AP Biology

## Learning Outcomes

- Distinguish prokaryotic cells from eukaryotic cells using structural evidence.
- Identify major eukaryotic organelles and describe their main functions.
- Explain how organelle structure supports organelle function.
- Compare plant and animal cell structures, including cell walls, chloroplasts, vacuoles, and centrioles.
- Describe how the endomembrane system produces, modifies, and transports proteins and lipids.
- Connect mitochondria and chloroplast structure to energy transformations.
- Use organelle evidence to justify predictions in AP Biology data and diagram questions.

## AP Biology Syllabus

In AP Biology Unit 2, cells are studied as living systems whose structures support transport, energy use, information processing, and homeostasis. This subtopic focuses on recognizing major cell components and explaining why their structures suit their functions. Unit 2 makes up about 10–13% of the AP Biology exam, so organelles often appear in multiple-choice questions, diagrams, and free-response prompts that ask for structure-function reasoning rather than simple recall.

- Prokaryotic cells lack a nucleus and membrane-bound organelles.
- Eukaryotic cells contain membrane-bound organelles that divide cellular work into compartments.
- All cells contain a plasma membrane, cytoplasm, genetic material, and ribosomes.
- Plant cells have a cellulose cell wall, chloroplasts, and usually a large central vacuole.
- Animal cells commonly contain centrioles, while most plant cells do not.
- Mitochondria and chloroplasts have double membranes and are linked to endosymbiotic origins.
- Internal membranes increase surface area and create specialized environments.
- Structure-function explanations are often tested through unfamiliar examples.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What structural feature separates prokaryotic cells from eukaryotic cells?
2. Which organelles are involved in making, modifying, and exporting a secreted protein?
3. Why do mitochondria and chloroplasts have folded or stacked internal membranes?
4. How do plant cells differ from animal cells in organelle structure?
5. What evidence supports the claim that mitochondria and chloroplasts originated from prokaryotic cells?

## Introduction

Cells are the basic units of life. Whether an organism is a bacterium, a fungus, a plant, or an animal, its life processes depend on cells maintaining internal conditions, using energy, storing information, and exchanging materials with the environment. AP Biology expects you to know the major structures inside cells and to explain how those structures support life processes.

The most useful rule for this topic is: structure determines function. A folded membrane, a fluid-filled compartment, a rigid wall, or a protein fiber is not just a label on a diagram. Each structure helps the cell carry out a task. Exam questions often describe a mutation, poison, disease, or experimental treatment and ask you to predict which organelle would be affected.

> **Key Term: organelle**
> A specialized subcellular structure that carries out a specific function within a cell.

### Prokaryotic and eukaryotic cell organization

The two major cell types are prokaryotic cells and eukaryotic cells. Bacteria and archaea are prokaryotes. Animals, plants, fungi, and protists are eukaryotes. The main distinction is that eukaryotic cells have membrane-bound organelles, while prokaryotic cells do not.

> **Key Term: prokaryotic cell**
> A cell that lacks a nucleus and membrane-bound organelles; its DNA is usually found in a nucleoid region.

Prokaryotic cells are generally smaller and simpler than eukaryotic cells, but they are still fully living cells. They contain cytoplasm, a plasma membrane, ribosomes, and genetic material. Their DNA is commonly one circular chromosome located in a region called the nucleoid, which is not surrounded by a nuclear membrane. Many prokaryotes also have a cell wall, often containing peptidoglycan in bacteria. Some have flagella for movement or a protective capsule outside the cell wall.

> **Key Term: eukaryotic cell**
> A cell that contains a nucleus and other membrane-bound organelles.

Eukaryotic cells are larger and more internally divided. Their organelles allow different cellular processes to occur in separate compartments. This separation helps a eukaryotic cell control chemical conditions. For example, lysosomes can maintain an acidic interior for digestion, while the cytosol remains closer to neutral pH. Mitochondria can build proton gradients across internal membranes, while the rest of the cell continues other reactions.

All cells share certain basic structures:

- **Plasma membrane:** selectively controls movement into and out of the cell.
- **Cytoplasm:** fluid-filled interior containing dissolved molecules and cell structures.
- **Genetic material:** DNA stores hereditary information.
- **Ribosomes:** sites of protein synthesis.

The plasma membrane is made mainly of a phospholipid bilayer with proteins. Its hydrophobic interior blocks many charged or polar substances, while small nonpolar molecules such as oxygen and carbon dioxide can cross more easily. Although membrane transport is a separate AP Unit 2 focus, it connects closely to organelles because every membrane-bound organelle uses membranes to create controlled internal conditions.

> **Test Tip:**
> If a question asks whether a cell is prokaryotic or eukaryotic, look for a nucleus or membrane-bound organelles. Do not classify a cell as eukaryotic just because it has DNA, ribosomes, cytoplasm, or a plasma membrane; all cells have those.

### The genetic information and protein production pathway

The nucleus is the control center for genetic information in eukaryotic cells. It contains most of the cell’s DNA and is surrounded by a double membrane called the nuclear envelope. Nuclear pores regulate movement of RNA and proteins between the nucleus and cytoplasm.

> **Key Term: nucleus**
> A membrane-bound organelle in eukaryotic cells that contains most of the cell’s DNA.

The nucleolus, located inside the nucleus, produces ribosomal RNA and helps assemble ribosomal subunits. These subunits leave the nucleus and function in the cytoplasm as ribosomes.

> **Key Term: ribosome**
> A cellular structure made of RNA and protein that synthesizes polypeptides during translation.

Ribosomes may be free in the cytosol or attached to rough endoplasmic reticulum. Free ribosomes usually make proteins that function in the cytosol. Bound ribosomes usually make proteins that will be secreted, inserted into membranes, or sent to organelles such as lysosomes. This distinction is a common AP Biology structure-function point.

The endoplasmic reticulum, or ER, is a membrane network connected to the nuclear envelope. It has two major regions:

- **Rough ER:** has ribosomes attached to its surface. It helps synthesize, fold, and begin modification of proteins destined for secretion, membranes, or certain organelles.
- **Smooth ER:** lacks attached ribosomes. It is involved in lipid synthesis, detoxification, carbohydrate metabolism, and calcium ion storage in some cells.

The Golgi apparatus receives molecules from the ER in transport vesicles. It modifies, sorts, and packages proteins and lipids. Vesicles leaving the Golgi may move to the plasma membrane for secretion, become part of the membrane, or travel to other cellular destinations.

> **Key Term: endomembrane system**
> A group of interacting membranes and organelles in eukaryotic cells, including the nuclear envelope, ER, Golgi apparatus, vesicles, lysosomes, vacuoles, and plasma membrane.

A secreted protein follows a typical pathway:

1. DNA in the nucleus is transcribed into mRNA.
2. mRNA exits through a nuclear pore.
3. A ribosome on the rough ER translates the mRNA into a polypeptide.
4. The rough ER folds and modifies the protein.
5. A vesicle carries the protein to the Golgi apparatus.
6. The Golgi further modifies, sorts, and packages the protein.
7. A vesicle moves to the plasma membrane.
8. The protein is released by exocytosis.

This pathway is highly testable because it links several organelles into one process.

> **Exam Warning:**
> Do not say that the Golgi apparatus “makes” proteins. Ribosomes synthesize polypeptides. The rough ER helps fold and modify them, and the Golgi modifies, sorts, and packages them.

Lysosomes are membrane-bound sacs containing digestive enzymes. They break down macromolecules, damaged organelles, and materials taken into the cell. Their enzymes work best in acidic conditions, so the lysosome membrane helps maintain an internal environment different from the cytosol. If a lysosomal enzyme were released into the neutral cytosol, it would usually be less active than inside the lysosome.

Peroxisomes are also involved in breakdown reactions. They detoxify certain substances and produce hydrogen peroxide as a byproduct. They contain enzymes that convert hydrogen peroxide into water and oxygen. In animals, peroxisomes are common in liver and kidney cells, where detoxification is a major function.

### Energy, storage, digestion, and plant-specific organelles

Mitochondria are the main sites of aerobic cellular respiration in eukaryotic cells. They help convert energy stored in organic molecules into ATP, the energy currency used by cells.

> **Key Term: mitochondrion**
> A double-membrane organelle that carries out cellular respiration and produces much of the ATP used by eukaryotic cells.

Mitochondria have an outer membrane and a highly folded inner membrane. The folds of the inner membrane are called cristae. These folds increase surface area for proteins involved in oxidative phosphorylation and ATP production. The matrix, the fluid-filled interior inside the inner membrane, contains enzymes involved in respiration, as well as mitochondrial DNA and ribosomes.

This is an ideal AP Biology structure-function example: more inner membrane surface area allows more electron transport chain proteins and ATP synthase molecules to be embedded, which can increase ATP production capacity.

Chloroplasts are found in plants and photosynthetic algae. They carry out photosynthesis, converting light energy into chemical energy stored in sugars.

> **Key Term: chloroplast**
> A double-membrane organelle in plants and photosynthetic algae that carries out photosynthesis.

Chloroplasts contain chlorophyll, the pigment that absorbs light and gives many plants and algae their green color. Inside chloroplasts are membrane sacs called thylakoids, often stacked into grana. The thylakoid membranes contain molecules needed for the light-dependent reactions of photosynthesis. The stroma, the fluid surrounding the thylakoids, contains enzymes used in carbon fixation reactions.

Like mitochondria, chloroplasts show the benefit of internal membranes. Thylakoid membranes increase surface area for light-absorbing pigments, electron transport proteins, and ATP-producing machinery.

Mitochondria and chloroplasts are linked to the endosymbiotic theory, which proposes that they evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Over time, the engulfed cells and host cell became dependent on each other.

Evidence for this idea includes:

- Mitochondria and chloroplasts have double membranes.
- They contain their own DNA.
- They contain ribosomes.
- They reproduce by a division process similar to bacterial binary fission.
- They are similar in size to some prokaryotes.

> **Key Term: endosymbiosis**
> The theory that mitochondria and chloroplasts originated from prokaryotic cells that lived inside ancestral eukaryotic cells.

Vacuoles are membrane-bound storage compartments. They may store water, ions, food molecules, pigments, or wastes. Plant cells often have a large central vacuole that takes up much of the cell volume. This vacuole stores cell sap and helps maintain turgor pressure, the pressure of the cell contents against the cell wall.

A plant with a full central vacuole is firm. If the plant loses water, the vacuole shrinks, turgor pressure decreases, and the plant wilts. This links organelles to osmosis and homeostasis, two common AP Biology themes.

> **Test Tip:**
> When explaining plant wilting, connect water movement to the central vacuole and turgor pressure. A strong answer does not just say “the plant lacks water”; it explains that reduced water in the vacuole lowers pressure against the cell wall.

Plant cells have a cell wall outside the plasma membrane. In plants, the wall is made mainly of cellulose. Cell walls provide support, help maintain shape, and protect against osmotic swelling. Fungi also have cell walls, usually made of chitin. Bacterial cell walls differ in composition and commonly contain peptidoglycan.

> **Key Term: cell wall**
> A rigid layer outside the plasma membrane that provides structural support and protection in plants, fungi, bacteria, and some protists.

Plant and animal cells share many organelles, including nuclei, mitochondria, ribosomes, ER, Golgi apparatus, plasma membranes, cytoskeletons, and vesicles. Major differences include:

- Plant cells have cell walls; animal cells do not.
- Plant cells have chloroplasts; animal cells do not.
- Mature plant cells usually have a large central vacuole; animal cells may have smaller vacuoles.
- Animal cells commonly have centrioles; most plant cells do not.

### Cell support, movement, and surface structures

The cytoskeleton is a network of protein fibers that helps maintain cell shape, organize the cell interior, move materials, and separate chromosomes during cell division.

> **Key Term: cytoskeleton**
> A network of protein fibers that supports cell shape, movement, intracellular transport, and cell division.

Two major cytoskeletal components emphasized in AP Biology are microtubules and microfilaments.

Microtubules are made of tubulin protein. They help maintain cell shape, form tracks for organelle movement, and build spindle fibers during cell division. They are also major components of cilia and flagella. In animal cells, centrioles are found in microtubule-organizing centers and help organize the spindle during cell division. Most plant cells do not contain centrioles.

Microfilaments are made of actin. They are involved in cell movement, changes in cell shape, muscle contraction, cytoplasmic streaming, and cytokinesis. During cytokinesis in animal cells, actin-containing microfilaments help form the cleavage furrow that divides the cytoplasm.

Cilia and flagella are extensions involved in movement. In single-celled organisms, they may move the whole cell. For example, many protists use cilia or flagella to move through water. In multicellular organisms, cilia may move substances across tissues. Human respiratory tract cells have cilia that help move mucus and trapped particles away from the lungs. Sperm cells use a flagellum for movement.

Although prokaryotic and eukaryotic flagella can both move cells, they are structurally different. AP Biology questions usually focus more on function and cell type than on detailed molecular differences.

Surface area is another recurring structure-function theme. Cells and organelles often use folds, stacks, branches, or thin shapes to increase surface area. Examples include:

- Cristae in mitochondria.
- Thylakoid membranes in chloroplasts.
- Extensive ER membrane networks.
- Microvilli on certain animal cells.
- Root hairs in plants.

Greater surface area allows more exchange, more membrane proteins, or more reaction sites. However, cell volume increases faster than surface area as a cell grows, which limits cell size. This is why small cells generally exchange materials more efficiently than large cells.

> **Exam Warning:**
> Avoid vague answers such as “folding helps the organelle work better.” State the mechanism: folding increases surface area, which allows more membrane-bound proteins or more exchange sites, increasing the rate or capacity of the process.

Compartmentalization is one of the main advantages of eukaryotic organization. Membrane-bound organelles allow different reactions to occur at the same time in different parts of the cell. They also separate incompatible processes, concentrate enzymes and substrates, and create local conditions such as low pH or ion gradients.

For example, the lysosome can maintain acidic conditions for digestion, while mitochondria can maintain proton gradients for ATP synthesis. The nucleus protects and organizes DNA, while ribosomes in the cytoplasm translate mRNA. The ER and Golgi direct proteins through a controlled production and shipping pathway.

AP Biology rarely rewards memorizing organelle lists alone. Instead, strong answers connect an organelle to a biological process, then connect its structure to that process. If a prompt asks about a cell that secretes large amounts of protein, predict abundant rough ER, Golgi apparatus, vesicles, and mitochondria. If a cell carries out high rates of photosynthesis, predict many chloroplasts with extensive thylakoid membranes. If a cell detoxifies chemicals, predict abundant smooth ER or peroxisomes, depending on the context.

## Key Point Checklist

This article has covered the following key knowledge points:

- Prokaryotic cells lack a nucleus and membrane-bound organelles.
- Eukaryotic cells contain membrane-bound organelles that allow compartmentalization.
- All cells contain a plasma membrane, cytoplasm, genetic material, and ribosomes.
- The nucleus stores most eukaryotic DNA, and the nucleolus helps assemble ribosomal subunits.
- Ribosomes synthesize polypeptides; bound ribosomes usually make secreted, membrane, or lysosomal proteins.
- Rough ER, smooth ER, Golgi apparatus, vesicles, lysosomes, and the plasma membrane are part of the endomembrane system.
- Mitochondria produce ATP during cellular respiration, and cristae increase inner membrane surface area.
- Chloroplasts carry out photosynthesis, and thylakoid membranes contain light-absorbing and electron transport components.
- Plant cells have cell walls, chloroplasts, and usually a large central vacuole; animal cells commonly have centrioles.
- Central vacuoles help maintain plant turgor pressure.
- The cytoskeleton supports cell shape, movement, transport, and chromosome separation.
- Mitochondria and chloroplasts support endosymbiotic theory because they have double membranes, DNA, ribosomes, and divide similarly to prokaryotes.

## Key Terms and Concepts

- organelle
- prokaryotic cell
- eukaryotic cell
- nucleus
- ribosome
- endomembrane system
- mitochondrion
- chloroplast
- endosymbiosis
- cell wall
- cytoskeleton


Cell structure and function - Major organelles and their functions