Eukaryotic Cell Study Notes

THE CELL

  • Definition: The basic unit of all animals and plants

  • Role:

    • Cells and their products make up all tissues of the body

    • All functional activities of the body are carried out by cells

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

    • Growth and development occur through an increase in cell number and the differentiation of cells into different tissue types

THE BASIC STRUCTURE OF THE CELL

  • Components:

    • Smooth endoplasmic reticulum

    • Chromatin

    • Nucleolus

    • Nuclear envelope

    • Nucleus

    • Plasma membrane

    • Cytosol

    • Mitochondria

    • Lysosomes

    • Centrioles

    • Centrosome matrix

    • Cytoskeletal elements (Microtubule, Intermediate filaments)

    • Rough endoplasmic reticulum

    • Ribosomes

    • Golgi apparatus

  • Further Processes: Secretion being released from the cell via exocytosis

THE CELL (PLASMA) 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

  • Maintains molecular organization inside the cell

  • Regulates molecular transport into and out of the cell

  • Maintains a unique intracellular pH environment

  • Responds to internal and external changes

THE CYTOSKELETON

Structure

  • Comprises filaments and tubules, providing a framework for the cell

  • Features double strands of actin that create a mesh beneath the plasma membrane

  • Links with proteins embedded in the membrane, which interact with the extracellular matrix (ECM)

Functions

  • Serves as the protein scaffolding of a cell, preserving its shape

  • Enables the cell to change its shape as necessary

  • Facilitates movement within the cell

INSIDE THE CELL: THE ORGANELLES

THE CYTOPLASM
Structure

  • A viscous, aqueous solution within the cell

  • Crossed by cytoskeleton fibers

  • Occupies space between the cell membrane and nucleus

  • Contains a component known as cytosol

Functions

  • Supports and suspends organelles and cellular molecules

  • Numerous cellular processes take place, including:

    • Protein synthesis

    • Initial stage of cellular respiration (glycolysis)

    • Mitosis and meiosis

  • Assists in the movement of materials (e.g., hormones) within the cell

  • Dissolves cellular waste

THE NUCLEUS
Structure

  • Enclosed by a double membrane called the nuclear envelope

  • Features a smooth inner nuclear membrane linked to the endoplasmic reticulum (ER)

  • Contains chromatin (DNA and proteins forming chromosomes)

  • Houses one or more nucleoli (ribosome assembly areas rich in RNA and proteins)

  • Surrounded by nucleoplasm, a gel-like substance

Fate of DNA in the Cell

  • Stored in chromosomes within the nucleus

  • Dependent on cellular signals, DNA can:

    • Remain quiescent

    • Be translated to RNA for new protein synthesis

    • Be replicated for cell division

INHERITANCE OF DNA

  • Genes composed of DNA: Each gene has 2 copies (one from each parent)

    • One copy from mother

    • One copy from father

FROM DNA TO PROTEIN

  1. Transcription: DNA information is converted into mRNA (messenger RNA)

  2. Exit: mRNA exits the nucleus via nuclear pores into the cytoplasm

  3. Translation: Assembly of ribosomes around the mRNA occurs

  4. Amino Acids: mRNA bases are translated into amino acids

THE ENDOPLASMIC RETICULUM (ER)

Structure

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

  • Two types of ER:

    • Rough ER: Characterized by ribosomes on its membranes

    • Smooth ER: Granule-free and continuous with Rough ER

Functions

  • Supports scaffolding for protein synthesis

  • Rough ER is prominent in cells with high protein synthesis demand

  • Smooth ER involved in lipid metabolism

THE GOLGI APPARATUS

Structure

  • Composed of stacks of flattened containers known as cisterns or sacs

  • Located in the cytosol, extending from the ER toward the cell membrane

  • Divided into parts:

    • Cis-Golgi: Closest to the nucleus

    • Trans-Golgi: Closest to the cell membrane

Functions

  • Modifies proteins and lipids through glycosylation (attachment of carbohydrates)

  • Sorts and distributes them to organelles or for secretion

  • Packages contents for further processing

THE MITOCHONDRION

Structure

  • Sausage-shaped structure, approximately 1 µm wide and 7 µm long

  • Double membrane separated by the intermembrane space

  • Outer membrane permeable to small molecules; inner membrane has folds (cristae) projecting inward

Function

  • Contains enzymes for:

    • Electron transport chain

    • Krebs cycle

    • Beta-oxidation of fatty acids

  • Generates most ATP through oxidative phosphorylation

  • Abundant in energy-consuming cells

LYSOSOMES

Structure

  • Spherical or oval organelles enclosed by a single-layer membrane

  • Contains acid hydrolases (degradative enzymes) that digest biological molecules

  • Optimal pH of approximately 5.0 for enzyme activity

Function

  • Compartmentalization of intracellular degradative enzymes

  • Facilitates protein degradation through denaturation in acidic environment

  • Fuses with vesicles from the Golgi apparatus to degrade contents

  • Destroys aged cell organelles engulfed by the ER

PROTEASOME

Summary

  • A multi-subunit enzyme complex, referred to as the cell's "trash bin"

  • Protein complexes arranged in rings around a central core

  • Degrades cytosolic proteins

  • Plays a role in cell cycle regulation and apoptosis

  • Proteins destined for destruction are tagged with ubiquitin, directing them into the proteasome core for breakdown

PEROXISOMES

Summary

  • Similar in structure to lysosomes with a single lipid bilayer

  • Contain various enzymes, particularly oxidases and catalases

  • Functions include:

    • Lipid metabolism

    • Detoxification of cellular compounds

OUTSIDE THE CELL: JUNCTIONS, ADHESION AND RECOGNITION

CELL JUNCTIONS

  • Facilitate cell adherence forming tissues and organs

  • Types:

    • Tight Junctions

    • Anchoring/Adhesive Junctions (Desmosomes)

    • Gap/Communicating Junctions

TIGHT JUNCTION

  • Protein complex forming a seal between two adjacent cells

  • Prevents leakage of substances across cell membranes

ANCHORING/ADHESIVE JUNCTIONS

  • Attach the cytoskeleton to neighboring cells or the extracellular matrix

  • Formed by intracellular anchor proteins and transmembrane adhesion proteins

GAP/COMMUNICATING JUNCTIONS

  • Constructed from proteins forming tunnels (connexins) between cells

  • Allow communication between cells

  • Facilitate sharing of nutrients and transfer of chemical/electrical signals

CELL ADHESION

  • Capacity of one cell to adhere to another cell or ECM

  • Important for:

    • Cell communication

    • Regulation

    • Maintenance and development of tissue

  • Influences signals regulating differentiation, cell cycle, migration, and survival

  • Involves proteins called Cell Adhesion Molecules (CAMs)

CELL RECOGNITION

  • Glycoproteins on the cell surface have sugar chains attached

  • Serve as identification tags, recognized by other cells

RESOURCE

  • Textbook Reference:

    • Medical Sciences

    • Check out chapter 2 available through the library

    • Authors: Jeannette Naish, Denise Syndercombe Court

    • Medical Sciences, Third Edition, 2019, Elsevier Limited

    • All rights reserved.