Diffusion

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Learning Intentions

By the end of this topic, you should be able to:

Describe the composition of the cell membrane and how it acts as a semipermeable membrane barrier.
Discuss diffusion in the context of different cellular processes.
Describe the importance of a high SA : V ratio, in terms of cell processes.

Glossary

Key terms and definitions.

aerobic respiration: Enzyme controlled process which requires oxygen to produce 38 ATP from the breakdown of glucose.

cell membrane: Semipermeable barrier surrounding the cytoplasm of a cell.

channel proteins: Transmembrane protein that is open to both ends.

concentration gradient: When a solute is more concentrated in one area compared to another.

diffusion: Passive movement of molecules down a concentration gradient.

facilitated diffusion: Transport of substances across a membrane from an area of higher concentration to an area of lower concentration by means of a transmembrane protein.

passive transport: Ratio of the cell’s surface area and its volume.

photosynthesis: Enzyme-controlled process occurring in plants that use energy from sunlight to combine carbon dioxide and hydrogen to produce glucose.

SA : V ratio: Ratio of the cell’s surface area and its volume.

semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

surface area: Total area occupied by the surface of an object.

volume: Amount of space in a 3D object.

Diffusion is Passive

Diffusion is the movement of molecules from an area of high concentrations to an area of low concentrations (so they are moving down a concentration gradient) until they are equally distributed. Diffusion is a form of passive transport, because it does not require energy in the form of ATP.

The difference in concentration between the area of high concentration and the area of low concentration is called the concentration gradient. Diffusion doesn’t require energy because it is driven by this concentration gradient.

A concentration gradient only exists until the molecules are evenly distributed on both sides of the cell membrane. So the concentration gradient is directly proportional to the rate of diffusion.

Example of Diffusion: Aerobic Respiration

Oxygen concentration is lower in the muscle cells than in the bloodstream because that is where it is being used. So oxygen diffuses from the bloodstream to the muscle cell.

Carbon dioxide concentration is higher in the muscle cells than in the bloodstream because that is where it is being produced. So carbon dioxide diffuses from the muscle cell to the blood stream.

Diffusion is needed for aerobic respiration - In case you're interested...
Aerobic respiration is the process of producing the energy or ATP plants and animals use to grow, move and survive. It happens in organelles called mitochondria and uses glucose and oxygen to release energy, carbon dioxide and water.
Oxygen diffuses from the bloodstream and through the cell membranes of muscle cells, where it is used in the mitochondria to carry out aerobic respiration. This is because the concentration of oxygen is much higher in the bloodstream, and much lower in the cells as it is constantly used up inside the muscle cells to produce energy.

Example of Diffusion: Photosynthesis

Carbon dioxide concentration is lower in the leaf cells than in the air because that’s where it is being used up.

Therefore, carbon dioxide diffuses from the air into the leaf (via the stomata).

Diffusion is needed for photosynthesis - In case you're interested...
Photosynthesis produces the sugar glucose from water and carbon dioxide. Glucose can be broken down during respiration into energy the plant can use to grow, reproduce, and survive.
Carbon dioxide diffuses from the air and into the leaf through small pores called stomata. This is because the concentration of carbon dioxide is much higher in the air, and much lower in the leaf and plant cell because it is constantly being used up within the leaves during photosynthesis.
Once it is inside the leaf, carbon dioxide diffuses through the plant cell membrane and into the chloroplasts where photosynthesis happens.

Video: Fluid Mosaic Model. Old school video, but a good overview of the fluid mosaic model.

Video for Experts: Cell Membranes are Way More Complicated than You Think (TED-Ed). More informative video of the fluid mosaic model, but more complex. You do not need to know about cell signalling and endocytosis.

Video: Inside the Cell Membrane (Amoeba Sisters). Describes surface area to volume ratio in cells, and fluid mosaic model of cell membrane. Don’t need to know about cholesterol.

Video: Diffusion (Amoeba Sisters). Comprehensive video on diffusion.

Interest Only Video: Oxygen's surprisingly complex journey through your body (TED-Ed). Puts diffusion into the context of the transport of oxygen through the human organ systems.

Facilitated Diffusion

FACILITATED DIFFUSION, which is the movement of molecules from an area of high concentration to an area of low concentration (down a concentration gradient) through CHANNEL or CARRIER PROTEIN in the CELL MEMBRANE.

Its only difference from simple diffusion is that these molecules are unable to diffuse through a cell membrane without the help of their specific carrier protein or protein channel.

Channel and carrier proteins help the diffusion of molecules, such as:

Glucose and amino acids (too large; carrier protein)
Amino acids (some are polar/charged; carrier protein)
Ions (charged; carrier protein)
Bulk flow of water (polar; channel called ‘aquaporin’)

Without aquaporins, water takes a long time to diffuse across the cell membrane.

Channel proteins and carrier proteins - In case you're interested...The channel and carrier proteins that span the phospholipid bilayer facilitate the diffusion of these molecules into or out of a cell.
CHANNEL PROTEINS: Large molecules, polar molecules (e.g. water) and ions (e.g. Na+, K+ or Cl-) use channel proteins to diffuse across a cell membrane. The channel proteins are open at both ends of the membrane. They act as a corridor, allowing these molecules to diffuse in and out of a cell from areas of high concentration to areas of low concentration.
CARRIER PROTEINS: Carrier proteins are only open at one end at a time. Molecules such as glucose and amino acids bind to the carrier proteins, which then change shape to allow these molecules to diffuse into a cell. Proteins are highly specific, meaning they can only transport one kind of molecule through the cell membrane.

Video: How Facilitated Diffusion Works. Good overview of facilitated diffusion and carrier proteins

Simulation 1

Explore how molecules can cross a cell membrane and learn about the nature of their movement. Set up the model with high oxygen and low carbon dioxide outside the cell and low oxygen and high carbon dioxide inside the cell.

Factors Affecting the Rate of Diffusion

There are many different factors that can affect the rate of DIFFUSION. We will focus a little on temperature, but the most important factor is SURFACE AREA TO VOLUME RATIO (SA : V).

Temperature

Molecules will diffuse quicker at higher temperatures than at lower temperatures, because increasing the temperature causes the molecules to move faster.

Simulation 2: If you place a drop of food colouring into a glass of water, eventually the entire glass is coloured evenly with that dye. Use this model to observe how food colouring diffuses throughout the water.

Simulation 3: Observe the model of diffusion below. Notice the position of blue, green, and white particles. In this simplified model, the blue molecules represent perfume molecules, and the white and green molecules represent the nitrogen and oxygen molecules in the air.

Surface Area to Volume Ratio (SA : V)

When a cell grows, the contents of the cell (its volume) increases at a much greater rate than the CELL MEMBRANE (its SURFACE AREA). This is because the surface area increases by a square factor (cm^2) and the VOLUME increases by a cube factor (cm^3).

So we say that as the cell grows larger, the SURFACE AREA TO VOLUME RATIO (SA : V) decreases.

Because the SA : V ratio plays a vital role in the efficiency of DIFFUSION, cells are dependent on an optimal SA : V ratio to function properly.

If cells get too big, there is comparatively less membrane per volume available for essential molecules such as oxygen to diffuse through and more cell contents that require these molecules in order to function. So diffusion is less efficient for larger cells because essential molecules will not be able to reach the centre of the cell fast enough. The cell that’s too large will also be overloaded with toxic waste products like carbon dioxide in plant cells because waste molecules are unable to diffuse out of the cell quickly enough.

Therefore, when a cell gets too large, the cell will divide to keep high surface area to volume ratio.

The cubes in the diagram above represent a cell. The larger 2 cm cube represents a large cell above has a much lower surface area to volume ratio than the small 1 cm cube which represents a small cell.

Different cells have developed different adaptations to increase their surface area to volume ratio. The most common adaptation are tiny hair-like projections called MICROVILLI.

Specific adaptations to increase surface area - In case you're interested...Some cells, such as root hair cells, increase their surface area by having an elongated shape. This increases the amount of membrane available for diffusion while reducing the volume of the cell.
Other cells, such as red blood cells, have a biconcave shape. This is a disc-like shape with a flattened centre, almost like a donut! For red blood cells, this increases the surface area available for oxygen to diffuse into the cell.
Other cells and organelles have many folds in their cell membranes to increase the surface area. For example, the cells that line our small intestine are responsible for absorbing nutrients from food that we eat. To do this, they require a large surface area. They are covered with many folds called microvilli, which are long, finger-like extensions on the top of the cells. They create a very large area, allowing for maximum diffusion.

Video: Surface Area to Volume Ratio Explained (Science Sauce). Easy to understand video explaining surface area to volume ratio.

We will be doing an experiment very similar to this phenolphthalein experiment in this video above. The experiment demonstrates how diffusion becomes less efficient as the surface area to volume ratio decreases.

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