Cell Physiology Study Notes

Cell Physiology Overview

Prof. M.O. Welcome MD, PhD
Department of Human Physiology, College of Health Sciences, Nile University of Nigeria, FCT - Abuja

Outline

  1. Definition

  2. Cell Components

  3. Plasma Membrane

    • Intracellular Constituents

Definition

  • Cell: The cell is the basic structural and functional unit of an organism, containing the fundamental constituents of life.

Cell Components

  • Major components of a cell include:

    • Plasma membrane

    • Cytoplasm

    • Membrane-bound organelles such as:

    • Nucleus

    • Centrosomes/Centrioles

    • Mitochondria

    • Ribosomes

    • Endoplasmic Reticulum

    • Golgi Complex

    • Lysosomes

    • Peroxisomes

Plasma Membrane

Definition and Function

  • The plasma membrane is a physico-biological membrane that separates the interior of the cell from its external milieu. It is composed of:

    • Phospholipids

    • Proteins

    • Cholesterol

    • Carbohydrates

Structure of the Plasma Membrane

  • Key structural elements:

    • Glycolipid

    • Single-pass transmembrane protein

    • Glycoprotein

    • Multipass transmembrane protein

    • Integral proteins

    • Peripheral proteins

    • Cholesterol

    • Cytoskeletal filaments

    • Protein channels/receptors

Membrane Lipids

  • The entire lipid composition of the cells of an organism is termed its lipidome. The study of a cell’s lipidome is called lipidomics. Approximately 10910^9 lipid molecules exist in the plasma membrane of a typical animal cell, categorized into three main groups:

    • Phospholipids: The most abundant lipids in biological membranes, featuring a hydrophilic head and hydrophobic acyl chains. Common types include:

    • Phosphatidylcholine

    • Phosphatidylethanolamine

    • Phosphatidylserine

    • Sphingomyelin

    • Glycolipids

    • Cholesterol

Lipid Rafts

  • Membrane components may aggregate to form structures known as lipid rafts, which are small (10-200 nm in diameter) domains rich in sterols and sphingolipids, compartmentalizing cellular functions. Example: Caveolae formed by caveolin proteins.

Functions of Membrane Lipids

  • Functions include:

    • Cell attachment and support

    • Cellular signaling: Lipid rafts serve scaffolds for signaling molecules that relay information between extracellular and intracellular environments.

    • Regulation of cellular homeostasis: Including diffusion regulation and response to various stimuli.

    • Vesicle budding and fusion: Essential for endocytosis and exocytosis.

    • Transport functions

    • Membrane remodeling

Membrane Proteins

  • The plasma membrane contains a multitude of proteins, categorized as:

    • Integral proteins: Embedded within the membrane; includes single-pass and multi-pass transmembrane proteins.

    • Peripheral proteins: Loosely associated with one leaflet of the membrane. Examples include G proteins, Ras family proteins, and various protein kinases.

Functions of Membrane Proteins

  • Receptors (e.g., ion channels)

  • Transporters (e.g., membrane carriers)

  • Enzymes (e.g., phosphatases, protein kinases)

  • Storage proteins (e.g., ferritin for iron storage)

  • Signaling mechanisms: Translating cues into cellular responses.

  • Immune defense: Recognizing foreign entities.

Membrane Carbohydrates

  • Class of information-encoding biological macromolecules, forming a component of the cell known as glycocalyx, which is rich in glycoproteins and serves in cell recognition and signaling.

Models of Plasma Membrane Structure

  1. Bilipid Layer Model: Proposes that the bilipid layer structure is due to phospholipid behavior in an aqueous environment.

  2. Fluid Mosaic Model: Introduced by Singer and Nicolson, describes the fluid properties of the membrane due to protein and lipid dynamics.

  3. Dynamic Fluid Mosaic Model: Accounts for the shifts in protein and lipid distributions within the plasma membrane, highlighting microdomains and interactions.

Intracellular Components

  • Cytoplasm: Comprising 75-95% water and various biomolecules, it functions in chemical reactions and synthesis.

  • Organelles: Specialized structures with specific functions, including:

    • Endoplasmic Reticulum (ER): Dynamic membrane-bound organelle involved in protein and lipid synthesis, quality control, and more, characterized by rough (ribosome-studded) and smooth (lipid-synthesizing) regions.

    • Golgi Complex: Central hub for protein modification, sorting, and transport.

    • Peroxisomes: Involved in the synthesis and degradation of cellular components.

    • Lysosomes: Acidic organelles degrading biomolecules via hydrolytic enzymes.

    • Mitochondria: Powerhouses of the cell, responsible for ATP synthesis and containing their own genetic material.

    • Nucleus: Houses genetic material and is essential for RNA synthesis and regulation of cellular functions.

Cytoskeleton

  • Described as a dynamic network of filaments with roles in providing structure, facilitating transport, and enabling cellular processes. Types include:

    • Microfilaments: Composed of actin, involved in shape and motility

    • Microtubules: Hollow structures important for transport and division

    • Intermediate Filaments: Provide mechanical strength to the cell

    • Septins: Rod-like structures important in cell division

Functions of the Cytoskeleton

  • Key roles include:

    • Cellular transport

    • Cell growth and division

    • Differentiation

    • Signal transduction

    • Maintenance of cell shape

Intracellular Vesicles

  • Small transport units utilized to move molecules between cellular compartments, with examples including clathrin-coated and endocytic vesicles.

Conclusion

  • Understanding the structure and function of cellular components is critical for comprehending the physiological processes essential for life.