Module 4 Transport Mechanisms (1) PDF

UNIT 1: CELL General Biology 1 1st Se...

UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 TRANSPORT MECHANISM Topics: 1. Structure of the Plasma Membrane 2. Functions of the Plasma Membrane 3. Membrane Transport The plasma membrane that surrounds the cell can be considered the edge of life, the boundary that separates a living cell from its surroundings and controls all inbound and outbound traffic. Like all biological membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross it more easily than others. The ability of the cell to discriminate in its chemical exchanges is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible. Structure of the Plasma Membrane In all organisms, cell membranes are lipid bilayers made up mostly of phospholipids. The polar head of a phospholipid interacts with water molecules; the nonpolar fatty acids tails do not. Because of these properties, they organize themselves as a lipid bilayer sheet or bubble in a liquid medium. Figure 1. Structure of a Cell Membrane In all organisms, cell membranes are lipid bilayers made up mostly of phospholipids. The polar head of a phospholipid interacts with water molecules; the nonpolar fatty acids tails do not. Because of these properties, they organize themselves as a lipid bilayer sheet or bubble in a liquid medium. Figure 2. Hydrophobic and Hydrophilic Regions of the Cell Membrane Embedded in or attached to the lipid bilayer are other molecules like cholesterol and proteins. Cholesterol molecules prevent the plasma membrane from becoming too fluid at higher temperature and too solid at lower temperatures. A cell membrane has been described as a fluid mosaic. The “mosaic” part is attributed to the mixed composition of the cell membrane, and the “fluid” part come from the ability of the bilayer to drift sideways and spin around their long axis. This happens because the phospholipids in a typical membrane are not bonded to one another. 1 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 3. Fluid Mosaic Model Many types of proteins are associated with a cell membrane, and each type adds specific function to it. Thus, a cell membrane can have different characteristics depending on the proteins present in it. The types of proteins that may be present in the cell membrane and the functions they carry out are as follows: Figure 4. Types of Proteins in the Cell Membrane 1. Cell Membrane Lipids A. Phospholipids are a major component of cell membranes. Phospholipids form a lipid bilayer in which their hydrophilic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane. Figure 5. Structure of Phospholipid Bilayer 2 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 B. Cholesterol is another lipid component of animal cell membrane. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps to keep cell membranes from becoming stiff by preventing phospholipids from being too closely packed together. Cholesterol is not found in the membranes of plant cells. Figure 6. Structure of Cholesterol C. Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body. Figure 7. Structure of Glycolipid 2. Cell Membrane Proteins The cell membrane contains two types of associated proteins: A. Peripheral membrane proteins are exterior to and connected to the membrane by interactions with other proteins. B. Integral membrane proteins are inserted into the membrane and most pass through the membrane. Portions of these transmembrane proteins are exposed on both sides of the membrane. Figure 8. Associated Proteins in the Cell Membrane 3 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 The cell membrane contains four types of transmembrane proteins: A. Structural proteins help to give the cell support and shape. B. Cell membrane receptor proteins help cells communicate with their external environment using hormones neurotransmitters, and other signaling molecules. C. Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion. D. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane. Figure 9. Transmembrane Proteins in the Cell Membrane Functions of the Plasma Membrane 1. It protects the integrity of the interior of the cell by allowing certain substances into the cell while keeping other substances out. 2. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus, the cell membrane also serves to help support the cell and help maintain its shape. 3. Another function of the membrane is to regulate cell growth through the balance of endocytosis and exocytosis. Types of Substances: Hydrophobic substances – Similar to the phospholipid center of the membrane cab easily diffuse across membranes without consuming energy. Polar molecules – Chemically incompatible with the center of the membrane, require an expenditure of energy for their transport. Non-charged molecules – Can freely cross the membranes since they can easily pass through the hydrophobic tails of the membrane because they are also nonpolar. o Example: Carbon dioxide, Oxygen, Glycerol, and Alcohol Figure 10. Different Types of Membrane Transport 4 | STEM – General Biology 1 Module  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Membrane Transport 1. Passive Transport A transport protein moves substances from a region of higher concentration to one of lower concentration. This is the reason why passive transport is also called facilitated transport. Energy – Independent mechanism of the cell, allowing small molecules to enter into it without energy consumption A. Simple Diffusion The spontaneous movement of molecules from higher concentration to a lower concentration, that is, down their concentration gradient, until molecules are distributed equally. This is a process that results from the random motion of molecules. For instance, when a crystal of dye is dropped in water, the molecules of both dye and water move in different directions, but their net movement, that is the sum of their motion, is toward the region with lower concentration. Figure 11. The diffusion of solutes across a synthetic membrane Solution – made up of both a solute, usually a solid, and a solvent, usually a liquid. In this case, the solute is the dye, and the solvent is the water. Once the solute and the solvent are evenly distributed, their movement continues, but there is no net movement in either direction. The speed of mixing between molecules depends on the following factors: 1. Size – Takes more energy to move bigger molecules, thus, the smaller the size, the faster the rate of diffusion, and vice versa. 2. Temperature – Molecules moves faster at high temperature, making them collide more often. Thus, the higher the temperature, the faster the rate of diffusion. 3. Concentration – The difference in solute concentration between adjacent regions of a solution. Solutes – Tends to diffuse “down” their concentration gradient, that is, from a region of higher concentration to one of lower concentration. As the concentration of a solution increases, the molecules become more crowded, and the collision between them become more often. Thus, during a given interval of time, more molecules are bumped out of region of higher concentration than bumped into it. 4. Charge – Charged particle of matter (ion or molecule) in a fluid add up to the fluid’s overall electrical charge. A difference in charge between two regions of the fluid can influence the rate and direction of diffusion between them Example: Positively–charged substances like sodium ions will tend to diffuse toward a region with an overall negative charge 5. Pressure – A change, or difference in pressure between two adjoining regions may affect the rate and direction of diffusion. Pressure squeezes molecules together, and the more crowded the molecules become, the more frequent molecules collide and rebound among them, thus, the faster the diffusion. 5 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 B. Facilitated Diffusion or Facilitated Transport - The solute simple binds to the transport protein and gets released to the other side of the membrane Transport protein – Moves substances down their concentration gradient by: 1. Changing its shape when it bonds to the molecule like glucose, and then reverting to its original shape after releasing the molecule to the other side of the membrane 2. Forming permanently open channels through membrane 3. Forming gated channels that open and close in response to a stimulus such as binding to a signaling molecules or a shift in electrical charge. Figure 12. Two types of transport proteins that carry out facilitated diffusion C. Osmosis A special example of diffusion It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution – Down the water potential gradient) Note: Diffusion and osmosis are both passive, i.e. energy from ATP is not used. A partially permeable membrane is a barrier that permits the passage of some substances but not others; it allows the passage of the solvent molecules but not some of the larger solute molecules. 6 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 13. Effects of Osmosis on Water Balance Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of solutes stimulates the membrane to open specific channels or trigger active transport mechanisms to allow the passage of those chemicals across the membrane. Some major examples of osmosis: Absorption of water by plant roots. Re-absorption of water by the proximal and distal convoluted tubules of the nephron. Re-absorption of tissue fluid into the venule ends of the blood capillaries. Absorption of water by the alimentary canal — Stomach, small intestine and the colon. Osmoregulation Osmoregulation is keeping the concentration of cell cytoplasm or blood at a suitable concentration. a. Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm (thus, need more O2/ATP than isotonic (marine) Amoebae). b. The kidneys maintain the blood (thus, whole body) at the correct concentration. Osmosis and Plant Cells a. Plant Cells in a hypotonic (weaker) solution – Cells have lower water potential. The plant cells gain water by osmosis. The vacuole and cytoplasm increase in volume. The cell membrane is pushed harder against the cell wall causing it to stretch a little. The plant tissue becomes stiffer (turgid). b. Plant Cells in a hypertonic (stronger) solution – Cells have higher water potential. The plant cells lose water by osmosis. The vacuole and cytoplasm decrease in volume. The cell shrinks away from the cell wall. Shrinkage stops when the cell sap is at the same concentration as the external solution. The plant tissue becomes flaccid, it has shrunk slightly. May go on to become plasmolyzed. 7 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 14. Effects of Different Solutions in Cells Turgor - The pressure of the swollen cell contents against the cell wall when the external solution more dilute than the cell sap of the vacuole. Role of Turgor in Plants: Mechanical support for soft non-woody tissue like leaves. Change in shape of guard cells forming the stomatal opening between them. Enlargement of young immature plant cells to mature size. Figure 15. Osmosis in Animal and Plant Cells 2. Active Transport - Substances are move against their concentration gradient, i.e., from lower concentration to one of higher concentration - The transport protein uses energy from ATP to pump solute against its concentration gradient. Example: Calcium pumps – Moves calcium ions from across the cell membrane This process is important to maintain the concentration gradient of a particular solute at a certain level. Calcium ions – act as potent messenger inside the cells and they affect the activity of many enzymes 8 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 16. Active Transport in the Cell Membrane A. Primary Active Transport - Movement of substances against the concentration gradient through a membrane protein. 9 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 B. Secondary Active Transport - Movement of one substance against its concentration gradient by coupling it to the movement of another molecule. - The concentration of the other molecule was generated using ATP. Figure 17. Cotransport: Active transport driven by a concentration gradient. Figure 18. Primary and Secondary Active Transport in the Cell Membrane C. Bulk/ Vesicular Transport - Some molecules or particles are just too large to pass through the plasma membrane or to move through a transport protein. So, cells use two other active transport processes to move these macromolecules (large molecules) into or out of the cell. Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane. - There are two types of vesicle transport, endocytosis and exocytosis. Both processes are active transport processes, requiring energy. 10 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 1. Endocytosis – The process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane. The membrane folds over the substance and it becomes completely enclosed by the membrane. At this point a membrane-bound sac, or vesicle, pinches off and moves the substance into the cytosol. Two Kinds of Endocytosis: Phagocytosis – (Cellular eating) occurs when the dissolved materials enter the cell. The plasma membrane engulfs the solid material, forming a phagocytic vesicle. Pinocytosis – (Cellular drinking) occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell, when the channel is closed, the liquid is encircled within a pinocytic vesicle Receptor-mediated endocytosis is a form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule. Receptor proteins, receptors, and other molecules from the extracellular fluid are transported in the vesicles. Emptied receptors are recycled to the plasma membrane. Figure 19. Endocytosis in the Cell Membrane 11 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 2. Exocytosis – Describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin. Newly made membrane proteins and membrane lipids are moved on top of the plasma membrane by exocytosis. Figure 20. Exocytosis in the Cell Membrane