CELL TRANSPORTATION Cell membranes are selectively permeable, controlling the entrance and exit of all nutrients, wastes and other molecules in order to maintain homeostasis.

SOLUTIONS Everything in an organism is in solution. A solution is always made of a solute dissolved in a solvent. In a cell the solvent is always water. The main cell solutes are sugars, salts, ions like Na +, Cl -, K +, or the energy molecule ATP.

CONCENTRATION GRADIENTS In a solution in a bottle the solute mixes evenly in the solvent. Often there is a concentration gradient maintained by the cell. This means that the cell must do work to ensure that there is more solute on one side of the cell membrane. For example, more Na + outside the cell is crucial for nerve cell function.

FOUR MAIN FACTORS There are four main factors influencing how a solute molecule is moved across a cell membrane: 1. The size of the molecule - small molecules move more easily 2. The charge of the molecule - neutral molecules can pass through the phospholipid bilayer more easily 3. The concentration gradient of the solute molecule across the cell membrane - it is always easiest to move a solute molecule from an area of high concentration to low concentration 4. Distance - it is harder for a molecule to move to the inside of a large cell - in a small cell the area > volume of the cell - in a large cell the area < volume of the cell

Three Main Transport Methods There are also three main methods for moving molecules across a cell membrane: 1. Passive Transport which does not need any energy 2. Active Transport which requires energy 3. Bulk Membrane Transport

LE 7-17 Diffusion Facilitated diffusion Passive transport ATP Active transport

1. Passive Transport A. Diffusion – the movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. Ex. Perfume in a room

1. Passive Transport B. Osmosis – the movement of water molecules from an area of high water concentration to an area of low water concentration across a semi-permeable cell membrane. This is a very special type of diffusion.

1. Passive Transport C. Facilitated Diffusion – the movement of molecules across a semi-permeable cell membrane from an area of high concentration to low concentration using a protein the carrier protein glucose permease is the right shape to enable a large glucose molecule to move from an area of high glucose concentration to low the glucose can move either in or out of the cell a channel protein allows ions to pass the semi- permeable cell membrane

Facilitated Diffusion: Carrier Protein Carrier protein Solute A carrier protein, glucose permease, facilitates diffusion of glucose.

Facilitated Diffusion: Channel Protein EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein facilitates diffusion of ions like H +, Ca +2

2. ACTIVE TRANPORT molecules are moved against their concentration gradient this requires the cell to supply energy the energy can be supplied by a molecule of adenosine triphosphate (ATP) which is hydrolysed to adenosine diphosphate (ADP) and phosphate, releasing energy sometimes the energy is supplied by an electrical gradient if ions are involved for example the sodium-potassium pump

3. BULK MEMBRANE TRANSPORT A. Endocytosis - the cell membrane folds inwards trapping some extra-cellular material and forming a vesicle - the vesicle breaks from the cell wall and the contents are processed by the cell I. Pinocytosis - also called “cell drinking” - occurs when a small vesicle is formed - only fluid and small dissolved particles are ingested II. Phagocytosis - also called “cell eating” - a larger vesicle forms around a small particle of matter like a bacteria cell - used by many protists to obtain food -sometimes specific receptor molecules control what particles are engulfed. Ex. cholesterol

Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). 0.5 µm Vesicle PINOCYTOSIS

CYTOPLASM Pseudopodium “Food” or other particle EXTRACELLULAR FLUID Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) Pseudopodium of amoeba 1 µm Food vacuole PHAGOCYTOSIS

LE 7-20c Receptor RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coated pit Coated vesicle Coat protein Coat protein Plasma membrane 0.25 µm A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs).

Bulk Membrane Transport B. Exocytosis this is the reverse of endocytosis and is used when the cell wants to export material a vesicle containing material produced in the cell fuses with the cell membrane the cell contents are expelled into the extra- cellular fluid especially important for hormones that are transported in the blood like insulin from the pancreas

Exocytosis