Biology Teach Yourself Series - VCE Unit 3 & 4 biology - Homeunit3and4biology.weebly.com/uploads/5/3/5/9/53596141/...Topic 4: Plasma Membranes and Material Transport A: Level 14, 474

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Biology Teach Yourself Series

Topic 4: Plasma Membranes and Material Transport

A: Level 14, 474 Flinders Street Melbourne VIC 3000

T: 1300 134 518 W: tssm.com.au E: [email protected]

mailto:[email protected]


Contents

Plasma membranes and material transport…………………………………………………………………….3

Interactions between plasma membranes and water …………………………………………………………..4

The role of lipids in plasma membranes ………………………………………………………………………4

Structure of the plasma membrane…………………………………………………………………………….4

Factors affecting the rate of diffusion…………………………………………………………………………6

Review questions………………………………………………………………………………………………7

Material transport………………………………………………………………………………………………9

Review questions …………………………………………………………………………………………….10

Placing cells into solutions…………………………………………………………………………………...12

Plant cells in solutions………………………………………………………………………………………..12

Animal cells in solutions……………………………………………………………………………………..13

Review questions…………………………………………………………………………………………….14


Plasma Membranes and Material Transport

All living cells are enclosed in a plasma membrane. The membrane controls the entry and exit of substances

into and out of the cell. It also ensures that the internal environment of the cell is kept separate from the

external environment and this allows the cell to maintain an internal environment that differs from its

surroundings. The plasma membrane thus stops the free exchange of materials between the two

environments.

Interactions between the plasma membrane and water As it appears in Units 1 and 3

To understand the role of the plasma membrane one must understand the way that in which it interacts with

the substances around it. One particularly important molecule that cells need is water. All life requires water

to survive. Around 80% of a cell’s weight is made up of water and almost every important metabolic process

occurs in a watery medium.

Water is a polar molecule, which means that one end, the oxygen end, is negative and the other end, the

hydrogen end, is positive. The negative oxygen end of one water molecule attracts the positive hydrogen end

of another water molecule, forming a hydrogen bond.

O -

H + H +

O -

H + H +

Hydrogen Bond


As water is a polar molecule, it is able to interact with other polar molecules or ions. When these molecules

encounter water, they form hydrogen bonds and dissolve. These molecules are referred to as hydrophilic

(water loving). However, not all substances are able to dissolve into water. Non-polar substances are unable

to form hydrogen bonds with water and consequently do not dissolve. These molecules are referred to as

hydrophobic (water hating).

The role of lipids in the plasma membrane As it appears in Units 1 and 3

There are several different types of lipids located in the plasma membrane. Information relating to each is

shown below.

Type Description Function

Phospholipid A phosphate group is added to the

glycerol molecule. The phosphate is

hydrophilic and the fatty acids

hydrophobic.

Form the lipid bilayer of the membrane.

Glycolipid

A carbohydrate group attaches to the

glycerol

Vital for communication as they project from the

cell membrane and can detect and bind with

signaling molecules

Cholesterol Belongs to the group of lipids known as

steroids. Component of cell membranes

(except inner membrane of mitochondria

and chloroplasts) and myelin sheaths

around nerve cells

Important for maintaining a level of rigidity in cell

membranes

Cholesterol and phospholipids (like those found in the plasma membrane) are non-polar molecules that do

not dissolve in water, but they do dissolve into each other. These types of molecules can also be referred to

as lipophilic (lipid loving) and this allows the plasma membrane to control which substances can enter and

leave the cell.

Structure of the plasma membrane As it appears in Units 1 and 3

The plasma membrane is composed of a phospholipid bilayer, into which proteins and glycoproteins are

embedded.

The phospholipids that make up the bilayer have two distinct parts, a hydrophilic head and a hydrophobic

tail as shown below.


The hydrophilic head is attracted to the aqueous solution inside and outside the cell so that the head of one

layer faces out while the head of the second layer faces in. The hydrophobic tails repel water and face each

other in the centre of the two layers (as shown above). This prevents large water-soluble molecules from

passing freely through the plasma membrane.

Some of the proteins create channels that permit specific substances to pass through the membrane in either

direction. Other proteins act as receptors, which are able to bind to hormones and other substances that

interact with the cell. There are also proteins that attach to carbohydrate molecules to create antigens which

are used by the immune system to distinguish self from non-self. If the immune system determines that the

antigen is non-self it will destroy the cell. The plasma membrane also comprises cholesterol molecules

which help to maintain the membrane’s stability.

Hydrophilic Head

Hydrophobic Tail


Factors affecting the rate of diffusion As it appears in Units 1 and 3 Biological membranes are semi-permeable (selectively permeable). This means that some substances will

diffuse across a membrane easily, others will do so with difficulty and some will be completely unable to do

so unaided. There are a range of factors that will affect the rate at which a substance diffuses across a

membrane. These include:

Size: small particles such as oxygen gas, water and carbon dioxide are able to diffuse directly across the

plasma membrane. Large hydrophilic, uncharged molecules such as sugars do not freely diffuse across the

plasma membrane.

Polarity: Non polar substances diffuse more easily than polar substances.

Charge: Charged particles do not diffuse across the plasma membrane.

State: a substance will diffuse faster through a gaseous medium than a liquid or solid medium.

Temperature: heating particles gives them more kinetic energy causing the rate of diffusion to increase.

Surface area to volume ratio: increasing the surface area to volume ratio increases the rate at which

material transport occurs.

Concentration gradient: this is the difference in concentration between the internal environment of the cell

and the external environment. The greater the difference in concentration, the more rapid the rate of

diffusion will be.

It is important to remember that there may be more than one of these factors in operation, for example a

large lipid soluble substance such as a steroid will diffuse directly across the plasma membrane but a large

water soluble substance such as a protein will not.


Review Questions

1. A typical plasma membrane is made up of a phospholipid bilayer with a variety of proteins embedded

within the membrane on either the surface or penetrating the whole structure.

a. Channel proteins are one example of the proteins present in the plasma membranes. What is the

function of channel proteins?

b. What would happen to a cell if its recognition proteins were damaged so that they could no

longer carry out their function?

c. Why is the plasma membrane a bilayer? Why isn’t it simply a single layer?

2. Why are large water-soluble molecules unable to move through the plasma membrane?

3. Which type of organic molecule is not part of the plasma membrane?

A. Protein

B. Lipid

C. Carbohydrate

D. Nucleic Acid


4. What type of bonds form between water molecules?

A. Water Bonds

B. Hydrogen Bonds

C. Ionic Bonds

D. Covalent Bonds

5. The diagram that most accurately portrays the arrangement of phospholipids in a plasma membrane is:

A. B.

C. D.


Material Transport As it appears in Units 1 and 3

Substances move through plasma membranes in a range of different ways diffusion, facilitated diffusion,

osmosis and active transport.

Passive processes are governed by the concentration gradient and do not provide an input of energy. Active

processes require an input of energy and substances are transported against the concentration gradient.

Diffusion is a passive process that transports a substance through the phospholipid bilayer from an area of

high substance concentration to an area of low substance concentration.

Facilitated diffusion is a passive process that transports a substance through a protein channel from an area

of substance high concentration to an area of low substance concentration.

Osmosis is a passive process that transports water through the phospholipid bilayer from an area of high

water concentration to an area of low water concentration.

Active transport is an active process that transports material through a protein channel against the

concentration gradient.

Lipophilic (lipid soluble or non-polar) substances such as chloroform are able to diffuse directly through the

phospholipid bilayer, as shown by arrow A above, because they are lipid soluble. Small hydrophilic

substances such as water, O2 and CO2 are small enough to diffuse between individual phospholipids (shown

by arrow B). Larger hydrophilic substances such as glucose must be transported through a protein channel

(shown by arrow C) as they are too large to fit between the individual phospholipids.

A B C


Review Questions

6. Every cell found within an organism must be able to allow substances to pass through the plasma

membrane. These substances are commonly found in solution; however, occasionally they are

transported as a solid. The plasma membrane is selectively permeable, allowing the movement of certain

substances. This movement through the plasma membrane relies on a range of transport methods

including diffusion, osmosis and active transport.

a. Complete the table below by circling the appropriate answer for each process.

Diffusion Osmosis Active Transport

Movement occurs with a

concentration gradient/

Movement occurs

against a concentration

gradient



Movement occurs


gradient



Movement occurs


gradient

Active /

Passive

Active /

Passive

Active /

Passive

Uses energy/

Does not use energy

Uses energy/

Does not use energy

Uses energy/

Does not use energy

b. Outline the pathway a substance would use to enter a cell via active transport. Make sure that you

use a specific example in your answer.


c. Chloroform is a lipid soluble substance that has an almost immediate effect on humans. Explain

why being lipid soluble allows it to act so rapidly. Make sure that you use a labeled diagram as

part of your answer.

7. How is water able to pass directly through the plasma membrane despite being a polar molecule?


Placing cells into solutions As it appears in Units 1 and 3

The internal environment (cytosol) and the external environment (interstitial fluid) both contain high

concentrations of water. Water moves into or out of cells based on the difference in water concentration

between the cytosol and the interstitial fluid.

It is important to remember that the water and solute concentrations are effectively opposite to each other

e.g. if a solution has a high water concentration then the solute concentration will be low.

There are 3 types of solutions:

Isotonic: Water concentration is the same inside and outside the cell. The amount of water entering

and leaving the cell will be the same, so there is no net diffusion.

Hypotonic: the solution outside the cell has a higher water concentration and lower solute

concentration than the cytosol e.g. distilled water. In this situation there will be a net diffusion of

water into the cell.

Hypertonic: the solution outside the cell has a lower water concentration and higher solute

concentration than the cytosol e.g. solutions with a high salt or sugar concentration. In this situation

there will be a net diffusion of water out of the cell.

The effect on the cell depends upon whether the cell is a plant cell or an animal cell.

Plant cells in solutions As it appears in Units 1 and 3

This diagram represents a plant cell placed into a hypotonic

solution. Water diffuses into the cell along the concentration

gradient. The plasma membrane presses against the cell wall,

but the cell does not lyse (burst) because the cell wall prevents

too much pressure being placed on the cell membrane. The cell

becomes turgid (swollen).


Animal cells in solutions As it appears in Units 1 and 3 Animal cells respond slightly differently than plant cells when placed into solutions. The main reason for

this is the absence of the cell wall, although the absence of the vacuole also plays a small role.

The diagram below shows red blood cells placed into the three different types of solutions.

This diagram represents a plant cell placed into a hypertonic

solution. Water diffuses out of the cell along the concentration

gradient. The plasma membrane shrinks away from the cell

wall. The cell plasmolyses and becomes flaccid (limp).

This diagram represents a plant cell placed into an isotonic

solution. There is no nett diffusion of water; therefore the cell

retains its natural shape

This diagram represents red blood cells placed into a hypotonic

solution. Water diffuses from an area of high water concentration

outside the cells to an area of low water concentration inside the

cells. As a result the cell swells, since there is no cell wall to prevent

expansion of the plasma membrane, the cell will eventually lyse (in

the case of red blood cells this is called haemolysis).


Review Questions

8.

a. Outline what would happen to cell A if it were placed into a high salt solution.

Cell A

This diagram represents red blood cells placed into a hypertonic

solution. Water diffuses from an area of high water concentration

inside the cells to an area of low water concentration outside the

cells. As a result the cell shrinks. This is called crenation.

This diagram represents animal cells placed into an isotonic

solution. There is no nett diffusion of water, therefore the cells

retains there natural shapes.


b. Outline what would happen to cell B if it were placed in fresh water.

Cell B

c. What would prevent cell A from doing the same thing if placed in fresh water?


9. Four hollow tubes have one end sealed with a partially permeable membrane, and are filled with

solutions of differing sugar concentrations. The tubes are placed in a container filled with a 10% sugar

solution and are observed until the fluid levels become stable, as shown in the diagram below. Provide

appropriate original concentrations for solutions A, B, C and D? Explain.

Initial 10% Sugar Solution Final

A B C D A B C D


The following information relates to Questions 10 and 11.

Figure 1 shows part of the cell membrane of an animal cell.

Region Y

Region X

Figure 1

10. In Figure 1,

A. Region X represents the phospholipid bilayer and Region Y represents carbohydrates.

B. Region X represents a glycoprotein and Region Y represents the phospholipid bilayer.

C. Region X represents carbohydrates and Region Y represents the phospholipid bilayer.

D. Region X represents the phospholipid lipid bilayer and Region Y represents a protein channel.

11. A hydrophilic molecule, such as glucose, would:

A. Pass through the plasma membrane at Region X.

B. Pass through the plasma membrane at Region Y.

C. Need to be broken down into its sub units to pass through the plasma membrane.

D. Not be able to enter the cell.


Solutions to Review Questions

1.

a. Answer: Channel proteins allow large polar molecules like glucose to move into and out of the cell.

Explanation: As large polar molecules are unable to dissolve through the phospholipid bilayer and

they are too large fit in between the individual phospholipids, they must pass through a protein

channel.

b. Answer: The cell would no longer be recognized as self by the immune system and as a result, the

body’s white blood cells would destroy the cell.

Explanation: Recognition proteins are required by the body to differentiate self from non-self.

Without the presence of these proteins the body’s immune system would not be able to tell that a

cell was self.

c. Answer: The tail of a phospholipid is water hating, which means that it must face away from water.

As both the internal and external environments of a cell are aqueous, the tail of a single

phospholipid would not be able to face anywhere and it would create a sphere, see diagram below.

2. Answer: Large water-soluble molecules are polar molecules, which are insoluble in lipids and therefore

are unable to dissolve through the phospholipid bilayer.

3. Answer: D

Explanation: The plasma membrane is composed of phospholipids, so it contains lipids. It has protein

channels, so it contains proteins. Some of the proteins embedded within the plasma membrane attach to

carbohydrates to form antigens, so it contains carbohydrates. It does not, however, contain nucleic acids.


4. Answer: A

Explanation: Water is a polar molecule; this means that the oxygen end of the water molecule is

negatively charged, while the hydrogen end is positively charged. When the negative end of one water

molecule meets the positive end of another water molecule a hydrogen bond forms.

5. Answer: A

Explanation: Both the external and internal environments of a cell are aqueous, which means that the

hydrophilic heads of the phospholipids face into and out of the cell leaving the hydrophobic tails to face

into the centre of the bilayer.

6.

a. Answer:

Diffusion Osmosis Active Transport


concentration gradient


concentration gradient

Movement occurs


gradient

Passive Passive Active

Does not use energy Does not use energy Uses energy

b. Answer: Active transport is used to move large polar molecules such as glucose. These molecules

are transported through a protein channel, commonly from an area of high concentration to an area

of low concentration.

c. Answer: Lipid soluble substances such as chloroform are able dissolve directly through the plasma

membrane (as shown in the diagram below) without using a protein channel; this increases the

speed at which they can enter a cell.

7. Answer: Water molecules are very small; this means that they are able to pass between individual

phospholipids without being repelled by their hydrophobic tails.

Phospholipids

Protein Channel


8.

a. Answer: The plant cell would lose water and the plasma membrane would shrink away from the cell

wall. The large central vacuole would also empty of water and decrease in size.

Explanation: A high salt solution has a low concentration of water. The water inside the cell will

move along the concentration gradient and leave the cell via osmosis causing the plasma

membrane to shrink away from the cell wall.

b. Answer: Water would flood into the animal cell, which would burst.

Explanation: Water would move along the concentration gradient and into the cell, and this

would cause the cell to swell until the plasma membrane could no longer withstand the building

pressure, at which time it would burst.

c. Answer: A plant cell would also swell if placed in fresh water, however, the cell wall would prevent

the plant cell from expanding to the point that it would burst.

9. The answers provided here are approximations.

Answer: A – 5% as the level of solution has decreased, water must have left the tube. This means that the

tube must have had a higher water concentration (lower sugar concentration) than the container.

B – 2.5% Like tube A the level of solution has decreased; however, it has decreased more than A so the

concentration of sugar must be even lower than that in tube A.

C – 10%, as the level of solution has not changed it must have been the same concentration as the

solution in the container.

D – 20%, as the level of solution has increased water must have entered the tube. This means that the

tube must have had a lower water concentration (higher sugar concentration) than the container.

10. Answer: D

Explanation: Region X is pointing to the phospholipid bilayer as you can clearly see the two layers of

hydrophilic heads and hydrophobic tails. Region Y is clearly pointing to the protein channel as you can

see that it provides a pathway directly through the phospholipid bilayer.

11. Answer: B

Explanation: As glucose is hydrophilic it is unable to diffuse through the phospholipid bilayer so it must

move through the protein channel, which is represented by Region Y.

Biology Teach Yourself Series - VCE Unit 3 & 4 biology - Homeunit3and4biology.weebly.com/uploads/5/3/5/9/53596141/...Topic 4: Plasma Membranes and Material Transport A: Level 14, 474

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