Types of transport

What is simple diffusion?

The movement of molecules from an area of high concentration to a region of lower concentration down a concentration gradient

What kind of molecules can diffuse though the cell membrane?

molecules that are small and/or lipid soluble are able to diffuse through the phospholipid bilayer

Give an example of molecules that can move through the cell membrane via simple diffusion

Oxygen and carbon dioxide can diffuse into cells via simple diffusion.

Why can oxygen and carbon dioxide diffuse through via simple diffusion

they are:

Small - this means that they can pass through the spaces between phospholipids

Non-polar - this means that they can dissolve in the hydrophobic core of the cell membrane

What type of molecules move through the cell membrane via facilitated diffusion?

Large or polar (ions) molecules

Which processes are passive and which are active?

Simple diffusion, facilitated diffusion and osmosis is passive and active transport is an active process

Why do large or polar ions move through the cell membrane via facilitated diffusion

Ions or molecules that carry a charge or are too large cannot pass through the phospholipid bilayer, they can only pass through the membrane with the help of proteins

What proteins do facilitated diffusion involve?

Facilitated diffusion involves two types of proteins:

Carrier proteins

Channel proteins

How do carrier proteins work?

Carrier proteins are specific to certain kinds of ions or molecules.

A large molecule attaches to a carrier protein which causes the carrier protein to change shape and so the carrier protein releases the molecule on the other side of the membrane

How do channel proteins work?

Channel proteins mainly transport ions across the cell membrane and they have a hydrophilic channel through them that allows specific molecules through

State and explain the factors effecting the rate of diffusion

Temperature -

At higher temperatures, particles have more kinetic energy and diffuse faster.

Concentration gradient -

The steeper the concentration gradient, the faster the rate of diffusion.

Thickness of membrane

The particles travel shorter distances through thin exchange surfaces, so diffuse faster.

Surface area

Larger surface areas mean more particles can cross the membrane at once, making diffusion faster.

Number of carrier or channel proteins

The more of these proteins, the faster the rate of facilitated diffusion.

How do water molecules travel through the cell membrane?

Water molecules are small and although they carry charges they can pass between the phospholipid molecules in the bilayer via osmosis

What is osmosis?

Osmosis is the net movement of water molecules across a partially permeable membrane from an area of high water potential to an area of lower water potential

What is water potential and what is it measured in

Water potential (Ψ) is the pressure exerted by water molecules on the membrane (or container) surrounding a solution. It is measured in kiloPascals (kPa)

High water potential:

A lot of water molecules are available to move

Low water potential:

Not a lot of water molecules available to move due to presence of solute molecules or just not enough water

What has the highest water potential?

Pure water has the highest water potential of 0kPa and as more solute is added the value decreases (becomes more negative)

What does it mean for a solution to be isotonic?

If two solutions have the same water potential, they're said to be isotonic. There is no net movement of water into or out of the cell

What is a hypotonic solution?

A hypotonic solution has a higher water potential than the cell so water molecules move into the cell and the cell swells and bursts

In plant cells the cell would swell and become turgid

What is a hypertonic solution?

A hypertonic solution has a lower water potential than the cell so water molecules move out of the cell. This causes the cell to shrink

In a plant cell the cell would shrink and become plasmolysed

What are the factors effecting the rate of osmosis?

Temperature

At higher temperatures, water molecules have more kinetic energy and diffuse faster.

Water potential gradient

The steeper the gradient, the faster the rate of osmosis.

Thickness of membrane

Water molecules travel shorter distances through thin exchange surfaces, so diffuse faster.

Surface area

Larger surface areas mean more water molecules can cross the membrane at once, making osmosis faster.

When is active transport used?

Sometimes cells take up substances against their concentration gradient via active transport

What is active transport?

Active transport is the movement of particles from an area of lower concentration to an area of higher concentration. This process requires energy from respiration in the form of ATP

What is a similarity between active transport and facilitated diffusion?

Like facilitated diffusion, active transport involves the use of carrier proteins to transport molecules or ions across membranes

Explain how active transport occurs?

The molecule or ion binds to the carrier protein.

ATP binds to the carrier protein

The hydrolysis of ATP to ADP and phosphate (Pi) causes the carrier protein to change shape. This releases the molecule or ion on the opposite side of the membrane

The phosphate (Pi) is released from the carrier protein, causing the carrier protein to return to its original shape, ready to be used again

What are the factors affecting the rate of active transport?

Temperature

At higher temperatures, particles have more kinetic energy and travel faster. Respiration also increases with temperature. Very high temperatures denature carrier proteins, decreasing the rate of active transport.

Thickness of membrane

Particles travel shorter distances through thin exchange surfaces, so travel faster.

Number of carrier proteins

The more of these proteins, the faster the rate of active transport.

Rate of respiration

The more respiration, the more ATP available for active transport.

What is co-transport?

Some carrier proteins can bind to two molecules at once, these proteins are known as co-transports. They use the concentration of one molecule to move the other molecule against its own concentration gradient

Example of where co-transport occurs?

The absorption of glucose in the ileum

Explain how glucose is abosorbed into the ileum

look at sheet of Dr Copeland