Topic 1.1 CELL THEORY Topic Outline AUDIO Functions of Life Cell Theory Microscopes Magnification Surface Area : Volume Ratio Multicellular Organisation Cell Differentiation Stem Cells The evolution of multicellular organisms allowed cell specialisation and cell replacement Functions of Life All living things carry out 7 basic life functions: • Metabolism (undertakes essential chemical reactions) • Reproduction (produces offspring – sexually or asexually) • Sensitivity (responsive to internal and external stimuli) • Homeostasis (maintains a stable internal environment) • Excretion (able to remove toxic waste products) • Nutrition (exchanges material with the environment) • Growth / movement (changes shape / size / position) Organisms consisting of only one cell carry out all the functions of life Mnemonic: MR SHENG Life Function Examples Paramecium Chlorella Heterotrophic M Autotrophic Asexual (mitosis) R Asexual (mitosis) Chemotaxis (towards food) S Phototaxis (towards light) Keeps equilibrium H Keeps equilibrium Via anal pore E Via diffusion Food vacuoles N Photosynthesis Moves via cilia G Non-motile Investigate the functions of life in Paramecium and one named photosynthetic unicellular organism The Cell Theory The cell theory describes the structural organization of all living organisms According to the cell theory: • Living things are composed of cells (or cellular products) • The cell is the smallest unit of life • Cells only arise from pre-existing cells Watch the above video describing the ‘wacky’ history of the cell theory According to the cell theory, living organisms are composed of cells Cell Theory Exceptions Certain types of cells and tissues do not conform to the standard notion of a cell Striated Muscle Giant Algae Aseptate Fungal Hyphae Individual cells fuse to form long multinucleated fibres Certain species can be very large (Acetabularia: >7cm) Hyphae may be connected by a continuous cytoplasm Questioning cell theory using atypical examples – striated muscle, giant algae, aseptate fungal hyphae Cell Scale In science, objects are typically Cells and their components are usually measured via the metric system measured in micrometres & nanometres Unit Prefix Symbol 1 metre m milli mm 10 –3 ÷ 1000 ÷ 1000 10 –6 micro Molecule: 1nm Membrane: 7.5nm Virus: 100nm μm ÷ 1000 10 –9 10 –12 nano pico nm pm ÷ 1000 Bacteria: 1μm Organelle: 10μm Cell: 100μm Microscopes Objects that are too small for the naked eye may be visualized with microscopes Light Microscopy • Views living specimens in natural colour • Has a lower resolution and magnification Electron Microscopy • Views dead specimens in monochrome • Has a higher resolution and magnification Light Microscope Electron Microscope Light Microscopy Bacteria Plant Cells Animal Cells Muscle Tissue Use of a light microscope to investigate the structure of cells and tissues, with drawings of cells Electron Microscopy Bacteria Plant Cells Virus (green), bacteria (red), animal cell (blue) Electron microscopes have a much higher resolution than light microscopes (1.2) Magnification To calculate linear magnification of a drawing or image, use the following calculation: • MIA: Magnification = Image size ÷ Actual size MIA To calculate the actual size of a drawing or image, use the following calculation: • AIM: Actual size = Image size ÷ Magnification AIM Calculation of magnification and the actual sizes of structures shown in drawings and micrographs Magnification – Worked Example Calculate the magnification of the image: • Magnification = Image size ÷ Actual size ➜ Image size = 130,000μm 50 μm (13 × 104) ➜ Actual size = 350μm (50μm × 7) ➜ Magnification = 370 (130,000 ÷ 350) Total length = 13 cm Calculation of magnification and the actual sizes of structures shown in drawings and micrographs Surface Area : Volume Ratio Cells need to produce energy to survive and this requires material exchange • The rate of metabolism is a function of cell volume • The rate of material and heat exchange is a function of surface area As a cell grows in size, volume (units3) increases faster than surface area (units2) • This leads to a decreased surface area : volume ratio (⬇︎ SA:Vol) If metabolic rate exceeds the capacity to exchange materials, the cell will die • Hence growing cells typically divide and remain small in order to survive Surface area to volume ratio is important in the limitation of cell size Surface Area : Volume Ratio Surface area increases Volume stays constant Total surface area 150 (height width sides number of boxes) 750 Total volume 125 1.2 (height width length number of boxes) SA:Vol ratio (surface area volume) 125 Cells / tissues may increase their surface area to optimise transfer 6 (e.g. microvilli = ⬆︎ SA:Vol ratio) Multicellular Organisms Multicellular organisms form when groups of individual cells function together These organisms are capable of completing new functions (emergent properties) due to the collective action of many cells combining to create synergistic effects Cell Tissue Organ System cardiac heart cardio vascular (muscle) Multicellular organisms have properties that emerge from the interaction of their cellular components Cell Differentiation All cells in a multicellular organism share an identical set of genetic instructions • Individual instructions (called genes) form a totality called the genome The activation of different instructions in specific cells will cause these cells to differentiate and become specialised (possessing distinctive functionality) Gene A expressed Identical cells Gene B expressed Specialised tissues can develop by cell differentiation in multicellular organisms Gene Packaging Within the nucleus, active genes are packaged in an expanded and accessible form called euchromatin The inactive genes are packaged in a condensed and inaccessible form called heterochromatin Differentiated cells will have different regions of DNA packaged according to their specific function Differentiation involves the expression of some genes and not others in a cell’s genome Stem Cells Stem cells are unspecialised cells that possess two key qualities: • Self-Renewal – They can continually divide and replicate • Potency – They have the capacity to differentiate into specialised cell types Embryonic stem cells (totipotent / pluripotent) can form any cell type, whilst adult stem cells (multipotent / unipotent) have a limited capacity for differentiation Self-Renewal Potency The capacity of stem cells to divide and differentiate is necessary in embryonic development Stem Cell Therapy Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones • Harvest Stem cells are extracted from an appropriate source (embryos, umbilical cord blood, certain adult tissues) • Biochemical solutions trigger cell differentiation • New cells are implanted into the host’s tissue • Immune system is suppressed to prevent rejection • New cells monitored to ensure they are not cancerous Differentiate Implant The capacity of stem cells to divide and differentiate makes them suitable for therapeutic use Examples of Stem Cell Therapy Stargardt’s Disease • An inherited form of juvenile macular degeneration that leads to blindness • Treated by replacing dead cells within the retina with functioning ones Parkinson’s Disease • A degenerative disorder caused by the death of dopamine-secreting cells (CNS) • Treated by replacing dead cells in the midbrain with functioning ones Use of stem cells to treat Stargardt’s disease and one other named condition Ethics of Stem Cell Use Harvesting Potency Tumor Risk Limitations Embryos Are specially created (SCNT) Highest Higher Involves destruction of an embryo Umbilical Cord Blood Easy to extract cells from cord Lower Lower Cells must be stored from birth at cost Adult Tissues Cells obtainable at any life stage Lowest Lower May be difficult to extract (and painful) Ethics of the therapeutic use of stem cells from embryos, umbilical cord blood (newborn) & adult tissues Topic Review Can you do the following? • List the functions of life • State the cell theory (and exceptions) • Contrast light and electron microscopes • Calculate magnification of images • Explain how SA:Vol ratio limits cell size • Outline the process of cell differentiation • Describe stem cell use (ethics and examples)
