Unit 2 Table of Contents

  1. Introduction: Sequencing Unit 2
  2. Week 5: Surface Area and Volume, Cell Parts and Functions
  3. Week 6: Cell Membrane Structure and Function
  4. Week 7: Endomembrane System, Cellular Compartmentalization
  5. Summative Activities for Unit 2

Introduction: How to Sequence Unit 2

Unit 2 of AP Bio focuses on cell structure and function. It’s a lot of detail, but also super fun, because it’s a very lab-rich unit.

In terms of sequence, I suggest that you don’t follow the College Board.  Instead, do what I do.

  1. I start with Topic 2.3 (Cell Size), which revolves around the agar cube surface-area-to-volume lab.
  2. This is followed by Topics 2.1, 2.2, 2.10, and 2.11  Cell Structure and Function, which revolves around several class periods of microscopic observations of cells.
  3. In the second week of this unit, you can address Topics 2.4 – 2.8, Cell Membrane Structure and Function (including osmosis).
  4. Use part of a 3rd week for pulling this all together (AP Classroom, review, etc.)

Week 5, Surface Area and Volume; Cell Parts and Functions

Objectives for Cell Size (Topic 2.3)

You can see the College Board’s original objectives for Topic 2.3 (Cell Size) in their Course and Exam Description, or in my condensed version of the same document. Here are these objectives in a student-friendly form:

  1. Explain how surface area-to-volume ratios affect the ability of biological systems (cells, organisms, and groups of organisms) to obtain resources; eliminate wastes; or absorb or dissipate heat or other forms of energy from the environment.
  2. Explain how membrane surface area influences the size and shape of cells and organisms.
    1. Explain why smaller size enables cells to increase their surface area-to-volume ratio to more efficiently exchange materials and energy with their environment.
    2. Explain how, as cells and organisms increase in size, their surface area-to-volume ratio decreases, which affects properties like diffusion of nutrients and wastes or heat exchange with the environment.
    3. Explain how various adaptations can increase or decrease surface area-to-volume ratios. These adaptations and strategies can be used to increase the efficiency of, or limit the exchange of, materials or energy with the environment

Illustrative examples include root hair cells, the surface folds (microvilli) of gut epithelial cells, the folds (villi of the gut epithelium), the shape of leaves, etc. When you teach cellular respiration and photosynthesis in Unit 3, you can also talk about how the inner membrane of mitochondria is also an adaptation for increasing surface area, as is the flattened shape of the thylakoid sacs within chloroplasts.

Start with the Cell Size/Surface Area-to-Volume Lab

To teach about surface area-to-volume relationships,  I strongly suggest that you do the surface area-to-volume lab first. After the lab, use the tutorial to consolidate what’s learned in the lab.  In this lab your students will use phenolphthalein agar cubes, cut to various sizes, and measure that amount of diffusion that happens in a given period of time. The lab is great, and it’ll communicate the concepts related to the lab in a very powerful (and very fun) way.

Since you’ve just taught about standard error, there’s a great opportunity for a lab extension (which is not part of the handout). Collect measured data from multiple groups, work out an average, calculate standard error, and graph the data with error bars. It’ll be interesting to see (and discuss) how observed values differ from expected values.

Then do the tutorial on Learn-Biology.com

Once you’ve done the lab, my tutorial, Surface Area, Volume, and Life, will help your students consolidate their learning. The tutorial includes an embedded video, and it’s supported by this student learning guide.

Completing the activities in the cell size lab and going over it can take up to two class periods. That leaves you the rest of the week for Cell Structure and Function (Topics 2.1 and 2.2).

Cell Structure and Function learning objectives

Here’s the thinking that went into the suggested objectives below (as always, if you want to see the original objectives, you can consult the College Board’s Course and Exam Description, or my condensed version of the objectives).

  • The College Board put a lot of detail about chloroplasts and mitochondria into topic 2.2. My suggestion (and the approach I follow in my tutorials) is to save this detail until unit 3, when you can teach about the structure of mitochondria and chloroplasts in relationship to their roles in cellular respiration and photosynthesis, respectively.
  • Some cell parts (nucleus, cytoplasm, membrane) were left out of the College Board’s list. I’ve added them.
  • My students have never taken any biology at the high school level (we’re a physics-first school: my students are mostly juniors who have taken physics in 9th grade and chemistry in 10th grade). So I’m throwing some very general objectives about cells into the list below.

With that in mind, here are my suggested objectives for Topics 2.1 and 2.2, Cell Structure and Function.

  1. Explain the basic ideas of the cell theory (cells are the basic units of life, all living things are made of cells, and cells come from other cells).
  2. Define the term “organelle.”
  3. Compare and contrast the basic features of prokaryotic and eukaryotic cells.
  4. Describe the structure and functions of the following cell parts
    1. nucleus
    2. cytoplasm
    3. cell membrane
    4. ribosomes
    5. Rough endoplasmic reticulum
    6. Smooth endoplasmic reticulum
    7. Golgi complex
    8. mitochondria
    9. vacuoles
    10. Chloroplasts
    11. cell wall

Cell Structure and Function Tutorials on Learn-Biology.com

To teach the material above, use these tutorials, supported by this student learning guide (a continuation of the same on you used for Surface Area-to-Volume).

  1. Introduction to cells
  2. Animal Cells: Parts and Functions
  3. Plant Cell Parts and Functions

Other resources for teaching about cell structure and function

The highlight of this topic is viewing cells through a microscope. Here’s the lab handout that I use.

When teaching about cells, it can be difficult to to give students a sense of how dynamic cells are. To communicate that, I like to start with this excerpt from Bill Bryson’s A Short History of Nearly Everything (which is a fabulous survey of pretty much all of science: I can’t’ recommend it highly enough).

After my students do the tutorials, I do a lot of careful checking for understanding. That involves interspersing readings from this outline (a continuation of the Bill Bryson reading above) with images and learning tasks from a google slideshow that you can access in this folder. Note that we’ll be using the same handout next week.

Week 6: Cell Membrane Structure and Function


Teaching about membranes is simultaneously fun and challenging. The fun is that there are a bunch of labs that usually work (meaning that they yield pretty clean, understandable results). The challenge is that understanding how membranes work requires some molecular imagination on the part of students. That can be difficult without giving students a lot of practice with visual representations of membranes, and that’s precisely where the tutorials on Learn-Biology.com can be of help.

Learning Objectives for Topics 2.4 – 2.8 (Cell Membrane Structure and Function)

To see the original learning objectives and recommended essential knowledge for these topics, you can consult the College Board’s Course and Exam Description, or my condensed version of that document. In what’s below, I’ve boiled these down into a more teacher and student friendly form.

Note that my curriculum has a separate tutorial about water transport in plants. This tutorial is connected to a lab on transpiration, and that’s where I do most of my teaching about water potential. You can do that lab now, but I usually do it a bit later in the year (when the broccoli seeds I had my students plant during week 2 have grown large enough to do a whole plant transpiration lab).

  1. Describe the fluid mosaic model of the cell membrane. Your description should include
    1. the overall function of the membrane
    2. phospholipids (and how their structure results in the formation of phospholipid bilayers.
    3. embedded proteins (how they fit into the bilayer, and their various roles)
    4. the functions of cholesterol, glycolipids, and glycoproteins.
  2. Define the term “selective permeability,” and explain how selective permeability arises from the fluid mosaic structure of the membrane. Your explanation should include
    1. how small, nonpolar molecules like N2, CO2, and O2 can pass across the membrane (simple diffusion)
    2. how ions and large polar molecules move across the membrane (facilitated diffusion)
    3. how small polar molecules (like water) pass through the membrane (also addressed below).
  3. Compare and contrast passive transport, active transport, and facilitated diffusion. Connect each process to membrane structure (how protein pumps use energy for active transport; the role of protein channels in facilitated diffusion).
  4. Compare and contrast endocytosis and exocytosis.
  5. Explain membrane potential, and connect it to processes such as ATP synthesis and nerve impulses.
  6. Define the term osmosis, and be able to predict and explain water flow into or out of cells in hypotonic, hypertonic, and isotonic environments.
  7. Explain the movement of water into or out of cells (and entire organisms) in relationship to water potential (but see my note above about transpiration). This involves teaching two water potential equations:
    1. the general water potential equation (Ψ = ΨS + ΨP : water potential = pressure potential + solute potential)
    2. The equation for solution potential:   ΨS = – iCRT .

Membrane Structure and Function Tutorials on Learn-Biology.com

If you’ve been following this scope and sequence, then your students should have already covered the structure of phospholipids back in unit 1. A fun fact for us biology teachers is that the phospholipid bilayer is not the universal basis for cell membranes. It’s what’s found in Bacteria and Eukarya. The cell membranes found in life’s third domain, Archaea, are built from a different lipid. Hang on to that until we get to the end of unit 7 and cover the origin of life.

Here are links to membrane structure and function tutorials on Learn-Biology.com

  1. Understanding Phospholipids (tutorial, flashcards, quizzes)
  2. Membrane Structure (tutorial, flashcards, quizzes)
  3. Transport across Cell Membranes  (tutorial, flashcards, quizzes)
  4. Osmosis 1: Key Concepts Tutorial, Flashcards
  5. Osmosis 2: Osmotic Pressure Tutorial, Interactive Diagrams, Quiz

Other resources for teaching about cell membrane structure and function

Here’s a link to my version of the Diffusion and Osmosis Lab. There’s a lot of material to set up, so read through the handout carefully. Here are two tips.

  1. You’ll probably vastly increase the chance that you’ll have good results if you make up the solutions ahead of time.
  2. You can simplify the lab, reduce the setup time, and increase success by reducing the number of solutions. For example, you’d do fine with 0.0 molar, 0.33 molar, 0.67 molar, and 1.0 molar. If you do that, make sure that you adjust the tables in the lab handout.

In addition, I also put together two virtual labs related to membranes and osmosis. If you successfully do the actual lab, they might be redundant. But just in case, here they are:

  1. Cell Membrane Model Demonstration Using Dialysis Tubing (virtual lab)
  2. Osmosis with Thistle Tubes (virtual Lab)

I have several videos that’ll help you teach this material.

  1. 1. Membranes: Structure and Function, which covers the functions of membranes, the structure of phospholipids, how phospholipid bilayers form, the fluid mosaic model, and the roles of membrane proteins, carbohydrates, and cholesterol.
  2. If you want a musical version of the same material, you can use my music video Membranes!, which also has a Karaoke version 
  3. Cell Membrane Model
  4. Osmosis with Thistle Tube Demonstration
  5. My music video Osmosis!, and its Karaoke version.

Flinn’s AP Bio level POGILs on Membrane Structure and Membrane Function.

Week 7: Endomembrane System, Cellular Compartmentalization

The last topic in Unit 2 is Cell Compartmentalization. This topic focuses on the endomembrane system of eukaryotic cells. Explaining the origins of that system involves diving into the emergence of the eukaryotic cells about 1.8 billion years ago. While Lynn Margulis wasn’t the first biologist to propose that mitochondria and chloroplasts are endosymbionts — the descendants of once-independently living bacterial cells that took up residence inside another prokaryotic (probably archaeal) cell— she was that theory’s main modern proponent.

As to how that endosymbiosis actually happened, I follow Chapter 2 of Nick Lane’s Life Ascending. If you’re reading this during the school year, you might have to save reading that fabulous book about “the ten great inventions of evolution” for your next vacation, but I strongly recommend it.

The basic idea is that two ancient prokaryotic cells (1 and 2 below) lived in close association, each consuming each other’s metabolic waste products. Cell 1 was an archaeal cell. Cell 2 was a bacterium. At some point, the bacterial cell slipped inside of the archaeal cell (creating cell 3). Secretion of vesicles (a) from the bacterial cell led to the formation of what would become a nuclear membrane around the genetic material of the archaeal cell. Other vesicles would develop into the endomembrane system (at d) in what would become a eukaryotic cell (at 5)

For more on this you can read this article by Carl Zimmer (written for the lay public, and completely accessible to any AP Bio student). You can also watch a great video abstract of a much more detailed paper (which is unfortunately behind a paywall).

Cell Compartmentalization Learning Objectives

To see the original learning objectives and recommended essential knowledge for these topics, you can consult the College Board’s Course and Exam Description, or my condensed version of that document. In what’s below, I’ve boiled these down into a more teacher and student friendly form.

  1. Define the term “endomembrane system,” and describe that system’s overall function. Descriptions should include:
    1. Creating cellular compartments that segregate cellular functions such as hydrolysis, export of cell materials through exocytosis, import through endocytosis, and assembly of macromolecules
    2. Creating compartments with optimal conditions for enzymatic reactions
    3. Increasing surface area for membrane-bound enzymatic reactions.
  2. List and describe the function of the key membrane-bound structures found within eukaryotic cells (rough E.R., smooth E.R., Golgi, lysosomes, vacuoles, vesicles, mitochondria, and chloroplasts.
  3. Explain how compartmentalization is different in eukaryotic and prokaryotic cells
    1. Extensive compartmentalization is primarily a feature of eukaryotic cells.
    2. Prokaryotic cells do have some internal compartments (such as the thylakoids in cyanobacteria)
  4. Explain the evolutionary origins of cellular compartmentalization in eukaryotes. Explanations should include
    1. The endosymbiotic theory of the origins of mitochondria and chloroplasts
    2. The evidence for the endosymbiotic theory
    3. The origin of other endomembrane structures (ER, Golgi, etc).

Cell Compartmentalization/Endosymbiosis Tutorials on Learn-Biology.com

These tutorials are linked off of this page, or they can be accessed directly through the links below,

  1. The Endomembrane System (ER, Golgi, Lysosomes)
  2. The Evolution of Cellular Compartmentalization

Summative Activities for Unit 2

To help your students pull together what they’ve learned from unit 2…

  1. Have your students study the Unit 2 AP Bio Review Flashcards . Note that these were primarily designed for AP exam review, so some of the material relates to material that comes later in the course. Therefore, you have to prep your students to be kind to themselves as they go through these cards: they shouldn’t expect themselves to get these cards right on the first try. However, spending a few hours working on reciting answers to these 39 cards will help your students consolidate what they’ve learned, and set them up for success in concepts that they’ll learn later in the course.In order to just work on Unit 2, students need to apply the settings below.
  2. Complete the Unit 2 progress check items on AP Classroom.

Link back to Teacher’s Guide Table of Contents.