Cellular Transport – 1.15

Hi. My name is Doctor. Matt ivory.

I'm one of the lecturers in the School of Farm and

pharmaceutical sciences. And in this series of talks,

we're gonna look at the ways that cells and organisms

both transport things into their cells and bodies and how

they transport things out.

And we'll also have a look at the exchange surfaces that are

used in the human body and the transport system.

And we'll have a look at respiration as well.

So in this first talk,

we'll have a look at how substances are transported into

and out of cells.

Molecules

and ions in the gas phase tend to show a net movement

from areas of higher concentration

to areas of lower concentration.

And this is because the particles are constantly

moving. So if you think about the molecules of air in the atmosphere,

you think about the molecules of solute in a solution.

They have energy and so they're constantly moving all over the

place And so if you allow this to continue, then the

particles will eventually even out.

So we'll establish an equilibrium where the solute or

the gas molecules are evenly spread across the space that they're in.

And so I like to think of it like if you imagine Zorb

football and all the different molecules are people wearing

Zorb football equipments and they're all blindfolded and

just let them loose in a big field and just told them to

run-in whatever direction they wanted. They'd start running.

They'd bump off each other and it be very random because

they'd be blindfold and they wouldn't know where they're

going. So the molecules in solution and in gas act a

little bit like that.

When you have an area of higher concentration

and an area of lower concentration in a different

area, this creates what's known as a concentration gradient.

So the concentration

gradient is down towards the area of lower concentration

and so particles will therefore move down the concentration gradient.

And so you can think about if you let a ball go on top of a hill,

it'll move down the slope and just like that particles will

move from an area of high concentration to an area of low concentration.

And this net movement because it's not unidirectional.

It's not just that the particles of high concentration

move in one direction. Sometimes they'll bounce back.

Sometimes they'll stay in place because they're moving in circles.

But the net movement is from high concentration to low

concentration and this is called diffusion.

And a practical example that you may be aware of.

So if you're getting ready for PE and someone sprays deodorant

at one side of the room, then

fairly soon afterwards you'll be able to smell it at the

other side of the room despite the fact that the spraying was quite far away.

And that's because the deodorant creates aerosol

particles which are at a higher concentration next to the

person who sprayed it.

But eventually it'll equilibrate in the room so it

won't be as strong.

Concentration will have fallen when it reaches you,

but it will equilibrate and become equal across the whole room.

The bigger the difference in concentration,

the steeper the concentration gradients and the faster

diffusion will occur.

If we think about this in terms of cells,

so there's a constant movement of substances needed by the

cell into the cell.

And also a constant movement of things that are unwanted by the

cell out of the cell.

And so this ensures that the cell has all of the oxygen and

the nutrients that it needs and also allows it to get rid of waste products.

Some molecules are able to pass freely So they're able to pass

through the cell membrane without the cell doing anything.

They just move down a concentration gradient and

naturally diffuse into the cell. So things like oxygen

diffuse in. And when respiration happens,

carbon dioxide is produced,

and that will diffuse out of cells because oxygen and carbon

dioxide are small molecules and are able to pass through the

cell membrane just on their own.

Not all molecules are able to move through the cell membrane,

And so the cell membrane is called either semi

permeable or partially permeable.

So whereas oxygen is able to move in and carbon dioxide side

out or vice versa if the concentration gradient was reversed.

Other molecules like

salts and sugars are too big to fit through the cell membrane by themselves.

And so for this reason, this creates a concentration

gradient of water molecules.

So this kind of flips what you normally think about as a

concentration gradient in diffusion on its head So

instead of being the concentration of things

dissolved in water,

it instead becomes the concentration of water.

So if you've got a solution with very little solute in it,

then that has a higher concentration of water because

it has more water molecules per volume than if you have a

solution that has a lot of solute dissolved in it.

So very high solute concentration will mean that

there are fewer water molecules per volume you can think about

it as a lower water concentration.

Water will therefore move through a semipermeable

membrane because it's able to diffuse through the membrane.

So just like up gen and carbon dioxide, water's a very small

molecule and so is able to move through the cell membrane freely.

So we'll do so down that concentration gradient,

so from an area of high water concentration to an area of

low water concentration.

So if you're thinking about that in terms of classical

solute concentrations,

it moves from an area of low solute concentration

to an area of high solute concentration.

And so this works just like diffusion.

So if you think about how diffusion created that

equilibrium and evened out the concentration on two sides of

the room, then Osmosis,

which is this movement of water molecules across a

semipermeable membrane, does the same.

So it establishes the same concentration of water

molecules on both sides of the membrane. And again,

it's reliant on the net movement of molecules.

So water molecules are moving randomly, bumping all around,

So some will go against the concentration gradient, not

many. The net movement overall,

the net movement will be from an area of low solute

concentration to an area of high solute concentration.

If we think about how this affects a cell, then if you

have a cell that's sitting quite happily and there's no

net movement of water into or out of the cell,

then the cell's gonna stay the same size.

Think about it like a water balloon.

If there's no water going into the water balloon,

then the water balloon stays the same size.

If you've got an environment surrounding the cell that has a

higher concentration of water in it,

so very low solute concentration,

then water will move across the cell membrane and into the

cell. So that's gonna cause it to increase its volume as it

fills up with more water.

So that's what's called a hypotonic environment.

We spoke about it briefly in another talk and neither way to

remember it is the o in Hypo because the cell is gonna swell

and become a ball, a filled with water, just like that

The balance conditions are called isotonic conditions

where there's no net movement of water,

but if there was a situation where there was a lower

concentration of water outside the cell,

so very high solute concentration,

then the net movement of water would be out of the cell to try

and establish an equilibrium between the inside and out

outside of the cell.

So this would cause water to be lost from the cell and just

like water going in causes it to swell. If water's going out,

then it's gonna cause the cell to shrink.

The effect that this has on cells will vary between the types of cells.

So in animal cells where they've just got a lipid cell

membrane, if it absorbs too much water,

lipid cell membranes not particularly strong and so the

cell is liable to burst.

And release all of its contents out into the environment.

Whereas if it's in a hypotonic environment,

then the animal cell is gonna lose water and will shrink down

and it might not be able to function anymore.

On the other hand,

with plant cells they're built a bit differently.

So they have a rigid cell wall around the outside of the cell,

so they're not just reliant on their cell membrane to be able

to resist this extra pressure.

They also have a big vacuole in the middle of the cell and so

that's where this excess water will go. And so if you put a

plant cell or plant tissue into hypotonic environment,

then the volume of cell sap within the back door will

increase and so this will put outward pressure on the cell,

like we saw outward pressure on the animal cell that caused it

to burst, but instead of bursting,

the rigid cell wall will keep everything inside the cell So

the outward pressure will be balanced out by the inwards

force from the rigidity of the plant cell wall and this is

what makes plant cells turgid and if you think about a whole plant organism,

if all of its cells are nice and full of water and nice and

turgid, then that plant is gonna be able support itself,

it's gonna stand up.

But if the opposite is true,

so if you have a hypertonic environment and you put a plant

cell in it, then water is lost from the vacuole.

And so this causes the whole celled and flaccid because the

vacuole shrinks sometimes you get the cell membrane pulling

away from the cell wall a bit.

And this causes the whole cell to come flaccid and so the

plant itself would become flaccid and would droop.

And so a plant that hasn't been watered enough,

you'll see it looks a bit sad,

and doesn't have the pressure inside of it to support itself.

Any move to increase the amount of water inside a cell

will increase its mass because there's more stuff inside it.

And so if you have that happening across a whole bunch of cells,

then you'll have an increase in the mass of tissue as well.

And so therefore,

osmosis can cause an increase or a decrease in the mass of

plant tissues. And in the next talk,

we'll talk about an experiment that you can do where you

measure the net change in mass based on the osmotic pressure

of different environments for plant tissue.

If we think about the effect of osmosis on a whole plant

organism, land plants rely on having a supply of fresh water

in the soil and they draw it up through their roots.

And the way that that works is that root hair cells transport

mineral ions into their cell side plasm and that creates an

osmotic gradient,

which water will therefore move from the soil into the plant.

So if you were to replace the fresh water in the soil with

salt water, then you'd find that even though the root hair

cells are act transporting in some mineral ions.

The concentration of mineral ions outside the cell would be

higher. And so therefore,

the concentration of water would be lower outside the cell

than it is inside the cell.

So this would therefore lead via osmosis to a net loss of

water from the roots And so plant cells are reliant on that

absorption of water being drawn up through a transpiration

stream to service the leaves and then the leaves can use it

in photosynthesis.

But if you reverse that flow of water out of the roots

then the transpiration stream's broken so they can't absorb

water anymore and the plant is very quickly gonna dry out and

die And so that's why you have specifically adapted plants

that live either in the sea or in areas where sea water

It's more likely to occur. So things like coastal areas.

And these sea plants are called halophytes.

An example is sea grass,

And the way that they get around this issue is that they

maintain a much higher salt concentration in their cells

than land plants would.

So that re establishes the osmosis gradient and so they're

able to absorb water from their environment,

and use it rather than losing water through osmosis

We've spoken about diffusion and about osmosis and both of

these are passive processes

that rely on the random movement of molecules

to move from areas of higher concentration to lower concentration.

And in particular the importance of the semipermeable

membrane in cells.

An issue that cells have is that bigger molecules so things like glucose,

some irons,

some other things that need to be transported into cells are

too big or to charge,

they'll have some property that doesn't allow them to move

through the lipid membrane of the cell.

Another issue that cell might face as well is that it wants

to establish a higher concentration of something

inside a cell than exists outside.

So if you think about food molecules,

then a cell isn't gonna only wanna have half of the

available food if it just allows an equilibrium to be established.

It's gonna want to absorb as much food as possible and therefore

be able to grow and to survive for as long as possible.

So in these two situations where something can't pass

through the cell membrane passively,

or well where the cell wants to absorb more than would be

possible via the concentration gradients,

the cell will use active transports and it does so using

transport proteins in the cell membrane.

And so this gives it an opportunity to move things that

would normally be too big to fit through the normal cell membrane.

And these transport proteins rely on energy from the cell to work.

So you can think of them as pumping machines that pumps

specific molecules into the cell.

And so this is why it's called active transport because it requires energy

and you can differentiate that from the passive processes of

diffusion and osmosis.

So as well as ensuring that very large molecules can be

imported into the cell, It allows us to ensure that

absorption of food molecules like glucose is one way.

So if it was passive,

not only would we not be able to absorb all of it,

but if we missed a meal and

glucose concentration was much higher in the blood than it was in the guts,

you'd have glucose entering the gut from the blood and you'd

have lost food that you'd already had.

So your kind of missed meal would very quickly evolve into starvation.