passive cellular transport -...

Passive Cellular Transport Unit 2 Lesson 4

Passive Cellular Transport

Students will be able to: • Define passive cellular transport • Describe the structure of the cell membrane and how molecules

move through it via simple diffusion. • Define surface area to volume ratio. • Calculate the surface area to volume ratio for a hypothetical cell. • Explain why surface area to volume ratio is important to the survival

of the cell. • Define osmosis. • Describe how different environments affect the water content of the

cell. • Describe facilitated diffusion


Key Vocabulary:

• cell membrane • concentration gradient • crenation • diffusion • equilibrium • extracellular space • facilitated diffusion • flaccid • hypertonic • hypotonic • hydrophilic • hydrophobic

• intracellular space • isotonic • lipid bilayer • lysis • osmosis • partially permeable membrane • passive transport • phospholipid • plasmolysis • surface area to volume ratio • transport protein • turgid


• As cells perform their functions, they need to absorb nutrients and food from their environment and remove waste products.

• Thus, cells must have mechanisms to move materials in and out of the cell through the cell membrane.

THE CELL MEMBRANE AND

DIFFUSION


Passive Cellular Transport The Cell Membrane Lipid Bilayer

• Recall that the cell membrane is a double-layered structure made of phospholipids.

• This is called the “lipid bilayer”. The phospholipids in the membrane have two parts:

– a hydrophilic (attracted to water) head

– a hydrophobic (repelled by water) tail


The Cell Membrane Lipid Bilayer

• The hydrophilic heads and tails line up.

• The phospholipids heads are attracted to the cytoplasm and the watery extracellular fluid.

• The hydrophobic tails are repelled by the cytoplasm and the extracellular fluid because they are watery so they point their tails pointing inwards towards each other.


The Cell Membrane Lipid Bilayer

• Small molecules such as oxygen and water are able to pass through, while other molecules are kept out.

• The cell membrane is referred to as “semi” or “partially” permeable.

• Other structures inside the cell which also have membranes have a similar structure surrounding them.


Comprehension Check

Can you…

Identify the two parts of a fatty acid?

Describe why the cell membrane is semi or partially permeable?


ANSWERS…

Identify the two parts of a fatty acid?

Describe why the cell membrane is semi or partially permeable?

The membrane is able to permit some molecules to pass through while preventing others.


Other Structures found in the Cell Membrane

• Cells also need other substances such as glucose, certain ions and other macromolecules to be brought into the cell.

• Many of these cannot pass directly through the membrane and therefore require assistance to be absorbed.

• These include…

Passive Cellular Transport Cholesterol prevents the membrane from becoming stiff by ensuring

that the phospholipids are not too closely packed together.


Glycolipids are responsible for recognizing other cells within the

organism.


Glycoproteins are essential for cell to cell communication and assist

with transport of molecules through the cell membrane.


Peripheral proteins - maintain the cell’s cytoskeleton.

Passive Cellular Transport • Channel/Integral proteins - are transport proteins which assist

specific molecules with entering and exiting the cell.


• Globular proteins – also act as transporters during facilitated diffusion.


• Structural proteins – provide the cell with support and shape.


Alpha helix proteins – these are able to ‘plug’ any gaps in the

phospholipid bilayer.


Structures

1. Cholesterol

2. Alpha helix proteins

3. Glycoproteins

4. Glycolipids

5. Channel Proteins

Functions

a) Cell to cell communication

b) Preventing stiffness

c) Cell to cell recognition

d) Transporting molecules

e) Plugging gaps

Comprehension Check... Match the structure in the cell membrane with its function


Structures

1. Cholesterol

2. Alpha helix proteins

3. Glycoproteins

4. Glycolipids

5. Channel Proteins

Functions

b) Preventing stiffness

e) Plugging gaps

a) Cell to cell communication

c) Cell to cell recognition

b) Transporting molecules

Comprehension Check: Answers


Moving Molecules into and out of the Cell

• This is called cellular transport.

• Passive cellular transport expends no energy.

• Passive cellular transport relies on:

– the concentration gradient to move substances across the cell membrane

– the kinetic energy of the particles involved.


Concentration Gradient

• occurs when there is a greater number of particles of the same type in one area than another.

• The bigger the difference in concentration, means a steeper concentration gradient which means a faster diffusion rate.


There are three types of passive transport which are used to move

particles into and out of the cell:

1. Diffusion

2. Osmosis

3. Facilitated Diffusion



1. In which direction do molecules usually move?

2. What does passive cellular transport mean?

3. What are the three types of passive cellular transport?


Comprehension Check

1. In which direction do molecules usually move?

2. What does passive cellular transport mean?

3. What are the three types of passive cellular transport?

From an area where there is a high concentration of molecules to an area of low concentration of molecules

The movement of molecules into and out of the cell without using energy.

Diffusion, osmosis and facilitated diffusion.


Diffusion is the net movement of small or uncharged particles from an area of high concentration to an area of low concentration.


• Molecules and ions are in constant motion

• As they collide with each other they can spread through a medium and spread out evenly.

• Once the substance has fully dispersed through the medium it is said to be at equilibrium.


How molecules diffuse through the cell membrane.

1. There is a high

concentration of particles outside the cell (in the extracellular space), creating a steep concentration gradient.

Passive Cellular Transport How molecules diffuse through the cell membrane.

2. Over time as the particles move through the cell membrane into the cell (the intracellular space), the concentration gradient will decrease.

Passive Cellular Transport How molecules diffuse through the cell membrane.

3. Once the same number of particles are in both the extracellular and intracellular spaces equilibrium is reached.

Passive Cellular Transport Factors which affect the Rate of Diffusion

Several other factors also affect diffusion rates. These are:

1. Molecule size – smaller molecules move faster than larger ones.

2. Temperature – higher temperature makes molecules move faster.

3. Membrane thickness – a thicker membrane means slower diffusion rate.

4. Cell surface area–greater surface area of the cell, means more area available for diffusion.


Comprehension Check:

1. Draw a simple diagram to show how diffusion works

2. Describe the arrangement of particles when equilibrium is reached.

3. How does a thicker membrane affect diffusion?



1. Draw a simple diagram to show how diffusion works

2. Describe the arrangement of particles when

equilibrium is reached.

3. How does a thicker membrane affect diffusion?

High concentration Low concentration

Diffusion

Evenly spread out, uniform

It slows down the rate of diffusion

SURFACE AREA TO VOLUME RATIO



Surface Area to Volume Ratio

• The surface area of a cell is the area that is in contact with the extracellular space.

• As the cell increases in size, it's surface area will also increase accordingly, allowing for greater diffusion.

• However, this change in size also changes the cell’s volume. In fact, increasing the size of the cell has a much greater effect on the volume of the cell than it does on its surface area.

• If a cell becomes too large, substances are unable to reach the parts of a cell that need them.



• The surface area to volume ratio of a cell is defined as the area on a cells surface compared to the area inside the cell.

• Smaller cells have a much greater surface area to volume ratio which allows for the efficient diffusion of substances throughout the entire volume of the cell.



• Surface area to volume ratio can be used to justify the shape of many cells and cell surfaces. For example: – folds inside the mitochondria or the

flat pancake-like structures inside chloroplasts provide a greater surface area on which specific reactions can occur



• Surface area to volume ratio can be used to justify the shape of many cells and cell surfaces. For example: – folds in the lining of the human stomach or

the tiny, finger-like projections that extend out from the intestinal wall both act to increase the surface area without increasing the overall size or volume of the organ


Calculating Surface Area to Volume Ratio

• Cells are often depicted as cubes for surface area to volume calculations, with their dimension in centimeters.

• For a ‘cell’ with sides of 1 cm in length, the ratio can be calculated as follows: – Surface area = 6 x area of a single side = 6 x 1 x 1 = 6 cm2

– Volume = width x depth x height = 1 x 1 x 1 = cm3

– Surface area to volume ratio (SA:V) = 6:1


Calculating Surface Area to Volume Ratio


Comprehension Check:

1. How does surface area affect diffusion?

2. Calculate the surface area to volume ratio for a cell which is 2.5cm x 2.5 cm x 2.5 cm and one which is 1.5cm x 1.5cm x 1.5cm .

3. Which has the greatest efficiency? Why?



1. How does surface area affect diffusion?

2. Calculate the surface area to volume ratio for a cell which is 2.5cm x 2.5 cm x 2.5 cm and one which is 1.5cm x 1.5cm x 1.5cm .

3. Which has the greatest efficiency? Why?

A greater surface area allows more molecules to diffuse into and out of the cell at the same time, increasing efficiency.

Surface area: 2.5 x 2.5 x 6 = 37.5 cm2 1.5 x 1.5 x 6 = 13.5 cm2 Volume = 2.5 x 2.5 x 2.5 = 15.625 cm3 1.5 x 1.5 x 1.5 = 3.375 cm3 SA:Vol Ratio = 2.4: 1 4:1

1.5cm cell has greater efficiency, as it has a higher SA:Vol ratio

OSMOSIS AND FACILITATED

DIFFUSION



Osmosis is the diffusion of water through the partially permeable membrane from a region of high concentration to a region of low concentration.


• Cell membranes are completely permeable to water, and the amount of water in the environment has a large effect on the survival of a cell.

• Water cannot be compressed and the pressure it creates when inside the cell helps to keep the cell firm (or turgid).

• There are 3 types of solutions that involve water, each of which affects the cell differently:


Hypertonic Solutions

• In a hypertonic solution, there is a higher concentration of water inside the cell than outside the cell.

• A hypertonic solution has more solute (salt, sugar, etc.) than the cell, which causes there to be less water in the solution.

• Water flows from an area of high concentration to an area of low concentration and leaves the cell. This loss of water causes the cell to shrivel.


• In animal cells, the shriveling is called crenating.

• In plant cells, plasmolysis occurs, and the cell membrane shrinks away from the cell wall.

• Death will result in both cells.


Hypotonic Solutions

• In a hypotonic solution, the solution contains a higher concentration of water than the cell.

• A hypotonic solution has less solute than the cell, and this causes the solution to have more water than the cell.

• When a cell is placed in a hypotonic solution, water flows from an area of high concentration to an area of low concentration and rushes into the cell.


Hypotonic Solutions

• In animal cells, the cell bursts or will lyse, killing the cell. This is because animal cells lack a cell wall to support the cell membrane.

• In plant cells, the cell membrane is pressed up against the cell wall, but the cell wall does not allow the cell to expand anymore and the plant cell does not die.


Isotonic Solution

• In an isotonic solution, there is the same concentration of water on the outside of the cell as the inside of the cell.

• An isotonic solution has the same amount of solute as the inside of the cell.

• Water moves at a constant rate into and out of the cell, and the cell maintains its original shape.


Isotonic Solution

• In animal and plant cells, the cell keeps its shape when in an isotonic solution.

• Most cells live in an isotonic environment, and are able to maintain their water levels and survive.

Passive Cellular Transport Comprehension Check: 1. The diagrams to the right show 3 cells in

different sugar solutions. For each indicate the overall direction of water movement.

2. For each cell indicate the overall direction to

sugar movement.

3. Which of the cell drawings pictured right

would best represent each cell after 5 minutes?

Cell A Cell B Cell C

0% sugar solution

5% sugar solution

10% sugar solution

Left Center Right

Cells in different sugar solutions

Cells after 5 minutes

Passive Cellular Transport Comprehension Check: Answers 1. The diagrams to the right show 3 cells in

different sugar solutions. For each indicate the overall direction of water movement.

2. For each cell indicate the overall direction to

sugar movement.

3. Which of the cell drawings pictured right

would best represent each cell after 5 minutes?

Cell A Cell B Cell C

0% sugar solution

5% sugar solution

10% sugar solution

Left Center Right

Cells in different sugar solutions

Cells after 5 minutes

Cell A – water in, Cell B - no movement, Cell C - water out

All cells would show no movement

Cell A – right, Cell B – left, Cell C – center


Facilitated Diffusion

• Some molecules, such as sucrose are too large to pass through the cell membrane by simple diffusion.

• These molecules require the assistance of other molecules such as channel or carrier proteins which are found embedded in the cell membrane.



• This process is called facilitated diffusion and is…

the movement of large molecules from an area of high concentration to an area of low using proteins in the cell membrane.



• Glucose enters our cells from the bloodstream by facilitated diffusion.

• Large molecules cannot pass through the phospholipid bilayer and therefore require the assistance of a protein channel to help them enter the cell.

Passive Cellular Transport Comprehension Check… True or False? 1. Facilitated diffusion expends energy. 2. Facilitated diffusion uses glycoproteins to transport

molecules through the cell membrane. 3. Facilitated diffusion moves molecules down their

concentration gradient (from a high to low concentration).

4. Facilitated diffusion generally moves large molecules which are unable to pass through the membrane themselves.

Passive Cellular Transport Comprehension Check… Answers 1. Facilitated diffusion expends energy. 2. Facilitated diffusion uses glycoproteins to transport molecules

through the cell membrane. 3. Facilitated diffusion moves molecules down their concentration

gradient (from a high to low concentration). 4. Facilitated diffusion generally moves large molecules which are

unable to pass through the membrane themselves.

False

False

True

True

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