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Physiology 2130
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Physiology 2130 Module 3 Online Notes (Sec 3.1 to 2.7) Human Cell Introduction  Each cell is a living entity  Our organs and systems rely upon the complex functioning of these cells Basic Cell Organelles Cell Membrane – Primary function of plasma membrane is to regulate passage of substances into and out of the cell. It allows certain molecules to cross the membrane while excluding other molecules. It also plays in important role in detecting chemical signals from other cells and in forming physical links with adjacent cells. Endoplasmic Reticulum – Continuation of the cell’s nuclear membrane, it is the site for the synthesis, storage and transport of proteins and molecules.  Rough ER – Covered with rows of ribosomes, it is the site of protein synthesis. These proteins are packaged into vesicles that transport them to the Golgi Apparatus.  Smooth ER – Lacks ribosomes, responsible for the synthesis of lipids and fatty acids Nucleolus – Dense body within nucleus contains specific DNA that produces RNA found in ribosomes. Mitochondrion – Membrane organelle where most of the ATP is generated  Often called the power house of the cell since ATP is the primary mechanism for energy storage and transfer  Number of Mitochondrion in a cell is determined by the cell’s energy needs  When a cell has increased energy demands over a period of time such as muscle cells that are regularly exercised, the mitochondria can replicate themselves even if the cell in not undergoing division Golgi Apparatus – Responsible for packaging proteins from rough ER into membrane-bound vesicles  Secreting Vesicles – transport proteins to the cell membrane for release into the extra cellular environment  Storage Vesicles – Such as lysosomes, contents are stored for use within cell Lysosome – Storage vesicle produced by Golgi Apparatus; act as digestive system of cell. Contains several kinds of enzymes that are used by the cell to destroy damaged organelles, kill bacteria, and break down other kinds of bio molecules. Ribosomes – Dense granules of RNA and Protein  Responsible for manufacturing proteins from amino acids under the control of the cell’s DNA  When free ribosomes are in groups of 10 or 20 they are polyribosomes The Cell Membrane  Separates the intracellular environment from the extracellular environment  Proteins, nucleotides, and other large molecules needed for structure and function of the cell cannot penetrate the membrane  Meanwhile, other molecules and ions can penetrate this membrane to varying degrees  Therefore, it is Selectively Permeable - provides two way traffic for nutrients and waste needed to sustain metabolism, while preventing the passage of other substances between the intracellular and extracellular compartments Cell Membrane Structure 1. Phospholipid Molecule: Composed of a phosphate head and a lipid tail; the primary structure of the cell membrane is a double layer of phospholipid molecules. Hydrophilic Head – The hydrophilic heads of the phospholipid molecules face out into the water base solutions inside and outside the cell. 2. Hydrophilic Tail – Hydrophobic tails are oriented away from the aqueous and extra and intracellular solutions into the cell membrane. 3. Cholesterol Molecule – These molecules are found inserted into the non- polar lipid layer of the membrane. The cholesterol helps make the membrane impermeable to some water-soluble molecules and also helps to keep the membrane flexible over a wider temperature range. 4. Associated Protein: Enzyme – Associated proteins can be attached to either the intracellular or extracellular surface of the membrane. Enzymes are a form of associated protein which act as catalysts for certain reactions immediately inside or outside the membrane. 5. Carbohydrate Molecule – Groups of carbohydrate molecules can be found associated with extracellular membrane proteins or lipids. They form a protective layer called the glycocalyx, which plays a key role in the immune response of the cell and in recognition of other cells in the body. 6. Membrane Spanning Protein – Certain proteins are embedded in the phospholipid bilayer such that they span the entire width of the membrane. These membrane-spanning proteins can act as gates or channels that control movement of certain substances into and out of the cell. 7. Associated Protein: Structural – Structural proteins are generally attached to the inside surface of the cell membrane. These proteins can support and strengthen the membrane while others may anchor some organs to the intracellular side of the membrane. Phospholipids  Phospholipid molecules are molecules made up of a phosphate head and a fatty acid tail  The fatty acid tails of this molecule are hydrophobic while the phosphate heads are hydrophilic  When many phospholipids are thrown into water, they will align themselves into a lipid bilayer so that the head groups all face out towards the water and the tails away from the water o This is why they are arranged this way in the cell membrane  Since the fatty acid tails are hydrophobic, they are the major barrier to water and water soluble substances such as ions, glucose, urea, and most other molecules found in living organisms  Fat-soluble substances like oxygen, carbon dioxide, and steroid hormones can penetrate this portion of the membrane easily, as they can dissolve through the lipid region of the membrane Membrane Proteins The other important components of the cell’s membrane are the proteins: 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.  A main function of the proteins is to transport substances across the membrane  There are many ways substances cross the membrane – some require proteins and some that do not: 1. Endocytosis/exocytosis (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 Diffusion  Diffusion is the movement of molecules from an area of high concentration to low concentration due to the molecules’ random thermal motion  Example: When a drop of dye is added to a glass of water, the dye molecules will be localized to an area of high concentration. These molecules are constantly moving in a random manner bumping into each other and the water molecules. Slowly, the dye will spread from an area of high concentration to the area of lower concentration down what is called the dye’s chemical concentration gradient. This concentration gradient is like a ski hill; skier moves down the hill from high elevation to low elevation. They dye will continue to move until its concentration is uniform throughout the glass of water, at which point there is no more concentration gradient and the net movement is zero: chemical equilibrium (net diffusion = zero). However, the dye and water molecules are still randomly moving about. +  Electrically charged molecules like (Na ) tend to move towards areas of opposite charge, tend to move towards areas of opposite charge down their electrical gradient  Therefore, the charged ions can move down both their chemical concentration gradient and their electrical gradient  If the gradients are in opposite directions, the movement of the ion will depend on the balance of the two gradients and will stop when the molecules reach electrochemical equilibrium (electrical force equal to and in opposite direction to the chemical force) Diffusion of Lipid-Soluble Substances  Substances that are lipid soluble can pass right through the cell membrane, while those that are water soluble have a tougher time  Lipid-soluble (or fat-soluble) substances include oxygen, carbon dioxide, fatty acids, and some steroid hormones. These molecules can diffuse right through the membrane's lipid bilayer and are not stopped by the hydrophobic fatty acid chains. Diffusion of Water-Soluble Substances  Substances that are water soluble cannot diffuse directly through the fatty acid region of the cell membrane but may still cross  Some of these substances like water and many ions (K , Na , etc.) may cross cell membranes through special protein channels or pores  Each pore or channel is quite specific and will generally only allow one type of ion through Diffusion Factors  The rate of movement of molecules through protein channels is limited by several factors: Facilitated Diffusion  Other water-soluble substances (such as sugars) that cannot diffuse through the lipid bilayer and are too large through protein channels still cross the membrane fairly quickly  These molecules attach to specific protein carriers on the membrane and cause a change in the protein’s shape  The result of this is either an opening of the protein channel through which the molecule passes, or the protein rotates the molecule to the inner surface of the membrane where it is released *watch videos on web*  The process of facilitated diffusion is similar to simple diffusion in that it does not require energy and is powered by the concentration gradient of the molecule  It differs from simple diffusion because the rate of transport is limited by the number of available proteins  Once the carriers are occupied, the system becomes saturated and cannot operate any faster  The speed at which the carrier can change shape or configuration is also limited; once all the carriers are working and occupied, they are said to be saturated  Facilitated diffusion shows chemical specificity (a given carrier protein will only interact with specifically shaped molecule) and may be competitively inhibited by molecules that are very similar in shape  Shown below is another type of facilitated diffusion Active Transport  Active transport requir
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