2. THE CELLULAR LEVEL OF ORGANISATION

THE CELLULAR LEVEL OF ORGANISATION

Introduction

  • The focus of this material is on the cellular structure and functions, specifically related to human biology as presented by Dr. Anna Maceri from Western Sydney University.

Overview of the Cell

  • Cells are the basic structural and functional units of living organisms.

  • Cells can be organized into three main compartments:

    1. Plasma Membrane

    • Acts as the cell boundary.

    • Controls the movement of substances in and out of the cell.

    1. Nucleus

    • Serves as the control center of the cell.

    • Houses the DNA.

    1. Cytoplasm

    • Contains all the materials between the plasma membrane and the nucleus.

Details of Cell Compartment Structures

  • Cytoplasm

    • Made up of:

    • Cytosol: The liquid component mostly composed of water, in which organelles are suspended.

    • Organelles: Specialized structures within the cell that each perform particular functions.

    • Inclusions: Granules or vesicles that may contain various substances.

  • Plasma Membrane

    • The outer boundary of the cell, critical for the selective movement of substances between the cell and its environment.

  • Nucleus

    • The control center of the cell, responsible for housing genetic material (DNA).

    • Contains:

    • Nuclear Envelope: A membrane that surrounds and protects the nucleus.

    • Nuclear Pores: Openings that allow materials to enter and leave the nucleus.

    • Nucleolus: Site of ribosome production within the nucleus.

Cellular Structures and Functions

  • Ribosomes

    • Sites for protein production, including:

    • Free Ribosomes: Float in the cytoplasm and produce proteins for internal use by the cell.

    • Membrane-bound Ribosomes: Attach to the surface of the endoplasmic reticulum (ER), producing proteins for packaging and either secretion or use in the plasma membrane.

  • Endoplasmic Reticulum (ER)

    • A network of interconnected sacs or tubules associated with the nuclear envelope.

    • Smooth ER: Lacks ribosomes; involved in the synthesis of lipids and cholesterols.

    • Rough ER: Studded with ribosomes; synthesizes proteins which are packaged into vesicles.

  • Mitochondria

    • Known as the powerhouses of the cell; responsible for ATP production, which is used as energy by the cell.

  • Golgi Apparatus

    • A stack of flattened membranes that modify and package proteins received from the rough ER for secretion out of the cell.

  • Lysosomes

    • Small vesicles containing digestive enzymes that help break down worn-out organelles and unwanted substances.

  • Peroxisomes

    • Small vesicles containing enzymes that neutralize toxins and free radicals.

  • Cilia

    • Cell extensions that move substances along the cell surface through a beating action.

  • Flagellum

    • A long cellular extension that propels the cell itself to enable movement.

  • Microvilli

    • Extensions that increase the surface area of the cell to enhance absorption capabilities.

Variability in Cell Types

  • Not all cells are identical; they vary significantly in structure and function depending on their roles in the body.

Chemical Reactions in Cells

  • Types of Reactions

    • Synthesis Reactions: Combine molecules to form more complex structures.

    • Breakdown Reactions: Decompose complex structures into simpler ones.

    • Exchange Reactions: Exchange components between molecules.

    • Energy Production, Usage, and Transfer: Involves cellular metabolism and energy transformations.

  • Some chemical reactions are facilitated by enzymes, which are proteins that act as biological catalysts, quickening the reaction without altering the substances involved.

    • Enzymatic Action: Sensitive to environment changes like temperature and pH, and typically end in -ase (e.g., lipase, amylase).

Human Genetics

  • Chromosomes

    • Found in the nucleus, they carry genetic information in DNA.

  • DNA (Deoxyribonucleic Acid)

    • Serves as the genetic blueprint, containing instructions for inheritable features.

  • Genes

    • Segments of DNA that code for specific proteins or features, such as enzyme production or eye color.

  • Chromosomal Composition in Humans

    • Humans have 46 chromosomes, arranged in 23 pairs:

    • 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females and XY for males).

Fertilization Process

  • In humans, fertilization involves:

    • 23 chromosomes from sperm and 23 from ovum combine to form a zygote with 46 chromosomes in 23 pairs.

    • This continues to develop into an embryo.

Cell Division & Specialisation

  • Cells begin as a single fertilized egg (zygote) that undergoes mitosis to divide into identical cells.

    • Cells differentiate and specialize into specific cell types.

    • The process of specialization involves the regulation of gene expression (turning genes on and off).

    • Stem Cells: Adult cells capable of differentiating to replace other cell types (e.g., blood, skin cells).

Types of Cell Division

  • Mitosis:

    • A form of cell division that occurs in somatic (general body) cells.

    • Characteristics:

    • Produces two identical daughter cells.

    • Used for growth and cellular replacement (e.g., skin cells).

    • The chromosome number remains consistent with the parent cell.

  • Meiosis:

    • Exclusively occurs in ovaries and testes for producing gametes (sperm and ova).

    • Characteristics:

    • Produces four daughter cells with half the chromosome number (haploid).

    • Introduces genetic variability through recombination.

Comparative Summary of Mitosis and Meiosis

  • Mitosis vs Meiosis:

    • Mitosis produces 2 identical cells (no genetic variation), while meiosis produces 4 varied cells (half the chromosome number).

    • Both processes commence with a single parent cell but serve different purposes in cellular biology.

The Cell Cycle

  • Phases of the Cell Cycle:

    • Interphase (G1, S, G2): The non-dividing phase where the cell grows and performs normal activities.

    • G1: First gap phase; cell grows and prepares for DNA replication.

    • S Phase: DNA replication occurs, and the cell synthesizes DNA.

    • G2: Second gap phase where the cell prepares for division.

    • Mitotic Phase: The phase where cell division occurs, leading to the creation of two identical daughter cells.

  • G0 Phase: A resting phase where cells do not actively divide.

Conclusion

  • This overview covers the fundamental aspects of cell structure and function, genetic inheritance, and the processes of cell division and specialization. Understanding these topics is critical for studying human biology.