REVIEW – MEMBRANE

TRANSPORT

Passive and Active transport across a cell membrane

Thin, flexible barrier

Cell (plasma) membrane

Membranes also organize the interior of a

cell. Cell organelles are defined by

membranes.

Cell (plasma) Membrane

What does it do for cell?

Controls what goes in and out

Protects/isolates

Communicates

Identification

Every cell is covered by a membrane

that controls what can enter and leave

the cell.

Homeostasis – Maintaining a Balance

The plasma membrane is selectively

permeable – it will allow some things to

pass through, while blocking other things.

Cells must keep the proper concentration

of nutrients and water and eliminate

wastes.

Structure of the Plasma Membrane

Lipid bilayer – two sheets of lipids (phospholipids).

Embedded with proteins and strengthened with cholesterol molecules.

Membrane Proteins

Determine what particles can pass through the membrane.

Serve as enzymes (may speed reactions).

Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell.

Proteins

Proteins help things get across membrane

Some go all the way through like a

channel

Some use energy

Cell Function:

transport

osmosis and diffusion

homeostasis and

transport

• Cell membranes help organisms maintain

homeostasis by controlling what substances may

enter or leave cells

• To stay alive, a cell must exchange materials with its

environment

• These materials must cross the cell (plasma)

membrane

Cell membranes have protein channels

Selectively permeable: Allows some

molecules in and keeps other molecules

out

Remember…

The structure helps it be selective!

• simple diffusion

• facilitated diffusion

• osmosis

NO expenditure of Energy

uses concentration gradient

expenditure of Energy

does NOT use

concentration gradient

the cell regulates movement in and out by two main methods:

• protein pumps

• endocytosis

• exocytosis

Biological membranes are…

SEMI-PERMEABLE -- A.K.A.

Selectively permeable

Cell membranes are picky

What diffuses across the

membrane???

Small, uncharged

(non-polar) molecules

Examples:

Carbon dioxide

Oxygen

Types of Cellular Transport

Passive Transport

cell does not use energy

diffusion

facilitated diffusion

osmosis

Active Transport

cell does use energy protein pumps

exocytosis

endocytosis

high

low

This is

gonna be

hard

work!!

high

low

Weeee!!!

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Diffusion Particles constantly move

Collide randomly

Spread out randomly

Diffusion is moving from area of HIGH conc. to

area of LOW conc.

This is what we call the CONCENTRATION

GRADIENT

diffusion

Caused by Brownian motion

(movement of particles because of

the movement of their atoms).

Continues until an equilibrium is

reached (no gradient).

Dynamic equilibrium – particles

continue to move freely and are

evenly distributed.

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biology1111/animations/transport1.html

What happens when we reach

equilibrium?

Particles continue moving across membrane but

in both directions!

***No more changes in concentration

Osmosis

Diffusion of water across a selectively permeable membrane.

Occurs until water is balanced on both sides of the membrane.

How Osmosis works…

We have water molecules and some other molecules, let’s

say sugar

If we have more solute concentration on one side of the

membrane, then the water will diffuse across…

Membrane will let water thru but not sugar

Water can move back and forth (not sugar)

Cell Concentrations

Hypertonic solutions – more dissolved solute.

Hypotonic solutions – less dissolved solute.

Isotonic solutions – the same dissolved solute.

hypotonic

solutions

hypertonic

solutions

isotonic

solutions

Why don’t all our cells

burst?

What are our cells (animal) contained in? Fresh water?

No…..blood or other fluids, which are ISOTONIC The conc. Of sugars, salts, proteins and molecules in these

fluids is the same as the conc. in the cell

Bacteria and plant cells They DO come in contact with fresh water…what do

they have PROTECTING them?

CELL WALL Prevent cells from expanding even when there’s a lot of

osmotic pressure

But, cell walls are prone to injuries when there is too much osmotic pressure

Overcoming Osmosis

Contractile vacuoles – expel excess water from bacterial cells that live in water.

Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis).

Osmotic Pressure

Central vacuole fills with water and exerts and

out ward pressure on cell membrane and cell

wall

Cell wall does NOT allow cell to expand past a

certain size

Osmoregulation

Osmoregulation

means by which cells keep the

concentration of cell cytoplasm or blood

at a suitable concentration.

Problems in Plant Cell

How do big molecules diffuse

across a membrane so

quickly if the membrane is

selectively permeable?

Problems for diffusion…

Things too large (like Glucose!)

Charged molecules and Polar molecules

Positive/negative

Opposites attract but likes do not…

These all present serious problems for things

getting across membranes…

Facilitated Diffusion

What does facilitate mean?

Proteins are the extra help

“escorts” across the membrane

We call these membrane proteins…

Protein channels

Carrier proteins

LIGAND receptor proteins

Facilitated Diffusion

•takes place through proteins, or assemblies of proteins, embedded in

the plasma membrane

Protein channels

Carrier proteins

LIGAND receptor proteins

Facilitated Diffusion

(continued)

FAST

SPECIFIC

Still diffusion

so we only see

it from high

concentration

to low

Does NOT

require energy

What about when we want to

go against the

concentration?

(From low concentration to

high?)

What do we need???

ENERGY!!!!

Cellular Transport [2]

Active transport – energy is needed to move particles.

Carrier proteins – embedded proteins change shape to open and close passages across the membrane.

Endocytosis – taking something into the cell.

Exocytosis – expelling something from the cell.

Exocytosis

Exo- means…

Cyto means…

Sis means…

When cell releases large amounts of material

Excretes stuff

Membrane of vacuole surrounding particle inside cell fuses with the cell membrane

The contents in vacuole are then forced out of the cell

We see this in removal of water by contractile vacuoles