TOPIC 2A 2 CELL TRANSPORT AND DIFFUSION

What substances pass through the cell membrane easily?

1. Substances that can dissolve easily in lipids.

2. Other small molecules, such as gases like oxygen and carbon dioxide pass freely across the membrane.

What substances cannot pass through the cell membrane freely?

1. Large molecules, such as steroid hormones.

2. Charged particles, such as sodium ions.

How does passive transport take place in the cell membrane.

1. Pressure or electrochemical gradients involve no energy from the cell.

How does active transport take place in the cell membrane?

1. It is when substances move into or out of the cell by using Adenosine triosephosphate ATP which is produced during cellular respiration.

List some passive transport processes.

1. Diffusion

2. Facilitated diffusion

3. Osmosis

List some active transport mechanisms.

1. Endocytosis

2. Exocytosis

What is the definition of diffusion?

1. It is the movement of particles in a liquid or gas down a concentration gradient.

2. They move from low concentration to high concentration area, cell membrane is not a barrier to oxygen and carbon dioxide,

What is facilitated diffusion?

1. Facilitated diffusion is diffusion that takes place through carrier proteins or protein channels.

2. The protein-lined pores of the cell membrane make facilited diffusion possible.

What is Osmosis?

1. It is the free movement of water molecules down a water potential gradient through a partially permeable membrane.

When in active transport is carrier protein involved?

1. always.

What is endocytosis?

1. Endocytosis is the movement of large molecules into cells through vesicle formation.

2. The fluid nature of the cell membrane makes it possible.

What is exocytosis?

1. The movement of large molecules out of cells through fusion of vesicles to the membrane.

What causes diffusion of molecules?

1. Molecules contain kinetic energy, which causes them to vibrate. When they do so, they collide with each other, pushing each other in different directions. Hence diffusion.

2. They will collide with each other until they have reached equal distribution.

For what molecules IS DIFFUSION ENOUGH TO TRANSPORT ACROSS CELL MEMBRANES?

1. Small molecules such as oxygen and carbon dioxide can diffuse freely across the membrane.

2. Hydrophilic molecules and ions that are larger than carbon dioxide molecules can’t move across freely.

What is facilitated diffusion?

1. It is the diffusion of a substance across a differentially permeable membrane with the help of proteins down a concentration gradient.

Give an example of the proteins that help with facilitated diffusion.

1. Channel proteins form pores through the membrane; each channel protein allows a specific molecule to pass through passively, depending on its shape and charge.

2. Some channel proteins are for potassium, some are for sodium. Some channels open if a specific molecule is present. Some open if there is an electrical charge across the membrane.

3. Channels that open and close based on certain conditions are called gated channels.

Asides from channel proteins that create pores, what other proteins may be involved in facilitated diffusion?

1. Carrier proteins float on the surface of the membrane and are found out and inside the cell membrane.

2. When a molecule is to be carried into the cell membrane or out. The carrier protein picks up the molecule; it rotates to the other side of the cell membrane (changes shape) bringing the molecule with it and releases the molecule.

3. This process depends on the shape of the protein carrier and the shape of the molecule to be carried.

4. This process can only take place down a concentration gradient, from high concentration to low. It does not require ATP; therefore it is considered a form of diffusion.

Explain why transport systems are needed across membranes?

1. The properties of the membrane and the properties of the substances entering or leaving a cell (such as their size, solubility in water or lipids, and their charge) mean that many substances cannot cross cell membranes by simple diffusion.

2. Concentration gradients across cell membranes can also prevent movement by simple diffusion.

3. Any substances that cannot cross a cell membrane by diffusion need a specific transport system, whether carrier molecules or specific pores, to get from one side of the membrane to the other.

Describe the conditions needed for the passive transport of molecules into a cell.

1. The molecules need to be able to cross the membrane,

2. And there needs to be a concentration gradient for those molecules from one side of the membrane to the other.

Water and ions often enter the cell through protein pores but they cannot pass through the lipid layer in the same way that oxygen and carbon dioxide do. Why not?

1. Water can’t pass across the hydrophobic region in the middle of the membrane, and ions are not lipid-soluble.

Explain the differences between simple diffusion and facilitated diffusion.

1. Simple diffusion is through the lipid part of the membrane, so it can happen at any part of the membrane.

2. Facilitated diffusion can only happen where there are suitable carriers in the membrane.