Crossing the Cell Membrane

When I first decided to write a post about membrane transport,  it seemed like an uncomplicated topic to tackle. But once I began the actual writing process, I realized that the subject was far too intricate to cover in a one page blog post. After all, cell membranes are responsible for maintaining ion concentrations between the cell and its environment. They provide a mechanical barrier that traps cellular constituents unique to each cell type. And they are selectively permeable and regulate what molecules enter the cell and what molecules leave the cell. The cell membrane is a complex, multi-functional structure that communicates with matrices inside and outside the cell as well as with other cells.  So for the sake of simplicity, I will focus on only a few types of transport processes and save the more complicated energy dependent multi-step transfers for another time.

phosp

Before talking about membrane transport,  let’s first take a look at the membrane’s basic structure shown at the left. A cell membrane is comprised of a phospholipid bilayer with the polar (hydrophilic, or “water-loving”) heads facing into the cell (intracellular) and outside of the cell (extracellular). The non-polar (hydrophobic, or “fat-loving”) ends of the phospholipid face inside the bilayer. This bilayer structure completely surrounds the cell and is filled with proteins, cholesterol and glycoproteins.  Many of the proteins transverse the membrane, some do not, and they are in constant communication with other proteins in the complex extracellular and intracellular matrices.  Membranes are fluid, that is they are in constant motion, and it takes about 1 second for a phospholipid to travel the length of a bacterial cell (about 2 μm).

Let’s take a look at the one-dimensional depiction shown below on the left. This over-simplified drawing illustrates the lipid bilayer with three proteins embedded into it, but keep in mind that this is hardly representative of what is actually there. The lipid bilayer acts as a cell membrane“gate-keep,” allowing only small, hydrophobic molecules such as O2 and CO2 to move through it unaided through the process of passive diffusion. This is illustrated in “A.” In this way, O2 is delivered quickly and efficiently into the cells when needed and its entry doesn’t depend on the presence of a protein, which might ultimately slow down the process because it takes a while for the proteins to make their way to the cell membrane. Despite their size, steroid hormones, such as testosterone and progesterone, can also pass through the cell membrane this way because they are fat-soluable and are not impeded by the membrane’s hydrophobic interior.

Ions and small polar molecules such as water require a transport protein to facilitate their movement through the membrane. The interior of the transport protein is charged, thereby attracting these molecules. This is illustrated at “B.” Each protein is unique in that it will only transport one type of molecule. For instance, the transport protein shuttling H+ across the membrane is different from that used for Ca++ .  Many of these proteins, such as  the aquaporins used to transport water molecules, are brought to the cell surface only when needed.

The drawing at “C” illustrates a transport protein that undergoes a conformational change upon the binding of its substrate. Similar to those shown at “B,” these proteins are specific for one particular substance, and many are brought to the surface only when needed.  As shown in the picture, the substrate binds to the binding site of the transport protein, and this interaction causes a “flip-flop,” or conformational change in the protein that results in the release the molecule into the cytoplasm. This release then causes the protein to resume its original structure. Glucose enters the cell by this means. There are several types of glucose transport proteins, and many of them are tissue specific. For example, the glucose transport proteins found in the brain are not dependent upon insulin. They are embedded in the cell membrane at all times to ensure the brain receives a constant supply of glucose regardless of the individual’s diabetic status.

The last mode of membrane crossing illustrated here is called receptor-mediated endocytosis, the process by which cells internalize large molecules that bind to cell surface receptors. This is illustrated  at “D.”  Once a molecule binds to the receptor on the cell surface, the membrane will invaginate, encasing the bound molecule within its own membrane. Enzymes take over from there, and the molecule is released into the cytoplasm and the membrane and receptor may then be recycled back into the cell membrane, thereby maintaining the size of the cell. Insulin, some vitamins and the HIV virus are examples of substances that gain entry into the cell by this method.

The survival of every organism is dependent upon this complex bilayer of their cells. One genetic mutation may prevent a particular protein from making its way to the cell membrane and result in serious consequences to the life of the organism. I am in constant awe at that premise, and once again it reveals that the mammalian species is truly remarkable.

Thanks for stopping by!

The Nature of DNA

Untitled

All living things grow and reproduce, and there must be a blueprint, directions if you will, that enables the organism to carry out these processes and maintain the integrity of the species. That blueprint, in most organisms, is deoxyribonucleic acid, better known as DNA. There are some viruses that do not have DNA, and instead use RNA as their blueprint. HIV is one of them. But many scientists argue that a virus is not a living organism, a topic to be discussed at a later date.

The structure of DNA was elucidated in 1953 by James Watson and Francis Crick. I had the honor of meeting Dr. Watson, the only survivor from that era, and I must say it was like meeting Mic Jagger. Watson stands as a symbol to perhaps the greatest achievement known to science, but make no mistakes, there were others involved in the discovery who never reaped the honors bestowed upon Watson and Crick. Rosalind Franklin deduced the structure of DNA prior to Watson and Crick through her X-ray diffraction photograph. Sadly, she passed away in 1958 at the age of 38 before the Nobel prize was awarded.

So what exactly is DNA? All proteins in the body are coded from DNA through a multistep process. DNA is a polymer, a molecule comprised of repeating similar parts. It is a double-stranded helix containing a series of the 5-carbon sugar deoxyribose, a nitrogenous base and a phosphate group. The phosphate group is what makes DNA acidic.

DNA 1

Let’s take a closer look at the double-stranded structure on the right. The backbone of DNA consists of alternating sugar and phosphate groups. The two strands run antiparellel. The sugar is chemically bonded to a nitrogenous base, which in turn forms hydrogen bonds with its complimentary base, forming complimentary base pairs. There are four different nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). A always pairs with T, and G always pairs with C. This molecule can replicate itself through a complex series of steps involving enzymes that are coded using its DNA. It’s the replication of this molecule that maintains the integrity of the dividing cell, and ultimately the species. If it were replicated erroneously, then the proteins that are coded from it will be altered, and they will not perform their job very well. As a result, genetic disorders may occur, as well as cancers.

DNA is separated into genes, the unit of hereditary information that codes for one particular protein. It is the sequence of nitrogenous bases, commonly known as the nucleotide sequence, that denotes what protein will be translated from that particular gene. Only one strand of the DNA codes for a protein, and that nucleotide sequence must remain constant. For example, a section of the nucleotide sequence that partially codes for a particular DNA repair protein is 5′ – GGC AAT CCT GTC CCC ATC – 3′. 5 Altering just one of these bases (creating a point mutation) may decrease the activity of this protein, or render it nonfunctional. That could be devastating, considering its job is to repair damaged DNA, or it could be beneficial in cases where you want to destroy the cell by way of DNA modulation, as in cancer chemotherapy. If just one point mutation in one particular gene could cause such drastic consequences, it is a wonder the human species is sill thriving. But there are many safeguards in place to circumvent the consequences caused by DNA damage, and that is what makes the cell so fascinating.

The human body is very complex and each cell contains 20,000 – 25,000 genes, and roughly 50 – 250 million base pairs. This is a lot of DNA, but fear not, DNA is efficiently packaged in the nucleus of most cell types (the exception being prokaryotic cells, which include bacteria).  Before cell replication, the genetic material condenses further to form chromosomes. Each chromosome is comprised of a single strand of DNA,  measuring on average 1.5 x 108 base pairs, that is tightly coiled and packaged along with proteins called histones. There are 23 pairs of chromosomes in human somatic cells (any cell except a sperm or egg). That makes 46 chromosomes per cell. Lined up end to end, the DNA in those chromosomes would extend approximately 2 meters in length. Not bad for a string of molecules.

histones

The understanding of DNA has come a long way from the days of Watson and Crick, but we still have miles to go until we fully unravel this marvelous structure.

How Insects Breathe

UntitledFor my first post, I would like to say something about insects. There are an estimated 10 million species of insects on Earth, and their success may be attributed to their simple, yet efficient, body structure. Their segmented exoskeletons, a topic we will discuss at a later date, provides armor that protects their bodies from trauma that would otherwise harm them. They have complex social structures and are fascinating creatures to watch.

Insects do not have lungs, but like other organisms within the Animalia kingdom, they require oxygen to carry on life processes.

Insects breathe through tiny pores on the sides of their abdomens. These pores, known as spiracles, are found on each segment of the abdomen and open and close in response to the concentration of oxygen within the insect’s tissues. They also close to prevent water loss within the insect. Just like humans, when an insect respires there is an exchange of gases, (O2 for CO2) and a loss of water.  When air passes through the spiracle, it will then travel through a trachea. These tracheae connect in a pipe-like network of air tubes  that extends the length of the insects body. They terminate in close association with an insect cell where the gas exchange will then take place between the membrane of the tracheoles and the cell membrane through the process of diffusion. This system of respiration is considered the simplest and most efficient respiratory system in animals. Besides the spiracles found on the abdomen, most insects also have three pairs of spiracles at the base of their legs.

Insects belong to the phylum Arthropoda. Most terrestrial arthropods (spiders,  ticks, centipedes) have this system of respiration. Aquatic arthropods, such as lobsters and crabs, breathe by a specialized internal gill system.

Please feel free to comment below, or if you wish to add anything, please do.