= Solution
The <valence quark> distributions describe net <quark> content: $\int u_v(x)dx=2$ and $\int d_v(x)dx=1$ for a <proton>. Their <momentum> densities $xu_v,xd_v$ are small at very small $x$, peak at moderate <Parton momentum fractions>[momentum fractions], and vanish toward $x=1$, where one <parton> carrying almost all <proton> <momentum> is unlikely. The larger $u$ distribution reflects two valence <up quarks> and its comparatively harder shape; its pointwise value need not be exactly twice the down distribution.
The <sea quark> distribution is generated largely by <gluon> radiation and $g\to q\bar q$ splitting. It grows strongly toward small $x$ and falls at large $x$. The plotted sea curve is scaled by $0.05$, so it must be multiplied by twenty before numerical use. The total up-quark <Parton distribution function>[PDF] is
$$
\boxed{u(x)=u_v(x)+\bar u(x),}
$$
using the valence-up distribution and the up-sea distribution. The summed sea curve alone does not fix $\bar u$ without a <Flavor eigenstate>[flavour] assumption. Under the later stated four-flavour symmetry, $\bar u=\Sigma/8$.
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