Unit 2: Cell Structure and Function

Learning Objective

You will be able to:

Explain the chemical composition of the plasma membrane (phospholipid bilayer)
Predict which regions of a protein are hydrophobic or hydrophillic based on the way it embeds in the membrane
Predict and explain whether a molecule will have some permeability, full permeability or no permeability with the plasma membrane due to its size and polarity
Understand the difference between passive diffusion and facilitated diffusion
Explain whether a cell is in a hypertonic, isotonic or hypotonic environment based on its "before and after" shape
Use the water potential equation (provided on AP reference table) to calculate missing values
Utilize the solute potential equation to find missing values

AP Bio Course and Exam Description for Unit 2.

Slides

APB_Unit02_1_Cell Structure

APB_Unit2_2_Membranes

APB_Unit2_3_Water_Potential

APB_Unit2_4_Endosymbiosis

Homework

Plant vs. Animal Homework

Organelles

A helpful organizational guide with a review slide show and video:

Cell organelles chart

APB_Unit2_Organelle Review

APB_Unit02_Lesson02_Water_Potential

Key

APB_Unit02_Lesson02_Water_Potential Key

Cell_Transport_Problems

Cell Transport Problems Key.pdf

Unit 2 Equations

Water Potential Problems F15

Water Potential Problems Key

Review Cells and Cell Transport

Lab Exercises

APB_Unit02_Lab01_Potato_Tonicity

Potato Core Osmosis

What to include in the 2.1 Write-up:

We are not testing any hypothesis. What we are doing is determining the solute concentration and potential for the potato. So if you'd like a statement of purpose for the lab, that is reasonable to include.

Procedure: This is three parts.

Create the solutions (weight out sugar, dissolve)
Cut the potatoes (push the core cutter through the potatoes, cut to the same length -- 2.5 cm)
Do the actual osmosis experiment (which is basically mass the potatoes before putting them into the solution, remove then 15 minutes later, dry them off, and remass them.)

Data is just initial masses, time, and final masses.

Calculations and Analysis

If you want to be very complete, you could include the computations for the solution concentrations. But, that was really a little chem review worked into the lab (because the assumption of the AP writers is that you know how to do those calculations, though they really don't focus on testing them.)
So the main thing that you want is a graph of % change in mass over time to study change in mass due to osmosis.
- % change in mass = (final mass - initial mass)/initial mass -- you should include one sample calculation of this in the write-up (one data point) as an example of how the points to be graphed were computed.
- Please make sure to label the axes and title the graph.
- The graph will have a linear trendline with the equation displayed.
Your analysis for the lab includes
- calculating the solute concentration in the potato (from the trendline of the graph)
- calculating the solute potential (Ψs) of the potato. Temperature in the room on that day was 21℃ (not the 25 in the handout).
- Show/explain your work for each. (Yes, this is a sentence for the first and a brief calculation for the second, but show your work!)

APB_Unit02_Lab02_Cell_Size

What to include in the 2.2 Write-up:

We did have a pre-lab hypothesis. What did you expect to see based on size?

Procedure: Pretty straightforward here.

Data is just initial size of the cube, time, and remaining pink inside the cube.

Calculations and Analysis

Again, we've got a graph on a spreadsheet. If you haven't noticed, AP Bio really spends a lot of time focusing on data analysis and this is one of the most transferable skills of the class. There are many disciplines that use spreadsheets for data presentation and analysis. And even if they use a different, more specialized, piece of software, the same sorts of analysis are done all the time. So what makes sense to graph?
- We want to show how the diffusion (of the acid into the call) varies. We'd like to to reflect at time 0 there is no diffusion and we'd like to scale it relative to the cell size.
- After playing with the data, I have found that the interested trend to look at is volume of the cell remaining. I know that the "cells" were not perfect cubes, but it is a decent enough approximation. The collected raw measurement data are below.
- Compute the volume for each cell at each time (that is volume remaining which has not been diffused into)
- Plot Volume/Initial Volume vs time (min) for each cell. You don't need to take averages, just plot all of the volumes vs. time for each cell.
- Add an exponential trendline (label should be set to equation). If you think about it, it does makes sense that penetration of the acid into the cell by diffusion behaves as an exponential decay process. As you approach equilibrium, the concentration differences are getting smaller and smaller.