Page 644 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

608 part IV Soils, ecosystems, and Biomes
can form large colonies—were fundamental to the cre- ation of Earth’s modern atmosphere. These organisms were also critical in the origin of plants, since free-living cyanobacteria eventually became the chloroplasts used in plant photosynthesis. Although these bacteria are called blue-green algae (mainly because they are photo- synthetic and aquatic), they are not related to other or- ganisms we know as algae. True algae are a large group of single-celled or multi-celled photosynthetic organisms that range in size from microscopic diatoms (a type of phytoplankton) to giant sea kelp.
Land plants (and animals) became common about 430 million years ago, according to fossilized remains. Vascular plants developed conductive tissues and true roots for internal transport of fluid and nutrients. (Vascu- lar is from a Latin word for “vessel-bearing,” referring to the conducting cells.)
In plants, leaves are solar-powered chemical facto- ries wherein photochemical reactions take place. Veins in the leaf bring in water and nutrient supplies and carry off the sugars (food) produced by photosynthesis. The veins in each leaf connect to the stems and branches of the plant and to the main circulation system.
Flows of CO2, water, light, and oxygen enter and exit the surface of each leaf. Gases move into and out of a leaf through small pores, the stomata (singular: stoma), which usually are most numerous on the lower side of the leaf. Each stoma is surrounded by guard cells that open and close the pore, depending on the plant’s changing needs. Water that moves through a plant exits the leaves through the stomata in the process of transpiration, thereby assist- ing the plant’s temperature regulation. As water evapo- rates from the leaves, a pressure gradient is created that
allows atmospheric pressure to push water up through the plant all the way from the roots, in the same manner that a soda straw works.
Photosynthesis and Respiration Powered by energy from certain wavelengths of visible light, photosynthesis unites CO2 and hydrogen (hydrogen is derived from water in the plant). The term is descriptive: photo- refers to sunlight and -synthesis describes the “manufacturing” of starches and sugars through reactions within plant leaves. The process releases oxygen and produces energy- rich food for the plant (Figure 19.3).
The largest concentration of light-responsive, pho- tosynthetic structures in leaf cells is below the leaf’s upper layers. These specialized units within the cells are the chloroplasts, and each chloroplast contains a green, light-sensitive pigment called chlorophyll. Light stimulates the molecules of this pigment, produc- ing a photochemical, or light-driven, reaction. Conse- quently, competition for light is a dominant factor in the formation of plant communities. This competition is expressed in the height, orientation, distribution, and structure of plants.
Only about one-quarter of the light energy arriv- ing at the surface of a leaf is useful to the light-sensitive chlorophyll. Chlorophyll absorbs only the orange-red and violet-blue wavelengths for photochemical opera- tions, and it reflects predominantly green hues (and some yellow). That is why trees and other vegetation look green.
Photosynthesis essentially follows this equation: 6CO2 + 6H2O + Light S C6H12O6 + 6O2
Sun
Sunlight
H2O
Minerals, water, and other nutrients
(a) Plant photosynthesis
CO2
Sugars and other carbohydrates consumed (oxidized)
Heat
CO2
Energy stored in sugars and other carbohydrates
Water transpiration
H2O
Minerals, water, and other nutrients
(b) Plant respiration
Water transpiration
O2
O2
▲Figure 19.3 Photosynthesis and respiration. (a) in the process of photosynthesis, plants consume light, carbon dioxide (CO2), nutrients, and water (H2O) and produce outputs of oxygen (O2) and carbohydrates (sugars) as stored chemical energy. (b) Plant respiration, illustrated here at night, approximately reverses this process. The balance between photosynthesis and respiration determines net photosynthesis and plant growth.
(carbon dioxide)
(water) (solar (glucose, energy) carbohydrate)
(oxygen)