5.2 - What is a Cell Made Up Of

A cell is the basic structural and functional unit of life. It is the smallest unit of an organism that can carry out all the necessary functions of life, such as metabolism, growth, reproduction, and response to stimuli. Cells are made up of several components, each with its specific function. The components of a cell can be broadly categorized into two main parts: the cell membrane and the cell organelles.

1. Cell Membrane

The cell membrane, also called the plasma membrane, is a thin, flexible layer that surrounds the cell. It is primarily made of lipids and proteins. The cell membrane has selective permeability, which means it allows certain substances to enter or exit the cell while blocking others. This helps in maintaining the internal environment of the cell. The lipid bilayer of the membrane is made up of phospholipids, with hydrophilic heads facing outward and hydrophobic tails facing inward. Embedded proteins assist in transport, signaling, and cell recognition.

2. Cytoplasm

The cytoplasm is a jelly-like substance found between the cell membrane and the nucleus. It consists of cytosol (the fluid portion), organelles, and other structures. It serves as the site for various chemical reactions and is involved in the transportation of materials within the cell. The cytoplasm contains enzymes that catalyze reactions involved in energy production, protein synthesis, and cell division.

3. Nucleus

The nucleus is the control center of the cell. It contains the genetic material (DNA) in the form of chromatin. The nucleus is surrounded by a double membrane known as the nuclear envelope, which contains pores that allow the exchange of materials between the nucleus and the cytoplasm. The nucleolus, found within the nucleus, is responsible for the production of ribosomes.

4. Organelles

Organelles are specialized structures within the cell that perform specific functions. Some of the major organelles include:

• Endoplasmic Reticulum (ER): The ER is a network of membranes involved in the synthesis and transport of proteins and lipids. It comes in two forms: rough ER (with ribosomes on its surface) and smooth ER (without ribosomes).
• Golgi Apparatus: The Golgi apparatus is involved in the modification, sorting, and packaging of proteins and lipids for secretion or delivery to other parts of the cell.
• Mitochondria: Often referred to as the "powerhouse" of the cell, mitochondria are the site of cellular respiration. They generate ATP (adenosine triphosphate), the cell’s energy currency, through oxidative phosphorylation.
• Ribosomes: Ribosomes are tiny structures found either floating freely in the cytoplasm or attached to the rough ER. They are the sites of protein synthesis, where RNA is translated into proteins.
• Vacuoles: Vacuoles are membrane-bound sacs involved in storing water, nutrients, and waste products. In plant cells, the central vacuole plays a major role in maintaining turgor pressure.
• Lysosomes: Lysosomes are membrane-bound organelles containing digestive enzymes that break down waste materials, cellular debris, and foreign substances.
• Cytoskeleton: The cytoskeleton is a network of protein filaments and tubules that provide structural support, shape, and facilitate cell movement and division.

5. Chemical Composition of a Cell

Cells are primarily composed of organic and inorganic compounds, including water, proteins, lipids, carbohydrates, and nucleic acids.

• Water: Water makes up a large portion of the cell and is vital for maintaining cellular processes and the proper environment for biochemical reactions.
• Proteins: Proteins are made of amino acids and perform a wide variety of functions in the cell, including enzyme catalysis, structural support, and transport.
• Lipids: Lipids, including phospholipids and cholesterol, are important components of cell membranes and serve as energy reserves.
• Carbohydrates: Carbohydrates provide energy to the cell and are also found in the cell membrane, where they play a role in cell recognition.
• Nucleic Acids: DNA (deoxyribonucleic acid) stores genetic information, while RNA (ribonucleic acid) plays a role in protein synthesis.

6. Chemical Formula of ATP

The energy currency of the cell is ATP (adenosine triphosphate). The chemical formula for ATP is:

$C_{10}H_{16}N_{5}O_{13}P_3$

ATP stores energy in its high-energy phosphate bonds, which can be broken down to release energy for cellular processes.