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DYNAMIC CELL MOLECULES ON THE MOVE

Structure and function of plasma membrane Membranes are vital because they separate the cell from the outside world. They also separate

compartments inside the cell to protect important processes and events.

Prokaryotes Eukaryotes

Plasma membrane Present Present

Function Boundary of the cell and maintains the internal environment of the cell by

controlling the movement of substances into and out of the cell

Fluid mosaic model of cell membrane (by Singer and Nicolson 1980)

According to this model, a plasma membrane consists of a double layer of lipid (phospholipid

bilayer), and proteins that are embedded in this layer forming channels that allow certain substances

to pass across the membrane in either direction.

Phospholipids

Phospholipids are one of the principal types of lipid in the

membrane. These have a polar head group and two hydrocarbon

tails. The polar head (the phosphate group) is hydrophilic and the

two fatty acid tails are hydrophobic. This means that the head tends

to dissolve in water whereas the tails are repelled and forced to face

inwards away from the watery environment and towards each other

forming a phospholipid bilayer. The lipid structure of the membrane

gives it the unique property of being flexible and being able to repair itself if, for example, it is

pierced; punctures that are not too extreme can be sealed.

Membrane Cholesterol Another type of lipid in the membrane is cholesterol. The amount of

cholesterol may vary with the type of membrane. Plasma membranes

have nearly one cholesterol per phospholipid molecule. Cholesterol

molecules make the lipid bilayer less deformable and decreases its

permeability to small water-soluble molecules. Cholesterol

regulates the fluidity of cell membranes, preventing them from

becoming both too rigid and too fluid.

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Membrane proteins Proteins are embedded in the bilayer. They may pass through the bilayer as transmembrane

proteins, or they may be inserted at the cytoplasmic or exterior face (peripheral membrane

proteins). Transmembrane proteins are amphipathic, in that they have hydrophobic and hydrophilic

regions that are oriented in the same regions in the lipid bilayer.

Types of membrane proteins Membrane proteins Description and Functions

Transport proteins

These have openings on both sides of the membrane, forming channels

that allow some substances to move through the membrane. An example

of this occurs when ions (charged particles) move through the

membrane. This can cause a rapid change in the electric potential

difference (the difference in positive and negative charges) across the

membrane and explains how the electrical charge of a nerve impulse is

transmitted between nerve cells

Receptor proteins

Proteins are also involved in cellular communication. Receptor

proteins on the surface of membranes detect hormones and other

chemical molecules (neurotransmitters) to control the transmission of

messages within and between cells.

Recognition proteins (glycoproteins)

The protein molecules embedded in the plasma membrane have particular

functions and many of them carry a sugar molecule, giving them their

collective name of glycoproteins. Glycoproteins are often receptors and

marker molecules that identify the cell as belonging – each kind of

organism has it own kind of glycoprotein and even different individuals

of the same species can be distinguished by the glycoproteins that they

have on the surface of their cell membranes. These marker molecules

identify self and are important in cell recognition. For example, the

immune system can recognise invaders by the self or non-self

glycoproteins they have. Membrane proteins are important for the

regulation of cell behaviour and the organisation of cells in tissues.

Adhesion proteins

These are located on the cell surface and are involved with the binding

with other cells or with the extracellular matrix (ECM) in the process

called cell adhesion

Hydrophilic, hydrophobic and lipophilic substances Substances that can dissolve readily in water are termed hydrophilic, or ‘water-loving’. Some

substances that have low water solubility or do not dissolve in water are able to dissolve in or mix

uniformly with lipid. These substances are termed lipophilic (sometimes called hydrophobic).

Lipophilic substances such as alcohol and ether can cross plasma membranes readily. The rapid

absorption of substances, such as alcohol across plasma membranes, appears to be related to the

ability of alcohol to mix with lipid.

Examples Absorption of water from food in the gut

Reabsorption of water during urine formation in the kidneys

The opening and closing of stomata in leaves

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Substances that move in and out of a cell

Movement of different molecules through cell membrane

Substances How they are transported

• Lipid-soluble substances of various

sizes, such as chloroform and alcohol

simply dissolve into the phospholipid bilayer and pass

easily through membranes (diffusion)

• Tiny molecules, such as water, carbon

dioxide, oxygen, urea and other small

nonpolar molecules.

pass through easily can pass between the phospholipid

molecules (diffusion, osmosis)

• Small uncharged molecules, such as

oxygen and carbon dioxide,

can pass through the phospholipid bilayer.

(diffusion)

• Larger water-soluble substances,

including amino acids and simple

sugars (glucose)

pass through channels made from protein molecules.

Protein channels may be selective for particular

substances, and they may require the expenditure of

energy for transport to occur.(active transport)

Charged particles like sodium and

potassium

transported by facilitated diffusion

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Functions of cell membrane can be protective

can regulate transport in and out of cell or subcellular domain

allows selective receptivity and signal transduction by providing transmembrane receptors

that bind signaling moleculesallow cell recognition

provides anchoring sites for cytoskeletal filaments or components of the extracellular matrix.

This allows the cell to maintain its shape and perhaps move to distant sites.

helps compartmentalize subcellular domains or microdomains

provides a stable site for the binding and catalysis of enzymes.

regulates the fusion of the membrane with other membranes in the cell via specialized

junctions )

provides a passageway across the membrane for certain molecules, such as in gap junctions.

allow directed cell or organelle motility

Processes involved in cell transport Because a plasma membrane allows only some dissolved materials to cross it, the membrane is said

to be a partially permeable boundary (‘Partially permeable’ is also known as selectively or

differentially or semi permeable.) Cell transport

Need energy Don’t need energy

Active transport Passive transport

Active transport Bulk transport Diffusion Facilitated diffusion Osmosis

Uses protein pumps Transports large Transporting Transporting Transporting

to transport ions molecules using vesicles small molecules molecules using water from

high to low protein pumps high to low

concentration from high to low concentration

Exocytosis Endocytosis

transporting transporting

out of cell into cell

Pinocytosis Phagocytosis

transporting transporting

liquid into cell solid into cell

Passive transport Movement of materials across a membrane without requiring energy is called passive transport.

This type of movement relies on process called

diffusion,

osmosis

facilitated diffusion.

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Diffusion Diffusion is the passive movement of molecules through a partially permeable membrane from

a region of high concentration to a region of low concentration.

The difference in concentration between the two regions is called the concentration gradient

or diffusion gradient.

If the numbers of molecules (concentrations) are the same on both sides of a membrane,

there will always be about the same number passing in either direction. That is, there will be

no net movement from one side to the other.

If the concentrations of a particular molecule are different on either side of the membrane,

more molecules will move from the more concentrated region to the less concentrated

region than in the opposite direction. There will be a net movement of molecules into the

more dilute solution (until equilibrium is reached).

Examples In active tissues, oxygen moves out of the blood into the surrounding fluid (interstitial

fluid) and carbon dioxide moves into the blood by diffusion along concentration gradients.

In the lungs, the reverse exchange takes place along the concentration gradients for oxygen

and carbon dioxide between blood and air.

Factors that increase the rate of diffusion • greater concentration gradient

• greater temperature (as molecules move faster)

• smaller molecules

• gaseous medium (as molecules are further apart)

Substances such as water, oxygen, carbon dioxide and other small, uncharged particles pass easily

through the lipid molecules of the cell membrane by simple diffusion.

Osmosis

Osmosis is the passive movement of water through a partially permeable membrane, from a

region where there are more free water molecules to a region where there are fewer free water

molecules

When dilute and concentrated solutions are separated by a partially permeable membrane, free water

molecules cross the membrane in both directions. Because there are more free water molecules in the

less concentrated solution, there will be a net movement of water from the dilute to the

concentrated solution.

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Types of solutions Concentrated and dilute solutions If you add more sugar or salt to water, you are adding more solute to the solvent and making the

solution more concentrated.

Solute-a substance dissolved in another substance (the solvent) to create a solution (salt or sugar)

Solvent- a substance in which solutes can be dissolved to create a solution. Water, owing to its polar

nature, is the universal solvent for living things

Solution = solvent particles + solute particles

Concentrated solution = high concentration of solute+ low concentration of solvent +

Dilute solution = low concentration of solute + high concentration of solvent

Osmosis in animal and plant cells

Osmosis in animal cells Osmosis in plant cells

(a) In fresh water, elodea cells swell and become turgid (b) In salt water, plasmolysis occurs and gaps develop

between the cytoplasm and the cell wall

Water moves in

Water moves out

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Solutions Animal cells Plant cells

Isotonic solution

the concentration of solute

is the same on both sides

of the membrane

water diffuses equally in both

directions, resulting in no net

movement of water into or out

of the cell. The cell shape and

size remains the same

water diffuses equally in both

directions, resulting in no net

movement of water into or out of the

cell. The cell shape and size remains

the same

Hypotonic solution

has less solute and more

water. (dilute)

water will move into the red

blood cell which expand and

burst. This condition is called

haemolysis.

Water moves in and the vacuole

increases in volume, stretching and

pushing against the cell wall and

develops tension. The tough cell wall

prevents the cell from bursting. The

cell is said to be turgid.

Hypertonic solution

has a more solute and less

water(concentrated).

Water moves out of RBC and

it shrinks. This process is

called crenation

Water moves out of the plant cell and

it shrinks. This process is called

plasmolysis

Opening and closing of stomata by osmosis

Stomata Many plants regulate the exchange of gases, including water vapour, between their internal and

external environment by means of small openings called stomata, which are usually found on the

underside of leaves. Each stoma consists of two guard cells, which surround the stomatal pore.

Each guard cell has a thick, rather inelastic cell wall bordering the inside of the pore but a thinner

and elastic cell wall on the outer side. This difference in thickness is significant. As water is taken in

by endosmosis, the guard cells become turgid and bend to become rather sausage-shaped, opening

the pore. If the guard cells lose water, they become flaccid and the pore is closed.

Guard cells swell in response to changes in the concentration of solutes within the cells.

During periods of light, the concentration of solutes inside the guard cells increases,

causing water to move into the cells by osmosis so they swell, opening the pore.

During periods of darkness, the solute concentration drops so water moves out of the cells

by osmosis and the pore closes.

The guard cells can take up potassium ions (K+) from adjacent cells in the epidermis or

lose them to the epidermal cells. The mechanism of moving potassium ions is thought to be

triggered by receptors in the cell membrane that are sensitive to blue light. Guard cells are

packed with chloroplasts and mitochondria to provide energy to move potassium ions

against their concentration gradient.(active transport)

Stomata open Stomata closed

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Facilitated diffusion Facilitated diffusion is a passive process. Molecules move through protein channels in the cell

membrane and is faster than passive diffusion. Facilitated diffusion does not need energy.

Charged particles, such as sodium and chloride ions, and relatively large molecules, such as

glucose, do not pass through readily. The channel proteins and carrier proteins assist such

particles to diffuse into the cell.

Facilitated diffusion through channel proteins Channel proteins form narrow passageways through which small ions can diffuse rapidly from

where they are in high concentration to where they are in low concentration.

Facilitated diffusion through carrier proteins carrier protein takes up ions of a specific size and shape on one side of the plasma membrane,

change shape and release them on the other side.

Active transport by carrier proteins Active transport is the net movement of dissolved substances into or out of cells against a

concentration gradient. Because the net movement is against a concentration gradient, active

transport is an energy-requiring (endergonic) process. It involves carrier proteins similar to those

responsible for facilitated diffusion. However, in active transport, the carrier protein is coupled

with a source of energy, which enables it to transport molecules or ions against a concentration

gradient.

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Sodium-Potassium Pump

Outside there are high concentrations of sodium and low concentrations of potassium, so

diffusion occurs through ion channels in the plasma membrane. In order to keep the appropriate

concentrations, the sodium-potassium pump pumps sodium out and potassium in through active

transport. A large protein in the plasma membrane provides the doorway through which sodium and

potassium ions can move. ATP is the energy source. Three sodium ions inside the cell bind to the

protein. The addition of a phosphate group from ATP changes the shape of the protein and

3 sodium ions are expelled outside the cell. The phosphate is released and, as the protein returns to

its former shape, two potassium ions are moved across the membrane into the cell. Sodium can

once again bind to the protein, and the process repeats as long as there is a supply of ATP.

http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/i

on_pump/ionpump.html excellent site for animation

Differences between facilitated diffusion and active transport

Active transport requires the expenditure of energy whereas facilitated diffusion does not

require energy

Active transport can move substances against a concentration gradient whereas facilitated

diffusion cannot.

Similarities- Facilitated diffusion and active transport occur through protein channels.

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Differences between different types of cell transport Diffusion Osmosis Facilitated

diffusion

Active

transport

Movement of

particles

Move from high

concentration to

low concentration

Move from high

concentration to

low concentration

Move from high

concentration to

low concentration

Move from low

concentration to

high concentration

Molecules

transported

Urea, oxygen, CO2

Water

amino acids

glucose

Na, K, Cl, and urea

Energy

requirement

none none none required

Protein channel

involvement

none none channel proteins

carrier proteins

carrier proteins

Speed of

transport

slow slow fast fast

Examples of active transport Active transport enables cells to get rid of unwanted waste substances

Animal cells contain high concentrations of potassium ions, but low concentrations of

sodium ions. The mechanism responsible for this is the sodium–potassium pump, which

moves these two ions in opposite directions across the plasma membrane. The sodium–

potassium pump has a particular significance for excitable cells, such as nerve cells, which

respond to stimuli.

Other substances such as vitamins, amino acids and hydrogen ions are also pumped across

membranes.

uptake of ions by the roots of plants

Bulk transport Sometimes, cells need to move large quantities of materials into or out of their cytoplasm all at one

time. These are too large to pass through the pores in the plasma membrane. The process of bulk

transport may achieve this. As with active transport, large amounts of energy are necessary for this

to function.

1 Endocytosis- Material is transported into a cell by vesicles

a. Phagocytosis- It is a process in which cells take in large solid particles, clumps of food and

even other cells! In phagocytosis, extensions of cytoplasm surround and engulf the object that it

is trying to take in. White blood cells (phagocytes) engulf bacteria by phagocytosis.

b. Pinocytosis- It is a process in which a cell takes up water. Tiny pockets along the cell

membrane, and then fill with liquid. Those tiny pockets then break off into the cell to form tiny

vacuoles filled with water.

2. Exocytosis-Bulk transport out of cells (for example, the export of material from the Golgi

complex is called exocytosis. In exocytosis, vesicles formed within a cell fuse with the plasma

membrane before the contents of the vesicles are released from the cell. If the released material is a

product of the cell (for example, the contents of a Golgi vesicle), then ‘secreted from the cell’ is the

phrase generally used.

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Exocytosis- moving out of cell

Materials are moved out of a cell by vesicles Endocytosis- moving into cell

Material is transported into a cell by vesicles

Pinocytosis

Cells take in liquids by endocytosis Phagocytosis

Cells take in liquids by endocytosis

Cystic fibrosis due to defective proteins

Cystic fibrosis is the most common inherited

disease in Western society. It occurs because of

a missing membrane protein, the CFTR

protein, in the glands that secrete body fluids

(exocrine glands). This large protein, which is

composed of 1480 amino acids, acts as a

chloride channel and regulates the function of

many other proteins.

If the CFTR protein is overactive, as seen in

individuals infected by cholera toxin, it induces

diarrhoea, which can kill due to excessive

water loss. When the CFTR protein is not

working or is absent from the membrane,

chloride ions, and therefore water, will not

flow from the bloodstream into the mucus.

This causes secretions to become thick and sticky, blocking pancreatic and reproductive ducts.

Mucus secretions become too thick and the cilia lining the lungs cannot remove it. The airways

become blocked and this makes it difficult to breathe. Production of CFTR protein requires a series

of steps as it moves through the endoplasmic reticulum (ER) and the Golgi apparatus. In the ER,

chaperone proteins assist in trafficking and folding the polypeptide and carbohydrates essential for

its function are added. Generally, only one-third of the CFTR protein being produced moves from

the ER to the Golgi apparatus. Only one-third will eventually reach the cell membrane. The rest is

destroyed in the ER by proteases, protein-digesting enzymes. The most common mutant form of

the CFTR protein, which is found in 90% of persons with the inherited disease cystic fibrosis, is

delta-F508 CFTR. This mutation causes the CFTR polypeptide to get stuck in the ER so that it

cannot move to the cell membrane to function as a chloride channel. Scientists believe that the

delta-F508 CFTR polypeptide adheres too strongly to the chaperone proteins in the ER or that it

does not fold quickly enough. As a result, too many copies are destroyed by the action of proteases

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and very few make it to the Golgi apparatus and cell membrane. The mutant protein is able to

function as an ion channel so a dysfunctional protein does not cause cystic fibrosis. Rather, it’s a

problem of not enough CFTR protein getting to the plasma membranes.

Cell movements and connections The cell cytoskeleton – a protein network Only cells of eukaryotes have a cytoskeleton. It consists of a network of protein fibres that give

shape to the cell, hold and move organelles and coordinate cell movement.

The cytoskeleton is an internal skeleton of microtubules that extends throughout eukaryotic cells,

giving them their shape,

their ability to move and to arrange organelles.

the centrioles, are involved in moving chromosomes apart in cell division.

Three types of protein fibres criss-cross the cytosol of the cell:

• microtubules: these are polymers of tubulin and are involved in the movement of

chromosomes, organelles, cilia and flagella

• intermediate filaments: these provide tensile strength for the attachment of cells to each

other and their external environment to help maintain tissue shape and to support long nerve cell

extensions

• microfilaments: these are composed of contractile fibres of actin that associate with myosin to

control muscle contraction, maintain cell shape and carry out cellular movements, such as gliding,

contraction and cell division.

These three structures combine to assist in:

• maintaining the shape of a cell

• providing a support structure for other components within a cell

• the movement of materials within a cell

• movement of the cell itself if required.

Dystrophin is a protein that forms a scaffold in cells that are essential for muscles to function.

Patients with muscular dystrophy cannot replenish the protein dystrophin in their muscle cells. As a

result, their muscle cells fall apart, causing progressive muscle weakness and wasting, and

eventual death from respiratory failure.

Another drug, taxol, from the Pacific yew tree, prevents cells from dividing by affecting the

breakdown of microtubules. Because of this property it is now used as a localised cancer therapy to

stop tumour cell growth.

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The extracellular matrix – glue that connects cells Most cells have some structures that are external to the plasma membrane but are still an integral part

of the cell, both in structure and function. The cell wall in plants, fungi and bacteria is one such

structure. Many kinds of cells, including most of our own body cells, are connected in some way to

neighbouring cells. This is how body tissues form. Cells in body tissues are surrounded by

extracellular matrix (ECM). This consists of various macromolecules that have been produced and

secreted by fibroblast cells found in the matrix. The ECM in animal cells usually consists of long

flexible fibres that are embedded in a matrix made up of glycoproteins and glycolipids. Bone and

cartilage are types of connective tissues that are largely composed of ECM materials. ECM plays an

important role in determining the shape and mechanical properties of tissues and organs.

Connections between cells-Animal cells There are three different types of junctions in animal cells: Occluding, communicating and anchoring

junctions

Occluding junctions Occluding junctions involve cell membranes coming together in contact with each other. There is

no movement of material between cells.

Communicating junctions- gap junctions Communicating junctions are also called gap junctions.

They consist of protein lined pores in the membranes of

adjacent cells. The proteins are aligned rather like a series of

rods in a circle with a gap down the centre. Communicating

junctions permit the passage of salt ions, sugars, amino

acids and other small molecules as well as electrical

signals from one cell to another.

One example of the latter is the control of the beating of the

heart. A small area of your heart, called the pace maker,

receives an electrical impulse. This electrical impulse is

transmitted to all cells of the heart through communication

junctions so that the heart ‘beats as one’.

Anchoring junctions- desmosomes Anchoring junctions are the most common form of junction between epithelial cells in areas such as

the skin or uterus. They are also called desmosomes. Dense plaques of protein exist at the junction

between two cells. Fine fibrils extend from each side of these plaques and into the cytosol of the two

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cells involved. These are intermediate filaments that use the plaques as anchoring sites. This structure

has great tensile strength and acts throughout a group of cells because of the connections from

one cell to another.

Connections between cells: plant cells Plasmodesmata Plants have rigid cell walls. In addition, the primary walls of adjacent cells are held together tightly

by a layer of pectin, a sticky polysaccharide. Hence, plant cells have no need for a structure such as

the anchoring junctions of animal cells. Secondary walls are laid down in each cell on the cytosol

side of the primary wall so that the structure across two cells is relatively wide. The junctions that

exist in plant cells to allow communication between adjacent cells in spite of the thick wall are

plasmodesmata.

Because of the way in which plant cell walls are built up, the gap or pore between two cells is lined

with plasma membrane so that the plasma membrane of the two cells is continuous. A structure

that bridges the ‘gap’ is also continuous with the smooth endoplasmic reticulum of each cell.

Plasmodesmata exist in virtually all plants and hence cell-to-cell communication can occur between

large numbers of cells that are in effect connected via their cytoplasm.

Diffusion, cell size and cell shape As a cell grows, its volume (cytoplasm) expands and will increase more rapidly than its surface

area (plasma membrane). It is the volume of cytoplasm within a cell that determines the amount of

requirements and waste removal that is needed. Yet removal of wastes and supply of nutrients is

dependent on the surface area size. This is because diffusion takes place across the surface of a cell,

through the plasma membrane. Consequently, the cell is limited in the size to which it can grow.

Consider the surface area of cells compared with their volumes. This value is sometimes called the

surface area- to-volume ratio (SA:V ratio). The SA:V ratio of any object is obtained by dividing its

area by its volume. ‘Area’ refers to the coverage of a surface. One unit of measurement of area is a

square centimetre (cm2). ‘Volume’ refers to the amount of space taken up by an object. One unit of

measurement of volume is the ‘litre’ (L), but the volume of solid matter, such as a brain, is

sometimes expressed in units such as ‘cubic centimetres’ (cm 3 ).

The formulae to calculate surface area and volume of a sphere”

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Surface area to volume ratio of some hypothetical cells Hypothetical cell Cell A Cell B Cell C

Diameter (cm) 1.0 2.0 3.0

Surface area (cm2) 3.14 12.57 28.28

Volume (cm 3) 0.52 4.19 14.14

SA: V ratio 6:1 3:1 2:1

Cell A has a volume of 0.52 cm3 and a surface area of 3.14 cm2 to service it. This is a surface

area to volume ratio of 6:1.

Cell C, has a volume of 14.14 cm3 and a surface area of 28.28 cm

2 to service it, a surface area

to volume ratio of only 2:1

Notice that as a sphere increases in size, its surface-area-to-volume ratio decreases. The SA:V ratio

of a shape identifies how many units of external surface area are available to ‘supply’ each unit of

internal volume. As a sphere increases in size, the amount of surface area for each unit of

volume decreases.

In general, as a particular shape increases in size, the SA:V ratio of the shape decreases. As part

of staying alive, cells must take in supplies of essential material from outside to meet their energy

needs. Cells have to move wastes from inside to the outside. Efficient uptake and output of

material is favoured by a higher surface-area-to-volume ratio. It is reasonable to suggest that

cells are limited to small sizes so that their surface areas are large enough to let in essential material

fast enough to meet their needs, and to allow waste materials to diffuse out fast enough to avoid

the cells being poisoned by their own wastes.

Same volumes- different shapes SA:V ratios of different shapes with the same volume.

Shape Surface area Volume SA:V ratio

Flat sheet(100x100x0.1cm ) 10 040 1000 10:1

Cube (10x10x10 cm) 600 1000 0.6:1

Flat pancake (height 1cm, radius 17.8 cm) 2112 1000 2.1:1

Sphere (radius 6.2 cm) 483 1000 0.5:1

Cells differ in shape. The shape of the cell is also a limiting factor. In a small, round cell, substances

can become distributed easily throughout the cytoplasm, but this is not the case in larger cells and it

is found that often these cells become thinner, longer or have increased folding in the cell surface.

By increasing the surface area available for movement, all of these shapes increase the efficiency of

movement of substances across the cell surface.

Although they differ in shape, they have the same volume of one litre (1000 cm 3). The SA:V ratio

varies according to shape. The flat sheet has about 10 units of surface area for each unit of volume.

The sphere has just half a unit of surface area for each volume unit. Cells with out foldings can

exchange matter with their surroundings more rapidly than cells lacking this feature.

Summary As a structure increases in size, its surface-area-to-volume ratio (SA:V) decreases.

Various shapes differ in their SA:V ratios, with this ratio being highest in flattened shapes and

lowest in spheres.

The size of cells is limited by SA:V ratios since these ratios influence the rate of entry and

exit of substances into and out of cells.

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The importance of SA:V ratio in multicellular organisms

The surface-area-to-volume-ratio (SA:V) of a cell is important in determining the cell’s efficiency

to move materials across its membrane and that the higher the SA:V ratio of a cell, the more efficient

it is in carrying out those functions. The need for small cells can be graphically demonstrated with

regard to groups of cells. Exchange of materials between tissues and their environments has the

potential to be far more efficient if the tissue is made up of many small cells rather than fewer

larger cells. This potential for efficiency of small cells becomes a reality only if each of the cells in a

group of cells is close to a delivery mechanism, capable of providing material to and removing

material from the cells. A mass of small cells without a delivery system has no advantage over a

single large cell.

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