Question

Large particles or large volumes of fluid can be brought into the cell by a. facilitated diffusion. b. active transport. c. endocytosis. d. exocytosis. e. all of the above.

Short answer

The correct answer is c. endocytosis.

Step-by-step solution

Understand the options

Each option represents a different type of cell transport mechanism. Facilitated diffusion and active transport generally handle small molecules, while endocytosis and exocytosis deal with larger molecules and particles. 'All of the above' suggests that all mentioned procedures can manage the task. But we need to identify which mechanism is specifically designed for the transport of large particles or volumes of fluid into the cell.

Eliminate options

Looking at the options, facilitated diffusion and active transport are usually involved in the transport of small, specific molecules across the cell membrane, therefore, can be eliminated from the options. Exocytosis is a process by which cells move substances out of the cell, not into the cell, so this option can be eliminated too.

Identify the correct answer

Endocytosis is the process by which cells absorb large particles or volumes of fluid by engulfing them, making it the mechanism that fulfills the conditions presented in the exercise.

Key concepts

Endocytosis

Endocytosis is a form of active transport where cells take in large particles or volumes of extracellular fluid through an inward folding of their plasma membrane. This process results in the formation of vesicles that encapsulate the substances to be absorbed.

There are different types of endocytosis:

• Phagocytosis: Often referred to as 'cell eating,' it involves the ingestion of large particles like bacteria or dead cells.
• Pinocytosis: Known as 'cell drinking,' it is the ingestion of fluid containing various solutes.
• Receptor-mediated endocytosis: This is a highly selective process where cells intake specific molecules bound to receptors on their surface.

The specificity and active nature of endocytosis mean that it requires energy, usually in the form of ATP, to facilitate the transport of substances into the cell.

Facilitated Diffusion

Facilitated diffusion is a type of passive transport that allows substances to cross the cell membrane with the help of membrane proteins. Unlike simple diffusion, which occurs freely across the membrane's phospholipid bilayer, facilitated diffusion utilizes specific proteins to create a pathway for charged or large molecules.

Two main types of transport proteins are involved in this process:

• Channel proteins: They provide a hydrophilic tunnel that can be used by molecules like ions to pass through.
• Carrier proteins: These proteins change shape to move a target molecule from one side of the membrane to the other.

Since facilitated diffusion is a form of passive transport, it does not require energy and moves substances down their concentration gradient.

Active Transport

Active transport is a cellular process in which substances move against their concentration gradient through the cell membrane, requiring the use of energy. This is opposite to passive transport, which occurs along the concentration gradient and does not require energy.

Active transport is vital for maintaining concentrations of ions and other substances necessary for life processes, often achieved through:

• Protein pumps: These are membrane proteins that use ATP to transport ions and molecules.
• Co-transport: It involves coupled passage where one substance moves down its gradient while another moves against.

Through active transport, cells can uptake necessary nutrients, expel waste, and maintain an internal ionic balance conducive to life's biochemical processes.

Exocytosis

Exocytosis is the process by which cells move materials out of the cell in vesicles that fuse with the plasma membrane. This is an active transport mechanism important for various cellular functions such as releasing neurotransmitters, hormones, and other cell products.

Key steps in exocytosis include:

• The vesicle moves towards the cell membrane.
• Vesicle and plasma membrane proteins help the vesicle to dock and fuse with the membrane.
• Contents of the vesicle are expelled into the extracellular environment.

Just like its counterpart endocytosis, exocytosis is essential for cells to communicate and maintain a balanced internal environment.

Cellular Transport

Cellular transport refers to the mechanisms by which substances move into and out of cells. It plays a crucial role in maintaining homeostasis within the cell and the organism as a whole.

Types of cellular transport include:

• Passive transport: This includes diffusion, osmosis, and facilitated diffusion, where substances move without the input of cellular energy.
• Active transport: This includes endocytosis and exocytosis, as well as protein pumps, which require energy, usually in the form of ATP.

Understanding cellular transport mechanisms is fundamental to appreciating how cells interact with their environment and regulate their internal conditions for survival.