LITMIR.BIZ

      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 organ could occur.

      Membrane transport

      To stay alive and function optimally, each cell has to take up useful molecules (such as oxygen, water, salts, sugars and amino acids), and eliminate waste products such as carbon dioxide, urea and uric acid. Movement of materials across the plasma membrane is called membrane transport.

      Simple diffusion

      Since the plasma membrane is a fluid structure consisting predominantly of phospholipid, molecules that are fat-soluble are able to dissolve in the phospholipid bilayer and pass rapidly across by a process called simple diffusion.

      Simple diffusion can be defined as:

      The passive movement of molecules from a region of high concentration to a region of low concentration until an even distribution of molecules (equilibrium) is achieved.

      Gases such as oxygen and carbon dioxide and lipid-based hormones such as steroids including testosterone and oestrogen are highly soluble in the fluid phospholipid bilayer and pass readily into and out of cells via simple diffusion. Simple diffusion also occurs rapidly in the lungs with oxygen inspired at high concentration from the atmosphere before diffusing rapidly across the alveolar air sacs into the blood. Conversely, carbon dioxide is at high concentration in the blood and passes across into the alveoli by simple diffusion before being eliminated during expiration. Concepts such as diffusion are often very abstract in nature, so to help consolidate your understanding of this process, attempt Activity 1.2.

      Activity 1.2 Team working

      Add a small amount of perfume, aftershave or nail varnish remover (acetone) to a piece of tissue and place it in the centre of the room.

      What do you notice?

      Now that you have had the opportunity of exploring simple diffusion via the diffusion of odours, we can examine a variant of diffusion.

      Facilitated diffusion

      Since many of the molecules required by cells are water soluble and not particularly soluble in lipid, they cannot pass across the plasma membrane by simple diffusion. Facilitated diffusion makes use of channel proteins which function as physical passageways to carry molecules across the plasma membrane.

      Facilitated diffusion can be defined as:

      The passive movement of molecules across the plasma membrane from a region of high concentration to a region of low concentration aided

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