ap bio module 11: water movement

Identify 5 reasons water is critically important to life

1. Solvent: Water dissolves many substances, allowing nutrients and other molecules to move through organisms.

2. Chemical reactions: Water participates in important reactions, including hydrolysis.

3. Transport: Water carries nutrients, gases, and wastes throughout organisms.

4. Temperature regulation: Water absorbs and releases heat slowly, helping organisms maintain stable temperatures.

5. Osmosis and homeostasis: Water movement across cell membranes helps maintain cell volume and proper water and solute balance.

What is a solute?

• Differences in water concentration occur when substances such as ions, amino acids, and sugars are dissolved in water

• These dissolved substances are called solutes

What is a solvent?

Water

What is molarity?

• A concentration of solutions which is equal to moles of a solute per liter of the solution

• Abbreviated as M

What is impermeable mean?

A membrane that blocks diffusion of water and solutes entirely

What does selectively permeable mean?

When a membrane allows movement of some molecules

What does permeable mean?

When a membrane allows free diffusion

What term describes the cell membrane?

Selectively permeable

Identify properties of the cell membrane that determines its permeability

1. Phospholipid bilayer: Allows small, nonpolar molecules through.

2. Membrane proteins: Help specific substances cross.

3. Molecule size: Smaller molecules pass more easily.

4. Polarity and charge: Polar and charged molecules usually need transport proteins.

5. Cholesterol: Regulates membrane fluidity and permeability.

Why is there a water carrier in the cell membrane and what is the water carrier called?

Aquaporins allow water to diffuse freely across the cell membrane much more quickly than would be possible by simple diffusion

How does water move across a cell membrane?

• By diffusion

How does water move in water concentration areas?

• Water moves from area where water concentration is high to areas where water concentration is low

  
  

How does water move in areas of solute concentration?

• Water moves from areas of low solute concentration to high solute concentration

What is osmosis?

The net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration

Identify where water concentration is greater

Side A

Identify where solute concentration is greater

Side B

If water can move, but solutes cannot, the membrane is _______

Selectively permeable

Identify which way water flows in this figure

Left to right

describe why there is a smaller arrow from right to left

Water concentration

Predict what occurs when concentration equalizes on both sides of a selectively permeable membrane

The net movement of water molecules by osmosis stops, although water molecules continue to move in both directions

One solute that easily crosses the cell membrane. One solute that does not

Not able to cross cell membrane: ions (sodium + potassium ions)

Able to easily cross cell membrane: lipids (steroids)

Describe why water, even though it is polar, is able to cross cell membranes easily by facilitated diffusion

Most membranes have channel proteins called aquaporins that allow the rapid diffusion of water across a membrane by facilitated diffusion

Define water potential

A parameter that combines all the physical + chemical factors that influence the movement of water, like pressure, osmosis, and gravity

• Water moves from regions of higher water potential to regions of lower water potential

What is osmotic pressure

The tendency of water to move from one solution to another by osmosis

Describe what happens when water is added to the water side of the tube

Water moves from the water side into the solute side through the selectively permeable membrane, causing the solute side to rise

Describe why the column on the side with the solutes stops rising

When the hydrostatic pressure caused by the height of the solution balances the osmotic pressure

Identify what the difference in column height between left and right sides of the tube represents

The osmotic pressure of the solution

Measuring differences in a U-shaped tube is one method for measuring osmotic pressure. Describe a second method

To apply pressure to the solution side until it stops the movement of water. The pressure required is a measure of osmotic presure

Define tonicity

The measure of osmotic pressure; the higher the osmotic pressure, the higher the tonicity

A hypertonic solution has a ____ solute concentration than another

Higher

A hypotonic solution has a ______ solute concentration than another

Lower

Isotonic solutions have ______ concentrations of solutes

Same

What will happen to a red blood cell that is placed in distilled water

Water will move into the red blood cell by osmosis bc distilled water is hypotonic compared with the cell.

• the cell will burst

What will happen to a red blood cell placed in a 10% salt solution

Water will move out of the red blood cell bc the salt solution is hypertonic.

• the cell will shrink

Describe the role of the contractile vacuole in protists, single celled eukaryotes

A contractile vacuole takes up excess water from inside the cell and expels it into the external environment

• helps maintain single celled eukaryotes maintain water balance —> cell will not burst

Plant cells exhibit turgor pressure, when water flows into a cell and presses against the ___.

Water will continue to enter the cell until the turgor pressure increases to a level sufficient to stop _____

• Cell wall

• Osmosis

4 groups of organisms in addition to plants that have cell walls

• bacteria

• Fungi

• Protists

• Algae

A plant in your classroom is drooping. Describe the problem at the cellular level.

The plant lost water, causing turgor pressure to decrease. As a result, the cells became less rigid, causing the plant to droop.

A plant in your classroom is drooping. Predict what will happen after watering the plant.

After watering, Water will enter the plant cells by osmosis

• turgor pressure will increase → cells become firm and the plant will stand upright

Define water potential

A measure of all of the factors that influence the movement of water (potential energy of water)

Water potential specially describes the ____ of water compared to a reference.

• Potential energy

• Determines how water moves due to solutes, turgor pressure, gravity

• Higher → lower water potential

Define each term for water potential:

Ψ = Ψₛ + Ψₚ

• Water potential measured in mits of pressure expressed as bars

• Solute potential - effect of solutes on the movement of water

• Pressure potential - effect of pressure in the movement of water

What would happen to pressure potential, Ψₚ , in an open container

An open container has a pressure potential Ψₚ of 0. The water of the open container is exposed to the atmosphere, no additional pressure being applied to the water.

equation for solute potential and define each term

Ψ𝑠=−𝑖𝐶𝑅𝑇

• measures the effect of solutes in a solution

• i= the ionization constant, equals 1 for a substance that does not ionize in water

• C = the solute concentration (water concentration/molarity)

• R = the pressure constant (0.0831 L x bars / mol x K)

• T = the temperature in Kelvin (C + 273)

State why pure water has a solute potential of 0

Does not contain any solutes

Predict what happens to water potential as concentration of solutes increases

Adding solutes increases osmotic pressure. The more concentrated a solution is, the higher the osmotic pressure, the lower + negative the solute potential → more likely to draw water across the membrane by osmosis

Water will move from areas of _____ water potential to areas of ____ water potential.

• higher

• Lower

Describe what happens with regard to the movement of water when an animal cell is placed in a hypotonic solution

water moves into the cell by osmosis and the cell swells/bursts

Describe what happens with regard to the movement of water when an animal cell is placed in a hypertonic solution

Water leaves the cell by osmosis and cell shrinks

3 factors that influence the movement of water

1. Solute concentration - Differences in solute concentration between two sides of a selectively permeable membrane lead to water movement by osmosis

2. Turgor pressure - Cell wall pushes back + creates pressure

3. Gravity - influences flow of water

State what drives bulk flow of solutions

Driven by differences in pressure

Describe the direction of bulk flow.

Flow is always from higher to lower pressure

Describe how animals can maintain high pressures

A mechanical pumps the heart and raises the pressure of the blood, causing it to flow from the heart through your arteries and veins

Describe how plants can maintain high pressures

Water loss in leaves lowers the pressure of the water at the top of the tree, causing water to flow from soil to leaves

What is the property of water that allows it to withstand large negative pressures

Hydrogen bonds → allows tensile (pulling) forces to be transmitted from one water molecule to another

Define osmoregulation

The regulation of water and solute levels to control osmotic pressure

Why is osmoregulation a form of homeostasis?

It maintains water balance inside cells and organisms

Identify two strategies for osmoregulation

1. Osmoconformers

2. Osmoregulation

Define osmoconformers

Organisms (animal) that keep their internal fluids at the same osmotic pressure as the surrounding environment

• solute concentration inside + outside are similar → not a lot energy regulating osmotic pressure

Describe the environments where osmoconformers live

Tend to live in environments that have stable solute concentrations (seawater)

• maintaining stable internal solute concentrations is easier in stable environments compared to fluctuating environments

Describe conditions in which osmoconformers might have fluctuating solutes

When the solute concentration of their surrounding environment changes

(Internal osmotic concentration tends to match the external environment)

5 osmoconformers species

• sea stars

• Mussels

• Lobsters

• Scallops

• Hagfish

Define osmoregulators

Expend considerable energy pumping ions across cell membranes to regulate the movement of water into or out of their bodies

Describe one advantage and one disadvantage of osmoregulation

Advantage: maintains a stable internal solute concentration despite changes in the environment

Disadvantage: requires ATP to actively regulate water + solutes

Describe what is meant by osmoregulation

Maintenance of constant osmotic pressure inside of cells or organisms

Describe the difference between osmoconformers and osmoregulators

Osmoconformers - animals that regulate their internal osmotic pressure by maintaining their initial solute concentration at a level similar to that of their environment

Osmoregulators - actively regulate their internal osmotic pressure, expending considerable energy to maintain an internal solute concentration that is different from that of their environment

One animal that is a osmoconformers, one annal that is an osmoregulators

Osmoconformers - hagfish

Osmoregulators- humans