Cell Basics

Cells are the basic unit of all living organisms or are living organisms in and of themselves. They come in various sizes, shapes, and carry out a massive range of functions. From the nervous system to the microorganisms that inhabit our skin they abound with cells.

They are surrounded by a membrane that separates their inner compartment from the outer compartment. This separation allows cells to carry out and control a wide variety of chemical reactions. This includes metabolic reactions that allow the cell to sustain itself, the reactions of central dogma that direct the cell and tell it what to do, and even reaction that tell the cell to kill itself.

Domains of Life

When looking at the whole array of cells out in the world we will start to notice some pretty big difference between them. The most significant division is between eukaryotes and prokaryotes. Eukaryotes are cells considered to be more complex and contain membrane-enclosed organelles that carry out different functions for the cell. This includes a nucleus that contains all of the chromosomal DNA of the organism.

In contrast, prokaryotes lack organelles and a nucleus allowing their DNA to float about in their cell. Due to this prokaryotes carry out their chemical reactions within their cytoplasm or with the help of their cellular membrane.

Application: Cell-Based Questions

Throughout this material we will learn how to answer a multitude of different cell based questions. Before getting into the specific details I want to discuss some keys things to watch out for and outline a basic approach for cell based questions.

Organism First

First and foremost always, always check your organism. If the question asks about a prokaryotic organism you need to make sure you aren’t picking an organelle as an answer. Since they don’t have them.

Let’s look at a question to see how we can apply this:

E. Coli is a rod-shaped gram-negative bacteria found in healthy intestinal microbiomes. However, pathogenic O157:H7 E. Coli variants produce the Shiga toxin and lead to the development of severe and sometimes life-threatening diarrheal illnesses. Scientists trying to develop a new drug to treat E. Coli caused illnesses should target which cellular structure if they wish to interfere with its electron transport chain (ETC)?

A. Peroxisome
B. Cytoplasm
C. Mitochondria
D. Cell Membrane

First and foremost we need to recognize that we are dealing with a bacteria which is a type of prokaryote. Since prokaryotes don’t possess peroxisomes or mitochondria both of which are organelles these answers are incorrect. From here we need to know that the ETC of a bacteria is carried out using its cell membrane thus D is correct. By focusing on the organism first we can already get to a 50/50 question greatly increasing our chances of getting the answer right. Furthermore, we avoid choosing mitochondria since this is where the ETC is carried out in eukaryotic cells.

Facts Lists

Additionally, it is helpful to generate fact lists when approaching many of the cell-based questions. Often times a process or structure will be described in the passage but not stated outright. By generating a facts list we can recognize more easily what they are referring to and select the correct answer.

For example, let’s look at another question to determine how to use a facts list to help us stay organized and identify the correct answer.

Signal sequences are commonly found on proteins destined for secretion from the cell. If scientists fluorescently tagged peptide signal sequences where would you expect a cell to fluoresce?

I. Nucleus
II. Golgi Apparatus
III. Rough Endoplasmic Reticulum

To begin we will collect a list of facts that have been presented in the answer stem then match them up to the roman numeral above. Here is what we know:

  • Secretory Protein – will need to be packaged and placed into a vesicle for secretion
  • Contains Signal Sequence – signals a ribosome to transition from the cytoplasm to the RER and finish translation there

In this case, I wrote out some basic definitions for each of the facts in the question stem but on your actual exam that is unnecessary. So long as you can recall the definition or a little about each fact you should be good to go.

Now we can answer the question relating the facts to each of the roman numerals to see if they fit. First up, nucleus. Since the nucleus does not involve signal sequences or secretory proteins it wouldn’t fluoresce thus our answer shouldn’t include I. The Golgi and the RER of the other hand involve signal sequences and secretory proteins and would fluoresce. Therefore the correct answer should be both II and III.

We don’t need to create a fact list in order to tackle this problem but it is often helpful to do since it organizes our approach to the question and helps us avoid making errors.