Oxygen and carbon dioxide move by diffusion across moist membranes. In animals, gas exchange follows the same general pattern as in plants.
Physiological changes in the surrounding guard cells account for the opening and closing of the stomata (see Chapter 20). Stomata are abundant on the lower surface of the leaf, and they normally open during the day when the rate of photosynthesis is highest. Gases do not pass through the cuticle of the leaf they pass through pores called stomata in the cuticle and epidermis. In the leaf of the plant, an abundant supply of carbon dioxide must be present, and oxygen from photosynthesis must be removed. In terrestrial plants, air enters the tissues, and the gases diffuse into the moisture bathing the internal cells. In aquatic plants, water passes among the tissues and provides the medium for gas exchange. Sponges and hydras have water-filled central cavities, and planaria have branches of their gastrovascular cavity that connect with all parts of the body.Īlthough plants are complex organisms, they exchange their gases with the environment in a rather straightforward way. Liverworts, for instance, have numerous air chambers in the internal environment. In larger organisms, adaptations bring the environment closer to the cells. Even in simple multicellular organisms, such as green algae, their cells may be close to the environment, and gas exchange can occur easily. Gas exchange occurs by diffusion across their membranes. Single-celled organisms, such as bacteria and protozoa, are in constant contact with their external environment. In living systems, the molecules move across cell membranes, which are continuously moistened by fluid. Diffusion is the movement of molecules from a region of greater concentration to a region of lesser concentration, in the direction following the concentration gradient. The basic mechanism of gas exchange is diffusion across a moist membrane. Gas exchange is thus an essential process in energy metabolism, and gas exchange is an essential prerequisite to life, because where energy is lacking, life cannot continue. Carbon dioxide must be available to plant cells, and oxygen gas must be removed. In plant cells, carbon dioxide may appear to be a waste product of respiration, too, but because it is used in photosynthesis (see Chapter 5), carbon dioxide may be considered a by-product. In the process, carbon dioxide gas is produced and must be removed from the body. In the majority of organisms, this metabolism takes place by respiration, a process that requires oxygen (see Chapter 6). Introduction to Nutrition and DigestionĪll living things obtain the energy they need by metabolizing energy-rich compounds, such as carbohydrates and fats.Vascular Plants: Structure and Function.Quiz Domains and Kingdoms of Living Things.Quiz Structure of Prokaryote and Eukaryote Cells.The Structure of Prokaryote and Eukaryote Cells.Quiz Movement through the Plasma Membrane.
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