- Understand the overall cell structure and the structure of each cell organelle that constitutes the cell.
- Learn about the functions of each organelle in a cell.
Did you know you shed about five hundred million skin cells per day? Sounds like a lot, but five hundred million is child’s play compared to the thirty trillion cells that make up the human body. Cells are often called the fundamental unit of life, meaning that they’re the building blocks for all other living organisms on earth. But that kind of leaves you wondering…if cells are the building blocks that make up everything else, what are the building blocks that make up a cell? Lucky for you inquisitive little scientists, that’s exactly what we’re here to talk about today–cells, and all of the structures that make them so good at what they do. Let’s get into it.
“There are two types of people in this world: those who divide the world into two kinds of people, and those who don’t”. As much as we wish we came up with that, it’s actually a quote from a guy named Robert Benchley (who’s honestly pretty funny for a dude born in the 1800s). There are also two types of cells in this world: prokaryotic and eukaryotic. Prokaryotic Cells and Eukaryotic Cells are similar in many ways, but differ by one main thing: the presence or absence of a nucleus. Prokaryotes don’t have one, and eukaryotes do.
Eukaryotic cells are even further divided into two categories: Plants and Animals. Plant and animal cells are also super similar to one another, minus the chloroplasts, central vacuole, and rigid cell wall that plants have and animals do not. But let’s table that for now. Today we’ll be focusing on animal cells, so if you want to learn more about the structures that differentiate plant cells, check out this study guide here.
There are twelve key parts that make up an animal cell: a cell membrane, nucleus, nucleolus, nuclear membrane, cytoplasm, endoplasmic reticulum, golgi apparatus, ribosomes, mitochondria, vacuoles, and vesicles. Let’s break each one down below to get a better idea of what they are and how they help a cell function.
The Cell Membrane
The Cell Membrane, also referred to as the plasma membrane, is the structure that separates a cell from its environment. Inside are a bunch of Organelles, tiny little membrane-bound structures that carry out the basic tasks a cell needs to function. They’re like our organs–each performs a special job that helps keep us alive.
The cell membrane is composed of a Lipid Bilayer. Lipids are fats with a hydrophilic head (attracted to water) and a hydrophobic tail (repelled by water). In an aqueous environment like our bodies, this bilayer forms spontaneously as the hydrophilic heads group together toward the outside and the hydrophobic press together on the inside, forming a flexible bubble that keeps water from entering the cell.
The cell 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 lipid bilayer that enforce this semipermeability, including glycoproteins that allow cells to communicate with one another and bind to other molecules, and channel proteins that act as gates through which certain materials are allowed to pass through.
A gel-like substance called Cytoplasm fills the majority of the cell membrane. A subtype of cytoplasm called Cytosol contains all of the cell’s organelles except the nucleus. Also found in the cytoplasm is a structure called the Cytoskeleton, which functions pretty much like our skeleton does. It helps the cell maintain its shape and keeps everything inside organized.
The biggest and most well-known structure in a cell is the Nucleus, membrane-bound organelle that translates and stores a cell’s genetic material. They’re also the big boss of the cell, coordinating all of its activities including metabolism, growth, protein synthesis, and cell division. The nucleus is composed of 4 parts: the nuclear membrane, nucleoplasm, nucleolus, and chromosomes.
The Nuclear Membrane, or nuclear envelope, functions much like the outer plasma membrane of a cell, maintaining its shape and regulating the flow of molecules in and out through Nuclear Pores.
It is composed of a double lipid bilayer and is filled with a gelatinous substance similar to cytoplasm called Nucleoplasm, which cushions the contents of the nucleus and provides a medium through which nucleotides and enzymes can be transported to and from the nucleus and cytoplasm.
Inside the nucleus is another organelle called the Nucleolus, which is in charge of producing and assembling all of the cell’s ribosomes for protein synthesis (we’ll get to ribosomes later on, don’t worry).
The nucleus houses Chromosomes, x-shaped structures made of DNA that carry the cell’s genetic info. When a cell is not dividing, chromosomes exist in the form of long entangled threads called Chromatin.
Mitochondria are membrane-bound organelles that generate most of the energy a cell uses to function. Though we mostly know them as the powerhouse of the cell (you’ve been living under a rock if you haven’t heard that one before), they also store calcium and activate apoptosis if they decide a cell is too old or has ceased to function. Mitochondria are composed of four main structures: the outer membrane, inner membrane, intermembrane space, and the mitochondrial matrix.
The Outer Membrane is the gateway to the mitochondria, allowing ions and molecules to move in and out of the organelle through channels called Porins.
Once through the outer membrane, you reach the Intermembrane Space, which holds all of the important enzymes used during the citric acid cycle.
The Inner Membrane follows the intermembrane space and forms folds that extend into the matrix called Cristae, which you can see in the 3D model as the squiggly red part. Embedded throughout the inner membrane are proton pumps and permease proteins that regulate the transport of molecules into and out of the matrix.
The Mitochondrial Matrix is the innermost compartment of the organelle, and is composed of a gel-like material where the citric acid cycle takes place. It also contains ribosomes (yellow spheres in the model) that make proteins for the mitochondria to use, as well as mitochondrial DNA (not pictured).
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 from amino acids which 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.
👉 To learn more about ribosomes, check out this study guide here!
The Endoplasmic Reticulum (ER)
Up next we have the Endoplasmic Reticulum (ER), an organelle formed by a network of interconnected membrane-enclosed sacs called Cisternae. The cisternae are attached to the nuclear membrane and fold in on themselves to each form an internal space called a Lumen.
The endoplasmic reticulum is an important manufacturing site for both lipids and proteins, many of which are made for and exported to other organelles. It is divided into two regions: the Rough ER and the Smooth ER. Let’s discuss below.
The Rough ER
The Rough ER is the site of membrane and protein production in a cell. It works in conjunction with ribosomes to synthesize, transport, and fold proteins that will either become a part of the membrane or be released from the cell.
Ribosomes translate mRNA to synthesize polypeptide chains. When synthesis is complete, they embed themselves in the rough ER’s membrane and release the chains, which will then find their way to the lumen and fold into their final protein form.
In a process called Glycosylation, the finished proteins are tagged, indicating they are ready for transport to the Golgi apparatus, an organelle that we’ll discuss shortly.
The Smooth ER
The Smooth ER is the other region found within the endoplasmic reticulum, distinguished from the rough ER by its lack of ribosomes. Its main job is to make lipids and steroids, which are important for energy storage, membrane structure, and communication.
The smooth ER is also in charge of calcium regulation and detoxifying the cell. It releases calcium ions when the nervous system triggers a muscle cell to make the muscle contract, and removes metabolic waste and drugs from the body.
We mentioned the Golgi apparatus (aka Golgi bodies) when discussing the protein production in the rough endoplasmic reticulum. The Golgi Apparatus is composed of four to eight folds of cisternae that are not interconnected like they are in the endoplasmic reticulum.
The Golgi apparatus is pretty much the post office of the cell, responsible for packaging proteins and lipids into tiny membrane-bound pouches called Vesicles and delivering them to various locations within the cell.
Some proteins and lipids are transported to the cell membrane where their vesicles fuse to the lipid bilayer. They will either become part of its structure or be secreted from the cell to other parts of the body.
Others are transported to Lysosomes, where their vesicles fuse to the organelle’s membrane and release the molecules inside to be broken down through the process of hydrolysis.
Vacuoles are pretty simple relative to some of the other organelles we’ve talked about. They’re small, membrane-bound storage pockets containing either fluid like water or gas like CO2 and oxygen. Vacuoles are mainly used for storing materials to be used in the future or need to be removed from the cell. Their four main functions are as follows:
Removing and storing parts of the cell that have broken down due to age or damage.
Removing and storing harmful products to protect the cell from damage.
Storing nutrients like lipids, proteins, and carbohydrates.
Vesicles, Lysosomes, and Peroxisomes
Vesicles are membrane-bound organelles similar to vacuoles in structure, but instead of serving as a storage locker, they’re involved in activities like transporting molecules, secreting substances, digesting materials, or regulating cell pressure. There are two specialized types of vesicles: lysosomes and peroxisomes.
Lysosomes (depicted above) are the recycling plant of a cell. These membrane-bound organelles are full of enzymes ready to hydrolyze (aka break the chemical bonds of) any molecule that crosses their membrane so that the raw material can be used for other purposes within the cell (you heard the lysosomes–reuse, reduce, recycle!).
Lysosomes are acidic, and their enzymes are only able to function in environments that have a pH of five (which is way more acidic than the cell’s usual pH of seven).
This characteristic actually serves as a safety mechanism for the cell. If a lysosome were to burst, the enzymes released would cease to function before they had the chance to digest anything else that the cell still needed.
Peroxisomes are similar to lysosomes in the sense that they also break down cellular materials, but instead of proteins, they focus on fatty acids and reactive oxygen species (abbreviated ROS). ROS molecules can cause some serious damage in a cell. They’re a normal byproduct of cellular metabolism, but can also be created by radiation, tobacco, and drugs (and that’s why we say no to drugs, kids).
All organelles of a cell are found in the cytoplasm and are enclosed by the selectively permeable plasma membrane.
The nucleus is responsible for translating and storing a cell’s genetic material. It is composed of a nuclear membrane, nucleoplasm, nucleolus, and chromosomes.
The mitochondria are responsible for generating most of the energy a cell needs to function, storing calcium, and instigating apoptosis. They are composed of an outer membrane, intermembrane space, inner membrane, and mitochondrial matrix.
The endoplasmic reticulum manufactures proteins and lipids. It is divided into two subunits: the rough ER and the smooth ER.
The rough ER works with ribosomes to produce proteins. The smooth ER produces lipids and steroids, molecules important for energy storage, membrane structure, and communication.
The Golgi apparatus is the post office of the cell and packages proteins and lipids into vesicles to be transported to various locations for various purposes around the cell.
Proteins and lipids delivered to the cell membrane either become part of its structure or are secreted from the cell. Those delivered to lysosomes are broken through hydrolysis and their parts are used for other purposes.
Vacuoles are small membrane-bound structures that contain either fluid or gas and are mainly used for storage within the cell.
Vesicles are similar to vacuoles in structure. They transport molecules, secrete substances, digest materials, and regulate cell pressure.
Lysosomes are the organelles responsible for recycling unneeded parts of the cell. They are full of enzymes that hydrolyze molecules that cross their membrane.
A lysosome’s enzymes only function in acidic environments. This ensures that if a lysosome were to burst, the enzymes would cease to function before they could damage any other part of the cell.
Peroxisomes break down cellular materials like fatty acids and reactive oxygen species (ROS).
1. How does the nuclear membrane regulate the flow of molecules in and out of the nucleus?
Through the nuclear pores.
2. What are the four main structures of mitochondria?
The outer membrane, intermembrane space, inner membrane, and mitochondrial matrix.
3. What is the purpose of the rough ER?
The rough ER’s main purpose is membrane and protein production, the latter process assisted by ribosomes.
4. What are the main purposes of the smooth ER?
The main purposes of the smooth ER are lipid and steroid production, calcium regulation, and cell detoxification.
5. Which organelle serves as the post office of the cell?
The Golgi apparatus, which is responsible for packaging proteins and lipids into vesicles and delivering them to various locations around the cell.
6. What is the difference between a vacuole and a vesicle?
Vacuoles are used for storage, and vesicles are involved in transport, secretion, digestion, and pressure regulation.
7. What is the purpose of a peroxisome?
The peroxisome breaks down fatty acids and reactive oxygen species which can damage the cell.
Expert Question: What cell structures are present in plant cells that are not present in animal cells?
The cell wall, central vacuole, and specialized plasmids such as chloroplasts.
We hope you enjoyed studying this lesson and learned something cool about the Cell Structure! Join our Discord community to get any questions you may have answered and to engage with other students just like you! We promise, it makes studying much more fun 😎]]>