3.3 Active transport (3)
Resources |
Revision Questions |
Biology
Login to see all questions
Click on a question to view the answer
1.
Describe active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration. In your answer, explain the role of carrier proteins and the process of saturation.
Active transport is the movement of molecules across a cell membrane against their concentration gradient – meaning from an area of lower concentration to an area of higher concentration. This process requires energy, which is supplied by respiration, typically in the form of ATP (adenosine triphosphate).
The movement is facilitated by specific carrier proteins embedded in the cell membrane. These proteins bind to the molecule being transported. The binding causes the protein to change shape, and this conformational change allows the molecule to be moved across the membrane. Some carrier proteins are always in the 'on' state, while others can switch between 'on' and 'off' states depending on the availability of ATP.
A key feature of active transport is saturation. There are a finite number of carrier proteins in the cell membrane. As the concentration of the transported molecule increases, more and more carrier proteins become occupied. Eventually, all the carrier proteins are bound to the molecule, and the rate of transport reaches a maximum. At this point, the transport is said to be saturated. Increasing the concentration of the molecule beyond this point will not increase the rate of transport.
In summary, active transport uses energy to move molecules against their concentration gradient, relying on carrier proteins that bind to the molecule and undergo a conformational change to facilitate movement. The process is limited by the number of available carrier proteins, leading to saturation.
2.
Explain the difference between active transport and facilitated diffusion. In your answer, describe how the process of facilitated diffusion works and identify a factor that can affect its rate.
Both active transport and facilitated diffusion involve the movement of particles across a cell membrane, but they differ significantly in their energy requirements and the mechanisms involved.
Facilitated diffusion is a type of passive transport. It involves the movement of molecules *down* their concentration gradient – from an area of high concentration to an area of low concentration. It does not require energy input. The movement is facilitated by carrier proteins or channel proteins embedded in the cell membrane. These proteins provide a pathway for the molecule to cross the membrane. The molecule simply diffuses through the protein, following the concentration gradient.
Active transport, as previously discussed, requires energy (ATP) to move molecules *against* their concentration gradient. It uses carrier proteins and the energy from respiration to power this movement.
A factor that can affect the rate of facilitated diffusion is the temperature. Increasing the temperature generally increases the rate of diffusion, as the molecules have more kinetic energy and move faster. However, excessively high temperatures can damage the cell membrane and denature the carrier proteins, reducing the efficiency of facilitated diffusion. Also, the concentration gradient itself is a key factor; a steeper gradient will lead to a faster rate of diffusion.
3.
State that protein carriers move molecules or ions across a membrane during active transport. Explain how this process requires energy and describe the role of the protein carrier in this process.
Statement: Protein carriers facilitate the movement of specific molecules or ions across a cell membrane during active transport.
Energy Requirement: Active transport requires energy because the movement of molecules against their concentration gradient necessitates overcoming the natural tendency for molecules to diffuse down their concentration gradient. This energy is typically supplied in the form of ATP (adenosine triphosphate). ATP hydrolysis provides the energy needed to change the conformation of the protein carrier.
Role of the Protein Carrier: The protein carrier forms a specific complex with the molecule or ion being transported. This binding causes the carrier protein to undergo a conformational change, which physically moves the molecule/ion across the membrane. The conformational change is driven by the hydrolysis of ATP. The carrier then releases the molecule/ion on the other side of the membrane, returning to its original conformation, ready to bind another molecule/ion. The specificity of the carrier ensures that only the intended molecule/ion is transported.