Phospholipid: has a hydrophilic polar head made from glycerol and phosphate, other end has two non-polar fatty acid tails that are hydrophobic.. The two phospholipids are arranged with the polar heads facing out and the tails on the inside so that the heads will associate with both the cellular and extra-cellular environments. These two layers are held by hydrophobic interactions (fairly weak) . This weak interaction allows the membrane to break, reform and to move across each other. The shorter the unsaturated fatty acid, the more fluid the phospholipid.
Along the membrane are many types proteins that allow for the exchange of materials. The membrane is semi-permeable so only a select number of materials can cross the membrane. Molecules such as water, waster, oxygen, CO2, metabolites, ions, amino acids and nucleotides can be moved using these proteins:
- Transport: facilitated and protein pumps
- Receptors: peptide based hormones, specific shape that allows certain molecules to bind to it
- Cell Recognition: allows body to recognize which cells are foreign or domestic. This makes transplants and blood transfusions difficult.
- Channel Protein: certain molecules or ions diffuse across the concentration gradient
- Carrier protein: combine with a specific ion or molecule, changes shape, then transports through the membrane.
Using these proteins, molecules will go through a variety of important processes to enter or exit the cell. Those that move from a high concentration to a low concentration do not require ATP:
- Diffusion: particles move from a high concentration to an area of low concentration until it reaches equilibrium
- Osmosis: movement of water from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration) until it reaches equilibrium
- Simple Diffusion: small, non-polar (lipophilic) molecules diffuse across the membrane
- Facilitated diffusion: Large, polar subtances (ions, macromolecules) require transport proteins, such as carrier or channel proteins, to help them move across the membrane
- Active transport: Molecules passed against concentration gradient from low to high, utilizes protein pumps that hydrolyze ATP to force the molecule across the membrane
Vesicle formation and transport of materials within the cell:
- Proteins are synthesized by the ribosomes of the rough ER and enter the lumen of the rough ER
- The protein modified as it travels along the rER. At the end of the ER a vesicle is formed with the protein inside
- This vesicle is transferred to the golgi apparatus where it fuses with the membrane so that the contents are released into the golgi's lumen
- The golgi further modifies the protein by adding lipids or polysaccharides
- The golgi forms another vesicle around the contents and to migrates to the cell membrane
- The cell membrane and the vesicle fuses together to secrete the contents out of the cells in a process celled exocytosis
The weak hydrophobic interaction between the phospholipids of the membrane allow it to change shape, break and reform. This weak interaction allows specific processes that allow the exchange of materials between the membrane, its organelles and the environment
- Endocytosis: large molecules enter the cell without travelling across plasma membrane. A dip in the membrane forms around the molecule to form a vesicle
- Phagocytosis: solid substances are ingested, usually transported to the lysosome to be broken down
- Pinocytosis: liquids and solutions are ingested
- Exocytosis: large substances leave the cell without travelling across plasma membrane. Vesicles fuse with the cell to expel the contents into the external environment.
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