Discover the Cell: AP Biology Crash Course on Cell Organelles 

What do we know about the cells and cell organelles? Luckily, much more than people several centuries ago. Still, you might face some issues with this topic on the AP Biology exam. Why? The main reasons are diverse material, specific terms, and scientific definitions. Yes, all those can be overwhelming and make you feel anxious before the exam. 

But the truth is that to understand cells, you don’t need to look for the most revolutionary biology course. The only thing you actually need is to take your time, read this blog, find a bit of additional materials, and practice as much as you can. Just like that, you can nail your exam (at least the cell-related questions). 

With that said, let’s deep dive into organelles and cell structure with our AP Biology crash course. 

AP Biology Exam: Cell Structure Topics

The AP Biology exam covers 8 units, one of which is called “Cell Structure and Functions.” As you might have guessed, it is all about cells. This part will make up 10-13% of your exam score. That’s why, whether you like it or not, you should know how organelles work, including their structure and functions.

Let’s see what topics this section covers:

1. Cell Structure: Subcellular Components
2. Cell Structure and Function 
3. Cell Size 
4. Plasma Membranes 
5. Membrane Permeability 
6. Membrane Transport 
7. Facilitated Diffusion 
8. Tonicity and Osmoregulation 
9. Mechanisms of Transport 
10. Cell Compartmentalization
11. Origins of Cell Compartmentalization

Still, keep in mind that each of the AP units involves dozens of terms and definitions you should remember. And to be honest, most of the topics are interconnected. So, to better understand the Cell Structure and Functions unit, you should understand the fundamentals from another unit, “Chemistry of Life.” 

You can start by reading about Gilbert Lewis and his octet rule, carbon and hydrogen bonds, and electron orbitals. If you are a visual learner, try to find educational channels that have the information you need. For example, Hank Green teaches most of these topics in his Chemistry crash course on YouTube. 

What Do We Know About Cells Today?

Now, let’s get to our AP Biology crash course on cells, where we explain the basics you should know to succeed in the exam. Over the centuries, our perceptions of cell structure have changed a lot. Robert Hooke discovered them in 1665. A bit later, more complex microscopes helped us learn that there are many cells of various shapes. 

Despite the long way of evolution, most cells have the same or similar inner structure. Speciation among living organisms is amazing. And today, we know that the human body has over 36 trillion cells!

Cell Theory by Schwann & Schleiden

In 1839, after analyzing thousands of cells and their functions, German scientists Theodor Schwann and Matthias Schleiden created the first Cell Theory. These are the postulates for the existence of living organisms in all populations and taxonomy. Let’s check their modern interpretations:

• All cells of similar species are basically identical in chemical composition;
• Energy flow occurs in the cells;
• All living things are composed of at least one cell;
• All living cells arise from preexisting ones due to cell division;
• Cells contain DNA in the chromosomes and RNA in the cell nucleus and cytoplasm;
• A cell is a fundamental unit of all living things;
• The activity of the organisms depends on the total activities of all its cells.

Before moving to cell organelle, we should cover one more thing. There are two main types of cells: 

• Prokaryotic cells are considered to be “simpler” cells. They don’t have a defined nucleus and a variety of membrane-bound structures. They have limited organelles and their genetic material resides in a so-called nucleoid (e.g., bacteria, archaea, and cyanobacteria);
• Eukaryotic cells are more complex. They have a proper nucleus and membrane-bound organelles. These can be a single-cell organism (e.g., amoeba and paramecium) or multi-cell (e.g., plants, animals, fungi, etc.). 

Source: Biology Notes

Of course, there are other differences that we will check further below. 

Prokaryotic Cells: The Basics

Source: Freepik

Prokaryotes are single-cell organisms, representatives of Bacteria and Archaea. Here are the main features that differentiate prokaryotic cells from eukaryotic cells:

• Prokaryotes have cell walls above cellular membranes, slime layers, or capsules.
• Most have single circular chromosomes (at the same time, eukaryotes have many chromosomes, and they are not circular; they are linear) and plasmids, which are smaller, additional circular DNA molecules.
• Prokaryotes are much smaller in size than eukaryotic cells.
• They have no nucleus and lack other organoids. 
• They have additional membrane structures (pili) that can be used for DNA exchange between two cells.
• Many of them have flagella to move. 
• They do not have compartmentalization ability.

Eukaryotic Cells: Animals vs. Plants

Eukaryotes are more complex and diverse. They can be both single-celled and multicellular organisms:

1. Single-celled organisms, like some protists (e.g., infusoria), bacteria, and certain algae, function independently within a single cell, performing all necessary life processes. They do not require specialized tissues or organs.
2. Multicellular organisms, such as fungi, plants, and animals (e.g., fungi, a tulip, a cat, etc.), are composed of multiple cells organized into tissues, organs, and organ systems. Each type of cell within these organisms has specialized structures and functions, helping organisms eat, breeze, run, sleep, or even make decisions.

While plant and animal cells are both eukaryotic and share some fundamental structures (a nucleus, cytoplasm, and organelles), they are still different. For example, plant cells have additional structures like chloroplasts and a cell wall made of cellulose. At the same time, animal cells typically lack these features but may have specialized structures like centrioles.

Source: Freepik

Now, let’s compare animal and plant cells in detail:

Cell Structure and Organization

Now, let’s discover the main cellular domains. According to their functions, we divide them into three elements:

• Cell membrane. It shapes, protects, and exchanges substances with the environment;
• Nucleus. It stores genetic information and controls all cellular processes;
• Organelles. They ensure the cell’s vital activity at the level of metabolic reactions.

Note: This breakdown is relevant for eukaryotic cells because, as you already know, prokaryotes lack a defined nucleus and membrane-bound organelles. Still, they do have a plasma membrane, which basically refers to the same structure as the cell membrane. 

So, we’ve already talked a lot about the nucleus and cell membrane. But what are organelles? They are subcellular structures with one or more specific functions that are vital for cell existence. There are several types of cell organoids (aka organelles):

• Double-membraned: nucleus, mitochondria, and chloroplasts (only found in eukaryotes);
• Single-membraned: vacuole, lysosome, Golgi apparatus, and endoplasmic reticulum;
• Non-membrane: ribosomes and centrioles.

Overall, exploring organelle function sheds light on the inner workings of cellular biology.

Cellular Membrane: Structure and Functions

The cellular membrane has many names, including plasma membrane, cytoplasmic membrane, and plasmalemma. Still, the idea is the same – the layer that separates the inside of a cell from the external environment. It consists of many macromolecules, including lipids, peptides, and carbohydrates, and has the following main components:

• Double-layer of phospholipids. This is the fundamental structure of the membrane. Lipids are essentially building blocks for the cell membrane. Apart from phospholipids, the membrane might also have other types of lipids (sterols and glycolipids). Also, the phospholipid bilayer may contain another lipid called cholesterol. It helps regulate the fluidity and stability of the membrane. 
• Proteins. Proteins have many functions. But overall, they act like signal transmitters, transporters, and structural support. Proteins are located in the plasmalemma (aka plasma membrane). But there are two main types of them:
  1. The first one is integral or transmembrane proteins located inside the membrane, in between the double layer of lipids. 
  2. The second type is peripheral proteins attached to the integral ones or just to the membrane itself, on the inner or outer surface. 
• Carbohydrates. Together with lipids or peptides, they make complex compounds called glycolipids (lipids with a carbohydrate attached) and glycoproteins (proteins with a carbohydrate attached). Together, glycolipids and glycoproteins form an additional extracellular layer known as glycocalyx. Glycocalyx can exist in the form of a capsule or a slime layer. Its main function is to protect the cell.

Source: Freepik

Depending on the cell types and their functions, cell membranes might have different types of receptors, ions, and other molecule channels. However, the general structure is the same for bacteria as it is for whales. 

Nucleus or Cell Brain

Source: Genome

Usually, eukaryotic cells have a single nucleus. Still, there are several exceptions. For example, mammalian red blood cells don’t have any, while their bone cells (osteoclasts) might have several nuclei.

The nucleus is a double-membrane organelle. It has a nuclear matrix or nucleoplasm and an envelope between two membranes. The nucleoplasm contains nucleolus, DNA molecules (chromatin), transcriptional factors, and some ribosomes.

The nuclear membrane has large pores and many ion channels. These are necessary for transporting different molecules, components, and ions between the nucleus and the cytoplasm.

The nucleus has 2 vital functions:

• Storage of genetic material (DNA molecules);
• Regulation of gene expression.

Well, as you see, the nucleus acts as a brain. In addition, its presence allows the separation of gene expression (in the nucleus) and translation (in the cytoplasm) in the cell space. 

7 Main Components of Cytoplasm Environment

What is cytoplasm? It is the entire internal content of a living cell, except for the nucleus. It has several components: 

#1 Cytosol or Cytoplasmic Matrix

Cytosol is an inner liquid or semi-solid substance around all organelles. Actually, it has a gelatinous consistency because of the many macromolecules (especially proteins) and ions suspended in water. Cytosol performs many functions:

• Signal transduction;
• Site for cytokinesis and many metabolic reactions;
• Metabolite transportation.

#2 Endoplasmic Reticulum and Transportation

Source: Genome

Endoplasmic reticulum (ER) is a single membrane organelle that is usually present in the eukaryotic cells (but some cells are an exception, like red blood cells (erythrocytes)). ER consists of a wide network of cisternae that are used mainly for the storage or transportation of something. Thus, macromolecules can migrate from one cell district to another via the endoplasmic reticulum.

There are two types of endoplasmic reticulum: rough (associated with ribosomes) and smooth (without ribosomes). The main functions of the endoplasmic reticulum are the following:

• Protein folding;
• Calcium storage;
• Protein synthesis;
• Lipid metabolism.

#3 Golgi Apparatus and Processing

Source: Britannica

A single-membrane organelle is pretty common for most eukaryotic cells. It looks like many vesicular clusters that form a wide network. These clusters are made of cisternae and stack. The space inside such a cisternae is called a lumen.

Golgi Apparatus receives protein from the endoplasmic reticulum and processes it to the final structure. Then, via vesicles, they are transported to the other organelles in cells or cellular membranes. In addition, Golgi Apparatus participates in the lysosome creation.

#4 Mitochondria or Cell Energy Station 

Source: Genome

Mitochondria is one of the most interesting organelles. Why? First, it has a double membrane and its own genome, called the mitochondrion. All this is due to its bacterial origin. Millions of years ago, mitochondria were a single bacteria. It was eaten by another cell and, for some reason, not digested. As a result, they entered into symbiosis and coevolved together. 

Therefore, one mitochondrial membrane belongs to this bacterium and another one – to the predator cell. This is the basic idea of the endosymbiotic theory. Mitochondria has the following structure:

• The outer membrane;
• The space between two membranes;
• The inner membrane;
• The cristae space, which looks like infoldings of the inner membrane;
• The liquid matrix inside (has a mitochondrial genome, RNA molecular, peptides, and ribosomes). Note: The mitochondrial genome is relatively small and contains only a fraction of the genes necessary for mitochondrial function. Most mitochondrial proteins are encoded by nuclear DNA and imported into the mitochondria.

The main functions of the mitochondria are:

• ATP production via cellular respiration (the main energy source for the cells);
• Metabolism regulation (affects citric acid cycle);
• Involved in calcium regulation (regulates calcium levels within their own matrix, which can affect signaling pathways);
• Influence on apoptosis (involved in the regulation process of the programmed cell death).

#5 Chloroplasts and Other Plastids

Source: Britannica

Chloroplasts or plastids are double-membraned green organelles common to plants and algae. Similar to mitochondria, they have their own genome and unique heredity characteristics. Due to the endosymbiotic theory, the main role was performed by the cyanobacterium, which was eaten by another cell.  

The main function of chloroplasts is to convert light energy into chemical compounds. This is possible due to the photosynthesis reaction. Chloroplasts have thylakoid structures (as flat tanks) that form granum. Thylakoids connect with each other via lamellae. The space inside the thylakoid is called a lumen, while the semi-liquid matrix inside the organelle is the stroma. 

Besides chloroplasts, there are other types of plastids:

• Chromoplasts synthesize and store pigments;
• Amyloplasts store starch;
• Proteinoplasts store and process some proteins;
• Elaioplasts collect fats.

#6 Vacuoles and Their Functions

A vacuole is a single-membrane organelle, which is pretty common for plant and animal cells. In the plant cell, this organelle can take up to 90% of the space. Inside, it has the cell sap, a water solution with ions, and some other nutrients. A vacuole performs the following functions:

• Turgor or cell pressure regulation;
• Pigment storage;
• Waste management (acts like a trash bin). 

As for the animal cells, there are also several types of vacuoles:

• Food vacuoles – digest food particles;
• Secretory vacuoles – transport some cell’s substances to release them outside;
• Excretory vacuoles – contain waste and toxic components to be released outside.

Note: vacuoles in animal cells are much smaller than the ones in the plant cells.

#7 Lysosomes and Enzymatic Activities

Source: Shutterstock

Lysosomes are the organelles that are very common in animal cells. As mentioned before, they are produced via the Golgi Apparatus and presented as single-membrane vesicles with a variety of enzymes. Their main function is breaking down proteins, nucleic acids, and other biological molecules (lysis process). 

While lysosomes break down macromolecules into simpler compounds that can be reused by the cell, they do not utilize these compounds themselves. Instead, lysosomal enzymes break down unwanted materials into small molecules that can be recycled or excreted by the cell.

Conclusion

Well, that wasn’t so hard, was it? Now, you better understand cell structure and organelles, as well as their functions. With this information, you will have answers to many AP Biology exam questions. You can use our crash course Biology as a cheat sheet that will support you while preparing for the test. Thus, you’ll become more confident and master this topic.