lecture 2-2 (cell transport)

• Cells communicate via signals that originate within or external to the cell which are recognized by the cell via membrane-bound receptors or receptors found within the cell.
• Label this cell communication diagram.

  A) reception

  B) transduction

  C) response

  D) signalling molecule

  E) receptor

  F) signal transduction

  G) relay

  H) activation

  I) cellular

  J) response

  
• Induction of a signal cascade stimulated by a receptor molecule requires energy (ATP) from the cell. An example of this is when epinephrine binds to a receptor on the cell membrane.

  
• Signal molecule can interact with protein kinases in the cytoplasm through receptors

  
• A signal cascade is a chain reaction involving many different proteins, starting with a signal molecule which can be hormones or growth factors. For example, testosterone is recognized by a receptor protein in the cytoplasm and forms a hormone-receptor complex to interact with the DNA in the nucleus. This stimulates the production of a new protein in the cytoplasm.
• A signal transduction pathway is a series that involves many proteins, such as kinases, that occur before a cellular response occurs.
• Testosterone can bind to the receptor that triggers genes to be expressed in the cytoplasm.
• Epinephrine can interact with transduction kinases as well as activation of enzymes before a cellular response occurs.
• Insulin activates kinases, which activate other signal molecules, which trigger metabolic pathways in the body.
• The plasma membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell.
• Endomembranes are the membranes that surround the cellular organelles.
• The endoplasmic reticulum and golgi apparatus are folded membranous structures that are involved in the synthesis and transport of proteins.
• The cell membrane is composed of a phospholipid bilayer, with a hydrophobic interior and a hydrophilic exterior.

  
• Cholesterol can be found in the hydrophobic interior of phospholipid bilayer to reduce membrane fluidity of animal cells and allowing the membrane to function under a broader range of temperatures. They are not found in plant cells.
• The plasma membrane is fluid and not rigid, containing phospholipids, proteins, and carbohydrates and glycoproteins. It can be called the "fluid mosaic" model as it is fluid and contains both lipids and proteins.
• An amphipathic molecule is a molecule that has both hydrophilic and hydrophobic regions.
• There are two types of proteins found on the membranes: integral and peripheral. Integral proteins are amphipathic and typically serve as transport proteins such as carriers, channels and ion pumps. Peripheral proteins are non-amphipathic.

  
• Carbohydrates on the cell membranes are oligosaccharides. Oligosaccharides can be involved in cell-to-cell signalling or recognition. They also determine a person's blood type.
• Hydrophobic molecules can move easily through the membrane. Neutral molecules such as oxygen and carbon dioxide can move easily. Water has limited movement and needs specialized channels. Organic molecules and sugar cannot get through the membrane. Ions such as sodium, potassium, calcium cannot get through. Proteins and other large molecules cannot get through.

  
• Passive transport is the movement of substances across the membrane without the need for energy input. Goes down the concentration gradient.
• Active transport requires energy input and goes against the concentration gradient.
• Diffusion: movement of molecules down a concentration gradient to occupy available space until equlibrium is reached
• Osmosis: movement of water from a region of low solute to a region of high solute
• Facilitated diffusion. Transport proteins in the cell membrane provide channels through which molecules can move. This occurs along a concentration gradient (high to low). The proteins are called channel proteins or carrier proteins.

  ex. Potassium ion channels, water channels, glucose transporters
• Active transport occurs against a concentration gradient (low to high). This requires energy in the form of ATP. The proteins are called transport proteins. Ex: proton pump and co-transport and the sodium-potassium pump.
• A proton pump causes protons (hydrogen ions) to be moved from a low concentration inside the cell to the outside where there is a higher concentration. This creates a positive charge outside the cell and a negative charge inside (differential charge).

  
• Sodium inside the cell can build up to toxic levels and has to be taken out. To balance the ionic charge, potassium ions are taken in. This occurs against a gradient, where sodium may already be high outside the cell. To maintain the negative charge inside the cell, three of Na+ leaving the cell is balanced by two K+ entering the cell.
• Peripheral proteins help stabilize and shape the plasma membrane. Meanwhile, integral proteins determine a membrane's specific functions. Example of integral protein functions are: channel, carrier, cell recognition, receptor and enzymes.
• Exocytosis: a process by which the contents of a cell vacuole are released to the exterior through fusion of the vacuole membrane with the cell membrane.
• Endocytosis: the taking in of matter by a living cell by invagination of its membrane. Examples of which include phagocytosis, pinocytosis, and receptor-mediated endocytosis.
• If membrane receptors or protein channels don't work, it can result in high cholesterol or cystic fibrosis. Cystic fibrosis is caused by chloride transport proteins in the cell membrane not functioning. It is also caused by a double recessive mutation.
• The build up chloride caused by malfunctioning membranes due to cystic fibrosis can cause a build up of mucus in lungs, leading to respiratory problems.