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Physiology 1021
Bob Larose

Homeostasis, Membrane Transport and Excitable Cells Physiology is the study of the normal functioning of a living organism and its components. Homeostasis The cell is the smallest form of life. Cells come together to make tissues. Tissues then come together to form an organ. Multiple Organs form an organ system. When multiple organ systems work together, an organism is created. The cells in a multicellular organism play a critical role in the Homeostasis of the organism as a whole. Homeostasis is the ability of the body to maintain a relatively constant internal environment. Homeostasis is derived from the words homeo= similar and stasic= constant. Our body’s are constantly being exposed to external environments, such environments can cause the body’s internal environment to leave homeostasis. The body uses a system called Negative Feedback to control the changes brought upon by the external environment. When the body senses a change in pH, temperature etc. a response is sent to counteract the external environment. An example is that when the external environment is cold and our body’s become cold, we start shivering to warm ourselves up. There is as well Positive Feedback which sends signal to continue certain functions of the body in the needed time. An example is oxytocin during labor. Cells 67 percent of the total body water is found inside the cell, 7 percent in the plasma of blood vessels and 26 percent in the area outside cells and blood vessels (Intracellular fluid). If there was 42L of water, 28L would be in the intracellular fluid, 14L in the extracellular fluid and 3L in the plasma. Body fluid composition:  Our body fluids act as “exchange sites” with the external environment. Cells within the body are surrounded by extracellular fluid  Serves not as a transition between the external environment and the intracellular fluid inside a cell  Extracellular fluid= fluid inside of the cell (cytoplasm)  Extracellular fluid= interstitial fluid (fluid surrounding cell) and plasma (liquid component of blood) Chemical Component of Cells: (remember salty banana) Substance Plasma Interstital Intracellular Majority Fluid Fluid located: Sodium ion 148 150 15 Outside cell Potassium ion 4.8 5.0 150 Inside cell Calcium ion 2.5 2.4 1 Inside cell Chloride ions 102 125 9 Outside cell Proteins 1.2 0.2 4 Inside cell Cellular Structure: Nucleus: contains DNA Mitochondria: produces energy (ATP) Lysosome: Contains enzymes which breaks down uselessness Cytoskeleton: Structures cell Rough ER: makes proteins Smooth ER: makes lipids Golgi apparatus: adds sugars/ folds proteins (modifies proteins) Centrioles: Helps with cell division Cell membrane: Phospholipid bilayer formed by hydrophilic phosphate heads facing water while fatty acid heads try to hid from water. In the cell membrane there are trans membrane proteins, peripheral proteins, cholesterol, carbohydrate molecules, glycolipids and glycoprotein molecules. Transportation: Long distance communication is essential for maintaining homeostasis  The nervous and endocrine system coordinate many function within body to maintain homeostasis Neurotransmitter: Chemical signals release by neuron onto a target call  Has a rapid affect Affect of cells that has receptors for that chemical Neuron transmitters travel long distances Neurotransmitter (the chemical affecting the cell) travels short distance. Hormones: Produce by an endocrine cell Structural difference to neuron Chemical signals secreted into bloodstream Affect only cells with receptors for specific hormones Unlike the neuron, the hormones (the chemical) travels, not the cell itself Slower than the neurotransmitter Neurohormones: Chemical signals made by a neuron and secreted into the blood stream Only affect cells with the correct receptor. Cells that are not of endocrine or nervous origin can also communicate with one another through both chemical and physical means. Chemical- mediated cell- cell communication: Autocrine: signals out on the call that made it  Short distance travels  It self motivates the cell.. the cell releases a chemical which interacts with its self. Paracrine: signals out on the neighboring cell. Cytokine: a chemical signal made by any type of cell in response to stimulation Contact- dependent signals: Surface molecules on one cell membrane bind to a membrane protein of another Gap Junction: Protein channels that make cytoplasmic “bridges” between adjacent cells. Can directly transfect chemicals and electrical signals. Cells are constantly receiving inputs and messages from their environments and from one another. Frequently, messages that the cell “receives” either begins to be processed at the cell membrane or need to pass through the cell membrane in order to be interpreted. Recall that the cell membrane has a highly hydrophobic core and a number of embedded proteins. Functions of the protein membranes: 1. Ion Channels 2. Enzymes 3. Receptors  Hormone/ neurotransmitter 4. Membrane carried  Transportation of glucose  Proteins that can attach and carry molecules (physically move them) Mechanisms of membrane transport: 1. Endocytosis/Exocytosis  Cell membrane forms membrane around molecules, pass through membrane and releases  Pinocytosis of small molecules 2. Diffusion through lipid by layer  Fat soluble 3. Diffusion through protein channels 4. Facilitated Diffusion 5. Active transport  Only form of transport that required energy. Simple Diffusion:  Movement of molecules from an area of high concentration to an area of lower concentration due to a molecules rendon thermal motion  Passive process (no energy is required)  Can reach equilibrium  At equilibrium, concentration are equal but molecules are still moving (no net movement) Factors that affect the rate of diffusion: 1. Concentration gradient  Molecules want to move from areas of high concentration to areas of low concentration 2. Surface area  More surface area, more places for diffusion to occur  Alveoli in the lungs 3. Size of molecules  Smaller molecules, faster diffusion 4. Membrane thickness  Thicker membrane, slower rate of diffusion 5. Lipid solubility of a molecule  More soluble to fat, faster rate Diffusion of fat-soluble molecules through the lipid bilayer: Can pass through the bilayer because fat soluble molecules are hydrophobic and nonpolar. There is no help needed. Naturally move from an area of high concentration to an area of low concentration Example: Oxygen and Carbon Dioxide. Diffusion of water- soluble molecules through protein channels: Ions cannot pass through gradient because of charge therefore use protein channels instead. Protein channels block off the bilayer which blocks off the hydrophobic core allowing hydrophilic molecules to pass through Protein channels have certain charge:  If channel is negatively charged, positively charged ions will pass through  If channel is positively charged, negatively charged ions will pass through The channels have a certain diameter that ions must pass through. Anything bigger than the diameter of the channel cannot pass through making the channel to be selectively permeable. The greater concentration gradient, the faster the movement is The greater number of channels, the faster the movement is. Facilitated Diffusion: Protein carrier grabs a specific molecule on one side of the membrane and drops it on the other The concentration has to be higher on one side than the other Competitive inhibition can occur if a non- specific molecule tries to bind to it. Limited in capacity as it can only turn so fast that it can only transport so many molecules at once  Not all the glucose we need can get into the cell. Active Transport: Needs energy  Opposing to others where they do not use energy Must be chemically specific Moves things against concentration gradient  Other types of transport move with concentration gradient Limited in capacity Osmosis: Movement of water across a membrane in response to a solute gradient  Water moves to dilute the more concentrated solution 3 things affect osmosis across a cell membrane 1. Permeability to the membrane  Water must be permeable. 2. Concentration gradient of solution  Must be a concentration gradient for water to diffuse/ t
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