Cell Membrane (3)

Transport of Substances in & out of Cells across Membrane
Passive Transport

• Does not require ATP
• Driven by concentration gradient (concentration difference) – moving down its concentration 
• Occurs in both living and non-living systems
• Mechanism
• Simple Diffusion / Diffusion (Oxygen, CO₂, minerals in soil etc)
• Types of substances that undergo diffusion are:
• Small, hydrophobic
• Lipid, soluble
• Small, hydrophilic (slower)
• Diffusion
• Spread of particles through random motion from regions of higher concentration to regions of lower concentration
• Simple Diffusion
• Hydrophilic molecules diffuse into cells very slowly, because of repulsion faced in the hydrophobic interior core of the lipid bilayer
•       Osmosis (water only)
•        Net movement of water molecules across a selectively permeable membrane from an area of less negative (high) water potential, to an area of more negative (low) water potential
•       Facilitated Diffusion (carrier proteins and pores which increase diffusion rate)
•        Type 1: Transmembrane proteins (Changes shape to facilitate entry and exit of some nutrients, e.g. fructose) create a water-filled pore through which ions and small hydrophilic molecules can pass by diffusion. Channel protein can open and close like gates
  Some molecules are too large for ion channel (e.g. amino acid, glucose etc)
•        Type 2: Some molecules are too large for ion channel (e.g. amino acids, glucose). Solution = carriers. Carrier protein has a specific binding site. Once substance binds-> conformation change of protein occurs. Move substance across. Release and protein returns back to original conformation

Active Transport

• Requires the use of energy (ATP) (E.g. Proton pump)
• Substances are transported against concentration gradient by specific proteins
• Occurs only in living systems
• Transport is specific-- carrier can only recognize certain molecules / ions
• Glucose and amino acids taken in by active transport to the villi cells of small intestine where there is lower concentration of these molecules in the lumen of the small intestines than the blood capillaries 

Endocytosis (Bulk Transport)

• Phagocytosis
• Formation of pseudopodia observed
• Pinocytosis
• Invagination of cell membrane

Exocytosis (Bulk Transport)

• Where secretory vesicles fuse with cell surface membrane
• E.g. Secretion of extracellular enzymes, hormones and antibodies, removal of waste products of digestion
• Are Endocytosis and Exocytosis a form of Active Transport?
• Yes. They require ATP and go against the concentration gradient.

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