Which of the following accurately describes an example of how materials can move across the cell membrane through active transport? a protein acts as a channel to allow large, hydrophilic molecules to diffuse across the cell membrane. a protein acts as a pump and moves calcium ions from an area of low concentration to an area of high concentration. the cell membrane folds to surround a substance and bring the substance inside the cell during exocytosis. water moves by osmosis from higher concentration to lower concentration by passing through proteins called aquaporins.

A protein acts as a pump and moves calcium ions from an area of low concentration to an area of high concentration.

Explanation:

An active transport means a transport which utilizes ATP i.e. energy input to move substances against their concentration gradient.

Naturally, substances like ions have a tendency to move from higher concentration to lower concentration. This is know as passive transport  which does not require any energy input. But, if we want to move an ion against its concentration gradient then we will have to provide energy input and such transport is known as active transport. Pumps are transport proteins which are involved in active transport in a cell. They may be located on cell membrane or plasma membrane of organelles like sarcoplasmic reticulum (ER) etc.

Examples of pumps are Na⁺ - K⁺ pumps in cell membrane, calcium pumps in  sarcoplasmic reticulum (SR).