The Plasma Membrane and Transport across it

Cell Membrane Controls what enters and leaves the cell Like water, nutrients and waste The membrane is Selectively Permeable Membrane selects which molecules can permeate (pass through) into the cell. Like a window screen – allows what you want in and keeps others out.

Structure of Plasma Membrane Lipid Bilayer (2 layers of phospholipids) Lipids with phosphate “heads” attached Head of phospholipid is polar Hydrophilic (points outward, likes watery environment) Fatty acids (tails) are nonpolar Hydrophobic (points inward, avoids water)

Other Membrane Molecules Membrane Proteins - (transport) Allow substances/waste to move in and out of the cell. Creates a tunnel that can be closed or open. Carbohydrates/ Other Proteins- Stick out of cell surface (id card for other cells to see)

Cell Membrane Structures Carb/Protein (ID Cards) Membrane Proteins (tunnels) Fatty Acid Tails Polar Heads

How stuff gets into the cell 2 ways cell gets nutrients it needs and gets rid of waste Passive Transport Requires no energy Active Transport Requires an input of energy

Passive Transport Occurs due to concentration gradient Molecules naturally move from areas of higher concentration to lower concentration (no energy required) Diffusion – natural movement of molecules Facilitated Diffusion – through protein channels Osmosis – diffusion of water molecules

Diffusion In nature, molecules ALWAYS move from areas of higher concentration to areas of lower concentration, as is shown in this picture. They move to reach equilibrium This also happens across the cell membrane! These odor molecules are diffusing from the bottle (area of high concentration) to the surrounding air (lower concentration)

Facilitated Diffusion Diffusion of molecules through protein channels in the cell membrane Used for substances that the cell needs but cannot pass through the membrane Large molecules Charged (ions)

Osmosis The diffusion of water molecules is called osmosis Just like diffusion Water molecules move from areas of higher conc. to areas of lower water conc. They will move until there are equal concentrations of water (and solute) molecules everywhere Called equilibrium Molecules still diffuse, but at equal rates

Osmosis The movement of water molecules across a membrane Water moves from areas of high water conc. to areas of lower water conc. This is how cells maintain a stable internal environment Homeostasis – reacting to environment Blue – water molecules, easily pass through membrane Red – large (or charged) molecules, can’t pass through membrane

Passive Transport Video

Isotonic Solutions - “Iso” equal Cells that are in equilibrium with the surrounding solution do not experience osmosis. These cells have the same concentration of solute (and water) as the surrounding solution

Hypotonic Solutions – “hypo” lower [solute] (solute concentration) is lower outside the cell More water outside, so water moves into the cell, to try to reach equilibrium The cell swells with the extra water. Animal cells may swell so much, they may burst (especially in pure water) Plant cells – cell swells and membrane pushes against the cell wall Like celery in water – crisp/ rigid from pressure on cell walls

Hypertonic Solutions - “hyper” higher [solute] is higher outside of the cell. Less water outside the cell, more inside. Water moves out of the cell. Animal cells shrivel because of water loss. Plant cell membranes move away from cell wall as water moves out. Why plants wilt.

Other Forms of Transport Active Transport – (needs energy) Large particle transport Endocytosis – cell surrounds molecule(s), which are then engulfed by cell doesn’t pass through membrane Exocytosis - expulsion/secretion of materials. Opposite of endocytosis

Active Transport (con’t) Movement of molecules against (up) the concentration gradient From areas of lower concentration to areas of higher concentration For instance, if a cell needs a high concentration of something (that would normally diffuse OUT of cell)

Summary!