SBI4U Movement Across the Cell Membrane

How do particles go in and out of the cell if the membrane is

selective?

• Chemicals in our body must be kept in balance in order for us to survive

• Maintenance of constant conditions is called homeostasis

• To achieve homeostasis, materials must be exchanged from the inside of the cell to the extracellular environment

• There are two types of transport mechanisms across the cell membrane:– Passive transport (no energy used)– Active transport (needs energy)

• Animations of Active Transport & Passive

Transport

• The extracellular fluid is a mixture of water and dissolved materials that surrounds the cell

cellECF (extracellular fluid)

• Many small and uncharged molecules can move easily through the membrane (i.e. oxygen and water)

• Brownian motion - molecules are constantly moving randomly and colliding with each other – This is the force behind diffusion

Passive Transport

• Is the movement of materials (randomly) across the cell membrane without the expenditure of cell energy

• 3 types of passive transport:–Diffusion–Facilitated Diffusion (diffusion with the

help of transport proteins)–Osmosis (diffusion of water)

Diffusion1. Diffusion: random movement

of particles from an area of high concentration to an area of low concentration.

(High to Low)• Diffusion continues until all

molecules are evenly spaced (equilibrium is reached)

-Note: molecules will still move around but stay spread out.

http://bio.winona.edu/berg/Free.htm

Simple Diffusion Animation

2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane a.Transport Proteins are

specific – they “select” only certain molecules to cross the membrane – WHY?

b.Transports larger or charged molecules

Channel Protein hydrophilic channelNa+, K+, Ca2+ and Cl-

(voltage-channels in eukaryotes)

2. Facilitated Diffusion

Carrier Protein

• http://bio.winona.edu/berg/Free.htm

High Concentration

Low Concentration

Cell Membrane

Glucosemolecules

Proteinchannel

Passive Transport: 2. Facilitated Diffusion

Go to Section:

Transport Protein

Through a

Cellular Transport From:High

Low

• Channel Proteins animations

Osmosis

• Water moves freely through pores.

• Solute (green) too large to move across.

• diffusion of water through a selectively permeable membrane

• water from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration)

Osmosis animation

Factors that Affect the Rate of Diffusion

• Any ideas?• Molecule size• Molecule polarity• Charge on particle –

charged molecules/ions CANNOT diffuse across the cell membrane

• Temperature

H2O

Which way will water move?

• HYPOTONIC conditions: when the water concentration outside the cell is greater than inside the cell…water moves INTO the cell

Outside Inside

Hypotonic conditions contd.• If an animal cell is placed in a hypotonic

solution, it could burst through a process called lysis

• Plant cells become turgid (normal)

H2O

Which way will water move?

• HYPERTONIC conditions: when the water concentration inside the cell is greater than outside the cell…water moves OUT of the cell

Outside Inside

Hypertonic conditions contd.• Cells placed in hypertonic

solutions will shrink/shrivel (water exits)

• Plasmolysis is a condition in plants where the cell membrane shrinks away from the cell wall– Plants will wilt because of a loss

of turgor pressure (the pressure of the cell contents on the cell wall)

– Salt on roads – good thing?

H2O

Which way will water move?

• ISOTONIC conditions: When water concentrations outside and inside the cell are equal, equal amounts of water move in and out of the cell

H2O

Outside Inside

Isotonic conditions contd.

Isotonic conditions contd.

• Animal cells in isotonic conditions are normal

• Plant cells in isotonic conditions are flaccid (lacks stiffness)

• Side A is hypotonic to side A – why? – Side A has more water molecules than B

• Water will always diffuse from a hypotonic solution to a hypertonic solution until both sides have equal concentrations

A ABB *Solutes

cannot pass through the membrane

hypotonic hypertonic isotonic

hypertonic isotonic hypotonic

Why are cells so small?

• Copy the following table for a sphere into your notes and fill it out:

Diameter (cm)

Radius (cm) Surface Area (cm2)

=4r2

Volume (mL)

=4/3r3

Surface Area ÷ Volume

2

4

6

8

10

What can you conclude?• How does surface area change as the radius increases?• As the size of an object increases, does the surface area

or volume increase more rapidly?• What does this have to do with cells?

– The amount of nutrients that a cell can take in and the amount of waste that can be expelled depends on the amount of surface area

– Thus, as the cell size increases, at a certain point, the cell will not have enough surface area to support its volume

Active Transport

•cell uses energy

•actively moves molecules to where they are needed

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

(Low High)•Use carrier proteins

•Three Types:

Protein changes shape to move molecules: this requires energy - ATP!

Types of Active Transport

1. Protein Pumps -transport proteins that require energy to do work• Example: • Sodium / Potassium

Pumps are important in nerve responses.

Sodium Potassium Pumps (Active Transport using proteins)

A QUESTION!

HOW DO THE REALLY LARGE MOLECULES (Hormones,

polysaccharides etc.) move in and out of cells??

An Answer!!

By two processes called ENDOCYTOSIS AND EXOCYTOSIS. Both methods require the use of

vesicles and ATP!

Types of Active Transport

• 2. Endocytosis: taking bulky material into a cell • Uses energy• Cell membrane in-folds

around food particle• “cell eating”• forms food vacuole &

digests food• This is how white blood

cells eat bacteria!

Types of Active Transport

3. Exocytosis: • Forces material out of cell in

bulk• membrane surrounding the

material fuses with cell membrane

• Cell changes shape – requires energy

• EX: Hormones or wastes released from cell

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Endocytosis & Exocytosis

animations