The Amazing Plasma Membrane Structure and Function

Overview of functions ► Container—holds the cytoplasm & organelles in ► Protective barrier ► Allows cell to cell communication ► Gatekeeper—regulates what goes in and out of the cell*

How??? ► How does it determine what should come in, what should go out? ► How does it determine how much of something should come in, go out? ► How does it let them in and out, while not letting other things in and out as well?

Structure ► Phospholipid bilayer What is a phospholipid? previously: lipid = glycerol + 3 fatty acids (long chains of C, H, O) (long chains of C, H, O) phospho = phosphate PO 4

Phospholipid properties Phosphate end: polar, hydrophilic Lipid end (now only 2 fatty acids): nonpolar, hydrophobic

Phospholipid bilayer Bi- = two ► Double layer of phospholipids ► Oriented so hydrophilic ends are near water and hydrophobic ends are away from water

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Another look

Membrane is semipermeable

But cells need those other molecules—so how do they get in? ► Proteins embedded in the phospholipid bilayer They act as channels and play other and play other important roles as well

FLUID- because individual phospholipids and proteins can move within the layer MOSAIC- because of the patchwork pattern produced by the scattered protein molecules FLUID MOSAIC MODEL

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Review ► What is the basic structure of the plasma membrane? ► Why does it form a bilayer? ► What does it mean to say the membrane is semipermeable (or selectively permeable)? ► So how does anything else get through? ► What is the fluid mosaic model?

Function Transport of materials across the membrane Passive transportActive transport No Energy Required!Energy Required!

Passive transport ► Diffusion ► Facilitated Diffusion ► Osmosis

Equilibrium (everything is balanced) Simple Diffusion

Cell membrane Solute moves DOWN concentration gradient from area of HIGH concentration (many molecules per mL) to area of LOW concentration (fewer molecules per mL) Diffusion through a membrane

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Random movement ► In diffusion, solute molecules move randomly, bumping into each other as they go. ► Through random movement, they spread out to every corner of the space. ► Equilibrium is reached when the molecules are evenly spread throughout the space. However, they continue to move—there is just no net change in concentration.

Two tasks-simultaneously ► ► Two of you set up diffusion activity ► ► The other two set up for egg osmosis lab: Get a scale, 2 beakers, 100 mL graduated cylinder, 2 strips of masking tape, piece of string, ruler, sheet of paper Use masking tape to label one beaker A and the other B and put your initials on both labels Weigh Beaker A and record weight Get egg from Ms. Z: use string to measure egg around its widest point and record Put it gently into beaker A and record new weight Measure 100 mL of vinegar and pour into beaker Repeat with Beaker B

Facilitated diffusion Facilitated = helped Some molecules are helped by channel or carrier proteins ► Channel – a tunnel for substances like glucose

Facilitated diffusion ► Carrier protein – changes shape to facilitate molecule passing through Facilitated diffusion also involves moving across a concentration gradient (from high to low)—like diffusion, does not require energy

Osmosis Osmosis = diffusion of water through a cell membrane Definition: the movement of water from a region of high water potential (low solute concentration) to low water potential (high solute concentration)

Osmosis: Diffusion of Water

Three osmotic conditions ► ► Isotonic: iso = the same --two solutions have the same solute concentration --so the water flow between them is in equilibrium For example, if one solution is a cell and the other solution is the surrounding environment, they have the same salt concentration

Cell in Isotonic Solution CELL 10% NaCL 90% H 2 O 10% NaCL 90% H 2 O ENVIRONMENT NO NET MOVEMENT What is the direction of water movement? The cell is at equilibrium.

Three osmotic conditions ► ► Hypertonic: hyper = higher than --comparing two solutions, the one with a higher solute concentration is hypertonic to the other: it will gain water --so water will flow into the hypertonic solution For example, if the outside environment has a higher salt concentration than inside the cell, the outside environment is hypertonic to the cell: water will flow out of the cell and the cell will shrink

Cell in Hypertonic Solution CELL 15% NaCL 85% H 2 O 5% NaCL 95% H 2 O What is the direction of water movement? ENVIRONMENT copyright cmassengale

Three osmotic conditions ► ► Hypotonic: hypo- = lower than --comparing two solutions, the one with the lower solute concentration is hypotonic to the one with higher solute concentration: it will lose water --so water will flow out of the hypotonic solution For example, if the outside environment has a lower salt concentration than inside the cell, then the outside is hypotonic to the cell: water will flow into the cell and the cell will swell

Cell in Hypotonic Solution CELL 10% NaCL 90% H 2 O 20% NaCL 80% H 2 O What is the direction of water movement? copyright cmassengale What is the direction of water movement?

Three osmotic conditions

Isotonic: the same amount of water leaves the cell as enters Hypotonic: the pure water solution loses water to the red blood cell, which expands Hypertonic: the red blood cell loses water to the salt solution, causing it to shrink Tonicity

Cells in Solutions copyright cmassengale

Cytolysis & Plasmolysis Cytolysis Plasmolysis

Review ► How is passive transport different from active transport? ► What are the three types of passive transport? ► What is an example of diffusion? ► In facilitated diffusion, what does the facilitating and how does it do it? ► How is osmosis different from diffusion? ► How are hypotonic, isotonic, and hypertonic conditions different?

Active Transport ► ► Requires energy -- why? Because it is moving materials against a concentration gradient: from low to high concentration instead of from high to low

Ion pump ► ► Example: Sodium-Potassium pump Proteins in membrane change conformation to pump sodium ions (Na+) in one direction and potassium ions (K+) in the other direction

Endocytosis and Exocytosis Sometimes the cell has to move big stuff! Exo- = out: move it out Endo- = in: move it in

For example, this is how hormones are sent out

Endocytosis ► ► Phagocytosis-cell eating ► ► Pinocytosis-cell drinking ► ► Receptor-mediated endocytosis

Phagocytosis Projections from the membrane called pseudopods (false feet) surround and engulf an external object. In this picture the cell is “eating” a bacterium.

Pinocytosis ► This time the membrane pinches in to surround an object and forms a vesicle around it.

Receptor mediated endocytosis Only molecules that are specifically recognized by the receptors in a particular region of the membrane are taken up and enclosed in a coated vesicle.

Other membrane functions Carried out by membrane proteins ► Cell to cell recognition (important in forming tissues) ► Cell signaling: hormone binds to membrane protein causing it to change shape and relay signal inside the cell ► Enzyme activity related to plasma membrane