Homeostasis and Cell Membrane Functions - Biophysics Lecture 2 - PDF

# Homeostasis and Functions of Cell Membrane ## Biophysics -Physiology Lecture 2 ### Prof. Sahar El Agaty | Professor of Physiology ## Intended Learning Outcomes - Define homeostasis and describe its importance. - Recognize the components of homeostatic mechanism. - Differentiate the negative an...

# Homeostasis and Functions of Cell Membrane ## Biophysics -Physiology Lecture 2 ### Prof. Sahar El Agaty | Professor of Physiology ## Intended Learning Outcomes - Define homeostasis and describe its importance. - Recognize the components of homeostatic mechanism. - Differentiate the negative and positive feedback mechanisms. - Identify examples of negative and positive feedback mechanisms. - Describe the functions of cell membrane. ## Homeostasis ### Definition: Homeostasis is the maintenance of nearly constant conditions in the internal environment. ### Importance of homeostasis: - For the body to function optimally, conditions within the body must be very carefully regulated. Therefore, many important variables, such as body temperature, blood pressure, blood glucose, oxygen, and carbon dioxide content of the blood as well as electrolyte balance are maintained within narrow physiological limits. - Homeostasis increases the chance of survival and the ability to live in different environmental conditions. ### Contribution of some body systems to homeostasis: All organs and tissues of the body perform functions that help maintain these relatively constant conditions. | Organ system | Function | |-----------------------|-------------------------------------------------------------------------------------------------| | Nervous System | Regulates muscular activity and glandular secretion; responsible for all activities associated with the mind | | Endocrine System | Regulates metabolic processes through secretion of hormones | | Muscular System | Allows for body movement; contributes to thermoregulation | | Circulatory System | Transports nutrients, oxygen, waste, carbon dioxide, electrolytes, and hormones throughout the body | | Respiratory System | Obtains oxygen and eliminates carbon dioxide; regulates acid-base balance (pH) | | Gastrointestinal Tract | Digests and absorbs food substances to provide nutrients to the body | | Renal System | Eliminates waste products from the body; regulates blood volume and blood pressure; regulates acid-base balance (pH) | - There are two regulatory systems in the body that influence the activity of all the other organ systems so that homeostasis is maintained. - These are nervous system, and endocrine system. ### Examples of Homeostatically regulated factors: 1. Concentration of nutrients. Cells need a constant supply of nutrient molecules for energy production. 2. Concentration of O2 and CO2. Cells need O2 to form energy. The CO2 produced during these reactions must be removed so that acid-forming CO2 (H2CO3) does not increase the acidity of the internal environment. 3. Concentration of waste products. The end products of some chemical reactions have a toxic effect on body cells if these wastes are allowed to accumulate. 4. pH. Changes in the pH of the ECF adversely affect nerve cell function and the enzyme activity of all cells. 5. Temperature. Body cells function best within a narrow temperature range. If cells are too cold, their functions slow down too much; if they get too hot, their structural and enzymatic proteins are destroyed. ## Homeostatic control system - The human body has thousands of powerful control systems act to maintain the constancy of internal environment despite wide environmental variations from injury and diseases. ### Components of homeostatic control mechanisms: - **Receptor:** detect the change in internal or external environment and send input to a control center. - **Control center:** compare the change to a set point and produce orders to effector organ. - **Effector organs:** give response to the center to correct the change ### Example of homeostatic control mechanisms: Regulation of body temperature. - **Afferent:** Thermoreceptors (brain and skin) - **Efferent:** Muscles (+ shivering), Gland (+ thyroid gland) - **Thermostat (brain):** Set point 37° - **Change is compared to the set point.** - **Decreased body temperature** - **Correction:** Increased body temperature - **Response:** Change is corrected. ### Components of homeostatic control mechanisms: - **Afferent:** Receptors - **Efferent:** Effectors (muscles or glands) - **Control center (set point):** - **Stimulus:** Change occurs in internal environment - **Change is compared to the set point** - **Response:** Change is corrected. ### Negative feedback mechanism: Control of blood pressure. - **Increased Arterial blood pressure** - **Baroreceptors in vessels** - **Cardiovascular center in brain** - **Decreases heart rate and causes dilatation of blood vessels** - **Decreased Arterial blood pressure** - **Coordinating center** - **Afferent pathways** - **Detector: Baroreceptors** - **Mean arterial pressure** - **Medulla** - **Efferent pathways** - **Effector: Heart Vessels** ### Positive feedback mechanism: Childbirth. - **Labor begins with contraction of uterus** - **The baby is pushed toward the cervix** - **Stretch receptors in the cervix** - **Nerve impulse to brain and pituitary gland** - **Release the hormone oxytocin into the mother's bloodstream** - **Oxytocin causes strong uterine contractions, moving the baby farther down the birth canal. This continuous cycle of stretching and releasing of oxytocin stops only when the baby is expelled from the birth canal.** - **The baby pushes against the cervix, causing it to stretch.** - **Uterus** - **Oxytocin causes the uterus to contract.** - **Stretching of the cervix causes nerve impulses to be sent to the brain.** - **The brain stimulates the pituitary to release oxytocin.** ## Characteristics of the control system | Mechanism | Description | |-----------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | **Negative feedback** | The response is opposite to the stimulus. | | | Reduces the effect of the stimulus. | | | More frequent in the body e.g., Control of body temperature, blood pressure, hormones levels. | | | More associated with stability. | | | Mostly results in restoring homeostasis and is life saving. | | **Positive feedback** | The response is in the same direction of the stimulus. | | | Amplify the effect of the stimulus. | | | Less frequent in the body e.g., blood clotting and childbirth. | | | Less associated with stability. | | | May result in vicious circle and death. | ## Examples of negative feedback control mechanism - **Decreased Body temperature** - **Thermoreceptors** - **Signals to thermostat in the brain** - **Muscle shivering** - **Release of thyroxine hormone** - **Heat gain** - **Increased Body temperature** - **Increased Body temperature** - **Thermoreceptors** - **Signals to thermostat in the brain** - **Sweating** - **Vasodilatation of skin blood vessels** - **Heat loss** - **Decreased Body temperature** ## Feedforward mechanisms initiate responses in anticipation of a change. - When a meal is still in the digestive tract, a feedforward mechanism increases secretion of a hormone (insulin) that increases the cellular uptake and storage of ingested nutrients e.g., glucose; after they have been absorbed from the digestive tract. These prevent marked rise in blood glucose level after meal. ## Feedback Vs Feedforward mechanism | Mechanism | Description | |--------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | **Feedback** | Generate response after the variation is detected. | | | Certain time lag between variation and response. | | **Feedforward** | Generate response anticipatory to variation. | | | No time lag between variation and response. | ## Homeostasis - Most control systems in the body are negative feedback - Positive feedback (vicious circle): in some cases, if the negative feedback mechanism failed to correct the change, a positive feedback mechanism can run out of control and can result in the collapse of the system. This is called vicious circle. For example: - If positive feedback mechanism operates when Arterial blood pressure (ABP) is decreased, ABP will decrease more Blood flow to the brain and heart death. ## Advantage of homeostasis: 1. Enables the human to work and live in different external environmental conditions. 2. It is important for body survival ## Homeostasis is not unlimited The body can control the change in the internal environment but with a limited range and time. Thus, diseases may occur when homeostatic mechanisms fail to maintain the constancy of the internal environment. Disease is a state of disrupted homeostasis. ## Clinical Note - Disruptions in homeostasis can lead to illness and death. - When one or more of the body's systems malfunction, homeostasis is disrupted, and all cells suffer because they no longer have an optimal environment in which to live and function. - Medications are designed to help the body in maintenance of homeostasis when its own regulatory mechanisms fail to do that (in diseases). - For example: - Insulin injection in patients with type 1 diabetes mellitus to keep the blood glucose within the normal range. - Diuretics is used to decrease blood volume in patients with heart failure. ## Student Activity ### True and false - Homeostasis is unlimited process. **F** - Negative feedback mechanism does not maintain stability. **F** - Positive feedback mechanism is less common in human body. **T** ## Cell membrane (Plasma membrane) - Each cell is enclosed within a plasma membrane that separates the cytoplasmic contents of the cell (ICF), from the fluid outside of the cell (ECF). - It is semipermeable membrane that allow the passage of certain substances and prevent others. ### Function: 1. It protects the cytoplasm from rapid changes in the surrounding environment. 2. It allows the cell to maintain and modify the cytoplasmic composition (ICF) that is very different from that of the ECF. ### Structure and function of the plasma membrane - The major components of the plasma membrane include: - Lipids - Proteins - Carbohydrates ### 1.Lipids: #### a)Phospholipids: - The principal membrane lipid is phospholipid. - Phospholipids are amphipathic molecules that have both polar (charged and water-soluble) and nonpolar (uncharged and water-insoluble) regions. - The molecules are arranged to form **lipid bilayer.** - The polar regions are directed toward the exterior and the interior of the cell, where they can interact with water. - The non-polar region is directed toward the center of the membrane away from the water. #### Function: - **Allow the membrane to act as semipermeable barrier** (has selective permeability), that determine the composition of ICF. - It allows the passage of **lipophilic substances** (uncharged or nonpolar) easily through the membrane e.g., oxygen, carbon dioxide and fatty acids. - It prevents the passage of **hydrophilic substances** easily through the membrane (they use transport mechanisms) e.g., glucose, amino acids, ions [Na+, Ca++, H+, Cl-, and HCO3-]. - **Maintain the membrane fluidity** because the molecules are not connected together by chemical bonds. ### b) Cholesterol: - It is lipid soluble and present in the interior of the membrane #### Function: - **Maintain the membrane fluidity** - **Importance of membrane fluidity** - It is important for the function of many cells, for example: - Muscle cells contract and relax easily - White blood cells leave the blood vessels and squeeze through the capillary pores (without rupture) to reach tissues and attack the infection ### 2. Proteins #### A) Integral (intrinsic or transmembrane) proteins: - They span the membrane #### B) Extrinsic or peripheral proteins - They are attached to the inside or to the outside surface of the membrane. ### Function: #### 1.Channel - Ligand gated - Leaky channel - Voltage gated #### 2.Carrier #### 3.Receptor #### 4.Enzyme - Adenylate cyclase - Choline esterase #### 5.Anchor #### 6.Pump #### 7.Antigen ### Function: 1. **Channels:** allow passage of ions (water soluble) to enter or leave the cell 2. **Carriers:** transport larger water-soluble molecules, such as glucose or cellular products, across the membrane along their concentration gradient. (from area of high concentration to area of low concentration) 3. **Enzymes:** which regulate specific chemical reactions, and are found on either the internal (e.g., adenylate cyclase) or the external (e.g., acetylcholinesterase) surfaces of the membrane. 4. **Receptors:** Bind neurotransmitters and hormones, changing cell function. 5. **Pumps:** they are protein carriers that transport substances against their concentration gradient and require energy (Adenosine triphosphate, ATP) 6. **Antigens:** they are cell markers that allow the immune system to differentiate between our cells and foreign cells like viruses and bacteria. 7. **Cell adhesion molecules (anchor protein):** attach cells together ### 3. Carbohydrate: - They are present on the outer surface of the membrane and attached to proteins (glycoproteins) and lipids (glycolipids). ### Function: 1. Receptors for certain hormones. 2. Antigens e.g., blood group antigens. 3. Acts as a surface marker which is important in cell recognition, and cell surface antigenicity ## Student Activity ### True and false - Cell membrane allows the passage of water-soluble substance easily. **F** - Carrier proteins facilitate the transport of large water soluble molecules across the membrane. **T** - Enzymes in cell membrane are carbohydrates. **F** ## References 1. Barrett KE, Barman SM, Brooks HL, and Yuan JX. (2019). Ganong's Review of Medical Physiology. 26th ed. ebook by McGraw-Hill Education. 2. Hall JE, and Hall ME. (2021). Guyton and Hall Textbook of Medical Physiology. 14th ed. eBook by Elsevier, Inc. 3. Sherwood L, (2016). Human Physiology From Cells to Systems. 9th ed. eBook by Nelson Education, Ltd. ## It's so lovely to meet all of you! Thank you for listening.