The Cell Membrane and Diffusion

Cell MembranesCell Membranes• Think back to the cell city activity from

yesterdayyesterday• Remember how there was an outer wall

bordering your city?bordering your city? • This is similar to the cell membrane

( )– The cell membrane (aka plasma membrane) is a border that separates the interior of the cell from its surroundingsits surroundings

– Plant cells also have a supporting structure around their cell membranes, known as a cell wall, which provides support and protection for plant cells

Cell MembranesCell Membranes

Cell WallsCell Walls• Cell walls are commonly made up of

cellulose, a strong structural carbohydrate

Cell MembranesCell Membranes

• Cell membranes regulate what enters andCell membranes regulate what enters and leaves the cell, and also provides protection and supportprotection and support – The cell wall even more so…

Our cell membranes are known as lipid• Our cell membranes are known as lipid bilayers as there are two lipid layers that make up our cell membranesmake up our cell membranes

Cell Membrane – Lipid BilayerCell Membrane Lipid BilayerPhospholipidsp p

1st Layer

2nd Layer

Phospholipidsp pPhosphate Head• Consist of a

phosphate headphosphate head and a fatty acid tailtail

• The tails, when d iarranged in a

bilayer, is the t l “b i ” factual “barrier” of

the cellFatty Acid Tails

Phospholipid BilayerPhospholipid Bilayer• Think of oil and water. What happensThink of oil and water. What happens

after you pour oil into water?

Phospholipid Bilayer• A similar phenomenon occurs with the

phospholipid bilayer– The inner layer is consists of lipids, or “fat/oil

molecules”. Thi t b i i t l l l– This creates a barrier against polar molecules

LIPID CENTER (Hydrophobic)

Regulation of WaterRegulation of Water• As water is a polar

molecule, regulation of water by the cell is done th h ithrough aquaporins,small protein channels that allow water to flow inthat allow water to flow in and out of a cell

Osmosis and DiffusionOsmosis and Diffusion• Because of aquaporins and other protein

h l id th ll b t bchannels, we consider the cell membrane to be selectively permeable as certain molecules can easily cross iteasily cross it

• When these molecules move from an area of higher concentration to that of a lowerhigher concentration to that of a lower concentration we call this diffusion.

• The diffusion of water across a selectively e d us o o ate ac oss a se ect e ypermeable membrane is known as osmosis– In other words, osmosis refers to the diffusion of

water and only water

Diffusion and OsmosisDiffusion and Osmosis

Diffusion and Osmosis

Concentration refers to the number of solute molecules in a given volume of solvent. There are more water molecules on the right side than

Therefore, when given the opportunity, there is a net movement of water from the right side to the left!are more water molecules on the right side than

the left! So the concentration of water is higher on the right.

of water from the right side to the left!

Diffusion and Osmosis• Both diffusion and osmosis stop once

both sides have reached equilibrium, or qhave the same concentration.

• Note that molecules will still move around,Note that molecules will still move around, but they are now evenly spaced apart

Hyper-, Hypo-, and Isotonic Solutions

Solute Molecules Keep in mind the terms refer to the solution outside of the cell!

Hyper-, Hypo-, and Isotonic Solutions

A hypotonic solution will have fewer solute molecules than its reference

An isotonic solution will have the same number of solute molecules as its

A hypertonic solution will have the more solute molecules as its referenceits reference solute molecules as its

referencemolecules as its reference

Which Way Would Water Molecules Move?

No net movement!

Effect on CellsEffect on Cells

Membrane Transport• Cell membranes have a variety of proteins

to facilitate molecules other than water across their membrane

• Passive transport does not use anyPassive transport does not use any energy while active transport does

• Simple diffusion is the first method cells• Simple diffusion is the first method cells use to facilitate molecular transport

Molecules simply diffuse from a higher– Molecules simply diffuse from a higher concentration to a lower concentration and go through the plasma membraneg p

– These are normally restricted to very small molecules

Membrane TransportMembrane Transport• Facilitated diffusion is another form of passive

transporttransport• Involves the use of carrier proteins to move specific

molecules across the membrane

Facilitated diffusion Diffusion Carrier Proteindiffusion (Channel Protein)

Diffusion (Lipid

Bilayer)

Carrier Protein

Facilitated DiffusionGlucosemolecules

HighHigh ConcentrationHigh

Cell Membrane

Low Concentration LowLow Concentration Proteinchannel

Transport

Low

Transport ProteinThrough a

Active Transport• All forms of active transport require

energygy• This is because they are able to “pump”

molecules from an area of lowmolecules from an area of low concentration to an area of high concentrationconcentration

• In other words, this form of transport often goes against the direction diffusion wouldgoes against the direction diffusion would occur, known as the concentration gradientgradient

Protein Pumps

Protein Pumps -transport proteins that

i drequire energy to do work

P t i hProtein changes shape to move molecules: this requires energy!

Endocytosis

• Endocytosis: taking bulky t i l i t llmaterial into a cell

• Uses energy• Cell membrane in-folds

around food particle“ ll ti ”• “cell eating”

• forms food vacuole & di t f ddigests food

• This is how white blood cells eat bacteria!cells eat bacteria!

Endocytosis

• Phagocytosis: How a cell takes in “bulky” material

Endocytosis

• Pinocytosis: Cell takes up liquid from the di i tsurrounding environment

Exocytosis

• Exocytosis: Forces ymaterial out of cell• membrane surrounding the g

material fuses with cell membrane

• Cell changes shape –requires energy

• Hormones or wastes are released from cell this wayway

EndocytosisEndocytosis

Exocytosis

Membrane TransportMembrane TransportMembrane Transport

Passive Transport (No E R i d)

Active Transport (Energy R i d)Energy Required)

Simple

Required)

Protein Endocytosis ExocytosisDiffusion

Facilitated Diffusion

Pumps Endocytosis

Phagocytosis

Exocytosis

Diffusion g y

Pinocytosis

Animal Cell MembraneMembrane