The movement of food and water across short distances in plants takes place either passively by diffusion or by active transport.
In diffusion, the molecules move from a region of high concentration to a lower concentration, with no expenditure of energy. The rate of diffusion is affected by several factors such as the size of molecules, their solubility in lipids and the concentration gradient. As for the size, the bigger the molecule, the slower the rate of diffusion. Since all membranes are made up of lipids, lipid-soluble substances diffuse faster. In a concentration gradient, diffusion of hydrophilic substances is facilitated by special proteins called transport proteins, without any energy expenditure. This type of diffusion is called facilitated transport.
Proteins are channels in the membrane, which allow molecules to pass through them. Some of these channels are always open while others are controlled. Some transport proteins, called porins, form pores in the outer membranes of cell organelles, to allow small proteins to pass through.
Based on the direction, transport of molecules can be differentiated into three types – symport, antiport and uniport. Both molecules cross the membrane in the same direction in a symport, while they move in opposite directions in an antiport and a single molecule diffuses across a membrane in a uniport.
Active transport moves molecules against the concentration gradient with no expenditure of energy. It is carried out by the membrane proteins called pumps that use energy and are specific to the type of substances they carry across. Of all the means of transport, both diffusion and facilitated diffusion always take place along a concentration gradient, without the expenditure of energy while active transport uses energy against concentration gradient.