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Ways materials can cross the membrane:

Diffusion

Osmosis

Facilitated Diffusion

Active Transport

Bulk Transport: Endocytosis and Exocytosis

Passiv

e t

ran

sp

ort

(

no

en

erg

y)

Diffusion: Spreading out of molecules from higher conc. to lower

conc. until equilibrium is reached

Examples: • Perfume spill • Oxygen molecules

equilibrium

Caused by RANDOM MOTION OF MOLECULES

Diffusion is due to the random motion of molecules within a substance. Movement of molecules is GREATEST when there is

more SPACE between them.

solid liquid gas

Factors that affect Diffusion are: Temperature

Increasing temp faster diffusion

Pressure

Increasing pressure speeds up diffusion (more collisions)

Concentration Gradient – difference in concentration from one area to another

Molecular size

Small molecules move faster than larger ones, therefore have faster diffusion

2 H20 10 H20 2 H20 5H20 **diffuses

faster**

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Phospholipid Structure

fatty acid #1

fatty acid #2

fatty acid #3

glyc

ero

l

fatty acid #1

fatty acid #2

glyc

ero

l

phosphate group

Polar

Head

phosphate

glycerol

Fatty acid chains

Polar head

H20

Hydrophilic =

water loving

Hydrophobic =

water fearing

The Plasma Membrane The plasma membrane controls the passage of materials in

and out of the cell.

This means that the plasma membrane is responsible for maintaining HOMEOSTASIS or the CHEMICAL BALANCE of the cell.

[Amounts of water, glucose, amino acids, O2, and CO2, and wastes must be kept at appropriate levels for cell survival.]

Outside cell

Inside cell

Phospholipid molecule

Composed of two layers of PHOSOPHOLIPIDS. Lipids make a great cell boundary (does not dissolve in

water)

Cholesterol molecules (in between phospholipids) stabilize/solidify the membrane’s structure.

cholesterol

Ph

osp

ho

lipid

bila

yer

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Contains channels made of PROTEINS.

Act as pores for certain molecules to travel through the membrane.

Protein (function depends on unique shape)

Type and pattern of carbohydrate chains is unique in every person, acting like ID tags for the body to recognize its own cells from other cells.

If a cell with a different pattern of carbohydrates is present, the immune system will detect it as foreign and will seek to destroy it.

The outer surface of the membrane is covered with CARBOHYDRATES (glycocalyx).

carbohydrate

Membrane is semi-permeable. Blocks some molecules, while others are allowed to

pass through.

This allows the cell to control most of what gets in/out based on the size and charge of the molecules entering/leaving.

OSMOSIS

The Diffusion of Water Across a

Membrane

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Environment of Cells

Cells exist in an environment of mostly water.

Water can move across the phospholipid bilayer

directly.

Cells are constantly subjected to the flow of water

across the membrane; they CANNOT CONTROL IT

because water is

Small

Fast

Always Around

Three Different Situations

More water on the outside of the cell: the cell is in a

hypotonic solution.

More water on the inside of the cell: the cell is in a

hypertonic solution.

Even amount of water inside and out: the cell is in an

isotonic solution.

Hypotonic Solution

More water on the outside of the cell: the cell is in a

hypotonic solution.

Outside the cell: 99% water, 1% other molecules

Inside the cell:

97% water,

3% other

molecules

water

(99%)

Hypotonic Solution – Result

Water diffuses into the cell (osmosis) until a state of

dynamic equilibrium is reached. (a state in which molecules still move in and out, but at an equal rate)

Animal cells will swell and may burst (cytolysis).

Plant cells will experience high turgor pressure

(water fills vacuole and the plasma membrane

gets pushed against the cell wall). The plant is

fresh – does not wilt.

Hypertonic Solution

More water on the inside of the cell:

the cell is in a hypertonic solution.

Outside the cell: 95% water, 5% other molecules

Inside the cell:

97% water,

3% other

molecules

water (95%)

Hypertonic Solution – Result Water will diffuse out of the cell (osmosis) until a state of dynamic equilibrium is reached.

Animal cells In plant cells, water leaves the shrivel. vacuole and the plasma membrane shrinks away from the cell wall (plasmolysis). Turgor pressure is low – plant wilts.

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Isotonic Solution

Even amount of water inside and out:

the cell is in an isotonic solution.

Outside the cell: 97% water, 3% other molecules

Inside the cell:

97% water,

3% other

molecules

water (97%)

Remember: Water is ALWAYS

moving.

Isotonic Solution – Result

Water will move in and out of the cell at an equal rate.

No net movement of water (osmosis) will occur.

Animal cells and plant cells will

maintain normal conditions.

Summary

Type of

Solution

Description Net mov’t of

water

(osmosis)

Effect on

Animal Cell

Effect on

Plant Cell

Hypotonic

Hypertonic

Isotonic

More water

on the

outside

Water enters

cell

Cell swells,

possible

cytolysis

High turgor

pressure

More water

on the

inside

Water leaves

cell

Cell

shrivels

Plasmolysis

Equal amt

water inside

and out

Water enters

and leaves

at equal rate

No change No change

Proteins transport molecules and ions that cannot

cross the membrane directly by diffusion either

because of size or charge.

Some proteins can also pump molecules and ions

against the concentration gradient (from low to

high concentration) if necessary.

• The function of the protein channels is due to its shape.

•There must be a unique protein for each type of

molecule that crosses the membrane.

• Some of these protein channels are gated, meaning

they require certain signals in order for them to open or

close.

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Protein channel assists and speeds up diffusion

Does not require energy – movement of molecules is due to concentration gradient alone

Transport Across Membrane with Protein Channels,

Method Number 1: FACILITATED DIFFUSION

High [ ]

Low [ ]

Protein channel moves molecules AGAINST the concentration gradient

(from an area of LOW concentration to an area of HIGH concentration)

Requires energy from ATP (the cell’s only usable energy source)

Transport Across Membrane with Protein Channels,

Method Number 2: ACTIVE TRANSPORT

Osmosis (water)

Osmosis (water) Sodium/Potassium Pump

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ENDOCYTOSIS (endo = in)

• Transport of large quantities of substances into a cell , requiring ATP energy.

• The ingested material gets surrounded by the phospholipids of the cell membrane. Once inside the cell, it is considered to be a vacuole.

Bulk Transport (Endocytosis and Exocytosis)

EXOCYTOSIS (exo = out)

• Transport of large quantities of substances out of a cell, requiring ATP energy.

• A vacuole inside the cell fuses with the plasma membrane and releases its contents outside of the cell.