Movement of materials into and out of cells takes place either passively or actively. Passive movement includes diffusion and osmosis, passive movement requires no energy input from the cell – since the molecules follow a concentration gradient. Active transport requires energy as the molecules are forced against the concentration gradient.
Passive transport
Diffusion and osmosis result from the random movement of particles, in accordance with Brownian motion. When they are in higher concentration in one region, this random movement slowly results in an overall spreading out the most concentrated point – finally bringing about their even distribution within a space. Limitations include that it depends on the presence of a concentration difference and that it is relatively slow, particularly when the concentration gradient isn’t steep.
Diffusion is the movement of any molecule from regions of high concentration to low concentration, until equilibrium is reached. No energy input is required. Osmosis is the movement of water molecules from areas of high water concentration to low water concentration, through a semi-permeable membrane.
Within the kidney the movement of substances, between the bloodstream and excretory fluid, occurs in the microscope tubules (nephrons) and involves both active and passive transport. Within the kidney tubules there is a two-way movement of substances – waste substances pass from the bloodstream into the nephrons to be excreted and substances required by the body and removed from the urine and returned into the bloodstream. Passive transport moves water (by osmosis), and some nitrogenous wastes (by diffusion) into the kidney. Only excess salts and water are excreted, homeostasis requires that sometimes water and salts should be conserved to maintain their appropriate levels. Salt movement is through active transport and this in turn draws water through osmosis.
Active Transport
It is the movement of molecules from an area of low concentration to high concentration – requiring energy input. Sometimes in organisms there is a need to move chemicals against the concentration gradient. Active transport involves a carrier protein which spans the membrane; this carrier molecule can actively move chemicals from low to high concentrations.
Active transport moves mainly sodium ions, glucose, amino acids and hydrogen ions across the wall of the nephron. All glucose and amino acids are reabsorbed by kidney cells, so that they are not lost in urine; thus they move against the concentration gradient. Additional nitrogenous wastes and hydrogen ions are added to urine (from the blood capillaries) in the kidney tubules. A “sodium pump” mechanism operates in the tubules, actively transporting ions from the urine back into the kidney cells. Besides conserving salts this process also converses water within the body – as the active transport of salts draws water out of the urine, as water follows due to osmosis (passive transport). Water is drawn by the osmotic pull of salts in the kidney.