Membrane separates inside/outside Inside cell = aqueous Outside cell = aqueous Solutes are mostly polar How do we keep molecules where they need to be

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1 Biology Physical isolation Separates inside of cell from extracellular fluid Lecture 6: Cell Membranes 2. Regulates intracellular-extracellular extracellular exchange Controls ions, nutrients, waste, and secretory product exchange 3. Sensitivity to the extracellular environment Receptors allow cell recognition/response to molecules in the environment 4. Attachment (within cell) Cytoskeleton Microfilaments (actin) Microtubules (tubulin) Intermediate filaments Animal cells only >60 proteins 5. Attachment (outside cell) Cells don t live floating in fluid must attach to surface and to each other Molecules secreted by cells Provide structural and biochemical support, stiffness, and elasticity Examples: fibronectin, proteoglycan, cadherins 1

2 Membrane separates inside/outside Inside cell = aqueous Outside cell = aqueous Solutes are mostly polar How do we keep molecules where they need to be? Solution: cell membrane must be fundamentally non-polar, but able to exist in aqueous environment Molecule that makes this possible = phospholipid Polar heads face both outside and inside Hydrophilic heads face aqueous environments Non-polar tails protected in between Membrane must be very fluid Fluidity adjusted by changing saturation of fatty acid tails More saturated = less fluid Problem: system is too effective Like a room with no doors and no windows How does anything get in or out? 2

3 Solution: membranes not 100% phospholipid Phospholipid Cholesterol (Carbohydrate) Proteins 50% dry weight of membrane Adds stiffness Cholesterol Straightens phospholipid tails Prevents small polar molecules from passing through membrane Membrane Carbohydrates Carbohydrate functions Membrane lubrication and protection Hygroscopic (attract water) Carbohydrate functions Carbohydrate chains on glycoproteins provide fingerprint Cell can recognize self vs non-self Genetically determined Membrane Proteins Proteins = major functional component Two configurations Integral Span entire width of membrane Part of membrane structure Peripheral Membrane Proteins Bind to inner or outer surfaces Distinct from membrane 3

4 Transporters Channels Integral proteins Central pore Permits entry of small molecules, water Transporters Carriers Bind solutes, transport across membrane Protein changes shape May or may not require ATP Enzymes Can be integral or peripheral Catalyze reactions inside and outside cell Example: membrane-associated associated enzymes in small intestine break down dipeptides into amino acids Receptors Bind to external chemicals to regulate processes within cell Shape-specific Not necessary for steroid hormones Example: insulin (peptide hormone) Anchors Intercellular Attach cells to each other Anchors Cytoskeleton and/or extracellular matrix Hold membrane to cytoskeleton Hold cell to matrix 4

5 Materials must constantly cross the cell membrane Example Cell Membranes Food energy molecules in Waste molecules out Some things can cross membranes freely; others cannot Cells have developed a number of ways around this Cell membrane is semi-permeable Some things can cross, others can not Primary influences: Size Polarity Charge Cell Membranes Diffusion Some solutes can cross the cell membrane along a diffusion gradient Will equalize concentrations on both sides Costs no cellular energy = passive transport Several types of diffusion Simple diffusion Substance flows freely across membrane Mostly small, non-polar molecules Diffusion Examples: O 2, CO 2 Facilitated diffusion Membrane protein facilitates movement across membrane May be a channel Based on size, charge Example: ions Diffusion Facilitated diffusion May be carrier Highly specific Based on Shape Example: glucose Diffusion DOES THIS REQUIRE ENERGY?? 5

6 Facilitated Diffusion Both channels and carriers can be regulated Turned on and off Number on membrane changed Diffusion of water molecules only Occurs when 1. Concentration gradient exists 2. Membrane is not permeable to solute(s) 3. Membrane is permeable to water Osmosis Takes place through the membrane protein aquaporin A type of facilitated diffusion Osmosis Transport Across Cell Membranes Review: several types of passive transport Simple diffusion Channel-mediated diffusion Carrier-mediated diffusion Osmosis Transport Across Cell Membranes What happens when these methods don t work? What if we need to move solutes AGAINST a concentration gradient? Requires energy Transport of Small Solutes Active transport Moves substances through carrier proteins specific to solute Energy provided by ATP Works AGAINST concentration gradient 6

7 Transport of Small Solutes Active transport Used to Concentrate substances in the cell or in organelles Get rid of unneeded things Create gradients that drive other processes Example: sodium-potassium pump Filtration Transport of Small Solutes Hydrostatic pressure forces water and solute across a porous membrane Process based on pore size of membrane Example: cardiovascular system Transport of Large Solutes Some substances are too large to pass through a membrane protein or pore Examples: whole proteins, bulk transport Must be moved by vesicular transport Membrane reorganizes to move substances through the membrane in vesicles REQUIRES ATP Transport of Large Solutes Substances moving into the cell = endocytosis Endocytosis Several types of endocytosis Receptor-mediated endocytosis Very specific substances Endocytosis Several types of endocytosis Phagocytosis cell eating Very large objects such as bacteria 7

8 Phagocytosis Process is very effective but sometimes it goes wrong Endocytosis Several types of endocytosis Pinocytosis cell drinking Small, random samples of extracellular fluid Exocytosis Essentially the opposite of endocytosis Substances moved through the membrane to the outside of the cell Example: insulin, neurotransmitters Energy-Dependent Membrane Transport Recall: all of these types of transport require energy ATP Pressure gradient Solute gradient 8