Cell Structure and Function: Study Notes for Anatomy & Physiology

Module 3: Cells

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. In human anatomy and physiology, understanding cell structure and function is essential for grasping how the body operates at the microscopic level.

• Cells are the smallest unit of a living thing.

• All organisms are composed of one or more cells.

• Human bodies contain trillions of cells, each specialized for particular functions.

The Cell Theory

Core Principles of Cell Theory

The cell theory is a foundational concept in biology, describing the properties shared by all cells.

• Cells are the building blocks of plants and animals.

• Cells are the smallest functioning units of life.

• All cells are produced through division of preexisting cells.

• Each cell maintains homeostasis, regulating its internal environment.

Understanding cells is crucial for learning how anatomical structures are maintained and how physiological functions are performed.

Overview of Cell Anatomy

Diversity and Specialization of Cells

Cells in the human body are highly diverse, with each type adapted for specific functions. The study of cell structure and function is known as cytology.

• Examples of specialized cells include blood cells, smooth muscle cells, bone cells, ovum, and sperm.

• Each cell type has unique structural features that support its role in the body.

Microscopy is a key tool in cytology, allowing detailed examination of cell components.

Components of Cells

Basic Parts of a Cell

Cells are generally divided into three main components:

• Plasma membrane (cell membrane)

• Cytoplasm (includes cytosol and organelles)

• Organelles (specialized structures within the cytoplasm)

Plasma Membrane

Structure and Function

The plasma membrane surrounds every cell, providing a barrier and regulating the movement of substances in and out of the cell.

• Composed primarily of lipids and proteins.

• Functions include protection, structural support, and selective permeability.

• Separates the intracellular fluid (ICF) from the extracellular fluid (ECF).

Example: The plasma membrane allows nutrients to enter the cell while keeping harmful substances out.

Cytoplasm and Cytosol

Internal Environment of the Cell

The cytoplasm is the material inside the cell, excluding the nucleus. It consists of cytosol (the fluid portion) and organelles.

• Cytosol is the intracellular fluid containing dissolved proteins, enzymes, carbohydrates, amino acids, and lipids.

• Stores nutrients such as glycogen and lipid droplets.

Example: Enzymes in the cytosol facilitate metabolic reactions essential for cell survival.

Organelles

Types of Organelles

Organelles are specialized structures within the cell, each performing specific functions. They are classified as either membranous or non-membranous.

• Membranous organelles: Surrounded by a plasma membrane, creating compartments. Examples: Nucleus, Mitochondria, Lysosomes, Endoplasmic Reticulum, Peroxisomes.

• Non-membranous organelles: Not surrounded by a membrane; components are in direct contact with cytoplasm. Examples: Ribosomes, Cytoskeleton, Cilia.

Nucleus

The nucleus is typically the largest organelle and serves as the control center of the cell.

• Enclosed by a double-layered nuclear envelope with nuclear pores.

• Contains DNA organized into chromatin (loose) or chromosomes (condensed during cell division).

• Includes one or more nucleoli, which produce ribosomal RNA (rRNA).

• Stores genetic instructions for protein synthesis.

Example: Human cells contain 23 pairs of chromosomes in the nucleus.

Ribosomes

Ribosomes are non-membranous organelles responsible for protein synthesis.

• Composed of two subunits made of rRNA and proteins.

• Found free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

• Primary site of translation (protein synthesis).

Example: Ribosomes translate mRNA into polypeptide chains.

Endoplasmic Reticulum (ER)

The ER is a network of membranous sheets and channels connected to the nuclear envelope.

• Rough ER (RER): Studded with ribosomes; involved in protein synthesis and modification.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, carbohydrate metabolism, and detoxification.

Example: Liver cells have abundant SER for detoxifying drugs.

Golgi Apparatus

The Golgi apparatus is a stack of flattened membranes that processes and packages proteins and lipids.

• Acts as the "post office" of the cell, sorting and shipping molecules.

• Receives proteins from the RER, modifies them, and directs them to their destinations via vesicles.

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

Lysosomes and Peroxisomes

These are vesicular organelles involved in digestion and detoxification.

• Lysosomes: Contain hydrolytic enzymes for breaking down waste, damaged organelles, and foreign particles.

• Peroxisomes: Contain enzymes that break down fatty acids and neutralize free radicals (e.g., hydrogen peroxide).

Example: White blood cells use lysosomes to digest bacteria.

Mitochondria

Mitochondria are double-membraned organelles known as the "powerhouses" of the cell.

• Site of cellular respiration and ATP synthesis.

• Contain an inner membrane with folds called cristae to increase surface area.

• Matrix and cristae house enzymes for energy production.

Equation:

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

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

Cytoskeleton

The cytoskeleton is a network of protein filaments providing structural support and facilitating movement.

• Microfilaments: Thinnest, composed of actin; involved in cell movement.

• Intermediate filaments: Medium size, provide stability; e.g., keratin.

• Microtubules: Largest, made of tubulin; involved in cell shape and transport.

Functions: Maintains cell shape, anchors organelles, and enables movement of cell and organelles.

Cilia and Flagella

Cilia and flagella are extensions of the cell surface made of microtubules.

• Cilia: Short, numerous; move substances across cell surfaces (e.g., respiratory tract).

• Flagella: Long, usually singular; enable cell movement (e.g., sperm cell).

Example: Cilia in the respiratory tract move mucus toward the throat.

Microvilli

Microvilli are microscopic, finger-like projections of the plasma membrane that increase surface area for absorption.

• Found on cells involved in absorption, such as intestinal epithelial cells.

Example: Microvilli in the small intestine enhance nutrient absorption.

Centrioles and Centrosome

Centrioles are cylindrical organelles arranged at right angles, forming the centrosome, which is essential for cell division.

• Organize microtubules and facilitate chromosome movement during mitosis.

Example: Centrioles help form the mitotic spindle during cell division.

Summary Table: Membranous vs. Non-Membranous Organelles

Organelles	Membranous	Non-Membranous
Nucleus	Yes (double membrane)	No
Mitochondria	Yes (double membrane)	No
Endoplasmic Reticulum	Yes	No
Golgi Apparatus	Yes	No
Lysosomes	Yes	No
Peroxisomes	Yes	No
Ribosomes	No	Yes
Cytoskeleton	No	Yes
Cilia & Flagella	No	Yes
Centrioles	No	Yes

Key Definitions

• Homeostasis: The maintenance of a stable internal environment within the cell.

• Organelle: Specialized structure within a cell that performs a specific function.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Chromatin: The loose, thread-like form of DNA found in the nucleus when the cell is not dividing.

• Chromosome: Condensed form of DNA visible during cell division.

Additional info: Some details were inferred and expanded for completeness and clarity, such as the summary table and definitions.