3.2.3 Transport across cell membranes 1

What is the primary limitation of simple diffusion across the phospholipid bilayer?

• The presence of protein channels

• The nature of the phospholipid bilayer

• The size of the cell

• The temperature of the environment

Answer explanation

The primary limitation of simple diffusion is the nature of the phospholipid bilayer, which is selectively permeable. This means only certain small, nonpolar molecules can pass through easily, restricting the movement of larger or polar substances.

Which type of diffusion involves carrier proteins?

• Simple diffusion

• Facilitated diffusion

• Osmosis

• Active transport

Answer explanation

Facilitated diffusion involves carrier proteins that help transport molecules across the cell membrane. Unlike simple diffusion, which does not require proteins, facilitated diffusion is essential for moving larger or polar molecules.

What is the role of channel proteins in facilitated diffusion?

• To provide energy for transport

• To create a passageway for specific molecules

• To change the shape of the cell

• To increase the surface area of the membrane

Answer explanation

Channel proteins facilitate diffusion by creating specific passageways in the cell membrane, allowing certain molecules to cross without the need for energy. This is essential for transporting substances like ions and glucose.

Osmosis is best explained in terms of which concept?

• Concentration gradient

• Water potential

• Membrane permeability

• Temperature

Answer explanation

Osmosis is best explained by water potential, which considers the movement of water across a semipermeable membrane due to differences in solute concentration. This concept directly relates to how water moves in response to concentration gradients.

How can cells be adapted for rapid transport across membranes?

• By decreasing the number of protein channels

• By increasing the surface area of the membrane

• By reducing the number of carrier molecules

• By thickening the phospholipid bilayer

Answer explanation

Increasing the surface area of the membrane allows for more protein channels and carrier molecules to be present, facilitating faster transport of substances across the membrane.

Which of the following is NOT a method of movement across membranes?

• Simple diffusion

• Facilitated diffusion

• Osmosis

• Endocytosis

Answer explanation

Endocytosis is a process where cells engulf substances, which is different from the other methods listed. Simple diffusion, facilitated diffusion, and osmosis are all passive transport methods that move substances across membranes without energy.

What is the main difference between simple diffusion and facilitated diffusion?

• Simple diffusion requires energy, facilitated diffusion does not

• Facilitated diffusion requires protein channels, simple diffusion does not

• Simple diffusion occurs only in plant cells, facilitated diffusion occurs in animal cells

• Facilitated diffusion is faster than simple diffusion

Answer explanation

The main difference is that facilitated diffusion requires protein channels to help transport substances across the membrane, while simple diffusion does not involve any proteins and occurs directly through the lipid bilayer.

Which of the following increases the rate of osmosis?

• Increasing the temperature

• Increasing the water potential gradient

• Decreasing the surface area

• Decreasing the number of carrier proteins

Answer explanation

Increasing the water potential gradient enhances the difference in concentration between two areas, driving water movement through osmosis. This is the primary factor that increases the rate of osmosis.

What is the primary function of carrier proteins in facilitated diffusion?

• To provide structural support to the cell

• To bind and transport specific molecules across the membrane

• To synthesise ATP

• To increase the membrane's fluidity

Answer explanation

Carrier proteins facilitate diffusion by binding to specific molecules and transporting them across the cell membrane, allowing for selective permeability and efficient movement of substances.

Which of the following statements about osmosis is true?

• It requires energy in the form of ATP

• It involves the movement of solutes

• It occurs from a region of high water potential to a region of low water potential

• It is a type of active transport

Answer explanation

Osmosis is the movement of water across a semipermeable membrane from a region of low water potential (high water concentration) to high water potential (low water concentration), making the correct statement true.