Transport Across The Membrane

Transport Across The Membrane

PRESCRIBED LEARNING OUTCOMES (Chapter 4, pp. 65-76) G1. Apply knowledge of organic molecules to explain the structure and function of the fluid-mosaic model G2. Explain why the cell membrane is described as “selectively permeable” G3. Compare and contrast the following: diffusion, facilitated transport, osmosis, and active transport. G4. Explain factors that affect the rate of diffusion across a cell membrane. G5. Describe endocytosis, including phagocytosis and pinocytosis, and contrast it with exocytosis. G6. Predict the effects of hypertonic, isotonic, and hypotonic environments on animal cells G7. Demonstrate an understanding of the relationship and significance of surface area to volume, with reference to cell size.

VOCABULARY _____ Osmotic pressure _____ Active transport _____ Phagocytosis _____ Phospholipid _____ Pinocytosis _____ Polarity _____ Protein channels _____ Receptor sites _____ Selectively permeable _____ Solubility _____ Solute _____ Solvent _____ Surface area/volume ratio _____ Thyroid gland _____ Thyroxin _____ Tonicity _____ Turgor pressure _____ Villi _____ Viscosity _____ Active transport _____ ATP energy _____ Blood antigen _____ Concentration gradient _____ Crenate _____ Diffusion _____ Endocytosis _____ Exocytosis _____ Facilitated transport _____ Glycolipid _____ Glycoprotein _____ Hemolysis _____ Hypertonic _____ Hypotonic _____ Isotonic _____ Na/K pump _____ Neuron _____ Osmosis

PLASMA MEMBRANE SELECTIVELY PERMEABLE: Controls what comes in and out of the cell. Does not let large, charged or polar things through without help.

FLUID MOSAIC MODEL: The phospholipids move, thus allowing small non-polar molecules to slip through.

PLASMA MEMBRANE GLYCOLIPIDS and GLYCOPROTEINS: Act as receptors – receive info. from body to tell cell what to do.

PLASMA MEMBRANE INTEGRAL PROTEINS: assists specific larger and charged molecules to move in and out of the cell. Can act as ‘tunnels’ or will change shape.

PLASMA MEMBRANE CHOLESTEROL: Reduces membrane fluidity by reducing phospholipid movement. Also stops the membrane from becoming solid at room temperatures.

PLASMA MEMBRANE CYTOSKELETON: A cytoskeleton acts as a framework that gives the cell it's shape. It also serves as a monorail to transport organelles around the cell.

TRANSPORT ACROSS THE MEMBRANE Everything that is transported across the cell membrane takes place by one of two fundamental processes: Passive transport moves molecules from a [high] to [low] in order to establish equilibrium. The molecules may or may not need to use a protein channel or carrier.

2. Active transport moves molecules from [low] to [high], AGAINST the concentration gradient and this process requires energy in the form of ATP.

SIMPLE DIFFUSION

SIMPLE DIFFUSION Simple Diffusion is a passive process ( no energy required). Some substances will diffuse through membranes as if the membranes weren’t even there. Molecules diffuse until they are evenly distributed. The molecules move from an area of [high] to [low]. EXAMPLES of molecules that easily cross cell membranes by simple diffusion are: oxygen, carbon dioxide, alcohols, fatty acids, glycerol, and urea.

towards equilibrium

Fatty Acids, Glycerol, Alcohol & Urea

SIMPLE DIFFUSION The rate of diffusion will be increased when there is : Concentration: the difference in [ ] between two areas (the [ ] gradient) causes diffusion. The greater the difference in concentration, the faster the diffusion. Molecular size: smaller substances diffuse more quickly. Large molecules (such as starches and proteins) simply cannot diffuse through. 3. Shape of Ion/Molecule: a substance’s shape may prevent it from diffusing rapidly, where others may have a shape that aids their diffusion.

SIMPLE DIFFUSION 4. Viscosity of the Medium: the lower the viscosity, the more slowly molecules can move through it. Movement of the Medium: currents will aid diffusion. Like the wind in air, cytoplasmic steaming (constant movement of the cytoplasm) will aid diffusion in the cell. Solubility: lipid - soluble molecules will dissolve through the phospholipid bilayer easily, as will gases like CO2 and O2. 7. Polarity: water will diffuse, but because of its polarity, it will not pass through the non-polar phospholipids. Instead, water passes though specialized protein ion channels.

where is diffusion important?

simple/passive diffusion facilitated diffusion

OSMOSIS

OSMOSIS Osmosis is the diffusion of water across a selectively permeable membrane driven by a difference in the concentration of solutes on the two sides of the membrane. A selectively permeable membrane is one that allows unrestricted passage of water, but not solute molecules or ions. So it is the WATER THAT MOVES to create equilibrium!!!

OSMOSIS Osmosis requires NO ENERGY. Osmosis is the net movement of WATER molecules from the area of [high] of water to the area of [low] of water until it is equally distributed. Because membranes often restrict or prevent the movement of some molecules, particularly large ones, the water (solvent) must be the one to move.

To cross the membrane, water must move through a protein ion channel. In certain cellular conditions, these protein channels can be opened or closed (ie: in the kidneys, large intestines) depending on how much water is needed by the body.

just remember that ...

TONICITY OF A SOLUTION ISOTONIC SOLUTION: The tonicity of a solution will affect the size & shape of cells: ISOTONIC SOLUTION: the solution concentration is equal on both sides of the membrane . There is no net concentration difference across the cell membrane Water moves back and forth, but there is no net gain or loss of water.

the cell stays the same

TONICITY OF A SOLUTION HYPERTONIC SOLUTION: The solution outside the cell is more concentrated than inside. There is more water inside the cell and the water will move out of the cell. This causes the cell to shrink *Memory Trick... Hyper people get skinny!

                                                                                                                                                        the cell shrinks

TONICITY OF A SOLUTION HYPOTONIC SOLUTION: The concentration inside the cell is more concentrated than outside. Therefore there is more water outside of the cell, and water will move into the cell. This will cause the cell to swell. *Memory Trick... Hippos are FAT!

the cell expands or bursts

hyper, hypo, and isotonic

OSMOSIS Hypertonic Isotonic Hypotonic In Biology we usually talk about the SOLUTION’S tonicity, NOT the cells!

MEMORY TRICK: If you eat a lot of sugar (ie: solute) you get HYPER *MEMORY TRICK: If you eat a lot of sugar (ie: solute) you get HYPER. The solution with a lot of solute is called HYPEROSMOTIC.

water always moves towards HYPER-OSMOTIC HYPEROSMOTIC = MORE [solutes]'d

hypotonic sol'n isotonic sol'n hypertonic sol'n

hyper, hypo, or isotonic solution? hypertonic causes CRENATION

hyper, hypo, or isotonic solution? hypotonic causes HEMOLYSIS

hyper, hypo, or isotonic solution?

When we speak of plants, we speak of TURGOR PRESSURE. Which of these cells is in a hypertonic solution? Hypotonic?

where is osmosis important?

FACILITATED DIFFUSION

FACILITATED DIFFUSION Facilitated Transport: Some molecules are not normally able to pass through the lipid membrane, and need channel or carrier proteins to help them move across. This does not require energy when moving from [H] to [L] (with the concentration gradient). Molecules that need help to move through the plasma membrane are either charged, polar, or too large.

EXAMPLES: sugars, amino acids, ions, nucleotides …. If molecules are POLAR, CHARGED, or TOO LARGE they need a protein the help them across the membrane EXAMPLES: sugars, amino acids, ions, nucleotides ….

The protein channels and carriers are very SPECIFIC Each protein channel or protein carrier will allow only ONE TYPE OF MOLECULE to pass through it.

Many channels contain a “gate” which control the channel's permeability. When the gate is open, the channel transports, and when the gate is closed, the channel is closed. These gates are extremely important in the nerve cells.

where is facilitated transport important?

ACTIVE TRANSPORT

ACTIVE TRANSPORT Active Transport: the movement of polar, large, and charged molecules moving against the [ ] gradient (uphill). EXAMPLES of molecules that move this way are all of the things that require protein carriers to move across the plasma membrane. ions (like Na+ and K+ in cells, and iodine) and sugars, amino acids, nucleotides...

requires

LOW [ ] TO HIGH

EXAMPLES OF ACTIVE TRANSPORT Example 1: the thyroid gland accumulates iodine as it is needed to manufacture the hormone thyroxin. The iodine concentration can be as much as 25 times more concentrated in the thyroid than in blood.

GOITER

EXAMPLES OF ACTIVE TRANSPORT Example 2: a Na/K pump (mostly in nerve membranes). These function to restore electrical order in a nerve after an impulse has traveled along it. the Na/K Pump

EXAMPLES OF ACTIVE TRANSPORT Example 3: In order to make ATP in the mitochondria, a proton pump (hydrogen ion) is required.

where is active transport important?

ENDOCYTOSIS & EXOCYTOSIS

requires

ENDOCYTOSIS Endocytosis: (“Endo” means “in”). Endocytosis is the taking in of molecules or particles by invagination of the cell membrane forming a vesicle. This requires energy.

ENDOCYTOSIS There are two types of endocytosis: 1. pinocytosis (cell drinking): small molecules are ingested and a vesicle is immediately formed. This is seen in small intestine cells (villi) 2. phagocytosis (cell eating): large particles, (visible with light microscope) are invaginated into the cell (ie: white blood cells ‘eat’ bacteria).

phagocytosis

EXOCYTOSIS Exocytosis: (“Exo” means “out”.) Exocytosis is the reverse of endocytosis. This is where a cell releases the contents of a vesicle outside of the cell. These contents may be wastes, proteins, hormones, or some other product for secretion. This also requires energy. Example: vesicles from the Golgi fuse with the plasma membrane and the proteins are released outside of the cell.

CELL SIZE Cells exist individually, or in groups forming tissues, organs, and eventually organisms. Their size is a very important factor for their survival. Cells are the size they are because that is the size they have to be in order to function effectively. The materials it needs to use, and the wastes it needs to remove must pass in and out of the cell through the cell membrane.

CELL SIZE As a cell grows, it suffers the possibility of the volume increasing faster than the surface area. The bigger the cell, the more it needs and the more it needs to get rid of. If the cell gets too big, there will not be enough room on the plasma membrane for things to get in and out quickly enough to maintain the cell. Why? Its surface area has not kept up to its volume size. When the volume of a cell increases, the amount of surface area does not increase in the same proportion.

CELL SIZE When cells grow larger, if they don’t want to die, the only real solution is to divide. Once a cell has divided, the surface area to volume ratio will increase and the problem is solved.

*When you see questions regarding surface area to volume ratio, think of a water balloon. (ie: when you squish a water balloon, it has the same volume, but the surface area is increased!) versus versus