Prokaryotes
Section 4-8/9
PROKARYOTIC CELLS: Structures Internal to the Cell Wall – The plasma membrane
- Plasma Membrane = a thin structure inside of the cell wall and enclosing the cytoplasm of the cell. It consists of phospholipids and proteins, and it is more fluid than the cell wall. The major function of the plasma membrane is to serve as a selective barrier to protect the cell from harmful chemicals. Due to selective permeability, the cell only receives desired molecules and ions, and stops the rest from entering.
- Due to the importance of the plasma membrane barrier, it is a common target for therapeutics and antibiotics. Destroying the cell membrane, causes the cell to die.
- The phospholipids in the membrane arrange in such a way that only the phosphate heads are exposed to water, and the lipid tails are hidden from water. This creates a lipid bilayer.
- The proteins found in the cell membrane consist of two main groups:
- Peripheral proteins = membrane proteins that adhere only temporarily to the biological membrane with which they are associated. They do not interact with the hydrophobic core of the phospholipid bilayer. Instead they are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid polar head groups.
- Integral proteins = a type of membrane protein that is permanently attached to the biological membrane. Transmembrane proteins penetrate the lipid bilayer completely and are exposed on both sides of the membrane. Transmembrane proteins are the only class of proteins that can perform functions both inside and outside of the cell.
- Glycolipids and glycoproteins are important for membrane stability, interactions with other cells, and lubrication of the cell surface to aid in motility.
- Glycoproteins = any of a class of proteins that have carbohydrate groups attached to the polypeptide chain.
- Glycolipid = any of a class of lipids that have carbohydrate groups attached.
- It is thought that all of the proteins in the cell membrane move fluidly around. This called the fluid mosaic model = The lipid bilayer has the viscosity of olive oil, and is self-healing in the presence of water; thus, proteins can move freely through the lipid bilayer dynamically.
- How do things move across the membrane barrier?
- Materials move across the plasma membrane by two categories of processes:
- Passive = substances cross the plasma membrane by moving down a concentration gradient. Meaning that substances move from areas of high concentration to areas of low concentration. This movement does not require energy.
- Active = a transport method that requires energy to transport substances in opposition to the concentration gradient.
- Materials move across the plasma membrane by two categories of processes:
- Types of Passive transport:
- Simple diffusion = occurs when small molecules pass through the lipid bilayer of a cell membrane along the concentration gradient. The movement of molecules continues until the concentration is evenly distributed (equilibrium), and the gradient disappears. Oxygen and Carbon dioxide are examples of molecules that moves through the cell membrane by simple diffusion.
- Facilitated diffusion = Integral membrane proteins called transporters function as channels or carriers to facilitate the movement of ions and molecules across the plasma membrane. The process still relies on the concentration gradient and does not require energy. Facilitated diffusion is required when the molecule being transported is:
- 1) charged, making it too hydrophilic to pass through the lipid portion of the bilayer,
- 2) polar, also making it hydrophilic,
- 3) too large to pass through the membrane easily.
- Osmosis = the diffusion of water through the lipid bilayer. Water can pass through the membrane by simple diffusion or through protein channels called aquaporins.
- There are three kinds of osmotic solutions:
- Isotonic = a solution in which the overall concentration of solutes is equal to those found inside of the cell. Thus water moves in and out of the cell at an equal rate because there is no gradient.
- Hypotonic = solution outside of the cell has less solutes than inside the cell. Thus water flows into the cell from the environment, causing the cell to swell. Unless the conditions are severe, the cell wall usually prevents bacteria from absorbing too much water and bursting.
- Hypertonic = solution outside of the cell has more solutes than inside the cell. Thus water flows out of the cell and into the environment, causing the cell to shrink. Unless the conditions are severe, the cell wall usually prevents bacteria from losing too much water.
- There are three kinds of osmotic solutions:
- Active Transport is required when resources in the environment are low, but the cell still needs to acquire those resources. Thus, the cell must use energy to move the substance against the concentration gradient. A cell has a different active transporter for each substance required.