Cell Membranes and Transport Ch. 5

Structure of membranes Bilayer membrane ~7 nm wide Contains embedded proteins

Fluid Mosaic Model Membrane described as fluid b/c both phospholipids and proteins can move about by diffusion Bilayer has fluidity we associate with olive oil Move sideways in their own layers Proteins move like icebergs in sea

Fluid Mosaic Model Some phospholipid tails are saturated, some are unsaturated More unsaturated = more fluid WHY???

Factors affecting fluidity Longer tail = less fluid membrane Lower temp. = less fluid membrane

Phospholipids Tails of phospholipids are nonpolar, so it is difficult for polar molecules (water soluble) to pass through the membrane Ex: sugars, amino acids, and proteins cannot leak out of the cell

Cholesterol hydrophilic heads + hydrophobic tails Fits in between phospholipids Animal cells: amount cholesterol= amount phospholipids in cell membrane Plant cells: little to no cholesterol; Prokaryotes = none! Regulates fluidity, stabilize membranes Hydrophobic regions: prevent ions/polar molecules from passing through membrane Important in myelin sheath around nerve cells: ion leaks would slow signals

Cholesterol

Two types of proteins in Cell membraneS: Intrinsic (aka integral): found in inner layer, outer layer, or most commonly spanning the entire membrane (transmembrane proteins- cross both sides of membrane) Extrinsic (aka peripheral): found on either the inner or outer surface of the membrane

Intrinsic Proteins Stay in membrane due to its hydrophobic and hydrophilic regions Most float like mobile icebergs although some are fixed like islands to structures inside or outside the cell and do not move about

Extrinsic proteins Many are bound to intrinsic proteins Some are held in place by binding to molecules inside or outside the cell

Proteins’ roles in membranes Transport proteins: hydrophilic channels Moving Ions, polar molecules Enzymes: on plasma membrane on small intestine surface hydrolyze disaccharides Proteins for photosynthesis + cell respiration: E- transport chains in mitochondria membrane and thylakoid membrane of chloroplast Receptors: bond to antigen and begin communication pathway

Glycolipids and Glycoproteins Many proteins and lipids in membrane have carbohydrate chains attached that face the outside of the membrane Glycolipids = carb. (polysaccharide) attached to lipid Glycoproteins = carb. attached to protein

Glycolipids and Glycoproteins Form H-bonds with water to stabilize the membrane Form sugary coating on membrane called glycocalyx Act as receptor molecules for cell-cell recognition (immune cells)

Signaling Receptors Coordinate activities of animal cells Recognize messenger molecules like hormones and neurotransmitters When molecule binds with receptor, it triggers a series of chemical reactions in the cell

Example: cell Insulin Receptors

Endocytosis receptors Bind to molecules that are parts of the structures that are to be engulfed by cell

Cell Marker Receptors Aka antigens; Allow cell-cell recognition Each cell type has its own specific antigen (similar to how different countries have different flags) Ex: ABO blood types

Transport Proteins Provide channels or passageways for ions and polar molecules to pass through cell membrane Two main types: Channel proteins – Facilitated diffusion (NO ATP!) Carrier proteins – Active transport (ATP REQUIRED!)

Review and practice! http://www.wisc-online.com/objects/ViewObject.aspx?ID=ap1101 KAHOOT!

Transport across the CelL MEmBRANe Ch. 5

Cell Surface Membrane (CSM) transport Phospholipid bilayer creates effective barrier against water soluble molecules and ions Prevents aqueous contents from escaping Some essential transport is achieved through: Diffusion Facilitated diffusion Osmosis Active transport and bulk transport

Diffusion (simple diffusion) Diffusion: net movement, as a result of random motion of its molecules or ions, of a substance from a region of high concentration to a region of low concentration Particles move down a concentration gradient

Factors that affect Rates of Diffusion ‘steepness’ of concentration gradient: greater difference = higher rate Temperature: higher temperature = higher rate Surface area of diffusion: greater surface area = higher rate (more contact space) Nature of molecules/ions: small molecules = greater rate

Respiratory gases cross membrane by diffusion (uncharged and nonpolar)

Facilitated diffusion Diffusion that takes place with the help “assistance” of a certain protein molecule Channel proteins Carrier proteins

Channel proteins Water-filled pores Fixed shape Allow charged substances (usually ions) to diffuse through membrane Gated to allow for selectivity and control of movement

Carrier proteins Flips between shapes to alternately open binding sites on interior/exterior of membrane Direction of movement depends on concentration gradient inside and outside of cell

Osmosis Type of diffusion involving water molecules only

Water potential Water potential: tendency of water molecules to move from one place to another Measured by Greek symbol psi Ψ

Water Potential Problems In beaker A, which has a higher water potential, distilled water or the beet core? Where will water flow in the diagram B? Explain why.

Water potential Water potential for pure water (distilled) is 0 Solute potential - amount that solutes lower the water potential Solutes make water potential less than 0, the more solute, the more negative the water potential! Pressure potential (symbol Ψp) Increasing pressure increases water potential Pressure potential makes water potential less negative (more positive)

Water potential

Water potential in animal cells

Osmosis in plant cells

Active Transport (ATP needed) Active transport moves molecules/ions against concentration gradient Achieved by carrier proteins (specific for structure of particular ion/molecule) ATP supplied by cellular respiration Can occur into or out of the cell

Sodium-potassium pump Found in CM of all animal cells Use ~30% cell energy and ~70% neuron cell energy Pump 3 Na+ ions out of the cell and 2 K+ ions into the cell Net result: inside of cell become more negative than the outside

Active transport Important in re-absorption in the kidneys, where certain useful molecules and ions have to be re-absorbed into the blood after filtration into the kidney tubules Involved in absorption of products of digestion in the gut

Active transport Used to load sugar from photosynthesizing cells of leaves into phloem tissue for transport around the plant

Bulk transport (ATP Required!) Bulk transport involves the mechanism of moving large quantities of molecules into the cell (endocytosis) or out of the cell (exocytosis) Large molecules such as proteins or polysaccharides, part of cells, or even whole cells may be transported across the membrane

phagocytosis ‘cell eating’ Bulk up take of solid material Cells specializing in this are called phagocytes and the vacuoles are called phagocytic vacuoles Ex: engulfing of bacteria by immune cells

Pinocytosis ‘cell drinking’ – bulk uptake of liquid into cell Vacuoles ( vesicles) formed are often extremely small, in which the process is called micro-pinocytosis Human egg cells take up nutrients from cells that surround it by pinocytosis

exocytosis Ex: how plants transport materials out of CM to make cell wall Ex: secretion of digestive enzymes of the pancreas Secretory vesicle from the Golgi carry the enzymes to the CSM and release their contents