CELL PHYSIOLOGY
Introduction Physiology refers to the branch of biology that deals with the study of functions and activities of life or of living matter such as organs, tissues or cells. It aims at understanding the mechanism of living. In simpler terms, physiology refers to the processes and functions that take place inside the body cells of organisms. Cell physiology refers to the study of functions of the cell structures. The cell structures perform various functions of life. In particular: Chloroplasts play a vital role in carbohydrate synthesis. Mitochondrion produces energy required to carry out life processes. Ribosomes manufacture of proteins. These physiological processes require various raw materials for them to take place. For photosynthesis to occur, carbon (IV) oxide, mineral salts and water have to be taken into the chloroplasts. For respiration (energy production) to take place, food substrate such as glucose and oxygen have to be taken into the mitochondrion. Energy, carbon (IV) oxide, water and alcohol (in plants) are some of the end products of respiration. Some of the end products of the physiological processes such as carbon (IV) oxide can be harmful when allowed to accumulate in the cells. They, thus, have to be eliminated from the cells. This implies that there is a constant flow of materials in and out of the cells and the cell organelles where these physiological processes are taking place. There is a constant movement of materials across the cell membrane in the cells. This chapter discusses the properties of the cell membrane and the processes through which materials move in and out of the cells. Structure of the membrane A membrane is a surface structure that encloses the cell and cell organelles. The membranes include the cell membrane, tonoplasts, nuclei membrane, mitochondrial membrane and chloroplast membrane. The membranes have a common basic structure which regulates the movement of materials in and out of the cells. The cell membrane is made up of a phospholipid layer sandwiched by two protein layer (it is a lipoprotein layer) the overall thickness of the cell membrane is about 7.5 nm thick. The membrane is perforated by small pores that allow the passage of substances in and out of the cells. Properties of the cell membrane The cell membrane is semi permeable– The pores that occur on the cell membrane allows the passage of the small size molecules but does not allow the passage of the large sized molecules. Such a membrane is said to be selectively permeable or semi-permeable. In particular, when a cell is surrounded by a dilute sugar solution, the small sized water molecules will enter the cell but the larger sugar molecules will not pass through the cell membrane. In contrast, the cell wall is permeable as it allows both sugar and water molecules to pass through it; it has larger pores. This property of selectively permeability enables the cell membrane to select what enters and leaves the cell. The cell membrane is sensitive to changes in temperature and pH– Cell membranes are made up of protein. Proteins are adversely affected by extreme changes in temperature and pH. Changes in temperature and pH will alter the structure of the cell membrane thereby hindering the normal functioning of the cell membrane. High temperature denatures (destroys) the proteins thereby impairing the functions of the cell membrane. The cell membrane possesses electric charges– The cell membrane has both positive and negative charges. These charges affect the manner in which substances move in and out of the ells. The charges also enable the cell to detect changes in the environment. Physiological Processes of the Cell membrane In this section, we discuss the various physiological processes through which materials move in and out of the cells across the cell membrane. Materials move in and out of the cells through three main physiological processes: Diffusion Osmosis Active transport Diffusion From kinetic theory, matter is made up of particles that are in continuous random motion. In solids, the particles are at fixed positions and can only vibrate at these fixed positions. In liquids and gases, the particles are loosely held and are free to move from one region to another randomly. This movement of gas or liquid particles is observed to be from regions of high concentration to a region of low concentration. The process by which particles move from a region of high concentration to a region of low concentration is known as diffusion. In particular, the scent of a flower or perfume experienced by an individual is as a result of the flower scent particles or perfume particles move from a region of high concentration. Diffusion occurs until the regions have an even concentration of the liquid or gas particles. The difference in concentration of particles between the region of high concentration and region of low concentration is known as the diffusion gradient/concentration gradient. Demonstration of the process of diffusion using potassium manganate (VII) Requirements: potassium manganate (VII) crystals, glass tubing, 100 cm3 beaker and water. Procedure Hold the glass tubing vertically in a beaker so that one end of the tubing rests on the bottom of the beaker. Cautiously and quickly drop a crystal of potassium manganate (VII) through the upper opening of the glass tubing. Close the upper hand of the glass tubing with the thumb. Half fill the beaker with water. Carefully withdraw vertically the glass tubing so that the crystal is left undisturbed at the bottom of the beaker. Record your observations for the first 15 minutes. Explain your observations. Expected observations After some time, the purple colour of the potassium manganate (VII) spread throughout the water and eventually all the water turned purple. Explanation The crystals of potassium manganate (VII) are highly concentrated with the potassium manganate (VII) particles. The potassium manganate (VII) particles break away from the crystals, dissolve in water and then diffuse through the water until they are evenly distributed. The Role of Diffusion in Living Organisms In Plants Diffusion plays an important role in plants in that: It