4.1.3 Transport in cells

4.1.3.1 Diffusion

Diffusion Definition

• Diffusion is the passive movement of a substance in a solution from an area of high concentration to an area of lower concentration.
• Passive means that it does not require energy.
• Diffusion occurs in liquids and gases and so it can occur in the cytoplasm.

Diffusion in the Lungs

Diffusion occurs in the alveoli.

Oxygen diffuses from a high concentration in the alveoli into the blood across the capillaries.

Carbon dioxide diffuses from the blood into the alveoli, down the concentration gradient.

Diffusion in the Liver and Kidney

Urea is produced by the liver as a waste product of the body.

• It diffuses from the liver cells into the blood, from an area of high concentration to an area of low concentration.
• The urea then goes to the kidneys to be excreted.

Concentration Gradient

The greater the concentration gradient, the quicker diffusion takes place.

However, if there is an almost equal concentration in both, the rate of diffusion will be

very low.

Temperature

As the temperature increases, particles gain more kinetic energy and so can diffuse

across a membrane more quickly.

Therefore, as the temperature increases, the rate of diffusion increases.

Surface Area

As the surface area of the membrane increases, the rate of diffusion also increases

This is because there is more space for molecules to diffuse across the membrane.

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Simple Organisms

Single celled organisms do not have a specific transport.

• They instead can meet their demands of substances through diffusion.
• They are able to do this as they have a large surface area to volume ratio.

Calculating SA:Volume Ratio

Divide the surface area by the volume.

Comparing Smaller and Larger Organisms

Single celled organisms have a very large surface area to volume ratio.

• However large organisms have a greater volume, with a smaller increase in surface area.
• Therefore, the rate of diffusion would be too slow in large organism to provide all of the transport.

• Thus these organisms require a transport system.

The Small Intestine

The small intestine contains villi.

• These villi even have microvilli.
• This increases the surface area for transport.
• Moreover, it has a very rich blood supply to increase the rate of diffusion and a short
• diffusion path.

The Lungs

The lungs have alveoli, to increase their surface area for diffusion.

The alveoli also have a rich blood supply, thus keeping the concentration gradient going.

Fish Gills

The gills are a part of the fish that act as an exchange surface between blood and the water.

• They have an operculum, a flap, that keeps the water flowing over the gills.
• This maintains the concentration gradient.

Plant Roots

These roots are very long.

• This increases their surface area.
• The surface area is further increased by root hair cells.

Plant Leaves

The leaves of plants are very flat, which increases their surface area.

They also have stomata to maintain the concentration gradient and allow gases to move

into and out of the leaves.

4.1.3.2 Osmosis

Osmosis Definition

• Osmosis is the net movement of water from an area of high concentration to low concentration.
• It only occurs in the movement of water.
• Importantly, it occurs only across a partially permeable membrane.

Water Potential

The concentration of water can also be referred to as water potential.

• A high concentration of water is a high water potential and a low concentration of water is a low water potential.
• Therefore, in osmosis, water moves from a high to a low water potential.

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Percentages

• Percentages can be used to easily compare values from the start and finish of osmosis.
• You can get a value for the percentage change in mass of an object.
• This will allow you to then compare values between different specimens.

Using the data from before, the percentage change would be:

Using the data from before, the percentage change would be:

(62-50)/50 x 100. This gives you 24%.

If you get a negative value, this is a percentage loss

If you get a positive value, it is a percentage gain.

4.1.3.3 Active transport

Active Transport Definition

Active transport is the movement of substances against a concentration gradient using

energy provided by respiration.

Root Hair Cells

• Plants require mineral ions for growth.
• Nitrates are especially needed for protein synthesis.
• Mineral ions are in a higher concentration inside the root hair cells than in the soil.

The Gut Wall

• Active transport helps sugar to be transferred from a low concentration in the gut to a higher concentration in the blood.
• This allows the glucose from the gut to be absorbed.
• This glucose is used for respiration.