AP Biology

## Learning Outcomes

- Distinguish prokaryotic cells from eukaryotic cells using nucleus, organelles, DNA organization, and size.
- Identify structures shared by all cells: plasma membrane, cytosol, genetic material, and ribosomes.
- Describe the nucleoid, plasmids, cell wall, capsule, and flagella in prokaryotic cells.
- Explain how membrane-bound organelles allow compartmentalization in eukaryotic cells.
- Compare plant, animal, fungal, protist, bacterial, and archaeal cell features.
- Use evidence supporting the endosymbiotic origin of mitochondria and chloroplasts.
- Avoid common AP Biology mistakes about ribosomes, cell walls, and nuclei.

## AP Biology Syllabus

For AP Biology Unit 2, you should be able to connect cell structure with cell function. Prokaryotic versus eukaryotic cells is a high-yield comparison because it tests whether you can identify what all cells share, what only eukaryotic cells contain, and how internal organization affects cellular processes.

- All cells have a plasma membrane, cytosol, genetic material, and ribosomes.
- Prokaryotic cells lack a nucleus and lack membrane-bound organelles.
- Eukaryotic cells have a membrane-bound nucleus and many membrane-bound organelles.
- Bacteria and archaea are prokaryotic; animals, plants, fungi, and protists are eukaryotic.
- Prokaryotic DNA is usually circular and located in a nucleoid region; eukaryotic DNA is organized into linear chromosomes inside a nucleus.
- Ribosomes occur in both prokaryotic and eukaryotic cells, but their structures differ slightly.
- Mitochondria and chloroplasts provide evidence for endosymbiosis.
- Cell size and surface area-to-volume ratio affect exchange of materials across membranes.

## 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 four basic structures are found in both prokaryotic and eukaryotic cells?
2. Why is the nucleoid of a bacterial cell not the same as a nucleus?
3. Which cell type contains membrane-bound organelles: prokaryotic, eukaryotic, or both?
4. What evidence supports the idea that mitochondria and chloroplasts were once independent prokaryotic cells?
5. Why would a dye that stains nuclei distinguish eukaryotic cells from prokaryotic cells?

## Introduction

Cells are the basic units of life, but not all cells are organized in the same way. The two major cell types are prokaryotic and eukaryotic. Prokaryotic cells are generally smaller and structurally simpler. Eukaryotic cells are generally larger and contain internal membrane-bound compartments called organelles.

This comparison appears often in AP Biology because it links structure to function. A cell with a nucleus can separate DNA storage and transcription from many cytoplasmic activities. A cell with mitochondria, chloroplasts, endoplasmic reticulum, or Golgi bodies can carry out specialized reactions in separate compartments. A prokaryotic cell, despite lacking these compartments, can still perform all basic life processes.

> **Key Term: prokaryotic cell**
> A cell type found in bacteria and archaea that lacks a nucleus and lacks membrane-bound organelles.
>
> **Key Term: eukaryotic cell**
> A cell type found in animals, plants, fungi, and protists that contains a nucleus and membrane-bound organelles.

### Features Shared by All Cells

Although prokaryotic and eukaryotic cells differ in complexity, they share several core features. These shared structures are strong evidence for common ancestry among living organisms.

All cells contain genetic material. DNA stores hereditary information and provides instructions for making RNA and proteins. In prokaryotes, DNA is usually found as a circular chromosome in the nucleoid region. In eukaryotes, DNA is packaged into linear chromosomes inside the nucleus.

All cells contain ribosomes. Ribosomes are the sites of protein synthesis, where amino acids are assembled into polypeptide chains according to messenger RNA instructions. Ribosomes are made of ribosomal RNA and proteins. Prokaryotic and eukaryotic ribosomes both have a large subunit and a small subunit, but the exact sizes and molecular components differ.

All cells contain cytosol, the fluid portion of the cytoplasm where many biochemical reactions occur. The term cytoplasm includes the cytosol plus cellular contents inside the plasma membrane. In eukaryotic cells, cytoplasm generally refers to the region outside the nucleus.

All cells are enclosed by a plasma membrane. The plasma membrane is a phospholipid bilayer with associated proteins. It regulates movement of substances into and out of the cell. Small hydrophobic molecules such as oxygen and carbon dioxide can pass through the hydrophobic interior of the membrane relatively easily, while large or charged substances usually require transport proteins or vesicle-based transport.

> **Key Term: ribosome**
> A cellular structure made of ribosomal RNA and proteins that carries out protein synthesis.
>
> **Key Term: plasma membrane**
> The selectively permeable phospholipid bilayer surrounding a cell that regulates exchange with the environment.
>
> **Test Tip:**
> If an AP question asks what all cells have, do not choose “nucleus,” “mitochondria,” or “membrane-bound organelles.” The safer universal features are plasma membrane, cytosol, DNA, and ribosomes.

Cell size also matters. Smaller cells usually have a more favorable surface area-to-volume ratio. As a cell gets larger, its volume increases faster than its surface area. Since exchange with the environment occurs across the plasma membrane, a cell must maintain enough membrane surface area to meet its needs for nutrient uptake, waste removal, and gas exchange. Prokaryotic cells are often small, which helps them exchange materials efficiently despite lacking many internal compartments.

> **Key Term: surface area-to-volume ratio**
> The relationship between a cell’s outer surface area and its internal volume; smaller cells generally exchange materials more efficiently.

### Prokaryotic Cells: Small Cells Without Internal Membrane-Bound Compartments

Prokaryotic cells include bacteria and archaea. They are typically much smaller than eukaryotic cells and have a simpler internal organization. The key AP Biology distinction is that prokaryotes do not contain a nucleus or membrane-bound organelles. Their only major membrane is the plasma membrane, though some prokaryotes have internal membrane folds or specialized regions that are not the same as eukaryotic organelles.

The DNA of a prokaryotic cell is usually one continuous, circular chromosome. It is located in a region called the nucleoid. The nucleoid is not surrounded by a membrane, so it is not a nucleus. Many bacteria also contain plasmids, which are small circular DNA molecules separate from the main chromosome. Plasmids may carry genes that provide advantages, such as antibiotic resistance, and they can sometimes be transferred between bacteria.

> **Key Term: nucleoid**
> The non-membrane-bound region of a prokaryotic cell where the main chromosome is located.
>
> **Key Term: plasmid**
> A small, usually circular DNA molecule found in some prokaryotes in addition to the main chromosome.

Many prokaryotes have cell walls outside the plasma membrane. In bacteria, the wall commonly contains peptidoglycan, a polymer made of sugars and peptides. This feature matters on exams because some antibiotics target bacterial cell walls or bacterial ribosomes—structures that differ from those of animal cells. Archaea may have cell walls too, but their wall chemistry differs from bacterial peptidoglycan.

Some bacteria have a capsule outside the cell wall. A capsule is a thick protective outer layer that can help the cell resist drying out, avoid host immune defenses, or attach to surfaces. Some prokaryotes also have flagella, long projections used for movement. Bacterial flagella are structurally different from eukaryotic flagella, even though both can contribute to motility.

> **Key Term: cell wall**
> A rigid layer outside the plasma membrane that provides support and protection in many bacteria, plants, fungi, and some protists.
>
> **Key Term: flagellum**
> A long cellular projection used for movement in some prokaryotic and eukaryotic cells.

Because prokaryotes lack a nucleus, their genetic processes are not separated into a nuclear compartment. In bacteria, transcription and translation can occur in the same general cellular space. This differs from eukaryotic cells, where transcription occurs in the nucleus and translation occurs on ribosomes in the cytosol or on the rough endoplasmic reticulum.

> **Exam Warning:**
> Do not describe the nucleoid as a “primitive nucleus.” A nucleus is membrane-bound. A nucleoid is a DNA-containing region with no surrounding membrane.

A useful AP-style example involves cell staining. If a student has one dye that stains nuclei and another that stains cytosol, the nuclear dye is better for distinguishing eukaryotic cells from prokaryotic cells. Both cell types contain cytosol, but only eukaryotic cells contain nuclei.

### Eukaryotic Cells: Nuclei, Organelles, and Compartmentalization

Eukaryotic cells include animal, plant, fungal, and protist cells. They are more internally organized than prokaryotic cells because they contain membrane-bound organelles. These organelles create separate spaces for different functions. This separation is called compartmentalization.

> **Key Term: organelle**
> A specialized structure within a cell; in eukaryotes, many organelles are membrane-bound compartments with specific functions.
>
> **Key Term: compartmentalization**
> The organization of a eukaryotic cell into internal compartments that allow different processes to occur in specialized locations.

The nucleus is the defining eukaryotic structure. It contains the cell’s DNA, which is arranged into linear chromosomes. The nuclear envelope separates the DNA from the cytoplasm. The nucleus also contains the nucleolus, where ribosomal RNA is produced and ribosomal subunits begin to assemble.

The endoplasmic reticulum, or ER, is a network of membrane channels. Rough ER has ribosomes attached to its surface and is involved in synthesis of proteins that may be secreted, inserted into membranes, or sent to certain organelles. Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification of harmful substances.

The Golgi complex is a stack of flattened membrane sacs. It modifies, sorts, and packages proteins and other molecules into vesicles for transport. If a cell cannot modify and package proteins for secretion, the Golgi complex is a likely structure to examine.

Lysosomes are membrane-bound sacs containing hydrolytic enzymes. They digest macromolecules, break down worn-out cell parts, and help destroy bacteria or viruses that enter the cell. A disease involving failure to break down large molecules may point to lysosome malfunction.

Vacuoles are membrane-bound storage compartments. They may store food, water, wastes, salts, or pigments. Plant cells usually contain a large central vacuole. When the central vacuole is full of water, it helps produce turgor pressure, supporting the plant cell. When plant cells lose water, vacuoles shrink and the plant may wilt.

Mitochondria are sites of ATP production through cellular respiration. Cells with high energy demand, such as muscle cells, often contain many mitochondria. Chloroplasts are found in plants and photosynthetic algae. They contain chlorophyll and carry out photosynthesis.

Plant and animal cells share many eukaryotic organelles, but they are not identical. Plant cells have a cellulose cell wall, chloroplasts, and a large central vacuole. Animal cells lack a cell wall and chloroplasts and commonly contain centrioles, which help organize microtubules during cell division. Fungal cells are eukaryotic and usually have cell walls made of chitin, not cellulose.

A compact comparison helps organize these differences:

| Feature                   | Prokaryotic cells | Plant cells        | Animal cells       |
| ------------------------- | ----------------- | ------------------ | ------------------ |
| Nucleus                   | No                | Yes                | Yes                |
| Membrane-bound organelles | No                | Yes                | Yes                |
| Ribosomes                 | Yes               | Yes                | Yes                |
| Plasma membrane           | Yes               | Yes                | Yes                |
| Cell wall                 | Usually yes       | Yes                | No                 |
| Main DNA form             | Usually circular  | Linear chromosomes | Linear chromosomes |
| Chloroplasts              | No                | Often yes          | No                 |

> **Test Tip:**
> When asked to identify a cell type from a diagram, look first for a nucleus and membrane-bound organelles. Then check for a cell wall, chloroplasts, and a large central vacuole to distinguish plant cells from animal cells.

### Endosymbiosis and High-Yield Comparisons

The endosymbiosis hypothesis explains the origin of mitochondria and chloroplasts in eukaryotic cells. According to this idea, these organelles were once independent prokaryotic cells. An ancestral eukaryotic cell engulfed them, and over evolutionary time they became permanent parts of the host cell.

> **Key Term: endosymbiosis**
> The theory that mitochondria and chloroplasts originated from formerly free-living prokaryotic cells that became permanent residents inside ancestral eukaryotic cells.

Several lines of evidence support endosymbiosis. Mitochondria and chloroplasts have their own circular DNA. They contain their own ribosomes. They reproduce by a process similar to binary fission, the way many prokaryotes divide. They also have double membranes, which fits the idea that one cell engulfed another. Their ribosomes resemble prokaryotic ribosomes more than eukaryotic cytosolic ribosomes.

Be careful with wording. “Mitochondria and chloroplasts are found in all eukaryotic cells” is not good evidence for endosymbiosis. Chloroplasts are found in plants and photosynthetic algae, not animal cells or fungal cells. Mitochondria are present in most eukaryotic cells, but the presence or absence of an organelle in all eukaryotes is not the main evidence. The strongest evidence involves DNA, ribosomes, division, and membranes.

> **Exam Warning:**
> Do not say prokaryotes “have no organelles” without context. They lack membrane-bound organelles, but they still have ribosomes and other cellular structures. On AP questions, “membrane-bound” is often the key phrase.

Another high-yield comparison involves cell walls. Prokaryotic cells may have cell walls, but bacterial cell walls are chemically different from plant and fungal cell walls. Plant cell walls contain cellulose. Fungal cell walls contain chitin. Bacterial cell walls commonly contain peptidoglycan. Animal cells do not have cell walls.

Ribosomes are another common trap. Both prokaryotic and eukaryotic cells have ribosomes because all cells need to synthesize proteins. However, prokaryotic ribosomes differ slightly from eukaryotic ribosomes. You do not need to memorize all subunit details for AP Biology, but you should know that the difference exists. This difference helps explain why some antibiotics can target bacterial protein synthesis without directly targeting eukaryotic cytosolic ribosomes in the same way.

Surface area also connects to the prokaryote-eukaryote comparison. Prokaryotic cells can remain small and exchange materials efficiently across the plasma membrane. Eukaryotic cells are often larger, but internal membranes increase the surface area available for biochemical processes. For example, folded mitochondrial inner membranes provide more area for reactions involved in ATP production, and ER membranes provide large surfaces for protein and lipid-related processes.

A strong AP Biology answer should always link structure to function. Do not merely list that eukaryotic cells have organelles. Explain that organelles allow different chemical environments, separation of incompatible processes, increased membrane surface area, and more regulated movement of molecules inside the cell. Likewise, do not merely state that prokaryotes are simpler. Explain that their small size and lack of internal membrane-bound compartments still allow rapid exchange, growth, and reproduction.

## Key Point Checklist

This article has covered the following key knowledge points:

- Prokaryotic cells include bacteria and archaea; eukaryotic cells include animals, plants, fungi, and protists.
- All cells contain genetic material, ribosomes, cytosol, and a plasma membrane.
- Prokaryotic cells lack a nucleus and lack membrane-bound organelles.
- Prokaryotic DNA is usually circular and located in the nucleoid region.
- Some bacteria contain plasmids, which are small circular DNA molecules separate from the main chromosome.
- Eukaryotic cells contain a nucleus with linear chromosomes.
- Eukaryotic cells use membrane-bound organelles for compartmentalization.
- Ribosomes occur in both prokaryotic and eukaryotic cells, but their structures differ.
- Plant cells have cellulose cell walls, chloroplasts, and a large central vacuole; animal cells do not.
- Bacterial cell walls commonly contain peptidoglycan, while fungal cell walls contain chitin.
- Mitochondria and chloroplasts support endosymbiosis because they have circular DNA, ribosomes, and divide in a prokaryote-like way.
- Smaller cells generally have a more favorable surface area-to-volume ratio for exchange.

## Key Terms and Concepts

- prokaryotic cell
- eukaryotic cell
- ribosome
- plasma membrane
- surface area-to-volume ratio
- nucleoid
- plasmid
- cell wall
- flagellum
- organelle
- compartmentalization
- endosymbiosis


Cell structure and function - Prokaryotic versus eukaryotic cells