Question

A researcher carefully inserts an electrode into an algal cell. He determines that the electrical charge across the cytoplasmic membrane is \(-70\) millivolts. Then he slips the electrode deeper into the cell across another membrane and measures an electrical charge of \(-90\) millivolts compared to the outside. What large organelle does the second membrane surround? Explain your answer.

Short answer

The second membrane likely surrounds the nucleus of the cell.

Step-by-step solution

Understand the Basic Structure of a Cell

Eukaryotic cells have a nucleus enclosed by a membrane, and other organelles that each perform specific functions. Each organelle is surrounded by membranes. But, what sets the organelles apart is the number of membranes that surround them.

Deduce Possible Organelle

The nucleus is surrounded by two membranes, while some other organelles like mitochondria and chloroplasts are double-membraned as well. However, the question hints that the organelle is large. Among these organelles, the nucleus is considered the largest.

Identify the Organelle

From the information above, it can be concluded that the second membrane likely surrounds the nucleus since it's the largest organelle and is also enclosed by another membrane.

Key concepts

Eukaryotic Cell Structure

To understand the discovery made by the researcher, it's crucial to know what eukaryotic cells are. Eukaryotic cells are complex cells with a defined nucleus and other specialized structures, known as organelles. Each organelle performs a unique function necessary for the cell's survival. Some common organelles include:

• Nucleus: Contains genetic material and acts as the control center of the cell.
• Endoplasmic Reticulum: Synthesizes proteins and lipids.
• Mitochondria: Produce energy for the cell.
• Chloroplasts: Present in plant cells and conduct photosynthesis.

Eukaryotic cells are more organized than prokaryotic cells, thanks to their compartmentalized organelles. Understanding this structure is a vital step in studying their functionality.

Cytoplasmic Membrane

The cytoplasmic membrane, also known as the plasma membrane, forms the outer boundary of the cell. It's a dynamic structure made of a phospholipid bilayer embedded with proteins. This membrane is crucial because it regulates the entry and exit of substances, maintaining the internal environment of the cell. Key functions of the cytoplasmic membrane include:

• Regulating nutrient and waste exchange through selective permeability.
• Facilitating communication through receptor proteins.
• Maintaining structural support with the help of the cytoskeleton.

In the original exercise, the membrane with the measured potential of (-70 mV) refers to this cytoplasmic membrane, showcasing its role in creating and maintaining a charge difference, or potential, across its surface.

Nucleus in Cells

The nucleus is a standout organelle in eukaryotic cells, as it is the largest and contains the cell's genetic material in the form of DNA. It's often dubbed the "control center" because it regulates gene expression and mediates the replication of DNA during the cell cycle. The nucleus is enveloped in a double membrane called the nuclear envelope, which has nuclear pores allowing substances to move in and out. In the exercise, the larger charge difference of (-90 mV) could indicate the passage into the nucleus past its membrane, due to the modifications in the electrical charge as more membrane layers were crossed. The charge difference between the nucleus and its surroundings assists its operation as a secure container for genetic information.

Cellular Membrane Potential

Membrane potential refers to the voltage difference across a cellular membrane. This is crucial for the functionality of nerve impulse transmission, muscle contraction, and the cell's longevity. The disparity in charge arises due to the unequal distribution of ions on either side of the membrane. Factors affecting membrane potential:

• Ion Channels: Proteins that allow ions to move in and out, altering the voltage.
• Ion Pumps: Use energy to transport ions against the concentration gradient.
• Cell Permeability: Allows selective movement of ions contributing to the voltage.

As seen in the exercise, the different readings of membrane potential provide insight into the cell's internal architecture, identifying the double-membraned nature of the nucleus. This underscores the significance of this concept in identifying cellular structures.