Cells: The Living Units

Cellular Basis of Life 1. A cell is the basic structural and functional unit of living organisms. They range from single-celled “generalists”, like amoebas, to complex multi-cellular organisms such as humans. 2. The majority of cells consist of the same general parts. A generalized cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm. The membrane separates a cell from its surroundings and decides what comes in or out of the cell. The nucleus determines a cell’s basic function and structure. The cytoplasm, which separates the nucleus and membrane, holds organelles which perform certain tasks for the benefit of the cell as a whole.

The Plasma Membrane: Structure 3. The fluid mosaic model of a membrane is structured in a way that allows the diffusion of proteins and lipids. 4. The plasma membrane is made up of two layers of phospholipids. The membrane has many proteins embedded within it, which allow the passage of certain things into and out-of the cell. The hydrophilic (water loving) heads of phospholipids are on the surfaces of the membrane with their hydrophobic (water fearing) tails stay sheathed within.

The Plasma Membrane: Functions 5. In passive transport a membrane is permeated by carriers, channels, or direct diffusion. Secondary active transport requires ATP, the cell’s energy, to perform tasks such as the sodium-potassium pump. 6. Simple diffusion is the act of substances passing through a membrane without the aid of an agent, like the embedded membrane protein, to drive it though. Facilitated diffusion occurs only with the aid of proteins. Osmosis is the movement of solvent molecules through a selectively permeable membrane into a higher solute concentration. During filtration, solid particles are separated from the fluid which they are suspended in through a porous membrane. 7. Primary active transport uses direct energy to transport molecules across a membrane. An example of this type of direct activity is the sodium-potassium pump. In contrast to primary active transport, secondary active transport does not directly use ATP, and instead the cell pumps out ions 8. Vesicular transport is an active process in which materials move into or out of the cell enclosed as vesicles. There are two basic types of vesicular transport: endocytosis and exocytosis. Endocytosis involves the cell bringing materials into itself by enveloping them. Exocytosis involves the cell sending out materials. 9. Membrane potential is created the difference in electric potential between the interior and exterior of a cell. This difference is created by the concentration of extracellular sodium and intracellular potassium. It is maintained by the sodium-potassium pump which pumps out excess potassium. 10. Cell adhesion molecules are proteins located on the cell surface with the purpose to bind to other cells. Cell receptors are composed of two or more protein subunits, which take part in communicating between the cell and the outside world.

The Cytoplasm 11. The cytoplasm is the fluid substance within the cell membrane that contains the cell’s organelles except for the nucleus. Organelles have specific functions. Some major organelles are the mitochondria, Endoplasmic Reticulum, Golgi apparatus, lysosomes, and chloroplasts. 12. Certain cells have more of a certain type of organelle than the average cell. This is due to the conditions said cell may undergo. For example, cells of the liver, which detoxifies drugs, have higher amounts of peroxisomes.

The Nucleus 13. The nucleus contains a cell’s genetic material in the form of DNA. Its function is to maintain these genes and control the cell’s activities. The nuclear envelope completely encloses the nucleus and separates the cell’s genetic material from the surrounding cytoplasm. The nucleolus of a cell