3.3 Movement through the plasma membrane

The cell membrane separates the cell from the external environment. In order to communicate with the external environment for the purpose of survival (e.g., for the consumption of food, minerals etc.), there is movement in the cytoplasm and plasma membrane. This movement occurs through several mechanisms which are listed below:

Osmosis : One method of movement through the membrane is osmosis. Osmosis is the movement of water. Osmosis often occurs across a membrane that is semipermeable. A semipermeable membrane allows only certain molecules to pass through while keeping other molecules out. Osmosis is really a type of diffusion involving only water molecules.

Diffusion : Another method of movement through the membrane is diffusion. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. This movement occurs due to molecules which constantly collide with each other. The total effectual momentum of the molecules is away from the region of high concentration to the region of low concentration.

Diffusion is the random movement of molecules. The exchange of molecules (taking place from a higher concentration region to a lower concentration region) leads to the formation of concentration gradient. Diffusion phenomena can be seen by letting a drop of dye into water. The color of the dye gets diffused throughout the water.

Facilitated diffusion : A third method is facilitated diffusion which occurs across the plasma membrane. This type of diffusion is very specialized. This occurs only in cases where specific proteins in the membrane permit only certain molecules across the membrane. These membrane proteins allow movement in the direction that diffusion would normally take from a region with a higher concentration of molecules to a region with a lower concentration of molecules. No energy use is required for facilitated diffusion.

Active transport : A fourth method for movement across the membrane is active transport. When active transport takes place, a protein moves a certain substance across the membrane, usually from a region of lower concentration to a region of higher concentration. As you know this movement is against the concentration gradient, hence energy is required for this movement. Normally the cell gets its energy from ATP (adenosine triphosphate). For example, in cardiac muscles, active transport takes place. In these cells, sodium ions are constantly transported out of the cell. The cellular compartment is a region of high concentration of sodium ions. Buildup of electrically charged ions allows changes in voltage over the cell membrane which affects contraction of muscle cells.

Endocytosis : This another mechanism of movement across the plasma membrane. In this type, a small patch of plasma membrane encloses particles or tiny volumes of fluid which are at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and breaks off from the membrane, forming a vesicle that moves into the cytoplasm. When the vesicle contains particulate matter, the process is called phagocytosis. When the vesicle contains liquids or droplets of fluids the process is called pinocytosis.

Table of Contents

3.0 Introduction
3.1 Modern cell theory
3.2 Structure of cell
3.3 Movement through the plasma membrane

Chapter 4

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