9.1: Introduction
The cell theory states that all living things are composed of cells and that cells only arise from other cells. Some cells are fairly simple in structure, while others are extremely complex. For example, some organisms are unicellular—they exist as a single cell, while multicellular organisms are composed of many cells that form tissues and organs. In either case, all cells share some common properties: the presence of DNA, intracellular proteins that enable the cell to perform its functions, and a plasma membrane. Some cells, known as eukaryotic cells, also contain membrane-bound organelles that allow a more complex level of functioning.
Homeostasis is defined as the maintenance of a stable internal environment. In order to maintain homeostasis, cells continually transport substances in and out across their cell (plasma) membrane.
The cell membrane serves as a “gatekeeper” and is the cellular structure that regulates the transport of materials into and out of the cell. The phospholipid bilayer architecture of the cell membrane allows certain molecules to pass through while keeping others out, therefore the cell membrane is selectively permeable (or semipermeable) . Things that need to enter a cell for it to function properly include ions, nucleotides, sugars, oxygen, amino acids, water, vitamins, and some hormones. Cells also allow certain molecules like water, ions, and secreted proteins to leave. Additionally, cells must eliminate waste products like urea and carbon dioxide
In the following exercises, you will examine the semipermeable nature of the cell membrane. You will also explore the concept of tonicity , which refers to the solute concentration of a solution, and its inherent ability to influence the rate and direction of osmosis.