The Cell: Nutrient Intake

It has been a while since I last delved into the inner workings of the human cell. The series I intended to write on the cell has been protracted and has at least one further installment to come.  This particular installment will focus on the mechanisms cells use to intake nutrients. These processes are critical to cellular function. Without them, the cell would starve from lack of nutrients. However, they are also key to our understanding of the origin of the cell. This article will explore both aspects of cellular nutritional intake.

Cells uptake nutrients through a variety of methods, but before going into those, it is important to recognize the parts of the cell that are utilized in the processes. The most important part in human cells is the cell membrane. Human cells lack a cell wall, which is only found in plants.  The cell membrane is composed of something called a phospholipid bi-layer. This incredible structure is key to how the cell intakes nutrients and minerals. The structure is a double layer of phospholipids which rings the outside of the cell. Embedded throughout this layer are specialized proteins we will discuss in a moment. The individual phospholipids are composed of a phosphate “head” and a lipid “tail”. The phospholipid is a unique molecule becuase it is both hydrophilic and hydrophobic. The phosphate end, which resembles a pinhead in shape, is hydrophilic. It seeks the presence of water. The lipid tail is hydrophobic, meaning it does its best not to find water. Thus the phospholipid bi-layer is constructed in such a way that both the lipid and the phosphate get their wishes.  The phosphate molecules face into the cell, towards the cytoplasm and out towards the extracellular interstitial fluid.  the lipid molecules form the tie that binds the membrane together, far from the water.  This unique structure grants the cell flexibility but also gives it sufficient strength to hold together under pressure.  However, this specialized cellular membrane is not enough. There must be a way to get nutrients through the membrane.

The first and perhaps most well known way to bring nutrients into the cell is called osmosis. Osmosis relies on two ideas: concentration and permeability. A concentration is the amount of a given substance, perhaps carbon, in a given area. Permeability is used to describe how easily, if at all, it is for molecules to pass through a given substance.  Osmosis is a completely natural process and not regulated by the cell in any way. It relies on the principle of diffusion. Diffusion is what happens when you place a drop of food coloring into a glass of water. At first, the food coloring is only in the place you dropped it in. It has a high concentration in that location. However, given enough time, the food coloring will diffuse throughout the glass, until its concentration is equal in every area of the glass.  Osmosis works the same way except it involves crossing a membrane. The more permeable the membrane, the faster the diffusion and the larger molecules can cross it.  The phospholipid bi-layer is considered semi-permeable, meaning only molecules smaller than a certain size can pass through it via osmosis.  Osmosis can only occur if the concentration of our example molecule is lower inside the cell than it is outside.  This is because concentrations will always attempt to diffuse from an area of high concentration to an area of low concentration in an attempt to reach equilibrium.

Of course, osmosis also poses problems to the cell. If the concentration of our example carbon is lower outside the cell than it is inside the cell, then carbon will attempt to leave the cell. To prevent catestrophic loss of minerals or nutrients and to intake molecules larger than can pass across the phospholipid bi-layer, cells use ion transporters. Ion transporters are the proteins mentioned earlier found in the phospolipid bi-layer.  Their function has a similar goal to osmosis but works opposite the principle of diffusion. The cell is now attempting to bring in something which either has a higher concentration inside the cell than out, or is too big to pass through the membrane. An example is the potassium pump. This requires the cell to expend energy, which to the cell is ATP. Given a certain amount of ATP, the cell will create an ionic gradient on the outside of the potassium pump which will attract positively charge potassium ions. The pump will then capture the ions and transport them across the membrane. However, not all pumps work this way. Some, such as the hydrogen pump, actually create ATP by interfacing with a specialized enzyme as they are pumped across and activating ADP into ATP.  The cell has complete control over the activation of these pumps. When specialized sensors in the cell detect that potassium is low, they send a message to the potassium pumps which turn them on and cause them to begin to uptake potassium.

The above barely even scratches the surface of the depth involved in the cell up taking nutrients. It is an incredibly complex process that is still not entirely understood. However, its very complexity defies evolution. The chemical process of osmosis, something that would appear to have no value to life at all, yet the cell has co-opted it to allow nutrients to enter the cell at no energy cost. This kind of hand in glove fit requires forethought and design. The phospholipid bi-layer is embedded with proteins that serve as transporters for minerals and atoms. Did the first cell have them? If not how did it gain minerals it needed to survive? If it did, how did evolution have the foreknowledge to equip the cell with exactly what it needed to function? Evolution is supposed to be a blind, unguided process, yet it had to have equipped the first cell with exactly what it needed, or the cell would have died. Dead cells do not evolve. The ion pumps require a designer. No blind process could have foreseen the need for them when it put together the cell.  The cell demands a designer. Evolution could not have provided all the parts necessary for the first cell to function. It had to have all been in place at once, it could not have evolved slowly because every feature is essential to the cells survival.