Cells are the fundamental building blocks of all living things. Whether you're looking at a single-celled organism like bacteria, or a complex multicellular organism like a human, everything is made up of cells. In this lesson, we'll examine the structure of a typical eukaryotic cell, which contains many specialized components called organelles.
The cell membrane is a selectively permeable barrier that surrounds the cell. It's composed of a phospholipid bilayer with embedded proteins. The phospholipids have hydrophilic heads that face outward and hydrophobic tails that face inward. This structure allows the membrane to control what substances can enter or leave the cell, which is essential for maintaining the cell's internal environment or homeostasis.
The nucleus is often called the control center of the cell. It's surrounded by a double membrane called the nuclear envelope, which has pores that allow materials to move in and out. Inside the nucleus, we find the nucleolus, which is involved in ribosome production. The nucleus also contains the cell's DNA, organized into chromosomes. This genetic material directs all cellular activities by controlling protein synthesis and gene expression.
Mitochondria are often called the powerhouses of the cell because they generate most of the cell's supply of adenosine triphosphate or ATP. This process is called cellular respiration. Mitochondria have a double membrane structure: an outer membrane and a highly folded inner membrane. These folds are called cristae and they increase the surface area for chemical reactions. Inside is the matrix, which contains enzymes for the citric acid cycle. The unique structure of mitochondria allows them to efficiently produce energy for the cell.
The endoplasmic reticulum or ER is a network of membranes that extends throughout the cytoplasm. There are two types: rough ER and smooth ER. The rough ER has ribosomes attached to its surface, giving it a bumpy appearance. It's primarily involved in protein synthesis and modification. The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage. Together, these membranes form a transportation system that moves proteins and lipids to different parts of the cell.
The Golgi apparatus is like the cell's post office. It receives proteins and lipids from the ER, modifies them, and then sorts and packages them into vesicles for transport to their final destinations. The Golgi consists of a series of flattened membrane sacs called cisternae. Materials move through these sacs from one end to the other, undergoing various modifications along the way. Once processed, the materials are packaged into vesicles that bud off from the Golgi and are sent to different parts of the cell or even outside the cell.
Ribosomes are the cellular structures responsible for protein synthesis. They read messenger RNA or mRNA and translate the genetic code into sequences of amino acids to form proteins. Ribosomes can be found floating freely in the cytoplasm or attached to the rough endoplasmic reticulum. They consist of two subunits - a large subunit and a small subunit - which work together to assemble amino acids into proteins. This process is called translation and is essential for all cellular functions.