04-27-16: Lecture 9 Membrane Structure and Function All Membranes are semipermeable Proteins can move about the membrane – not a static structure Fluid Mosiac Model http://www.youtube.com/watch?v=Qqsf_UJcfBc 04-27-16: Lecture 9 Membrane Structure and Function All Membrane are semipermeable Fundamental unit is phospholipid It allows some substances to cross it more easily than others Proteins can move about the membrane – not a static structure Fluid Mosiac Model Proteins move back and forth Proteins inside the lipid bilayer should have non-polar side chains (R groups) Interactions with hydrophobic interior Importance of Fluidity Adjust the fluidity by changing the degree of saturation Cholesterol is also helpful in maintaining fluidity 04-27-16: Lecture 9 Membrane Structure and Function Fission of Membranes Fusion of Membranes Examples: Examples: Vesicle leaving ER or leaving Golgi Vesicle from ER fusing to Golgi Cell Division (cytokinesis) fertilization Endocytosis Virus infection Exocytosis Membrane Structure and Function04-27-16: Lecture 9 Membrane Interaction – an Up-close view Apposition Hemifusion Full fusion (content mixing) Membrane Structure and Function04-27-16: Lecture 9 Membrane Interaction – an Up-close view Invagination Hemifusion Separation 04-27-16: Lecture 9 Membrane Structure and Function Endocytosis Brings nutrients into the cell Brings signals into the cell Invagination of the plasma membrane - Vesicle fission Phagocytosis Brings foods into cell vesicle lysosome (digested) Pinocytosis Brings soluble components into cell water vacuole 04-27-16: Lecture 9 Membrane Structure and Function Receptor mediated Endocytosis Involves proteins in plasma membrane – called a receptor transmembrane Binds a molecule (ligand) on the outside of the cell (e.g. sugar) Clusters on cell surface – a pit is formed Then coated pits formed – by attachment of clathrin (a protein) inside cell Invagination form vesicle coat falls off vesicle can go to its destination 04-27-16: Lecture 9 Membrane Structure and Function Endocytosis Receptor mediated Endocytosis •Ligand • •Receptor RECEPTOR-MEDIATED ENDOCYTOSIS •Receptors cluster Coat protein Receptor Coated vesicle •Invagination •Recruit clathrin (coat protein) Coated pit Ligand •Pinch off vesicle (fission) Coat protein Transmission •Clathrin falls off EM of a coated pit Plasma membrane 0.25 µm 04-27-16: Lecture 9 Membrane Structure and Function Permeability of Membranes Most permeable: CO2, CH4, O2 Least permeable: Sugars, amino acids, proteins 04-27-16: Lecture 9 Diffusion Rate of diffusion is proportional to concentration of a solute Osmosis is the diffusion of water across a selectively permeable membrane Isotonic solution: equal concentrations Hypertonic solution: cell in a solution w/ higher solute concentration Hypotonic solution: cell has greater solute conc than solution. 04-27-16: Lecture 9 Diffuson One solute Net diffusion Net diffusion Equilibrium Two solutes Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium 04-27-16: Lecture 9 Membrane Structure and Function Types of proteins that reside at membranes Integral membrane Proteins – example of structure Extracellular side N-terminus Note: N-term (amino) and C-term (carboxyl) can be on either side and same side depending how the protein is inserted into membrane C-terminus CYTOPLASMIC a Helix Figure 7.8 SIDE 04-27-16: Lecture 9 Membrane Structure and Function Types of proteins that reside at membranes Integral membrane Proteins •Penetrate the hydrophobic core •Transmembrane – span the entire lipid bilayer - ~10-20 amino acids in length Peripheral Proteins ~20 a.a •Loosely bind to plasma membrane •Typically by interacting with integral membrane proteins •Some peripheral proteins can partially embed in plasma membrane 04-27-16: Lecture 9 Membrane Structure and Function Integral membrane Proteins Six major functions 1. Transport ATP Enzymes 2. Enzymatic Activity Signal 3. Signal Transduction 04-27-16: Lecture 9 Membrane Structure and Function Integral membrane Proteins Six major functions 4. Cell Recognition Glyco- protein 5. Intercellular joining 6. Attachment of the cytoskeleton to the extracellular matrix 04-27-16: Lecture 9 Internal Cytoskeleton •Microtubules (MT) Polymers of tubulin •Microfilaments (MF) Polymers of actin •Non-covalent •Intermediate filaments Polymers of keratin 4˚ structure : protein-protein interactions which make long chains •Cell shape •Rigidity/flexibility •Transport “roadway” •Movement 04-27-16: Lecture 9 Internal Cytoskeleton •Microtubules •Microfilaments •Intermediate filaments •Cell Shape (compression- •Cell Shape (tension- •Cell Shape (tension- resisting) bearing) bearing) •Cell motility •Cell shape changes! •Organelle anchorage •Chromosome movement in •Cell motility cell division •Muscle contraction •Organelle movement (vesicle movement thru •Cell Division endomembrane system) 04-27-16: Lecture 9 Cytoskeleton 04-27-16: Lecture 9 Internal Cytoskeleton Microtubules: cell transport and motility •Central assembly point for MT in the cell is called the centrosome (MT organizing center) •Motor proteins move along MT •Kinesin move things away from the nucleus •Dynein move things towards the nucleus •Cell motility – Flagella, or cilia – use Dynein motor Show Kinesin Animation Show Kinesin Animation See Video links on website : neuron migration 04-27-16: Lecture 9 Internal Cytoskeleton Microfilaments: cell shape and motility •Changes in cell shape is related to motility •Three types of cell shape •Microvilli – projections on surface – increase surface area •Lamellipodia – membrane ruffles help sense environment – and direct movement •Filopodia – like microvilli but less stable – also sense environment (can turn into lamellipodia) 04-27-16: Lecture 9 Actin-based motility – Filopodia and Lammellipodia Growth cone of a neuron 5 uM Betz et al., 2009 See Video links on website : neuron migration 04-27-16: Lecture 9 Actin-based motility Actin polymerization at the leading edge (lamellipodia) 04-27-16: Lecture 9 Actin-based motility – neuron migrating past another neuron See Video links on website : neuron migration 04-27-16: Lecture 9 Linking the Extracellular Matrix (ECM) to the Cytoskeleton – SUPPORT! ECM made of: •Glycoprotein (proteins modified with sugars) •collagen (most predominant) •Proteoglycans Plus Integrin (integral membrane protein) •fibronectin EXTRACELLULAR FLUID Polysaccharide Collagen molecule A proteoglycan complex Carbo- hydrates Core protein Fibronectin Proteoglycan Plasma molecule membrane Integrins CYTOPLASM Integrin Micro- filaments Figure 6.29 04-27-16: Lecture 9 Linking the Cells together by Cytoskeleton – Why? Fluid Mosaic Model •proteins are not stationary - THEY MOVE in the MEMBRANE Tight Junction: Physical Barrier to proteins •Link neighboring cells together so components Can’t get thru. •Anchored by cytoskeleton •Barrier to movement in membrane •Have to send proteins to the right membrane surface Tight Junction: Physical Barrier to proteins 04-27-16: Lecture 9 •Link neighboring cells together so components Can’t get thru. 04-27-16: Lecture 9 Moving molecules across a membrane: Transport Diffusion: Entropy at work - molecules move from high to low concentration •Small and nonpolar molecules can freely diffuse across membrane (CO2, O2, etc) •Protein dependent diffusion •Facilitated Diffusion channel protein through which small molecules (such as water) can pass. Carrier protein - integral membrane protein switches between 2 conformation states – moving molecules across as the shape of protein changes •Movement in Both directions – WHY? •Depends on the concentration gradient!! 04-27-16: Lecture 9 Moving molecules across a membrane: Transport Facilitated Diffusion - EXTRACELLULAR FLUID •Example of a channel protein Channel protein Solute CYTOPLASM Figure 7.15 •Example of a carrier protein Carrier protein Solute Figure 7.15 04-27-16: Lecture 9 Moving molecules across a membrane: Transport Facilitated Diffusion Rate of transport 04-27-16: Lecture 9 Moving molecules across a membrane: Transport Active Transport :transport of a molecule against its concentration gradient •A way to concentrate molecules in the cell – against conc. Gradient •Involves a transporter or pump •Uses Energy •Primary active transport •Uses ATP as energy (ATP ADP + Pi) •Conformational change in pump due transfer of phosphate to protein (phosphorylation) •Secondary Active transport •Energy is provided by a concentration gradient Previously made using ATP! •Co-transporter protein •Example: Sugar-H+ pump which moves high of H+ and sugar together Don’t forget Allosteric regulation! 04-27-16: Lecture 9 Cell Signaling Phosphorylation: Why is it important in signaling? from ATP Serine O Threonine OH + HO P O- Tyrosine O- Enzyme (Kinase) H20 O O P O- O- 04-27-16: Lecture 9 Moving molecules across a membrane: Transport Active Transport :transport of a molecule against its concentration gradient •Primary active transport •Example – Sodium-Potassium Pump!! •Sodium-Potassium Pump exchanges sodium (Na+) for potassium (K+) across the plasma membrane •Membrane Potential •Voltage across their plasma membranes •Voltage – is electrical potential energy – separation of opposite charges •Cytoplasm of cells is negative compared to extracellular fluid •Unequal distribution of cations and anions •Membrane potential is like a battery that effects the traffic of all charged molecules. •Cell is negative so this membrane potential helps to drive transport of cations into cell •ELECTROCHEMICAL GRADIENT