2.6: The Cytoplasm and Cellular Organelles

Inside the cell, a coordinated system of organelles works together to produce energy, build molecules, transport materials, and keep the cell alive.

Master this section and you'll be able to

• Describe the structure and function of the cellular organelles associated with the endomembrane system, including the endoplasmic reticulum, Golgi apparatus, and lysosomes
• Describe the structure and function of mitochondria 

Inside the Cell 

Now that you know every cell is wrapped in a protective cell membrane, let us take a tour inside and see what is going on beneath the surface.

The entire content (excluding the nucleus) within the cell membrane of a typical human cell is a busy, jelly-like interior called the cytoplasm, the cell's working interior. The cytoplasm includes the cytosol, organelles, and other inclusions:

• Cytosol is the fluid portion inside the cell, excluding organelles and other structures. It is composed primarily of water which makes up about 70% of its volume and serves as the site for many metabolic (biochemical) reactions. The cytosol is also the liquid in which the organelles "float".
   
• Organelles are the cell’s mini-organs, each with its own job to do. Just like your organs work together to keep your body functioning, the cell’s organelles team up to keep the cell alive, healthy, and productive. Some organelles make proteins, some generate energy, and others help clean up cellular waste.

All eukaryotic cells (like those in your body) have this organized internal setup. At the center is the nucleus, the “command center” of the cell, which safely stores the DNA, the instructions for building and running the entire cell. So yes, even a tiny cell is like a bustling city, full of activity and teamwork—with the nucleus acting as city hall, and organelles taking on roles like power plants, recycling centers, factories, and more.

Figure \(\PageIndex{1}\): Prototypical Human Cell.  While this image is not indicative of any one particular human cell, it is a general example of a human cell containing the standard organelles and internal structures. 

Organelles of the Endomembrane System

The endomembrane system is a group of connected membranes and organelles inside eukaryotic cells that work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products (proteins and lipids). Think of it as the cell’s internal shipping and processing center, making sure molecules get to the right place, both inside and outside the cell. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.
 

Endoplasmic Reticulum 🏭🧪

The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Endoplasmic Reticulum (ER). (a) The ER is a winding network of thin membranous sacs found in close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance and function (source: mouse tissue). (b) Rough ER is studded with numerous ribosomes, which are sites of protein synthesis (source: mouse tissue). EM × 110,000. (c) Smooth ER synthesizes phospholipids, steroid hormones, regulates the concentration of cellular Ca++, metabolizes some carbohydrates, and breaks down certain toxins (source: mouse tissue). EM × 110,510. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

Endoplasmic reticulum can exist in two forms: rough ER (RER) and smooth ER (SER). These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. Rough ER is so-called because its membrane is dotted with embedded granules — organelles called ribosomes, giving the RER a bumpy appearance. A ribosome is an organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Smooth ER (SER) lacks these ribosomes.

One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular Ca⁺⁺, a function extremely important in cells of the nervous system where Ca⁺⁺ is the trigger for neurotransmitter release. The smooth ER additionally performs a detoxification role, breaking down certain toxins.

In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.
 

The Golgi Apparatus: The Cell's Post Office 🏣

The Golgi apparatus is like the cell’s own post office. It sorts, modifies, and ships the proteins and other products that arrive from the rough endoplasmic reticulum (RER).

This organelle looks like a stack of flattened, squishy pancakes — each "pancake" is actually a membrane-bound sac. Like the ER, the Golgi apparatus is made of membranes, but it is arranged in a more compact, stacked form.

The Golgi apparatus has two distinct sides, each with a different job:

• One side receives incoming products in vesicles (tiny membrane bubbles).

• Inside the Golgi, those products are processed and sorted. Some are modified with sugars or other molecules.

• Then, they are repackaged into new vesicles and sent out from the opposite side.

If a product is meant to be exported out of the cell, the vesicle carrying it will move to the cell membrane, fuse with it, and release its cargo outside the cell. The cargo is thereby secreted in a process called exocytosis  (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Golgi Apparatus. (a) The Golgi apparatus manipulates products from the rough ER, and also produces new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes (or packaged and sent for fusion with existing lysosomes). (b) An electron micrograph of the Golgi apparatus.

Lysosomes: The Cell’s Recycling and Demolition Crew 🗑️🔧💣

Some of the proteins packaged by the Golgi apparatus include digestive enzymes that stay inside the cell. These enzymes are packaged inside special vesicles known as lysosomes.

A lysosome is a membrane-bound organelle filled with enzymes that break down unwanted or damaged parts of the cell — like a worn-out organelle. Think of lysosomes as the cell’s garbage disposal or even a demolition crew that clears out old, unsafe structures.

The process of a cell "cleaning up" its own components is called autophagy, which literally means "self-eating."

Lysosomes are also key players in defense against invaders. When immune cells like white blood cells engulf (or phagocytize) harmful bacteria, the engulfed bacteria are sent to a lysosome, where they are broken down and destroyed by enzymes. Because of this, defense cells are packed with lysosomes to help them do their job.

But lysosomes are not always so tidy. In severely damaged or dying cells, lysosomes may burst open and release their enzymes into the cell’s interior. This results in the destruction of the entire cell, a process called autolysis (“self-destruction”). Unlike apoptosis (a clean and controlled form of cell death), autolysis is messy, it can spill damaging enzymes into the surrounding tissue, especially during trauma, severe infection, or after death.
 

Interactive Link

Watch this video(opens in new window) to learn about the endomembrane system, which includes the rough and smooth ER and the Golgi body as well as lysosomes and vesicles. What is the primary role of the endomembrane system?

 

 

---

Mitochondria: The Cell's Power Plant ⚡

In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert to chemical energy that can be used to power biochemical reactions. 

A mitochondrion (plural: mitochondria) is a bean-shaped, membrane-bound organelle known as the “energy transformer” of the cell. Just like the nucleus, each mitochondrion has two membranes:

• An outer membrane (made of a lipid bilayer),

• And a highly folded inner membrane (also made of a lipid bilayer), which forms winding structures called cristae.

These folds give the inner membrane a large surface area, which is important because this is where the critical steps of cellular respiration take place.

During cellular respiration, mitochondria use oxygen and nutrients (like glucose) to produce ATP (adenosine triphosphate) — the molecule that powers almost all of the cell’s work. Think of ATP as the "energy currency" your cells spend to do anything from contracting muscles to sending nerve signals.

Because your body constantly needs energy, mitochondria are always working. Some cells need more energy than others, so they have more mitochondria:

• Muscle cells need lots of ATP to keep contracting, so they are packed with mitochondria.

• Nerve cells also need a steady supply of ATP to run their sodium-potassium pumps, and a single neuron may contain over a thousand mitochondria.

• In contrast, a bone cell, which is less active metabolically, might have just a few hundred mitochondria.

And yes, this is why you need to breathe! Oxygen is essential for your mitochondria to keep the ATP flowing in the process called aerobic respiration!

Figure \(\PageIndex{4}\): Mitochondrion. The mitochondria are the energy-conversion factories of the cell. (a) A mitochondrion is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. (b) An electron micrograph of mitochondria. EM × 236,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)