part of the endomembrane system), but they differ in size, number, and main functions. In plant cells there’s usually one large central vacuole that stores water, ions, and nutrients and maintains turgor pressure to support cell shape and rigidity (EK 2.1.A.7.i). In animal cells vacuoles are smaller, more numerous, and mainly store temporary materials or transport substances (EK 2.1.A.7.ii). Functionally, the large plant vacuole also helps with waste breakdown, pigment storage, and can affect cell growth by taking up volume; animal vacuoles rarely drive cell shape or turgor. On the AP exam, expect to connect vacuole structure (size/number, membrane-bound) to those functions when explaining how subcellular components contribute to cell function (LO 2.1.A). For a quick review, see the Topic 2.1 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9), and try practice questions at Fiveable (https://library.fiveable.me/practice/ap-biology).
What's the connection between chloroplasts and photosynthesis?
Chloroplasts are the organelles where photosynthesis happens in plants and photosynthetic algae (EK 2.1.A.8). They have a double membrane and internal thylakoid membranes stacked into grana. The light reactions occur in the thylakoid membranes: chlorophyll captures light, water is split (releasing O2), and ATP and NADPH are made. The Calvin cycle happens in the stroma (the fluid inside the chloroplast): it uses ATP and NADPH to fix CO2 and build sugars. So chloroplast structure—double membrane, thylakoids, and stroma—creates compartments that separate light-driven energy capture from carbon fixation, making photosynthesis efficient (useful CED keywords: chloroplasts, thylakoids, membranes). This fits AP goals in Units 2–3 (Cells and Cellular Energetics). For a quick review, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and try practice questions at (https://library.fiveable.me/practice/ap-biology).
Why do all living things have ribosomes if they're so different from each other?
Ribosomes are in every living cell because making proteins is a universal requirement—proteins do almost all the work (structure, enzymes, channels, signals). Ribosomes are the molecular machines that read mRNA and build polypeptides from amino acids; they’re made of rRNA plus proteins (EK 2.1.A.1). Even though organisms look different, they all use proteins, so they all need ribosomes. Also, ribosomes are extremely conserved (similar across life), which supports the idea of common ancestry—a key AP idea about shared molecular machinery (LO 2.1.A). For the exam, expect questions about ribosome structure/function and where they’re found (free vs. membrane-bound on rough ER) and how that links to protein synthesis and trafficking (EK 2.1.A.3, A.4). Review this Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2). For extra practice, check Fiveable’s practice problems (https://library.fiveable.me/practice/ap-biology).
How does the ER help transport stuff around the cell?
The ER helps move materials around the cell by being the first stop in the endomembrane system. Rough ER (with membrane-bound ribosomes) synthesizes proteins and folds them into a separate compartment so they don’t mix with cytosolic proteins. It buds off transport vesicles that carry those proteins to the Golgi for modification and packaging. Smooth ER makes lipids, detoxifies, and provides membrane surface and channels that help shuttle molecules and maintain cell shape (mechanical support). Together with the nuclear envelope, Golgi, lysosomes, vacuoles, and plasma membrane, these membranes create compartments and vesicle routes that modify, sort, and deliver proteins, lipids, and polysaccharides to their destinations (EK 2.1.A.2–A.4, A.3). For a quick CED-aligned review, check the Topic 2.1 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and practice questions (https://library.fiveable.me/practice/ap-biology).
What does it mean when proteins get modified in the Golgi complex?
When proteins are “modified in the Golgi complex,” it means the Golgi chemically changes and finishes proteins that were made on the rough ER so they can work properly and get sent to the right place. Modifications include folding checks, trimming or adding sugar chains (glycosylation), adding phosphate groups, and sorting/packaging proteins into transport vesicles for secretion, the plasma membrane, or other destinations (EK 2.1.A.4; illustrative example: glycosylation). These changes can determine a protein’s final shape, stability, activity, and where it ends up in the cell. On the AP exam, expect to connect Golgi modification + packaging to protein function and trafficking (Topic 2.1). For a quick review of this whole endomembrane workflow, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and more unit resources (https://library.fiveable.me/ap-biology/unit-2). For extra practice, try problems at (https://library.fiveable.me/practice/ap-biology).
I don't understand how organelles work together - how does a protein go from ribosome to where it needs to be?
Proteins follow the “secretory pathway” from where they’re made to where they work. If a ribosome is bound to the rough ER, the new protein is pushed into the ER lumen as it’s made. In the ER it folds and gets initial modifications (and the rough ER helps with intracellular transport). Vesicles pinch off and carry the protein to the Golgi apparatus, where it’s further folded/modified (e.g., glycosylation) and packaged. The Golgi sorts proteins into different transport vesicles that deliver them to the plasma membrane (for secretion or membrane insertion), to lysosomes, or to other places. Non-membrane ribosomes make proteins that usually stay in the cytosol. This pathway—ribosomes → rough ER → transport vesicles → Golgi → final vesicles—is exactly what the CED emphasizes (rough ER, Golgi, transport vesicles, glycosylation, plasma membrane, lysosomes). For a clear AP-level summary and practice, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and try related practice problems (https://library.fiveable.me/practice/ap-biology).
Why are mitochondrial inner membranes folded and wrinkled?
Mitochondrial inner membranes are folded into cristae to increase surface area so the cell can make more ATP efficiently. The folds provide lots more membrane space for the electron transport chain complexes and ATP synthase enzymes that drive oxidative phosphorylation. More surface area means more places to pump protons into the intermembrane space and more ATP synthase units to let protons flow back into the matrix—so higher ATP production per mitochondrion. The double-membrane also creates separate compartments (intermembrane space and matrix) so different steps of aerobic respiration can be localized and controlled (EK 2.1.A.5). For more on organelle structure/function and AP-style review, check the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and practice questions (https://library.fiveable.me/practice/ap-biology).
What would happen to a cell if its lysosomes stopped working?
Lysosomes are membrane-enclosed sacs full of hydrolytic enzymes that digest macromolecules and damaged organelles (CED EK 2.1.A.6). If lysosomes stopped working, the cell would accumulate undigested material and worn-out organelles, disrupting intracellular recycling and the endomembrane system (ER → Golgi → lysosome). That buildup impairs cellular homeostasis, can block normal trafficking/turnover of proteins and lipids, and reduces programmed cell death (apoptosis) signaling; over time the cell can become dysfunctional and may die by necrosis. Clinically, nonworking lysosomes cause lysosomal storage disorders where specific substrates accumulate. For AP exam connections, this links directly to LO 2.1.A (structure ↔ function) and may appear in conceptual or free-response questions about organelle roles and consequences of disruptions. For a quick review, see the Topic 2.1 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9) and Unit 2 resources (https://library.fiveable.me/ap-biology/unit-2).
How do cells maintain their shape without a skeleton like we have?
Cells don’t need a bone skeleton because they use internal structures to hold shape and resist forces. The endoplasmic reticulum (especially rough ER) helps provide mechanical support and compartmentalization (EK 2.1.A.3). More importantly, the cytoskeleton—a dynamic network of microfilaments (actin), microtubules, and intermediate filaments—physically supports the cell, maintains shape, anchors organelles, and enables movement and intracellular transport. Plant cells also rely on a large central vacuole that creates turgor pressure to press the cytoplasm against the cell wall, keeping the cell rigid (EK 2.1.A.7.i). Membranes (plasma membrane + endomembrane system) and organelle placements further stabilize cell architecture (EK 2.1.A.2). This stuff is fair game on Topic 2.1 (LO 2.1.A), so review the CED terms and examples in the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9). For more practice, check the AP problems on Fiveable (https://library.fiveable.me/practice/ap-biology).