CHAPTER 5 Membrane Structure Function Plasma Membrane Selectively

CHAPTER 5 Membrane Structure & Function

Plasma Membrane • Selectively permeable- allows certain substances to pass through more easily than others • Ability of a membrane to discriminate in its chemical exchanges with the env’t is essential to life • We’re going to focus on the plasma membrane, but the same principles apply to most of the membranes in the cell

Components of membranes • Most important are lipids and proteins • Carbs also very important • Phospholipids- most abundant lipids in membranes • Phospholipids are amphipathic- have both hydrophilic and hydrophobic regions • Fluid mosaic model- membrane is a fluid structure with various proteins embedded and attached to phospholipid bilayer

• Membranes with different functions will differ in chemical composition & structure • Proteins are amphipathic as well • They are embedded in the membrane so that the hydrophobic portions are covered and the hydrophilic portions are exposed

• Membranes are not composed of molecules locked in place • They move fluidly • Held together primarily by hydrophobic interactions (weaker than covalent bonds) • Most lipids and some proteins can float laterally • But proteins usually do not flip flop sides of the membrane

• Lateral movement of phospholipids is very rapid • Proteins are much larger than lipids and move more slowly • Some proteins drift or seem to move directionally, others are immoble b/c attached to the cytoskeleton • Membrane remains fluid as temp decreases until finally phospholipids settle closely packed and solidify (think bacon grease)

• In animal cells, cholesterol (steroid) is wedged between phospholipids in plasma membrane • Cholesterol is a temperature buffer

• Membranes need to be fluid to work properly • When it solidifies, permeability changes and enzymes may become inactive

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Membrane Proteins Membrane is a collage of different proteins embedded in the lipid bilayer Proteins determine most of membrane’s function Integral proteins- penetrate the lipid bilayer (many are transmembrane proteins) Hydrophobic portions of proteins are embedded Hydrophilic protein portions are exposed to aqueous the solution on either side

Peripheral proteins – not embedded They are loosely attached to the surface of the membrane Sometimes attached to exposed portion of integral proteins

• On cytoplasmic side, some proteins are anchored by attachment to the cytoskeleton • On exterior side, some proteins are attached to portions of the ECM • These attachments give the plasma membrane a stronger framework

• Membrane proteins have many different functions • A single cell may have proteins carrying out all of these functions • A single protein can have multiple functions

6 main functions of membrane proteins:

6 main functions 1) Transport – a transmembrane proteins may provide a hydrophilic channel to help transport a specific solute into/out - Others can move molecules back and forth by changing shape (need ATP) 2) Enzymatic activity –some proteins may be enzymes, some are located close together and catalyze sequential steps in a metabolic pathway

3) Signal transduction- may have a binding site specific for a chemical messenger (ie hormone)- the signal causes a change in the shape of the receptor protein to relay the message 4) cell-cell recognition- some glycoproteins are “ID tags” and are specifically recognized by other cells

5) Intercellular joining- can hook together with those of other cells to form junctions like gap or tight junctions 6) Attachments to the cytoskeleton & ECM – Help maintain cell shape and stabilizes location of certain membrane proteins

Membrane Carbs • Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cell from another • Crucial to the functioning of an organism • ex. Helps cells differentiate in embryos and allows for immune response to recognize foreign cells

• Cells recognize other cells by binding to the surface molecules (often carbs) on the plasma membrane • Membrane carbs are usually short, branched chains (15 sugar units or less) • Some are covalently bonded to lipids(glycopilids) • Most bonded to proteins (glycoproteins)

• Carbs on outside of plasma membrane vary from species to species/ among individuals/ even different cell types • Ex. Blood types A, B, AB & O differ in the carbs on the surface of RBCs

• Membranes have distinct inside and outside surfaces • Proteins within a membrane have directional orientation • When vesicles fuse with the plasma membrane, the outside layer of the vesicle becomes part of the cytoplasmic side of the plasma membrane & contents (glycoproteins/glycolipids) are incorporated onto the exterior of the membrane

• This means that the distribution of proteins/ lipids and their associated carbs is determined by what the ER synthesizes and how the Golgi modifies

Selective Permeability • Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer of the membrane and cross very easily • Ex. CO 2 , oxygen, hydrocarbons • The hydrophobic core of the membrane impedes the direct passage of ions and polar molecules which are hydrophilic • Ex. Polar molecules such as Glucose and other sugars pass slowly through; even water (very small) can not pass rapidly across the lipid layer

• Charged atoms or molecules find it even harder to cross the lipid bilayer • This is why we have proteins built into the layer to help out….

Transport Proteins • Cell membranes are permeable to specific ions and a variety of polar molecules • These hydrophilic molecules can get through with the help of transport proteins • Some of these proteins are channel proteins • Ex. Aquaporins are channels specifically to enable water to pass in and out

• Other transport proteins are carrier proteins • Both channel and carrier proteins are very specific • Each type will only work for one specific molecule

Diffusion • Tendency of any substance to spread out evenly into the available space • Each molecule moves randomly, but the movement of a population of molecules may be directional

• In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated • Basically, every substance will diffuse down its concentration gradient • This is spontaneous and does NOT require energy • Each substance diffuses down its own concentration gradient, regardless of the concentration gradients of other substances

• Most membrane movement is due to diffusion • Diffusion of a substance across a biological membrane is called passive transport because the cell does not need to expend any energy • Concentration gradient represents potential energy and drives diffusion

• Because membranes are selectively permeable, they will have different effects on the rates of diffusion of different molecules

Osmosis • Diffusion of water across a selectively permeable membrane • Water diffuses across the membrane from a region of lower solute concentration to that of higher solute concentration • This occurs until the solute concentrations are equal on both sides of the membrane

Water balance in cells without walls: • Tonicity- ability of a solution to cause a cell to gain or lose water • The tonicity of a solution depends in part on its concentration of solutes that can not cross the membrane, relative to that in the cell itself • If there are more nonpenetrating solutes in the surrounding solution, water will leave the cell.

• If a cell without a wall is placed in an isotonic environment there will be no NET movement of water across the membrane • Molecules will still move, just the rate of water movement will be the same in both directions • An animal cell’s volume will remain stable when placed in an isotonic solution

• If a cell is placed in a solution that is hypertonic (having more nonpenetrable solutes) to the cell – cell will lose water, shrivel and probable die • If animal cell is placed in a solution that is hypotonic, water will enter faster than it can leave and the cell will swell and lyse open

• A cell without a cell wall can not tolerate an excessive uptake or loss of water • An easy way to solve this is for a cell to live in isotonic surrounding to its env’t • Ex. Many marine invertebrate cells are isotonic to seawater • Most land animals have cells that are isotonic to their extracellular fluid

• Those cells living in hypo or hypertonic environments need adaptations for osmoregulation (control of water balance) • Ex. Paramecium – contractile vacuole

Water balance in cells with walls: • When immersed in hypotonic solution, the cell wall helps maintain the cell’s water balance • Plant cell will swell when water enters • But the wall will only expand so much before it exerts pressure back on the cell, preventing further water uptake • This results in a cell being turgid (firm) and this is the healthy state for plant cells

• Nonwoody plants especially rely on this turgor for mechanical support • So the cells are continuously bathed in hypotonic solution • If a plant’s cells and their surroundings are isotonic, the cells will become flaccid (limp)

• In hypertonic solution- the cell wall has no advantage, so the same thing will happen as does in an animal cell – lose water and shrink • As plant cell shrivels, plasma membrane pulls away from the wall • This is called plasmolysis and causes the plant to wilt and may cause cell death

Facilitated Diffusion - When polar molecules and ions diffuse through the plasma membrane passively with the help of transport proteins Some types of protein channels – Aquaporins Ion channels (gated channels)- open and close in response to a chemical or electrical stimulus

Gated channel example • Nerve cells stimulated by neurotransmitter molecules open gated channels that allow sodium into the nerve cell • Facilitated diffusion is still diffusion (down concentration gradient) • It speeds diffusion by providing an efficient passageway through the membrane

Active Transport • Pumping molecules across the membrane against their concentration gradient • This requires work (ATP) • Carrier proteins move solutes against their gradient • This allows cells to maintain internal concentrations of certain molecules very different from the environment

Ex. Animal cells have much higher K and lower Na concentrations than their surroundings Plasma membrane helps this by pumping Na ions out and K ions in This is the sodium-potassium pump Most active transport needs ATP energy In this case ATP transfers a phosphate directly onto a transport protein thus changing its shape and allowing the ions to be moved across the membrane

• All cells have voltages across their plasma membranes • Voltage = electrical potential energy • Cytoplasm is (-) compared to the EC fluid • Membrane potential- voltage across a membrane. Ranges from -50 to -200 m. V • Membrane potential acts as an energy source that affects movement of charged substances across the membrane

• Electrogenic pump- transport protein that generates voltage across a membrane • Ex. Sodium potassium pump- each pump sends 3 Na out and 2 K in (this produces a net transfer of one positive charge out into the EC fluid) • This stores energy in the form of voltage

• By generating voltage across membranes, electrogenic pumps store energy that can be tapped into for cellular work

How really large molecules get in and out of cells… Exocytosis – when vesicles from the Golgi fuse with the plasma membrane letting macromolecules out of the cell Many secretory cells use this Ex. Pancreatic cells secrete insulin into the blood in this way

Endocytosis – when the cell takes in macromolecules and particles by forming new vesicles from the plasma membrane 3 types – Phagocytosis Pinocytosis Receptor Mediated

• Human cells use receptor mediated endocytosis to take in cholesterol to be incorporated into membranes and as precursors for synthesis of other steroids • Cholesterol travels in the blood as LDLs (low density lipoproteins) • These act as ligands (specific binding molecules) • LDLs bind to LDL receptors on membranes and then enter the cells by endocytosis • If LDLs are not taken into cells, they build up in the blood and lead to atherosclerosis

The amount of plasma membrane of a cell remains relatively constant Due to endo and exocytosis, about the same amount of plasma membrane is lost from some vesicles as is gained by others