queries about using updatepanel its data post to server

== case 1:

<asp:UpdatePanel ID="UpdatePanel3" runat="server">                 <ContentTemplate>

   <asp:Button runat="server" ID="cmdgetdata" OnClick="cmdgetdata_Click" Text=""/>

<Triggers>                     <asp:AsyncPostBackTrigger ControlID="cmdgetdata" />                 </Triggers>

</ContentTemplate>                 </asp:UpdatePanel>

==case 2:

<asp:UpdatePanel ID="UpdatePanel3" runat="server">                 <ContentTemplate>

<Triggers>                     <asp:AsyncPostBackTrigger ControlID="cmdgetdata" />                 </Triggers>

</ContentTemplate>                 </asp:UpdatePanel>

<asp:Button runat="server" ID="cmdgetdata" OnClick="cmdgetdata_Click" Text=""/>

== case 3:

<asp:UpdatePanel ID="UpdatePanel3" runat="server">                 <ContentTemplate>

<grid control>

   <asp:Button runat="server" ID="cmdgetdata" OnClick="cmdgetdata_Click" Text=""/>

<Triggers>                     <asp:AsyncPostBackTrigger ControlID="cmdgetdata" />                 </Triggers>

</ContentTemplate>                 </asp:UpdatePanel>

== case 4:

<asp:UpdatePanel ID="UpdatePanel3" runat="server">                 <ContentTemplate>

<grid control>

   <asp:Button runat="server" ID="cmdgetdata" OnClick="cmdgetdata_Click" Text=""/>

</ContentTemplate>                 </asp:UpdatePanel>

whts the actaul working , processing difference between the 4 cases & in all the 4 how much amt of data will go to the server on postback

i want to post back only that data of the grid whose cell is clicked i get that data in javascript in a variable

suppose i put the entire grid in updatepanel & postback on cmdgetdata_Click the entire grid data will be posted back

how do i postabck only the amt of data which is needed & not any other data avaliable on page