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Sarah Taylor

Parts of a Cell Study Guide

🗻 Big Picture: There are a lot of different kinds of cells, but no matter the type, they all have at least 5 structures in common: DNA, a cell membrane, cytoplasm, a cytoskeleton, and ribosomes. Today we’re going to talk about each of these commonalities in detail to see what makes all cells alike!

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Introduction

What’s the difference between you and me? We both have skin, we both have a heart, we both love to learn (🤞) and we both have a brain. We might have different hair colors, or different noses, but for the most part, we’re made up of pretty similar stuff. Now, what about the difference between you and your cat? Well, there are a lot of things that each of us have that the other doesn’t. But once again, cats also have skin, and a heart, and a brain, and we’re pretty sure they like to learn too. The point is that even though we’re really different, there are some things that are always conserved between us. Cells are the same way. Though there are a lot of differences across them, there are some things that always stay the same no matter what. And that’s what we’re here to talk about today–what all cells have in common. Let’s get going.


Lesson Objectives

  • Brief look into the discovery of the cell and cell theory.
  • Understand that all cells have some key, common components to their structure.
  • Understand each of those structures in detail.

Discovery of the Cell

The cell was first discovered by scientist Robert Hooke when he put a slice of cork under a microscope and noticed that it was divided into small sections that, according to him, looked similar to the small rooms that monks inhabited. What Hooke was really seeing was the dead cell walls of the plant cells that make up cork. Check out the image below to see what Hooke saw when looking at these cells up close.

cork cell

Source

In 1674, the first live cell was witnessed by Anton van Leeuwenhoek, a Dutch scientist and businessman. He was also the first person to view bacteria, which he did by scraping plaque off his teeth and looking at it under a microscope (a groundbreaking scientific contribution, but respectfully, ew).

Both of these discoveries led to the proposition of the Classical Cell Theory by Theodor Schwann in 1839. This theory has three parts:

  • All organisms are made up of one or more cells.

  • All of the life functions of a living thing occur inside cells.

  • All cells arise from pre-existing cells.

Pretty smart right? We still use this cell theory today, but a modern version with a few additions. First (fourth?), DNA is passed between cells during cell division. Second, the cells of all organisms of the same species are structurally and chemically the same. And third, energy flow occurs within cells.


What Do All Cells Have in Common?

Besides being super cool and interesting, all cells share some universal features, which include a plasma membrane, cytoplasm, cytoskeleton, DNA, and ribosomes. Let’s go over each below to learn about their structures and functions within a cell.


The Cell Membrane

The Cell Membrane, also known as the Plasma Membrane, is the structure that separates a cell from its environment. It’s kind of like how our skin separates us from our environment (and thank god it does, without it a singular step into the filth of a New York City street would probably kill you on the spot). Two universal features of all membranes are semi-permeability and the lipid bilayer. Let’s go over them below.

  • The Lipid Bilayer is composed of lipids (shocking, we know). Lipids are fats that have a hydrophilic head and a hydrophobic tail (a fancy way of saying that the heads are attracted to water and the tails are repelled by it). Our bodies are made mostly of water, which causes the lipid bilayer to form spontaneously as the heads of a bunch of lipids group together on the outside while their tails press together on the inside, forming a flexible bubble that keeps water (and pretty much anything else) from entering the cell.

  • The plasma membrane is Semi-permeable, meaning that it’s picky about what it lets in and out. A bunch of different proteins are scattered across the membrane, and they serve as the bouncers that decide who gets in and who gets tossed to the curb. These include glycoproteins that allow cells to communicate with one another and bind to other molecules, and channel proteins that act as gates that allow certain molecules to pass through (the VIPs, or, in this case, the VIMs).

While the core functions of the cell membrane are protection, providing a fixed, constant environment inside the cell, allowing cell-to-cell interaction, and structural support, there are many different versions of plasma membranes that are customized to each type of cell to support its functionality.


Cytoplasm

So what is cytoplasm anyway? Well, “cyto” means “cell”, and “plasm” means “stuff”, so we guess it’s…cell stuff? Honestly, it sort of is. Cytoplasm is a thick, gelatinous substance made mostly of water, salts, and proteins surrounded by the cell membrane. It is composed of all individual components inside the cell, including the cytoskeleton, cytosol, and every organelle but the nucleus (which has its own membrane with a substance inside called nucleoplasm).

  • Cytosol is the interior layer of the cytoplasm that contains all of the cell’s organelles. If the cell were a watermelon and the seeds were the organelles, the cytoplasm would be everything within the skin, while the cytosol would be just the red interior.

Cytoplasm might seem free flowing and unstructured, but it’s actually highly coordinated by a component called the cytoskeleton that gives it structure and organization. Speaking of the cytoskeleton, let’s take a look at that next.


Cytoskeleton

The Cytoskeleton of a cell is a network of protein filaments that differ in both size and protein composition. They fall into three classes: microtubules, actin filaments, and intermediate filaments. Let’s briefly go over them below.

  • Microtubules are the largest of these filaments, and are composed of a protein called tubulin.

  • Actin Filaments are the smallest type of filament, and are composed of, well, actin.

  • Intermediate Filaments, as the name suggests, are mid-sized, and are composed of many different types of proteins.

The cytoskeleton has a lot of functions (more than our skeleton does for sure). It’s linked to the plasma membrane and creates an internal structure for the cell, providing support and giving it shape (which is important for animal cells that don’t have a cell wall or capsule to give them structure).

It also organizes and tethers the organelles, and occasionally helps them move throughout the cell when they need to. And as if that wasn’t enough, it also has a role in molecular transport, cell division, and cell signaling. Not to make it a competition, but, overall, cytoskeleton > human skeleton.


Ribosomes

Ribosomes are organelles composed of RNA molecules and proteins that carry out the process of protein synthesis. Each is divided into two subunits, one large and one small. When not synthesizing proteins, these subunits remain unattached (💔). During protein synthesis, the two units combine and work together as one to translate mRNA and form a polypeptide chain that will eventually become a functional protein (❤️).

  • The Small Subunit is responsible for translating mRNA, basically the instruction manual that tells the large subunit how to create the protein.

  • It attaches to the Large Subunit, which is responsible for using the mRNA instructions to synthesize a polypeptide chain (a long string of amino acids that make up a protein).

  • The two units transport the chain to the rough ER where it will make its way to the lumen and fold into its final, functional form.

Basically, if a ribosome was you and your friend trying to assemble a chair, the small subunit would be the one reading the instruction manual, and the large subunit would be the one doing the actual work (not to say the small subunit isn't pulling its weight, those instruction manuals might as well be written in hieroglyphics).

If you want to learn more about ribosomes, check out this study guide here!


DNA

I think we can all name at least ONE thing about DNA–it’s the part of our bodies that carries our genetic information. Basically it’s what makes you, you. DNA, or deoxyribonucleic acid, is composed of chemical building blocks called Nucleotides. This composition is true for all organisms, but there are a few differences in the actual structure of DNA between prokaryotes and eukaryotes.

  • Eukaryotic DNA is double-stranded, linear, and stored within the nucleus.

  • Prokaryotic DNA is double-stranded, circular, and stored within the nucleoid (which is basically the prokaryotic remix of a nucleus) (actually, because eukaryotes evolved from prokaryotes, the nucleus would be the remix in this case).

In both prokaryotes and eukaryotes, DNA specifies the structure and function of the organism that houses it. It’s found in pretty much every cell an organism possesses and is passed along from parent to offspring, which is why everyone says you have your dad’s eyes and your mom’s nose (customize that as you will).


💡 Summary

  • Robert Hooke discovered the cell in 1665. The first live cell was witnessed by Anton van Leeuwenhoek in 1674. Both of these discoveries led to the proposition of Cell Theory by Theodor Schwann in 1839.

  • The three parts of Classical Cell Theory are as follows: all organisms are made up of one or more cells, all of the life functions of a living thing occur inside cells, and all cells arise from pre-existing cells.


  • All cells have a plasma membrane, cytoplasm, cytoskeleton, DNA, and ribosomes.

  • The cell membrane is semi-permeable and composed of a lipid bilayer. It’s functions are protection, support, to separate a cell from its environment and determine which materials are allowed to enter and exit the cell, and to provide a fixed environment inside the cell.

  • Cytoplasm is a thick, gelatinous substance made of water, salts, and proteins that contains everything in a cell including the cytosol, cytoskeleton, and every organelle but the nucleus. Cytosol is part of the cytoplasm and contains all of the cell’s organelles.

  • The cytoskeleton is a network of protein filaments that include microtubules, actin filaments, and intermediate filaments. Its functions are to provide structure, support, give cell shape to a cell, organize and tether its organelles, and help with molecular transport, cell division, and cell signaling.

  • Ribosomes are organelles composed of RNA and proteins that carry out the process of protein synthesis. They are divided into two subunits: the large subunit and the small subunit. The small subunit translates mRNA, and the large subunit synthesizes polypeptide chains.

  • DNA is the carrier of genetic information. It determines the structure and function of the organism, and is passed along from parent to offspring.


FAQs:

1. What are the five universal parts found in every cell?

A cell membrane, cytoplasm, cytoskeleton, ribosomes, and DNA.

2. What are the three parts of Cell Theory?

  • All organisms are made up of one or more cells.
  • All of the life functions of a living thing occur inside cells.
  • All cells arise from pre-existing cells.

3. What is the difference between prokaryotic and eukaryotic DNA?

Prokaryotic DNA is circular and found within the nucleoid, while eukaryotic DNA is linear and found within the nucleus.

4. What are the functions of the cytoskeleton?

Providing cell structure, shape, and organization, organizing and tethering organelles, moving organelles throughout the cell, and assisting with molecular transport, cell division, and cell signaling.

5. What is cytoplasm?

Cytoplasm is a thick substance composed of water, salts, and proteins encased by the cell membrane in which all organelles but the nucleus are found.

6. What are the functions of the small and large subunit of a ribosome?

The small subunit translates mRNA which the large unit uses to synthesize polypeptide chains.

7. What are the two universal features of cell membranes?

Semipermeability and a lipid bilayer.


We hope you enjoyed studying this lesson and learned something cool about the Parts of a Cell. Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don't forget to download our App to experience our fun, VR classrooms - we promise, it makes studying much more fun! 😎

Sources:

  1. Ribosomes. https://byjus.com/biology/ribosomes/. Accessed Nov 10, 2021.

  2. Ray et al. Systems Biology in Toxicology and Environmental Health. 2015.

  3. Introduction: What Is DNA? https://www.nature.com/scitable/topicpage/introduction-what-is-dna-6579978/. Accessed Nov 10, 2021.

  4. Parts of the Cell. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/2.1/primary/lesson/common-parts-of-the-cell-bio/. Accessed Nov 10, 2021.