What is a 'phagocytic vacuole'?

A phagocytic vacuole is a membrane-bound sac formed during phagocytosis that contains a solid particle, such as a bacterium, which has been engulfed by the cell.

What is the fundamental difference between endocytosis and exocytosis?

Endocytosis is the bulk intake of materials into the cell by engulfing them in a membrane-bound vacuole, while exocytosis is the bulk release of materials out of the cell via vesicle fusion with the plasma membrane.

How does phagocytosis differ from pinocytosis?

Phagocytosis involves the intake of solid materials (cell eating), often forming large vacuoles, whereas pinocytosis involves the intake of liquids (cell drinking) into smaller vesicles.

Why is bulk transport considered an active process?

It requires metabolic energy in the form of ATP to move vesicles through the cytoplasm and to facilitate the physical rearrangement and fusion of the phospholipid bilayer.

What role does the Golgi body play in exocytosis?

The Golgi body modifies and packages proteins or lipids into secretory vesicles, which then transport these substances to the cell surface membrane for release.

What would happen to bulk transport if a cell was treated with a respiratory inhibitor?

Bulk transport would stop because the inhibitor prevents the production of ATP, which is the essential energy source required for vesicle movement and membrane fusion.

True or False: Bulk transport moves substances down their concentration gradient.

False. Bulk transport is an active process that moves materials regardless of concentration gradients; it is defined by the size and quantity of the material rather than the gradient direction.

Define 'micropinocytosis'.

Micropinocytosis is a specific form of pinocytosis where the cell internalizes liquids into extremely small vesicles, typically visible only under high-magnification electron microscopy.

How does exocytosis affect the total surface area of the cell membrane?

Exocytosis increases the total surface area of the cell membrane because the membrane of the secretory vesicle fuses with and becomes part of the plasma membrane.

Why is membrane fluidity essential for endocytosis?

Fluidity allows the phospholipids to move and flow, enabling the membrane to bend inward, pinch off, and seal itself to form a vesicle without rupturing.

Endocytosis & Exocytosis | OCR A-Level Biology

2. Foundations in Biology

Endocytosis & Exocytosis

Summary

Endocytosis and exocytosis are active bulk transport mechanisms used by cells to move large quantities of materials or large molecules across the plasma membrane. These processes rely on the fluid nature of the phospholipid bilayer and require metabolic energy in the form of ATP to facilitate membrane reshaping and vesicle movement.

1. Definition & Core Concepts

Bulk transport refers to the movement of large substances, such as proteins, polysaccharides, or even whole cells, into or out of a cell. Unlike diffusion or active transport via carrier proteins, bulk transport involves the physical manipulation of the cell membrane.
These processes are active, meaning they require energy. This energy is provided by the hydrolysis of ATP (adenosine triphosphate) produced during cellular respiration.
The ability of the cell to perform these actions depends entirely on the fluidity of the cell membrane, which allows it to break and reform without losing its integrity.

Diagram showing a secretory vesicle moving toward the cell surface membrane for exocytosis.

2. Endocytosis: Mechanisms of Intake

Endocytosis is the process by which the cell surface membrane engulfs external material to form an internal vesicle or vacuole. The membrane folds inward (invaginates) around the substance until it pinches off into the cytoplasm.
Phagocytosis is the 'cell eating' mechanism used for solid materials. Specialized cells called phagocytes use this to engulf large particles like bacteria, which are then digested by enzymes within the resulting phagocytic vacuole.
Pinocytosis is the 'cell drinking' mechanism for the intake of liquids. When the resulting vesicles are extremely small, the process is specifically referred to as micropinocytosis.

3. Exocytosis: Secretion & Removal

Exocytosis is the functional reverse of endocytosis, used to export materials from the cell. This is essential for secreting substances like hormones, digestive enzymes, or neurotransmitters.
Substances destined for export are typically packaged into secretory vesicles by the Golgi body. These vesicles travel through the cytoplasm toward the cell surface membrane.
Upon contact, the vesicle membrane fuses with the plasma membrane. This fusion allows the contents to be released into the extracellular environment while the vesicle membrane becomes part of the cell surface membrane.

4. Key Distinctions

5. Exam Strategy & Tips