Chapter 4: Cell membranes and transport

Micelles - Stable ball-like structures in the water, hydrophilic tails project inwards away from the water 

Fluid mosaic model 

• Fluid - Phospholipids move sideways 
• Mosaic - Pattern produced by the scattered protein molecules 

Fluidity depends on: 

1. Unsaturated ↑, fluid ↑
2. Longer the tail, fluid ↓  
3. Temperature ↓, fluid ↓ 
• When temperature decreases, cholesterol intercalates between phospholipid and prevents them from moving around 

Ion transportation 

• Unable to pass through the hydrophobic core of the phospholipid bilayer. 
• Lined with amino acids with hydrophilic R group 

Aquaporin 

• Water is polar, so it is unable to pass through the hydrophobic part of the phospholipid tail. 
• Channel proteins are hydrophilic
• Aqauporins increase permeability of membrane to water 

Roles of the components of cell membrane 

Phospholipids 

• Forms a barrier to charged particles 
• Can modify chemically to act as signalling molecules 
• Activate other enzymes 

Cholesterol 

• When temperature is too low, it prevents from becoming too rigid 
• Prevents close-packing of the tails 
• The hydrophobic region of cholesterol prevents myelin sheath leaking of ions through the membrane 

Glycolipids, glycoproteins and proteins 

• Cell to cell recognition + have own set of glycoprotein on the cell surfaces 
• Recognition sites: 
• Acts as receptor molecules 
• Receptor - specific shape which allows them to bind to other molecules with complementary shapes 

Cell signalling 

• Getting message from one place to another 
• Hormones as chemical messenger circulates in the blood and bind to specific target cells with correct receptor sites 
• Cell adhesion - firmly attach to adjacent cells to one another 
• Cells have receptors that bind to signalling molecule and initiate physiological response 
• Regulatory sequence - a segment of DNA that is capable of controlling the expression (production) of specific genes 

Steroids - thyroid hormones, testosterone, oestrogen and progesterone 

1.  Hormones bind to receptor molecules inside target cells. Easily pass through the phospholipid bilayer since they are lipid soluble 
2. Hormone binds to receptor, carries hormone into nucleus
3. Attaches to a specific gene 
4. Genes carry instructions for the manufacture of specific polypeptide chain 
5. Hormone-receptor complex attaches to DNA and alters the protein 

Adrenaline, glucagon and insulin 

• Signal-transduction mechanism 
• Signalling molecule attaches to the receptor protein 
• 2nd messenger transmit signal and change takes place 
• Hormone-receptor complex activates a protein in the membrane 
• G-protein acts as a relay between the complex and the enzyme 
• Adenyl cyclase catalyse ATP to cAMP (2nd messenger)
• Chain reaction ends when enzyme catalyses the breakdown of glycogen to glucose phosphate. 

Movement of substances into and out of cells 

Diffusion 

• The net movement of molecules from high to low concentration down a gradient, as a result of the random movements of the particle 
• Depends on:
• Steepness of concentration gradient 
• Temperature ↑ KE ↑ random movement ↑
• Surface area ↑ Faster 
• Size and nature of the diffusing molecules 
• ↓ smaller molecules, diffusion ↑
• Non-polar, small size 

Facilitated diffusion 

• Diffusion through transport proteins, the proteins provide hydrophilic areas for ions to pass through along the concentration gradient (passive) 

Channel Protein	Carrier Protein
Fast transport	Slow
Pore facilitates solute transportation	Change in shape
Only allows passive	Active + passive
Cannot transport against concentration gradient	For active transport
Transport water molecules	Transport both water and insoluble molecules (Na+/K+ pump)

Osmosis 

• Net movement of particles from high to lower water potential, along water potential gradient through partially permeable membrane 
• Water potential - the tendency of water to move out of a solution 
• Addition of solutes to pure water lowers its water potential 

External solution	Higher (less negative) Hypotonic solution	Equal Isotonic	Lower (more negative) Hypertonic solution
Net movement of water	Enters cell	NO change	Leaves cell
State of cell	Swells and burst (Turgid)	No change	Shrink (Plasmolysis)

Incipient plasmolysis - about to occur 

Active transport 

• Movement of molecules through transport proteins against concentration gradient energy in the form of ATP
• Carrier protein 

Bulk transport 

• Endocytosis - engulfing material by the cell surface membrane 
• Phagocytosis (cell-eating)
• Intucking of cells to form cup-shaped depression which large bacterium is contained 
• Requires ATP 
• Pinocytosis (cell-drinking) 
• Exocytosis - bulk movement of material out of the cell