AUIB+BIO101+Chapter+4+Membrane+Transport+CK+2024+%282%29.pdf

Mechanisms of Membrane Transport Mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, Introductory Biology (BIO 101) Prof Chee Kai CHAN...

Mechanisms of Membrane Transport Mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, Introductory Biology (BIO 101) Prof Chee Kai CHAN Spring 2024  Mechanisms of Membrane Transport Learning Outcomes: 1. Identify different types of transporters present at cell membrane 2. Understand Passive Transport across the membrane 3. Understand Active Transport across the membrane  Mechanisms of Membrane Transport All Cells have a plasma membrane: 1. Prokaryotes have a cell wall + cell membrane 2. Eukaryotes: a) Animal Cells (cell membrane only) b) Plant cells (cell membrane + cell wall) PLASMA MEMBRANE A cell membrane is made of: Phospholipids, proteins, FLUID MOSAIC MODEL and other lipids (cholesterol)  Mechanisms of Membrane Transport Cell membrane / Plasma membrane It’s a double layer (bilayer) of phosphates, and fats (lipids) A single phospholipid has: 1. Hydrophilic (water loving) phosphate heads 2. Hydrophobic (water hating) fatty acid tails  Mechanisms of Membrane Transport The cell membrane is responsible for maintaining homeostasis within the cell Homeostasis is the maintenance and regulation of a stable, internal environment The cell membrane maintains homeostasis through balancing the pH, temperature, glucose (sugar intake), water balance It does this through membrane proteins. Example active and passive transport Mechanisms of Membrane Transport  Mechanisms of Membrane Transport 1. Passive transport NO ATP NEEDED GLUT  Mechanisms of Membrane Transport 1. Passive transport Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. During diffusion, no energy is used to make the molecules move Based on the presence or absence of facilitator molecules, there are two types of diffusion processes. They are: 1. Simple Diffusion 2. Facilitated Diffusion.  Mechanisms of Membrane Transport 1. Passive transport 1.1. Simple Diffusion Simple diffusion The diffusion occurs directly through the phospholipid bilayer without the aid of an intermediary molecule such as channel proteins or carrier molecules. Small uncharged molecules can diffuse freely through a phospholipid bilayer. However, the bilayer is impermeable to larger polar molecules (such as glucose and amino acids) and to ions.  Mechanisms of Membrane Transport 1. Passive transport 1.1. Simple Diffusion 1. Oxygen – Non-polar and small so diffuses very quickly. 2. Carbon dioxide – Polar but very small so diffuses quickly. 3. Water – Polar but also very small so diffuses quickly.  Mechanisms of Membrane Transport 1. Passive transport 1.2. Facilitated Diffusion Facilitated diffusion is the process of passive transport of molecules or ions across a cell’s membrane via trans-membrane proteins thru a transporter. Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also charged ions such as Na+ or Cl- cannot pass through phospholipid bilayer. These molecules pass through protein channels or transporters instead. Movement of molecules is still PASSIVE just like simple diffusion (No energy is needed), the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.  Mechanisms of Membrane Transport 1. Passive transport 1.2. Facilitated Diffusion CYSTIC FOBROSIS GLUT  Mechanisms of Membrane Transport 1. Passive transport 1.3. Osmosis Osmosis is the diffusion of water from an area of high concentration of water molecules (high water potential) to an area of low concentration of water (low water potential) across a partially permeable membrane.’ Occurs when there is a difference in concentration on opposite sides of the membrane. Water will move to the side where there is more solute  Mechanisms of Membrane Transport 1. Passive transport 1.3. Osmosis (movement of solvent: water) Osmosis Osmosis can take place via both: 1. Simple diffusion of water through lipid bilayer 2. Transport through aquaporins  Mechanisms of Membrane Transport 1. Passive transport 1.3. Osmosis  Mechanisms of Membrane Transport 1. Passive transport Is Osmolarity is not the same as tonicity? 1.3. Osmosis Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Isotonic Solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.  Mechanisms of Membrane Transport 1. Passive transport If you are lost in the sea on a floating raft, you are thirsty, Should you drink seawater? Why? 1.3. Osmosis Mechanisms of Membrane Transport  Mechanisms of Membrane Transport 2. Active transport +ATP  Mechanisms of Membrane Transport 2. Active transport  Mechanisms of Membrane Transport 2. Active transport 2.1. Protein Pumps Protein pumps are transmembrane proteins that actively move ions/solutes against the gradient of concentration across membranes. They use ATP.  Mechanisms of Membrane Transport 2. Active transport 2.1. Protein Pumps UNIPORT SYMPORT ANTIPORT Na+ K+  Mechanisms of Membrane Transport 2. Active transport 2.1. Protein Pumps Sodium-potassium (Na+/K+) pump Na+/K+ Antiport  Mechanisms of Membrane Transport 2. Active transport  Mechanisms of Membrane Transport 2. Active transport 2.2. Bulk Transport Cells need bulk transport mechanisms, in which large particles (or large quantities of smaller particles) are moved across the cell membrane. These mechanisms involve enclosing the substances to be transported in their own small globes of membrane (vesicles), which can then bud from or fuse with the membrane to move the substance across. Endocytosis is the process by which substances are internalized into the cell. Exocytosis is the reverse; the process by which substances are released from the cell.  Mechanisms of Membrane Transport 2. Active transport 2.2. Bulk Transport 2.2.1 Endocytosis Endocytosis, the process by which large molecules are internalised into the cell, occurs when external material is engulfed within the cell membrane, which then forms a vesicle containing the ingested material. There are three main subtypes of endocytosis 1. Phagocytosis 2. Pinocytosis 3. Receptor-mediated endocytosis  Mechanisms of Membrane Transport Bacteria proteins 2. Active transport 2.2. Bulk Transport 2.2.1 Endocytosis 1. Phagocytosis Macrophage This is the process of engulfing large, solid particles such as bacteria into the cell for immune purposes. Extensions of the cytoplasm, termed psuedopodia (‘false feet’), sense, surround and enclose the target, creating a vesicle or vacuole or phagosome on the inside of the cell membrane. This allows the process of phagocytosis to be highly specific.  Mechanisms of Membrane Transport 2. Active transport 2.2. Bulk Transport 2.2.1 Endocytosis 2. Pinocytosis It is the process of non-specific uptake of fluid surrounding the cell, allowing it to take in nutrients such as ions, enzymes and hormones. In this process, the cell membrane invaginates, before budding off to create a vesicle known as a pinosome.  Mechanisms of Membrane Transport LDL Receptors LDL (Cholesterol) 2. Active transport 2.2. Bulk Transport 2.2.1 Endocytosis 3. Receptor-mediated endocytosis This is a process of uptake of specific target substances, such as iron, and LDL( cholesterol) via their receptor. Receptors cluster in regions termed coated pits, as they are coated with proteins such as clathrin. Clathrin causes the coated pit to invaginate and become a vesicle, bringing the desired ligand into the cell.  Mechanisms of Membrane Transport 2. Active transport 2.2. Bulk Transport 2.2.2 Exocytosis Exocytosis is a form of active transport through which large molecules are moved from the interior to the exterior of the cell. Vesicles are packaged within the cell and transported to the cell membrane, where their phospholipid bilayers fuse. This allows the contents to be released outside the cell.  Mechanisms of Membrane Transport 2. Active transport 2.2. Bulk Transport Summary: The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called bulk transport. There are two types of bulk transport, exocytosis and endocytosis, and both require the expenditure of energy (ATP). Endomembrane system Mechanisms of Membrane Transport