[AP Biology 2.1] Cell Structure: Subcellular Components

The first section of Unit 2 in the AP Biology curriculum focuses on the subcellular components of cells, specifically the organelles within cells that allow them to function. This section covers ribosomes, the endoplasmic reticulum, the Golgi complex, mitochondria, lysosomes, vacuoles, and chloroplasts. Check it out!

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Living systems are organized in a hierarchy of structural levels that interact.

Describe the structure and/or function of subcellular components and organelles.

Ribosomes comprise ribosomal RNA (rRNA) and protein. Ribosomes synthesize protein according to mRNA sequence.

Ribosomes are found in all forms of life, reflecting the common ancestry of all known life.

Endoplasmic reticulum (ER) occurs in two forms – smooth and rough. Rough ER is associated with membrane-bound ribosomes-

  1. Rough ER compartmentalizes the cell.
  2. Smooth ER functions include detoxification and lipid synthesis.

The Golgi complex is a membrane-bound structure that consists of a series of flattened membrane sacs-

  1. Functions of the Golgi include the correct folding and chemical modification of newly synthesized proteins and packaging for protein trafficking.
  2. Mitochondria have a double membrane. The outer membrane is smooth, but the inner membrane is highly convoluted, forming folds.
  3. Lysosomes are membrane-enclosed sacs that contain hydrolytic enzymes.
  4. A vacuole is a membrane-bound sac that plays many and differing roles. In plants, a specialized large vacuole serves multiple functions.
  5. Chloroplasts are specialized organelles that are found in photosynthetic algae and plants. Chloroplasts have a double outer membrane.


Specific functions of smooth ER in specialized cells are beyond the scope of the course and AP Exam.

The role of the Golgi in the synthesis of specific phospholipids and the packaging of specific enzymes for lysosomes, peroxisomes, and secretory vesicles are beyond the scope of the course and the AP Exam.

2.1 Cell Structure: Subcellular Components Overview

The big picture of section 2.1 is that life exists in a hierarchy. In this section, we are going to look specifically at cellular organelles. Organelles are tiny components inside of cells that complete specific actions, allowing cells to complete the many processes and chemical reactions that allow them to grow and reproduce. Besides ribosomes, all organelles are covered or created by a lipid bilayer. Let’s start with the most ubiquitous cellular component – ribosomes.

Ribosomes are tiny cellular components made of ribosomal RNA and proteins. They complete the process of translation by connecting amino acids based on the information they receive from messenger RNA. Let’s consider their structure.

Ribosomes are created out of multiple proteins and ribosomal RNA molecules, which weave together into a complex – but specific – structure. The ribosomal RNA and proteins weave together to form subunits of a ribosome. These subunits then come together around a messenger RNA molecule to function. The subunits come together perfectly, allowing the ribosome to grab onto a piece of messenger RNA. Once a piece of mRNA is found, the ribosome can begin its work.

Each ribosome has three areas where transfer RNAs can fit, known as “sites.” Transfer RNAs enter at the “A” site. If they can hydrogen bond to the codons presented on the mRNA molecule, they can advance to the “P” site. Here, the ribosome catalyzes a reaction that removes the amino acid from the tRNA molecule and attaches it to the growing polypeptide chain. Finally, the used tRNA is discarded through the “E” site. You can remember the sites like this: A = acceptance, P = peptide bond formation, and E = exit!

The ribosome will slowly move along the mRNA molecule, reading each codon and attaching the appropriate amino acid to the chain. The polypeptide is completed when the ribosome reads the “stop” codon, allowing the ribosome to release the chain and start on translating a new mRNA molecule.

Ribosomes are the only cellular components aside from DNA that are visible in prokaryotic cells AND eukaryotic cells. In fact, studies of the molecular structure of ribosomes in species as different as a human and a bacteria show that there is not much difference between their ribosomes. This suggests that ribosomes are one of the most ancient cellular components, and adds evidence that all life on Earth originated from a common ancestor!

Next, let’s consider an organelle found in all eukaryotic organisms – the endoplasmic reticulum (sometimes just ER for short). There are two types of endoplasmic reticulum found in eukaryotes, the rough ER and the smooth ER – which have slightly different functions.

The rough endoplasmic reticulum is a series of flattened sacs that extend directly from the lipid bilayer that surrounds the nucleus. These flattened sacs are covered with ribosomes, which are able to deposit newly created polypeptides directly into the sac they are connected to. Inside each sac, a specific microenvironment is formed with the proper pH and chemical constituents to help fold the proteins into the proper shape and make any chemical additions that are necessary. For instance, some proteins require the addition of inorganic atoms such as iron or copper before they can function. Those bits and pieces are added in the rough ER.

By contrast, the smooth endoplasmic reticulum has no ribosomes attached to its surface. The smooth endoplasmic reticulum is a series of sacs that extend out of the rough endoplasmic reticulum. However, the smooth ER has a slightly different function. These chambers are usually reserved for creating lipids – like phospholipids needed for membrane construction or fat molecules needed to store energy. The smooth ER is also responsible for detoxifying cells, since the toxins can be broken down here without affecting the rest of the cytosol in a negative way.

The next organelle, the Golgi Complex, is also made of a series of flattened sacs. However, these sacs are not physically connected to the endoplasmic reticulum. Rather, the Golgi complex sits closer to the cellular membrane, where it carries out several important functions.

Some proteins need even more modification than the endoplasmic reticulum can provide, or they need to be distributed to specific places on the cell membrane. These proteins are packaged up in a transport vesicle and are sent to the Golgi complex. Here, the proteins are fully modified and mixed with other chemical constituents. There are 3 important functions that the Golgi complex completes (in addition to many minor functions).

First, proteins can be packaged into secretory vesicles for exocytosis. These can be defensive proteins or proteins needed by other parts of an organism, but the important point is that they are expelled from the cell. Second, some proteins are needed to break down incoming nutrients – these go to lysosomes (covered further down). Lastly, some proteins need to be added to the cellular membrane. Proteins headed for the cellular membrane are embedded into the vesicle, which is then merged with the current lipid bilayer of the cell membrane – adding the proteins directly to the surface of a cell.

Mitochondria are likely one of the most important organelles within eukaryotes, though they are also one of the smallest. In fact, they are about the size of an average bacterial cell. Interestingly, this may be where mitochondria originated from.

Mitochondria have two membranes – the inner membrane and the outer membrane. While the outer membrane is smooth, the inner membrane contains many folds (called cristae) which provide more surface area for important reactions. Specifically, the inner membrane of mitochondria is home to the electron-transport chain – an essential part of the process that extracts energy from glucose and stores it in the bonds of ATP for use elsewhere in cells. Mitochondria also have their own DNA and ribosomes which is further evidence that these organelles may have originated from bacteria that evolved to live within larger cells. This is called the endosymbiotic theory, though we will not go into more detail here. The important thing to know is that mitochondria provide energy for all eukaryotic cells – plants, animals, and single-celled eukaryotes alike.

Next up are lysosomes. As discussed earlier, lysosomes are created by the Golgi complex. The Golgi packs a small vesicle full of protein enzymes that can break down various substances. These substances are referred to as hydrolases since they act to break apart polymers by catalyzing hydration reactions. Plus, the Golgi complex loads the surface of the new lysosome with transport proteins and receptors that help the lysosome make it to a specified target.

Lysosomes are pulled around the cell to connect with vesicles that contain nutrients, smaller organisms, and other substances that can be digested. The lysosome fuses with these vesicles, releasing the cocktail of enzymes. The enzymes digest the material into smaller monomers and usable pieces, which are released into the cytosol so the cell can access them. The membrane of this fused vesicle can then be recycled by the Golgi complex into new lipids to create new lysosomes or to repair the cell membrane.

A vacuole is a very simple organelle that serves a wide variety of purposes in different organisms. In general, a vacuole is simply a spherical membrane that holds whatever the cell needs it to hold. In animal cells and many single-celled organisms, the vacuole is an organelle that holds excess water and sometimes waste products. Some organisms have a contractile vacuole that can expel water from the cell if it takes on too much. Toxins, wastes, and byproducts are often stored in vacuoles so they cannot affect the chemistry of the rest of the cell.

Plants also use a vacuole to store some byproducts, but plants use their vacuole for another purpose altogether. In most plant cells, there is one large vacuole that sits in the middle of the cell. When it is filled with water, it pushes outward on the cell wall. This is known as turgor pressure, and it gives plants the ability to stand tall without any bones or solid support structures. When you forget to water your plants and they droop, this is because their vacuoles do not have enough water to put pressure on the cell wall and create a supportive structure!

The last organelles we will look at in this lesson are chloroplasts. Chloroplasts are only found in algae and plants, and they have the ability to convert light, carbon dioxide, and water into sugar molecules!

If we look closely at the structure of a chloroplast, you will notice that – like mitochondria – these organelles also have a double membrane. Also similar to mitochondria, chloroplasts have their own DNA and ribosomes, so it is theorized that these organelles were also once free-living cells that evolved to live inside of larger cells. Inside of the inner membrane of a chloroplast is a series of sacs known as thylakoids, which have the right proteins and molecules for completing the process of photosynthesis. The sugar created is exported to the cytosol of the cell, where it can be broken down by mitochondria to create energy in the form of ATP. Cells then use ATP to power all of their other important biochemical reactions!