Study Notes on Cellular Structure & Membranes
Cellular Structure & Membranes
Learning Objectives for this Lecture
Understand history of cell discovery and Cell Theory.
Know methods used to study cells.
Describe the three functions of cell membranes.
Identify differences between prokaryotic and eukaryotic cells.
Explore cellular organization of animal and plant cells.
Understand organization and function of the eukaryotic nucleus.
Discuss cytoplasmic organelles and the functions they carry out.
Know components of biological membranes and their functions.
Understand models that explain membrane structure and function.
Cell Theory
Historical Context: Cells were first observed using a microscope in 1665 by Robert Hooke.
Modern Cell Theory Principles (Schleiden and Schwann):
Cells are the smallest LIVING things, and they are the basic units of organization of all organisms.
All organisms are composed of one or more cells, and life processes of metabolism and heredity occur within these cells.
Cells arise only by division of a previously existing cell.
All cells today represent a continuous line of descent from the first living cells.
Methods to Study Cells
Microscopes:
Not many cells are visible to the naked eye; most are <50 μm.
Objects must be 100 μm apart for the naked eye to resolve them as two distinct objects.
Types of Microscopes
Light Microscopes:
Use magnifying lenses with visible light.
Resolve structures that are 200 nm apart.
Limited resolution using light.
Electron Microscopes:
Use a beam of electrons.
Resolve structures that are 0.2 nm apart.
Transmission Electron Microscopes (TEM): Transmit electrons through the material.
Scanning Electron Microscopes (SEM): Beam electrons onto the specimen surface.
Cell Size and Diversity
The human body consists of approximately 1 quadrillion cells and around 300 different types.
Cells within the same organism can exhibit significant diversity in size, shape, and internal organization.
Cells are limited in size by their surface area to volume ratio:
As a cell increases in size, its volume increases much more rapidly than its surface area.
This limits the cell’s efficiency in transporting substances across memebranes.
Size Comparisons
Water molecule: 0.28 nm
Red blood cells: 7-8 μm
Biological cell: 10-100 μm
Atom: ≈0.1 nm
DNA: =2 nm
Adenovirus: 90-100 nm
Antibody: 10 nm
Bacterium: 1-3 μm
Chromosome: 1-10 μm
Paramecium: 50-330 μm
Frog eggs: >1 mm
Surface Area and Volume Formulas
Cell radius (r):
Surface area: 4 ext{π}r^2
Volume: ext{π}r^3
As size increases:
1 unit = 10 units
Area for 1 unit = 12.57 units²
Area for 10 units = 1257 units²
Volume for 1 unit = 4.189 units³
Volume for 10 units = 4189 units³
Cellular Organization
Basic Structural Similarities:
Cellular organization varies between different organisms; however, all cells share basic structural similarities, including:
Centrally located genetic material: Nucleoid or nucleus where DNA is housed.
Cytoplasm: A semi-fluid matrix containing organelles and cytosol.
Ribosomes: Sites of protein synthesis.
Cell Membrane (Plasma Membrane): Phospholipid bilayer.
Centrally Located Genetic Material
DNA (Deoxyribonucleic Acid): Long molecule that contains the genetic code for the organism.
Functions as a recipe book by holding the instructions for making all proteins required by the organism.
Consists of four basic building blocks (or bases): Adenine, Cytosine, Guanine, and Thymine.
The sequence of these bases forms the genome instructions.
Cell Membrane Structure
The cell membrane encloses the cell and separates it from its surroundings, composed of a phospholipid bilayer that is 5-10 nm thick with proteins embedded throughout.
Functions:
Transport proteins facilitate the movement of molecules and ions across the plasma membrane.
Receptor proteins induce changes within the cell upon contact with specific external molecules (e.g., hormones).
The Cytoplasm and Cytosol
Cytoplasm: Contains cytosol and organelles (e.g., Golgi apparatus, mitochondria).
Functions:
Supports and suspends organelles and cellular molecules.
Aids in transport of genetic material and products of cellular respiration.
Cytosol: The fluid environment for organelles, containing enzymes that break down molecules for use and a cytoskeleton that gives the cell shape.
Cytoskeleton
Function: Framework of protein fibers embedded within the cytoplasm.
Maintains cell shape and organization.
Provides structural support (like scaffolding).
Serves attachment points for organelles or cell membrane.
Facilitates movement within the cell (e.g., flagella in protists).
Types of Cytoskeletal Elements
Microtubules:
Structure: Hollow tubes made of tubulin molecules, diameter 25 nm.
Functions include maintaining cell shape, cell motility via flagella or cilia, moving chromosomes during cell division, assisting cell plate formation during plant cell division, and organelle movement.
Intermediate Filaments:
Structure: Fibrous proteins (keratin or vimentin) supercoiled into thick cables, diameter 8-12 nm.
Functions include maintaining cell shape and anchoring the nucleus and other organelles.
Microfilaments (Actin):
Structure: Two intertwined strands of protein actin, diameter 7 nm.
Functions include maintaining cell shape, facilitating cell motility (muscle contraction or “crawling”), dividing animal cells, and moving organelles and cytoplasm in plants and animals.
Cell Types
Two Basic Cellular Architectures:
Prokaryotic Cells
Always unicellular, small (1-10 microns), and have a cell wall.
Organelles: Usually none; has ribosomes that are not membrane-bound.
Nucleus: No nuclear envelope; genetic material is a single chromosome.
Division: Binary fission.
Eukaryotic Cells
Often multicellular, larger (10-100 microns).
Cell wall: Present in plants (not in animal cells).
Organelles: Many; have specialized functions (e.g., Golgi apparatus, mitochondria, and endoplasmic reticulum).
Division: Mitosis and meiosis; more than one chromosome enclosed within a nuclear envelope.
Specific Cell Structures
Prokaryotic Cell: Typically features a capsule, ribosomes, plasma membrane, and cytoplasm.
Eukaryotic Cell: Typically contains organelles such as the nucleus, mitochondria, plasma membrane, endoplasmic reticulum, Golgi complex, and various cytoplasmic structures.
Differences Between Plant and Animal Cells
Both animal and plant cells share largely the same structure; both possess a plasma membrane and most of the same organelles.
Plant Cells: Have extra components including a cell wall, chloroplasts, and specialized vacuoles.
Animal Cells: Lack cell walls but have centrosomes and lysosomes.
Organelles and Their Functions in Animal Cells
Cell Fractionation: A technique used to break cells apart and separate various organelles for examination of functions.
Organelle types include the Golgi apparatus, ribosomes, and mitochondria.
Cell extracts can be centrifuged to separate organelles based on density:
Pellet (e.g. ER, Golgi, plasma membrane)
Supernatant (e.g. molecules and ions).
The Nucleus
Functions:
Controls and stores hereditary characteristics of an organism via DNA.
Responsible for protein synthesis, cell division, growth, and differentiation.
Stores proteins and RNA in the nucleolus and is the site for transcription (production of mRNA).
Assists exchange of DNA and RNA between the nucleus and the rest of the cell.
Nucleus Structure
Nuclear Envelope: Composed of two membranes (phospholipid bilayers) protecting the nucleus from the cytoplasm.
Nucleolus: Large structure within the nucleus responsible for ribosome and RNA production.
Nucleoplasm: Liquid filling the interior of the nucleus.
Chromatin: DNA and proteins organized into chromosomes before cell division.
Nuclear Pore: Channels allowing RNA passage to cytoplasm while retaining larger DNA molecules.
Endomembrane System
Includes nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and cell membrane.
This system works to modify, package, and transport lipids and proteins, dividing the cell into functional and structural compartments.
Endoplasmic Reticulum (ER)
The ER is a set of tubes and sacs, called cisternae, enclosed in membranes extending from the nuclear membrane into the cell.
Rough Endoplasmic Reticulum (RER): Studded with ribosomes; produces proteins that travel elsewhere in the body.
Smooth Endoplasmic Reticulum (SER): No ribosomes; involved in lipid synthesis and detoxification activities.
Ribosomes
The cell's protein synthesis machinery, found in all cell types across all three domains of life.
Composed of two subunits: large and small, and can be free in the cytoplasm or associated with internal membranes.
The Golgi Apparatus
Acts as the cell's distribution and shipping department for the endomembrane system.
Processes proteins made in the ER before distributing them through transport vesicles.
Proteins enter at the cis face (adjacent to ER) and exit at the trans face.
Substances are tagged with molecular markers that direct them where to go within the cell.
Lysosomes
Function as the waste disposal unit of the cell, filled with digestive enzymes that break down cellular waste into simpler compounds.
Maintains a low pH environment internally for optimal enzyme activity.
Formed from enzymes synthesized in the rough ER, refined in Golgi apparatus, and then released into the cytoplasm.
Mitochondria
Power station of the cell; main function is to metabolize carbohydrates and fatty acids to generate energy in the form of Adenosine Triphosphate (ATP).
Functions:
Involvement in respiration processes (e.g., glycolysis).
Maintaining calcium ion concentration, detoxifying ammonia in the liver, contributing to blood component formation, and apoptosis processes.
Mitochondria are double-membraned organelles that maximize surface area for chemical reactions through their folded inner membrane.
Evolution of Mitochondria and Chloroplasts
According to the Endosymbiotic Theory, mitochondria and chloroplasts evolved from engulfed prokaryotes which once lived independently.
They have their own genomes, allowing them to divide independently, and most of their DNA has transferred to the host nucleus.
Cell Membrane
Description: Thin barrier structure (8 nm) also known as the plasma membrane.
Functions:
Maintains the integrity of the cell and controls the movement of particles into and out of the cell.
Structure: Composed of phospholipids, which are amphipathic, featuring hydrophobic and hydrophilic regions.
Biological Membranes
Critical for the origin of life and function as barriers, defining cellular compartments, protecting and supporting the cell, and allowing selective permeability.
Membrane proteins facilitate numerous functions including communication, energy transfer, and regulatory control over material passage.
Models of Membrane Structure
Davson-Danielli Model (1935): Proposed the lipid bilayer was sandwiched between two layers of hydrophilic proteins. This model is incorrect due to variations in membrane thickness and protein ratios.
Fluid Mosaic Model (1972): Proposed by Singer and Nicolson; membranes comprise a phospholipid bilayer with embedded proteins. The 'fluid' aspect reflects the movement of components within the membrane, while 'mosaic' depicts the disjointed arrangement of proteins.
Membrane Protein Movement Experiment
Larry Frye and Michael Edidin (1970) grew human and mouse cells in separate dishes, labeling their proteins with fluorescent antibodies. After cell fusion, the human and mouse proteins mixed over time, indicating protein mobility within plasma membranes.
Biological Membranes Function
Transport Mechanisms
Passive Transport: Movement of ions/molecules across a membrane without energy use (e.g., diffusion).
Active Transport: Movement of cells/ ions against a concentration gradient that requires ATP.
Types of Diffusion
Simple Diffusion:
Involves molecules moving down their concentration gradient without a semi-permeable membrane and can occur in any medium.
Osmosis:
Refers to the movement of solvent (water) through a semi-permeable membrane, driven by osmotic pressure related to solute concentration.
Defined states include isotonic, hypotonic, and hypertonic conditions concerning the solute concentrations on either side of the membrane.
Summary of Diffusion vs. Osmosis
Diffusion: Movement of solutes from high to low concentration, occurs in any medium, does not require a membrane.
Osmosis: Movement of solvent from high to low concentration through a semi-permeable membrane, occurs only in liquid medium.
Biological Membrane Functions
Regulate passages of materials into and out of the cell, chemical reaction facilitation, cell recognition and linkage, transducing signals between the environment and cell, and energy storage and transfer.
Facilitated Diffusion
Involves transport proteins aiding polar molecules and ions in crossing membranes, spanning the membrane as channels.
Passive process, allowing substances to move from high to low concentration without energy investment.
Active Transport
Movement against a concentration gradient, necessitating input of energy (ATP).
Example: Sodium-Potassium Pump which transports sodium out of the cell and potassium into the cell.
Conclusion
Understanding cellular structure and function is vital for comprehending biological systems and the mechanisms behind various life processes.