looping through column data to fit curve

I have a spreadsheet 'ANA2009_1ag.xlsx' with column 1= wavelength lambda and col2=specta1, col3=spectra2, col4=spectra3 and so on

I want to fit a curve to each of the spectra using the equation:

ag(lambda) = ag(lambda_o)*exp(-S(lambda-lambda_o)

So I can get the slope amplitude at lambda and slope S.

I have the code and I am able to run it if I have a single text file with 2 columns: column 1= wavelength lambda and col2=specta1 But as i said above I have several spectra so i would like to make a loop so that the routine that i have for fitting the curve (and getting the slope) loops from one column to the next. My code is as follows.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
[num,txt,raw]=xlsread('ANA2009_1ag.xlsx');

%Get the size of data to be plotted 
[r,c]=size(num);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% takeout range of interest
I=find(num(:,1)<650 & num(:,1)>400);

wl=num(I,1);
a_g=num(I,2);

%setting options for fmisearch
opts = optimset('fminsearch');      
opts = optimset(opts,'MaxIter',4000); 
opts = optimset(opts,'MaxFunEvals',2000);   %// usually 100*number of params
opts = optimset(opts,'TolFun',1e-9);
%opts = optimset('LevenbergMarquardt','on');

%guess for paramters (amplitude at 532 and slope)
x0=[0.1, 0.03];

%minimization routine
x1 = fminsearch(@least_squares,x0,opts,a_g,wl)

%plot data and fit
plot(wl, a_g, '.k', wl, x1(1)*exp(-x1(2)*(wl-412)),'b')

DATA IS FROM 300 NM (LAMBDA) TO 685 NM (lambda) BUT I CHOSE TO FIT THE CURVE FROM 650-400NM

EXAMPLE OF DATA:

Wavelength  Spectra-1 Spectra-2
300         1.561434    1.434769
300.5       1.549919    1.42786 
301         1.531495    1.414042
301.5       1.506162    1.400224 
302         1.483132    1.386406
302.5       1.467011    1.372588 
303         1.45089     1.356467
303.5       1.443981    1.342649 
304         1.42786     1.333437
304.5       1.414042    1.324225 
305         1.407133    1.31271

My expected output is: x1 a vector which contains the amplitude at 412nm and slope S