Chapter 5

Homeostasis and Transport

Homeostasis

The property of a system that regulates its internal environment and tends to maintain a stable, constant condition

Cell membranes help organisms maintain homeostasis by controlling what substances enter and leave cells

Passive Transport

Substances crossing the cell membrane without any input of energy by the cell – move down their concentration gradient

Types: Diffusion Osmosis Facilitated Diffusion Ion Channels

Concentration Gradient Concentration - how many of a

substance’s molecules are sitting in a specific volume of a solution

Gradient - a measurement of how much something changes as you move from one region to another

Concentration Gradient - the difference in the concentration of molecules across a space

Concentration Gradient

Diffusion

Movement of molecules from an area of high concentration to an area of low concentration

The process by which molecules intermingle as a result of their kinetic energy of random motion.

Diffusion

Diffusion

Add a sugar cube to

a beaker of water

It sinks to the bottom

making the concentration

of sugar higher there

Diffusion

As the cube dissolves

sugar molecules break away

and move from the bottom to

the top of the beaker

Diffusion

Because of their kinetic

energy, the molecules of

sugar are in constant motion They keep moving until

they hit something and then

they rebound

Diffusion

If no object blocks their

movement, molecules

continue on their path They move down their

concentration gradient from

areas of high concentration

to low concentration until

They reach equilibrium

Do Now

What is homeostasis? Do forms of Passive Transport require cell

energy? What is diffusion?

Equilibrium

Diffusion will eventually cause the concentration of molecules to be the same throughout

Equilibrium – when the concentration of the molecules of a substance is the same throughout a space

The molecules are still moving, but they are just as likely to move in one direction as the other – they balance each other

Equilibrium

Diffusion Through Cell Boundaries

Cell membrane regulates movement of dissolved molecules from the liquid on one side of the membrane to the other

Cell membrane - selectively permeable - some substances can pass across it and some cannot

Nonpolar molecules can diffuse through the lipid bilayer as well as small molecules

Diffusion Through Cell Boundaries

If a molecule can pass through a cell membrane, it will diffuse from an area of high concentration on one side to an area of low concentration on the other

The cell is not required to use energy for diffusion.

Diffusion in a Cell

Osmosis

Solution = solute + solvent

Osmosis

Both solute and solvent molecules can diffuse

In cells: the solutes are organic and inorganic

compounds the solvent is water

Osmosis

Osmosis - diffusion of water through a selectively permeable membrane (cell membrane)

Water moves down its concentration gradient

It moves from areas of high concentration of water to low concentration of water

Osmosis does not require energy

Osmosis

When water is more concentrated on one side of the membrane, water will move to an area of lower concentration in order to re-establish equilibrium.

Question?? If there is more salt outside of the cell, will

water move into the cell or out of the cell??

Osmosis

The net direction of osmosis depends on the concentration of solutes on the two sides of the semi-permeable membrane

In a cell, this can have important consequences

Types of Solutions

Solutions can be:IsotonicHypotonicHypertonic

Isotonic Solution

The concentration of solutes is the same inside and outside cell

Water will diffuse into and out of the cell at equal rate

No net movement of water

Hypotonic Solution

The solution has a lower solute concentration than the cell

Water diffuses into the cell until equilibrium is established

Net movement of water – into the cell

Hypertonic Solution

The solution has a higher solute concentration than the cell

Water diffuses out of the cell until equilibrium is established

Net movement of water is out of the cell

Osmotic Pressure

Osmosis exerts a pressure (osmotic pressure) on the hypertonic side of a selectively permeable membrane. (This could cause water to rush into cells and cells could bust)

This does not happen in animal cells because they are usually in isotonic fluids example: blood

Animal Blood Cells in Different Solutions

Isotonic Hypertonic Hypotonic

The Effects of Osmosis on Animal Cells

osmosis

Osmotic Pressure

Plant and Bacteria cells are usually in hypotonic environments (water wants to diffuse into the cell: exposed to tremendous osmotic

pressure Rigid cell wall keeps plant and

bacteria cells from bursting.

Isotonic HypertonicHypotonic

Plant Cells in Different Solutions

Facilitated Diffusion

Some molecules easily pass through the cell membrane because they dissolve in lipids (alcohols) - others can not (glucose)

Specific carrier proteins allow these other molecules to pass through the cell membrane easily

This does not require energy (type of diffusion) - only occurs when concentration is higher on one side of the membrane than the other.

Facilitated Diffusion

Carrier protein binds to molecule and changes shape

Carrier protein shields molecule from hydrophobic lipid bilayer

Carrier protein releases molecule inside cell

Carrier protein returns to its original shape

Facilitated Diffusion

Ion Channels

Some ions are important for cell functions (Na+, K+, Ca2+ and Cl-)

Since they are charged and hydrophilic, they can’t get across the lipid bilayer

Ion channels – specific membrane proteins that help ions get across cell membrane

Active Transport

Requires cell’s energy Cells must move substances up

their concentration gradient Substances go from areas of lower

concentration to areas of higher concentration

Types of Active Transport

Cell Membrane Pumps

Endocytosis

Exocytosis

Cell Membrane Pumps

Carrier proteins also help out in active transport and work against the concentration gradient: Low High

Similar to facilitated diffusion, but cell’s energy is required

Sodium (Na+) - Potassium (K+) Pump

Many types of animal cells need to have: a high concentration of Na+ outside the cell a high concentration of K+ inside the cell

The sodium-potassium pump uses cell energy to maintain this concentration difference

Sodium (Na+) - Potassium (K+) Pump

Three Na in the cytoplasm bind to the protein pump

At the same time, the protein splits a phosphate from an ATP molecule (energy)

Protein carries the three Na across the lipid bilayer and releases them out of cell

Sodium (Na+) - Potassium (K+) Pump

Next the protein binds two K from outside of cell

As K bind, the phosphate is released and the protein changes shape again

Finally the protein releases the K into the cytoplasm of the cell

This creates an electrical gradient across cell membrane (nerve and muscle cells)

Endocytosis and Exocytosis

Some substances (macromolecules and food particles) are too large to enter cell through membrane proteins

Endocytosis and exocytosis can move these substances across cell membrane

Requires cell energy – active transport

Endocytosis Taking material into

the cell by means of infoldings, or pouches in the cell membrane

Pouches pinch off and form vesicles (organelles) in cytoplasm.

Two Types of Endocytosis

Phagocytosis: cell membrane engulfs food or whole cells (bacteria and viruses)

Pinocytosis: cell membrane engulfs liquids

Exocytosis

Membrane of vesicle surrounding material fuses with cell membrane, forcing contents out of the cell

Reverse of endocytosis

Release of proteins