MEMBRANES AND CELL ORGANELLES 1 CELL STRUCTURE: MEETING THE NEEDS OF MOLECULES Molecules need to: – move in and around cell at a certain rate to reach sites of specific activity (ie where they will react with other molecules) – be in adequate concentrations (ie there needs to be enough of them) for chemical reactions to occur at the right rate. Cell structure therefore needs to facilitate the movement of molecules and maintain them in adequate concentrations to maintain cell function (ie so the cell doesn’t die) The Surface area conundrum Cells need to maximise their surface area to ensure the rapid movement of molecules Problem: – As volume increases, surface area decreases! – How do cells deal with this? Membranes! Prokaryotic Types of cells – Very small: less than 2mm in diameter – Lack internal compartments – Bacteria and archaeans Eukaryotic – Much larger: 10100mm in diameter – More complex structure – compartments called organelles – Animals, plants, fungi and protists Organelles Large eukaryotic cells increase their surface area by having folded membranes and internal compartments called organelles Organelles also allow different chemical reactions to occur at the same time in different places without interfering with each other Organelles maintain the concentration of molecules at levels that ensure they will react with each other at optimum rates CELL STRUCTURE We are now going to learn about the structure of eukaryotic cells and their various organelles in the context of cellular processes. What is a cell? A fluid filled compartment containing atoms and molecules INTRACELLULAR AQUEOUS ENVIRONMENT – CYTOSOL or CYTOPLASM EXTRACELLULAR AQUEOUS ENVIRONMENT CELL BOUNDARY (PLASMA MEMBRANE) Cell membrane - structure A plasma membrane is an ultra thin and pliable layer with an average thickness of less than 0.01 μm (0.00001 mm). Cell membrane - structure Called fluid mosaic model Lipids are the fluid part of the membrane Proteins are the mosaic part of the membrane Cell membrane - functions Define cell boundary Provide permeability barrier (acts like a sieve) Provide sites for specific functions Regulate transport of solutes Detect electrical and chemical signals Assists in cell to cell communication Summary: crossing the cell membrane Type Diffusion Osmosis Facilitated diffusion – carrier proteins Facilitated diffusion – channel proteins Active transport Endo/Exocytosis Description Molecules 1. Diffusion The movement of molecules from areas of high solute concentration to area of low solute concentration. i.e.. Down the concentration gradient. No energy is involved! Diffusion depends on… Permeability Surface Area Concentration Gradient Distance of Diffusion Which molecule will diffuse? Fick’s Diffusion Law Surface area of membrane Difference in concentration X across the membrane Length of the diffusion path (thickness of the membrane) Ways to increase diffusion Increasing concentration Increasing temperature Increasing surface area Permeable membrane If the membrane is permeable to both the solute and the solvent, the pattern of diffusion is unchanged. Concentration Gradients Diffusion High concentration Low concentration No net movement! Once diffusion is complete the molecules keep moving but the overall distribution remains constant = equilibrium. Partially Permeable Membrane If the membrane is partially permeable, the solvent can move through but the solute cannot. Concentration Gradients Partially permeable membrane High concentration Low concentration 2. Osmosis A special type of diffusion! Solute Water molecules The Add solute cannot cross the membrane. To try and Solute balance the concentrations, the water molecules move to dilute the solution. High concentration solute Low concentration solute The solute cannot cross the membrane. To try and balance the concentrations, the water molecules move to dilute the most concentrated solution. Osmotic Gradient Concentrated solute Dilute solute The pressure that makes the water move is called the osmotic pressure. Hypotonic = extracellular fluid lower concentration than intracellular fluid and water will diffuse into cell Isotonic = extra and intracellular fluid are same concentration and there will be no net movement of water Hypertonic = extracellular fluid higher concentration than intracellular fluid and water will diffuse out of cells The net movement of water from a region of low solute concentration to a region of high solute concentration is called: A. B. C. D. Osmosis Diffusion Facilitated diffusion Active transport 3. Facilitated Diffusion Most molecules are too large or too polar to cross membrane by simple diffusion Protein assisted movement down a concentration gradient – facilitated diffusion can occur in a few different ways HIGH CONCENTRATION GRADIENT LOW Facilitated Diffusion Special channels in the membrane help the diffusion. This channel or carrier mediated movement is selective and can become saturated. This may inhibit the movement of another molecule. No energy is used. Facilitated diffusion: carrier protein The molecule binds to its carrier protein, potentially changing its shape, and is carried to the other side Facilitated diffusion: channel protein Channel proteins form pores in the membrane that fill with water and dissolve hydrophillic molecules. Both simple diffusion and facilitated diffusion involve: A. B. C. D. Energy expenditure by the cell Movement of a substance down its concentration gradient A protein in the plasma membrane acting as a carrier molecule A substance moving from outside to inside a cell across the membrane. 4. Active transport When the cell spends energy to move molecules against the concentration gradient. Concentration Gradients Active transport High concentration Low concentration Against the concentration gradient! Transport/Carrier proteins Form a channel for molecules to pass through. They are selective, may become saturated and inhibit the movement of other molecules. Space filling model of rabbit calcium ATPase. Calcium ATPase is a membrane transport protein which transfers calcium after a muscle has contracted. Extracellular fluid Sodium-Potassium Pumps Na+ Na+ The sodiumpotassium pump is a protein in the membrane that exchanges sodium ions (Na+) for potassium ions (K+) across the membrane. K+ Na+ Plasma membran e Carrier protein K+ ATP Na+ Na+ moves to its binding site Cell cytoplasm K+ Proton Pumps Proton pumps use the energy from ATP to move hydrogen ions (H+) from inside the cell to the outside. Extracellular fluid H+ H+ H+ H+ H+ H+ Carrier protein Plasma membrane ATP H+ Cell cytoplasm Coupled Transport Coupled transport is also called cotransport. Plant cells use the hydrogen gradient created by proton pumps to actively transport nutrients into the cell. Extracellular fluid Diffusion of hydrogen ions down their concentration gradient H+ H+ Sucrose H+ Carrier protein H+ Plasma membrane H+ Cell cytoplasm Summary-Membrane Pumps Membrane pumps are proteins,which require energy (often as ATP) to transport molecules across the cell membrane. . Extracellular fluid Na+ Na+ K+ Na+ H+ H+ H+ H+ H+ Plasma membrane H+ ATP K+ Na+ Cell cytoplasm K+ ATP H+ H+ Summary-Membrane Pumps The activity of pumps may be coupled, e.g. the accumulation of H+ from the proton pump is used to drive the transport of sucrose against its concentration gradient. Extracellular fluid Na+ Na+ K+ Na+ H+ H+ H+ H+ H+ Plasma membrane H+ ATP K+ Na+ Cell cytoplasm K+ ATP H+ H+ The role of proteins and protein complexes in the plasma membrane of a cell includes their role as: A. B. C. D. A receptor protein A channel or pore An antigen All of the above 5. Cytosis When the cell spends energy to move LARGE molecules. Moving large molecules Sometimes, large molecules need to be moved around in the cell, stored within, or moved outside the cell To do this, cells make very small containers or sacs called vesicles from the plasma membrane Transporting out of the cell: exocytosis Transporting into of the cell: endocytosis Active Transport: Cytosis Membrane-bound vesicles or vacuoles are formed by infolding (invagination) or outfolding (evaginated) to transport substances across the membrane. Membranebound vesicle Plasma membrane folding inwards This cell is carrying out a form of endocytosis called pinocytosis in which the plasma membrane forms invaginations to enclose liquids and bring them into the cell. Phagocytosis During endocytosis the plasma membrane invaginates (folds in) around the molecules to be transported into the cell. Solid particle CDC Endocytosis Pinocytosis Membranebound vesicle Endocytosis 1 Materials that are to be collected and brought into the cell are engulfed by an invagination of the plasma membrane. 2 Plasma membrane Vesicle buds off from the plasma membrane. 3 Cell cytoplasm The vesicle carries molecules into the cell. The contents may then be digested by enzymes delivered to the vacuole by lysosomes. Types of endocytosis: phagocytosis: the engulfment of solid particles. pinocytosis: the engulfment of liquid particles. receptor mediated: engulfment of specific particles according to membrane receptors. Phagocytosis Food particle (cell eating) The particles are contained within a membrane enclosed sac (a vacuole). Digestion of the particles occur when the vacuole fuses with a lysosome containing digestive enzymes. Amoeba pseudopod Engulfed bacterium Pinocytosis Invaginations of the plasma membrane enclose the liquid droplets within small vesicles. Plasma membrane engulfing liquid substance. Membranebound vesicle The fluid within the vesicle is transferred to the cytosol. Pinocytosis by a capillary endothelial cell. TEM (X12,880) Receptor-Mediated Endocytosis The cell membrane has regions of specific receptor proteins exposed to the extracellular environment. The receptor proteins occur in clusters (called coated pits) and have binding sites that will only bind specific molecules. Extracellular fluid Receptor protein Cytoplasm Plasma membrane Receptor-Mediated Endocytosis The cytoplasmic side of the coated pit is lined with a special protein called clathrin protein, which provides membrane stability (right). Target molecule Clathrin protein Coated vesicle When the target molecule (ligand) binds to the receptor protein (left), a coated vesicle forms around it, allowing the molecule to be imported into the cell. A cell that is phagocytosing a bacteria cell could be expected to: A. B. C. D. Have a cell wall Be expending energy Be producing oxygen Contain a chloroplast Exocytosis Exocytosis releases molecules from the inside of the cell to outside of the cell. Exocytosis occurs by fusion of a vesicle membrane with the plasma membrane. The vesicle contents are then released to the outside of the cell. Transport vesicle Cross section through the plasma membrane of cardiac muscle showing the presence of transport vesicles. TEM X 162,000 Exocytosis 3 2 1 Vesicle carrying molecules for export moves to the perimeter of the cell. The contents of the vesicle are expelled into the intercellular space (which may be into the bloodstream). Vesicle fuses with the plasma membrane. Plasma membranes that are able to bend and fold are necessary for the movement of which substances into or out of a cell? A. B. C. D. Glucose molecules Sodium ions Fatty acid molecules Protein molecules Summary There are two types of transport in a cell. 1. Passive (not requiring energy) Plasma membrane Cell cytoplasm diffusion and facilitated diffusion osmosis 2. Active or energy requiring Active transport Cytosis (exocytosis, endocytosis etc) The plasma membrane is partially permeable, allowing some molecules to pass through, and preventing the passage of others. E The three types of movement across a membrane are correctly described as X Y Z A active transport diffusion facilitated diffusion B active transport facilitated diffusion diffusion C facilitated diffusion active transport diffusion D diffusion active transport facilitated diffusion SUMMARY