Lipids, Membranes, and the First Cells

the _ separates life from non-life

plasma membrane or cell membrane

plasma membrane

separates the cell’s interior from the external environment

membranes function to

keep damaging material out of the cell, allow entry of materials needed by the cell, facilitate the chemical reactions necessary for life

lipids are

carbon-containing compounds, found in organisms and are largely nonpolar and hydrophobic

hydrocarbonds

are nonpolar molecules that contain only carbon and hydrogens

lipids don’t dissolve in water because

they have fatty acids, which are a major component

fatty acid

a hydrocarbon chain bonded to a carboxyl function group

fatty acids and isoprene are

key building blocks of lipids

fatty acids can be

saturated or unsaturated

saturated hydrocarbon chains consist

of only single bonds between carbons

unsaturated hydrocarbon chains consist

of one or more double bonds exist in the hydrocarbon chains

bond saturation also profoundly affects

the physical state of lipids

highly saturated fats are

solid at room temperature

saturated lipids that have a extremely long hydrocarbon tails form

particularly stiff solids at room temperature

highly unsaturated fats are

liquids at room temperature

lipids structure is characterized by this physical property

their insolubility in water

their insolubility in water is based on

the high proportion of non-polar C-C and C-H bonds and relative to polar functional groups

the three most important types of lipids found in cells are

fats, steroids, and phospholipids

fats are composed of

three fatty acids linked to glycerol

when the fatty acids are polyunsaturated

they form liquid triacylglycerols called oils

the primary role of fats is

energy storage

fats form when

a dehydration reaction occurs between a hydroxyl group of glycerol and the carboxyl group of a fatty acid

the glycerol and fatty acid molecules become joined by

ester linkage

steroids are

a family of lipids and are distinguished by a bulky, four-ring structure

steroids differ from one another by

the functional group or side groups attached to different carbonds in those hydrophobic rings

a steroid example is cholesterol which is

a hydrophilic hydroxyl group attached to the top ring an an isoprenoid tail attached at the bottom

cholesterol is

an important component of plasma membranes in many organisms

membrane-forming lipids contain

a polar, hydrophilic region and a nonpolar, hydrophobic region

phospholipids are

amphipathic

the head region of phospholipids contain

highly polar covalent bonds and also consist of a glycerol, a phosphate, and a charged group

the tail region of a phospholipids is comprised of

two nonpolar fatty acid or isoprene chain

when placed in a solution, phospholipids form

membranes because the heads interact with water

phospholipides do not _ in water

dissolve in water because water molecules cannot form hydrogen bonds with the hydrocarbon tail

water molecules interact with

the hydrophilic heads

upon contact with water, phospholipids form either

micelles or phospholipid bilayers

phospholipid bilayers form when

two sheets of phospholipid molecules align, hydrophilic heads in each layer face a surrounding solution and the hydrophobic tails face one another inside the bilayer

phospholipid bilayers form

spontaneously

many factors influence the behavior of the membrane

number of double bonds between the carbons in the phospholipid’s hydrophobic tail, length of the tail, number of cholesterol molecules in the membrane and the temperature

double bonds in a hydrocarbon chain can cause a

kink in the hydrocarbon chain which prevent the close packing of hydrocarbon tails and reduces hydrophobic interactions

unsaturated hydrocarbons chain

have a least one double bonds which make the membranes much more permeable

saturated hydrocarbon chains are

without double bonds and have more chemical energy than unsaturated fats do which are much less permeable

lipid bilayer with short and unsaturated hydrocarbon tails

have higher permeability and fluidity

lipid bilayer with long and saturated hydrocarbon tails

have lower permeability and fluidity

membrane fluidity decreases with

temperature, and when molecules in the bilayer are moving more slowly

decreased membrane fluidity causes

decreased permeability

hydrophobic interactions become stronger

as saturated hydrocarbon tails increase in length

membranes containing phospholipids with longer tails have

reduced permeability

adding cholesterol to membranes increases

the density of the hydrophobic section

cholesterol decreases

membrane permeability

their permeability of a structures is

its tendency to allow a given substance to pass across it

lipid bilayers are

highly selective

phospholipids bilayers have

selective permeability

small or nonpolar molecules move across phospholipid bilayer

quickly

charged or large polar substances

cross slowly, if at all

individual phospholipids can move

laterally throughout the lipid bilayer

how quickly molecules move within and across membranes is a function of

temperature, the structure of hydrocarbon tails, and the number of cholesterol molecules in the bilayer

passive transport

does not require an input of energy

active transport

requires energy to move substances across the membrane

small molecules and ions in a solution

solutes

solutes have

thermal energy and are in constant, random movement

random movement of solutes are

diffusion

diffusion is a form of

passive transport

a concentration gradient is created by

a difference in solute concentrations

molecules and ions move randomly when

a concentration gradient exists and there is a net movement from high-concentration regions to low-concentration regions

diffusion along a concentration gradient

increases entropy and is spontaneous

equilibrium occurs when

the molecules or ions are randomly distributed but there is no more net movement

the movement of water is a special case of diffusion

osmosis

water moves from regions of _ solute concentration to regions of _ solute concentration

low and high

osmosis only occurs

across a selectively permeable membrane

an outside solution with a higher concentration is _ to the inside of a cell

hypertonic

a solution with a lower concentration is _ to the cell

hypotonic

if solute concentrations are equal on the outside and inside of a cell solutions are _ to each other

isotonic

in a hypertonic solution

water will move out of the cell by osmosis and the cell with shrink

in a hypotonic solution

water will move into the cell by osmosis the cell will swell

in an isotonic solution

there will be no net water movement and the cell size will remain the same

the fluid-mosaic model of membrane structure suggest

some proteins are inserted into the lipid bilayer thus making the membrane a fluid, dynamic mosaic of phospholipids and proteins

integral proteins are

amphipathic and able to span a membrane with segments facing both its interior and exterior surfaces

transmembrane proteins are

integral proteins that span the membrane that are involved in the transport of selected ions and molecules across the plasma membrane that are able to affect membrane permeability

peripheral proteins are

found only on one side of the membranes and are often attached to integral proteins

amphipathic proteins can integrate into

the lipid bilayers

transport proteins are

transmembrane proteins that transport molecules

three broad class of transport proteins

channels, carrier proteins or transporters, and pumps

transport proteins each affects

membrane permeability

ion channels are

specialized membrane proteins that circumvent the plasma membrane’s impermeability to small, charged compounds

electrochemical gradients occur when

ions build up on one side of a plasma membrane and they establish both of a concentration gradient and a charge gradient

ions diffuse through

channels down their electrochemical gradients

channel proteins are

selective and each has its own structure that permits only a particular type of ion or small molecules to pass through it

gated channels

open or close in response to a signal and the flow of ions and small molecules through membrane channels is carefully controlled

the movement of substances through channels is

passive and it does not require an input of energy

passive transport

is powered by diffusion along an electrochemical gradient

cells have many different types of

channel proteins in their membranes

channels are responsible for

facilitated diffusion

facilitated diffusion

the passive transport of substances that would not otherwise cross the membrane

facilitated diffusion can occur through

channels or through carrier proteins or transporters

carrier proteins or transporters

change shape during the transport process and move only down a concentration gradient, reducing differences between solutions

glucose is

a building block for important macromolecules and a major energy source

lipid bilayers are only moderately permeable to

glucose

cells can

transport molecules or ions

cells move _ an electrochemical gradient

against

cell require energy in the form of

ATP