Plants

Four major organs include roots, stems, leaves and flowers. Roots anchor the plant to the ground and are responsible for absorption of water and nutrients. Root hairs are important in increase the surface area of roots for absorption. Many roots also have mutualistic relationships with fungi called mycorrhizae which also increase their surface area. The stem of a plant is responsible for transportation of water and sugar. Flowers are the reproductive part of plants that include eggs and pollen. 

The primary role of a leaf is photosynthesis. Most of the photosynthesis occurs in the palisade mesophyll layer of the leaf which the spongy mesophyll creates intercellular space where gases and water can pass. Stomata are the opening in the leaves where gases move in (carbon dioxide) or out (oxygen) and water leaves through transpiration. Guard cells control the opening and closing of the stomata. When water moves into the guard cell it expands and becomes kidney shaped and opening the stomata. When water leaves the guard cell it loses this shape and closes the stomata. Guard cells take in water when the H+ ATPase proton pump is activated by sunlight and pumps H+ out of the cell and creates an electrical gradient. This gradient causes K+ and Cl- in move in which causes water to move in through osmosis 

The vascular tissue in plants include xylem and phloem. Xylem carries water up the plant and phloem carries sugar up and down the plant. 

Byrophytes are nonvascular plants that include mosses, liverworts and hornworts. They have few roots and get their water and nutrients through diffusion causing them to live in moist areas and preventing them from growing very tall. Gametophytes are their dominant stage with very small sporophytes. 

Pteridophytes include ferns, horsetails and club mosses. These are seedless vascular plants and require water for fertilization of the sperm with the egg. Sportophytes are the dominant stage. 

Gymnosperms include conifers and have naked seeds typically within cones. Angiosperm are plants with seeds within fruits and flowers. 

The cohesion-tension theory is used to explain how water moves through xylem. Cohesion keeps water molecules connected together and adhesion allows the water molecules to stick to the walls of the xylem. As water leaves through transpiration tension is created and will pull on the next water molecule and move it upward. 

Examples of plant response to abiotic factors include phototropism (growth towards the sun), gravitropism (growth of roots with gravity), and thigmotropism (growth with touch, such as climbing plants wrapping around structures to pull themselves upward). Plants may also become dormant if the environmental conditions will harm the plant. Seeds will remain dormant and not germination until the conditions (temperature, water, oxygen) will allow appropriate growth. Plants will loss leaves, a process called abscission, in the fall to conserve energy in the cold. 

Photoperiodism are responses to changes in photoperiods or the length of daylight. The circadian rhythm of a plant is an internal clock that measures the amount of daylight. The pigment phytochrome helps maintain this clock. Phytochrome has two forms, Pr and Pfr. When Pr is in red light (660nm) it will change to Pfr and when Pfr is in far red light (730nm) it changes to Pr. Pr is make in the cytoplasm of plant cells but Pfr is the active form. Pfr will form during dayligth when Pr is gets red light and is converted. Pr will form at night when it can not be changed to Pfr. Night length is responsible for resetting the clock if it is exposed to red light at night Pr will convert to Pfr. If the plant is then exposed to far red light at night it will reset it back to its original clock. 

Photoperiodism can control when a plant flowers. Some plants are considered long-day plants and will flower when daylight is above the critical length. These typically flower in the spring and early summer. Short-day plants flower in the late summer and fall when daylight is below the critical length. Day-neutral plants do not depend on daylight to flower; if the other environmental cues are correct they can flower anytime of the year. 

Many of the responses to biotic factors are forms of defense. Structural defense includes thorns or spines as well as thick cuticles or bark to keep herbivores away. Many plants also produce substances that are toxic to some predators and act as a chemical defense. Some plants are able to use other animals or plants to help protect them. For example, some plants release chemicals that attract parasitic wasp. These wasp lay their fertilized eggs on the unwanted insects (e.g. caterpillars) and when they hatch they feed on the insect from the inside out and kill it.