Cell Physiology

Introduction to Cell Physiology

Cell physiology is the study of the functions and processes that occur within living cells. Understanding cell physiology is fundamental to anatomy and physiology, as it explains how cells maintain homeostasis, communicate, and contribute to the overall function of tissues and organs.

Cytoplasmic Organelles

Overview of Cytoplasmic Organelles

Cytoplasmic organelles are specialized structures within the cell that perform distinct functions necessary for cellular survival and activity. These organelles can be classified as either membrane-bound or non-membrane-bound.

• Membrane-bound organelles: Enclosed by a lipid bilayer membrane, separating their internal environment from the cytosol.

• Non-membrane-bound organelles: Not enclosed by a membrane; instead, they are composed of protein complexes.

Classification of Organelles

Mitochondria

Structure and Function

The mitochondria are often referred to as the "power plants" of the cell. They are responsible for producing most of the cell's ATP (adenosine triphosphate), the primary energy currency.

• Double membrane: Consists of an outer and inner membrane; the inner membrane is highly folded into structures called cristae.

• Matrix: The innermost compartment containing enzymes and proteins for breaking down organic fuels.

• Own DNA: Mitochondria possess their own DNA and ribosomes, allowing some independent protein synthesis.

• Aerobic respiration: Mitochondria perform oxidative catabolism, generating ATP in the presence of oxygen.

Equation for Aerobic Respiration:

Example: Muscle cells have a high number of mitochondria due to their increased energy demands.

Peroxisomes

Functions of Peroxisomes

Peroxisomes are small, membrane-bound organelles involved in detoxification and lipid metabolism.

• Produce hydrogen peroxide () for oxidation reactions.

• Oxidize toxic substances (e.g., ethanol) for detoxification.

• Break down fatty acids for energy production or anabolic reactions.

• Synthesize certain phospholipids, especially for nervous system cell membranes.

Endomembrane System

Components and Functions

The endomembrane system is a network of organelles that work together to modify, package, and transport molecules within the cell.

• Includes the endoplasmic reticulum (ER), Golgi apparatus, and lysosomes.

• Forms vesicles for exchange of proteins and other molecules.

• Responsible for modification and packaging of cellular products.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; synthesizes and folds proteins, attaches sugars to form glycoproteins, packages proteins, and synthesizes cholesterol and phospholipids.

• Smooth ER (SER): Lacks ribosomes; involved in calcium storage, detoxification, and lipid synthesis (phospholipids, cholesterol, steroid hormones, lipoproteins).

Example: Liver cells have abundant SER for detoxification.

Golgi Apparatus

• Consists of flattened membranous sacs.

• Modifies, sorts, and packages proteins and lipids for export.

• Receives transport vesicles from ER and sends new vesicles to destinations.

Lysosomes

• Membrane-bound sacs containing water and acid hydrolases for digestion.

• Digest nutrients, worn-out organelles, and cellular debris (autophagy).

• Release products into cytosol for disposal or recycling.

Summary of Endomembrane System Function

• SER makes lipids; RER synthesizes proteins.

• Proteins enter RER for folding and modification.

• Products from RER and SER are packaged into transport vesicles and sent to Golgi apparatus.

• In Golgi, products are sorted, further modified, and packaged into vesicles.

• Vesicles may be sent to lysosomes, incorporated into cell membranes, or secreted from the cell.

Ribosomes

Structure and Function

Ribosomes are non-membrane-bound organelles responsible for protein synthesis.

• Composed of two subunits: large and small, each made of ribosomal proteins and ribosomal RNA (rRNA).

• Free ribosomes are suspended in the cytosol; bound ribosomes are attached to the ER or nuclear envelope.

• Ribosomes are the cell's protein factories.

Cytoskeleton

Structure and Functions

The cytoskeleton is a dynamic network of protein filaments that provides structural support, shape, and facilitates movement within the cell.

• Supports plasma membrane, nuclear membranes, and organelles.

• Functions in movement (motor proteins, cilia, flagella, muscle contraction).

• Performs specialized functions in different cell types.

Types of Cytoskeletal Filaments

• Actin filaments (microfilaments): 5-9 nm diameter; composed of actin subunits; provide structural support, bear tension, maintain cell shape, and are important for cellular motion with myosin.

• Intermediate filaments: 10 nm diameter; rope-like proteins such as keratin; form permanent structures, anchor organelles, and help cells withstand mechanical stress.

• Microtubules: 25 nm diameter; hollow rods composed of tubulin proteins; involved in movement of cilia and flagella.

Centrioles and Centrosome

Structure and Function

• Centrioles are a pair of tube-shaped structures located in the centrosome.

• Microtubules grow out from the centrosome, organizing the cytoskeleton.

• Each centriole is made up of a 9+3 pattern of microtubules.

• Play a critical role in nuclear division (mitosis and meiosis).

Cellular Extensions

Types and Functions

• Microvilli: Increase surface area for absorption (e.g., kidney and small intestine); can increase surface area up to 40 times.

• Cilia: Short, numerous extensions that sweep substances over the cell surface; composed of microtubules in a 9+2 arrangement.

• Flagella: Long, whip-like extensions; cells typically have only one flagellum (e.g., sperm cell).

Nucleus

Structure and Function

The nucleus is the master controller of the cell, housing the cell's DNA and directing cellular activities.

• Contains three main structures: nuclear envelope, DNA and associated proteins, and nucleolus.

• Nuclear envelope: Double membrane surrounding the nucleus; outer membrane covered with ribosomes; contains nuclear pores for transport between nucleoplasm and cytoplasm.

• Chromatin: DNA bound to histone proteins, forming a compact structure; during cell division, chromatin condenses into chromosomes.

• Chromosomes: Each chromosome is replicated before cell division, resulting in two sister chromatids joined at a centromere.

• Nucleolus: Dark-staining structure that assembles ribosomes; composed of protein, RNA, and DNA.

Example: Human cells have two sets of 23 chromosomes; a karyotype shows the full set of chromosome pairs.

Additional info: The notes cover foundational cell biology topics essential for understanding anatomy and physiology at the college level, including the structure and function of organelles, the cytoskeleton, and the nucleus.