Course: SAT 170 Introductory Biotechnology

Unit 2 Learning Module

Page 2 of 3

Cells -The Basic Unit of Life (continued)
Plasma Membrane
- The plasma membrane is a selectively permeable barrier which surrounds the cell as well as certain organelles as it regulates what enters and exits that cell or organelle. It also aids in cell-recognition and cell-cell communication.
- The cell membrane divides the internal contents of the cell from the external contents of the environment. These inner contents are referred to as the cytoplasm which is composed of the entire cell contents embedded within a cushy fluid known as cytosol. The flu d which surrounds the exterior of the cell is known as the extracellular fluid.
- The plasma membrane consists of the following components:
  - Phospholipids are a unique type of molecule which have both hydrophilic (water-loving) and hydrophobic (water-fearing) components. This unique structure allows these molecules to form the basic barrier of the membrane as they pair up to protect the hydrophobic “tails” toward the interior of the membrane and expose their hydrophilic “heads” toward both the cytoplasm and the extracellular fluid. This double layer of phoshpolipids is known as the phospholipid bilayer.
  - Cholesterol is a hydrophobic molecule which allows it to wedge between the phospholipid tails, thus preventing the phospholipids from packing too tightly together and causing the membrane to become less rigid and more fluid.
  - Another important component of the membrane is proteins. Proteins embed within and attach to the phospholipid bilayer to function in membrane transport, cell communication and cell recognition.
  - Carbohydrates also play a role in membrane structure as they attach to proteins or phospholipids to function in cell recognition. This is the basis for our immune response as it allows a cell to differentiate self from foreign.

See Figure 2: The Fluid Mosaic Model, shown here, reflects our current understanding of the structure of the plasma membrane. Notice the incorporation of a variety of different molecules such as lipids, proteins and carbohydrates that make up the membrane.

Figure 2

Movement Across the Plasma Membrane
- Simple diffusion is one form of transport across the plasma membrane. During simple diffusion, substances move from high concentration to low concentration until equilibrium is reached where the substance is present in equal concentrations on both sides of the membrane.
  - The substance being transported via simple diffusion is small and neutral and can pass through the phospholipid bilayer without assistance.
- During facilitated diffusion substances move from high concentration to low concentration until equilibrium is reached with the help of a transport protein.
  - The substance being transported is either too large or charged and cannot directly cross the phospholipid bilayer so the transport protein carries it across.
- Osmosis is the diffusion of water; water will move from high concentration to low concentration (concentration of water is indirectly related to the amount of solutes which are substances dissolved in the water). Since water is the most abundant molecule in living things, its diffusion can have huge impacts on whether or not a cell lives. When describing osmosis, we use terms that imply the relative concentrations of water and solutes both inside and outside of the cell.
  - Hypertonic is a term used to describe a solution which has more solutes, and less water compared to its surroundings.
  - A hypotonic solution has less solutes, and more water compared to its surroundings.
  - A solution is said to be isotonic when it has the same amount of solutes and water as its surroundings.
    - Water will always move from a hypotonic solution to a hypertonic solution. Isotonic solutions result in no net movement of water.
- Another type of membrane transport is known as active transport. During active transport, materials move from low concentration to high concentration with the help of a transport protein and the addition of energy. This energy comes in the form of Adenosine triphosphate (ATP).  ATP is a unique molecule because it contains three phosphate groups; each phosphate has a large negative charge. In order to bring these three negatively charged phosphates together in the same molecule, energy must be invested in the bonds that hold them together. When energy is needed these bonds can be broken to release that stored energy. For this reason, ATP is often referred to as the energy currency for our cells.
- At times it is important for the cell to move large amounts of materials across the cell. This type of bulk transport is known as endocytosis and exocytosis.
  - Large materials are moved into the cell by forming vesicles around that material using portions of the cell membrane via endocytosis.
  - During exocytosis, large materials are moved out of the cell by fusing a vesicle with the cell membrane to dump that material out of the cell.
  - A vesicle is simply a small membrane bound structure, and is often used for packaging and temporarily storing materials for the cell.

Cell Wall
- Some cells contain a cell wall which is a rigid outer barrier that is sometimes found outside of the cell membrane. This structure is found in prokaryotes (both Archaea and Bacteria) as well as plants, fungi and some protists.