Unit 3B Cell Membrane & Transport

What is Cell Transport?

• Transport- the movement of materials between an organism and its environment

• Cellular Transport- movement of materials into and out of a cell

• Materials move from the water based solution outside the cell (its environment) to the water based solution inside the cell (the cytoplasm/cytosol), or vice versa

• Movement occurs across the cell membrane

Solutions

• Solution- a type of mixture in which all components are evenly distributed

• Made of a– Solvent- the substance that does the dissolving

(ex: water is an excellent solvent)– Solute- the substance that is dissolved

• Cells are surrounded by aqueous (water based) solutions and contain an aqueous solution called cytoplasm

Cell Membranes

• Found surrounding all cells• Made of a lipid bilayer• Protects and supports the cell• Regulates what comes in and out of the cell

The Lipid Bilayer• Has 2 layers of lipids (“Bi” means two)• The layers are made up of molecules called

phospholipids • Each phospholipid has a HYDROPHOBIC fatty acid tail

region • and a HYDROPHILIC head region

• HYDRO = means water • PHOBIC = means afraid• PHILIC = means loving

Hydrophilic head region

Hydrophobic tail region

Lipid Bilayer (Continued)Because the fatty acid tails are “afraid” of water (hydrophobic), they turn towards each other so they won’t be exposed to the cytoplasm, or the outside of the cell (both of which contain water)

The Fluid Mosaic Model• “Fluid” means moving• “Mosaic” means many different things put

together– The Cell membrane contains different types of

molecules that can move around through the phospholipidsCarbohydrates attached to proteins and act as signals for molecules to attach (NOT AS MANY)

Proteins in Membrane- allow for transport of molecules across the membrane (ABUNDANT)

The Cell Membrane is Selectively Permeable (semipermeable)

• Some substances can pass through the membrane and some cannot.

• The structure of the membrane and the proteins in it decide which molecules can enter and which can leave.

How does a cell membrane help a cell maintain homeostasis?

–By transporting substances across the membrane to achieve equilibrium (the correct amount of each material inside and outside each cell)

Build a Cell Membrane Activity

2 Main Types of Transport

–passive -no energy needed from the cell–active -energy from cell needed

1. Passive Transport• Diffusion- particles move from an area of high

concentration to an area of low concentration to maintain equilibrium– This is called movement “down the concentration gradient”– A concentration gradient occurs where there are two

different concentrations of a particle on either side of the membrane

– Particles move straight through the lipid bilayer and no energy is required. Ex: O2 and CO2

Dynamic Equilibrium• Once diffusion has occurred and the cell has reached

equilibrium, will the solutes stop moving entirely?• No- they still continue to move across the membrane

but with equal amounts going in and out (no “net Movement”)

• This is called dynamic equilibrium (dynamic means movement)

What happens if a molecule/solute is too large and/or not dissolvable in lipids?

Facilitated Diffusion

-the molecule/solute will go through a carrier protein specific for that molecule.-still goes from high to low concentration;

so no energy needed.

Facilitated Diffusion

Water Can Use a Channel to get through the Cell Membrane

• Although possible, water has difficulty dissolving through the cell membrane by regular diffusion. Why do you think this is?

• The inside of the cell membrane is hydrophobic

• Water needs the aquaporin protein channel to get through quickly!

• This is an example of facilitated diffusion

Osmosis• The simple or facilitated diffusion of water

molecules across a cell membrane • From where there is more water (less solute) to

where there is less water (more solute)• Depends upon the solvent/solute ratio

Solutions can be classified by the amount of solutes they contain compared to other

solutions:• HYPERTONIC: A solution with a greater solute

[concentration]– (Ex: a solution with 20% sugar and 80% water is

hypertonic to a solution of 10% sugar and 90% water)• HYPOTONIC: a solution with a lower solute

concentration– (Ex: a solution of 5% NaCl and 95% water is hypotonic to a

solution of 20% NaCl and 80% water)• ISOTONIC: a solution with equal solute concentration

with another solution– (Ex: outside of a cell has 10% NaCl and the cytoplasm

inside the cell also has 10% NaCl)

What happens to our cells if they are placed into a/an…

HypErtonic Solution-• Since the [solute] is higher

and the [water] is lower outside the cell, the cell attempts to correct this by rushing water out

(osmotic pressure)• Result= cell shrinks and dies• E for water exiting the cell

HypOtonic Solution-• Since the [solute] is

lower and [water] is higher outside the cell, water rushes into the cell (osmotic pressure)

• Result= cell enlarges, can burst and die

• O for the cell “opens the door” to let water in

Isotonic Solution-• Since the solution has a

similar [solute] and [water] as the cytoplasm, water moves in/out at equal rates

• Dynamic equilibrium• In addition to osmosis,

solutes can also be moving at the same time by diffusion or facilitated diffusion

¨ Passive transport animations

Osmosis in Plant and Animal Cells

What is different about the plant and animal cells during the process of osmosis? _______________________________________________________Plant cells have cell walls which help them keep their shape .

The central vacuole in each plant cell stores the water.

Practice

• What is the solvent concentration of a solution with a 3% concentration of solute? __________

• What is the solvent concentration of a solution with a 15% concentration of glucose? __________

• What is the solute concentration of a solution with 98% solvent? _________________________

• What is the solute concentration of a solution with 75% water? _________________________

97%

85%

2%

25%

Practice

Conditions Water will…

(move in, out, or both?)

Environment is…

(Hypotonic, hypertonic, or isotonic?)

Solute concentration in the environment is

equal to that in the cell

Solute concentration in the environment is

greater than the cell

Solute concentration in the environment is less

than the cell

Predict how water will move in each of the following conditions. Name each environment with the correct solution type.

Fill in…

Conditions Water will… Environment is...

Solute concentration in the environment is equal to that in the cell

Solute concentration in the environment is greater than the cell

Solute concentration in the environment is less than the cell

Move in and out in equal amounts

No net movement

Isotonic to the cellCell is isotonic to its

environment

Move OUT of the cellThe Cell Shrinks

Hypertonic to the cellCell is hypotonic to its

environment

Move INTO the cellThe Cell Swells

Hypotonic to the cellCell is hypertonic to its

environment

Practice

Practice

Practice• Draw the following cells if placed in each type

of environment:Hypertonic

Hypotonic

Isotonic

Cell stays the same

Cell swells and possibly bursts

Cell shrinks

2. Active Transport• Cells move substances from low

concentrations to high concentrations• This is also called moving substances “UP

their concentration gradient” • Energy is needed in the form of ATP

A. Molecular Active Transport• Uses a protein in the cell membrane to carry the

substance across• Allows for a cell to purposely concentrate a

molecule in a particular location even when the forces of diffusion will want the molecule to move in the other direction

Example:Sodium-Potassium Pump

B. Bulk Transport (for large Molecules)

1. Some substances are too large to pass through any of the pores, channels, and pumps in the cell membrane

2. Or large quantities need to be transported into/out of the cell

3. Vesicles are produced by the membrane: pouches that are extensions of the cell membrane that pinch off and surround the substance

Bulk Transport: a. Endocytosis• Bring in large particles (ingestion)• 2 Types:Pinocytosis- transports large amounts of small solutes or

liquidsPhagocytosis- transport of large molecules or whole cells

(ex. white blood cells “eat” damaged cells)

Cell membrane surrounds the particles and pinches inward to bring the materials into the cell using a vesicle.

Bulk Transport: b. Exocytosis

• Reverse of endocytosis• Materials to be released

from the cell are taken into a vesicle

• The vesicle fuses with the cell membrane and releases the materials from the cell (ex: releasing proteins)

• This is how wastes are removed from many cells