Cell Structure & Organisation

Cell Structure & Organisation

Cell Theory

The Cell as a Unit of Life

  • Until the advent of powerful microscopes, the fundamental composition of living organisms remained unknown.

  • Robert Hooke is credited as the first individual to observe cells, coining the term 'cell' to describe these structures.

  • Subsequently, scientists Matthias Schleiden and Theodor Schwann studied cells in plants and animals, formulating the Cell Theory in 1837.

Cell Theory Overview

  • Cell Theory: A foundational principle in biology, universally accepted and consisting of three key tenets:

    1. All living organisms are composed of one or more cells.

    2. Cells are the fundamental functional units of life.

    3. New cells arise from pre-existing cells.

  • All cells share common characteristics including:

    • Cell surface membrane

    • Cytoplasm

    • DNA

    • Ribosomes

  • Structural disparities exist among different cell types:

    • Prokaryotic cells lack internal membranes and have smaller ribosomes.

    • Eukaryotic cells feature multiple membrane-bound organelles and larger ribosomes.

  • Cell Ultrastructure: The internal architecture of a cell, observable via microscopy.

Levels of Organisation of Cells

Multicellular Cell Organisation

  • Cell theory indicates cells are the functional units of life, capable of specialization for specific tasks.

    • Examples of Specialized Cells:

    • Epithelial cells in the small intestine for efficient nutrient absorption.

    • Red blood cells for oxygen transportation.

    • Xylem cells in plants for water transport.

  • In multicellular organisms, similar specialized cells group to form tissues.

    • Tissue: A collection of similar cells working collaboratively for a shared function.

    • Example of a tissue: Epithelial tissue in intestines for nutrient absorption.

Tissue Grouping into Organs

  • Different tissues merge to form organs.

    • Organ: A collection of tissues cooperating to perform a specific function.

    • Example: The heart, composed of cardiac tissue, blood vessel tissue, and connective tissue, which collectively pump blood.

    • In plants, tissues such as palisade mesophyll, spongy mesophyll, and vascular tissue aggregate to form leaves for efficient photosynthesis.

Formation of Organ Systems

  • Various organs collaborate to create organ systems.

    • Organ System: A system of organs cooperating to fulfill broad functions.

    • Example: The circulatory system, made up of the heart and blood vessels for blood circulation.

    • Example: The digestive system, encompassing the stomach, pancreas, small intestine, and large intestine for food digestion and nutrient absorption.

Eukaryotic Cells

Organelles and Structures

  • Cells are broadly divided into eukaryotic and prokaryotic categories.

  • Eukaryotic Cells:

    • Display more complex ultrastructures than prokaryotic cells.

    • Range in size from approximately 10-100 μm.

Ultrastructure Features

  • Eukaryotic cells, including animal and plant cells, possess:

    • Membrane-bound organelles

    • 80S ribosomes (larger than prokaryotic ribosomes)

  • Distinct Features of Animal vs. Plant Cells:

    • Animal cells: Contain centrioles, some have microvilli.

    • Plant cells: Possess a cellulose cell wall, large permanent vacuoles, and chloroplasts.

Organelle Functions

Cell Surface Membrane

  • All cells are encased in a cell surface membrane that manages material exchange between the internal and external environments.

    • Described as partially permeable; certain substances can permeate while others cannot.

    • Formed from a phospholipid bilayer, approximately 10 nm thick.

Nucleus

  • Present in all eukaryotic cells, the nucleus is a prominent structure separated from the cytoplasm by a nuclear envelope, featuring many pores.

    • Nuclear Pores: Critical for the transport of mRNA and ribosomes out of the nucleus and enzymes in.

    • Contains chromatin, the structure from which chromosomes are composed.

    • Often, one or more dark regions termed nucleoli can be observed, which are sites for ribosome production.

Mitochondria

  • Mitochondria, or singular mitochondrion, are the sites of aerobic respiration in eukaryotic cells.

    • Enclosed in a double membrane; the inner membrane forms folds known as cristae.

    • Contains enzymes for aerobic respiration and small circular DNA (mtDNA) necessary for mitochondrial replication.

Ribosomes

  • Ribosomes exist as free organelles in cytoplasm or attached to the rough endoplasmic reticulum (RER) in eukaryotic cells.

  • Composed of rRNA and proteins, with 80S ribosomes found in eukaryotic cells and 70S ribosomes in prokaryotes, mitochondria, and chloroplasts.

  • Ribosomes serve as sites for the translation process during protein synthesis.

Endoplasmic Reticulum (ER)

  • Two types:

    • Rough Endoplasmic Reticulum (RER):

    • Composed of folded membrane connected with the nuclear envelope, studded with ribosomes.

    • Functions in processing proteins produced on ribosomes.

    • Smooth Endoplasmic Reticulum (SER):

    • Similar membrane structure but lacks ribosomes.

    • Involved in the synthesis and processing of lipids, carbohydrates, and steroids.

Golgi Apparatus

  • Consists of stacks of flattened membrane sacs, often likened to the appearance of smooth ER, but distinguished by its regular arrangement.

  • Function: Modifying proteins and lipids before packaging them into vesicles for transport.

    • Modified proteins can be:

    • Exported from the cell (e.g., hormones like insulin)

    • Transported to lysosomes (e.g., hydrolytic enzymes)

    • Sent to other organelles

Lysosomes

  • Specialized vesicles containing hydrolytic enzymes for breaking down waste materials, including worn-out organelles.

  • Integral to immune system functions and programmed cell death (apoptosis).

Centrioles

  • Composed of microtubules, centrioles assist in the organization of spindle fibers during nuclear division in animal cells.

  • Typically absent in plant and fungal cells.

Prokaryotic Cells

Structural Overview

  • Prokaryotic cells are typically smaller than eukaryotic cells and lack membrane-bound organelles.

    • Unique attributes include:

    • Smaller ribosomes (70S) compared to 80S found in eukaryotes.

    • A circular, non-associated bacterial chromosome instead of a nucleus.

    • A cell wall composed of the glycoprotein murein (or peptidoglycan).

Additional Prokaryotic Features

  • Common prokaryotic structures may include:

    • Plasmids (circular DNA loops)

    • Capsules

    • Flagella

    • Pili

    • Mesosomes (folds in the cell membrane)

Electron Microscopy of Animal Cells

Types of Electron Microscope

  • Transmission Electron Microscopes (TEM):

    • Utilize electromagnets to focus an electron beam through a thin specimen.

    • Denser parts absorb more electrons, yielding darker images and high resolution in a 2D image.

  • Scanning Electron Microscopes (SEM):

    • Scan electron beams across a specimen, providing a 3D view of the surface.

    • Resolution is lower than that of TEM but does not require thin specimens.

Image Interpretation and Application

  • Images from TEM (e.g., stained micrographs of cells) can show intricate internal structures, while SEMs display 3D surface structures.

  • Essential for identifying organelles and observing cell interactions at the microscopic level.

Microscopy: Magnification & Resolution

Definitions and Calculations

  • Magnification: The enlargement factor of an image compared to its real-life size.

    • Light microscopes use eyepiece and objective lenses to achieve this.

    • Formula:
      Total Magnification=Eyepiece Magnification×Objective Magnification\text{Total Magnification} = \text{Eyepiece Magnification} \times \text{Objective Magnification}

  • Resolution: The ability to distinguish between two separate points.

    • Limited by light wavelength in optical microscopes (max resolution 200 nm).

    • Electron microscopes achieve much higher resolution through electron beams with smaller wavelengths.

Practical Considerations

  • Light microscopes can observe structures >200 nm, while electron microscopes can resolve details >0.5 nm, but require specimens to be dead.

  • Staining techniques enhance contrast in microscopy, allowing visibility of otherwise transparent structures.

Staining Techniques

  • Different stains are used based on the specimen type and observation needs, including:

    • Haemotoxylin: Stains nuclei purple/brown/blue.

    • Methylene Blue: Stains animal cell nuclei blue.

    • Acetocarmine: Stains chromosomes in dividing cells.

    • Iodine: Stains starch in plant cells blue-black.

Core Practical 5 - Light Microscopy

Preparing and Using Light Microscopes

  • Optical microscopes are essential for studying tissues, cells, and organelles.

  • Method for slide preparation includes:

    1. Add specimen with a pipette onto a slide.

    2. Cover with a coverslip and gently press to remove air bubbles.

    3. Utilize stains for better visibility.

  • Important to start with the low power lens to locate the specimen and switch to higher magnifications as needed.

Challenges in Microscopy

  • Inconsistencies may arise due to the 3D nature of tissues when viewed in a 2D format.

  • Optical microscopes have limitations in magnification and resolution compared to electron microscopes.

Drawing and Recording Observations

  • Biological drawings of observed structures must adhere to specific conventions, ensuring clarity and proportionality.

    • Notations should connect directly to the structures and utilize labeled lines placed aside from the drawings.

    • Measurements and calculations (e.g., magnification based on eyepiece graticules) should maintain unit consistency.

Worked Example for Magnification:
Given an image of an animal cell measuring 30 mm at a magnification of x3000, calculate the actual size as follows:
Actual Size=Image Size÷Magnification\text{Actual Size} = \text{Image Size} \div \text{Magnification}
Actual Size=30 mm÷3000=0.01 mm=10μm\text{Actual Size} = 30 \text{ mm} \div 3000 = 0.01 \text{ mm} = 10 \mu m

Using an Eyepiece Graticule & Stage Micrometer

  • Eyepiece graticules are calibrated against a stage micrometer to measure specimen sizes accurately.

  • Units must be consistent; for example, converting mm to µm (1 mm = 1000 µm) before calculations is crucial for accuracy in measurements.