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Movement Across the Cell Membrane

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Cells are bathed in extracellular fluid that is rich in nutrients such as oxygen, glucose and amino acids

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These nutrients are needed within the cell and must be able to cross the cell membrane

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The cell’s waste which accumulates within the cell must also be able to cross the cell membrane

Wastes are eventually eliminated from the body

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There are a number of mechanisms that help in the movement of water and dissolved substances across the cell membrane

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These transport mechanisms can be divided into two (2) groups

• Passive Transport

• Active Transport

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Passive Transport

Passive transport mechanisms do not require any additional energy in the form of ATP

Like a ball rolling down a hill

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Active Transport

Active transport mechanisms require an input of energy in the form of ATP

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Passive TransportThe passive mechanisms that move substances across the membrane include:

• Diffusion

• Facilitated diffusion

• Osmosis

• Filtration

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Diffusion

This is the most common transport mechanism

And involves the movement of a substance from an area of higher concentration to an area of lower concentration

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For example …

A tablet of red dye is placed in a glass of water

The tablet dissolves

The dye moves from an area where it is most concentrated to an area where it is less concentrated

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Diffusion continues until the dye is evenly distributed throughout the glass.

The point where no further net diffusion occurs is called eqilibrium

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Some examples of Diffusion in the human body ...

Diffusion causes oxygen to move across the membrane of an alveolus of the lung and into the blood

Oxygen diffuses from the alveolus and into the blood because the concentration of oxygen is greater within the alveolus than within the blood

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Carbon dioxide (a waste product) that accumulates in the blood diffuses in the opposite direction

Carbon dioxide moves from the blood into the alveolus

The lungs then exhale the carbon dioxide, eliminating it as a waste product

Diffusion moves oxygen into the blood and carbon dioxide out of the blood

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Facilitated Diffusion

This form of diffusion is responsible for the transport of many substances

Like diffusion, the substances move from a higher concentration to a lower concentration

In facilitated diffusion the substance is helped across the membrane within the membranehttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

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The helper molecule increases the rate of diffusion

See Herlihy (2007) Fig. 3-8, p. 38

In this illustration a glucose molecule is being carried ‘down hill’ (passive transport)

A glucose molecule is too large to pass through the cell membrane without help!

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Osmosis

Osmosis is a special case of diffusion and involves the diffusion of water through a selectively permeable membrane

A selectively permeable membrane allows the passage of some substances while restricting the passage of others

The dissolved substances do not move

See Herlihy (2007) Fig 3-9 p. 38

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Whenever dissolved substances like glucose and protein are confined in a space by a selectively permeable membrane they can pull water into the compartment by osmosis

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The strength of the osmotic pull is related directly to the concentration of the solution

The greater the concentration – the greater the pulling power

The more concentrated solution has more osmotically active particles

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Examples of Osmosis in the Human Body

Because osmotic pressure pulls water into a compartment it can cause swelling

For example, when tissue is injured this can cause proteins to leak and accumulate in the tissue space.

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The confined proteins act osmotically pulling water toward them

This process causes an accumulation of water in the tissue spaces. This accumulation of water is called oedema

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TONICITY

Tonicity is the ability of a solution to affect the volume and pressure within a cell

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This can be observed when a cell is placed in solutions of different concentrations

The following three terms are used to illustrate tonicity

• Isotonic solution• Hypotonic solution• Hypertonic solution

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ISOTONIC SOLUTION

An Isotonic solution has the same concentration as intracellular fluid

If a red blood cell is placed in an isotonic solution (0.9% NaCl) for example, no net movement of water occurs so the cell neither loses or gains water

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HYPOTONIC SOLUTION

If a red blood cell is placed in pure water (a solution containing no solute) water moves into the cell by osmosis (from where there is more water to where there is less

Pure water is more dilute than the inside of the cell so it is said to be hypotonic

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Putting a red blood cell into pure water (hypotonic) will cause water to rush into the cell and make it to burst.

Hypotonic solutions then cause red blood cells to burst or lyse

This process is called haemolysis and it is for this reason that pure water is not administered intravenously

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HYPERTONIC SOLUTIONS

If a red blood cell is placed within a very concentrated salt solution water diffuses out of the red blood cell and into the bathing solution causing the red blood cell to shrink (crenate)

The salt solution is referred to as a hypertonic solution

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Crenated Red Blood Cell

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FiltrationWater and dissolved substances move across the semipermeable membrane in response to difference in pressures

In diffusion and osmosis movement of these substances occurred because of concentration differences

In filtration pressure pushes substances across the cell membranes

See Herlihy (2007) Fig 3 -11 p.39

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Where does filtration happen in the body?

The capillaries are small blood vessels that have only one layer of cells with many little pores.

The pressure in the capillary pushes water and dissolved substances out of the blood and through the pores in the capillary wall into the tissue space.

This process is called filtration

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Active Transport

The active transport mechanisms include:

• Active transport pumps

• Endocytosis

• Exocytosis

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This kind of transport system requires an input of energy (ATP) to achieve its goal

Energy is required when the cell already contains a great amount of a substance and the only way to move additional substances into the cell is to pump them in!

Active Transport Pumps

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For example, the cell normally contains a large amount of potassium (K+), so the only way to move more in is to pump it in

To move K+ from an area of low concentration to an area of high concentration (up hill) requires energy.

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Endocytosis

This involves the intake of food or drink by the cell

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In Endocytosis the particle is too large to move across the cell membrane by diffusion

The particle is surrounded by the cell membrane which engulfs it and takes it into the membrane

There are two forms of Endocytosis:

• Phagocytosis

• Pinocytosis

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PhagocytosisThis is ‘cell eating’

This happens when white blood cells ‘eat’ bacteria and help the body to defend itself against infection

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Pinocytosis‘cell drinking’

This happens when a cell ingests a water droplet

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Exocytosis

Exocytosis moves substances out of the cell

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An example of exocytosis is when the pancreas makes proteins for use outside of the cell.

The pancreatic cells synthesise the protein and wrap it up in a membrane

This membrane-bound vesicle toward and fuses with the cell membrane.

The protein is then expelled into the surrounding space

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Exocytosis

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References

• Herlihy, B. ( 2014) The human body in health and illness. 5th (ed) China. Elsevier.