Question

Which statements are consistent with the known facts about membrane transport? (a) Active transport moves a substance from a region in which its concentration is lower to one in which its concentration is higher. (b) Transport does not involve any pores or channels in membranes. (c) Transport proteins may be involved in bringing substances into cells.

Short answer

Statements (a) and (c) are consistent; statement (b) is not.

Step-by-step solution

- Identify Active Transport

Active transport is a process that moves molecules against their concentration gradient. It moves molecules from an area of lower concentration to an area of higher concentration, often requiring energy in the form of ATP.

- Analyze Statement (a)

Statement (a) claims that active transport moves a substance from a region of lower concentration to one of higher concentration. This is consistent with the definition of active transport.

- Investigate the Role of Membrane Channels

Transport across membranes often involves pores or channels, such as ion channels or aquaporins, especially in passive transport. Thus, claiming that transport does not involve any pores or channels is inaccurate.

- Analyze Statement (b)

Statement (b) claims transport does not involve any pores or channels. This is incorrect because many forms of transport, including facilitated diffusion and some forms of active transport, occur through specific transport proteins that form these channels.

- Understand Transport Proteins

Transport proteins, such as carrier proteins or protein pumps, play a crucial role in moving substances across cell membranes. These proteins can either facilitate passive transport by channels or perform active transport using energy.

- Analyze Statement (c)

Statement (c) claims transport proteins may be involved in bringing substances into cells. This is correct, as transport proteins are essential for both passive and active transport in cells.

- Summarize Consistency

Statements (a) and (c) are consistent with known facts about membrane transport, while statement (b) is not.

Key concepts

active transport

Active transport is a vital process in cellular function where substances are moved against their concentration gradient.
This usually means moving molecules from an area of lower concentration to an area of higher concentration.
This movement requires energy, which is typically derived from ATP (adenosine triphosphate).
Unlike passive transport, active transport does not rely on the natural diffusion process.
It is crucial for maintaining cellular environments that differ from their surroundings and for functions like nutrient uptake and waste removal.
For example, the sodium-potassium pump is a well-known active transport mechanism that pumps sodium ions out of cells and potassium ions into cells, both against their concentration gradients.

transport proteins

Transport proteins are essential for moving substances across cell membranes.
These proteins can be divided into two main types: carrier proteins and channel proteins.

• Carrier Proteins: These proteins bind to the substance they are transporting, change shape, and then release the substance on the other side of the membrane.
• Channel Proteins: These form pores or channels that allow specific molecules or ions to pass through the membrane. They are like 'tunnels' for certain substances.

Transport proteins can facilitate both passive and active transport.
For instance, during active transport, protein pumps use energy to move substances, while in passive transport, proteins facilitate the diffusion of substances down their concentration gradient.

passive transport

Passive transport is the movement of substances across a cell membrane without the use of energy.
It relies on the natural tendency of molecules to move from an area of higher concentration to an area of lower concentration, a process known as diffusion.
There are several types of passive transport:

• Simple Diffusion: Movement of small or nonpolar molecules directly through the lipid bilayer.
• Facilitated Diffusion: Utilizes transport proteins like channels or carriers to help move substances across the membrane.
• Osmosis: Diffusion of water molecules through a semi-permeable membrane.

An example of passive transport is the movement of oxygen and carbon dioxide across cell membranes during respiration.

ion channels

Ion channels are a specialized type of transport protein that form pores in cell membranes.
These channels allow specific ions (like sodium, potassium, calcium, and chloride) to move across the membrane.
Ion channels are crucial for maintaining cellular homeostasis and for various physiological processes such as nerve impulse transmission and muscle contraction.
They are often gated, meaning they can open or close in response to certain stimuli, such as changes in voltage (voltage-gated channels) or the binding of specific molecules (ligand-gated channels).
For example, the opening of sodium channels in nerve cells is a key step in generating an action potential, which is necessary for nerve signal transmission.