The Plasma Membrane & The Cytoskeleton and ECM Lecture #3 The Plasma Membrane & The Cytoskeleton and ECM

The Plasma Membrane outside of cell (extracellular) extracellular matrix glycolipid carbohydrate tag on protein integral membrane proteins lipid bilayer – most abundant lipid is the phospholipid also contains cholesterol and glycolipids integral membrane proteins peripheral membrane proteins p.125 – theories on membrane structure fluid mosaic model applies to all biological membranes cholesterol peripheral membrane protein phospholipid cytoplasm of cell (intracellular) microfilaments of cytoskeleton

The membrane phospholipid the phospholipid one hydrophilic portion with a phosphate group 2 fatty acid side chains fatty acids can be saturated or unsaturated this will affect how the phospholipids sit in the bilayer and how fluid the membrane is unsaturated fatty acid saturated fatty acid

Phospholipids in a membrane WATER Hydrophilic head Hydrophobic tail WATER

Movement of Phospholipids membranes are not rigid they are not fixed in one position phospholipids and other lipids can move laterally within the membrane however, sidedness of the membrane is relatively set phospholipids won’t flip from the intracellular side to the extracellular side very easily Lateral movement (~107 times per second) Flip-flop (~ once per month)

Movement of Proteins + EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. EXPERIMENT RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour + proteins embedded within the membrane also move move more slowly than phospholipids however, this may be influenced by the cytoskeleton of the cell or the extracellular matrix as will be discussed later in the class The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. CONCLUSION

Membrane Fluidity Depends on Fatty Acid Saturation Viscous as a membrane cool, it becomes less fluid depends on the type of lipids in the membrane unsaturated fatty acids have a kink in the fatty acid chain cannot pack together as easily therefore remain fluid at lower temperatures olive oil – unsaturated fat butter or lard – saturated fat Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails

Cholesterol Affects Membrane Fluidity cholesterol is also an important lipid found in membranes can restrain movemments of the phospholipids at body temperature but it also prevents close packing of phospholipids – the membrane stays fluid at lower temperatures how would certain types of plants and animals adapt knowing this? Cholesterol

Integral Membrane Proteins N-terminus C-terminus a Helix CYTOPLASMIC SIDE lipid of membrane Phospholipid different types of cells have different sets of membrane proteins transmembrane portion of the protein is hydrophobic the alpha helix structure is perfectly suited for transmembrane proteins amino acids along one face of the helix are hydrophobic and like the lipis portion of the bilayer amino acids along the opposite face can bundle together and form a channel note: proteins are embedded in a particular orientation the outside face has a function specific to the outside inside face function is specific to the inside alpha helix of protein

Integral Membrane Proteins may be anchored by the ECM or the cytoskeleton extracellular matrix plasma membrane membrane is fluid but… proteins may have attachments to the ECM or cytoskeleton motor proteins may direct their movement talk about later intracellular cytoskeleton

How do plasma membrane proteins get to the plasma membrane? 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus 2 Vesicle glycoproteins and glycolipids sidedness of the membrane – important during fusion of vessicles 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid

The Cytoskeleton Microtubule Microfilaments 0.25 µm cytoskeleton – the skeleton of the cell mechanical support and maintains cell shape proteins can attach to the cytoskeleton – so a transmembrane protein attaches and can regulate the architecture of the cytoskeleton – transmit mechanical forces that alter what genes are expressed the cell adapts to the change can also provide a place for subcellular structures or organelles to anchor themselves – something to hold onto also provides guidance – like a track to follow if it is a vessicle Microfilaments 0.25 µm

vessicles are membrane enclosed structures Vesicle ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton Vesicles 0.25 µm vessicles are membrane enclosed structures have transmembrane and peripheral membrane proteins one of the TM proteins is receptors in this example, there is a receptor for a motor protein using energy from ATP (which comes from where?) the motor protein slides along the microtubule SEM – giant squid axon vessicles are moving along a microtubule

Components of the Cytoskeleton microtubules – hollow tubes composed of dimers of alpha and beta tubulin microtubules grow by adding tubulin dimers to the ends they can also be disassembled or broken down and reassembled elsewhere in the cell their general role is to resist compression (next slide) also important for beating of flagella and cilia on the cell surface cilia and flagella have a core of microtubules sheathed in an extension of the plasma membrane anchored in the cell by a basal body – similar to the centriole movement required ATP microfilaments are actin filaments – solid rods twisted double chain of actin in contrast to microtubules, microfilaments function to resist pulling forces helps support cell shape actin functions with myosin in muscle contraction intermediate filaments function in resisting pulling forces very diverse can be made of a variety of different proteins more permanent than microtubules and microfilaments important in keeping cell shape and position of organelles nucleus sits within a cage of intermediate filaments also part of the nuclear lamina – lamin protein neurofilaments give the axon projections of a nerve cell their shape microtubules microfilaments intermediate filaments

Centrosomes and Centrioles Intermediate filaments in nerve cells Microtubule Centrioles 0.25 µm Longitudinal section of one centriole Microtubules Cross section of the other centriole the centrosome is a region in the cell that houses 2 centrioles – which are 9 sets of triplet microtubules arranged in a ring centrioles are important in cell division – they organize the mitotic spindle which pulls the chromosomes apart during mitosis

Plant Cell Walls Central vacuole of cell Plasma membrane Secondary cell wall Primary Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata cell walls are composed of cellulose, other polysaccharides and proteins plasmodesmata are channels – intercellular junctions that allow the flow of materials between plant cells therefore plants can be one living continuum

Extracellular Matrix of Animal Cells Collagen Fibronectin Plasma membrane EXTRACELLULAR FLUID Micro- filaments CYTOPLASM Integrins Polysaccharide molecule Carbo- hydrates Proteoglycan Core protein Integrin A proteo- glycan complex main components of the ECM: glycoproteins collagen – makes strong fibers outside the cell collagen fibers are embedded in a network of proteoglycans also have fibronectin ECM proteins bind integrins in the plasma membrane integrins then have associated proteins on the intracellular or cytoplasmic side of the cell these proteins associate with the microfilaments of the cytoskeleton changes in the ECM translate into changes in the cytoskeleton these changes are not only in the cell shape or its movement, but also may lead to changes in gene expression

Intercellular Junctions Tight junctions prevent fluid from moving across a layer of cells Tight junction 0.5 µm Tight junctions Intermediate filaments Desmosome tight junctions – membranes of neighboring cells are pressed tightly against each other – specific proteins in both cells mediate this interaction desmosomes – anchoring junctions fastening cells together – desmosomes are anchored by intermediate filaments gap junctions – provide cytoplasmic channels between cells – special membrane proteins surround a pore through which small molecules may pass – also important in intercellular communication – heart muscle the signal to contract is passed efficiently through the gap junctions, allowing the heart muscle cells to contract in tandem. Gap junctions 1 µm Extracellular matrix Space between cells Gap junction Plasma membranes of adjacent cells 0.1 µm