2.3 cell requirements

• Extracellular fluid is the fluid that surrounds a cell. It allows for a continual exchange of materials into and out of cells.
• Extracellular fluids surround cells with a constant concentration, and keep their temperatures constant, maintaining homeostasis.
• Cells take in certain substances from the tissue fluid, while also producing materials that must be removed from the cell.
• During cellular respiration, glucose and oxygen are used to produce carbon dioxide, water and energy, which are removed from the cell.
• The currently accepted model for cell membrane structure is the fluid mosaic model.
• The membrane is said to be fluid as its molecules constantly change position, and mosaic as it is made up of many different kinds of molecules.
• The membrane is composed of phospholipid molecules (lipid molecules containing a phosphide group), which are arranged in two layers (bilayer).
• Each phospholipid molecule has a hydrophilic head and a hydrophobic tail.
• Phospholipids are arranged with their heads on the outside and tails on the inside. They drift from place to place with their heads and tails moving, keeping the membrane fluid.
• Cholesterol molecules are wedged between the phospholipids. They are essential for the function integrity and stability of the membrane.
• Functions of the cell membrane:

  • acts as a physical barrier: isolation of the cytoplasm from surrounding fluid is important as their compositions are very different.
  • regulates the passage of materials.
  • sensitive to changes: the cell membrane has receptors that are sensitive to particular molecules in its immediate environment (extracellular fluid).
  • helps support the cell: the cell membrane is attached to the microfilaments of the cytoskeleton, giving support to the whole cell. There are also connections to adjacent cells, giving support to the whole tissue.
• Cell membranes are differentially permeable, meaning they allow certain ions and molecules to pass through but restrict the movement of others.
• Materials may pass through a cell membrane through passive processes or active processes.
• Passive processes do not use energy.
• Active processes use the cell's energy in the form of adenosine triphosphate.
• Three basic processes result in transport of materials:

  • simple diffusion
  • facilitated transport
  • vesicular transport
• Simple diffusion is a passive process resulting from the random movement of ions and molecules.
• Facilitated transport is a transport mechanism that requires special proteins in the cell membrane, either carrier or channel proteins. Active or passive depending on the nature of the mechanism.
• Vesicular transport is an active process in which materials are moved in membrane-bound sacs (vesicles).
• Diffusion is the spreading out of particles so that they are evenly distributed over the space available.
• A diffusion gradient is the difference in concentration that brings about diffusion.
• Alcohol, steroids and other fat-soluble substances can easily enter cells as they can diffuse through the phospholipid bilayer.
• Oxygen diffuses through the phospholipid bilayer into the cell as it is continually used up inside the cell for respiration. Concentration of oxygen inside the cell is lower than concentration of oxygen outside of the cell, so there is net diffusion of oxygen into the cell.
• Carbon dioxide is continually produced inside the cell by respiration. The higher concentration of carbon dioxide inside the cell means that there is net diffusion of carbon dioxide out of the cell.
• Water-soluble substances are unable to pass directly through the lipid portion of the membrane.
• Osmosis is the diffusion of a solvent through a differentially permeable membrane in order to balance the concentration of another substance.
• Large polar molecules are unable to cross the cell membrane as they are repelled by the hydrophobic tails in the phospholipid bilayer.
• Water molecules are small enough to pass through the cell membrane, since they can fit between the lipid tails.
• In facilitated transport, proteins in the cell membrane allow molecules to be transported.
• To diffuse across a cell-membrane, water-soluble molecules must pass through protein channels in the membrane, allowing facilitated diffusion. Protein channels provide a pathway for the hydrophilic particles to cross the cell membrane without coming in contact with the hydrophobic inner portion.
• Carrier proteins are only open on one side of the cell membrane at a time. When a substance binds to the binding site within the protein, it changes shape and opens to the other side.
• Characteristics of carrier-mediated transport:

  • Carrier proteins are specific (only bond to certain molecules)
  • Carriers can become saturated (when all carriers are occupied, rate of movement cannot be increased)
  • Carrier activity is regulated by substances such as hormones (important in coordinating the activities of carrier proteins)
• Two types of carrier-mediated transport:

  • facilitated diffusion
  • active transport
• Facilitated diffusion occurs when substances are transported through a protein along the concentration gradient, from high to low concentration. Passive process as no energy input is required.
• Active transport requires energy in the form of ATP, as substances are transported against the concentration gradient. Similar to facilitated diffusion, but does not depend on a concentration gradient. Can take place regardless of concentration inside and outside cell.
• Vesicular transport is the movement of substances across a cell membrane in membraneous sacs (vesicles). An active process, as energy is needed to form the vesicles.
• Endocytosis is the process of taking liquids or solids into the cell by vesicular transport. The cell membrane encloses around a drop of liquid or solid, the vesicle pinches off and is suspended in the cytoplasm.
• Endocytosis of liquids is called pinocytosis.
• Endocytosis of solids is called phagocytosis.
• Exocytosis is when the contents of a vesicle inside a cell go outside. The vesicle inside the cell migrates and fuses with the cell membrane. The contents of the vesicle are pushed out into the extracellular fluid.