HL Membranes: Structure & Transport [IB Biology HL]

the cell has many specialized ways to

bring materials into and out of the cell

many of which depend on the fluidity in

the membrane as well as the proteins

present in this video we will discuss

the HL topics of fluidity in the lipid

Bayer including the presence of

saturated and unsaturated fats as well

as the function of cholesterol in

maintaining fluidity we will also

discuss vesicle formation in endocytosis

and exocytosis we'll also look at

voltage gated Channel and discuss how

the sodium pottassium pump operates

finally we'll see how sglts transport

glucose using concentration gradients

maintained by the sodium potassium pump

as well as how cams join cells together

to form tissues in a variety of ways

these key concepts are some of the most

important topics covered in the IB

Biology syllabus at the higher level the

fluidity of cell membranes depends

largely on the fatty acid composition of

the lipid Bayers lipid bilayers consist

of phospholipids which have two fatty

acid tails that can vary in saturation

unsaturated fatty acids contain one or

more double bonds causing kinks in the

tails that prevent tight packing thereby

lowering the membrane's melting point

this lower melting point results in

Greater fluidity and flexibility

allowing the membrane to adapt to colder

temperatures cells in cold habitats such

as those in polar organisms often have a

higher proportion of unsaturated fatty

acids to maintain fluidity conversely

saturated fatty acids lack double bonds

making the membrane more rigid and

stable which is beneficial at warmer

temperatures terrestrial animals and

hotter climates often exhibit higher

concentrations of saturated fatty acids

to stabilize their membranes against

temperature

fluctuations cholesterol is a critical

modulator of membrane fluidity in animal

cells positioned between the fatty acid

taals of of phospholipids in the Bayer

the ampath nature of cholesterol allows

the molecule to interact with both the

hydrophobic and hydrophilic portions of

the membrane the hydrophobic steroid

ring structure of cholesterol aligns

with the hydrophobic taals of the

phospholipids in the interior of the

Bayer while the polar hydroxy group at

the cholesterol's head is positioned

towards the hydrophilic surface of the

membrane cholesterol plays a dual role

at high temperatures it's stabilizes the

membrane by preventing excessive

movement of phospholipids thus reducing

the fluidity and at low temperatures it

has the opposite effect how does this

work at low temperatures the

phospholipids don't have a lot of

kinetic energy so they don't move around

a lot this causes them to be packed

tightly together the tight packing

causes the membrane's fluidity to be

much lower at high temperatures the

lipids have more kinetic energy and they

move around a little more this creates

more of a distance between the lipids

this increased distance creates more

fluidity in the membrane so how does

cholesterol affect this fluidity at high

temperatures if the membrane becomes too

fluid it may fall apart or melt and the

cell can no longer maintain a controlled

internal environment the insertion of

cholesterol helps pack together the

phospholipids so that the fluidity

decreases back to a more stabilized

membrane at low temperatures there's

less kinetic energy and cholesterol

prevents the fatty acids from packing

too tightly reducing membrane fluidity

can negatively affect cells as it

reduces the permeability of the membrane

barrier it affects the functions of

proteins and reduces the ability to form

vesicles for transport this unique

ability to adjust membrane fluidity

across temperature ranges makes

cholesterol crucial for maintaining the

Integrity of cellular membranes in

diverse environments in animal cells

cholesterol's balancing effect on

fluidity is essential for membrane

stability and function especially in

tissues exposed to variable temperature

changes similar to the idea that

saturated fatty acids stabilize

membranes lastly the amount of

cholesterol in the membrane affects

membrane fluidity at higher

concentrations the membrane fluidity is

reduced and the membrane is more rigid

conversely lower levels of cholesterol

maintain fluidity of the membrane for

example in the GGI apparatus the

cholesterol levels are low to moderate

in the GGI membrane as the GG apparatus

processes and transports lipids and

proteins which requires a more flexible

membrane for the formation of vesicles

mitochondria also have low levels of

cholesterol as the fluidity helps

maintain the function of the christe

where many reactions take place

ultimately making

ATP membrane fluidity is not only

essential for maintaining membrane

structure but also for enabling Dynamic

cellular processes

such as endocytosis and exocytosis in

endocytosis a portion of the cell

membrane engulfes external particles or

fluids forming a vesicle that transports

the contents into the cell exocytosis is

the reverse process where internal

vesicles fused with the cell membrane to

release their contents outside the cell

for these processes to occur seamlessly

the membrane must be sufficiently fluid

to allow bending and fusion this fluid

nature of the membrane allows cells to

intake essential nutrients and Export

waste products efficiently membrane

fluidity supported by specific lipid

compositions and cholesterol is

therefore crucial in maintaining the

cell's adaptability and responsiveness

to environmental and physiological

demands an example of exocytosis is seen

in the synapse where neurotransmitters

are released from the Press synaptic

neuron through exocytosis then diffuse

across the

synapse gated ion channels are

specialized protein channels in the cell

membrane that regulate the movement of

ions such as sodium and pottassium which

are critical for neuron functioning

there are two primary types of gated ion

channels Li gated channels and voltage

gated channels Li gated ion channels

open in response to a molecule acting as

a chemical messenger which binds to the

Target molecule to initiate a biological

response the LI may be a small molecule

or a larger molecule such as a hormone

or neurotransmitter in this example here

at the synapse the neurotransmitter

acetylcholine binds to a nicenic

acetycholine receptor of another neuron

opening a gated sodium Channel causing

sodium ions to enter the post synaptic

neuron the influx of ions generates a

nerve impulse sodium and potassium

channels on the other hand are voltage

gated whose Gates will open or close in

resp response to changes in membrane

potential these gated ion channels are

essential for transmitting electrical

signals the action potential across

neurons enabling functions like muscle

contraction heartbeat and cognition the

precise regulation of these channels

contributes to the rapid and controlled

responses required for Effective neural

communication the sodium potassium pump

is a key example of an exchange

transporter crucial for maintaining

membrane potential in animals this pump

uses ATP to actively transport three

sodium ions out of the cell and two

potassium ions into the cell this

activity creates a gradient with a

higher concentration of sodium outside

the cell and potassium inside this ion

distribution establishes an

electrochemical gradient contributing to

the cell's membrane potential which is

vital for nerve impulses muscle

contractions and overall cell Health by

maintaining ion balance and supporting

membrane potential the sodium potassium

pump is fundamental to the physiological

function of cells particularly excitable

cells like neurons and muscle cells in

the case of the sodium potassium pump

also known as sodium pottassium

atpa three sodium ions bind to the pump

protein from the interior of the neuron

ATB will convert to ADP when it releases

a phosphate group through hydrolysis of

the phosphate that will bind to the pump

the addition of the phosphate and sodium

ions cause es a shift in the protein's

three-dimensional shape which results in

it releasing the three sodium ions to

the exterior of the cell the change in

the protein shape with the release of

sodium allows two potassium ions from

the exterior of the cell to bind to the

protein this causes another change in

the protein shape which causes the

release of the potassium ions to the

inside of the cell the phosphate group

also releases which changes the protein

shape back to its original form and it's

ready to complete complete the cycle

again to pump more ions the pump allows

for the electrochemical gradient to be

established and maintained so the neuron

can transmit the action

potential sodium dependent glucose

co-transporters or sglts illustrate a

type of indirect active transport

crucial for the absorption and

reabsorption of glucose in the body

these co-transporters rely on the energy

of the sodium gradient which is

established by the sodium pot potassium

pump also known as sodium potassium atpa

actively transporting sodium out of the

cell creating a higher concentration

outside this gradient provides the

driving force that allows sglts to move

glucose into cells against its

concentration gradient even though sglts

themselves do not use ATP directly

instead as sodium ions flow back into

the cell down their concentration

gradient glucose is co-transported into

the cell with sodium

this process is essential in cells

lining the small intestine and in the

nefron tubules of the kidneys where

efficient glucose absorption and

reabsorption are vital in the small

intestine these co-transporters enable

cells to take up glucose from the

digestive tract in the membranes of the

microvilli providing the body with a

primary energy source in the nefron

sglts facilitate glucose reabsorption

from the filtrate ensuring that the

valuable glucose does not exit the body

and urine but is instead reclaimed for

energy use cell adhesion molecules or

cams are essential for the organization

and stability of tissues they enable

cells to adhere to one another and form

structured networks these molecules

mediate specific interactions between

cells allowing them to connect and

communicate which is crucial for tissue

function and integrity different types

of cams are specialized for various

forms of cellto cell Junctions for

example some cams are involved in tight

junctions which create a seal between

adjacent cells preventing substances

from passing between them for instance

tight junctions are seen in the small

intestine where having undigested

nutrients leaking across the gut would

be problematic others are used in

desmosomes which provide structural

support by anchoring cells together

which can allow tissues to stretch

without breaking the sheet of tissue

plants often produce plasmodesmata which

are cams with holot tubes that connects

a side plasm of cells this allows for

transport between the cytoplasm of cells

which often involves water and dissolve

solutes like sugars the diversity of

cams allows for the specialization

needed to form and maintain different

tissue types throughout the body in this

video we saw how the fats in the

phospholipid bilayer can help regulate

fluidity and that unsaturated fats

increase fluidity for organisms that

live in cold temperatures while

saturated fats make the membrane more

rigid for organisms that live in hot

temperatures we also discussed the

function of cholesterol in the membrane

and that it stabilizes the membrane at

high temperatures and keeps it fluid at

low temperatures we also looked at how

the membrane can form vesicles to bring

in bulk materials through endocytosis

and absorb vesicles to excrete materials

through exocytosis we looked at proteins

in the membrane including gated ion

channels that can be triggered to open

by ligans or changes in membrane

potential we also looked at the sodium

potassium pump as an example of an

exchange transporter that maintains the

membrane potential in animal cells we

reviewed sodium dependent glucose

co-transporters or sglts and saw how

they relied on the sodium potassium pump

to create a sodium gradient to

co-transport glucose finally we saw how

cell adhesion molecules create Junctions

between cells that can vary from

different cell types and tissues