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Western University
Physiology 2130
Anita Woods

Human cell Introduction  It is amazing to see that the functioning of our organs and systems relies upon the complex functions of the cells that look so simple when viewed though a light microscope Basic Cell Organelles  All cells in the body share similar features and organelles  Nucleolus: dense body within the cell nucleus which contains specific DNA that produces the RNA found in ribosomes  Endoplasmic Reticulum: it is a continuation of the cell’s nuclear membrane and is the site for the synthesis, storage and transport of proteins and lipid molecules. The two types of ER: o Rough Endoplasmic Reticulum  Covered with rows of ribosomes  Site for protein synthesis  Proteins manufactured here are packaged into vesicles that transport them to Golgi apparatus o Smooth or agranular Endoplasmic Reticulum  Lacks ribosomes  Site for synthesis of lipids and fatty acids  Golgi apparatus: it is responsible for packaging proteins from the rough endoplasmic reticulum into membrane-bound vesicles. Two types of vesicles are produced in the Golgi bodies: o Secretory vesicles: transport proteins to cell membrane for release into the extra-cell environment o Storage vesicles: vesicles whose contents are stored for use within the cell such as lysosomes  Lysosomes: a type of storage vesicles produced by the Golgi apparatus o Act as digestive system of the cell o Contain several kinds of enzymes that are used by the cell to destroy damaged organelles, kill bacteria and break down other kinds of biomolecules  Ribosomes: dense granules of RNA and protein that are responsible for manufacturing proteins from amino acids under the control of the cell’s DNA. There are two types of ribosomes: o Fixed ribosomes: attached to the endoplasmic reticulum o Free ribosomes: float in the cytoplasm and  Often form in groups of 10 to 20 known as polyribosomes  Mitochondrion: is the membranous organelle where most of the body’s adenosine triphosphate (ATP) is generated. o Mitochondrion is often called the powerhouse of the cell since ATP is the primary cellular mechanism for energy storage and transfer o The number of mitochondrion in a cell is determined by that particular cell’s energy needs o The mitochondrion can replicate themselves even if the cell is not undergoing division  Example: when a cell has increased energy demands over a period of time such as muscle cells that are regularly exercised  Centrioles: cylindrical bundles of microtubules that are responsible for directing the movement of DNA stands during the process of cell division  Cell (plasma) membrane: its primary function is to regulate the passage of substance into and out of the cell o It allows certain molecules to cross the membrane while excluding other molecules o It also plays an important role in detecting signals from other cells and in forming physical links with adjacent cells The Cell Membrane  The cell membrane separates the intracellular and extracellular environment o Cannot penetrate: proteins, nucleotides and other large molecules needed for structure and function of the cell o Can penetrate: other molecules and many ions to varying degrees  It is selectively permeable: o Provides a two-way movement for nutrients and waste needed to sustain metabolism o Prevents the passage of other substance between the inside and outside environment  The cell membrane is made up of o Proteins that form channels and pores o Carbohydrate molecules for cell recognition o Cholesterol for stability o The most abundant components are the phospholipid molecules  Phospholipid molecule: is composed of a phosphate head and a lipid tail. The primary structure of the cell membrane is a double layer of phospholipid molecules. o Hydrophilic heads: that make up the cells membrane face out into the water base solutions inside and outside of the cell o Hydrophobic tails: the molecules hydrophobic tails are oriented away from the aqueous and extra an intracellular solutions into the cell membrane  Cholesterol molecules o Helps make the membrane impermeable to some water soluble molecules o Helps to keep the membrane flexible over a wider temperature range. o They are found inserted into the non-polar lipid layer of the membrane  Carbohydrate molecules o Found associated with extracellular membrane proteins or lipids o They form a protective layer called the glycocalyx which plays a key role in the immune response of the cell and recognition of other cells in the body  Membrane spanning proteins: are embedded in the phospholipid bilayer such that they span the entire width of the membrane o These membrane spanning proteins act as gates or channels that control the movement of certain substances into and out of the cells  Associated proteins: Enzymes o Associated proteins can be attached to either the intracellular or extracellular surface of the membrane. o Enzymes are a form of associated protein which act as catalysts for certain reactions immediately inside or outside the cell membrane  Associated proteins: Structural o Structural proteins are generally attached to the INSIDE surface of the cell membrane o They can support and strengthen the membrane while others may anchor some cell organs to the intracellular side of the membrane Phospholipids  A phospholipid molecule is made up of: o Phosphate head: hydrophilic o Fatty acid (lipid) tail: hydrophobic  As a result, when many phospholipids are thrown into water, they will align themselves into a lipid bilayer so that the head groups face out towards the water and tails away from the water o This is how they are arranged in the cell membrane o The hydrophobic fatty acid tails are the major barrier to water and water-soluble substances such as ions, glucose, urea and most other molecules found in living organisms o Fat-soluble substances like oxygen, CO2, steroid hormones can penetrate this portion of the membrane sine they dissolve though the lipid region of the membrane Membrane Proteins The other important components of the cell's membrane are the proteins. Membrane proteins have many different functions, including the following: 1. Receptors for the attachment of chemical hormones and neurotransmitters 2. Enzymes that help with chemical reactions or breakdown molecules 3. Ion channels or pores that allow water-soluble substances, like ions, into the cell 4. Membrane-transport carriers that transport molecules across the membrane (this may include gated channels) 5. Cell-identity markers like antigens or glycoproteins. Antigens are foreign particles that can stimulate the immune system  One of the most important functions of the proteins is to transport substances across the membrane. Some require proteins and some that do not. Membrane-transport mechanisms include the following: 1. Exocytosis/endocytosis (pinocytosis for small molecules) 2. Diffusion through the lipid bilayer (in the case of fat-soluble molecules) 3. Diffusion through protein channels (in the case of water and water-soluble molecules) 4. Facilitated diffusion 5. Active transport  Exocytosis o Cell secretes large molecules that are unable to leave the cell through the plasma membrane; so they leave the cell through the process of exocytosis o After production in the system of membrane in ER, the molecules are packaged in a small sack of membrane called a vesicle o The vesicle moves to a stack of membranes called a Golgi apparatus and their membrane merge and the vesicles releases its contents for modification o As they leave this apparatus, they are packaged in a vesicle again o As the membranes of the vesicle and plasma membrane merge the products of the vesicles leave the cell without actually crossing the plasma membrane  Endocytosis o The above process can also happen in reverse that allows large molecules to enter the cell  Diffusion o Diffusion is the movement of molecules from an area of high concentration to low concentration due to the molecule’s random thermal motion o Example: A drop of dye (first localized at high concentration) will spread out to an area of lower concentration, down the concentration gradient. The dye will continue to move until it is uniform throughout and there is NO more concentration gradient (net movement is zero). Equilibrium is reached (diffusion ceases) and the net diffusion is zero although the dye molecules are still moving about. o Electrically charged molecules, including ions like sodium ions (Na ), tend to move toward areas of opposite charge  Positively charged ions move toward negatively charged areas (and vice versa) down their electrical gradient.  Therefore, charged ions can move down both their chemical concentration gradient and electrical gradient.  If the chemical and electrical gradients are in opposite directions, the movement of the ion will depend on the balance of the two gradients and will stop moving when the molecules reach electrochemical equilibrium (when the electrical force is equal to and in opposite direction to the chemical force)  Like ions repel; unlike attracts  Diffusion of Lipid-Soluble Substances o Substances that are lipid soluble can pass right through the cell membrane, while those that are water soluble have a harder time. Lipid soluble substances include:  Oxygen  Carbon dioxide  Fatty acids  Steroid hormones o These molecules can diffuse easily through the lipid bilayer and are not stopped by the hydrophobic fatty acid chains  Diffusion of Water-Soluble Substances o Water soluble substances will not pass through the fatty acid region of the lipid bilayer directly but can still cross the membrane o Substances like water and ions (Na+, K+) can cross the membranes through special protein channels or pores o Each pore or channel is quite specific and will generally allow one type of ions through (there are exceptions) o Diffusion of a water-soluble substance through the membrane requires both a pore or channel and a concentration gradient  Diffusion factors: the rate of movement of molecules through protein channels is limited by several factors o Size of the protein channel approximately 0.8 nm, will limit the size of the molecule. Sugar molecule, for example, is too large to diffuse through o Charge on the molecule: (Na+) will affect the rate of movement through channels because the proteins that make up the channels also have charges on them. Thus, a + ion will not go through a channel that has a + charge o Greater the electrochemical gradient of a molecule, the greater its rate of movement through the channels. Substances move both their concentration and electrical gradients (opposite charged attract) o Number of channels in the membrane: even if there is a large concentration gradient for an ion, the ion will not move across the membrane unless there is a channel for it. Thus the more channels, the more ions will diffuse through  Facilitated Diffusion o Other water-soluble substances (sugars) that cannot diffuse through the lipid bilayer and are too large to pass through the protein channels still cro
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