Cannot read property from Mark-up

I've got the following in my .aspx:
<input type="image" src="<%=PayPalButtonImage %>" onserverclick="RedirectToPayPal" runat="server" />
In the code-behind I've got this property:
protected string PayPalButtonImage
        {
            get { return PayPalExpressCheckoutButtonUrl;}
        }
why can't it see this property or the server method RedirectToPayPal?  I get a runtime error of :
'ASP.cart_aspx' does not contain a definition for 'RedirectToPayPal' and no extension method 'RedirectToPayPal' accepting a first argument of type 'ASP.cart_aspx' could be found
...