Chemfig: how to write vertically?

Question

I am trying to create a reaction scheme using chemfig. I can not understand though how to have text written vertically.

Say my set of reaction A+B -> AB, A+C ->AC

If I start writing

\startscheme
A+B \arrow{->}AB
\stopscheme

how can I write the + and the C vertically and aligned with A?

I am sorry if it is a naive question, I just discovered this package today!

Edit: I mean something looking approximately like:

A + B <=> AB
+
C
^
|
AC

Edit 2: (why EI is not aligned to E)?

\schemedebug{true}
\schemestart
\subscheme{E\phantom{I}} + S \arrow{<=>[$k_1$][$k_{-1}$]} ES \arrow{->[$k_\text{cat}$]} P
% insert arrow typ 0 (invisible) starting from compound `c2'
% scaled to 0.1 of the standard arrow length and pointing down
\arrow(@c2--){0}[-90,.1] +
% insert arrow typ 0 (invisible) scaled to 0.1 of the standard
% arrow length and pointing down
\arrow{0}[-90,.1] I
\arrow{<=>[$k_4$][$k_{-4}$]}[-90] \subscheme{EI} + S
\arrow(@c7--@c11){<=>[$k_3$][$k_{-3}$]}  
\arrow(@c3--){0}[-90,.1] + \arrow{0}[-90,.1] I  \arrow{<=>[$k_2$][$k_{-2}$]}[-90] EIS
\schemestop

I don't find very intuitive how this works :(

Answer 1

Simply use the optional argument of \arrow:

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
 A + B \arrow[-90] AB
\schemestop

\end{document}

Update responding to edit of the question: You can combine the use of \subscheme with the invisible arrow type 0:

\documentclass{article}
\usepackage{chemfig}

\begin{document}

% uncomment to see internal compound names:
% \schemedebug{true}
\schemestart
 \subscheme{A} + B \arrow AB
 % insert arrow typ 0 (invisible) starting from compound `c2'
 % scaled to 0.1 of the standard arrow length and pointing down
 \arrow(@c2--){0}[-90,.1] +
 % insert arrow typ 0 (invisible) scaled to 0.1 of the standard
 % arrow length and pointing down
 \arrow{0}[-90,.1] C
 \arrow[-90] AC
\schemestop

\end{document}

The \arrow command is a bit complex and you have to play a bit with it to get to know it. Here are a few examples of the syntax:

\arrow% simple arrow to the right

\arrow{<=>}% equilibrium arrow

\arrow{<=>[a][b]}% equilibrium arrow with labels

\arrow[-90]% arrow pointing down

\arrow{<=>[*{0}a][*{30}b]}[45]% equilibrium arrow pointing 45 degrees up with rotated labels

I repeat my advice from my answer to your previous question: study part V Reaction Schemes in the chemfig manual. Play with its lots of examples, copy them, change the parameters and see what happens...