Cell Transport

Cell Transport

Chapter 3(Sections 3.3 – 3.5)

Cell Membrane

• Function – regulates what enters and exits the cell (selectively permeable)

How does the membrane do that? Let’s look at…• Structure – double layer (bilayer) of

phospholipids– Phosphate head is POLAR –

attracted to water (hydrophilic)– Fatty acid tails are NONPOLAR –

repelled by water (hydrophobic)

• Both the extracellular fluid and the cytoplasm are watery, so…– The polar phosphate heads face the outside of the

membrane– The nonpolar fatty acid tails are on the inside of

the membrane

• Fluid Mosaic Model– Fluid: the phospholipids can move side to side– Mosaic: there are a variety of proteins embedded

into the membrane

Passive Transport• Passive transport = movement of molecules

across a cell membrane WITHOUT the input of energy from the cell

• Types of passive transport:– Diffusion– Osmosis– Facilitated diffusion

• Concentration gradient – the difference in the concentration of a substance from one location to another

DIFFUSION• Diffusion = the movement of molecules from

high concentration to low concentration (DOWN the concentration gradient)

• Why do molecules diffuse?– Molecules are constantly moving and colliding– Collisions make them spread out randomly– Results in movement from high concentration to

low concentration

• Which molecules can diffuse across the cell membrane?– Small (Why? They can squeeze between

phospholipids.)– Nonpolar (Why? Because the nonpolar fatty acid

tails will let them through the membrane.)

OSMOSIS• Osmosis = diffusion of water molecules• Water molecules are attracted to dissolved

substances can’t move around• Free water – not attracted to a dissolved

substance can move• Best definition of osmosis?

H2O travels from an area of high free water concentration to an area of low free water concentration

Example of OsmosisLots of

solute Low free H2O

Little solute

High free H2O

Equal free H2O Concentration

H2O moves

• Cells must maintain a balance of water or they could shrivel or explode

• How do cells deal with this problem?– Plant cells are constantly taking in water but

they have rigid cell walls– Paramecia (single-celled organisms that live

in fresh water) are constantly taking in water so they use contractile vacuoles to pump water out–Our body cells are surrounded by extra

cellular fluid (rather than pure water) which has approximately the same concentration of water as our cells

FACILITATED DIFFUSION• Some molecules cannot easily diffuse across

the membrane• If the substance is polar, the nonpolar fatty

acid tails in the center of the membrane won’t let it through

• So small, polar molecules get help from transport proteins

• Transport proteins…– Are SPECIFIC (They only let certain molecules

through)– Some are channel proteins (tunnel)– Some are carrier proteins (change shape)

• Still move from high to low with NO energy used!

Transport Processes that NEED Energy• If a substance cannot cross the membrane by

diffusion/osmosis/facilitated diffusion, the cell will use ENERGY to transport the substance

ACTIVE TRANSPORT• Active transport = cells use transport proteins

to pump materials from LOW to HIGH• Low to high = AGAINST the concentration

gradient• Energy supplied by ATP (the ready-to-use

energy molecule)

Facilitated diffusion vs. Active transport

• Uses transport proteins• Moves substances from

HIGH to LOW concentration• Does NOT need energy

• Uses transport proteins• Moves substances from

LOW to HIGH concentration• NEEDS energy (ATP)

• What if a transport protein can’t be used? (The substance it too big or there is too much)– Use endocytosis or exocytosis!

ENDOCYTOSIS• Endocytosis = the membrane engulfs a large

amount of material and brings it INTO the cell– When the membrane surrounds the material, the

membrane pocket pinches off to form a vesicle

• Types of endocytosis– Phagocytosis = the cell

membrane engulfs large particles• The term means “cell

eating”• Used by

white blood cells to fight infection

– Pinocytosis = the cell membrane engulfs a large amount of extracellular fluid• The term means “cell

drinking”

EXOCYTOSIS• Exocytosis = a vesicle fuses with the cell membrane

to transport materials OUT of the cell

• Used to… (Just a couple of examples)

– Send nerve signals from one neuron to another– Release cellular products (example: pancreas

cells releasing insulin)