Our tour today begins with an exciting overview of cell surface structures. There is enormous diversity of structures present on the surface of the cell and this helps the cell perform its many complex functions.You can think of the surface of the cell like the skin on our bodies. It provides a barrier to the outside world protecting us from harmful things, yet there are ways to let in the important things that we need. The cell surface allows us to detect and sense the environment around us, to ensure we can respond to the ever-changing surroundings.
The plasma membrane is a phospholipid bilayer that surrounds every cell and therefore represents the surface of all cells. The membrane is made of lipids which have hydrophobic (water-hating) and hydrophilic (water-loving) regions. Embedded in the membrane are various structures (proteinaceous and non-proteinaceous) which constitute the numerous structures that are found on the surface of cells.
Glycolipids are lipids that form part of the plasma membrane. They have a short carbohydrate chain covalently attached and this is exposed on the outer surface of the cell. The glycoplipids mainly have a communicative role, often acting as markers for cellular recognition. Additionally they provide stability for the cell and help cells join to other cells to form tissues.
One particularly interesting role glycolipids play in humans is their contribution to blood type. There are four main blood types: A, B, AB and O, and this variation stems from the different glycolipids present on the surface of red blood cells (erythrocytes).
Glycoproteins are integral membrane proteins. Like glycolipids, they have short carbohydrate chains covalently attached which are exposed on the outer surface of the cell. Integral proteins are those which penetrate the hydrophobic region of the plasma membrane. Most integral proteins are transmembrane proteins which span the entire plasma membrane.
Glycoproteins play a crucial part in cell-cell recognition, and have important roles in protection and the immune response, reproduction, structural integrity and cell adhesion.
Transport proteins are transmembrane proteins that provide hydrophilic channels through the membrane to allow specific substances in and out of the cell. Various nutrients, ions, gases, water and other substances need to be taken up by the environment, just as numerous waste products must be excreted from cells; this is the job of transport proteins present on the cell surface.
Channel proteins are one type of transport protein, which form a hydrophilic pore that perforates the membrane, allowing the movement of certain molecules to pass from one side of the membrane to the other. Aquaporins are a good example of a channel protein which allow the movement of water molecules through the membrane.
Carrier proteins are the other type of transport protein. They allow specific substrates to bind to the protein. This changes the conformation of the protein, moving the substance from one side of the membrane to the other.
A flagellum is a finger-like projection of a cell involved directly with cell motility. The flagellum is made of microtubules and is connected to the cell via the basal body. Flagella are long, thin and few in number, and most often associated with the surface of sperm cells. Flagella help cells to move through the internal environment with a spinning motion that drives a cell like a propellor.
Cilia are also finger-like appendages extending from the surface of the cell. Cilia are made from microtubules in the same arrangement as flagella; however they are usually a lot shorter than flagella and higher in number. One key difference between flagella and cilia is the way in which they move; flagella are involved with moving the cell through an environment, whereas cilia are involved with moving substances across a cell. Flagella act like a boat propellor pushing the cell forward, while cilia have a power and recovery stroke which passes substances over a cell in one direction. Cilia are often found on the lining of the windpipe (trachea), and their job is to sweep the mucus and dirt away from the lungs.
Cell signalling is a vital process in all organisms; it's how one cell talks to another cell to let them know what's going on. You can think of receptors as being like mobile phones; they allow you to pick up the information being sent to you. When you receive information from a friend the information gets sent to your brain where you process it, and then you respond accordingly by making a change. This is exactly the same in cells; the receptors act as mobile phones picking up the information, which causes signal transduction within the cell, which ultimately reaches the nucleus and changes in gene transcription occur to allow the cell to respond to the information. Without the receptors you can't process the information and therefore can't respond. This can often have significant consequences and can even result in cell death.
Molecules such as hormones and growth factors bind to receptors present on the surface of cells. This regulates the activity of intracellular proteins, and these transmit the signal from the receptor to intracellular targets, including transcription factors. These transcription factors then target the nucleus and cause changes in gene transcription, allowing the cell to respond to the relevant information received by the receptor. The ligand binding to the receptor thus causes a chain reaction of signalling events within the cell resulting in a programmed change which facilitates the cells overall growth, development and most importantly survival.
Neurotransmitters also bind to receptors on cell surfaces, however most neurotransmitter receptors are gated ion channels, like transport proteins, that control the influx and efflux of ions across the membrane to aid in chemical and electrical signalling (see transport proteins above and channel linked receptors below).
G protein-linked receptors are a type of receptor which upon ligand binding cause a conformational change in the receptor that ultimately activates a G protein. The activated G protein then binds to specific intracellular targets - for example an enzyme or transport protein -and alters its activity. The G protein-linked receptors are the largest family of cell surface receptors and are characterised by passing through the membrane seven times.
G protein-linked receptors are key to allowing us sense smells; these receptors are found in the nasal cavity and bind odorants which cause us to process smells in the environment.
Protein kinase-linked receptors are another type of receptor often present on cell surfaces. These receptors function not only as receptors but as protein kinases. When a specific ligand substrate binds to the receptor it's kinase activity is activated and the receptor phosphorylates a target protein within the cell. This often triggers a phosphorylation cascade.
Protein kinase-linked receptors are often involved with helping our bodies grow. During embryogenesis, specific growth factors bind to these receptors, which activate patterning processes and help our limbs to develop.
Channel-linked receptors are a type of receptor also known as gated ion channels. The channel-linked receptor functions both as a receptor and a channel on the cell surface. Upon the ligand binding to the receptor, the receptor causes the opening of a channel through the receptor, which allows ions to flow from one side of the membrane to the other, often controlling membrane potential and electrical signalling. Nervous communication depends heavily on the presence of ion channel-linked receptors on the cell surface which allows sodium and potassium ions to be transported across the membrane to permit the transmission of electrical signals.
It is imperative that cells are able to detect information in the environment so that they can transduce these signals within the cell to allow the cell to respond to the changes, so that they cell can grow, develop and survive. Even as you read these very words signal molecules are binding to receptors on cell surfaces and causing intracellular signal transduction so that we can process what each word means. Without the structures described above, we simply we not be able to stay alive.
Campbell, A & Reece, B (2008) Biology, Eighth Edition
Becker, W. M, Kleinsmith, L. J, Hardin, J, Bertoni, G. P (2009) The World of the Cell, Seventh Edition
Fastbleep © 2019.