Do Fat Cells Do More Than Store Energy?

For a long time fat tissue and their cells were viewed as somewhat inert containers of energy storage. However, today we know that adipose tissue functions as a gland with the capability to release a variety of factors relative to its size and endowed energy. As mentioned previously, some of these factors may promote the formation of more fat cells. Perhaps some of the most interesting released factors are those that circulate to the brain and provide insight to our energy storage status. One of the most important factors seems to be the hormone leptin. Fat cells release more and more leptin into our circulation when fat cells accumulate more fat. Leptin then signals the brain to reduce appetite. In addition, as fat cells swell due to excessive calorie consumption, some of the chemicals they release can promote the development and worsening of diabetes, high blood pressure and other medical conditions.

Are We Born with All of the Fat Cells We Will Ever Have?

We are not born with a full complement of fat cells as some scientists once thought. The number of fat cells in the body increases at various stages throughout growth, but by the time adulthood is reached the total number of these cells can become fixed. This means that if our body fat mass does not change, we probably would not produce new fat cells as adults. However, if we consume excessive calories, the number of fat cells can increase. In adipose tissue there is a small number of so-called pre-adipocytes or fat stem cells. When these cells are signaled, they will produce new fat cells. As you may have guessed, the signals are chemicals, many of which are released by existing fat cells when they become swollen with an increased bounty of stored fat.

How and When Do We Remove Fat from Our Fat Cells?

The fat stored in fat cells is available to us when food energy is not being absorbed (fasting) and when we exercise. Just as the hormone insulin promoted the storage of fat when energy was coming into our body, the process of mobilizing fat from fat cells is promoted by the hormones released into our blood when we are fasting and/or exercising (Figure 5.8). These hormones are glucagon, epinephrine, and cortisol, and all promote the release of fat from fat stores.

In order for fat to be released from fat cells, fat is first broken down to fatty acids and glycerol, which then enter our blood and circulate. However, because of their general water insolubility, the fatty acids will hitch a ride aboard a protein in the blood called albumin. On the contrary, glycerol is fairly water soluble and can dissolve into blood. In fact, researchers will measure the level of glycerol in the blood to estimate how much fat is being broken down.

Body fat is broken down to serve as energy in-between meals and during exercise.

Circulating fatty acids are removed by cells, especially skeletal muscle and our heart, liver, and other organs and then used by those tissues primarily for energy. However, keep in mind that cells of the brain and red blood cells (RBC) cannot use fatty acids for energy and will continue to use glucose. Conveniently the glycerol released from fat tissue can be used to make glucose in the liver and released into circulation to help maintain a desirable level of circulating glucose during prolonged exercise and fasting.

What Are Red Blood Cells?

Red blood cells (RBCs) have the responsibility of transporting oxygen throughout the body. About 33 percent of the weight of an RBC is attributed to a specialized protein called hemoglobin and thus RBCs are often referred to as “bags of hemoglobin.” Hemoglobin is a large and complex 

protein that contains four atoms of iron. Hemoglobin’s job is to bind to oxygen so that it can be transported in the blood. There are about 42 to 52 million RBCs per milliliter (or cc) of blood; and each RBC contains about 250 million hemoglobin molecules. Since each hemoglobin molecule can carry four oxygen molecules, the potential exists to transport one billion molecules of oxygen in each RBC.
 

There are two reasons for the need for such a large amount of hemoglobin in our blood. First, oxygen does not dissolve very well into our blood. Second, the demand for oxygen is extremely high in our body. Therefore, hemoglobin increases the ability of the blood to carry oxygen tremendously. Any situation that significantly decreases either the number of RBCs or the level of hemoglobin they carry can compromise oxygen delivery to our tissues and potentially compromise function and health.

Where Is ATP Made in Cells?

ATP is made in our cells by capturing some of the energy released from energy molecules when they are broken down in energy pathways. Most of the ATP made in our body is made in mitochondria (singular: mitochondrion). For this reason mitochondria are often referred to as the “powerhouses” of our cells. A relatively small portion of the ATP generated in our cells each day will be made in the intracellular fluid outside the mitochondria. As you might expect, cells with higher energy demands will have more mitochondria. This is certainly true for heart and skeletal muscle cells and cells within our liver.

Are Some Cell Membrane Proteins Receptors?

Last, but certainly not least, not all proteins in the plasma membrane function in transport operations. Some proteins function as receptors for special communicating substances in our body such as hormones and neurotransmitters. Typically, receptors will interact with only one specific molecule and ignore all other substances. In a way, then, these proteins can also be viewed as being involved in transport processes; however what’s being transported isn’t ions or molecules but information.

Do Individual Cells and Our Body as a Whole Attempt to Maintain an Optimal Working Environment?

Just as you clean your apartment or house and determine what kind of stuff is found within your living area, so too will our cells clean and regulate the contents in their intracellular fluid. This allows each cell to maintain an optimal operating environment. Scientists often use the term homeostasis to describe the efforts associated with the maintenance of this optimal environment. Furthermore, just as it is the responsibility of each cell to maintain its own ideal internal environment; at the same time many of our organs work in concert to regulate the environment within our body as a whole. These organs include the kidneys, lungs, skin, and liver. Many of our most basic functions, such as breathing, sweating, urinating, digesting, and the pumping of our heart, are actually functions dedicated to homeostasis (Table 2.2). Therefore, homeostasis is the housekeeping efforts of all our cells working individually as well as together to provide an environment conducive to optimal
function.

What Would We Expect to Find Inside of Our Cells?

Immersed in and bathed by the intracellular fluid are small compartments called organelles. The word organelle means “little organ.” Two of the more recognizable organelles are the nucleus and mitochondria. Other organelles include endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes (see Figure 2.1). The various organelles are little operation centers within cells. Each type of organelle performs a different and specialized job (Table 2.1). Each organelle has its own membrane with many similarities to the plasma membrane. Therefore, as we discuss the nature of the plasma membrane below you can keep in mind that some of these features also pertain to organelle membranes as well.

Cells contain special compartments called organelles, which have special functions to support total cell function. Also within the intracellular fluid of certain cells we would expect to find some energy reserves in the form of fat droplets and glycogen (carbohydrate) (see Figure 2.1). The amount of glycogen and fat will vary depending on the type of cell. Another important component of cells is ribosomes. Ribosomes are the actual site where proteins are constructed.

What Do Cells Look Like?

Human cells can differ in size and function. Some are bigger and some longer, some will make hormones while others will help our body move. In fact, there are roughly two hundred different types of cells in our body. Although these cells may seem unrelated, most of the general features will be the same from one cell to the next. Therefore, we can discuss cells by describing the features of a single cell. The unique characteristics of different types of cells such red blood cells, muscle cells, and fat cells will be described as they become relevant later in this chapter and book.
 

Let’s begin by examining the outer wall, or more scientifically the plasma membrane of cells. As shown in Figure 2.1, the plasma membrane separates the inside of the cell from the outside of the cell. The watery environment inside the cell is called the intracellular fluid. Meanwhile, the watery medium outside of cells is called the extracellular fluid. Previously, it was noted that our body is about 60 percent water. Of this 60 percent, roughly two-thirds of the water is intracellular fluid while the remaining one-third is extracellular fluid, which would include the plasma of our blood.

What Are Cells?

Among the millions of species on this planet, the cell is the common denominator. Cells are the most basic living unit. In many species, such as bacteria and amoeba, the entire organism consists of a single isolated cell. But for plants and animals, including us, the organism exists as a compilation of many cells working together. In fact, every adult human is a compilation of some 60 to 100 trillion cells.
 

As a rule of nature life begets other life and thus all cells must come from existing cells. This is to say that in order to create a new cell, an existing cell has to divide into two cells. It also suggests that all life-forms on Earth may be derived from the same cell or type of cell. The process of cell division is tightly regulated and, as we will discuss in later chapters, when this regulation is lost and cells divide out of control, cancer can arise.

When you and I were conceived, an egg (ovum) from our mother was penetrated by our father’s sperm. This resulted in the formation of the first cell of a new life. Therefore, everyone you know was only a single cell at first. That cell had to then develop and divide in two cells, which themselves divided to create four cells, and so on.
 

Our body is composed of 60 to 100 trillion cells, each of which contributes to overall health and well-being.
 

The term cell implies the concept of separation. Each cell has the ability to function on its own. In living things composed of numerous cells, such as humans, individual cells are also sensitive and responsive to what is going on in the organism as a whole. Therefore, these cells survive as independent living units and also cooperatively participate in the vitality of the organism to which they belong.