MEMBRANE PROTEINS
Membrane proteins - Proteins attached to the cell membrane
1) INTEGRAL - Mostly permanent structures; found through the membrane.
2) PERIPHERAL - Temporary structures; found on only one side of the membrane.
4 Functions of Membrane Proteins
1) Transport
2) Receptors
3) Membrane bound enzymes
4) Support
TRANSPORT ON A CELLULAR LEVEL
SIMPLE TRANSPORT
ACTIVE
PASSIVE
DIFFUSION
OSMOSIS
FACILITATED DIFFUSION
BULK TRANSPORT
(vesicles are formed; temporary vacuoles in animal cells)
PINOCYTOSIS
(liquids)
ENDOCYTOSIS (goes in)
(solids)
EXOCYTOSIS (goes out)
(solids)
Transport Proteins
Help maintain the concentration of nutrients and ions inside the cell.
1. Channel Proteins (Porins) - Help in facilitated diffusion for ions and polar bodies.
2. Ping Pong Proteins (Carrier Proteins) - Diffusion of larger molecules, binding to proteins causes change in shape (e.g. glucose molecule)
3. Pump Proteins - Use ATP to pump ions and molecules against concentration gradient (active transport - Sodium and Potassium Pump).
Channel Ping pong Pump
Figure 1. How a sodium-potassium pump works
1. The sodium-potassium pump binds three sodium ions and a molecule of ATP.
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2. The splitting of ATP provides energy to charge the shape of a channel. Sodium ions diffuse out.
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3. Two potassium ions bind to protein.
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4. Release of phosphate ion allows the channel to revert to its original form, releasing potassium ions on inside of membrane.
Receptor Proteins
Helps cell interact with organelles and with surrounding cells, also help maintain homeostasis by controlling expression of proteins within cell.
1. Recognition
2. Signalling
3. Chemical cascades
4. Hormone receptors
Figure 1. How a G-Protein coupled receptor works.
Membrane Bound Enzymes
- Produce different substances required by cell for internal and external secretions.
- Control secondary signal production along with receptor proteins.
E.x. Cyclin Dependent Kinase & ATP Dependent Kinase
Support
1) Cell Stability - Attach to integument tissue and surrounding cells to help maintain the position of the cell and also helps in cell to cell adhesion.
2) Organelle Stability - Connect with micro-filaments within the cell to help maintain the internal positioning of the organelle, also attach to microtubules for locomotion.