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8/3/2019 5 Unit 2A Transport
http://slidepdf.com/reader/full/5-unit-2a-transport 1/25
Unit 2A
Human Form & Function
Cells, metabolism & regulation
Transport
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Further information
• Further information aboutthis topic can be found inOur Human Species
(3rd edtn)
Chapter 3, section 5
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The cell membrane
Copyright - National Institute of Standards and Technology
Drawing by Dana Burns
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The cell membrane
• The cell membrane (or plasma membrane)is the outer boundary of a cell.
• The membrane is a phospholipid bi-layer.
• Proteins and other molecules areembedded in the membrane.
• The cell membrane is selectively
permeable –
i.e. it allows somesubstances to cross more easily thanothers.
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The bi-layer
TISSUE FLUID
CYTOPLASM
PHOSPHOLIPID
BI-LAYER
Hydrophilicphosphate
Hydrophobic
lipid tail
Membraneprotein
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Diffusion through the membrane
• Because the cell membrane isfatty, most water soluble
substances cannot diffuse throughit.
• Exceptions include oxygen &
carbon dioxide.
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Membrane proteins
• A variety of proteins areembedded in the bi-layer.
•These serve various functionsincluding the movement ofsubstances in and out of the cell.
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Membrane transport proteins
• Membrane proteins that aid themovement of substances in andout of the cell include:
• Channel proteins (ion channels) – open channels that allow simplediffusion.
• Carrier proteins that allow facilitateddiffusion (e.g. glucose) and activetransport (specific membrane pumps).
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Membrane transport
•
Transport processes are eitherpassive or active.
• Passive processes require no
cellular energy and includediffusion, osmosis & facilitateddiffusion.
• Active processes require ATP andinclude specific membrane pumps and phagocytosis/pinocytosis.
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Passive processes
• Require no cellular energy (ATP).
• Substances move from highconcentration to low concentration.
• E.g. – diffusion, facilitated diffusion& osmosis.
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Diffusion•
Diffusion is the tendency for particles tomove from an area of higherconcentration to an area of lowerconcentration.
• Diffusion results from the randommotion of atoms and molecules due totheir kinetic energy.
• Diffusion involves the movement ofatoms & molecules in gases and liquids
(specifically, solutes).
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Diffusion
Simple diffusionis the movementof particles froman area of high
concentration toan area wheretheirconcentration is
lower.
Diagram created by LadyofHats
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Concentration gradient• When the concentration of a substance is
different at two places, the substance willdiffuse along the concentration gradient untilthe concentration of the two areas becomesequal.
Diffusion
gradient
Highconcentration
Low
concentration
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High concentration
Low
concentration
Diffusiongradient
1
2
3
Uniform
concentration
Equilibrium
Concentrationat A
Concentrationat B
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Osmosis
•
This is the diffusion of water acrossa selectively permeable membrane.
• Water molecules move from highconcentration to low concentration.
• This is a passive process (does notneed ATP).
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Osmosis
High waterconcentration(dilute soln.)
Low waterconcentration(concentrated
soln.)
Semipermeablemembrane
Net water movementfrom high conc. To
low conc.
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Facilitated diffusion
• Facilitated diffusion is a process ofdiffusion where molecules diffuse
across cell membranes with theassistance of transport proteins.
• Diffusion takes place from high
concentration to low concentrationand does not require ATP.
• Examples: glucose & amino acids.
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Simple diffusion (left) &facilitated diffusion
Diagram created by LadyofHats
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Active transport
• Require energy (ATP).
• Substances move from lowconcentration to highconcentration (i.e. against the
concentration gradient).
• Example: membrane pumps.
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Active transport (sodium pump)
Diagram created by LadyofHats
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Endocytosis
•
Pinocytosis & phagocytosis arespecific types of endocytosis.
• Both processes involve cells
absorbing large particles such asproteins (or even whole organisms,such as bacteria & viruses) fromthe outside by engulfing them with
their cell membrane to form avesicle (like a bubble) within thecytosol.
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Endocytosis & exocytosis
• Endocytosis = things entering the cell
–Phagocytosis = cell eating
–Pinocytosis = cell drinking
• Exocytosis = things leaving the cell
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Endocytosis
Diagram created by LadyofHats
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Exocytosis
Diagram created by LadyofHats