The Golgi apparatus is a specialised region of smooth ER resembling a series of crescent-shaped stacked membranes (Figure 1.1). The Golgi is frequently referred to as the cell’s ‘packaging and export’ region since it is involved in preparing material for release from cells. Its key role is refining proteins from the rough ER; this usually involves adding sugar residues to the crude amino acid sequences via a process termed glycosylation. The refined proteins may be used within the cell or may leave the Golgi in membranous sacs called secretory vesicles which travel to the cell membrane before their contents are discharged out of the cell. Cells that have a secretory role such as those within endocrine glands may each have several well-developed regions of Golgi apparatus; a good example would be the insulin-producing beta cells of the pancreas. The Golgi is also responsible for packaging digestive enzymes required for intracellular digestion into small membrane-bound sacs called lysosomes (see below).
Mitochondria
Mitochondria are small bean-/boat-shaped cellular organelles (Figure 1.1) responsible for releasing energy within cells. Each mitochondrion consists of an outer smooth membrane and a highly folded inner membrane. The prominent folds of the inner membrane are termed cristae and associated with these folds are the enzymes responsible for cellular respiration. Within the mitochondria, glucose, which is derived from carbohydrate-rich foods, is reacted with oxygen acquired by our respiratory system to release energy. This energy is then used to synthesise the energy storage molecule adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and free phosphate. This process results in the production of water and carbon dioxide as waste products. Since these biochemical reactions occur in the presence of oxygen, the process is referred to as aerobic respiration.
Glucose (C6H12O6) + Oxygen (O2) → Carbon dioxide (CO2) + Water (H2O) + Energy (38ATP)
In theory each molecule of glucose can yield 38 molecules of ATP but in reality this is never achieved, and a yield of around 30 ATPs per glucose molecule is typical. From a nursing point of view the simple equation above tells us something essential about human physiology: to generate the energy necessary to keep us alive we must eat (glucose) and breathe (oxygen). Indeed, a key role that nurses play is in ensuring that their patients receive adequate nutrition and that oxygenation of the blood is maintained.
If the supply of oxygen is significantly reduced then aerobic respiration becomes impossible and the cell is forced into anaerobic respiration. This is a far less efficient process that results in only 2 ATP molecules being produced per molecule of glucose. The incomplete breakdown of glucose also leads to the accumulation of the metabolic waste product lactic acid (lactate). Many people experience the effects of anaerobic respiration when they participate in hard manual labour or when lifting weights in a gym. When muscles are forced into anaerobic respiration the accumulation of lactic acid is usually experienced as soreness, fatigue and sometimes pain.
The plasma (cell) membrane
All human cells are surrounded by an outer membrane referred to as the plasma membrane. This has a multitude of functions including: holding the cell together as a discrete intact unit, regulating the movement of materials into and out of the cell and communication and recognition between cells.
Figure 1.4 Structure of the plasma membrane
The plasma membrane is predominantly composed of a phospholipid bilayer within which are located a variety of proteins (Figure 1.4). Since phospholipid is a fluid, with a similar consistency to vegetable oil, and the denser proteins are positioned throughout its structure, under a microscope it has a mosaic-like appearance, hence the plasma membrane is frequently referred to as having a fluid-mosaic structure. The phospholipid molecules originate from the smooth ER while the integral proteins initially are synthesised by ribosomes, and refined in the Golgi before being transferred to and inserted into the membrane.
The plasma membrane is not a static structure; phospholipid and protein molecules are continually being added and removed depending on the current needs of the cell. The phospholipid bilayer is often referred to as being self-forming. Each phospholipid molecule consists of a hydrophilic (water-loving) head portion and two hydrophobic (water-hating) tails. Since the intracellular compartment of the cell is full of the water-based cytosol and the outside of the cell is surrounded by watery interstitial fluid, the phospholipid molecules naturally form a bilayer as the hydrophobic tails orientate themselves away from the aqueous environments of both the intracellular and extracellular compartments (Figure 1.4).
There are many different types of protein molecules within the phospholipid bilayer, including channel proteins which span the entire width of the membrane and form pores through which materials can enter and leave (see below), and receptor proteins which form three-dimensional pockets into which chemical signals such as hormones can fit.
The glycocalyx
Most of the proteins that are found embedded in the plasma membranes are actually glycoproteins since they have been refined by the addition of sugar (glyco) residues within the Golgi. Some of these sugar residues extend away from the outer surface of the plasma membrane in the form of large polysaccharides and these collectively form a thin shell of sugar around each cell called the glycocalyx (Figure 1.4). The glycocalyx includes a key set of human glycoproteins referred to as the major histocompatibility complex (MHC). These MHC proteins play a key role in cellular recognition. With the exception of genetically identical siblings, every person has their own set of MHC proteins which uniquely identify their cells as belonging within their body. MHC proteins can cause problems when organs are transplanted since the immune system of the recipient will immediately recognise the cells of the donor organ such as a kidney or heart as being foreign and begin to attack the transplant.
For this reason, most organ transplant patients will require immunosuppressive drugs to help reduce the speed of rejection. Unfortunately, because these medications reduce the patient’s natural immune responses, they can increase the risk of opportunistic infections. Even with immunosuppressive drugs, gradually the donated organ is usually rejected and some younger transplant patients may have to undergo several transplants during their lifetime. The only major cells that do not have MHC proteins on their surface are erythrocytes (red blood cells); this is fortunate because it allows for routine blood transfusions of cross-matched blood without the risk of transplant reactions and rejection. To help you understand the potential of transplanted organs to be rejected, explore Jack’s case study.
Case study: Jack – organ transplant rejection
Jack is a 32-year-old male who received a donor kidney following several years of renal dialysis. Five months after the transplant, he began to experience flu-like symptoms and was unusually tired. His temperature was slightly raised at approximately 38°C and he noticed that he was passing less urine than normal. Despite making a good recovery in the immediate post-operative period, Jack began to feel some tenderness over the transplant area. His wife made an appointment for Jack to see his GP who suspected that Jack may be rejecting the transplanted kidney. She contacted the transplant team who admitted Jack to hospital where a renal biopsy confirmed the GP’s suspicions. A high-dose steroid drug called methylprednisolone was given for three days and fortunately the rejection process was suppressed. After reviewing Jack’s medication and reminding him of the importance of taking his medication as prescribed, the team discharged him home.
Jack’s case study highlights the importance of patient vigilance following organ transplants; luckily the rejection of Jack’s transplanted kidney was suppressed before major damage to the transplanted
