SL Biology Unit 2 Transport 2025 PDF

Unit 2 Transport SL Biology M 2025 The Plasma Membrane and the Lipid Layers. Introduction. Biological membranes are fluid, flexible and dynamic. Questions that will be explored IB Guidelines How do molecules of phospholipids assemble into biological membranes? What are phosp...

Unit 2 Transport SL Biology M 2025 The Plasma Membrane and the Lipid Layers. Introduction. Biological membranes are fluid, flexible and dynamic. Questions that will be explored IB Guidelines How do molecules of phospholipids assemble into biological membranes? What are phospholipids made of? Do all substances pass through these membranes with equal ease? Does the structure of a membrane play a role in the movement of substances across it? B 1.1.12 Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions. Formation of a phospholipid Lipids- one of the four biological e w molecules in vi Re organisms. Formation of triglycerides B 1.1.12 Formation of phospholipid bilayers A phospholipid is a lipid that contains a phosphate as a consequence of the hydrophobic and group and is a major component of cell membranes. hydrophilic regions. The phospholipid is essentially a triglyceride in which a fatty acid has been replaced by a phosphate group. The hydrophilic head of the phospholipid molecule dissolves readily in water. The long fatty acid chains of a phospholipid are nonpolar and thus hydrophobic because of their insolubility. Fun Fac t! Phospholipids are synthesized by the smooth endoplasmic reticulum and transported as vesicles to the cell membrane. B 1.1.12 Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions. The presence of a ionized phosphate group makes it a polar compound and therefore hydrophilic. Other examples of polar compounds are water and glucose Like dissolves like B 1.1.12 Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions. Hydrophilic In water, phospholipids spontaneously form a double layer called a lipid bilayer in which the hydrophobic tails of phospholipid molecules are sandwiched between two Hydrophobic layers of hydrophilic heads In this way, only the heads of the molecules are exposed to the water, while the hydrophobic tails interact only with each other. Hydrophobic Molecules that have both hydrophilic and hydrophobic ends are called amphipathic. Hydrophilic B 1.1.12 Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions. Consequences for the amphipathic nature of the phospholipid. 1. They form a bilayer in water making it a suitable barrier. 2. Additional attractions in the lipid bilayer (between the hydrophobic carbon tails and the hydrophilic phosphate heads) make it even more stable. Exam Question. Explain why phospholipids form Exam Tip bilayers in water, with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails. Use a circle to represent phosphate head. Use two squiggly lines to represent the Draw a phospholipid bilayer. fatty acids tails. Draw two layers with the tails facing each other. B 1.1.12 Formation of phospholipid bilayers Exam Question as a consequence of the hydrophobic and hydrophilic regions. Explain why phospholipids form bilayers in water, with reference to a. There is water on either side of the hydrophilic phosphate heads and two cell membrane. hydrophobic hydrocarbon tails. Inside = cytoplasm Outside = extracellular fluid b. Phospholipids will arrange themselves as a bilayer so that the hydrophilic head associates with water inside and outside of the membrane c. and the hydrophobic tails face each other, within the membrane, away from the water. B 2.1.1 and B 2.1.2 Lipid bilayers as the basis of cell membranes. Lipid bilayers as barriers. 1972 Singer-Nicolson Model Membranes are flexible, supporting structures. All cellular membranes whether plasma membrane or organelle membranes have the same general structure. It hold the contents of the cell together. B 2.1.1 and B 2.1.2 Lipid bilayers as the basis of cell membranes. Lipid bilayers as barriers. Endocytosis The hydrophobic and hydrophilic regions cause phospholipids to naturally align as a bilayer if there is water present. Because fatty acids tails do not attract each other strongly, the membrane tends to be fluid or flexible. This allows animal cells to have a variable shape and allows the process of endocytosis. What maintains the overall structure of the membrane is the relationship between its chemical makeup and the chemical properties of water. Demo Time! Observe the model. Answer the following- 1. Why is the plasma membrane considered to be fluid in nature? 2. Describe the structure and orientation of phospholipids in the plasma membrane. 3. What properties of the membrane impact the physical structure? 4. What does it mean to say that the membrane is selectively permeable rather than totally permeable? B 2.1.1 and B 2.1.2 Lipid bilayers as the basis of cell membranes. Lipid bilayers as barriers. As the interior of the bilayer is hydrophobic, non-polar, lipid-soluble molecules like steroids can easily pass through the lipid bilayer. On the other hand, for the same reason, ions like Na+ cannot pass through the membrane. B 2.1.1 and B 2.1.2 Lipid bilayers as the basis of cell membranes. Lipid bilayers as barriers. Uncharged polar molecules like glucose are hydrophilic in nature. These molecules are ʻrepelledʼ by the cell membrane, i.e. the membrane is mostly impermeable to them. Small, uncharged molecules can readily pass through the lipid bilayer. Thus, polar molecules like water and ethanol or non-polar molecules like oxygen and carbon dioxide can easily enter or leave cells. B 2.1.1 and B 2.1.2 Lipid bilayers as the basis of cell membranes. Lipid bilayers as barriers. The permeability of biological membranes to molecules depends on the size of the molecules and their hydrophilic/hydrophobic nature. As most of the constituents (parts) of the cell are polar or charged, biological membranes form barriers, preventing the unneeded entry or exit of these molecules from the cell. Check your understanding! Where do the following fit in? Steroids Oxygen Water Sucrose K+ Cl- Complete it your note pack. B 2.1.10 Fluid Mosaic model of the membrane structure Membranes are “Fluid Mosaics” in which proteins move within layers of lipids – The phospholipid bilayer is the fluid portion of the membrane – A mosaic of proteins is embedded in the membrane – Membranes are dynamic, ever-changing structures B 2.1.10 Fluid Mosaic model of the membrane structure Why is it called a FLUID? Phospholipid bilayer is flexible, allowing for cellular shape changes. Membrane lipids (and some proteins) can drift laterally within the membrane. Individual phospholipid molecules are not bonded to one another. Proteins drift more slowly than lipids. Some membrane proteins are tethered to the cytoskeleton and Some fatty acids in the membrane cannot move far. are saturated while some are unsaturated. B 2.1.10 Fluid Mosaic model of the membrane structure The fluidity of the membrane allows for: formation of vesicles materials to be taken into cells by endocytosis or released by exocytosis Pinching in of the cell membrane during animal cell cytokinesis. B 2.1.10 Fluid Mosaic model of the membrane structure Why is it called a mosaic? Because it has proteins. Integral Proteins Span the entire membrane (transmembrane proteins are examples) Permanently attached to the cell membrane. Peripheral Proteins Not embedded- attached to one surface of the membrane. Attach only temporarily to the cell Carbohydrates membrane. Sometimes attach to integral proteins. Small variable amount of carbohydrates. B 2.1.10 Fluid Mosaic model of the membrane structure B 2.1.10 Fluid Mosaic model of the membrane structure Students should be able to draw a two-dimensional representation of the model and include peripheral and integral proteins, glycoproteins, phospholipids and cholesterol. They should also be able to indicate hydrophobic and hydrophilic regions. Cholesterol molecules should be added between the fatty acid chains. B 2.1.10 Fluid Mosaic model of the membrane structure B 2.1.4 Integral and peripheral proteins in membranes There are two major categories of proteins in the cell membrane: integral and peripheral proteins. B 2.1.4 Integral and peripheral proteins in membranes The protein content of membranes is very variable because the function of the membrane varies. Myelin Plasma Mitochondria sheath membrane and around around chloroplast neurons cells 75% 18% 50% You donʼt have to memorise this ! B 2.1.4 Integral and peripheral proteins in membranes Peripheral proteins are attached to Integral proteins are proteins that the outer surface of the bilayer are embedded in one or both of the lipid layers of the membrane. (outside the cells) or the inner surface (facing the cell cytoplasm) Integral proteins may act as Peripheral proteins may shuttle 1. Channels for transport of between integral proteins on the metabolites. surface of the membrane, act 2. Can be enzymes 3. Can be carriers 1. as scaffolding to hold shape 4. receptors 2. receptors for extracellular signals. B 2.1.4 Integral and peripheral proteins in membranes Integral proteins have hydrophobic amino acids on at least part of their surface and are therefore embedded in the hydrophobic fatty acid tails in the center of the membrane. They may fit in one of the two phospholipid layers or extend across both. Many integral proteins are transmembrane proteins- they extend across the membrane, with hydrophilic parts projecting through the regions of phosphate heads on either side. B 2.1.4 Integral and peripheral proteins in membranes Peripheral proteins are hydrophilic on their surface , so are not embedded in the membrane. Most of them are attached to the surface of the integral proteins and this attachment is often reversible. Some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the proteins to the membrane surface. B 2.1.4 Integral and peripheral proteins in membranes They have varied functions as seen below. Transport: Protein channels (facilitated) and protein pumps (active) Receptors: Protein-based hormones (insulin, glucagon, etc.) Anchorage: Cytoskeleton attachments and extracellular matrix Cell recognition: MHC proteins and antigens Intercellular joinings: Tight junctions and plasmodesmata Enzymatic activity: Metabolic pathways (e.g. electron transport chain) B 2.1.9 Structure and function of glycoproteins and glycolipids Glycoproteins are proteins with carbohydrates as the non protein component. They are a component of the plasma membrane of cells, with the protein part embedded in the membrane and the carbohydrate part projecting out into the exterior environment. Some functions of glycoproteins include 1. Cell recognition by immune system. 2. Hormone receptors 3. Cell adhesion. B 2.1.9 Structure and function of glycoproteins and glycolipids Cell Adhesion proteins Anchors the cell membrane to the inner cytoskeleton proteins outside the cell as well as to other cells.. Can be integral or peripheral A ligand is a molecule that can bind to a receptor. B 2.1.9 Structure and function of glycoproteins and glycolipids Receptor Protein Insulin Receptor Insulin is a hormone released by the pancreas when blood sugar levels are high. Insulin binds to the insulin receptor protein, which then causes the cell to open the typically closed glucose transport protein. This allows glucose to enter the cell from the blood. B 2.1.9 Structure and function of glycoproteins and glycolipids Cell Recognition proteins Serve as identification tags on the surface of a cell. Often times these are glycoproteins (proteins with an attached sugar molecule) Can be integral or peripheral. B 2.1.9 Structure and function of glycoproteins and glycolipids Cell Recognition protein example Major Histocompatibility Complex (MHC) Proteins: These are proteins on the surface of cells that belong to a particular individual. These MHC proteins interact with immune system cells to identify which cells belong to body and which cells are foreign. B 2.1.9 Structure and function of glycoproteins and glycolipids Glycolipids are molecules consisting of carbohydrates linked to lipids. The carbohydrate part is usually a single monosaccharide or a short chain of between two and four monosaccharide units. The lipid part usually contains one or two hydrocarbon chains which naturally fit into the membraneʼs hydrocarbon core. B 2.1.9 Structure and function of glycoproteins and glycolipids Glycolipids occur in the plasma membrane of all eukaryotic cells, with the attached carbohydrate projecting outwards into the extracellular environment of the cell. B 2.1.9 Structure and function of glycoproteins and glycolipids Cell recognition The carbohydrate chains that form on glycoproteins and glycolipids have specific shapes allowing the immune system to recognise the cells as self. Antigens are substances which stimulate an Bacteria immune response and the production of antibodies. Antigen Antibod y B 2.1.8 Selectivity in membrane permeability A semipermeable membrane allows the passage of certain small solutes and is freely permeable to the solvent. (water) A selectively permeable membrane allows the passage of particular particles, but not others. Wor d play ! B 2.1.8 Selectivity in membrane permeability Particles moves across the cell membrane by either Simple diffusion Facilitated diffusion Osmosis Active transport B 2.1.8 Selectivity in membrane permeability Membrane Transport Chain B 2.1.8 Selectivity in membrane permeability B 2.1.3 Simple diffusion across membranes. Describe what is happening in the image on your left. B 2.1.3 Simple diffusion across membranes. Diffusion is the passive net movement of particles from a high concentration to low concentration. This is often through a partially permeable membrane. Passive = does not require energy Net movement= overall movement ( all particles moving all the time) Concentration gradient: The difference in concentration of a substance between two locations. B 2.1.3 Simple diffusion across membranes. Simple diffusion across membranes involves particles passing between the phospholipids in the membrane. It can only happen if the phospholipid bilayer is permeable to the particles. B 2.1.3 Simple diffusion across membranes. Oxygen can pass into the cell by passive diffusion if there is a concentration gradient. Ions with positive or negative charges have a hard time passing through because the hydrophobic centre of the membrane. Polar molecules, which have partial charges, can diffuse at low rates. Small polar molecules (urea & ethanol) pass more easily than large ones B 2.1.3 Simple diffusion across membranes. B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins Osmosis may occur Aquaporin is an integral protein when there is a that, as its name suggests, acts as a partially permeable pore in the membrane that speeds membrane, such as the movement of water molecules a cell membrane. B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell) B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins Osmosis can happen in all cells because water molecules despite being hydrophilic are small enough to pass through the phospholipid bilayer. Some cells have water channels called aquaporins, which greatly increase membrane permeability to water. Examples are kidney cells that reabsorb water and root hair cells of plants that absorb water. At the narrowest part, the channel in an aquaporin is only slightly wider than water molecules , which therefore pass through in single file. B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins Describe what is happening in the image. B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins Solvents can be measured by either weight (i.e. kilograms) or volume (i.e. liters). An Osmole is the unit of osmotic concentration. It's the number of moles of solute that contribute to the oncotic pressure ( type of osmotic pressure) of a solution. Osmolarity, which is the measure of osmoles of solute per liter of solution (Osm/L). i e w! v Re B 2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins A solvent is a substance that can dissolve a solute. If you put sugar in your coffee, coffee is the solvent and sugar is the solute. Solutes are measured in weight (i.e. grams). ! w e vie R B 2.1.6 Channel proteins for facilitated diffusion. Ions and large molecules cannot get across the membrane via simple diffusion. Transmembrane proteins form _____ for these types of molecules to pass through. The diameter and chemical properties of the channel ensure that only one type of particles passes through. B 2.1.6 Channel proteins for facilitated diffusion. Facilitated diffusion is carried out by channel proteins. Facilitated diffusion is movement of molecules/ions along a concentration gradient through a protein channel. It does not require energy. B 2.1.6 Channel proteins for facilitated diffusion. These channels help proteins to move from high to low concentrations - so it is called facilitated diffusion. Cells can control what types of channels are found in the membrane and can therefore control what types of substance are able to diffuse in and out. B 2.1.6 Channel proteins for EXAMPLE facilitated diffusion. Extras!!!!!! The CFTR Channel moves chloride ions out of the cell. People with cystic fibrosis have a mutation that causes the CFTR channel to have the wrong shape and as a result the channel can not move the chloride ions. The ions build up in the cell. Exam Question Distinguish between simple diffusion and facilitated diffusion. Simple diffusion does not require any transport proteins. It needs a concentration gradient. The rate of facilitated diffusion depends on the number of transport proteins in the membrane whereas simple diffusion does not. Facilitated diffusion are carried out by proteins embedded in the plasma membrane. B 2.1.7 Pump proteins for active transport Primary active transport requires ATP. Proteins in membranes, called protein pumps, carry out active transport. Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane. Molecules are moved against their concentration gradient B 2.1.7 Pump proteins for active transport Active Transport is …...the movement of substances from areas low concentration to areas of high concentration through protein pumps …”against the gradient” Active transport requires energy in the form of ATP. B 2.1.7 Pump proteins for active transport ACTIVE TRANSPORT... Requires energy to happen (typically in the form of ATP) B 2.1.7 Pump proteins for active transport PROTEIN PUMPS... 1. Molecule to be transported enters the pump 2. Energy from ATP is used to change the shape of the protein 3. Molecule can now pass to the opposite side of the membrane