Medical Biology/ Lecture: 3 Cell Structures and Function (Cell Membrane) PDF

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 Cell structures and function )Cell Membrane) Learning Objectives: - Define the cell membrane. Describe the structure of cell membrane. Determine the functions of cell membrane. Identify specialization of the cell membrane....

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 Cell structures and function )Cell Membrane) Learning Objectives: - Define the cell membrane. Describe the structure of cell membrane. Determine the functions of cell membrane. Identify specialization of the cell membrane. Describe the process of diffusion, osmosis, and facilitated diffusion. Describe the process of active transport and vesicle transport. Cell Membrane Structure The cell membrane (plasma membrane) is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. The most widely accepted model of plasma membrane structure is the fluid mosaic model of Singer & Nicolson (1972). The fluid mosaic model indicates that the cell membrane is not solid. It is flexible and has a similar consistency to vegetable oil. The fluid-mosaic model is a working description of membrane structure. It states that the protein molecules form a shifting pattern within the fluid phospholipid bilayer. Cholesterol lends support to the membrane. The plasma membrane is a mosaic of phospholipids, cholesterol molecules, proteins and carbohydrates. 1 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 Components of the Plasma Membrane Phospholipids – Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards. Cholesterol – Found in animal cell membranes and functions to improve stability and reduce fluidity. Proteins – May be either peripheral or integral (trans-membrane) and serve a variety of roles : 1. Channel proteins: form pores for the free transport of small molecules and ions across the membrane. 2. Carrier proteins: for facilitated diffusion and active transport of molecules and ions across the membrane. 3. Cell recognition: that identifies a particular cell. 4. Receptor proteins: that binds specific molecules such as hormones. 5. Enzymatic proteins: that catalyzes specific chemical reactions. Carbohydrate– Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to protein, forming glycoproteins, or lipids forming glycolipids. Function of Cell Membrane  Protect a cell by acting as a barrier between cell contents and surrounding environment.  Identification.  Regulate the passage of substances into and out of the cell.  Communicate with other cells via membrane junctions and CAMs (cell adhesion molecules).  The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell. Cell Membrane Specialization Some membranes have components that are specialized for a specific purpose: - 1- Microvilli: Fingers like extensions of plasma membrane Particularly abundant on the surface of the cells Involved in the absorption, such as the epithelial cells lining the intestine. 2 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 2- Intercellular junctions: There are three main types of junction's tight junctions, gap junctions, and desmosomes.  Tight junctions: are proteins that hold adjacent cells together very tightly so nothing can penetrate between them. Cells that line the digestive and urinary tract contains many tight junctions to ensure the contents within those hollow organs do not leak out into the outer layers or body cavity.  Gap junctions: are channels between neighboring cells for transport of ions, water, and other substances. Cells that contain gap junctions are found within the heart muscle and smooth muscle.  Desmosomes: these junctions hold cells together by a fiber, which allows movement without separation. Cells that contain desmosomes are found within the muscle tissue and the skin. 3 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 Movement across Cell Membranes The movement of substances across the membrane by two ways: 1) Passive ways (without the input of cellular energy). i. Simple Diffusion ii. Facilitated Diffusion iii. Osmosis (water only) 2) Active ways (requiring the cell to expend energy). i. Active Transport ii. Vesicle Transport 1) Passive ways A. Diffusion Diffusion is the net passive movement of particles from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. An example: gas exchange for respiration. B. Facilitated Diffusion Facilitated diffusion is a type of diffusion in which the molecules move from the region of higher concentration to the region of lower concentration assisted by a carrier protein. Common molecules entering/leaving cells this way include glucose and amino-acids. It is passive and not requires energy. 4 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 C. Osmosis Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution. - For example: the red blood cell (RBCs) is placed in a medium, which is a water solution, the possible consequences are listed below: * If a cell is placed in an isotonic solution, there is no net water movement, so there is no change in the size of the cell. * If a cell is placed in a hypertonic solution, water will leave the cell, and the cell will shrink. * If a cell is placed in a hypotonic solution, water will enter the cell, and the cell will swell. 5 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 2) Active ways a. Active Transport Active transport is the energy-demanding transfer of a substance across a cell membrane, from lower concentration to higher concentration. Special proteins within the cell membrane act as carrier proteins. b. Vesicle Transport Some molecules or particles are just too large to pass through the plasma membrane or to move through a transport protein. Vesicles or other bodies in the cytoplasm move macromolecules across the plasma membrane. So cells use two other active transport processes to move these macromolecules into or out of the cell. 1. Endocytosis is the process of capturing particle from outside the cell by engulfing it with the cell membrane. There are two main kinds of endocytosis:  Pinocytosis (cell drinking): This is the uptake of large molecules (protein, DNA) from solution, by a form of endocytosis – the vesicles formed are minute and short-lived.  Phagocytosis (cell eating): This is the uptake of solid particles by a cell e.g., Phagocytes engulfing bacteria. 2. Exocytosis: describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin. 6 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 3 References: Madder, S., S., and Windelelspecht, M., Human biology, 15th ed. Mc Graw – hill, USA, 2018. Sylvia S. Mader, Biology, 6th ed. Mc Graw-Hill Education, USA,1999. 7