Topic 1: Cell Biology

Functions of Life (MR SHENG)

Functions of Life (MR SHENG)

• Metabolism

  • Undertakes essential chemical reactions

• Reproduction

  • Produces offspring sexually or asexually

• Sensitivity

  • Responsive to internal and external stimuli

• Homeostasis

  • Maintains stable inner environment

• Excretion

  • Able to remove toxic waste products

• Nutrition

  • Exchanges material with the environment

• Growth/movement

  • Changes shape/size/position

Cell Theory Exceptions

• Striated Muscle

  • Individual cells fuse to form long multi-nucleated fibres

  • Challenges idea that cells always function as autonomous units

• Giant Algae

  • Certain species can be very large

  • Challenges idea that larger organisms are always made up of many microscopic cells

• Aseptate Fungal Hypae

  • Hyphae may be connected by a continuous cytoplasm

  • Challenges idea that living structures are composed of discrete cells

Cell Theory (3 points)

1. Living organisms are composed of cells (or cell products)

2. The cell is the smallest unit of independent life

3. Cells can only arise from pre-existing cells (mitosis/meioses)

Functions of life in Autotroph (Scenedesmus)

• Chlorophyll pigments allow organic molecules to be produced via photosynthesis

  • Metabolism

• Daughter cells form as non-motile autospores via the internal asexual division of the parent cell

  • Reproduction

• Scenedesmus may exist as unicells or form colonies for protection

  • Sensitivity / Responsiveness

• Scenedesmus exchange gases and other essential materials via diffusion

  • Nutrition and Excretion

Functions of life in Heterotroph (Paramecium)

Paramecium (Heterotroph)

• Food particles are enclosed within small vacuoles that contain enzymes for digestion

  • Metabolism

• Paramecia divide asexually through fission, however horizontal gene transfer can occur via conjugation

  • Reproduction

• Surrounded by small hairs (cilia) which allow it to move

  • Sensitivity/Responsiveness

• Essential gases enter (Oxygen) exit (Carbon Dioxide) the cell to keep balance via diffusion

  • Homeostasis

• Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacuoles

  • Excretion

• Paramecia engulf food via a specialised membranous feeding groove called a cytosome

  • Nutrition

Autotroph

An organism that can produce its own food using light, carbon dioxide, or other chemicals.

Heterotroph

An organism that eats other plants or animals for energy and nutrients.

Cell Scale

Cells and their components are measured according to the metric system

Relative Sizes of Biological Material

Microscopes Function

Objects that are too small for the naked eye may be visualised with microscopes

Types of Microscopy: Light v.s. Electron

Light Microscopy

• Views living specimens in natural colour

• Lower resolution and magnification

Electron Microscopy

• Views dead specimens in monochrome

• Has a higher resolution and magnification

Cell Structures under Light Microscopy

Cell Structures under Electron Microscopy

Magnification Formula

MIA

Magnification = Imagine Size/Actual Size

M = I / A

A = I / M

Function of Cell’s Surface Area vs Volume

Volume / Mass

• The rate of metabolism of a cell is a function of its mass (larger cell needs more energy)

Surface Area

• The rate of material exchange is a function of its surface area (large membrane surface equates to more material movement)

SA:Vol Ratio and Cell Growth

• As a cell grows volume increases faster than surface area, leading to decreased SA:Vol ratio

• If metabolic rate > rate of vital material exchange cell dies

• Hence growing cells tend to divide and remain small to maintain high SA:Vol ratio

  • This way Volume stays constant with growth and SA can increase

High SA:Vol ratio is important to what functions

Cells and tissues specialised for gas exchange will increase surface area to optimise material transfer

• Villi of intestinal tissue

  • To maximise nutrient exchange from digestive tract to blood stream

• Microvilli in alveolar cells

  • To maximise gas exchange in lungs for consistent oxygen sypply

Drawing Microscopic Structures: Conventions of Diagram

• A title should be included to identify the specimen (e.g. name of organism, tissue or cell)

• A magnification or scale should be included to indicate relative size

• Identifiable structures should be clearly labelled (drawings should only reflect what is seen, not idealised versions)

Emergent Properties

• Arises when the interaction of individual components produce new functions.

• Help living organisms better adapt to their environments and increase their chances of survival.

“Emergent Property” is a characteristic an entity gains when it becomes part of a bigger system.

Multicellular Organisms: Function / Purpose

Multicellullar organisms are capable of completing functions that unicellular organisms could not undertake due to the collective actions of individual cells combining to create new synergistic effects

Multicellular Organisms: Organisation

• Cells may be grouped together to form tissues

• Organs are then formed from the functional grouping of multiple tissues

• Organs that interact may form organ systems capable of carrying out specific body function

• Organ systems collectively carry out the life functions of the complete organism

Cell Differtiation: Definition

Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature.

Cell Differentiation: Process

• All cells in a multicellular organism share an identical genome

  • Each cell contains entire set of genetic instructions for that organism

• The activation of specific genes within a cell will cause it to differentiate

  • Activation occurs by chemical signals

Gene Packaging

Inside nucleus of eukaryotic cells, DNA is packaged with proteins to form chromatin

Active genes - Packaged in expanded form called euchromatin

Inactive genes - Packaged in more condensed form called heterochromatin (saves space, not transcribed)

Differentiated cells will have different regions of DNA packaged as euchromatin and heterochromatin according to their specific function.

Stem Cells: Definition and Key Qualities

Stem cells are unspecialised cells that have two key qualities:

1. Self Renewal - They can continuously divide and replicate

2. Potency - They have the capacity to differentiate into specialised cell types

Necessary for embryonic development as they are an undifferentiated cell source from which all other cell types may be derive

Types of Stem Cells and Non-Stem Cells

Totipotent - Can form any cell type including extra-embryonic tissue (eg. zygote)

Pluripotent - Can form any cell type (eg. embryonic stem cells)

Multipotent - Can differentiate into a number of closely related cell types (eg haematopoietic adult stem cells)

Unipotent - Cannot differentiate, but are capable of self-renewal (eg. muscle stem cells)

Non-Stem Cells - Cells not capable of self-renewal (eg. amitotic nerve tissues). Cannot regenerate or replace, therefore damage often calls for stem cell therapy

Stem Cell Therapy: Process

1. Use of biochemical solutions to trigger differentiation of stem cells into desired cell type

2. Surgical implantation of cells into the patient’s own tissue

3. Suppression of host immune system to prevent rejection of cells (if stem cells are from foreign source)

4. Careful monitoring of new cells to ensure they do not become cancerous

Stem Cell Therapy Example: Stargardt’s Disease

What is Stargardt’s Disease

• Inherited form of juvenile macular degeneration (early vision loss)

• Causes progression vision loss to point of blindness

• Caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate

Stem Cell Treatment

• Treated by replacing dead cells in retina with functioning ones derived from stem cells

Stem Cell Therapy Example: Parkinson’s Disease

What is Parkinson’s Disease

• A degenerative disorder of the central nervous system caused by the death of dopamine-secreting cells in the midbrain

• Dopamine is a neurotransmitter responsible for transmitting signals involved in the production of smooth, purposeful movements

• Consequently, individuals with Parkinson’s disease typically exhibit tremors, rigidity, slowness of movement and postural instability.

Stem Cell Treatment

• Treated by replacing dead nerve cells with living, dopamine producing ones.

Stem Cell Therapy Examples: Others

• Leukemia:  Bone marrow transplants for cancer patients who are immunocompromised as a result of chemotherapy

• Paraplegia:  Repair damage caused by spinal injuries to enable paralysed victims to regain movement

• Diabetes:  Replace non-functioning islet cells with those capable of producing insulin in type I diabetics

• Burn victims:  Graft new skin cells to replace damaged tissue

Stem Cells: Sources

• Embryos - may be specifically created by therapeutic cloning

• Umbilical cord blood or placenta of new-born

• Certain adult tissues like bone marrow (not pluripotent)

Reasons for the therapeutic use of Stem Cells

1. Unspecialized/undifferentiated stem cells can divide / differentiate along different pathways; (Qualities of Stem Cells)

2. Stem cells are accessible as they come from embryos/bone marrow/umbilical cord blood/adult tissue; (Accessible Sources)

3. Stem cells can regenerate/repair diseased/damaged tissues in people;

4. valid specific example; (Qualities of Stem Cells)

5. drugs can be tested on stem cells (in laboratories to see if they are harmful); (Reduced Human Risk)

Stem Cell Therapy: Ethical Considerations

• Limited scope of application - Multipotent adult tissue only effective for certain conditions

• Availability and Access - Stem cells derived from umbilical cord blood need to be stored and preserved at cost (expensive and limited access)

• Embryo Destruction - Embryos hold the greatest yield of pluripotent stem cells, but their use means the destruction of a potential living organism

Artificial Stem Cell Techniques: Somatic Cell Nuclear Transfer (SCNT)

Involves the creation of embryonic clones by fusing a diploid nucleus with an enucleated egg cell (therapeutic cloning)

Advantages:

• Indistinguishable from embryo-derived cells

• Meaning Totipotent Stem Cells can be derived

Disadvantages:

• Involves ex vivo (out of body) creation of embryos

• More embryos are created than needed, raising ethical concerns about the exigency of excess embryos

Artificial Stem Cell Techniques: Nuclear Reprogramming

Induce a change in the gene expression profile of a cell in order to transform it into a different cell type (transdifferentiation)

Advantages:

• Stem cells are autologous to adult doner (derived from same person needing them) so no risk of incompatibility

Disadvantages:

• Involves the use of oncogenic retroviruses and transgenes, increasing the risk of health consequences (ie. cancer)

Prokaryotic Cells: Definition, How they divide, and Classification

Prokaryotes are organisms whose cells lack a nucleus, they divide by Binary Fission. They typically do not possess any membrane-bound organelles.

Kingdom: Monera

Domains:

• Archaebacteria - found in extreme environments like high temperatures, salt concentrations or pH (ie. extremophiles)

• Eubacteria - traditional bacteria including most known pathogenic forms (ie. E. coli, S. aureus, etc.)

Prokaryotic Cells: Features

• Cytoplasm: Internal fluid component of the cell

• Nucleoid: region of the cytoplasm where the DNA is located (DNA is circular and called “Genophore”)

• Plasmids: autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)

• Ribosomes: complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S)

• Cell membrane: Semi-permeable and selective barrier surrounding the cell

• Cell wall: Rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)

• Slime capsule: a thick polysaccharide layer used for protection against desiccation (drying out) and phagocytosis

• Flagella: Long, slender projections containing a motor protein that enables movement (singular: flagellum)

• Pili: Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)

Binary Fission: Definition and Process

A form of asexual reproduction used by prokaryotic cells.

Process:

1. The circular DNA is copied in response to a replication signal

2. The two DNA loops attach to the membrane

3. The membrane elongates and pinches off (cytokinesis) forming two cells

Eukaryotic Cells: Definition and Classification

Eukaryotes are organisms whose cells contain a nucleus and several membrane-bound organelles.

They have more complex structures and are believed to have evolved from prokaryotic cells.

Compartmentalised by membrane-bound structures that perform specific roles.

Kingdoms:

• Protista: Unicellular organisms; or multicellular organisms without specialised tissue

• Fungi: have a cell wall made of chitin and obtain nutrition via heterotrophic absorption

• Plantae: have a cell wall made of cellulose and obtain nutrition autotrophically (via photosynthesis)

• Animalia: no cell wall and obtain nutrition via heterotrophic ingestion

Animal Cell: Typical Structure and Features

Plant Cell: Typical Structure and Features

Universal Organelles (prokaryote & eukaryote)

• Ribosomes

• Cytoskeleton

• Plasma membrane

Ribosomes: Structure and Function

Structure:

• Two subunits made of RNA and Protein

• Larger in eukaryotes

  • Eukaryote: 80S

  • Prokaryote: 70S

Function:

• Site of polypeptide synthesis (translation)

Cytoskeleton: Structure and Function

Structure:

• A filamentous scaffolding within the cytoplasm (the fluid portion of the cytoplasm is the cytosol)

Function:

• Provides internal structure and mediates intracellular transport (less developed in prokaryotes)

Plasma Membrane: Structure and Function

Not an organelle per se, but a vital structure.

Structure:

• Phospholipid bilayer embedded with proteins

Function:

• Semi-permeable and selective barrier surrounding the cell

Eukaryotic Organelles (animal and plant)

• Nucleus

• Endoplasmic Reticulum

• Golgi Apparatus

• Mitochondrion

• Peroxisome

• Centrosome

Nucleus: Structure and Function

Structure:

• Double membrane structure with pores

• Contains an inner region called a nucleolus

Function:

• Stores genetic material (DNA) as chromatin

• Nucleolus is site of ribosome assembly

Endoplasmic Reticulum: Structure and Function

Structure:

• Membrane network that may be bare (smooth ER) or studded with ribosomes (rough ER)

Function:

• Transports materials between organelles

  • Smooth ER = lipids

  • Rough ER = proteins

Golgi Apparatus: Structure and Function

Structure:

• An assembly of vesicles and folded membranes located near the cell membrane

Function:

• Involved in sorting, story, modification, and export of secretory products

Mitochondrion: Structure and Function

Structure:

• Double membrane structure, inner membrane highly folded into internal cristae

Function:

• Site of aerobic respiration (ATP production)

Peroxisome: Structure and Function

Structure:

• Membranous sac containing a variety of catabolic enzymes

Function:

• Catalyses breakdown of toxic substances and other metabolites

Centrosome: Structure and Function

Structure:

• Microtubule organising centre (contains paired centrioles in animal cells but not plant cells)

Function:

• Radiating microtubules form spindle fibers and contribute to cell division (mitosis/meiosis)

Eukaryotic Organelles (Plant Cells Only)

• Chloroplast

• Vacuole

• Cell Wall

Chloroplast: Structure and Function

Structure:

• Double membrane structure with internal stacks of membranous discs (thylakoids)

Function:

• Site of photosynthesis - manufactured organic molecules are stored in various plastids (other plant cells, chloroplast is one type of plastid)

Vacuole: Structure and Function

Structure:

• Fluid-filled internal cavity surrounded by a membrane (tonoplast)

Function:

• Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)

Cell Wall: Structure and Function

Not an organelle per se, but a vital structure.

Structure:

• External outer covering made of cellulose

Function:

• Provides support and mechanical strength, prevents excess water uptake.

Lysosome: Structure and Function (Animal Cell Only)

Considered animal cell only, presence in plant cells is subject to debate.

Structure:

• Membranous sacs filled with hydrolytic enzymes

Function:

• Breakdown/hydrolysis of macromolecules

Electron Microscope: Definition, types, and advantages over light microscopes

Definition

Electron microscopes use electron beams focused by electromagnets to magnify and resolve microscopic specimens.

Types

• Transmission Electron Microscopes (TEM)

  • Generate high-resolution cross-sections of objects

• Scanning Electron Microscopes (SEM)

  • Display enhanced depth to map the surface of objects in 3D

Advantages (Over Light Microscopes)

• Much higher range of magnification (can detect smaller structures)

• Much higher resolution (provides clearer and more detailed images)

Disadvantage

• Cannot display living specimens in natural colour

Prokaryote Micrograph: Nucleoid

Prokaryote Micrograph: Cell Wall

Prokaryote Micrograph: Pili

Prokaryote Micrograph: Flagella

Eukaryote Micrograph: Nucleus

Eukaryote Micrograph: Endoplasmic Reticulum (ER)

Eukaryote Micrograph: Mitochondria

Eukaryote Micrograph: Golgi Apparatus

Eukaryote Micrograph: Chloroplast

Eukaryote Micrograph: Cell Wall

Eukaryote Micrograph: Vacuole

Label the Organelles

Deduce Cell Function Based on Abundance:

• Mitochondria

• Endoplasmic Reticulum

• Lysosomes

• Chloroplasts

Mitochondria:

• Cells with many mitochondria typically undertake energy-consuming processes (e.g. neurons, muscle cells)

ER:

• Cells with extensive ER networks undertake secretory activities (e.g. plasma cells, exocrine gland cells)

Lysosomes:

• Cells rich in lysosomes tend to undertake digestive processes (e.g. phagocytes)

Chloroplasts:

• Cells with chloroplasts undergo photosynthesis (e.g. plant leaf tissue but not root tissue)

Prokaryotic Cell: Label and Draw

Key Features:

• Pili – shown as single lines

• Flagella – shown as thicker and significantly longer lines than the pili

• Ribosomes – labelled as 70S

• Cell wall – labelled as being composed of peptidoglycan; thicker than cell membrane

• Shape – appropriate for bacteria chosen (e.g. E. coli is a rod-shaped bacillus)

• Size – appropriate dimensions (e.g. length of cell twice the width)

Prokaryotic Animal Cell: Label and Draw

Key Features:

• Nucleus – shown as double membrane structure with pores

• Mitochondria – double membrane with inner one folded into cristae ; no larger than half the nucleus in size

• Golgi apparatus – shown as a series of enclosed sacs (cisternae) with vesicles leading to and from

• Endoplasmic reticulum – interconnected membranes shown as bare (smooth ER) and studded (rough ER)

• Ribosomes – labelled as 80S

• Cytosol – internal fluid labelled as cytosol (‘cytoplasm' is all internal contents minus the nucleus)

Prokaryotic Plant Cell: Label and Draw

Key Features:

• Vacuole – large and occupying majority of central space (surrounded by tonoplast)

• Chloroplasts – double membrane with internal stacks of membrane discs (only present in photosynthetic tissue)

• Cell wall – labelled as being composed of cellulose ; thicker than cell membrane

• Shape – brick-like shape with rounded corners

Domain/Kingdom Classification: cell characteristics

Prokaryotes:

Kingdom - Monera

Domains - Archaea and Eubacteria

Eukaryotes:

Kingdom - Eukarya

Domains - Protist, Plant, Fungi, and Animal

Points of Comparison: Prokaryotic vs Eukaryotic Cells (DORA)

• DNA

• Organelles

• Reproduction

• Average Size

Points of Comparison: Animal vs Plant Cells

Both have:

• DNA stored within a nucleus

• Larger ribosomes (80S in size)

• A variety of membrane-bound organelles (e.g. mitochondria, ER, golgi apparatus)

Differences:

• Presence or absence of specific sub-cellular structures

• Composition of the plasma membrane

Structure of Phospholipids

Amphipathic molecules, meaning they have clearly discernible hydrophilic and hydrophobic regions

Phospholipids typically share a common basic structure that includes:

• A polar organic molecule (e.g. choline, serine)

• A phosphate group

• A glycerol molecule (replaced by sphingosine in sphingomyelin)

• Two fatty acid tails (may be saturated or unsaturated)

Main Parts

• Polar head

  • Hydrophilic

  • Composed of a glycerol and a phosphate molecule

• Two non-polar tails

  • Hydrophobic

  • Composed of fatty acid (hydrocarbon) chains

Phospholipid Arrangement in Membranes

• Phospholipids spontaneously arrange into a bilayer

• The hydrophobic tail regions face inwards

  • Thereby shielded from surrounding polar fluids

• The two hydrophilic head regions associate with the extra/intracellular fluids

  • Hydrophilic property allows association

  • Interstitial Fluid (Extracellular)

  • Cyrosolic Fluid (Intracellular)

Properties of the Phospholipid Bilayer

• Held together by weak hydrophobic interactions between the tails

• Hydrophilic / Hydrophobic layers restrict the passage of many substances

• Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility

  • Fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis/exocytosis)

Membrane Proteins: What and Integral vs Peripheral

Phospholipids are embedded with membrane proteins which may be either permanently or temporarily attached to the membrane

• Integral Proteins

  • Permanently attached to membrane

  • Typically transmembrane (span across bilayer)

• Peripheral Proteins

  • Temporarily attached by non-covalent interactions

  • Associate with one surface of the membrane

Structure of Membrane Proteins: Polarity

Aminos within the proteins are localised by polarity:

• Non-Polar (hydrophobic)

  • Associate directly with lipid bilayer (makes up outter lining of the protein)

• Polar (hydrophilic)

  • Located internally

  • Hydrophilic because face aqueous solutions passing through the membrane

Structure of Membrane Proteins: Tertiary Structures

Typically adopt one of two tertiary structures:

• Single helices/helical bundles

• Beta Barrels (common in channel proteins)

Functions of Membrane Proteins (JET RAT)

• Junctions

  • Serve to connect and join two cells together

• Enzymes

  • Fixing to membranes localised metabolic pathways

• Transport

  • Responsible for facilitated diffusion and active transports (pumps)

• Recognition

  • May function as markers for cellular identification

• Anchorage

  • Attachment points for cytoskeleton and extracellular matrix

• Transduction

  • Function as receptors for peptide hormones

Cell Membrane Cholesterol: Structure and Placement

Amphipathic molecule (like phospholipids)

• Hydroxyl (-OH) group is hydrophilic and alights towards the phorphate heads of phospholipids

• Remainder is hydrophobic and associates with phospholipids tails

Cell Membrane Cholesterol: Functions

Without Cholesterol

Phospholipid bilayers are fluid and in constant movement relative to one another.

Main Function

Cholesterol functions to maintain integrity and mechanical stability, only in animal cells because plant cells have rigid cell walls of cellulose to maintain structure.

Specific Functions

• Cholesterol immobilises the outer surface of the membrane, reducing fluidity

• It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross

• It functions to separate phospholipid tails and so prevent crystallisation of the membrane

• It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Regulation with Temperature

Cholesterol acts as a bi-directional regulator of membrane fluidity

• At high temperatures it stabilises the membrane and raises the melting point 

• At low temperatures it intercalates between the phospholipids and prevents clustering

Membrane Fluidity

Cell membranes are fluid, meaning they are not fixed in position and can adopt amorphous shapes.

Membrane fluidity is enhanced at higher temperatures.

Phospholipid Structure and Fluidity

Phospholipids may vary in the length and relative saturation of the fatty acid tails

• Shorter Fatty Acid Tails = more fluid

  • Less viscous and more susceptible. to changes in kinetic energy

• Unsaturated Fatty Acid Tails = less fluid

  • Lipid chains with double bonds (unsaturated) have kinked hydrocarbon tails

  • Harder to pack tightly

Fluid-Mosaic Model

Cell membranes are represented according to a fluid-mosaic model, due to the fact that they are:

• Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position

• Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

Davson-Danielli Model: Origin

Davson and Danielli made their model based of the 'trilaminar’ appearance of the cell membrane under the Electron Micrograph.

• Trilaminar = 3 layers (two dark outer layers and a light inner region)

This made them deduce it was a three layer membrane

• “Lipo-Protein Sandwich”

  • Lipid layer (lighter inner region) was sandwiched between two protein layers

  • The darker layers under the microscope were wrongly identified as protein layers.

Davson-Danielli Model: Problems

• Assumed all membranes were of uniform thickness and had constant lipid-protein ration

• Assumed all membranes would have symmetrical internal and external surfaces (not bifacial)

• Did not account for the permeability of certain substances (did not recognise need for hydrophilic pores like channel proteins)

• Temperatures at which membranes solifidied did not correlate with those expected under the model’s assumptions

Davson-Danielli Model: Falsification

Three Key Discoveries:

1) Membrane proteins were discovered to be insoluble in water and varied in size

• Indicated hydrophobic surfaces

• Varied protein sizes would not allow for a uniform and continuous layer around the outer surface of a membrane

• Showed that purely protein surface were unfeasible

2) Flourecent antibody tagging of membrane proteins showed they were mobile and not fixed in place

• Membranes from two different cells were tagged with red and green fluorescent markers respectively

• When two cells were fused, markers became mixed throughout the membrane of the fused cell

• Showed that the membrane proteins could move and did not form a static layer

3) Freeze fracturing was used to split open the membrane and reveal irregular rough surfaces within the membrane

• Rough surfaces were interpreted as being transmembrane proteins

• Showed that proteins were not solely localised to the outside of the membrane structure

Singer-Nicolson Model: Origin

The limitations found in the Davson-Danielli Model allowed for new understandings to be applied in the new Singer-Nicolson Model proposed in 1972

• According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers

  • Addresses the problems and limitations found in the Davson-Danielli Model

This fluid-mosaic model remains the model preferred by scientists today (with refinements)

Key Qualities of Cellular Membranes

• Semi-permeable

  • Only certain materials may freely cross - large and charged substances are typically blocked

• Selective

  • Membrane proteins may regulate the passage of material that cannot freely cross

Passive Transport: Definition and Types

• Movement of material along a concentration gradient (high to low)

• Does not require energy expenditure (ATP hydrolysis)

Three Main Types:

• Simple diffusion

  • movement of small or lipophilic molecules

• Osmosis

  • movement of water molecules

• Facilitated diffusion

  • the movement of large or charged molecules via membrane proteins

Active Transport: Definition and Types

• Movement of materials against a concentration gradient (low to high)

• Required expenditure of energy (ATP hydrolysis)

Two Main Types:

• Primary (direct) active transport

  • Direct use of metabolic energy to mediate transport

• Secondary (indirect) active transport

  • Coupling the molecule with another moving along an electrochemical gradient

Simple Diffusion: Definition and Factors

Diffusion - net movement of molecules from region of high concentration to region of low concentration

• Passive directional movement and continues until equilibrium is reached

• Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes

Factors

• Temperature

  • Affects the kinetic energy of particles in a solution

• Molecular Size

  • Larger particles are subjected to greater resistance within a fluid medium

• Steepness of Gradient

  • Rate of diffusion will be greater with a higher concentration gradient

Simple Diffusion: Definition and Solute Concentration

Osmosis - net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached).

• Water is universal solvent

  • Will associate with and dissolve polar or charged molecules (solutes)

• Because solutes cannot cross a cell membrane unaided, water moved to equalise the two solutions

• Higher solute concentration = less free water molecules in solution

• Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration.

Osmolarity: Definition and Categorisation

Osmolarity - measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol / L)

Categories

• Solutions with relatively higher osmolarity are categorised as hypertonic

  • high solute concentration ⇒ gains water

• Solutions with a relatively lower osmolarity are categorised as hypotonic

  • low solute concentration ⇒ loses water

• Solutions that have the same osmolarity are categorised as isotonic

  • same solute concentration ⇒ no net water flow

Estimating Osmolarity (Application)

The osmolarity of a tissue may be interpolated by bathing the sample in solutions with known osmolarities

• The tissue will:

  • lose water when placed in hypertonic solutions

  • gain water when placed in hypotonic solutions

• Water loss or gain may be determined by weighing the sample before and after bathing in solution

• Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Uncontrolled Osmosis: Negative Effects

Human / Animal Cells

• In hypertonic solutions:

  • Water will leave the cell causing it to shrivel (crenation)

• In hypotonic solutions:

  • Water will enter the cell causing it to swell and potentially burst (lysis)

Therefore, tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication

• This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring

Plant Tissue

• In hypertonic solutions:

  • Cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape

• In hypotonic solutions:

  • Cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)