Cellular Level of Organization: Structure and Function of Cells

Cellular Level of Organization

Learning Outcomes

• Describe the functions and structural features of the plasma membrane.

• Identify the organelles of a typical cell and their specific functions.

• Explain the functions of the cell nucleus and the importance of the genetic code.

• Summarize the role of DNA in protein synthesis, cell structure, and function.

• Describe cellular diffusion and osmosis and their roles in physiological systems.

• Explain carrier-mediated and vesicular transport mechanisms for substance movement.

• Discuss the origin and significance of the cell membrane potential.

Introduction to Cells

Cell Theory

Cell theory is a foundational concept in biology, stating that:

• Cells are the building blocks of all organisms.

• All cells arise from the division of preexisting cells.

• Cells are the smallest units that perform all vital physiological functions.

• Each cell maintains homeostasis at the cellular level.

Types of Cells

• Sex cells (germ cells or reproductive cells):

  • Male: sperm

  • Female: oocytes (develop into ova)

• Somatic cells: All body cells except sex cells.

Definition of Cytology

Cytology is a branch of cell biology focused on the study of cells, their structure, function, and chemistry.

Cell Structure Overview

Major Components of a Typical Cell

• Plasma membrane: The outer boundary of the cell, separating the cytoplasm from the extracellular environment.

• Cytoplasm: The material within the cell, excluding the nucleus, containing cytosol and organelles.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Organelles

• Nonmembranous organelles: Not surrounded by a membrane; include ribosomes, centrioles, cytoskeleton, microvilli, cilia, and flagella.

• Membranous organelles: Surrounded by membranes; include endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and mitochondria.

Example: Cell Diagram

The provided diagram labels key structures such as the plasma membrane, cytosol, nucleus, secretory vesicles, and centrioles. It distinguishes between membranous and nonmembranous organelles and highlights the centrosome and centrioles' role in cell division.

Key Terms and Definitions

• Plasma membrane: A selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Cytosol: The fluid component of the cytoplasm, containing dissolved nutrients, ions, proteins, and waste products.

• Organelle: Specialized structures within a cell that perform specific functions necessary for cellular life.

• Nucleus: The largest organelle, containing the cell's DNA and responsible for regulating gene expression and cell division.

• Centrosome: A region near the nucleus that contains centrioles, important for organizing microtubules during cell division.

• Centrioles: Cylindrical structures composed of microtubules, essential for the formation of the spindle apparatus during mitosis.

Summary Table: Cell Components and Functions

Component	Type	Main Function
Plasma Membrane	Membranous	Selective barrier; regulates entry/exit of substances
Cytosol	Nonmembranous	Fluid medium for cellular processes
Nucleus	Membranous	Genetic control center; stores DNA
Centrioles	Nonmembranous	Organize spindle fibers during cell division
Secretory Vesicles	Membranous	Transport and release cellular products

Additional Info

• Homeostasis at the cellular level refers to the cell's ability to maintain a stable internal environment despite changes in the external environment.

• Microvilli (as seen in the diagram) are extensions of the plasma membrane that increase surface area for absorption.