Chapter 3 : Movement of Substances Across the Plasma Membrane

A. Passive Transport

i) Simple Diffusion

• not selective: lipid-soluble molecules, gases and water.
• not control by cell.
• movement of the molecules from a region of higher concentration to a region of lower concentration.
• Factors affecting the rate of diffusion are temperature, size of molecules/ions, diffusion gradient, surface area and diffusion medium.
• example: diffusion of oxygen and carbon dioxide at the alveolus.

ii) Osmosis:

• only water molecules.
• not control by cell.
• movement of water from a region of higher concentration to one of lower concentration and often occurs across a semipermeable membrane.
• strong sucrose solution = less water molecule = low water potential.
• weak sucrose solution = more water molecule = high water potential.
• example: absorption of water by root hairs.

iii) Facilitated Diffusion:

• very specific: glucose, nucleic aicds, amino acids, protein and mineral ions.
• control by cell.
• transport of molecules (only certain molecules) across the outer membrane of living cell by a process of carrier protein (hydrophilic group) / channel protein (Ions: Na⁺, Ca²⁺, K⁺) within the cell membrane.
• normally take place from a region with higher concentration of molecules to a region of lower concentration.
• example: absorption of digested food in the villus.

B. Process of Active Transport

• very specific: minerals ions and amino acids.
• control by cell.
• This process needs carrier proteins and energy (due to against concentration gradient) from a region of lower concentration to a region of higher concentration).
• Cell must expend energy that derived from ATP (adenosine triphosphate)
• example: human nerve cells (sodium ions are constantly transport out of the cell) / ions intake by root hairs of a plant.