Cell Structure: Cytoplasm and Cell Organelles

The Cell Structure

Introduction

The cell is the basic structural and functional unit of all living organisms. Understanding the internal organization of the cell, including the cytoplasm and its organelles, is fundamental to the study of anatomy and physiology.

Cytoplasm

Definition and Components

• Cytoplasm is the region located between the plasma membrane and the nucleus. It is a gel-like material that fills the cell and contains various components essential for cellular function.

• Components of Cytoplasm:

  • Cytosol: The intracellular fluid (ICF), a semi-gelatinous material containing dissolved nutrients, proteins, ions, and waste products.

  • Organelles: Specialized structures ("little organs") that carry out the metabolism of the cell.

  • Inclusions: Non-living substances that vary with cell type, such as glycogen granules, pigments, lipid droplets, and vacuoles.

Example: Muscle cells contain many glycogen granules as inclusions for energy storage.

Cell Organelles

Overview

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival and activity. They can be classified as either membranous or non-membranous.

Classification of Organelles

Membranous Organelles	Non-membranous Organelles
Mitochondria Endoplasmic reticulum (ER) Golgi apparatus Lysosomes Peroxisomes	Cytoskeleton Centrioles Ribosomes

Additional info: Membranous organelles are surrounded by lipid bilayer membranes, while non-membranous organelles are not.

Membranous Organelles

Mitochondria

• Structure: Double-walled organelle with an outer and inner membrane. The inner membrane is folded into cristae, increasing surface area for metabolic reactions.

• Function: Known as the "power plants" of the cell, mitochondria produce most of the cell's adenosine triphosphate (ATP) supply through aerobic respiration.

• Genetic Material: Contain their own DNA, RNA, and ribosomes, allowing them to reproduce independently within the cell.

• Distribution: Numerous in cells with high energy demands, such as muscle and nerve cells.

Equation for ATP production (Cellular Respiration):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36-38\text{ATP}$

Example: Cardiac muscle cells have abundant mitochondria to meet their constant energy needs.

Endoplasmic Reticulum (ER)

• Structure: Network of membrane-bound tubes and sacs extending from the nuclear envelope.

• Types:

  • Rough ER: Studded with ribosomes; site of protein synthesis for proteins destined for membranes, lysosomes, or export.

  • Smooth ER: Lacks ribosomes; site of lipid, fatty acid, and steroid synthesis; detoxifies drugs (especially in liver and kidney cells); stores calcium ions in muscle cells.

Example: Liver cells have extensive smooth ER for detoxification.

Golgi Apparatus

• Structure: Stack of flattened, membrane-bound sacs (cisternae).

• Function: Receives proteins from the rough ER, modifies, sorts, and packages them into vesicles for secretion or delivery to other organelles.

Example: Secretory cells, such as those in the pancreas, have a prominent Golgi apparatus.

Lysosomes

• Structure: Small, spherical vesicles containing digestive enzymes.

• Function: Digest ingested bacteria, viruses, and toxins; break down nonfunctional organelles and cellular debris; release stored glycogen.

Example: White blood cells use lysosomes to destroy engulfed pathogens.

Peroxisomes

• Structure: Membrane-bound vesicles containing oxidative enzymes (oxidases and catalases).

• Function: Detoxify harmful substances and neutralize free radicals; abundant in liver and kidney cells.

Example: Peroxisomes break down hydrogen peroxide ($H_2O_2$) into water and oxygen.

Non-membranous Organelles

Cytoskeleton

• Definition: A network of protein fibers that provides structural support and shape to the cell.

• Components:

  • Microfilaments: Semi-flexible strands involved in cell movement and shape changes.

  • Intermediate filaments: Tough, insoluble proteins that resist pulling forces on the cell.

  • Microtubules: Hollow tubes made of tubulin, important for intracellular transport and cell division.

• Functions:

  • Maintain cell shape

  • Stabilize organelle position

  • Facilitate intracellular transport ("railroad tracks" for vesicles)

  • Assist in cell movement and assembly into tissues

Example: Microtubules form the mitotic spindle during cell division.

Centrioles

• Definition: Paired cylindrical structures composed of microtubules.

• Function: Direct chromosome movement during cell division by organizing the mitotic spindle.

Example: Animal cells use centrioles to form cilia and flagella.

Ribosomes

• Structure: Granules composed of protein and ribosomal RNA (rRNA).

• Function: Site of protein synthesis.

• Types:

  • Free ribosomes: Synthesize proteins for use within the cytosol or organelles.

  • Membrane-bound ribosomes: Attached to rough ER; synthesize proteins for membranes, lysosomes, or export.

Example: Pancreatic cells have abundant ribosomes for enzyme production.

Nucleus

Structure and Function

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

• Structure: Surrounded by a double-layered nuclear envelope with nuclear pores for material exchange.

• Nucleolus: Dense body within the nucleus responsible for producing rRNA for ribosome assembly.

Example: Most human cells have a single nucleus, but skeletal muscle cells are multinucleated.

Chromatin and Chromosomes

• Chromatin: Threadlike strands composed of 30% DNA, 60% histone proteins, and 10% RNA.

• Nucleosomes: Fundamental units of chromatin, consisting of DNA wrapped around histone proteins.

• Function: Histones help pack DNA and regulate gene expression. During cell division, chromatin condenses into visible chromosomes.

Example: Human cells have 46 chromosomes during metaphase of mitosis.