The cell membrane is referred to as a ‘fluid mosaic model’ because the protein part within the cell membrane used to be though of as an even layer spread over the outside and the inside of the phospholipid. Now we are starting to think that it is spread unevenly, more like a mosaic than a layer. The phospholipid part of the cell membrane is fluid; this means that its molecules are constantly moving about. Through the molecules constantly moving about it allows for things such as ‘transient gaps’ to occur, these are gaps within the phospholipids which allow molecules to pass through; they are only temporary. Here is a picture of the fluid mosaic model: [pic]
Both water-soluble (hydrophilic) and lipid soluble (hydrophobic) substances are able to pass across the cell membrane. It is easier for lipid-soluble compounds to pass relatively quickly through by dissolving in the lipid layer. Water needs to pass through via osmosis where as water-soluble substances cross the membrane through simple diffusion, facilitated diffusion and active transport.
Osmosis is thought of as the diffusion of water from an area of high concentration of water molecules to an area of low concentration, across a partially permeable membrane. To define osmosis more accurately we define it in terms of water potential. Water potential is a measure of how easy it is for water molecules to move. Diffusion occurs because substances attract a ‘cloud’ of polar water molecules around them. The cloud is held by weak chemical bonds, including hydrogen bonds; this means that these water molecules cannot move freely. This is a picture of osmosis:
Inside cells some solutes have a higher concentration inside than outside of the cell, this means in order to make it equal they need to move across the cell membrane against the concentration gradient. This means that they cannot get in by passive transport; passive transport is...