The Cell Membrane Ms. Napolitano Honors Biology

12/10 – Do Now  Please get your clickers!  Turn in your webquest if you did not hand it in last class.  Think about the following question: Why are cells small?

The Cell Membrane  Selectively permeable – allows some substances to cross more easily than others  8 nm thick  Fluid mosaic model  Fluid structure of phospholipids with a mosaic of various proteins embedded in it  Cholesterol maintains fluidity – acts as a temperature buffer  less fluid as temp increases (restricts movement)  More fluid as temp decreases (maintains space)

The Structure of the Cell Membrane

Membrane Proteins  Different types of cells contain different types of membrane proteins  Integral proteins – embedded into the hydrophobic core of the lipid bilayer  Can go all the way through the membrane  Peripheral proteins – on the surface of the membrane

Types of Membrane Proteins  Transport  Channel/carrier proteins allow polar molecules & ions to pass through hydrophobic layer  Channel & carrier proteins are substance specific  Enzymatic  Signal Transduction (chemical messages)  Receptor proteins transmit information from outside of the cell to inside of the cell  Cell Recognition  ID tags – glycolipids or glycoproteins  Intercellular Joining  Join together adjacent cells  Cell Support  Attach to cytoskeleton or extracellular matrix for stability

What type of membrane protein is this? 1. Integral 2. Peripheral

A cell that has been frozen will have a more fluid membrane than a cell at room temperature. 1. True 2. False

What would happen to a cell that lacks cholesterol in the membrane? 1. It would not be able to transport materials into/out of the cell 2. The membrane would change drastically with temperature 3. It would not be able to use enzymes effectively 4. Chemical messages would not be able to get into the cell

What is the function of membrane carbohydrates? 1. Transport 2. Signal transduction 3. Cell recognition 4. Intercellular joining

Passive Transport  Passive Transport – movement across the cell membrane that does not require energy  Diffusion – movement of particles from an area of high concentration to an area of low concentration until an equilibrium is reached  Said to go with/down its concentration gradient  Concentration gradient – the difference in the concentration of a substance across a space  Equilibrium – concentration of a substance is equal throughout a space  Doesn’t stop moving

Osmosis  Osmosis – the diffusion of free water across a selectively permeable membrane  Water diffuses across the cell membrane from a region of low solute concentration to that of a higher solute concentration until it reaches equilibrium

Really cool video! ycp0

Osmoregulation in Cells without Cell Walls  Osmoregulation – control of water balance  Tonicity – ability of a solution to cause a cell to gain/lose water  Isotonic solution – no net movement of water across the cell membrane  Hypertonic solution – more free water inside the cell  Cell loses water to environment and shrivels  Hypotonic solution – less free water inside the cell  Water will enter the cell and swell (and maybe lyse)  Some cells have a contractile vacuole to pump water out of the cell

Animal Cells

Osmoregulation of Cells with Cell Walls  Plant cells are healthiest in a hypotonic solution, osmotic pressure keeps cell walls turgid (very firm)  Plant cells are flaccid (limp) in an isotonic solution  In a hypertonic solution, the cell membrane will shrink and pull away from the cell wall  Called plasmolysis (wilting)

Plant Cell

U – Tube (not YouTube!)

Which of the following can readily diffuse across a lipid bilayer? 1. Glucose 2. Oxygen 3. Carbon dioxide 4. Both 2 & 3

If you immerse a living cell in a hypotonic solution, water will… 1. Diffuse into the cell 2. Diffuse out of the cell 3. Show no net movement

Plant 1 is in better shape than plant 2 because the cells of plant 1 are… 1. Turgid 2. Flaccid 3. Undergoing plasmolysis 4. The same as plant 2 Plant 1Plant 2

The image below is a U-Tube with a semi-permeable membrane. How will water flow? 1. It won’t 2. Left to right  3. Right to left 

12/ 16 Do Now:  Get Your Clicker!  Compare/Contrast Active & Passive Transport  Compare/Contrast Diffusion & Osmosis

Facilitated Diffusion  Facilitated diffusion: passive transport aided by proteins  Channel proteins – hydrophilic passageways  Some always open for diffusion  Rate of movement is determined by concentration gradient  (+) charged ions more likely to diffuse into the cell  (-) charged ions more likely to diffuse out of the cell  Some ion channels have gates and can be opened by:  Stretching of the cell membrane  Change in electrical charge  Binding of specific molecules

Voltage Gated Channels  Nerve cells transmit electrical signals by opening a series of Na + gated channels 1. Channel is closed 2. Area changes voltage 3. Channel opens briefly 4. Na + flood into cell – voltage changes 5. Channel closes and electrical signal passes on to the next voltage channel

Chemically (Ligand) Gated Channels  Nerve cells send out a neurotransmitter called acetylcholine (ACh) to nearby muscle cells to signal muscles to contract 1. ACh binds to ACh receptor proteins 2. Receptor gate opens for a microsecond to allow Na + in 3. Na + sets off muscle contraction

Facilitated Diffusion  Carrier Proteins 1. Specific substance binds to carrier protein 2. Protein changes shape and transports substances across cell membrane 3. Molecule is released into the cell, and carrier protein returns to its original shape

Which image best represents facilitated diffusion?

____are transported by a particular carrier or pass through a particular channel. 1. All water soluble ions or molecules 2. Certain water soluble molecules or ions 3. All insoluble molecules or ions 4. Certain insoluble ions or molecules 5. Only the smallest molecules or ions

Active Transport  Active Transport – uses energy to move solutes against the concentration gradient  Carrier proteins act as “pumps” powered by ATP  Examples:  Sodium Potassium Pump  Proton Pump  Cotransport

Sodium Potassium Pump  Pumps 3 Na + out of the cell and 2 K + into the cell  Actively transports both ions against their concentration gradient, powered by ATP  Prevents Na + from accumulating in the cell  Steps: 1. 3 Na + and a P (from ATP) bind to inside protein pump 2. Pump changes shape transporting 3 Na + across membrane and out 3. 2 K + bind to pump and are transported across membrane 4. 2 K + and P are released inside of cell

Sodium Potassium Pump

The sodium-potassium pump passes 1. More Na+ out than K+ in 2. K+ out and Na+ in on a one-for-one basis 3. Na+ out and K+ in on a one-for-one basis 4. K+ and Na+ in the same direction

Proton Pump  Actively transports protons (H + ) through the internal membranes of mitochondria and chloroplasts

Cotransport (Coupled Transport)  Cotransport – an ATP-powered pump that transports a specific solute  Can indirectly drive the active transport of several other solutes  Ex: as proton pumps pump H + out, H + diffuse back into the cell pulling sucrose molecules into the cell with it

By a mechanism called cotransport 1. sugar moves down (with) its concentration gradient while sodium moves up (against) its concentration gradient 2. sodium moves outward against is concentration gradient while sugar moves inward down its concentration gradient 3. sugar and sodium move inward down their concentration gradient 4. sodium moves inward down its concentration gradient while sugar moves inward up its concentration gradient 5. sodium and sugar move outward against their concentration gradient

Bulk Transport  Substances that are too large to be transported by carrier proteins  Exocytosis (export)  Secretion of macromolecules by fusion of vesicles with membranes, releasing the contents outside of the cell  Endocytosis (import)  Cell membrane engulfs particles and pinches off to form vesicles inside the cell. Vesicle may fuse with lysosomes or other organelles  3 Types:  Phagocytosis – engulf another organism  Pinocytosis – engulf fluid  Receptor-mediated - specific

Exocytosis (top) /Endocytosis (bottom)

A cell engaged in phagocytosis must be 1. engulfing a live organism 2. acquiring a liquid 3. engulfing a dead organism 4. transporting bulk dissolved nutrients 5. transporting bulk solid material

Substances transported by facilitated diffusion 1. move passively through specific channels from an area of greater concentration to one of lower concentration 2. are limited to solvents 3. must have movements coupled to those of other substances 4. may flow to a region of higher concentration by the expenditure of energy 5. are restricted to only one direction through the membrane

A contractile vacuole is an organelle that pumps excess water out of many freshwater protozoan cells. A freshwater protozoan was placed in solution A and observed to form contractile vacuoles at a rate of 11 per minute. The same protozoan was then placed in solution B and observed to form contractile vacuoles at a rate of 4 per minute. Based on this information, which of the following statements is correct? 1. Solution A is hyperosmotic to solution B. 2. Solutions A and B are isosmotic. 3. Solution B is hyperosmotic to solution A. 4. Solutions A and B are isosmotic to the protozoan cell. 5. Both a and b

T he methods of membrane transport that don't require protein channels or carriers are 1. osmosis 2. Diffusion 3. Phagocytosis 4. Exocytosis 5. all of the above