Transport Across the Membrane

The Cell Membrane This thin barrier controls traffic into and out of the cell. The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. The phospholipids and proteins in membranes are described as the fluid mosaic model. A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids. Mosaic: phospholipids, proteins, carbs (identification)

Fluid Mosaic Model In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water and the hydrophobic regions are in a nonaqueous environment.

Membranes are Fluid

Proteins Determine the Membrane’s Function

Transport Across the Membrane What needs to cross the cell membrane? What needs to enter? What needs to leave?

Passive Transport 3 Main Types Diffusion Osmosis (a type of diffusion) Facilitated Diffusion What do they all have in common? Ex. All diffuse down a concentration gradient

Some things pass directly through the phospholipids Hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily. Some water can also pass- why does this seem odd? These are examples of simple diffusion and osmosis

Osmosis Osmosis continues until the solutions are isotonic Differences in the relative concentration of solute in two solutions can lead to the movement of solutes from one to the other. The solution with the higher concentration of solutes is hypertonic. The solution with the lower concentration of solutes is hypotonic. These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water. Solutions with equal solute concentrations are isotonic. Osmosis continues until the solutions are isotonic

Osmosis

Facilitated Diffusion The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion. Many are channel proteins that allow fast transport, but others change shape to move molecules For example, water channel proteins, aquaprorins, facilitate massive amounts of diffusion.

Active Transport Some facilitated transport proteins can move solutes against their concentration gradient, from LOW to HIGH concentration. This active transport requires the cell to expend its own metabolic energy (in the form of ATP- adenosine triphosphate).

Protein Pumps These pumps store energy and help create potential energy that can be accessed for cellular work

Active Transport

Exocytosis and Endocytosis Allows for transport of large molecules into (endo) and out of (exo) the cell. During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane. Pinocytosis= cell drinking Phagocytosis= cell eating

Endocytosis

Exocytosis