* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
MEMBRANES AND CELL ORGANELLES 1 EL: INTRODUCTION TO CELL STRUCTURE AND FUNCTION, WITH A FOCUS ON THE PLASMA MEMBRANE 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. Highsolute concentration concentration The cannot cross theLow membrane. To try and solute balance the concentrations, the solute 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 Activity Complete chapter 1 quick check questions and booklet questions to hand at the end of the week Put in your PLJ to join wiki space and post on the discussion board Reflection What do you need to go over thouroughly before your SAC this week? 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 The activitypumps of pumps be coupled, e.g. Membrane are may proteins,which the accumulation of H+asfrom require energy (often ATP)the to proton transport pump is used to drive the membrane. transport of molecules across the cell 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 Summary: crossing the cell membrane Type Description Molecules Simple diffusion Unassisted (passive) movement of solutes down a concentration gradient (ie from area of high solute concentration to area of low solute concentration) Small polar or non polar molecules, eg oxygen, carbon dioxide Osmosis Simple diffusion of water from an area of low solute concentration to an area of high solute concentration Water Facilitated diffusion – carrier proteins Protein assisted movement down a concentration gradient molecule binds to its carrier protein, potentially changing its shape, and is carried to the other side Charged or polar molecules Facilitated diffusion – channel proteins Protein assisted movement down a concentration gradient Channel proteins form pores in the membrane that fill with water and dissolve hydrophillic molecules Molecules that dissolve in water eg ions (imp to note: channel proteins are selective to particular proteins) Active transport Protein assisted movement up (ie from low concentration to high concentration) a concentration gradient, requiring energy input Nutrients, glucose, waste products Endo/Exocytosis Movement of large molecules into (endocytosis) or out of (exocytosis) the cell Large molecules of groups of macromolecules (eg hormones, mucus) ACTIVITY Design and make a 3D model of a plasma membrane, which includes at least two of the ways to cross the membrane This will be assessed as part of SAC 1 and will be due on the first day back next term Reflection Develop a rhyme to remember the different ways molecules cross the plasma membrane. Homework: Work on your model and chapter 2 questions MEMBRANES AND CELL ORGANELLES 2 EL: To complete the experimental component to SAC 1 Reflection How well did your group work together today? MEMBRANES AND CELL ORGANELLES 3 EL: To complete the write up of SAC 1 Reflection How well did you work today? MEMBRANES AND CELL ORGANELLES EL: TO LEARN/REVISE THE STRUCTURE AND FUNCTION OF OTHER CELL ORGANELLES Organelles Within the EUKARYOTIC cell, various organelles work together to: move substances from one part of the cell to another prepare other substances for export from the cell Inside the cell Each living cell is a small compartment with an outer boundary, the plasma membrane. Within this one compartment that makes up a living eukaryotic cell is a fluid, called cytosol, that consists mainly of water containing many dissolved substances (see table 2.1, page 38) and membrane-bound organelles. NB Cytoplasm = cytosol+organelles Activity In groups of 2-3, randomly select an organelle. Spend 10 mins coming up with a way of explaining it to the class, which MUST be interactive (eg. Quiz, role play etc) You have 2-5 mins to deliver your lesson At the end, all students should be able to fill in following table Cell structure summary Organelle 1.Nucleus 2.Mitochondria 3. Ribosomes 4. Endoplasmic reticulum and Golgi complex 5. Lysosomes 6. Chloroplasts 7. Cytoskeleton and extracellular matrix Structure (can be a picture) Function Nucleus Information and control centre of the cell Controls production of all proteins via DNA in chromosomes Nucleus Nucleus contained within double membraned nuclear envelope, which: – is continuous with the endoplasmic reticulum (helps distribute materials through cell) – Contains numerous openings, called nuclear pores, channels for moving water soluble Nuceoli in the nucleus synthesise ribosomal RNA (rRNA) and ribosomes Protein pathways Eukaryotic cells have mechanisms to assemble, package and transport proteins within a cell Protein pathways: production RIBOSOMES Proteins are synthesised on extremely small organelles called ribosomes There are enormous numbers of ribosomes in a cell to make all the proteins needed Lack a membrane and are composed of 2 subunits – RNA and protein rRNA synthesised in the nucleolus passes through nuclear pores into the cytosol and to the ribosomes for protein synethesis Protein pathways: production Protein pathways: transport ENDOPLASMIC RETICULUM (ER) In eukaryotic cells, ribosomes are attached to membranes of the endoplasmic reticulum (ER: described as rough ER). The ER is a series of folded membranes and tubules found in the cytosol. Proteins produced by the ribosomes enter the tubules and are transported around the cell Proteins may also be modified in ER Protein pathways: packaging GOLGI BODY Receives proteins from ER, where they may undergo further modification and/or storage Proteins are placed in a vesicle and transported to other parts of the cell or the plasma membrane for exocytosis Protein pathways: packaging Protein secretory pathway GTAC: crossing the plasma membrane presentation See text figure 2.19 http://www.johnkyrk.com/er.html Cellular recyclers: Lysosomes Lysosomes are vesicles containing powerful digestive enzymes Can break down macromolecules and even organelles into simpler molecules. Any material that is not reused inside the cell is released from the lysosome by exocytosis into the extracellular fluid In white blood cells, they also digest pathogens (discussed later in Unit 3) Cellular recyclers: lysosomes http://highered.mcgrawhill.com/olc/dl/120067/bio01.swf Cell movements and connections: cytoskeleton Cell movements and connections: cytoskeleton The cytoskeleton consists of a network of protein fibres Fibre Function Microtubules Movement of chromosomes, organelles, cilia and flagella Intermediate filaments Provide tensile strength for the attachment of cells to each other and their external environment Microfilaments Composed of contractile filaments of actin that, together with myosin, control muscle contraction, maintain cell shape and carry out cellular movements Cell movements and connections: extracellular matrix Most cells have an extracellular matrix (ECM) that are an integral part of the structure and function of the cell: – eg cell wall in plants – Bone and cartilage in animals are connective tissues largely made up of ECM ECM has important role in determining shape and mechanical properties of tissues and organs Organelles for energy: mitochondria Organelles for energy: mitochondria Small, cigar-shaped organelles found in cytosol Consists of smooth outer membrane and highly folded inner membrane (the folds are called cristae) Fluid filled intermembrane space Protein-rich fluid called matrix in internal space Have own genetic material: mtDNA and RNA and ribosomes. This allows them to undergo division. Organelles for energy: chloroplasts Organelles for energy: chloroplasts Chloroplasts are found in green plant cells and some protists and are the site of photosynthesis Have an inner and outer membrane Enclosed by the inner membrane is the stroma – a gel-like enzyme-rich matrix Organelles for energy: chloroplasts Suspended in the stroma is a third membrane structure called the thylakoid membranes: flat sac-like structures called grana when grouped together into stacks Like mitochondria, have own genetic material: DNA and RNA and ribosomes Activity Look at some prepared slides under the microscope Sketch what you see and label visible organelles – Make sure you use pencil – Rule lines to label (don’t cross lines over) – Make sure you write the magnification down (e.g. x10) If timer permits/Homework Complete cell webquest on Wiki and email to me Complete cell quiz at www.gtac.edu.au – on student support page Quick check qu 7-18 Biochallenge qu 1&3 Chapter review qu 2, 3 & 11 (& 12 if you feel like it) Reflection What is one thing you really understood about YOUR organelle and one thing from another groups organelle? MEMBRANES AND CELL ORGANELLES EL: To learn about how cells connect with and communicate with each other and revise for your test Cell connection and communication Although some cells, (e.g. blood cells), are free to move as individuals, most cells remain as members of a group and need to communicate with each other Animal Cells There are three different types of junctions in animal cells: occluding, communicating (gap) and anchoring (desmosomes) junctions (see figure 2.25). Anchoring junctions are the most common form of junction between epithelial cells. Dense plaques of protein exist at the junction between two cells. Fine fibrils extend from each side of these plaques and into the cytosol of the two cells involved. This structure has great tensile strength and acts throughout a group of cells because of the connections from one cell to another. Occluding junctions involve cell membranes coming together in contact with each other. There is no movement of material between cells. Communicating junctions consist of protein-lined pores in the membranes of adjacent cells. The proteins are aligned rather like a series of rods in a circle with a gap down the centre and permit the passage of salt ions, sugars, amino acids and other small molecules as well as electrical signals from one cell to another. Plant cells Plants have rigid cell walls. Hence, plant cells have no need for a structure such as the anchoring junctions of animal cells. Secondary walls are laid down in each cell on the cytosol side of the primary wall so that the structure across two cells is relatively wide, at least 0.1 μm thick. The junctions that exist in plant cells to allow communication between adjacent cells in spite of the thick wall are plasmodesmata (singular: plasmodesma) Plant cells Because of the way in which plant cell walls are built up, the gap or pore between two cells is lined with plasma membrane so that the plasma membrane of the two cells is continuous. A structure that bridges the ‘gap’ is also continuous with the smooth endoplasmic reticulum of each cell. Activity Quick check qu 18-19 Biochallenge qu 2 In the last 15 minutes, we’ll play Cell Jeopardy revision game for your test next lesson Reflection What letter grade/% would you like to get on your test and how will YOU make it happen?