3.3

• Plants need to take in carbon dioxide and nutrients
• Waste products generated by plants need to be released
• Exchange of substances with the environment occurs at exchange sites, such as roots in plants for water and minerals
• Substances are considered to have entered or left an organism only when they cross the cell surface membrane
• Single-celled organisms like Chlamydomonas exchange substances directly with the environment due to their large surface area: volume ratio and short diffusion distances
• Larger organisms require specialized mass transport systems for transporting essential nutrients and molecules to all parts of the organism
• Surface area and volume are important factors in the exchange of materials in organisms
• As organisms increase in size, their surface area: volume ratio decreases
• Plants have adaptations to increase their surface area: volume ratio, such as branching body shape, flat and thin leaves, and root hairs
• Larger organisms have higher levels of activity and metabolic demands, requiring specialized mass transport systems for efficient nutrient transport
• Plants have evolved mass flow transport systems for efficient transport of nutrients and waste
• Plants have two separate mass transport systems: xylem for water and mineral ions, and phloem for sucrose and other nutrients
• Plants do not have a specialized transport system for oxygen and carbon dioxide due to adaptations like high surface area: volume ratio and chloroplasts in leaves and stems
• Xylem tissue in plants carries dissolved minerals and water, provides structural support, and stores food
• Xylem tissue is found in vascular bundles along with phloem tissue in plants
• Xylem tissue location varies in roots, stems, and leaves to provide support and transport functions
• Phloem tissue in plants transports organic compounds, particularly sucrose, from source to sink, and is a complex tissue with sieve tube elements and companion cells
• Phloem tissue location varies in roots, stems, and leaves to facilitate organic compound transport
• Xylem tissue is made up of tracheids, vessel elements, xylem parenchyma, and sclerenchyma cells
• Phloem tissue is made up of sieve tube elements, companion cells, parenchyma, and fibers
• Structure of phloem tissue:
• Phloem sieve tube elements structure & function table
• Sieve tube elements line up end to end to form a continuous tube
• Phloem companion cells structure & function table
• Difference between xylem and phloem tissue:
• Companion cells are associated with sieve tube elements and control their metabolism
• Companion cells play a role in loading and unloading sugars into the phloem
• Mature xylem tissue is dead with no evidence of organelles and has lignified cell walls
• Sieve tube elements have no lignin, have sieve plates, and their companion cells contain nuclei and dense cytoplasm
• Xylem transports water and mineral ions from roots to the rest of the plant
• Phloem transports substances from the source (e.g. leaf) to the sink (e.g. root)
• Xylem and phloem are arranged together in vascular bundles
• Vascular system involves a network of vessels running through leaves, stem, and roots
• Vascular system comprised of xylem and phloem
• Vascular bundles are laid out differently in leaves, stem, and roots
• Transpiration:
• Plants constantly take in water at roots and lose water via stomata in leaves
• Transpiration refers to the loss of water vapor from a plant to its environment by evaporation and diffusion
• Transpiration stream moves water from roots to leaves
• Evaporation of water vapor from leaves and cohesive/adhesive properties of water molecules cause water movement through a plant's xylem