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 TailsPolar 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

PassiveTransport

Occurs due to concentration gradientMolecules 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 membraneUsed for substances

that the cell needs but cannot pass through the membrane

Large molecules Charged (ions)

Osmosis The diffusion of water molecules is called

osmosisJust like diffusionWater 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 gradientFrom 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!