Microbodies (Cellular Biology)
Microbodies are specialized organelles found within plant cells, primarily comprising peroxisomes and glyoxysomes. These small vesicles, ranging from 0.3 to 1.5 micrometers in diameter, are crucial for various metabolic processes. Peroxisomes are involved in breaking down hydrogen peroxide and play a key role in photorespiration, which occurs under conditions of low carbon dioxide and high oxygen levels, commonly during hot, sunny weather. In this process, they help convert harmful by-products into usable metabolites, enabling plants to adapt to changing environmental conditions.
On the other hand, glyoxysomes are primarily associated with fat metabolism in germinating seeds. They convert stored fats into sucrose, which serves as an essential energy source for seedlings before they can perform photosynthesis. This conversion involves collaboration between glyoxysomes and surrounding lipid bodies, known as spherosomes. Glyoxysomes contain enzymes that break down fats, facilitating the transformation into more transportable forms like sucrose, ensuring that developing plants receive the necessary nutrients for growth. Overall, microbodies play a vital role in plant cellular biology, supporting growth and metabolic flexibility.
Subject Terms
Microbodies (Cellular Biology)
Categories: Anatomy; cellular biology; transport mechanisms
Peroxisomes and glyoxysomes are the two major types of microbodies in plant cells. Their vesicles (“packages”) vary in size from 0.3 to 1.5 micrometers in diameter and are self-replicating. New microbodies are formed by incorporation of required proteins and lipids from the cytoplasm and subsequent splitting when they reach a certain size. Although structurally similar, their roles, and thus their contents, are different.
Functions of Peroxisomes
Peroxisomes are present in leaves, and their oxidative enzymes are involved in the breakdown of hydrogen peroxide and, more important, in photorespiration. Photorespiration occurs when carbon dioxide levels in the leaves drop and oxygen levels increase, a typical phenomenon on hot, sunny days when a plant is experiencing some level of water stress. Under these conditions, the enzyme (Rubisco) that normally catalyzes the attachment of carbon dioxide to ribulose bisphosphate (RuBP) begins to have a higher affinity for oxygen than for carbon dioxide. When oxygen is used instead of carbon dioxide, RuBP is split into two molecules, phosphoglycolate and 3-phosphoglycerate (PGA). PGA can be used in another part of the Calvin cycle, but phosphoglycolate must be extensively processed to be useful.
Phosphoglycolate is hydrolyzed and converted to glycolate in the chloroplast. Glycolate is then transported out of the chloroplast and into nearby peroxisomes. Peroxisomal oxidase converts glycolate to glyoxylate, and hydrogen peroxide is produced as a by-product. Because hydrogen peroxide is toxic, it is quickly converted by a catalase to water and oxygen. Glyoxylate goes through several more steps which involve reactions in the mitochondria and then back again in the peroxisomes. Eventually, glycerate is formed in the peroxisomes. Glycerate is then transported out of the peroxisomes and into a chloroplast, where it is converted to PGA, which can reenter the Calvin cycle.
Functions of Glyoxysomes
Glyoxysomes are found in the cells of fat-rich seeds. Fats are synthesized and stored as oil bodies, sometimes called spherosomes. Spherosomes are surrounded by a single layer of lipids instead of a lipid bilayer and are therefore not organelles in the strict sense. Glyoxysomes are responsible for converting fats and fatty acids into sucrose. The fat used by glyoxysomes comes from spherosomes.
Glyoxysomes are considered to be a type of peroxisome. In some plants, small glyoxysomes are found in the cotyledons of developing seeds. During germination and seedling development, they mature into fully functional glyoxysomes. They function until the fats are completely digested into sucrose. The energy from sucrose is required to drive early seedling development before photosynthesis begins. Large fat molecules are difficult to transport into the plant embryo; they must be converted to the more mobile sucrose molecules.
Breakdown of fats is a collaborative effort between enzyme-containing glyoxysomes and fat-containing spherosomes. Direct contact between spherosomes and glyoxysomes must occur. Fats from spherosomes leak out close to the membrane of glyoxysomes. Most of the activity of lipase enzymes does not take place in spherosomes but rather in or near the membrane of glyoxysomes. Lipases in glyoxysomes hydrolyze the ester bonds of fats and release the three fatty acids and one glycerol from each fat molecule. Glycerol is converted, at the cost of adenosine triphosphate (ATP), to glycerol phosphate, which is then oxidized by nicotinamide adenine dinucleotide (NAD+) to dihydroxyacetone phosphate, most of which is converted to glucose.
Bibliography
Gunning, Brian E. S., and Martin W. Steer. Plant Cell Biology: Structure and Function. Sudbury, Mass.: Jones and Bartlett, 2000. Text integrates microscopy with plant cell and molecular biology. Includes more than four hundred micrographs and four pages of full-color plates.
Hay, R. K., and Alastair H. Fitter. Environmental Physiology of Plants. 3d ed. New York: Academic Press, 2001. Provides a synthesis of modern ecological and physiological thinking. Examines molecular approaches which can be harnessed to solve problems in physiology. Illustrations include color plates.
Hopkins, William G. G. Introduction to Plant Physiology. 2d ed. New York: John Wiley & Sons, 1998. Uses interactions between the plant and the environment as a foundation for developing plant physiology principles. Discusses the role of plants on specific ecosystems and global ecology and provides information on the cell, chemical background, plant growth regulators and biochemistry. Each chapter is illustrated with relevant graphs, figures, or photographs.