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How Our Body Works ...

How similar are we to other life forms?

It is obvious that humans are not the only life-form or organism residing on this planet. In fact, we are only one of several million different species of organisms. Organisms include everything from mammals, birds, reptiles, and insects, to plants, bacteria, fungi, and yeast.

 

But bear in mind that even though organisms such as a tomato plant and an octopus may seem completely different, they have numerous similarities which strongly suggest a common ancestry for all life-forms co-habilitating Earth, which includes humans. On the other hand, we humans have numerous features that are shared with only a few other species, namely apes, and further still we enjoy other features that no other species enjoys.

 

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 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. The term cell implies the concept of separation.

 

Each cell has the ability to function on its own. In living things comprised 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.

CELL STRUCTURE AND ORGANELLES

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 200 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. However, the unique characteristics of different types of cells such red blood cells, muscle cells and fat cells will be described elsewhere.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Let’s begin by examining the outer wall, or more scientifically the plasma membrane of cells. 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.

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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 Cell Figure). The various organelles are little operation centers within cells. Each type of organelle performs a different and specialized job (see Table below). 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. 

 

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 Cell Figure). 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.

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.

 

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. 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 is the composition of the plasma membrane?

Each cell is enveloped by a very thin membrane measuring only about 10 nanometers (nm) thick. A nanometer is one-billionth of a meter— pretty thin indeed. The makeup of the plasma membrane is a very clever combination of lipids and proteins with just a touch of carbohydrate and other molecules. Interestingly, plasma membranes use the basic principle of water solubility to allow for its barrier properties and it is the lipid that provides this character. Molecules that are somewhat similar to triglycerides (fat) called phospholipids are arranged to provide a water-insoluble capsule surrounding cells. What that means is that water-soluble substances such as sodium, potassium, and chloride, carbohydrates, proteins, and amino acids are not able to move freely through the membrane while some lipid substances and gases seem to move more freely. The plasma membrane will also contain the lipid substance cholesterol. Cholesterol appears to increase the stability of the plasma membranes Since the plasma membrane functions as a barrier between the outside and inside of the cell, there must be a means (or doorways) whereby many water-soluble substances can either enter or exit a cell. One of the roles of proteins in the plasma membrane is to function as doors, thereby allowing substances such as sodium, potassium, chloride, glucose, and amino acids to enter or exit a cell.

 

Do proteins in the plasma membrane have special roles?

If we were to weigh all of the components of the plasma membrane we would find that about half the weight of the membrane is protein. However, this is a bit misleading as the much smaller lipid molecules of the plasma membrane tend to outnumber protein molecules by about fifty to one. This means that the proteins tend to be larger and complex, which implies that they have important functions while phospholipids and cholesterol provide more structural support.

 

Are some membrane proteins involved in the movement of substances in/out?

Let us go into a little more detail about just how some of the proteins function as doorways in our plasma membranes. Some of these proteins function as channels or pores that will allow the passage of only one specific substance across the membrane. This is like opening the stadium doors for fans before a game. The concentration of fans outside the stadium is much higher than within and the natural flow is for the general movement of people into the stadium, an area of lower concentration.

 

Plasma membrane channels allow the passage of ions such as sodium, potassium, chloride, and calcium down their concentration gradient. However, the movement will be in mass amounts resulting in a sudden and significant change in a cell’s environment. As an example, ion channels are especially important in nerve and muscle cells, and drugs often prescribed for people with cardiovascular concerns are calcium-channel blockers.

 

We should stop for a moment and emphasize a very important concept. In nature, when provided the opportunity, things tend to move from an area of higher concentration to an area of lower concentration. This is referred to as diffusion. The movement of substances across our plasma membranes is an excellent example of diffusion. For example, skeletal muscle cells are told to contract by calcium. Thus for a muscle cell to be relaxed (not contracted) calcium must be pumped out of the intracellular fluid into the extracellular fluid as well as into a special organelle in muscle cells. In fact, the calcium concentration outside the muscle cell will be greater than ten times that ­inside when a muscle cell is relaxed. Then, when that muscle cell is told to contract, calcium channels on the plasma membrane and the organelle open and calcium diffuses into the intracellular fluid thereby allowing contraction to occur.

 

Let’s use calcium channel blocker drugs used to treat high blood pressure and angina as an example. Calcium-channel blockers (also called calcium blockers) inhibit the opening of calcium channels (pores) on heart muscle cells and muscle cells lining certain blood vessels. This reduces contraction of the muscle cells and as a result the heart pumps less vigorously and blood vessels relax, both contributing to a lowering of blood pressure and reduced stress on the heart.

 

Channels or pores are not the only types of proteins found in our plasma membranes. Other proteins can function as carriers that can “transport” substances across the membrane. Here again substances would be moving down their concentration gradient. These carrier proteins tend to transport larger substances such as carbohydrates and amino acids. Perhaps the most famous example of a carrier protein is the glucose transport protein (GluT) which is the primary concern in type 2 diabetes mellitus.

 

Do some membrane proteins function as pumps?

Not all substances move across the plasma membrane by down their concentration gradient. Since this type of movement seems to go against the natural flow of nature, to make this happen certain membrane proteins must function as pumps. Quite simply, pumps will move substances across a membrane against their concentration gradient or from an area of lower concentration to higher concentration. Pumps need energy which is derived from ATP. In fact, a very respectable portion of the energy that humans expend every day is attributed to pumping substances across cell membranes.

 

Are some cell membrane proteins receptors?

Some proteins in the plasma membrane 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.

 

What is DNA?

DNA (deoxyribonucleic acid) is found in almost all the cells of our body. Within those cells DNA is mostly housed in the nucleus, while a much smaller amount of DNA can be found in mitochondria. DNA contains the instructions (blueprints) for putting specific amino acids together to make proteins. You see, the human body contains thousands of different proteins, all of which our cells have to build using amino acids as the building blocks. Without the DNA’s instructions, our cells would not know how to perform such a task. DNA is long and strand like and organized into large structures called chromosomes. Normally we have twenty-three pairs of chromosomes in our nuclei. If we were to take a chromosome and find the end points of the DNA, we could theoretically straighten it out like thread from a spool. If we did so we would find thousands of small stretches called genes on the DNA. We have thousands of genes, which contain the actual instructions for building specific proteins. To oversimplify one of the most amazing events in nature, when a cell wants to make a specific protein, it makes a copy of its DNA gene in the form of RNA (ribonucleic acid). You see, DNA and RNA is virtually the same thing. However, one of the most important differences is that the RNA can leave the nucleus and travel to where proteins are made in cells, the ribosomes. At this point both the blueprint instructions (RNA) and the amino acids are available and it’s the job of the ribosomes to link (bond) amino acids together in the correct sequence.