How Cells Work: Membrane Transport

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Review of transport concepts (Please Review): :

http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport

Osmosis animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

The cell membrane acts as a barrier. However, cells need to obtain energy from food and get rid of waste from metabolism, so substances must move across the membrane. The process of substance movement across the cell membrane is called membrane transport.

Diffusion

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Def: Tendency of molecules (or ions) to scatter evenly in a solution due to random motion and collision of moleculesVariables include: distance, size, temperature, steepness of gradient and charge (positive or negative: like charges repel, unlike attract)

Random thermal motion due to kinetic energy “powers” diffusion

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All molecules have kinetic energy and are in constant motion.

• This motion is called Brownian movement.

• Random movement due to this motion leads to molecule collisions.

• Collisions redirect molecules so that eventually the molecules become evenly distributed in space.

• The process is called diffusion and requires no added energy.

Check it out: Random movement of small particles in water (here)http://galileoandeinstein.physics.virginia.edu/more_stuff/Applets/brownian/brownian.html

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Sometimes atoms pull hard enough to steal each other’s electrons ionic

bonds

What is this

and what is

happening

here?

What is this

and what is

happening

here?

What is this

and what is

happening

here?Cytoplasm

Interstitial

(extracellular fluid)

Summary: Membrane serves as a physical barrier to diffusion of water soluble substances

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Permeability: property of membrane that defines what substances can move across.

Impermeable – nothing can passPermeable – passes freely Selectively permeable – some things can cross.

Variables that determine permeability include: size, charge, shape, lipid solubility of molecule and proteins in the membrane.

Review: Types of Transport Across Cell Membranes

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Passive Transport: Diffusion Simple Diffusion

Substances other than water move: small lipid soluble substances including: oxygen, fat soluble vitamins down concentration gradientOsmosis: movement of water down concentration gradient

Facilitated Diffusion: requires a pathway through the membraneCarrier mediated: amino acids, sugarsChannel mediated: ions

Filtration: powered by blood pressure (hydrostatic pressure) e.g. bulk flow of substances across capillary walls, Active Transport (requires energy for transport)

Primary active transport: ATP hydrolysis provides energySecondary active transport: coupled to gradients established by ATP

Vesicle-mediated Transport: requires energy, substances move in and out of the cell via vesicle fusion with the cell membrane

Transport Paths

Figure 3.10

Active Transport: Primary

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http://www.youtube.com/watch?v=iA-Gdkje6pg

Primary active transport uses energy of ATP hydrolysis to move substances up energetic gradients . A membrane protein does the work.

EX: Na+,K + pump moves Na+

out of the cell (and K+ back into cell). Na+

concentration is high outside the cell, therefore the pump works against a Na concentration gradient and requires energy.

Sometimes atoms pull hard enough to steal each other’s electrons ionic

bonds

Transport maintains important biological gradients that are defined by selective membrane permeability to substances. Solution inside the cell ≠ solution outside the cell.

If you poison the pump that maintains the gradient>gradients will be disrupted and lead to eventual deathEX: anything that blocks the production of ATP (limits oxygen delivery to cells) e.g. cyanide, carbon monoxide,

Ion IN (mM) OUT(mM)

Na+ 5 145

K + 100 5

Ca + + .0000001 2-5

Mg + + 30 1-2

Cl - 4 110

Big Anions

High Low

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Mammalian Intracellular and Extracellular Solution compositions

IN:

[Na+] = low

[K+] = high

[Ca2+] = very low

[Cl-] = low

OUT:[Na+] = high[K+] = low[Ca2+] = low

[Cl-] = high

(blood, interstitial fluid)

Consequences of semi-permeable or SELECTIVE CELL MEMBRANE BARRIER: Concentration gradients

Energy stored in electrochemical gradient is used to do work.

EX: transport of food and water into body from intestinal lumen

Energy stored in electrochemical gradient is used to send signals quickly (2 – 300 miles/hour) along neurons

so that we can perceive and respond to our environment. These signals are used by the control center – the CNS

to send commands to effectors…

Excitable CellsWhat do you already know about neurons, resting potentials, action

potentials, synaptic transmission?

Explanation of Membrane Potential (Please Review):http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf