Facilitated diffusion across a membrane requires ______________ and moves a solute ______________ its concentration gradient. a. transport proteins . . . up (against) b. transport proteins . . . down c. energy and transport proteins . . . up d. energy and transport proteins . . . down

Verified step by step guidance

1. Identify the process being described: Facilitated diffusion is a type of passive transport that moves substances across a cellular membrane with the help of transport proteins.
2. Understand the role of energy: In facilitated diffusion, no external energy (like ATP) is required as it is a passive transport process.
3. Analyze the direction of movement relative to the concentration gradient: In facilitated diffusion, substances move down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration.
4. Determine the necessity of transport proteins: Transport proteins are essential in facilitated diffusion to help move substances across the membrane that cannot diffuse through the lipid bilayer directly.
5. Match the characteristics to the correct option: Since facilitated diffusion involves transport proteins and moves substances down their concentration gradient without the use of energy, the correct answer is 'b. transport proteins . . . down'.

Key Concepts

Facilitated Diffusion

Facilitated diffusion is a passive transport process that allows substances to cross membranes with the assistance of special proteins. Unlike simple diffusion, which occurs directly through the lipid bilayer, facilitated diffusion requires specific transport proteins that help move molecules across the membrane without the expenditure of energy.

Concentration Gradient

A concentration gradient refers to the difference in the concentration of a substance across a space or membrane. Molecules tend to move from areas of higher concentration to areas of lower concentration, which is known as moving 'down' the concentration gradient. This natural tendency is the driving force behind passive transport processes like facilitated diffusion.

Transport Proteins

Transport proteins are integral membrane proteins that facilitate the movement of ions and molecules across the cell membrane. They can be specific to certain substances and play a crucial role in processes like facilitated diffusion, where they provide a pathway for solutes to move down their concentration gradient without the need for energy input.