class 10 - membranes and transport

structure of cell membrane

• made of mostly proteins and lipids

  • types of lipids: mostly phospholipids, with some steroids like cholesterol)

  • contains glycolipids + glycoproteins (carbohydrate- modified lipids and proteins)

    • "glyco” = carbohydrate is fluid nad amphipathic

amphipathic

"two-faced"- phospholipid bilayer has polar/charged head, nonpolar fatty acid tail

function of cell membrane

cell membrane separates aqueous (watery) environment from external aqueous environment, maintaining homeostasis

is selectively permeable

homeostasis

keeping the internal environment stable by actively regulating it while the external environment is is changing

what structures do phospholipids spontaneously form in water?

micelles, lipisolmes, and bilayer sheets

micelles

single-layer lipid sphere, wedge shaped, bulky head

liposome

rectangular shape with less bulky phosphate head, sphere made out of phospholipid bilayer

bilayer sheet

basic structural arrangement of cell membrane before curving into closed shape (forms liposome)

• consists of polar head group facing the external environment, and nonpolar tail facing the internal environment

• is selectively permeable

selectively permeable

some molecules can move in and out freely, while other molecules are let in and out under certain conditions, and other molecules can't pass through at all

• small/nonpolar molecules: can diffuse freely through the membrane

• polar/charged molecules: must go through facilitated diffusion

describe the structure of the phospholipid bilayer

• flat sheet of phospholipids arranged tail-to-tail

• phospholipid molecules are amphipathic, which allow them to orient the polar end towards the water inside and outside the cell, while nonpolar tail faces inwards

what is solidification?

at low temperatures, linear fatty acid tails are packed closely together, making phospholipids vulnerable to fracturing 

how do you avoid solidification from occurring?

cholesterol spread among phospholipids, preventing crystallizing (cholesterol's bulky ring structure interrupts close packing)

cholesterol structure

• amphipathic, hydrophilic head with hydroxyl group, hydrophobic nonpolar tail where carbon rings are located

• kink in one of the two fatty acid tails occurs due to double bond, making an unsaturated C

phospholipid vs cholesterol

cholesterol has a hydroxyl head, phospholipids have a phosphate group head

fluid mosaic model

describes the cell membrane structure as fluid, as it is able to vibrate, flex back and forth, spin around long axis, sideways, etc

• this happens constantly; there’s enough interaction between amphipathic phospholipids and surrounding aqueous solutions that it all stays together

how does the fluid mosaic model stay fluid?

balance of unsaturated and saturated fatty acid tails

• unsaturated fatty acid tails → double bonds create kinks, preventing phospholipids from packing tightly and increasing fluidity

• saturated fatty acid tails → straight tails, tighter packing, which decreases fluidity

what does membrane fluidity allow the cell membrane to do?

• allows membranes to accommodate for cell growth, cell movement/motility, and surface stresses that cells or multi-cellviar organisms may come up against

how does the membrane stay fluid despite the temperature?

cholesterol - acts as a buffer

• at high temps, cholesterol stiffens the membrane, preventing it from getting too fluid.

• at low temps, cholesterol wedges between tails, preventing them from packing too tightly, preventing solidification

integral proteins

proteins that span the entire membrane, involved in transport, signaling, and maintaining cell structure

same as transmembrane proteins

transmembrane proteins

proteins that span the entire membrane, involved in transport, signaling, and maintaining cell structure

same as integral proteins

transporter protein

• moves material into/out of cell

• forms selectively permeable channels that allow certain polar molecules and ions to pass across the membrane

receptors

bind to chemical signals from other cells, organisms, the environment, and then transmit the signal they've received intracellularly- involved in all cellular signaling

enzymes

catalyze chemical reactions happening at the surface of the cell or inside of the cell

anchor proteins

help with stabilizing the cell+ attaching cell to surrounding extracellular matrix

cell-cell recognition proteins

identifies a cell as part of the same cell or a foreign, links cells, binds cells to extracellular matrix (ECM) to link ECM to cytoskeleton

• ex: immune systems determine if something is foreign invader (ex: bacteria, virus)

proteins embedded in fluid mosaic of cell's membrane

must have hydrophobic + hydrophilic regions

proteins in cell-cell junctions

bind cells tightly together

enzymatic proteins

allow chemical reactions to occur

glycoproteins

carbohydrate-modified protein, contains a sugar group usually involved in immune recognition

concentration gradient

high to low

occurs when there’s a higher concentration on one side of the membrane compared to the other

what does “moving down the concentration gradient” mean

moving from high to low concentration areas

diffusion

net movement of molecules driven by the natural kinetic energy of molecules (random motion), not by cellular energy (ATP)

what happens when there is no more concentration gradient (the concentration on both sides is the same)?

net movement of particles stops, but random motion of molecules in both directions continues

electrochemical gradient

uneven distribution of charged molecules

what type of energy do gradients contain?

potential energy

hypertonic solution

a solution with a higher solute concentration than another solution

hypotonic solution

a solution with a lower solute concentration than another solution

isotonic solution

a solution with the same solute concentration as another solution

passive transport

moving down the gradient by diffusion

simple diffusion

• small hydrophobic molecules diffuse directly through the membrane

• net movement continues until concentration is the same on inside and outside of cell, at which point net movement stops but diffusion continues

facilitated diffusoin

diffusion across cell membrane through a transmembrane transport protein

(channels or carriers), moves polar and charged molecules

channel proteins

• static core

• stable tertiary structure

• substance moves through channel that can go from closed to open state

• typically moves amino acids, sugars, and ions

aquaporins

• type of protein channel

• allow water to cross faster than diffusing through the lipid bilayer

osmosis

• net movement of a solvent (ex: water) across a selectively permeable membrane from low to high solute concentration (aka high to low water concentration)

osmotic pressure

tendency of water to move from one solution to another by osmosis

• higher solute concentration = higher osmotic pressure

relationship between water and solute concentration

water is inversely related to solute concentration

• high water concentration = low solute concentration

active transport

protein assistance moving against concentration gradient (low to high concentration), any substance is capable of active transport

where does the external energy come from to power active transport?

1. an energy carrying molecule (typically ATP)

2. the concentration gradient of another molecule or ion

primary active transport 

ATP is used as a fuel source to pump from a low to high concentration

secondary active transport

instead of breaking down ATP, a protein partners the passive movement of substance one with the primary active transport of substance two

types of secondary active transport

• symporter- moves two substances in the SAME direction across the membrane

• antiporter- moves two substances in OPPOSITE directions across the membrane

what is the membrane component involved in simple diffusion, facilitated diffusion, and active transport?

• simple diffusion: lipids

• facilitated diffusion: proteins

• active transport: proteins

does the transported substance have to bind to a carrier in simple diffusion, facilitated diffusion, and active transport?

• simple diffusion: no

• facilitated diffusion: yes

• active transport: yes

what is the energy source in simple diffusion, facilitated diffusion, and active transport?

• simple diffusion: concentration gradient

• facilitated diffusion: concentration gradient

• active transport: ATP hydrolysis or a different substance’s concentration gradient

what is the direction of transport in simple diffusion, facilitated diffusion, and active transport?

• simple diffusion: with gradient (high → low)

• facilitated diffusion: with gradient (high → low)

• active transport: against gradient (low → high)

can the mechanism cause saturation by simple diffusion, facilitated diffusion, and active transport? 

• simple diffusion: no

• facilitated diffusion: yes

• active transport: yes

when does protein saturation occur?

occurs when all the binding sites on a protein are fully occupied by a substrate or ligand

what happens when you add substrate and the protein is at max saturation?

adding substrate or ligand won’t increase protein activity

what is the impact of steroids/sterols (ex: cholesterol)?

function: stabilizes membrane

• low temps → prevents tight packing and solidification → increases fluidity

• high temps → keeps membrane together → decreases fluidity and permeability

what is the impact of double bonds on the cell membrane?

double bond function - reduces saturation of fatty acid tails

• more double bonds = more unsaturated = increased fluidity and permeability

what is the impact of unsaturated and saturated fatty acids on the cell membrane?

• more saturated (fewer double bonds) → tighter packing → less fluid and permeable

• less saturated (more double bonds) → creates kinks → looser packing → more fluid and permeable

what is the impact of protein content on the cell membrane?

• more transport proteins → increases selective permeability (larger amount of specific molecules/ions cross)

• more anchoring proteins → stabilize protein structure

• more receptors/enzymes → no impact on fluidity/permeability