Chapter 5: Biological MembranesAP BiologyChapter 5

Plasma Membrane

Functions: separates the cell's insides from outside regulates passage of materials into/out

of celltransmitting signals and info. between

the cell and environmentparticipates in chemical reactionsessential part of energy transfer and

storage systems

Composition:- lipid bilayer - proteins- in constant motion

Membrane Proteins

*Important area of research: how membrane proteins function in health/disease

many are enzymesfunction in transport of

materials/informationconnect cells together to form

tissues

Phospholipid BilayerPhospholipid: 2 fatty acid chains linked to

a glycerol molecule - nonpolar, hydrophobic ends (fatty acids) - hydrophilic ends (phosphate group)- hydrophobic "tails" turn to inside of membrane - hydrophilic "heads" turn to outer ends of membrane

- amphipathic - have distinct hydrophilic and hydrophobic regions

Phospholipid BilayerHydrogen bonds form between

the phospholipid "heads" and the watery environment inside and outside of the cell

Hydrophobic interactions force the "tails" to face inward

Phospholipids are not bonded to each other, which makes the double layer fluid

Fluid Mosaic ModelCell membrane consists of a fluid

bilayer of phospholipid molecules in which proteins are embedded

- like the tiles in a mosaic picture

NOT static - proteins can move

p.107 diagram of fluid mosaic model

Membrane ProteinsFunctions:

◦Transport of small molecules◦Enzymes ◦Information transfer ◦Identification tags - allow for cell-cell

recognition

Membrane Proteins

◦Integral Proteins - firmly bound to the membrane, usually do not extend all the way through amphipathic

◦hydrophilic regions extend out of the cell or into

cytoplasm - hydrophobic regions interact with fatty acid tails of the phospholipids

Membrane ProteinsTransmembrane Proteins -

extend through the membrane, also amphipathic

Membrane Proteins

Peripheral Proteins - not embedded in the lipid bilayer◦located on the inner or outer

surfaces of the plasma membrane◦can be removed from the membrane

without disrupting the structure

Cell Membrane is Selectively Permeable

Most membranes are permeable to small molecules and lipid-soluble or polar molecules

Water molecules may pass through the lipid bilayer◦gases: such as O2 and CO2

Transport Across MembranesPassive Transport - does not

require energy, moves with the concentration gradient

Active Transport - requires ATP, moves against the concentration gradient

Simple DiffusionProcess based on random motionParticles move down

concentration gradient -from an area of high concentration to low concentration◦can occur rapidly ◦occurs until equilibrium is reached

Types of Diffusion:Osmosis - diffusion of water

across a selectively permeable membrane ◦p.114 Figure 5-11

Dialysis - diffusion of a solute across a selectively permeable membrane◦p.113 Figure 5-10

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

Osmotic Pressure

the tendency of water to move into a solution by osmosis

solution with high solute concentration, low water, has a high osmotic pressure

solution with a low solute concentration, high water, low osmotic pressure

Isotonic - equal solute concentration◦ ex. blood plasma isotonic to blood cells

Hypertonic - higher solute concentration- if a cell is placed in a hypertonic environment, water will leave the cell, the cell shrinks- plasmolysis occurs: plasma membrane separates from cell wall

Hypotonic - lower solute concentration- a cell placed in a hypotonic environment will gain water, swell, and possibly burst

Turgor PressureTurgor Pressure - internal pressure of cells

with cell walls◦Plants, Algae, and Bacteria

Enables them to withstand a low solute concentration outside the cell

Cell is hypertonic to environmentWater moves into cell, cell swells, building

pressure -> turgor pressure against cell wallCell does not burst b/c of cell wall, resist

stretching and water molecules must stop moving into the cell

Carrier-Mediated TransportMembrane proteins move ions or

molecules across a membrane2 types:

1. facilitated diffusion (passive)2. carrier-mediated active transport

Carrier-Mediated TransportFacilitated diffusion - with

concentration gradient, requires transport protein

energy comes from concentration gradient

ex. glucose permease - transports glucose into red blood cells

Carrier-Mediated Active TransportCarrier-mediated Active Transport -

movement of solutes across membrane against concentration gradient

particles must be "pumped" from region or low conc. to region of high conc.

requires energy source - ATP, and transport protein

ex. sodium-potassium pump: in all animal cells, pump sodium ions out of cell and potassium ions into cell

Other Types of Active Transportlarge particles such as food, cell

partsrequires ATPEndocytosis and Exocytosis

Active Transport video clip

ExocytosisCell ejects waste products or

hormones by the fusion of a vesicle with the plasma membrane

Vesicle releases contents from the cell

EndocytosisMaterials are taken into the cell Types: phagocytosis, pinocytosis

Endocytosis Video clip

Phagocytosis"cell eating"cell ingests large solid particles

such as bacteria and foodex. protists, white blood cells plasma membrane folds enclose

particle, forms a vacuole, fuses, then enters the cell and fuses with lysosomes

Pinocytosis"cell drinking"cell takes in dissolved materialsdroplets of fluid are trapped by

folds in the membranepinch off into the cytosol as tiny

vesicles liquid is slowly transferred into

the cytosol vesicles become smaller, then

disappear