Cell Transport Mechanism PDF

**Cell Transport Mechanism** Cell transport is the movement of substances across the cell membrane. The cell membrane is selectively permeable, meaning it allows certain substances in and out while restricting others. Cell transport is crucial for maintaining homeostasis, supplying nutrients, and r...

**Cell Transport Mechanism** Cell transport is the movement of substances across the cell membrane. The cell membrane is selectively permeable, meaning it allows certain substances in and out while restricting others. Cell transport is crucial for maintaining homeostasis, supplying nutrients, and removing waste. **Types of Cell Transport** 1. **Passive Transport** - **Diffusion**: The movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. It occurs naturally and does not require energy. Examples include the movement of oxygen and carbon dioxide across cell membranes. - **Osmosis**: A specific type of diffusion involving water molecules. Water moves from an area of lower solute concentration (higher water potential) to an area of higher solute concentration (lower water potential) across a selectively permeable membrane. - **Facilitated Diffusion**: Some molecules cannot directly diffuse across the membrane due to their size or polarity. Carrier proteins or channel proteins assist in transporting these molecules. For example, glucose transport is facilitated by a protein carrier. 2. **Active Transport** - **Primary Active Transport**: This process requires energy in the form of ATP to move substances against their concentration gradient. The sodium-potassium pump is a classic example, where sodium ions are pumped out of the cell while potassium ions are pumped in. - **Secondary Active Transport**: It involves the use of an electrochemical gradient established by primary active transport. This gradient is then used to move other molecules across the membrane. 3. **Bulk Transport** - **Endocytosis**: The process where cells engulf substances to bring them inside. Examples include: - **Phagocytosis**: Engulfing large particles, often seen in immune cells that ingest pathogens. - **Pinocytosis**: Engulfing liquids or small particles. - **Receptor-mediated endocytosis**: Engulfing specific substances after binding to receptors on the cell membrane. - **Exocytosis**: The process of vesicles fusing with the plasma membrane to release their contents outside the cell. It's crucial for processes like neurotransmitter release in nerve cells. The cell membrane is composed of a phospholipid bilayer with embedded proteins. The hydrophilic heads of phospholipids face the water on either side, while the hydrophobic tails face inward, away from water. This structure: - Provides a barrier that regulates the entry and exit of substances. - Contains proteins that act as channels, carriers, and pumps to facilitate transport. - Contains carbohydrate molecules for cell recognition and signaling. **Applications in Human Physiology:** 1. **Simple Diffusion** - Gas exchange in alveoli: Oxygen (O₂) diffuses from the lungs into the blood, while carbon dioxide (CO₂) diffuses out of the blood into the lungs. - Absorption of alcohol: Alcohol diffuses from the stomach into the bloodstream. - Uptake of lipids in the intestines: Small lipid molecules diffuse through the intestinal wall into the blood. 2. **Facilitated Diffusion** - Glucose transport into cells via GLUT proteins: Glucose moves into cells down its concentration gradient. - Chloride ion (Cl⁻) transport in cystic fibrosis: Mutations in the CFTR protein affect chloride ion movement, leading to mucus buildup in the lungs. - Amino acid transport into muscle cells: Facilitated by specific amino acid transporters to support protein synthesis and muscle repair. 3. **Osmosis** - Water absorption in the large intestine: Water moves into the bloodstream to prevent dehydration and maintain fluid balance. - Water movement in kidney nephrons (Loop of Henle): Water moves out of the nephron back into the blood, concentrating urine. - Maintenance of cell volume: Water moves in or out of cells depending on the solute concentration in the extracellular fluid. 4. **Active Transport** - Sodium-potassium pump (Na⁺/K⁺ ATPase): Moves Na⁺ out and K⁺ into cells, maintaining an electrochemical gradient critical for nerve and muscle function. - Calcium pump in muscle cells: Actively pumps calcium ions back into the sarcoplasmic reticulum after muscle contraction to relax the muscle. - Proton pump in the stomach: Parietal cells actively pump hydrogen ions (H⁺) into the stomach, creating an acidic environment for digestion. 5. **Secondary Active Transport (Cotransport)** - Sodium-glucose cotransport in the intestines: Sodium ions moving down their gradient drive glucose uptake into intestinal cells. - Sodium-calcium exchanger in heart cells: Uses the sodium gradient to remove calcium from heart cells after contraction. - Amino acid uptake in the kidneys: Sodium-driven cotransport allows reabsorption of essential amino acids from the urine. 6. **Endocytosis** - Phagocytosis of bacteria by white blood cells: Macrophages and neutrophils engulf pathogens, digesting them inside the cell. - Uptake of nutrients by oocytes: The egg cell takes up nutrients and materials from its environment during development. - Receptor-mediated endocytosis of iron: Cells take in iron bound to transferrin through receptors. 7. **Exocytosis** - Secretion of insulin by the pancreas: Insulin is released into the bloodstream via vesicles to regulate blood glucose levels. - Release of digestive enzymes by the pancreas: Pancreatic cells release enzymes into the small intestine for digestion. - Neurotransmitter release at synapses: Neurotransmitters are released from nerve cells to pass signals to other nerves or muscles. 8. **Pinocytosis** - Absorption of extracellular fluid in kidney cells: Cells in the renal tubules take in fluid to regulate blood composition. - Nutrient uptake by intestinal cells: Intestinal cells ingest extracellular fluids containing nutrients for absorption. - Cell membrane recycling: Cells use pinocytosis to sample their environment and recycle parts of their plasma membrane. 9. **Receptor-Mediated Endocytosis** - Cholesterol uptake via LDL receptors: Cells bind LDL particles and bring in cholesterol for membrane synthesis and hormone production. - Iron uptake via transferrin receptors: Cells bind transferrin to take in iron needed for hemoglobin production. - Uptake of hormones such as insulin: Cells with insulin receptors bind and internalize the hormone to regulate glucose levels. **Neurotransmitters** 1. **Acetylcholine (ACh)** - Function: Involved in muscle activation, memory, and learning. - Key Roles: Helps transmit nerve impulses to muscles and is critical for neuromuscular function. 2. **Dopamine** - Function: Regulates mood, reward, motivation, and motor control. - Key Roles: Plays a crucial role in pleasure and addiction; important in movement control. 3. **Serotonin** - Function: Regulates mood, appetite, sleep, and digestion. - Key Roles: Linked to feelings of happiness and well-being; regulates sleep cycles. 4. **Norepinephrine (Noradrenaline)** - Function: Increases alertness, arousal, and speeds up the heart rate. - Key Roles: Plays a role in the \"fight or flight\" response to stress. 5. **Gamma-Aminobutyric Acid (GABA)** - Function: The major inhibitory neurotransmitter; calms the nervous system. - Key Roles: Reduces neuronal excitability, regulates anxiety, and contributes to muscle relaxation. 6. **Glutamate** - Function: The major excitatory neurotransmitter involved in cognitive functions like learning and memory. - Key Roles: Critical for synaptic plasticity and brain development. 7. **Endorphins** - Function: Natural painkillers, create feelings of euphoria. - Key Roles: Reduce pain and boost pleasure, often released during exercise. 8. **Epinephrine (Adrenaline)** - Function: Increases heart rate, blood pressure, and energy supply. - Key Roles: Involved in the \"fight or flight\" response to stress or danger. 9. **Histamine** - Function: Regulates immune responses and plays a role in the sleep-wake cycle. - Key Roles: Involved in allergic reactions and gastric acid secretion. **Hormones** 1. **Insulin** - Function: Regulates blood sugar levels by facilitating the uptake of glucose into cells. - Key Roles: Vital for maintaining normal glucose homeostasis. 2. **Glucagon** - Function: Raises blood sugar levels by stimulating the conversion of glycogen to glucose in the liver. - Key Roles: Works with insulin to balance blood sugar. 3. **Cortisol** - Function: Regulates metabolism and helps the body respond to stress. - Key Roles: Known as the \"stress hormone\"; increases glucose in the bloodstream and suppresses the immune system. 4. **Thyroxine (T4) and Triiodothyronine (T3)** - Function: Regulate metabolism, growth, and development. - Key Roles: Produced by the thyroid gland and control the rate at which cells burn fuels from food to produce energy. 5. **Adrenaline (Epinephrine)** - Function: Triggers the body\'s response to stress. - Key Roles: Enhances physical performance by increasing heart rate and blood flow. 6. **Oxytocin** - Function: Promotes bonding, social interactions, and maternal behaviors. - Key Roles: Known as the \"love hormone\"; stimulates uterine contractions and milk ejection. 7. **Prolactin** - Function: Stimulates milk production after childbirth. - Key Roles: Plays a role in reproductive health, including regulating the immune system and behavior.