Scaling up Study Guide

Stem cells are unique cells that have the ability to differentiate into various other cell types. In animals, there are two primary types of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells (ESCs) are found in the early stages of embryo development, specifically in the inner cell mass of a blastocyst. They are described as pluripotent, meaning they can differentiate into almost any cell type found in the body, ranging from nerve cells to muscle cells to skin cells. This broad differentiation potential makes them very versatile and a subject of intense research for regenerative medicine. However, their use raises ethical considerations because they are derived from human embryos.

In contrast, adult stem cells (ASCs) are found in various tissues throughout the body after embryonic development, both in children and adults. Examples include stem cells in bone marrow, skin, and the gut. Unlike embryonic stem cells, adult stem cells are generally multipotent, meaning they can only differentiate into a limited range of cell types within a specific lineage or tissue. For instance, hematopoietic stem cells in the bone marrow can develop into different types of blood cells (red blood cells, white blood cells, platelets) but not into nerve cells or skin cells. The primary role of adult stem cells is to maintain and repair the tissue in which they reside by replacing old or damaged cells. While their differentiation potential is more restricted than ESCs, they pose fewer ethical concerns and are already used in some medical treatments, such as bone marrow transplants. Understanding the differences in their source, developmental stage, and differentiation potential is crucial for appreciating their respective roles in biology and their applications in medicine.

Plants need a way to transport water and nutrients throughout their structure, as well as sugars produced during photosynthesis. This is achieved through two main processes: transpiration and translocation. Transpiration is the process by which water moves from the roots through the stem and leaves, eventually evaporating as water vapour from the stomata (small pores, primarily on the underside of leaves). This movement of water is driven by a concentration gradient, with water moving from an area of high concentration (the soil) to an area of lower concentration (the atmosphere). The evaporation of water from the leaves creates a 'pull' or 'transpiration stream' that draws more water up from the roots, through the xylem vessels. This process is essential for transporting water and dissolved mineral ions from the soil to all parts of the plant and for cooling the plant. Factors like temperature, humidity, wind, and light intensity affect the rate of transpiration. Translocation, on the other hand, is the movement of sugars (specifically sucrose, produced during photosynthesis) from the leaves, where they are made, to other parts of the plant where they are needed for growth or storage, such as roots, flowers, or developing fruits. This movement occurs through the phloem vessels. Unlike transpiration, which is largely passive, translocation is an active process requiring energy to load and unload sugars into the phloem. These two transport systems, xylem for water and phloem for sugars, are crucial for the survival and growth of plants.