Ch. 5

Homeostasis

&

Cell Transport

5.1

Homeostasis & Permeability

Homeostasis – ability of cell to maintain balance

needed for life

To maintain balance:

cells must transport needed materials into cells

&

waste materials out of cells

Movement in or out of a cell depends on

permeability

Permeability

the ability of a molecule to move across a membrane

If molecule can move across, membrane is permeable to molecule.

If molecule cannot move across, membrane is impermeable to molecule.

most membranes are Selectively (semi) Permeable:

some molecules move across, others can't

Membrane is “Choosy”

Two Main Types of Cell Transport

1. Passive Transport

2. Active Transport

Passive Transport

Movement of substances across a membrane

without using energy

Molecules move down the Concentration

Gradient

Concentration gradient - difference in

concentration of molecules across a membrane

Molecules move HIGH TO LOW!

3 forms of Passive Transport:

a. Simple Diffusion

b. Osmosis

c. Facilitated Diffusion

a. Simple Diffusion:

Movement of SOLUTE across a membrane!

No energy needed!

HIGH TO LOW!

Which way will the purple molecules move?

To the right!

Molecules move back and forth until balance is

reached.

Balance = Equilibrium

When equilibrium is reached:

molecules still move but concentration does not

change.

ex: 2 molecules in = 2 molecules out

5.2

Osmosis

b. Osmosis

Diffusion of water across a membrane

Passive: no energy needed

Water is the solvent in cells BUT movement of

water depends on concentration of solute

To determine which way water will move, 1st

describe the solution:

3 types of solutions:

– a. Hypertonic: MORE solute outside cell

– b. Hypotonic: LESS solute outside cell

– c. Isotonic: solute is EQUAL inside & outside

Hypertonic Solutions & Osmosis

cell

Sugar molecule

Which way will

the water move?

Water will move

OUT of the cell

Cell will shrivel

Hypotonic Solutions & Osmosis

cell

Sugar molecule

Which way will

the water move?

Water will move

INTO cell

Cell will swell

Isotonic Solutions & Osmosis

cell

Sugar molecule

Which way will

the water move?

Water will move

IN & OUT of cell

Cell will stay the

same

Osmosis & Red Blood Cells

What Makes Water Move?

Osmotic Pressure:

Pressure created by water molecules

If water concentration is , osmotic pressure is

If water concentration is , osmotic pressure is

Water moves from an area of HIGH osmotic

pressure to an area of LOW osmotic pressure

HIGH TO LOW!

Osmosis in Plants

Osmosis affects Turgor Pressure - pressure of water that presses cell membrane against the cell wall

if soil is hypertonic (dry): water moves out of cell, turgor pressure , membrane pulls away from wall, plant wilts

if soil is hypotonic (wet): water moves into cell, turgor pressure , membrane pushes against from wall, plant is crisp

c. Facilitated Diffusion

•

passive transport

molecules insoluble in lipids or too large

molecules need help to cross

2 types:

1. Carrier proteins

2. Ion channels

Carrier Proteins

Proteins in membrane

Move substances from high to low

Proteins are specific: transport only

1molecule

Glucose moves this way

Ion Channels

Na+, Cl-, ions are important to cell function

Insoluble in lipid bilayer, can’t cross alone

Proteins form channels

Passive transport: high to low

Some channels always open, others have

“gates” that open in response to stimuli

5.3

Active Transport

Active Transport

Move substances up(or against)

concentration gradient: from low to high

concentration

Requires input of energy

2 types: cell membrane pumps, and

movement of vesicles

Membrane Pumps

Protein pumps that move substances from

low to high concentration

Require energy of ATP

ex: sodium-potassium pump: moves Na+

and K+ ions low to high

important to nerve function

Movement in Vesicles

move molecules too large to diffuse

move large quantities at one time

ex: nutrients & macromolecules

2 types:

Endocytosis

Exocytosis

Endocytosis

cells ingest large materials INTO cell

materials fill pouch in membrane

pouch forms vesicle

lysosomes attach & spill in enzymes

Pinocytosis: endocytosis of liquids

Phagocytosis: endocytosis of solids

Phagocyte: cell that ingests solid material

ex: white blood cells – ingest bacteria

Exocytosis

Material moved OUT of cell

Vesicle carries materials to membrane

Fuses w/ membrane & opens to outside

Proteins & wastes removed this way

Endo

Exo