5 Unit 2A Transport

8/3/2019 5 Unit 2A Transport

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Unit 2A

Human Form & Function

Cells, metabolism & regulation

Transport



Further information

• Further information aboutthis topic can be found inOur Human Species

(3rd edtn)

Chapter 3, section 5



The cell membrane

Copyright - National Institute of Standards and Technology

Drawing by Dana Burns



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.



The bi-layer

TISSUE FLUID

CYTOPLASM

PHOSPHOLIPID

BI-LAYER

Hydrophilicphosphate

Hydrophobic

lipid tail

Membraneprotein



Diffusion through the membrane

• Because the cell membrane isfatty, most water soluble

substances cannot diffuse throughit.

• Exceptions include oxygen &

carbon dioxide.



Membrane proteins

• A variety of proteins areembedded in the bi-layer.

•These serve various functionsincluding the movement ofsubstances in and out of the cell.



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).



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.



Passive processes

• Require no cellular energy (ATP).

• Substances move from highconcentration to low concentration.

• E.g. – diffusion, facilitated diffusion& osmosis.



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).



Diffusion

Simple diffusionis the movementof particles froman area of high

concentration toan area wheretheirconcentration is

lower.

Diagram created by LadyofHats



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



High concentration

Low

concentration

Diffusiongradient

1

2

3

Uniform

concentration

Equilibrium

Concentrationat A

Concentrationat B



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).



Osmosis

High waterconcentration(dilute soln.)

Low waterconcentration(concentrated

soln.)

Semipermeablemembrane

Net water movementfrom high conc. To

low conc.



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.



Simple diffusion (left) &facilitated diffusion




Active transport

• Require energy (ATP).

• Substances move from lowconcentration to highconcentration (i.e. against the

concentration gradient).

• Example: membrane pumps.



Active transport (sodium pump)




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.



Endocytosis & exocytosis

• Endocytosis = things entering the cell

–Phagocytosis = cell eating

–Pinocytosis = cell drinking

• Exocytosis = things leaving the cell



Endocytosis




Exocytosis


5 Unit 2A Transport

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