Scaling up Study Guide

Stem cells are undifferentiated cells, meaning they have not yet specialised to perform a specific function. This unique characteristic allows them to divide and produce more stem cells, or differentiate into various types of specialised cells. In animals, there are two main categories of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells are found in very early embryos and are pluripotent, meaning they can differentiate into almost any cell type in the body. Adult stem cells, also known as tissue stem cells, are found in specific locations within developed tissues and organs. They are multipotent, meaning they can differentiate into a more limited range of cell types related to the tissue in which they reside. For example, bone marrow stem cells can produce different types of blood cells. Adult stem cells play a crucial role in growth, repair, and replacement of damaged cells throughout an organism's life.

In plants, the equivalent of stem cells are found in regions called meristems. Meristematic cells are perpetually embryonic, meaning they retain the ability to divide and differentiate into all cell types that make up the plant body. There are two primary types of meristems: apical meristems and lateral meristems. Apical meristems are located at the tips of shoots and roots and are responsible for primary growth, which increases the length of the plant. Lateral meristems, such as the vascular cambium, are responsible for secondary growth, which increases the girth of the plant. The continuous activity of meristems allows plants to grow throughout their lifespan, form new organs like leaves and flowers, and repair damaged tissues.

The human circulatory system is a closed system that efficiently transports blood, nutrients, oxygen, waste products, and hormones throughout the body. The heart and blood vessels are highly adapted to their specific roles to ensure this efficient transport. The heart acts as a pump, providing the necessary pressure to circulate blood. Its muscular walls contract rhythmically, pushing blood out into the arteries. The left side of the heart is more muscular than the right, as it pumps oxygenated blood around the entire body at high pressure, while the right side pumps deoxygenated blood to the lungs at lower pressure. Valves within the heart ensure that blood flows in one direction, preventing backflow and maintaining efficient circulation.

Blood vessels, including arteries, veins, and capillaries, also have structures tailored to their functions. Arteries carry blood away from the heart and therefore have thick, muscular, elastic walls to withstand and maintain the high pressure generated by the heart. Their elasticity helps to smooth out blood flow. Capillaries are the site of exchange of substances between blood and tissues. They are extremely narrow and have walls only one cell thick, which shortens the diffusion distance for oxygen, nutrients, and waste products, facilitating rapid exchange. Veins carry blood back to the heart. The blood pressure in veins is much lower, so their walls are thinner and less muscular than arteries. To prevent backflow of blood against gravity, especially in the limbs, veins contain valves at regular intervals. These coordinated adaptations ensure continuous and unidirectional blood flow, critical for sustaining life.