A Level Biology A (Salters-Nuffield) Edexcel

Water and Transport

Properties and Functions of Water

• Water is a polar molecule due to the uneven distribution of electrons, leading to partial positive and negative charges at different ends.
• The polarity of water allows it to form hydrogen bonds with other water molecules, creating a cohesion that aids in water transport. This is critical for the transpiration stream in plants.
• The polarity of water also contributes in its use as a solvent. It can dissolve a variety of solutes, which facilitates transport of nutrients and waste in organisms.
• Water's high heat capacity speaks for its ability to absorb significant amounts of heat before increasing in temperature. This helps organisms maintain stable internal body temperature.
• The evaporation of water causes a cooling effect. In animals, this helps with thermoregulation, such as sweating in humans.

Transport Systems in a Human Body

• The human body has a dual circulatory system, encompassing the systemic circulation and the pulmonary circulation.
• Blood vessels, including arteries, veins, and capillaries, play a central role in the transport of water, nutrients, gases, and wastes.
• The heart fuels the circulatory system through its regular contractions and relaxations, constituting the cardiac cycle.
• The systemic circulation transports oxygen and nutrients to cells around the body, while the pulmonary circulation offloads carbon dioxide and picks up oxygen from the lungs.
• Water forms a significant part of the plasma in blood, allowing the smooth transport of cells and molecules in the bloodstream.

Water and Transport in Plants

• Xylem vessels conduct water and dissolved minerals from roots to stems and leaves.
• Transpiration, the loss of water from plant surfaces, mainly through stomata in the leaves, drives water up the plant due to the resulting transpiration pull.
• Unlike in animals, plant cells are connected by plasmodesmata—small channels in the cell walls that allow the flow of water via osmosis.
• Root hair cells in plants absorb water from the soil. The high surface area to volume ratio of these cells allows efficient water absorption.
• Transpiration also aids in the cooling of the plant and creates a concentration gradient that facilitates absorption of nutrients from the soil.