overview: lab cell membrane the plasma membrane is the boundary that separates the living cell from...

Overview: Lab Cell membrane

• The plasma membrane is the boundary that separates the living cell from its surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

• Bi-lipid layer and proteins

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept : Cellular membranes are fluid mosaics of lipids and proteins

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it


Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Transport Chapter 5 Homeostasis

• Homeostasis – cell maintains internal equilibrium by adjusting its physiological processes

Movement across a membrane

• The movement of a substance across a membrane is active transport if it requires energy from the cell.

• The diffusion of a substance across a biological membrane is passive transport if it requires no energy from the cell to make it happen

Concept : Passive transport is diffusion of a substance across a membrane with no energy investment

• Diffusion is the tendency for molecules to spread out evenly into the available space

• Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction

• At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Animation: Membrane SelectivityAnimation: Membrane Selectivity Animation: DiffusionAnimation: Diffusion


• concentration gradient, the difference in concentration of a substance from one area to another

• Substances diffuse down their concentration gradient high to low

• No work must be done to move substances down the concentration gradient no energy


Molecules of dye

Fig. 7-11a

Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

(b) Diffusion of two solutes

Fig. 7-11b

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

Equilibrium

• Occurs when molecules of one substance is spread evenly through out another substance.

just so you really get it ….

• cells never reach a state of equilibrium. Instead, they achieve a "steady state". The chemical composition of the cell is "dynamic"; it is constantly in a state of flux.

• At dynamic equilibrium, as many molecules cross one way as cross in the other direction

• Chemical processes that reach equilibrium cease. If the chemical processes within the cell ceased the cell would die.

Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a selectively permeable membrane

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

• I like to say high water conc. to low water conc.


Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Brownian Motion

• The random movement of microscopic particles suspended in a liquid or gas, caused by collisions with molecules of the surrounding medium.

Water Balance of Cells Without Walls

• Tonicity is the ability of a solution to cause a cell to gain or lose water

• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water


Water Balance of Cells with Walls

• Turgor – pressure in Cell walls help maintain water balance

• A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)


Cytolysis

• breakdown of cells by the destruction of the animal cells outer membrane. If the cell membrane cannot regulate excessive influx of water

• Ex – RBC which is cytolysis ?

Video: PlasmolysisVideo: Plasmolysis

Video: Turgid Video: Turgid ElodeaElodea

Animation: OsmosisAnimation: Osmosis

• lethal effect called Plasmolysis when plant cells lose water; eventually, the membrane pulls away from the wall,

• Occurs in a hypertonic environment,


Fig. 7-13

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

• Polar molecules do not cross the membrane easily

• small – 02, C02 ,ethanol can pass

• large – sugars can’t pass

• ions – H+, NA + can’t pass

• So these large compounds and ions use …….

• carrier molecules - are transport proteins, called bind to molecules and change shape to shuttle them across the membrane

• A transport protein is specific for the substance it moves

1. carrier - facilitated diffusion

2. carrier- NA + K - pump


Facilitated Diffusion: Passive Transport Aided by Proteins

• In facilitated diffusion, transport proteins speed up the passive movement of molecules across the plasma membrane

• Facilitated diffusion is still passive because the solute moves down its concentration gradient

– Ex- move glucose and amino acids


• The sodium-potassium pump is one type of active transport system

• 3 Na + out, 2 K +in = maintains a – cytoplasm

• Active transport allows cells to maintain concentration gradients that differ from their surroundings


• Channel proteins include

– Gated channels - Ion channels that open or close in response to a stimulus

– also facilitated diffusion , passive because the solute moves down its concentration gradient

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(E) A gated channel protein

Solute CYTOPLASM

Solute Carrier protein

(D 1,2) A carrier protein

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

Fig. 7-16-7

Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles

• Bulk transport requires energy


Endocytosis

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

• Endocytosis is a reversal of exocytosis, involving different proteins

• Two types of endocytosis:

– Pinocytosis (“cellular drinking”)

– Phagocytosis (“cellular eating”)

Animation: Exocytosis and Endocytosis IntroductionAnimation: Exocytosis and Endocytosis Introduction


• In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

Animation: PinocytosisAnimation: Pinocytosis


Fig. 7-20b

PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

• In phagocytosis a cell engulfs a particle in a vacuole cell eating

• The vacuole fuses with a lysosome to digest the particle

Animation: PhagocytosisAnimation: Phagocytosis


Fig. 7-20a

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

• Many secretory cells use exocytosis to export their products

Animation: ExocytosisAnimation: Exocytosis


Fig. 7-UN3

Environment:0.01 M sucrose

0.01 M glucose

0.01 M fructose

“Cell”

0.03 M sucrose

0.02 M glucose

Fig. 7-UN4

overview: lab cell membrane the plasma membrane is the boundary that separates the living cell from...

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