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Asma Karameh

Omar Sami

Mohammad khatatbeh

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Happy day friends!

This lecture will be discussing what we have said in the previous lectures relating to

different mechanisms of transport across a biological membrane , these mechanisms

will give us some answers about how fluids are moving in and out of our body’s cells.

In this lecture we are mainly discussing

Secondary active transport.

Vesicular transport.

Control of transport and activity enzymes.

Secondary active transport :

Well, from the title we can recognize that it does need micro-energetic molecules because it is

“active”, but why is it secondary ? This is because of the way that ATP is used , in the secondary active

transport ATP is used indirectly through two mechanisms , either co-transport Or counter transport .

Hence :

1) Secondary and primary active transport are the same in their need of micro –

energetic molecules as a source of energy, but with different methods of ATP

consumption.

2) Both of them also have a substance to be transported against its

chemical/electrochemical gradient.

Differences between primary and secondary active transport:

Primary active transport Secondary active transport

Direct use of ATP Indirect use of ATP

Only one substance is transported Transport of two substances

ATP is used Directly , which means that an ATP molecule turns into ADP and the energy outcome results in moving substances AGAINST their concentration gradient .

No ATP consumption directly. However , the concentration gradient of one of the two substances across the plasma membrane drives this transport when it diffuses from high to low concentration (down its concentration gradient ).

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As mentioned earlier, the secondary active transport is either Co-Transport or

Counter Transport. However, the difference between the two mechanisms is : The

direction of the second substance , either towards the inside or towards the outside

of the cell , the first substance is Na+ as it in both mechanisms diffuse from outside

to inside.

Counter –transport:

In this mechanism a large concentration of Na+ is built outside the

membrane (as a result of Sodium Potassium pump), so these ions tend

to diffuse towards the inside of the cell . This movement is coupled with

the transport of another substance ( Ca++/H+) towards the exterior …

We can notice that sodium diffuses down it concentration gradient

(High to low) towards the inside, while the (Ca++/H+) is transported

against its concentration gradient ( low to high ) towards the outside .

Counter Transport may take to different ways , either with calcium or

with hydrogen :

1) Sodium-Calcium counter –transport

2) Sodium-Hydrogen counter –transport

** remember that Calcium is also transported via primary active transport methods.

Co-transport :

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In this mechanism we can notice that Na+ ( as usual ) diffuses from

outside to inside hand to hand with glucose and amino acids. And as

expected, when Na+ diffuses down it’s concentration gradient , glucose

and amino acids are moving against their concentration gradient, but

this time from outside to inside.

Some important notes about Co-Transport:

The transport protein must undergo conformational change in order to

transport both Na+ and Glucose towards the inside, and this will not

happen until both the sodium and glucose are attached to their sites.

Once they are attached the conformational change takes place and the

sodium and glucose to the inside of the cell at the same time.

However, sodium co-transport of amino acids occurs in the same manner

as for glucose, except that it uses different set of transport proteins, five

have been identified.

Sodium-glucose Co-Transporters are espicially important mechanisims in

transporting glucose across renal ( related to kidney ) and intestinal

epithelial.

*** Sodium-glucose Co-Transporters’ role in renal tubules of the kidney is :

To promote absorption of these substances into blood.

Vesicular Transport:

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After passive and active transport , we are now discussing Vesicular transport which ( from it’s name ) is through

vesicle.

Vesicular transport is considered to be also active (need energy)…

Well , you my be wondering why is it considered an active transport also?, as you know

that particles don’t “swim” inside the cytoplasm, instead they are moving within paths

called “microtubules”, when these particles are moving inside the cell along these

pathways , they are consuming energy because of motor protein that moves these

particles along the way.

And now lets have a closer look at these mechanisms:

1-Endocytosis : the transport of large particles into the cell by forming new vesicles

from the cell membrane when a small area of the membrane sinks inward to form a

pocket ,then as this pocket deepens(engulfs) ,it pinches in forming a vesicle containing

material that had been outside. There are three types of endocytosis :

1) Pinocytosis 2) phagocytosis 3) receptor–mediate endocytosis

2-Exocytosis:

The process of

engulfing water

(drinking process)

especially in

Bacteria.

This mechanism is related with

digestion of food particles and

destroying bacteria and viruses

(macrophages)

This type of endocytosis is designated for

special cells and not all of them , let us

clarify …

When insulin is secreted from pancreas , it is

now free in the blood , however cells can’t

use this hormone directly , only cells with a

receptor that mediate endocytosis can take

advantage of this hormone.

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Exo(means out ), indication of exporting products and transporting materials out of

the cell through vesicles that usually bud from the Golgi apparatus and move along

microtubules of the cytoskeleton to the plasma membrane where they will expel

their products outside the cell.

3-Transcytosis : This process combine both endocytosis and exocytosis, let us clarify …

During endocytosis , as mentioned earlier , plasma membrane surround the

substance that would be ingested by the cell then pinch off with the engulfed

materials and form a vesicle that is transported to the other pole of the cell

membrane and expel the contents into the extracellular fluid.

Regulation Of Transport: All of us know that all cellular activities are critical for maintaining

homeostasis and survival of living systems.

** However, there are control systems that maintain homeostasis such

as: endocrine system, nervous system and paracrine cells.

Well, now we are able to discuss what happens after the ligand is

bounded to its receptor:

***What is G-Protein ?

Ans: G-Protein is a bound intermediary protein composed of three units

{ Alpha , Beta , Gamma } , however the alpha subunit is attached to GDP,

when the whole G-Protein is activated the alpha subunit will be freed

and will have GTP instead of GDP.

1-Activation of channels:

EG: Activation of G-protein Activation of Na+ gated channelchanging of

membrane potentialopening of Ca++ channel (a voltage sensitive channel)

2-Activation of Second messenger system:

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Binding of a specific ligand to its receptor may result in activating

of second messenger, that undergoes series of biochemical events

to induce changes in cell activity.

A.C-AMP as second messenger:

Binding of ligand will induce activation of G protein freeing the

alpha subunit which binds to a membrane bound protein known as

adenylyl cyclase, this enzyme converts ATP to c-AMP , The formed

second messenger will activate c-AMP dependent protein kinase,

which phosphorylates particular protein which in turn bring

response inside the cell.

*notice that this process is amplified ; which means that activation

of one receptor may result in millions of end products.

B. Ca++ as second messenger:

In this pathway , phospholipase C is activated. This enzyme breaks

down Phosphotidyl inositol biphosphate(PIP2), the products of

PIP2 breakdown are : Diacylglycerol (DAG) and Inositol

triphosphate(IP3).The IP3 induces release of Ca++ from

endoplasmic reticulum into the cytosol where it binds to activate

another protein called calmodulin.

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To conclude…

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Never Dream For Success, Always work for it