between these lines
c_t(ind)= conv((a1 * exp(-b1 * (t(ind) - tmax))+ a2 * exp(-b2 * (t(ind) - tmax))) + a3 * exp(-b3 * (t(ind) - tmax)),(K_1*exp(-(k2+k3)*t(ind)+K_2)),'same');
plot(t,c_t);
add the following
c_t = c_t + normrnd(mu,sigma,length(c_t),1)
refer to the normrnd documentation for more information about the normrnd function. Or type
help normrnd
in the matlab console.
edit to correct this answer according to your last comment:
leave c_t as before, and you will have to create a new vector:
c_t_noise = c_t + normrnd(mu,sigma,1,length(c_t))
also changed the order of parameteres in normrnd to fit your dimension. to plot the two curves, just use the extended plot, like this:
plot(t,c_t, t,c_t_noise)
or do a hold on like this
plot(t,c_t)
hold on %tells matlab to put plots in the same figure
plot(t,c_t_noise)
hold off %this line if you pretend to make some other plot, desactivate hold on
more info about "hold" function in matlab console
help hold