# Cell Structure and Function

> AP Biology · AP Biology Unit 2
> Source: https://www.owlsprep.com/study/ap-biology-u2-cell-structure-and-function/

This sub-topic covers core structural and functional features of prokaryotic and eukaryotic cells, including key organelle roles, cell size constraints, and the core biological theme of structure-function relationships.

**Prerequisites:** [Basic understanding of cell theory as the fundamental unit of life](https://www.owlsprep.com/study/ap-bio-u1-cell-theory/)

## Learning objectives

- Distinguish between prokaryotic and eukaryotic cell structure
- Relate the structure of major eukaryotic organelles to their functions
- Compare and contrast plant and animal cell features
- Explain how surface area-to-volume ratio constrains cell size

## Prokaryotic vs Eukaryotic Cells

All cells share four common core features: a plasma membrane, cytoplasm, genetic material, and ribosomes. The primary evolutionary distinction between prokaryotic and eukaryotic cells is the presence of a membrane-bound nucleus and specialized membrane-bound organelles in eukaryotes.

**Prokaryote** — Unicellular organism that lacks a membrane-bound nucleus and specialized membrane-bound organelles. Genetic material is typically a single circular chromosome in the nucleoid region.

*Example:* Bacteria and archaea are all prokaryotes

**Eukaryote** — Organism with cells that contain a membrane-bound nucleus enclosing genetic material, plus specialized membrane-bound organelles that compartmentalize cellular functions.

*Example:* Plants, animals, fungi, and protists are eukaryotes

**Worked example:** A researcher observes a new cell under a microscope. The cell has a cell wall, ribosomes, and no membrane-bound nucleus. Is this cell prokaryotic or eukaryotic? Justify your answer.

1. Recall the defining characteristic that separates the two cell types
2. The defining feature is the presence of a membrane-bound nucleus in eukaryotes, and its absence in prokaryotes. Other features like cell walls and ribosomes are found in both groups.
3. The observed cell lacks a nucleus, so it cannot be eukaryotic
4. Conclusion: The cell is prokaryotic

## Organelle Structure and Function

Eukaryotic organelles compartmentalize different cellular processes, allowing incompatible reactions to occur separately and increasing overall cellular efficiency. Almost all AP Bio exam questions for this topic ask to connect structure to function, so this link is critical to master.

**Worked example:** Explain how the structure of mitochondria supports its core function of cellular respiration.

1. State the basic structural features of mitochondria:
2. Mitochondria have two phospholipid bilayers: a smooth outer membrane, and a highly folded inner membrane called cristae. The innermost compartment is called the mitochondrial matrix.
3. Connect folding to function:
4. Folding of the inner membrane dramatically increases surface area, which provides more space for the proteins of the electron transport chain (embedded in the inner membrane) that produce ATP.
5. Add the role of the matrix:
6. The matrix contains enzymes for the citric acid cycle (Krebs cycle), compartmentalizing these reactions away from other cellular processes.
7. Conclusion: This structure maximizes ATP production, supporting efficient cellular respiration.

> **tip**
>
> Always explicitly connect structural features to function on FRQs. Exam graders look for this explicit link to award points.

## Plant vs Animal Cell Differences

Both plant and animal cells are eukaryotic, but they have key structural differences adapted to their distinct lifestyles:

- Plant cells have a rigid cellulose cell wall outside the plasma membrane for structural support
- Plant cells contain chloroplasts, the site of photosynthesis
- Mature plant cells have a large central vacuole for water storage and turgor pressure maintenance
- Animal cells have centrioles (part of the centrosome for cell division) that are absent in most plants, and lysosomes that are rare in plant cells

**Worked example:** A student observes an unknown eukaryotic cell that has a nucleus, mitochondria, and a large central vacuole. Is this most likely a plant or animal cell?

1. Recall the distinguishing organelles between plant and animal cells
2. A large central vacuole is a defining feature of mature plant cells, used to store water and maintain turgor pressure against the cell wall.
3. Animal cells only have small, scattered vacuoles, and never have a single large central vacuole
4. Conclusion: This is a plant cell

## Cell Size and Surface Area-to-Volume Ratio

Cell size is constrained by the surface area-to-volume ratio (SA/V). As a cell grows, its volume increases faster than its surface area. Since the plasma membrane exchanges nutrients and waste with the environment, a lower SA/V means less surface area to support the needs of the larger volume, making exchange inefficient.

$$\text{For a sphere: } SA = 4\pi r^2, V = \frac{4}{3}\pi r^3, \frac{SA}{V} = \frac{3}{r}$$

**Worked example:** Two spherical cells have radii of 1 μm (Cell A) and 4 μm (Cell B). Which cell has more efficient nutrient exchange across its membrane?

1. Use the SA/V ratio formula for spheres
2. $$\frac{SA}{V} = \frac{3}{r}$$
3. Calculate SA/V for each cell:
4. Cell A: $\frac{3}{1} = 3$
5. Cell B: $\frac{3}{4} = 0.75$
6. A higher SA/V ratio means more surface area per unit volume, which allows more efficient exchange of materials.
7. Conclusion: The smaller Cell A has more efficient nutrient exchange.

## Common pitfalls

- **Wrong:** Claiming prokaryotes do not have ribosomes because they lack organelles
  - Why it fails: Ribosomes are non-membrane-bound, so they are present in all living cells
  - Correct: State that all cells (prokaryotic and eukaryotic) have ribosomes to synthesize proteins
- **Wrong:** Confusing the core functions of chloroplasts and mitochondria
  - Why it fails: Both organelles produce ATP, leading to mix-ups about their primary roles
  - Correct: Remember: chloroplasts carry out photosynthesis to make glucose; mitochondria carry out cellular respiration to make ATP from glucose
- **Wrong:** Claiming larger cells have a larger surface area-to-volume ratio than smaller cells
  - Why it fails: Total surface area is larger for big cells, but the ratio of surface area to volume decreases as size increases
  - Correct: Recall that as cell radius increases, SA/V decreases, reducing exchange efficiency
- **Wrong:** Claiming only plant cells have cell walls
  - Why it fails: Cell walls are present in other groups, just made of different materials
  - Correct: Know that cell walls are found in prokaryotes, plants, fungi, and some protists, not just plants
- **Wrong:** Forgetting that ribosomes can be free or bound in eukaryotes
  - Why it fails: Many students only associate ribosomes with the rough ER
  - Correct: Remember: free cytoplasmic ribosomes make proteins for use inside the cell; bound ER ribosomes make proteins for secretion or membranes

## Cheatsheet

| Cell/Organelle | Key Features & Function |
| --- | --- |
| Prokaryote | No nucleus, no membrane-bound organelles, 0.1-10 μm |
| Animal Eukaryote | Nucleus, no cell wall, no chloroplasts, small vacuoles |
| Plant Eukaryote | Nucleus, cellulose cell wall, chloroplasts, large central vacuole |
| Mitochondria | Double membrane, site of cellular respiration, ATP production |
| Chloroplast | Double membrane, site of photosynthesis, glucose production |
| Nucleus | Encloses genetic material, site of transcription |
| Ribosome | Site of protein synthesis, found in all cells |
| Small cell | Higher SA/V = more efficient material exchange |

## What's next

Understanding cell structure and function is the foundation for all subsequent units in AP Biology, from membrane transport to cell signaling and cellular energetics. The compartmentalization of functions by organelles we covered here directly underpins how cells carry out the reactions that sustain life, and how they interact with their extracellular environment. This subtopic also emphasizes the core biological theme of structure-function relationships, which appears repeatedly across AP Bio exam questions. Next, you will build on this knowledge to learn how the plasma membrane controls what enters and exits the cell, which connects to how cells maintain homeostasis in changing conditions.

- [AP Biology Cell Size](https://www.owlsprep.com/study/ap-biology-u2-cell-size/)
- [AP Biology Plasma Membranes](https://www.owlsprep.com/study/ap-biology-u2-plasma-membranes/)
- [Membrane Permeability](https://www.owlsprep.com/study/ap-biology-u2-membrane-permeability/)

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