Secondary Biology Reference Notes, Tutorials and Exercise Questions​TOPICS
​Lesson Objectives
By the end of the topic, the learner should be able to:
​Meaning of cell physiology
​The term physiology refers to the functions that occur in living organisms.
Cell physiology refers to the process through which substances move across the cell membrane. Several physiological processes take place inside the cell e.g. respiration. Oxygen and glucose required enter the cell while carbon (IV) oxide and water produced leave the cell through the cell membrane. ​Structure and properties of cell membrane
​Properties of cell membrane
Permeability
Concentration Gradient
An increase in the concentration of molecules at one region results in a steeper concentration gradient which in turn increases the rate of diffusion. Temperature High temperature increases kinetic energy of molecules. They move faster hence resulting in an increase in rate of diffusion, and vice versa. Size of Molecules or Ions The smaller the size of molecules or ions, the faster their movement hence higher rate of diffusion. Density The denser the molecules or ions diffusing, the slower the rate of diffusion, and vice versa. Medium The medium through which diffusion occurs also affects diffusion of molecules or ions. For example, diffusion of molecules through gas and liquid media is faster than through a solid medium. Distance This refers to the thickness or thinness of surface across which diffusion occurs. Rate of diffusion is faster when the distance is small i.e., thin surface. Surface Area to Volume Ratio The larger the surface area to volume ratio, the faster the rate of diffusion. For example, in small organisms such as Amoeba the surface area to volume ratio, is greater hence faster diffusion than in larger organisms. Factors Affecting Diffusion​Role of Diffusion in Living Organisms
Some processes that depend on diffusion include the following:
​Osmosis
​Factors Affecting Osmosis
Size of solute molecules-
Osmosis' occurs only when solute molecules are too large to pass through a semi-permeable membrane. Concentration Gradient. Osmosis occurs when two solutions of unequal solute concentration are separated by a semi-permeable membrane. Temperature. High temperatures increase movement of water molecules hence influence osmosis. However, too high temperatures denature proteins in cell membrane and osmosis stops. Pressure Increase in pressure affects movement of water molecules. As pressure increases inside a plant cell, osmosis decreases. ​Roles of Osmosis in Living Organisms
The following processes depend on osmosis in living organisms:
​Water Relations in Plant and Animal Cells
​Osmotic Pressure
​Plasmolysis
​Wilting
​Water Relations in Plants and Animals​Haemolysis
​Crenation
​Active Transport
Factors Affecting Active Transport:
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