Unit 2 Table of Contents

Introduction: Sequencing Unit 2
Week 5: Surface Area and Volume, Cell Parts and Functions
Week 6: Cell Parts and Functions Continued, Membrane Structure
Week 7: Diffusion and Osmosis Labs
Summative Activities for Unit 2

Essential Links

The College Board’s AP Bio Course and Exam Description.
My Condensed Version of the Course and Exam Description: takes the objectives, Enduring understandings, and key ideas of the 230-page CED and renders it into 40 pages.
My AP Exam Review Outline: Takes the Course and Exam description and renders it into student (and teacher) friendly language.
2023-24 AP Bio Scope and Sequence Calendar: A spreadsheet that lays out the entire course.

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.

I start with Topic 2.3 (Cell Size), which revolves around the agar cube surface-area-to-volume lab.
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.
In the second week of this unit, you can address Topics 2.4 – 2.8, Cell Membrane Structure and Function (including osmosis).
Use part of a 3rd week for pulling this all together (objectives, cumulative flashcards, and quizzes; AP Classroom, 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 my condensed version of the same document (links are above). Here are these objectives in a student-friendly form:

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.
Explain how membrane surface area influences the size and shape of cells and organisms. Specifically:
1. Why are cells small?
  - Cells are small because a 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, despite the limitation described above, organisms were able to increase in size.
  - The evolution of multicellularity required various adaptations to increase internal surface area to allow for the diffusion of nutrients and wastes into or out of cells, or to control heat exchange with the environment.
3. Explain various adaptations to increase or decrease surface area-to-volume ratios.
  - Increase: branch vessels or flat surfaces to increase the efficiency of exchange.
  - Decrease: the opposite of the above.

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 the 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.

If you can’t do the lab, I’ve made a video about it that I embedded into the Surface Area: Volume tutorial on Learn-Biology.com.

Here’s a recipe for making the agar cubes. I like to start with cubes that are alkaline and have the students pour vinegar to make them clear (which is much safer than having the students pour NaoH). Note that the agar needs to set, so you’ll want to do this a day or two before you do the lab.

CAUTION: Wear a hot glove and take all needed safety precautions when you’re working with the heated agar to prevent burns.

A 12″ by 9″ baking pan will require about 2.5 liters of solution to get to a depth of 3 cm. That should be enough to cut 18 3cm x 3cm x 5 cm cubes. If you organize your lab groups into groups of 4, than that’s enough for two classes of 36 students

Prepare a 2.5% solution of agar.
- You do NOT need lab grade agar for this. Food grade agar, available on-line and at many Asian grocery stores, is much cheaper than lab-grade agar, and it’ll work perfectly.
- Mix 75 g of agar with 2.5 L of water. I’ve always used tap water for this (no need to buy distilled water).
I use a non-stick soup pot to melt the agar. Heat almost to a boil. Stir frequently. If you have one, use a heater-stirrer and a stir bar.
Remove from heat and add 10 mL of 1% phenolphthalein solution. Keep stirring.
- 1 g phenolphthalein in 100 mL 95% ethanol
Stir in enough NaOH until the agar turns bright pink. You can use almost any concentration of NaOH.
Pour the melted agar/phenolphtalein/NaOH into a baking pan to a depth of 3 cm and allow it to set (overnight).
The next day, remove the agar from the baking pan (which can be a little tricky!) and then cut it into 3cm x 3cm x 5cm pieces for distribution to your students. Keep the cubes in a dilute solution of NaOH so they don’t dry out and so they don’t lose their color.

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. You can access both, and the associated student learning guide, on the Unit 2 Main Menu.

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 same document.

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 relation 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.

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.
Compare and contrast the basic features of prokaryotic and eukaryotic cells.
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. lysosomes
9. mitochondria
10. vacuoles
11. chloroplasts
12. cell wall

Cell Structure and Function Tutorials on Learn-Biology.com

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

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 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 other cell-related that I usually put on a slideshow. Note that we’ll be using the same handout next week.

Week 6: Cell Parts and Functions continued; Starting Membrane Structure and Function

In the previous week, you taught about the parts of cells and their functions. Through the Surface Area and Volume Lab, the tutorials on Learn-Biology.com, and the Viewing Cells Lab, you’ve established why cells are small (and established the basis for understanding a host of other adaptations).

We’ll start this week by finishing cell parts and functions. What’s left is the endomembrane system of eukaryotic cells. Explaining the origins of that system involves diving into the emergence of eukaryotic cells about 1.8 billion years ago. Our understanding of that process is usually credited to Lynn Margulis. While 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. It’s definitely worthwhile to learn more about her.

As to how that endosymbiosis happened, I follow Chapter 2 of Nick Lane’s Life Ascending. Nick Lane is the best: he’s a biochemist who writes for a biology-savvy general audience. His books are perfect for high school biology teachers and college faculty. You might be too busy to read that book right now but put it on your summer reading list.

The basic idea is this: about 1.8 billion years ago, two ancient prokaryotic cells (1 and 2 below) lived in close association, each consuming the other’s metabolic waste products. Cell 1 was an archaeal cell. Cell 2 was a bacterium. At some point, the bacterial cell slipped inside 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)

Note that this is somewhat different from how things are usually presented in textbooks. Check out my tutorial to see the difference.

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 (links are above). In what’s below, I’ve boiled these down into a more teacher and student-friendly form.

Define the term “endomembrane system,” and describe that system’s overall function. Descriptions should include:
- Creating cellular compartments that segregate cellular functions such as hydrolysis, export of cell materials through exocytosis, import of materials through endocytosis, the capture of food or pathogens in phagocytosis, and assembly of macromolecules.
- Creating compartments with optimal conditions for enzymatic reactions
- Increasing surface area for membrane-bound enzymatic reactions (in the E.R. and Golgi).
List the key membrane-bound organelles found within eukaryotic cells, and describe the structure and function of each.
- The list should include rough E.R., smooth E.R., Golgi, lysosomes, vacuoles, vesicles, mitochondria, and chloroplasts.
Explain how compartmentalization is different in eukaryotic and prokaryotic cells
- Extensive compartmentalization is primarily a feature of eukaryotic cells.
- Prokaryotic cells do have some internal compartments (such as the thylakoids in cyanobacteria)
Explain the evolutionary origins of mitochondria and chloroplasts, with supporting evidence.
- Both mitochondria and chloroplasts arose through endosymbiosis. Free-living bacterial ancestors of mitochondria and chloroplasts were taken up by and took up residence inside a larger archaeal cell.
- All eukaryotic cells originate from the uptake/incorporation of mitochondria into a larger cell.
- Plants and algae originate from an early eukaryotic uptake of a photosynthetic cyanobacterium.
Describe the evidence for the endosymbiotic theory.
- Both mitochondria and chloroplasts have their bacteria-like chromosomes, with their own DNA.
- Both have a double membrane (the inner one a vestige of their own ancestral membrane; the outer one a vestige of an ancient endocytotic vesicle)
- Both reproduce themselves through binary fission.

Cell Compartmentalization/Endosymbiosis Tutorials on Learn-Biology.com

These tutorials are linked off of the Unit 2 Main Menu or they can be accessed directly through the links below,

Then on to Membranes…

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). We’ll do those next week. 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 where the tutorials on Learn-Biology.com can be of help.

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

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). That’s also when I teach about water potential (which makes much more sense in the context of a process like transpiration).

Describe the fluid mosaic model of the cell membrane. Descriptions should include
- The overall function of the membrane
- The role of phospholipids (and how their structure results in the formation of phospholipid bilayers).
- The role of embedded proteins (how they fit into the bilayer, and their various roles)
- The functions of cholesterol, glycolipids, and glycoproteins.
Define selective permeability.
Explain how selective permeability arises from the fluid mosaic structure of the membrane.
1. How small, nonpolar molecules like N₂, CO₂, and O₂ can pass across the membrane
  - through simple diffusion through the phospholipid bilayer.
2. How ions and large polar molecules move across the membrane
  - facilitated diffusion through embedded protein channels
3. How small polar molecules (like water) pass through the membrane
  - Water passes through aquaporins.
Compare and contrast passive transport, active transport, and facilitated diffusion. Connect each process to membrane structure.
- Protein pumps use energy for active transport
- Protein channels allow for facilitated diffusion.
Compare and contrast endocytosis and exocytosis.
Explain membrane potential
- A charge is created by an imbalance in ions across a membrane.
Connect membrane potential to processes such as ATP synthesis.
- Proton gradients are used to drive ATP synthesis through chemiosmosis (more on this below in Topics 3.5 and 3.6.
Define the term osmosis, and be able to predict and explain the flow of water into or out of cells in hypotonic, hypertonic, and isotonic environments.
Explain the movement of water into or out of cells (and entire organisms) in relationship to water potential
- Water always flows from high water potential to lower water potential.
Be able to understand and use (but don’t memorize) the two water potential equations
1. the general water potential equation (Ψ = Ψ_S + Ψ_P : water potential = pressure potential + solute potential)
2. The equation for solute potential: Ψ_S = – iCRT.

Membrane Structure and Function Tutorials on Learn-Biology.com

If you’ve been following our AP Bio Scope and sequence (link is above) then your students should have already covered the structure of phospholipids 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

Other Cell Membranes Related Resources and Activities

A fun way to preview what you’ll be doing next week with osmosis is a really simple (and fun) gummy bear lab. You might have already done this with your 9th-grade students (but we don’t have a 9th-grade biology class at my high school). If you’re interested, here’s the link.

Also, Flinn’s POGILs about membranes are excellent and highly recommended.

Week 7: Diffusion and Osmosis Labs; wrapping up unit 2

In week 6, you established an understanding of membrane structure and function and osmosis. Now comes a series of important labs related to diffusion and osmosis.

Diffusion and Osmosis Labs

Here’s a link to the Diffusion and Osmosis Lab. This version of a “classic” AP bio lab uses four sucrose solutions (1.0M, 0.66M, 0.34M, and 0.0M) instead of the six (1.0M, 0.8M, 0.6M, 0.4M, 0.4M, and 0.0M) that are traditionally done. I think that this simplifying move will make your students more successful, and they will learn just as much. Even with this version, there’s a lot of material to set up, so read through the handout carefully. Note that you’ll probably vastly increase the chance of good results if you make up the 0.66 and 0.33 solutions ahead of time (instead of having your students do the dilutions).

In addition, I’ve 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:

Cell Membrane Model Demonstration Using Dialysis Tubing (Virtual Lab)
Osmosis with Thistle Tubes (Virtual Lab)

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

Membranes: Structure and Function is built into the tutorial, but you might want to show it to your entire class, pausing for questions and checking for understanding along the way.
If you want a musical version of the same material, you can use my music video Membranes!, which also has a Karaoke version. You can access both from the tutorial (the link is directly above).
To reinforce concepts related to the labs, you can use these two videos.
1. Cell Membrane Model
2. Osmosis with Thistle Tube Demonstration
My music video Osmosis!, and its Karaoke version, provide a really fun way to help your students consolidate their understanding of osmosis.

Summative Activities for Unit 2

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

Have your students study the objectives for Unit 2, and work with the flashcards, multiple-choice questions, and Click-On Challenges. These are accessible on this page.
Have your students complete the Unit 2 progress check items on AP Classroom.