Cellular Transport

Why must a cell control materials moving into and out of itself? The survival of a cell depends on its ability to maintain proper conditions within itself A cell has to be able to maintain balance regardless of internal and external conditions The process of maintaining the cells environment is called homeostasis

Cell Membrane Structure What is it composed of? –2 layers of phospholipids What 3 macromolecules are found in the cell membrane? –Lipids, carbohydrates & proteins Cholesterol (not a macromolecule)

The phospholipid is composed of 3 main parts: –A charged phosphate group –Glycerol –2 fatty acid chains Because the head bears a charge it is polar –This means the head forms H bonds with water molecules (likes water –hydrophyllic) fatty acid tails are nonpolar –as a result they are attracted to each other and repel water (hates water – hydrophobic) * So if a bunch of phospholipids were dropped in a container of water they would always form a cell membrane like structure.

Cell membrane structure The cell membrane has 2 major functions: –Regulate what enters/leaves a cell –Provide support Cell membrane is selectively permeable (allows some things to cross, others cannot) –Small substances, like water, carbon dioxide and oxygen can pass through without help

Cell Membrane Structure Why is cholesterol necessary in the cell membrane? –It helps stabilize the phospholipids –Prevents the fatty acid chains of the phospholipids from sticking together –You want the plasma membrane to be flexible!! Cholesterol molecule

Cell Membrane Structure Proteins are also found within the membrane –Act as channels for bigger objects to pass through –They help create that selectively permeable membrane that regulates what enters and leaves the cell Lastly, Carbohydrates also are found within the membrane –They attach to membrane proteins and serve as identification tags –This allows cells to distinguish one type of cell from another cell membrane Proteins Carbohydrate chains Cholesterol Cytoskeleton

Description of the Cell Membrane Scientists have developed a model that describes the arrangement/movement of the cell membrane. The fluid mosaic model. –It describes the membrane as a flexible boundary of a cell (fluid) –Also, the variety of molecules making up the membrane is similar to the arrangement of mosaic tiles.

Movement of materials across the membrane

There are two types of transport (movement) of molecules across a cell membrane: –Passive Transport (no energy from the cell required to move molecules) Diffusion, Osmosis, Facilitated Diffusion –Active Transport (energy from the cell is required to move molecules) First let’s learn why molecules don’t stay in one place……..

Concentration Gradient Molecules have natural motion (kinetic energy), which causes them to collide and scatter. Molecules can be concentrated in high numbers or low numbers in certain areas

Concentration Gradient (cont..) Concentration gradient is the name for this difference in the concentration of a substance from one location to another. Molecules will always diffuse down their concentration gradient – that is, from a region of high concentration to low concentration.

Example: Dye molecules are initially at a high concentration where they are added to water. Random movements of the dye and water molecules cause them to bump into each other and mix – Thus the molecules are moving to an area of High to low concentration. Eventually, they are evenly spread throughout the solution. This is referred to as dynamic equilibrium. – Equilibrium – molecules same throughout solution – Dynamic – molecules still moving

Diffusion Movement of molecules OTHER THAN WATER across a the cell membrane NO ENERGY REQUIRED Moves from areas of HIGH concentration to areas of LOW concentration –Until dynamic equilibrium is reached ***Equilibrium does NOT mean that movement across the membrane STOPS

Dynamic Equilibrium L H L H = Dissolved Molecule

Osmosis Movement of WATER across a selectively permeable membrane (basically diffusion of water) NO ENERGY REQUIRED Water moves from HIGH concentration to LOW concentration until equilibrium is reached Cell membrane Higher Concentration of Water Water molecules Lower Concentration of Water Sugar molecules

3 types of osmosis solutions Isotonic Hypertonic Hypotonic

ISOTONIC Isotonic = “equal strength” solution –Same concentration of dissolved particles on both inside and outside of cell – Water molecules move into and out of the cell at an equal rate, so size remains constant.

HYPERTONIC Hypertonic = “above strength” solution –Higher concentration of dissolved particles in solution –So water molecules are more concentrated inside the cell –Thus water flows out of the cell, causing it to shrivel or even die.

HYPOTONIC Hypotonic = “below strength” solution –Lower concentration of dissolved particles in solution –So water molecules are more concentrated outside cell, causing water to rush into the cell. –Cell membrane could potentially expand until it bursts. Hyp- O - tonic

Recap

Osmosis Plant cells and bacteria have cell walls, so they are rigid and CAN’T burst for the most part unless the cell wall is not very strong Some cells can pump out water as it comes in

Facilitated Diffusion Many molecules are too large to cross via diffusion They have a SPECIFIC protein channel that lets them pass through the membrane, this is called Facilitated Diffusion. No energy required Moves from high to low concentration Ex: ions, sugars, salts

Facilitated Diffusion