Cell Physiology

Introduction

Cell physiology is the study of the functions and processes that occur within living cells. Understanding the structure and function of cytoplasmic organelles is essential for comprehending how cells maintain life, perform specialized tasks, and interact with their environment.

Cytoplasmic Organelles

Overview of Organelles

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

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

• Non-membrane-bound organelles: Not enclosed by a membrane; typically composed of protein complexes.

Classification of Organelles

Mitochondria

Structure and Function

Mitochondria are often referred to as the "power plants" of the cell because they produce most of the cell's ATP through aerobic respiration.

• 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 oxidative catabolism.

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

• Aerobic respiration: Energy is released from organic fuels in the presence of oxygen to produce ATP.

Equation for aerobic respiration:

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

Peroxisomes

Functions

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

• Produce hydrogen peroxide (): Used to oxidize toxic substances.

• Detoxification: Break down substances such as ethanol.

• Fatty acid breakdown: Catabolize fatty acids for energy or anabolic reactions.

• Phospholipid synthesis: Important for cell membranes, especially in the nervous system.

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.

• Components: Endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and transport vesicles.

• Function: Synthesis, modification, and transport of proteins and lipids.

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

• Structure: Stack of flattened membranous sacs.

• Function: Modifies, sorts, and packages proteins and lipids for export or delivery to other organelles.

• Transport: Receives vesicles from ER; sends vesicles to destinations such as lysosomes, plasma membrane, or for secretion.

Lysosomes

• Structure: Membrane-bound sacs containing digestive enzymes.

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

• Role in metabolism: Release products for disposal or reuse in metabolic reactions.

Summary Table: Endomembrane System Pathways

Ribosomes

Structure and Function

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

• Composition: Made of ribosomal RNA (rRNA) and proteins; consist of large and small subunits.

• Types: Free ribosomes (in cytosol) and bound ribosomes (attached to ER or nuclear envelope).

• Function: Translate mRNA into polypeptides (proteins).

Equation for protein synthesis:

Cytoskeleton

Structure and Functions

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

• Maintains cell shape and size

• Supports plasma and nuclear membranes

• Enables movement: Motor proteins, cilia, flagella, muscle contraction

• Specialized functions: Vary by cell type

Types of Cytoskeletal Filaments

• Actin filaments (microfilaments): 5-9 nm diameter; composed of actin; provide structural support and are involved in cellular motion (with myosin).

• Intermediate filaments: 10 nm diameter; rope-like, composed of proteins such as keratin; provide mechanical strength and anchor organelles.

• Microtubules: 25 nm diameter; hollow rods made of tubulin; involved in movement of cilia, flagella, and organelle transport.

Centrioles and Centrosome

Structure and Function

Centrioles are tube-shaped structures found in the centrosome, which organizes microtubules and plays a key role in cell division.

• Centrosome: Region where microtubules originate.

• Centrioles: Each made of a 9+3 pattern of microtubules; critical for nuclear division.

Cellular Extensions

Types and Functions

Cellular extensions are projections formed by the cytoskeleton that increase surface area or enable movement.

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

• Cilia: Short, numerous; sweep substances over cell surface (e.g., respiratory tract).

• Flagella: Long, whip-like; usually one per cell; enables movement (e.g., sperm cell).

Nucleus

Structure and Function

The nucleus is the master controller of the cell, housing the cell's DNA and coordinating protein synthesis.

• Nuclear envelope: Double membrane with nuclear pores for transport between nucleoplasm and cytoplasm.

• DNA and associated proteins: Organized as chromatin and chromosomes.

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

Chromatin and Chromosomes

• Chromatin: DNA bound to histone proteins; loose and tangled between cell divisions.

• Chromosomes: Condensed chromatin during cell division; each chromosome is replicated to form sister chromatids joined at a centromere.

• Karyotype: Full set of 23 pairs of chromosomes in human cells.

Example: Human somatic cells have 46 chromosomes (23 pairs).

Nucleolus

• Function: Assembles ribosomal subunits.

• Composition: Protein, RNA, and DNA.

Additional info: Some details, such as the specific arrangement of microtubules in centrioles (9+3) and cilia/flagella (9+2), were inferred from standard cell biology knowledge.