Chapter 5

“Homeostasis and Cell

Transport”

Section 1: “Passive Transport”What part of a cell maintains

homeostasis? Cell Membrane is selectively permeable

Passive transportPassive transport

– movement of molecules across a cell membrane without energy expenditure.

Types of Passive transport:1. Diffusion

2. Osmosis

3. Facilitated Diffusion

4. Ion Channels

1. Diffusion – movement of molecules from an area of [high] to

an area of [low].

Concentration gradient – difference in concentration between two areas - No energy is required to go from high to low [ ].

Totally driven by molecules’ kinetic energy – constant motion.

Low

HighThe steeper the slope, the faster the rate of diffusion.

Therefore, the greater the concentration differential, the faster the rate of diffusion.

• Movement of molecules continue until they reach equilibrium.

• An increase in temperature increases the kinetic energy of the molecules, therefore diffusion occurs faster. – Why sugar “melts” faster in hot vs. cold water

• Diffusion occurs across the cell membrane• Dependent upon:

– size of molecules– size of membrane pores

– chemistry of molecules (CO2 and O2) – nonpolar and noncharged except WATER

2. Osmosis = diffusion of water across a membrane

- net movement of water from [high] ---> [low]- direction of water movement depends on the [ ] of solutes on two sides (inside the cell and outside the cell)

HYPERTONIC VS.

HYPOTONIC

Molecules move from [high] to [low].

Inside Outside

Red Blood Cells in isotonic, Red Blood Cells in isotonic, hypotonic and hypertonic hypotonic and hypertonic

solutions (left to right)solutions (left to right)

Cytolysis Crenation

Isotonic Hypotonic Hypertonic

Hypertonic• Hypertonic Solution:

[Solute] outside the cell is higher than inside the cytosol– More dissolved particles outside of cell than

inside of cell; more water inside • Hyper = more of something • Tonic = dissolved particles/solutes

– Water moves out of the cell into solution – Cell shrinks – What happens when you put salt on a slug?– Salting meat

Elodea

Plasmolysis

loss of turgor pressure and cytoplasmic structure due to water in a plant cell

Hypotonic Solution:

[Solute] outside is lower than inside cell – Less dissolved particles outside of cell than

inside of cell; less water inside • Hypo = less, under • Tonic = dissolved particles/solute

– Water moves into cell from solution – Cell expands (and may burst)

Hypotonic

• Isotonic Solution:

– Solute concentration of solution equal to that of cell • No net water

movement• Most cells exists in

an isotonic external environment

Quick Review…

PlasmyolysisTurgor Pressure

1) Paramecium live in freshwater. They have a 3% salt internal environment. How do they keep from exploding in this hypotonic environment?

2) Other organisms live in a salt water environment. How do they keep their cells from shriveling up?

3) What do plants do?4) How do your kidneys work?

Real Concentration Problems

Freshwater Problemhttp://www.youtube.com/watch?v=pahUt0RCKYc

• Plants usually exist in a hypotonic environment.

• Cells don’t burst due to flexible and strong cell wall.

Saltwater critters

• They pump out excess salt, or pump in

water.

Your kidneys

• Extremely important organs in maintaining

osmotic balance (isotonicity) with body

fluids.

Plants

3. Facilitated diffusion3. Facilitated diffusion

- Diffusion with assistance from carrier proteins.

-can’t occur fast enough, chemistry is different, or too large to pass through pores.

- transport may occur into or out of cell.

- carrier proteins are specific to one type of

molecule.

Animation

Steps in Facilitated Diffusion:

1. Carrier protein binds to molecule to be transported.

2. Carrier protein changes shape (shielding it from hydrophobic interior

of bilipid layer).

3. Carrier protein releases molecule and goes back to its original shape.

Occurs with glucose (that can’t pass through cell on its own).

4. Diffusion thru Ion Channels4. Diffusion thru Ion Channels

– Na+, K+, Ca2+, and Cl- cannot pass through the membrane.

- Recall that ions cannot pass since they

aren’t soluble in lipids.

- Therefore, they need ion channels to provide a small passageway for them.

- Conditions for ion channel gates to open.1. Stretching of cell membrane

2. Electrical signals

3. Chemicals in cytoplasm

or external environment

Section 5.2 “Active Transport”Section 5.2 “Active Transport”

• Movement of molecules from [Low] to [High]• Movement is against the gradient • Energy is needed• Cell Membrane “Pumps”

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

– Sodium – Potassium pump – – Transports Na+ and K+ up [ ] gradient– Important for transmission of signals between

nerve cells

ExocytosisExocytosis and Endocytosis Endocytosis * Engulfing/releasing of macromolecules * Engulfing/releasing of macromolecules and food particles/wastes into/out of the and food particles/wastes into/out of the cell membranecell membrane

* Uses the cell membrane to make vesicles

Animation 1Animation 2

Endocytosis

– Cells ingest external fluids, macromolecules and large particles.

• Cell membrane surrounds material into a pouch.• Cell membrane around pouch pinches together to

form a vesicle.• Vesicle may fuse with lysosome for digestion

Two Types of Endocytosis:– Pinocytosis (cell drinking) if solutes or fluids

are involved– Phagocytosis (cell eating) if large particles or

whole cells are involved Occurs with bacteria, viruses or phagocytes (WBC)

Exocytosis – Release of vesicles through cell membrane. May be proteins, waste materials, or indigestibles.

• Vesicles fuse with membrane, release contents into environment

• Proteins packaged by Golgi Apparatus into vesicles which fuse with membrane to release excess proteins

• Animation