Cell Structure and Function: Key Organelles and Processes in Human Cells

Cells Part 1 Recap

Learning Objectives

• Describe the basic structure of eukaryotic cells, including the plasma membrane, nucleus, and cytoplasm.

• Explain the structure and function of major organelles: ribosomes, rough endoplasmic reticulum (RER), Golgi apparatus, smooth endoplasmic reticulum (SER), lysosomes, nucleus, mitochondria, cilia, flagella, and cytoskeleton.

Generalized Cell Structure

Main Components of Human Cells

All human cells share common structural features that enable their diverse functions. The three basic parts of a cell are:

• Plasma membrane: A flexible outer boundary that separates the cell from its environment and regulates the movement of substances in and out.

• Cytoplasm: The intracellular fluid containing organelles, where most cellular activities occur.

• Nucleus: The control center containing DNA, which directs cellular activities and heredity.

Mitochondria

Structure and Function

Mitochondria (singular: mitochondrion) are often called the "power plants" of the cell because they generate most of the cell's energy in the form of ATP through aerobic (oxygen-requiring) cellular respiration.

• Enclosed by double membranes; the inner membrane is highly folded into structures called cristae.

• Cristae contain membrane proteins essential for cellular respiration.

• Mitochondria possess their own DNA, RNA, and ribosomes, resembling bacteria and capable of dividing by fission.

Example: Muscle cells have many mitochondria to meet high energy demands.

Ribosomes

Types and Roles in Protein Synthesis

Ribosomes are nonmembranous organelles responsible for protein synthesis. They are composed of protein and ribosomal RNA (rRNA) and exist in two forms:

• Free ribosomes: Float freely in the cytosol; synthesize proteins that function within the cytosol or other organelles.

• Membrane-bound ribosomes: Attached to the endoplasmic reticulum (ER); synthesize proteins destined for membranes, lysosomes, or export from the cell.

Example: Pancreatic cells have abundant membrane-bound ribosomes for secreting digestive enzymes.

Endoplasmic Reticulum (ER)

Rough ER

The rough ER is characterized by its surface studded with ribosomes. It is the site of synthesis for proteins that will be secreted from the cell, incorporated into membranes, or sent to lysosomes.

• Proteins enter the ER cisterns as they are synthesized and are modified during their passage through fluid-filled tubes.

• Final proteins are enclosed in vesicles and sent to the Golgi apparatus for further processing.

Smooth ER

The smooth ER lacks ribosomes and consists of a network of looped tubules continuous with rough ER. Its membrane contains enzymes that perform various functions:

• Lipid metabolism: Synthesis of lipids and steroid-based hormones (e.g., testosterone in testes).

• Detoxification: Neutralizes drugs, pesticides, and carcinogens, especially in liver and kidneys.

• Glycogen metabolism: Converts glycogen to free glucose.

• Calcium storage and release: Especially important in muscle cells, where the smooth ER is called the sarcoplasmic reticulum.

Example: Liver cells have extensive smooth ER for detoxification.

Golgi Apparatus

Processing and Packaging Center

The Golgi apparatus consists of stacked, flattened membranous sacs (cisterns) and functions to modify, concentrate, and package proteins and lipids made at the rough ER.

• Transport vesicles containing proteins move from ER and fuse to the Golgi cis face (receiving side).

• Proteins are further modified by addition or removal of sugar and phosphate groups.

• Proteins are tagged, sorted, and packaged into different types of vesicles that bud from the Golgi trans face (shipping side).

Example: Secretory cells use the Golgi apparatus to package hormones for release.

*Additional info: Further details on lysosomes, cytoskeleton, cilia, flagella, and nucleus are covered in subsequent slides and notes. These organelles are essential for cellular digestion, structural support, movement, and genetic control, respectively.*