Movement across cell membranes

Substances can move into and out of cells through the cell membrane. The three main types of movement are diffusion, osmosis and active transport.

Concentration gradients

The idea of concentrations and gradients within them is important when understanding the movement of substances across cell membranes.

Concentration

When sucrose is dissolved in water:

  • the soluteis sucrose

  • water is the solvent

The more particles there are in a certain volume, the more concentrated those particles are.

A solution with a low solute concentration has a high water concentration, and a high water potential. Pure water has the highest water potential.

A concentration gradient exists when there is a region of high concentration leading to a region of low concentration:

  • going from high to low concentration is going down the concentration gradient

  • going from low to high concentration is going against the concentration gradient

Diffusion

Dissolved or gaseous substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions - but diffusion does not occur in solids.

Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Examples of diffusion in living organisms

Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion. Their concentration is higher in the small intestine than their concentration in the blood, so there is a concentration gradient from the intestine to the blood.

Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:

  • oxygen moves down a concentration gradient from the air in the alveoli to the blood

  • carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli

The dissolved substances will only continue to diffuse while there is a concentration gradient.

Osmosis

Osmosis is the diffusion of water molecules, from a region of higher concentration to a region of lower concentration, through a partially permeable membrane. A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.

Partially permeable membranes are also called selectively permeable membranes or semi-permeable membranes. They let some substances pass through them, but not others.

Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls.

When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. At this point, the net exchange of water is zero and there is no further change in the liquid levels.

 

Osmosis in cells

The results of osmosis are different in plant and animal cells.

Plant cells

Plant cells have a strong cellulose cell wall on the outside of the cell membrane. This supports the cell and stops it bursting when it gains water by osmosis.

Water enters the cell by osmosis. The cytoplasm pushes against the cell wall and the cell becomes turgid.

Water leaves the cell by osmosis. The cytoplasm pulls away from the cell wall (plasmolysis) and the cell becomes flaccid and the plant wilts.

Turgid plant cells play an important part in supporting the plant.

Animal cells

Animal cells do not have a cell wall. They change size and shape when put into solutions that are at a different concentration to the cell contents.

For example, red blood cells:

  • gain water, swell and burst in a more dilute solution (this is called haemolysis)

  • lose water and shrink in a more concentrated solution (they become crenated or wrinkled)

These things do not happen inside the body. Osmoregulation involving the kidneys ensures that the concentration of the blood stays about the same as the concentration of the cell contents.

Active transport

Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient, using energy released during respiration.

Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteinspick up specific molecules and take them through the cell membrane against the concentration gradient.

Examples of active transport include:

  • uptake of glucose by epithelial cells in the villi of the small intestine

  • uptake of ions from soil water by root hair cells in plants

Active transport vs diffusion and osmosis

 

Diffusion

Osmosis

Active transport

Down a concentration gradient

Against a concentration gradient

Energy needed

Substance moved

Dissolved solutes

Water

Dissolved solutes

Notes

Gases and dissolved gases also diffuse

Partially permeable membrane needed

Carrier protein needed

 

Osmosis experiments

Visking tubing is an artificial partially permeable membrane:

  • smaller molecules like water and glucose pass through its microscopic holes

  • larger molecules like starch and sucrose cannot pass through it

The sucrose solution is hypertonic to the water – it is a more concentrated solution. There is a net movement of water molecules, by osmosis, from the water outside to the sucrose solution inside the Visking tubing. This makes the liquid level in the capillary tube rise.

A less concentrated solution is hypotonic to a more concentrated solution, while two solutions at the same concentration are isotonic.

The table summarises the results of the four combinations of water and 10% sucrose in the experiments, showing the movement of water and solute across a concentration gradient.

Liquid outside

Liquid inside

Water moves

Solute moves

Result

Water

10% sucrose

Outside → inside

Inside → outside

Liquid level rises

10% sucrose

Water

Inside → outside

Outside → inside

Liquid level falls

Water

Water

No movement

No movement

No visible change

10% sucrose

10% sucrose

No movement

No movement

No visible change

Living cells experiment

Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to:

  1. cut equal-sized pieces of potato

  2. blot with tissue paper and weigh

  3. put pieces into different concentrations of sucrose solution for a few hours

  4. remove, blot with tissue paper and reweigh

Example 1

A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end.

percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100 = 0.5 ÷ 2.5 × 100 = +20%

The plus sign shows that it has gained mass - it will have gained water by osmosis.

Example 2

A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end.

percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100 = –0.5 ÷ 2.5 × 100 = –20%

The minus sign shows that it has lost mass - it will have lost water by osmosis.

Example results

A graph of change in mass (vertical axis) against concentration of sucrose (horizontal axis) can be plotted.

Where the line crosses the horizontal axis at 0% change in mass, the sucrose concentration is equal to the concentration of the contents of the potato cells. The sucrose concentration is isotonic with the cells' cytoplasm, so there is no net movement of water by osmosis.

Source: www.bbc.com

Photo: www.startsmile.ru

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