CELLS AND THEIR ENVIRONMENT

PASSIVE TRANSPORT

Section 4-1

REVIEW What is homeostasis? Cells maintain homeostasis by

controlling the movement of substances across the cell membrane

RANDOM MOTION AND CONCENTRATION Passive transport is movement

across the cell membrane that does not require energy

Substances travel from areas of high concentration to areas of low concentration

The difference in the concentration of a substance across the space is the concentration gradient

An substance of high concentration will move into a lower concentration until it is in equilibrium or equal

MOVEMENT OF SUBSTANCES Particles of a substance will move

randomly If there is a concentration gradient

(uneven concentration) then diffusion occurs

Diffusion is the movement of a substance from an area of high concentration to low caused by the random movement of particles

MOVEMENT OF SUBSTANCES CONT… Many molecules and ions dissolve in

the cytoplasm and outside of the cell The concentration of substances is

different inside and outside of the cell Molecules will diffuse, to equalize,

and need to pass through the cell membrane

Selectively permeable

MOVEMENT OF SUBSTANCES CONT… The diffusion of molecules across the

cell membrane is the simplest type of passive transport

WATER DIFFUSES IN AND OUT Water molecules are small and can

diffuse through the cell membrane easily

Osmosis is the diffusion of water molecules through a selectively permeable membrane

OSMOSIS Osmosis is a form of passive

transport 3 possibilities of water movement

1. Water moves out2. Water moves in3. No net movement

WATER MOVES OUT When water diffuses out of the cell,

the cell shrinks A solution that causes a cell to shrink

is hypertonic (too much water goes out)

WATER MOVES IN When water moves into the cell, the

cell swells A solution that causes a cell to swell

because of osmosis is hypotonic Too much water goes in

NO WATER MOVES If the outside and inside of the cell have

the same concentration of water molecules, then water diffuses in and out at the same rateNo net movement

A solution that produces no change is isotonic

PROTEINS HELP TRANSPORT SUBSTANCES Transport proteins provide

passageways for polar molecules and ions to pass through the cell membrane

Each channel only allows a certain substance through

DIFFUSION THROUGH ION CHANNELS Sodium Na+, Potassium K+, Ca 2+ and

chlorine Cl-, are important in cell functions

Ion channels are a doughnut shaped transport protein for ions like Na+ to pass through

DIFFUSION THROUGH ION CHANNELS Some pores are always open, others

are closed by gates The rate of movement of a substance

across the membrane is determined by the concentration gradient

An ions movement across the membrane is determined by its charge

The inside of a cell is usually negative

FACILITATED DIFFUSION Carrier proteins carry specific

substances across the membrane They carry substances down their

concentration gradient Facilitated diffusion uses no energy

FACILITATED DIFFUSION

Step 1: molecule outside of cell binds to carrier protein

Step 2: carrier protein transports molecule across membrane

Step 3: molecule is released in the cell

ACTIVE TRANSPORT

Section 4.2

ACTIVE TRANSPORT Cells must transport certain amino acids,

sugars, and other substance from the cytoplasm to surrounding fluids

Low concentration to high concentration The concentration gradient will cause those

substances to move back into the cell The transport of a substance across a

membrane against its concentration gradient is active transport

SODIUM POTASSIUM PUMP This is one of the most important

membrane pumps in animal cells The sodium-potassium pump is a

carrier protein that transports 3 sodium ions out of a cell and two potassium ions into the cellSodium is more concentrated outside the

cell and potassium is more concentrated inside

Both ions are actively transported The energy needed to power the pump

is supplied by ATP

SODIUM-POTASSIUM PUMP

VESICLES MOVE SUBSTANCES Proteins and polysaccharides are too

big to be moved by carrier proteins These substances are moved by

vesicles Movement into the cell by a vesicle is

endocytosis Movement out of the cell by a vesicle

is exocytosis http://bcs.whfreeman.com/thelife

wire8e/content/cat_040/0504003.html

ENDOCYTOSIS

EXOCYTOSIS

HOW CELLS COMMUNICATE Cells need to be able to send and

receive signals to each other Cell release signal molecules that

carry info to other cellsHormones

HOW CELLS COMMUNICATE

The cell membrane contains special proteins that bind to signal molecules to receive messages

Receptor proteins bind to a specific signal molecule allowing the

cell to respond to the signal

RECEPTOR PROTEINS Most receptor proteins are embedded

in the lipid bilayer of cell membrane Binding a receptor protein to a signal

molecule changes the activity of the receiving cell

Changes 3 ways

CHANGES IN PERMEABILITY Receptor proteins may bind with ion

channels Channels open when signal molecules

bind to the protein Important in

nervous system

SECOND MESSENGERS Acts as a signal molecule in the

cytoplasm Amplifies the signal from the first

signal molecule

ENZYME ACTION Receptor proteins may act as

enzymes They may speed up reactions when

the receptor protein and signal molecule bind

May also activate other enzymes