Cell Membrane Structure
(the fluid mosaic model)
Structure and Function
The roles of the membrane;
- to provide attatchment sites for enzymes in metabolic reactions
- important in cell signaling
- to control what passes in and out of the cell
- allows the cell to change shape
Cholestrol; Helps to maintain the fluidity of the membrane, preventing it from becoming too stiff in low tempretures or too fluid in high tempretures
Channel Proteins; create pathways to allow larger or charged particles to pass through the membrane
Carrier Proteins; transport material into the cell that is too large or charged, they have a selective channel that moves to transport the material into the cell using ATP
Glycoprotiens and Glycolipids; act as receptors in cell signaling allowing communication between cells
Phospholipid bilayer; made up of phospholipids (fatty acids, glycerol, phosphate group) with heads (hydrophilic) and tails (hydrophobic)
Singals arrive from outside the cell (eg hormone) at the membrane receptors picks up the signal and brings about action within the cell.
There are three types of cell signalling;
Receptor acts as an ion channel; the signal is a chemical that attaches to a protein or gylcoprotein acting as an ion channel. When the chemical attaches to the receptor it makes the channel open and let the ions into the cell bringing about a response.
Receptor activates a G-Protein; the receptor in the plasma membrane interacts with another molecule a G-Protein, the signal molecule attaches to the receptor, the G-Protein is activated, which activates an enzyme bringing about action within the cell
Receptor acts as an enzyme; when the signal molecule arrives it slots into both parts of the receptor connecting them to one another, and forming an active enzyme.
Cell Signalling Diagram
(the mechanism of cell signalling)
Diffusion and Facilitated Diffusion
Diffusion; the net movement of of molecules or ions down thier concentration gradient, from a place of high concentration to a place of low concentration.
Facilitated diffusion; the diffusion of a substance across a cell membrane through a protein channel, an active process that requires ATP
each channel formed by a protein will only allow a specific ion or molecule to pass through, the protein can change its shape to be either open or closed (eg in cell signalling)
'the diffusion of water from an area of water from an area with high water potential to an area of low water potential, across a partically permeable membrane'
High water potential is usually caused by an abundance of another substance (a solution of sugar would have a negative water potential). Water moves down the water potential gradient (high Ψ to low Ψ)
Animal cells can become 'crenated' if the water potential inside the cell is higher than the water potential of the surround cell or alternativly it can undergo 'lysis' where the cell swells and bursts if the water potential outside the cell is much higher than inside it.
Plant cells in a solution that is more concentrated than the cell solution become 'plasmalysed', water diffues out of the cell, the contents of the cell do not push out on the walls and is said to be flaccid, the membrane is pulled away from the wall and becomes plasmalysed, some parts of the membrane do not pull away from the wall - if tears occur the cell will die. Cells in a less concentrated solution become 'turgid', water diffuses into the cell and the contents expand pushing against the membrane exerting a force known as pressure potential, when the cell reaches equlibrium it is fully turgid.
(the effects of osmosis on animal and plant cells)
Active transport; the movement of molecules or ions through transport proteins across a cell membrane against the concentration gradient, usuing ATP
For when a cell needs to take up or get rid of a substance, which goes against its concentration gradient (eg sodium and potassium).
Most cells need a higher concentration of potassium and a lower concentration of sodium ions inside the cell than outside, so the cell constantly pumps sodium out and potassium in, up thier concentraion gradients.
ATP is used to change the shape of transporter proteins, which moves three sodium out and teo potassium ions in.
Excoytosis and Endocytosis (BULK TRANSPORT)
- vesticle moves towards the plasma membrane
- vesticle joins with plasma membrane
- vesticle contents released - the vesticle membrane is now part of the plasma membrane
- the cell spreads around an object or area of the solution outside of the cell
- the area becomes enclosed in a vesticle
- the contents of the vesticle are absorbed into the cytoplasm and the membrane is recyled
(eg a phagocyte englulfs a bacterium) the fingers of cytoplasm extend around the bacterium and fuse together to form the vesticle