What does it mean to be selectively or semi-permeable

Selectively permeable or semipermeable means specific substances are allowed to pass through the plasma membrane.

Explanation:

The plasma membrane is selectively or semipermeable in nature because the plasma membrane allow only specific molecules such as nonpolar substances or hydrophobic substances such as steroidshormone to pass through it and prevent the passage of polar solute molecules such as water,glucose.

   To transport these polar solutes across the membrane an additional protein is needed in case of transport of water channel protein is needed whereas in case of glucose transport carrier protein is required.

answer:

the water molecules are transferred from the roots to the stem or top parts of the tall tree because of important properties of water which are adhesion, cohesion and capillary action. cohesion is the property of water molecules keeps water molecules in a combined state by sharing of bonds between them at the atomic level. adhesion allows the interaction of water molecules with other substances or substrate. adhesion will cause the attachment of water molecules to the wall of the vessel (xylem) of the plant. capillary action occurs when the adhesive force to the vessel wall is stronger than the cohesive force between water molecules. capillary action mimics the transfer of water from a tube or pipe. in this way water utilizes its property to transfer water from down to top level of the plant.