Unit 4 Overview: Cell Communication and Cell Cycle

4 min read•january 9, 2023

Haseung Jun

Annika Tekumulla

Haseung Jun

Annika Tekumulla

AP Biology 🧬

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Intro to Unit 4

Your body is made up of TRILLIONS of cells that all have different responsibilities. That is a LOT of cells!!

In order for your body to function correctly, these cells need to work in unison by communicating with each other.
Your cells also go through a process called the cell cycle in order to generate new cells.

Before we begin…

A good way to remember the importance of regulation in cell communication and the cell cycle is to think of a checklist. In order for these processes to be done correctly, there must be correct timing and coordination within the cell.

Image courtesy of Pixabay.

4.1 Cell Communication

Cells can communicate in various ways. In this section, you'll learn about juxtacrine, paracrine, autocrine, and endocrine signaling! Juxtacrine signaling is signaling a cell through direct contact. Paracrine signaling is communicating over short distances. Autocrine signaling is signaling yourself (and for cells, well, cellself?). Lastly, endocrine signaling is signaling a cell far far away using the bloodstream.

4.2 Introduction to Signal Transduction

Ah, a section of importance! Signal transduction is quite a thing! It's about how cells really do communicate, because they don't have phones to text 📱. Signal transduction is like a row of dominos. One domino falling leads to the next one falling and so forth. In signal transduction, one step leads to the next.

There are three steps in signal transduction: reception, transduction, and response. Reception is like the notification you receive when your friend texts you. It's the first stage where the ligand (signaling molecule) is received by the receptor protein in the target cell. Transduction is when the signal is transmitted through the cell and amplified. Finally, the response is when the signal is carried out. If the ligand told the cell to create a protein, the cell will create the end product of a protein.

4.3 Signal Transduction

There are multiple ways that the cell can respond to its environment. A good example is quorum sensing. Through quorum sensing, bacteria are able to determine its population in order to act accordingly. But instead of a protein counting, it's actually done through signal transduction.

Each bacteria basically releases a ligand so that the bacteria can sense each other. Other metabolism processes happening inside our body is a result of signal transduction. Insulin, for example is a ligand that tells the liver that the blood sugar level is too high. Without insulin, we would have difficulty regulating our blood sugar levels. Signal transduction is important for regulation. Even apoptosis, which is cell programmed death, is a form of signal transduction.

4.4 Changes in Signal Transduction Pathway

Changes in the signal transduction pathway can always happen. Two distinct and common ones are mutations and chemicals. Think about denaturing proteins. This is a similar to that. With signal transduction, we have mutations that prevent the cell from regulating the cell cycle. This can lead to unregulated cell division, or cancer. Chemicals can also change signal transduction, by activating something that shouldn't be activated.

4.5 Homeostasis and Feedback Loops

Homeostasis, one of the most important themes of biology! We have two loops: negative and positive feedback loops that happen within our system. Negative feedback loops try to sustain the current environment, meaning it doesn't like change. So if something goes up, the negative feedback loop will bring it down. If something goes down, the negative feedback loop will bring it up. Positive feedback loops are different. Instead, a stimulus creates more of something. It's almost like infinitely multiplying a number by 2.

4.6 Cell Cycle

Now we have mitosis! The cell cycle is comprised of 5 phases:

G1 - cell growing
S - DNA copies itself (now the cell has two copies of the same DNA)
G2 - cell continues to grow bigger
Mitosis - cell divides
Cytokinesis - cell cut into two new daughter cells

Within mitosis, we have prophase, metaphase, anaphase and telophase. You'll learn about each step and its role within cell division.

4.7 Regulation of Cell Cycle

Without regulations, cell division can lead to cancer. This is why there are checkpoints to make sure cell division is not happening too frequently. The Cdk-cyclin complex also have a similar role within cell replication. When the amount of cyclin goes up, mitosis is triggered. When the number is cyclin is down again after mitosis and the degradation of cyclin, mitosis does not happen.

p53 is also a protein that is important in regulating the cell cycle. Without p53, the cell's DNA is not properly checked and repaired for any damages. This could lead to again, cancer. There are other genes that also regulate the cell cycle, so you should check it out!

Regulation is a heavily emphasized theme in AP Biology, so I suggest that you pay close attention to this unit!