(12/10) Partner BR: (12/10) Partner BR: The Phospholipid BilayerThe Phospholipid Bilayer3. Plasma membranes are composed of

phospholipids that arrange themselves in two layers. Each phospholipid has a hydrophilic head and 2 hydrophobic fatty acid tails, which can be saturated or unsaturated. Explain which type of fatty acids would make the phospholipid bilayer more fluid and less rigid in its structure.

CH 5CH 5Cell Membranes & SignalingCell Membranes & Signaling

5.1 Biological Membranes Have a Common 5.1 Biological Membranes Have a Common Structure and Are FluidStructure and Are Fluid

Fluid mosaic model – the phospholipid bilayer (plasma membrane) is like a “lake” in which a variety of proteins “float”

Cholesterol

Peripheral membrane proteins do not penetrate the interior of the bilayer

Integral membrane proteins do penetrate the interior of the bilayer

Carbohydrates may attach to proteins (glycoproteins) or lipids (glycolipids)

5.1 Biological Membranes Have a Common 5.1 Biological Membranes Have a Common Structure and Are FluidStructure and Are Fluid◦ Hydrophilic heads face the aqueous

environment◦ Hydrophobic interior – nonpolar fatty acid

“tails”

5.1 Biological Membranes Have a Common 5.1 Biological Membranes Have a Common Structure and Are FluidStructure and Are Fluid

◦ Phospholipids may differ in: Fatty acid chain

length Degree of saturation Kinds of polar

groups◦ Fluidity is determined

by 2 factors: Temperature Lipid composition

(types of fatty acids & cholesterol)

Cholesterol restrains movement of phospholipids at both high and low temperatures, thereby stabilizing the plasma membrane

Unsaturated fatty acids increase fluidity (double bonds cause kinks that prevent hydrophobic interactions between neighboring fatty acid chains

5.1 Biological Membranes Have a Common 5.1 Biological Membranes Have a Common Structure and Are FluidStructure and Are Fluid◦ Peripheral membrane proteins lack

hydrophobic groups and are attached to one side of the bilayer

◦ Integral membrane proteins are at least partly embedded in the phospholipid bilayer

5.1 Biological Membranes Have a Common 5.1 Biological Membranes Have a Common Structure and Are FluidStructure and Are Fluid◦ A transmembrane protein extends through

the bilayer on both sides, and may have different functions in its external and transmembrane domains

◦ Plasma membrane carbohydrates (glycolipids & glycoproteins) are located on the outer membrane and can serve as ID markers for cell-to-cell recognition

ExitExitConsidering the fatty acid interior of the bilayer, which of the following would you expect to be able to travel through it?

a. Ionsb. Polar moleculesc. Nonpolar molecules

Read the following descriptions of 3 types of Read the following descriptions of 3 types of specialized cells. How do their shapes enable specialized cells. How do their shapes enable them to perform their functions more them to perform their functions more efficiently? efficiently? The epithelial cells that line the small

intestine absorb nutrients from the digestive tract that can then enter the blood. These cells contain a series of convolutions (ridges) called microvilli.

Plant root hairs are narrow, elongated parts of root cells that absorb water and nutrients from the soil.

Red blood cells (erythrocytes) have a flat, concave shape (like a thin Junior Mint). They absorb and release O2 and CO2 and carry them through the blood.

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Biological membranes are selectively permeable because they allow some substances, and not others, to pass through

Passive transport (diffusion) does not use energy – molecules move from an area of high concentration to an area of low concentration◦ Diffusion is the process of random movement

toward equilibrium. Speed is dependent on:

Size of molecules Temperature of the solution Concentration gradient (difference in concentration on each side of the membrane)

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Simple diffusion takes place directly through the phospholipid bilayer Small, hydrophobic molecules (e.g., O2) can pass through; polar molecules & ions cannot

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Osmosis is the diffusion of water through special membrane channels called aquaporins water moves toward side with higher solute concentration (dilutes it) hypertonic solution has higher solute concentration

isotonic solutions have equal solute concentrations

hypotonic solution has lower solute concentration In plants, turgor pressure is the internal pressure against the cell wall—as it builds up, cell becomes turgid and prevents more water from entering (this is ideal!)

(12/16) GROUP Bellringer: (12/16) GROUP Bellringer: OsmosisOsmosis1. Get out your Osmosis Practice WS and compare it

with those of the other members of your group. 2. Why does turgor pressure play a role in plant cells

but not animal cells?3. You are dangerously dehydrated and need

intravenous (IV) fluids to gradually build up your fluid levels. The label on the IV bag says “0.9% normal saline” and has a list of the solutes dissolved in the water. Why is it important for an IV solution to have salts (solutes) in it? What would happen if too much pure water were ingested/injected?

4. Describe a situation in which the blood plasma (the acellular liquid part of the blood) could become hypertonic to the red blood cells.

Cells lose water and shrivel (crenation)

Cells take in water and may burst (hemolysis)

Cell body shrinks away from cell wall (wilting)

Cells stiffen and become turgid

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2O

(1) Normal (2) Lysed (hemolysis)

H2O

H2O H2O H2O

Animalcell

Plantcell

(4) Flaccid (5) Turgid (6) Shriveled (plasmolysis)

(3) Shriveled (crenation)

Plasmamembrane

H2O

H2O

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Facilitated diffusion takes place with the aid of channel proteins or carrier proteins Ion channels are a type of channel protein that let specific ions through

Gated channels open when a stimulus causes the channel to change shape Ligand-gated channels respond to binding of chemical signals called ligands

Voltage-gated channels open or close in response to a change in voltage across the membrane

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Carrier proteins bind substances (e.g., glucose) Glucose molecules bind to glucose transporters and cause the protein to change shape and “carry” glucose across

5.2 Some Substances Can Cross the Membrane 5.2 Some Substances Can Cross the Membrane by Diffusionby Diffusion

Facilitated diffusion system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches its maximum rate

5.3 Some Substances Require Energy to Cross the 5.3 Some Substances Require Energy to Cross the MembraneMembrane

Active transport requires energy (ATP) to move substances against their concentration gradient◦ involves hydrolysis of ATP◦ The sodium–potassium (Na+–K+) pump is

an integral membrane protein in animal cells that pumps 3 Na+ out of a cell and 2 K+ in for each molecule of ATP

5.3 Some Substances Require Energy to Cross the 5.3 Some Substances Require Energy to Cross the MembraneMembraneThe sodium–potassium (Na+–K+) pump: pumps 3 Na+ out of a cell and 2 K+ in for each molecule of ATP

3 Na+ and 1 ATP bind to pump Hydrolysis of ATP

phosphorylates pump protein, changing its shape

Shape change releases Na+ outside the cell & enables K+ to bind to pump

Release of Pi returns the pump to its original shape, releasing K+ inside cell & exposing Na+

binding sites

5.3 Some Substances Require Energy to Cross the 5.3 Some Substances Require Energy to Cross the MembraneMembrane

Secondary active transport uses the energy from an ion concentration or electrical gradient established by primary active transport◦ uses energy that is “regained” by letting ions

move across the membrane with their concentration gradients

◦ Ex: Na+ diffusing back into cells allows glucose to be absorbed from digestive tract into bloodstream

5.4 Large Molecules Cross the Membrane via 5.4 Large Molecules Cross the Membrane via VesiclesVesicles

Endocytosis – membrane invaginates and forms a vesicle to bring molecules into a cell◦ Phagocytosis (“cellular eating”) – membrane

engulfs a large particle or cell, forming a phagosome Ex: white blood cells engulfing pathogenic

bacteria◦ Pinocytosis (“cellular drinking”) – vesicles are

smaller and bring in fluids and dissolved substances Ex: endothelial cells lining blood vessels taking

in fluids & solutes (like proteins) from blood

5.4 Large Molecules Cross the Membrane via 5.4 Large Molecules Cross the Membrane via VesiclesVesicles◦ Receptor–mediated endocytosis depends

on receptors binding to specific molecules (ligands)

Ex: cholesterol packaged into low-density lipoproteins

Receptor-mediated endocytosisReceptor-mediated endocytosis

Receptors are integral membrane proteins located in regions called coated pits

Cytoplasmic surface is coated by another protein (often clathrin)

5.4 Large Molecules Cross the Membrane via 5.4 Large Molecules Cross the Membrane via VesiclesVesicles

Exocytosis moves materials out of the cell in vesicles◦ Ex: hormones, neurotransmitters, enzymes

ExocytosisExocytosisNeurons release neurotransmitters at a synapse via exocytosis

ExitExit1. How are facilitated diffusion and active transport

different? How are they similar?2. In extreme cases, it is possible to die from drinking

too much water too fast. The consumption of several liters of water in a short amount of time can lead to brain edema (swelling) and death. Explain the effect of ingesting an extremely large amount of water at the level of the brain cells, including the role of osmosis in this process.

3. Dextran is a branched polysaccharide that is unable to cross plasma membranes. When administered into the blood, it reduces blood viscosity (thickness) and increases blood volume. Explain how dextran brings about this effect.

a. ________solution b. ________ solution c. ________ solution

H2O H2O

(1) Normal (2) Lysed (hemolysis)

H2O

H2O H2O H2O

Animalcell

Plantcell

(4) Flaccid (5) Turgid (6) Shriveled (plasmolysis)

(3) Shriveled (crenation)

Plasmamembrane

H2O

H2O

(12/17) BR: Is the solution outside the cell isotonic, (12/17) BR: Is the solution outside the cell isotonic, hypertonic, or hypotonic?hypertonic, or hypotonic?

a. Isotonic solution b. Hypotonic solution c. Hypertonic solution

H2O H2O

(1) Normal (2) Lysed (hemolysis)

H2O

H2O H2O H2O

Animalcell

Plantcell

(4) Flaccid (5) Turgid (6) Shriveled (plasmolysis)

(3) Shriveled (crenation)

Plasmamembrane

H2O

H2O

Type 2 diabetes, often associated with obesity, results in an increase in blood-glucose levels (hyperglycemia). One early sign that a person or animal might be developing type 2 diabetes is excessive thirst. Discuss a possible reason for this symptom based on what you have learned in this chapter about tonicity in cells.

Exit slip: Identify the form of membrane Exit slip: Identify the form of membrane transport that is utilized by the following:transport that is utilized by the following:

a. Cells taking in cholesterol from the bloodb. Cells in the pancreas releasing digestive enzymes

into the digestive tractc. The Na+/K+ pump maintaining a negative charge

(called membrane potential) inside the celld. Glucose transporters helping glucose enter a cell

along its concentration gradiente. Oxygen molecules leaving inhaled air in the lungs

and entering red blood cells in blood vesselsf. White blood cells called macrophages engulfing

pathogenic bacteria which are digested by lysosomes

g. Endothelial cells lining blood vessels take in fluids and dissolved proteins from the blood