Cellular Transport Notes - Level 2 Biology

Cells must have the ability to obtain and release substances in and out of themselves.

Living Cells require substances used for various life processes such as Respiration and Photosynthesis. They also require to dispose of waste material and can potentially be harmful.

There are different forms of Cellular Transport that cells can perform which are categorised into two: Passive Transport and Active Transport.

Passive Transport

Passive Transport is the transportation in and out of cells, that require no energy input.

There are 3 Types of Passive Transports cells can perform which are: Diffusion, Facilitated Diffusion and Osmosis.

Diffusion

Diffusion is the simplest form of passive transport. This is when substances from a high concentration area, move from low concentration area (down a concentration gradient) which can also be across a semi-permeable membrane until equilibrium is reached.

Diffusion occurs for many substances that goes in and out of cells. For Example:

Oxygen Gas (O2) diffuses across the cell membrane and inside cells as it used for Cellular Respiration in plants and animal cells. O2 is found in high concentrations from the atmosphere/environment. It is also diffused out of plant cells as it is a waste product of Photosynthesis.

Water (H2O) is also diffused in by plant cells as it is a reactant for Photosynthesis. Water Molecules are small and can therefore pass through the cell membrane easily.

Carbon Dioxide also diffuses out of cells and into various circulatory systems to be expelled out as it is a waste product of Cellular Respiration. Just like Oxygen Gas, it is a small molecule and can pass through the cell membrane easily.

Factors that affect the Rate of Diffusion

There are 3 Factors that affect the Rate of Diffusion, which are: Temperature, Particle Size and Viscosity.

Temperature

Temperature affects the rate of Diffusion as high-temperature input will provide the particle more kinetic energy. Therefore, they will get from one area to another faster and they move faster.

Particle Size

Smaller Particles tend to move faster than larger particles as they require less force to move.

Viscosity

Gas Substances diffuse faster than liquid substances (relates to particle size too…). Solids cannot diffuse.

Facilitated Diffusion

Facilitated Diffusion is similar to Simple Diffusion. It is the movement of substances from a high concentration area to a low concentration area (down a concentration gradient) ACROSS a semi-permeable membrane, until equilibrium is reached. The only difference is that Facilitated Diffusion requires the presence of a semi-permeable membrane.

Some particles like glucose and acids are very large molecules and require facilitation to move from one side of the cell membrane to the other. This is why in Channel Proteins are studded within cell membranes to help facilitate the movement of large substances in and out of cells. Charged Particles such as Ions also have a hard time going through the cell membrane (because the phospholipid bi-layer has polar heads).

Glucose (C6H12O6) uses Glut Transport Channel Protein to diffuse into the cells used for Cellular Respiration.

Although Water (H2O) can easily move in and out of cells through simple diffusion, large quantities of water need facilitation. Therefore a channel protein known as Aquaporin helps with the transport of large quantities of water, in and out of cells.

Factors that affect the Rate of Facilitated Diffusion

The only major factor that affects the rate of Facilitated Diffusion is Temperature.

Since Facilitated is controlled by proteins, which are temperature specific, when these proteins are at their optimal temperature environment, they will function efficiently. However, extreme high temperatures will cause these proteins to denature, stopping Facilitated Diffusion completely. Cold temperatures will slow down the rate of Facilitated Diffusion as particles are much slower.

Osmosis

Osmosis is a special type of Diffusion that involves the movement of WATER molecules from a high concentration area to a low concentration area (down a concentration gradient) ACROSS a semi-permeable membrane, until equilibrium is reached.
Osmosis can only occur in the presence of a semi-permeable membrane, similar to facilitated diffusion.

Osmosis can also be defined as the movement of water molecules from an area of low solute concentration, to an area of high solute concentration.

Solutions with high solute concentrations, tend to have lower water concentrations. This is because Water Molecules form what are known as Hydration Shells, which is responsible for dissolving solutes. When water molecules are part of these, they become immobile.

Solutions with low solute concentrations, tend to have higher water concentrations. This is because there are less Water Molecules participating in Hydration Shells, and are therefore free to move.

This is where Tonicity terms are used which are: Hypotonic, Isotonic and Hypertonic. (THESE TERMS CAN ONLY BE USED WHEN COMPARING 2 SOLUTIONS WITH EACH OTHER).

When a solution is classified as Hypotonic in comparison to another solution, this means that it has low solute concentration and high water concentration. Therefore, it has high water potential compared to the other solution.

When a solution is classified as Hypertonic in comparison to another solution, this means that it has high solute concentration and low water concentration. Therefore, it has low water potential compared to the other solution

This means that THE OVERALL NET MOVEMENT of water molecules would be from High water potential to low water potential OR from the Hypotonic Solution to the Hypertonic Solution.

When equilibrium is reached, this means that the water concentration on both solutions are equal, they are classified as Isotonic. This means that both solutions have equal water potential and there is ZERO OVERALL NET MOVEMENT of Water Molecules. (Although, water molecules will still be moving from one side to another.)

Osmosis in relations to: Animal Cells

Water is found in Animal Cells, therefore Osmosis can occur. The movement of water is vital for the functioning of animal cells, and when they are placed in various solutions their structures can be altered.

For Example: Blood Cells have water inside them. Solutes are also found inside blood cells.

When they are placed in a solution that is Hypotonic to the environment inside the cell. This means that the solution inside is Hypertonic and has low water potential compared to the solution outside which has high water potential. Therefore, the OVERALL NET MOVEMENT of Water would be from the outside solution to the blood cells. This can potentially cause the blood cell to burst as the excess amount of water will fill up the cell.

When they are placed in a solution that is Hypertonic to the environment inside the cell. This means that the solution inside is Hypotonic and had high water potential compared to the solution outside which has low water potential. Therefore, the OVERALL NET MOVEMENT of Water would be from the cell to the outside environment. This will cause the blood cell to lose water and shrivel.

This why blood cells must be placed in an environment that is Isotonic compared to the environment inside the cell so that there is ZERO OVERALL NET MOVEMENT of Water.

Osmosis in relations to: Plant Cells

Unlike animal cells, plant cells have rigid Cell Walls made of Cellulose and use Osmosis as a mechanism for support. This prevents cells from bursting.

The Central Vacuole of Plant Cells applies turgour pressure when water is inside it.

When Plant Cells are placed in an environment that is Hypotonic to the environment inside the cells. This means that the environment inside the cell is Hypertonic and has low water potential, whereas the environment outside has high water potential. Therefore, the OVERALL NET MOVEMENT of Water would be from the outside environment, to the cell. This will fill the central vacuole with water, increasing the turgour pressure making the cell TURGID by pushing the cell membrane against the rigid cell wall. This is vital as it gives the plant structural support which is important for survival. This is the likely environment plants should be in.

When Plant Cells are placed in an environment that is Hypertonic to the environment inside the cells. This means that the environment inside the cells is Hypotonic, with high water potential, whereas the environment outside has low water potential. Therefore, the OVERALL NET MOVEMENT of Water, will be from the cell to the outside environment. This will cause water from the central vacuole to move out, decreasing the turgour pressure of the cell. This will cause the cell membrane to move away from the cell wall, making the cell flaccid. Further water loss, then the cell would be Plasmolysed and die which is irreversible.

When Plant Cells are placed in an environment that is Isotonic to the environment inside the cells. Then both sides have equal water potential and ZERO OVERALL NET MOVEMENT of Water. Poses neither a threat or advantage for plants.

Active Transport

Active Transport is another transport category. Unlike passive transport, Active Transport requires the expenditure of energy.

This is because Active Transport is the movement of substances from a low concentration area, to a high concentration area (up a concentration gradient) ACROSS a semi-permeable membrane, with the expenditure of energy.

There are 3 Types of Active Transport, which are: Endocytosis, Exocytosis and Ion Pumps (Carrier Proteins).

Endocytosis

Endocytosis involves the absorption/extraction of substances from outside the cell to the inside (keep in mind Active Transport Principles)

There are 2 Types of Endocytosis: Pinocytosis and Phagocytosis

Pinocytosis is the absorption/extraction of liquid substances from outside to cell to the inside. This involves the cell membrane, engulfing the liquid substance, then pulling inwards towards the cell, forming a vesicle.
Phagocytosis is the absorption/extraction of solid substances from outside the cell to the inside. It also involves the cell membrane, engulfing the solid substance, then pulling inwards towards the cell, forming a vesicle.

Exocytosis

Exocytosis is the opposite of endocytosis, which involves the release/expulsion of substances from the inside of the cell to the outside. This involves the cell membrane engulfing the substance, then pulling outwards away from the cell, forming a vesicle. (Keep in mind Active Transport Principles)

Carrier Proteins (Ion Pumps)

Some substance requires facilitation from proteins, to go against their concentration gradient, with the expenditure of energy.

Carrier Proteins assist in the movement of some substances such as charged particles to go against their concentration gradient.

Ion Pumps are used to move ions against their concentration gradient. For example the exchange of Sodium and Potassium Ions.

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These are notes in preparation for the New Zealand NZQA Level 2 Biology External for Life Processes at a Cellular Level.