The Eukaryotic Cell

Learning Outcomes

At the end of this session, students will be able to:

  • Outline the structure of the cell, detailing:

    • Cell membrane: Structure and function

    • Cytoplasm: Structure and function

    • Cytoskeleton: Structure and function

    • Nucleus: Structure and function

    • Endoplasmic reticulum: Structure and function

    • Golgi apparatus: Structure and function

    • Mitochondria: Structure and function

    • Lysosomes: Structure and function

  • Describe the structure and function of cell junctions

  • Explain concepts of cell adhesion and cell recognition

The Hierarchy of Biological Organization

  • Levels of Biological Organization:

    • Atom

    • Molecule

    • Macromolecule

    • Organelle

    • Cell

    • Tissue

    • Organ

    • Organ System

    • Organism

    • Population

    • Ecosystem

    • Biosphere

The Cell

  • General Information:

    • The basic unit of all animals and plants.

    • Cells and their products constitute all body tissues.

    • All functional activities of the body are performed by cells.

    • Cells arise only from pre-existing cells (via cell division: meiosis and mitosis).

    • Growth and development involve an increase in cell number and differentiation into various tissue types.

The Basic Structure of the Cell

  • Cell components include:

    • Smooth endoplasmic reticulum

    • Chromatin

    • Nucleolus

    • Nuclear envelope

    • Nucleus

    • Plasma membrane

    • Cytosol

    • Mitochondrion

    • Lysosome

    • Centrioles

    • Centrosome

    • Cytoskeletal elements:

    • Microtubules

    • Intermediate filaments

    • Peroxisome

    • Rough endoplasmic reticulum

    • Ribosomes

    • Golgi apparatus

  • Note: Image references were mentioned but not elaborated.

Cell Membrane

  • Structure:

    • Composed of a lipid bilayer.

    • Proteins are embedded within the lipid bilayer.

    • Carbohydrates are attached to proteins and lipids.

  • Functions:

    • Acts as a barrier for physical isolation.

    • Retains molecules within the cell.

    • Regulates the movement of molecules into and out of the cell.

    • Maintains a distinct intracellular pH environment.

    • Responds to internal and external cellular changes.

Cytoskeleton

  • Concept: Think structure, movement.

  • Structure:

    • Comprised of filaments and tubules.

    • Filaments include double strands of actin forming a mesh beneath the plasma membrane.

    • Links with proteins in the membrane that interact with the extracellular matrix (ECM).

  • Function:

    • Acts as the protein scaffolding of cells.

    • Preserves cell shape.

    • Facilitates cell shape changes.

    • Provides a framework for cellular movement.

Cytoplasm

  • Concept: Acts as a solution supporting organelles; linked to processes like mitosis and meiosis.

  • Structure:

    • A viscous, aqueous solution filling the region between the cell membrane and the nucleus.

    • Contains a network of cytoskeleton fibers.

    • The area outside organelles is termed cytosol.

  • Functions:

    • Supports and suspends organelles and cellular molecules.

    • Site for multiple cellular processes, including:

    • Protein synthesis

    • The first stage of cellular respiration (glycolysis)

    • Mitosis and meiosis

    • Facilitates movement of materials such as hormones within the cell.

    • Dissolves cellular waste products.

Nucleus

  • Concept: Think DNA, RNA, chromosomes, protein.

  • Structure:

    • Enclosed by a double membrane known as the nuclear envelope.

    • The inner nuclear membrane is smooth; the outer membrane may link to the endoplasmic reticulum.

    • Allows passage of proteins and RNA via pores.

    • Contains chromatin, a complex of DNA and proteins, which forms chromosomes.

    • Houses nucleoli which are dark-staining areas for ribosome assembly, rich in RNA and proteins.

    • Surrounding the nucleoli and chromatin is the nucleoplasm, a gel-like substance.

  • What Happens to DNA in the Cell?:

    • DNA is stored in chromosomes within the nucleus.

    • The fate of DNA is influenced by cellular signals, leading to:

    • Quiescence

    • Translation into RNA for protein synthesis

    • Replication for cell division.

Inheritance of DNA

  • Key Points:

    • Genes are constructed from DNA.

    • For each gene, there are two copies: one inherited from the mother and one from the father.

    • Illustrative example:

    • Single copy from Father's DNA.

    • Single copy from Mother's DNA.

From DNA to Protein

  1. Transcription: Information in DNA is converted to mRNA (messenger RNA).

  2. Transport: mRNA exits the nucleus through nuclear pores and enters the cytoplasm.

  3. Translation: Ribosomes form around mRNA.

  4. Amino Acid Translation: mRNA bases are translated into amino acids.

Endoplasmic Reticulum (ER)

  • Concept: Think protein synthesis and lipid metabolism.

  • Structure:

    • A network of interconnected tubules and sacs extending from the nucleus into the cytoplasm.

    • Contains two types:

    • Rough ER: Ribosomes on membranes.

    • Smooth ER: Granule-free and continuous with rough ER.

  • Functions:

    • Serves as scaffolding for protein synthesis.

    • Rough ER is prominent in cells with high protein synthesis levels.

    • Smooth ER facilitates lipid metabolism.

Golgi Apparatus

  • Concept: Think protein and lipid modification and sorting.

  • Structure:

    • Comprised of stacked, flattened containers (cisterns or sacs).

    • Positioned in cytosol, extending from ER to cell membrane.

    • Sections:

    • Cis-Golgi: Closest to nucleus.

    • Trans-Golgi: Closest to cell membrane.

    • Remains in contact with cytoskeletal filaments.

  • Functions:

    • Modifies proteins and lipids via carbohydrate attachments (glycosylation).

    • Sorts and distributes molecules to other organelles.

    • Packages materials for secretion outside the cell.

Mitochondrion

  • Concept: Think of the powerhouse of the cell.

  • Structure:

    • Sausage-shaped, approximately 1 µm wide by 7 µm long.

    • Features a double membrane separated by an intermembrane space.

    • The outer membrane is permeable; the inner membrane has numerous folds (cristae) extending inward.

    • Interior is known as the matrix.

  • Functions:

    • Contains enzymes for the electron transport chain, Krebs cycle, and fatty acid beta-oxidation.

    • Generates most of the cellular ATP through oxidative phosphorylation.

    • Abundant in energy-consuming cells.

Lysosomes

  • Concept: Think degradation.

  • Structure:

    • Spherical or oval organelles within a single-layer membrane.

    • Contains acid hydrolases (degradative enzymes).

    • Internal pH is about 5.0, optimal for enzyme activity.

  • Functions:

    • Lysosomal membranes compartmentalize intracellular degradative enzymes.

    • The acidic environment denatures proteins aiding in degradation.

    • Vesicles from the Golgi apparatus merge with lysosomes to deliver materials for breakdown.

    • Aids in destroying aged organelles, which are engulfed by the ER prior to lysosomal fusion.

Proteasome

  • Concept: Think of it as the bin for protein degradation and apoptosis.

  • Structure:

    • Multi-subunit enzyme complex (the trash bin of the cell).

    • Proteins are arranged in rings surrounding a central core.

  • Functions:

    • Degrades cytosolic proteins.

    • Regulates the cell cycle and apoptosis.

    • Proteins flagged for destruction with ubiquitin are directed into the proteasome for breakdown.

Peroxisomes

  • Concept: Related to lipid metabolism and detoxification.

  • Structure:

    • Similar to lysosomes with a single lipid bilayer.

    • Contain various enzymes, notably oxidases and catalases.

  • Functions:

    • Involved in lipid metabolism.

    • Detoxifies chemicals within the cell.

Outside the Cell: Junctions, Adhesion, and Recognition

Cell Junctions

  • Cells adhere to each other, forming tissues and organs.

  • Common junction types include:

    • Tight junctions: Create a seal preventing leakage between adjacent cells.

    • Anchoring/Adhesive junctions (Desmosomes): Connect cytoskeletons between adjacent cells or to the ECM.

    • Gap/Communicating junctions: Form tunnels (connexons) allowing intercellular communication.

The Tight Junction

  • A protein complex that seals adjacent cells, averting leakage through cell membranes.

Anchoring/Adhesive Junctions

  • Secure the cytoskeleton to neighboring cells or to the ECM via intracellular anchor proteins and transmembrane adhesion proteins.

Gap/Communicating Junctions

  • Constructed from proteins (connexins) forming channels between cells.

  • Allow sharing nutrients and signaling molecules, such as ions and amino acids.

Cell Adhesion

  • Refers to the ability of one cell to adhere to another cell or ECM.

  • Important for cell communication, regulation, and the structural integrity of tissues.

  • Functions in differentiation, migration, and survival regulated by CAMs (Cell Adhesion Molecules).

Cell Recognition

  • Surface proteins with attached sugar chains (glycoproteins) allow for cell identification.

  • Glycoproteins serve as tags recognized by other cells to facilitate communication and identification.

Resource

  • Textbook: Chapter 2 of Medical Sciences

    • Authors: Jeannette Naish and Denise Syndercombe Court

    • Edition: Third Edition, 2019, Elsevier Limited.

  • Reference: Check online through the library for access.