Cell Theory

CELL THEORY FUNCTIONS OF LIFE All living things carry out seven basic funcCons integral to survival: • Metabolism: Living things undertake essenCal chemical reacCons • Reproduc>on: Living things produce offspring, either sexually or asexually • Sensi>vity: Living things are responsive to internal and external sCmuli • Homeostasis: Living things maintain a stable internal environment • Excre>on: Living things possess the capacity to remove waste products • Nutri>on: Living things exchange materials / gases with the environment • Growth / Movement: Living things can move and change shape or size CELL THEORY The cell theory describes the structural organisaCon of all living things. According to the cell theory: • The cell is the smallest unit of life (unicellular organisms are capable of all seven funcCons of life) • All living things are composed of cells (or their cellular products – such as hair, nails, etc.) • Cells only arise from pre-exisCng cells (spontaneous generaCon of life is no longer possible on Earth) CELLS All cells share four basic features: • They are enclosed by a membrane, which separates internal contents from the external environment • They contain an internal fluid called the cytosol, in which various biological processes are able to occur • There is gene>c material, which funcCons as a set of instrucCons (i.e. a blueprint) for cellular acCvity • They possess ribosomes, which funcCon to translate specific geneCc instrucCons within the cell ATYPICAL CELLS Certain types of eukaryoCc cells do not conform to the standard organisaCon of a typical cell: • Striated muscle fibres are formed from the fusion of individual muscle cells and so are mul>nucleated • Aseptate fungal hyphae lack internal par>>ons between cells and so have a conCnuous cytoplasm • Sieve tube elements are connected by plasmodesmata to form supracellular assemblies along the stem • Red blood cells have no nucleus and lack the capacity to replicate (new cells produced by bone marrow) CELL SIZE Cells and their components are measured according to the metric system. Most cells will be measured in micrometres (10–6 metres), while subcellular components may be measured in nanometres (10–9 metres). Unit 1 10–2 10–3 10–6 10–9 Prefix metre (m) cenCmetre (cm) millimetre (mm) micrometre (µm) nanometre (nm) MICROSCOPES As cells are typically too small to view with the naked eye, they may be visualised instead via the use of microscopes (i.e. light versus electron). Light Microscopy: • Views living specimens in natural colour (uses lenses to bend light) • Has a much lower resoluCon and magnificaCon (roughly 100-fold) Electron Microscopy: • Views dead specimens in monochrome (uses electromagnets) • Has a much higher resoluCon and magnificaCon (can view in nm) o Transmission electron microscopes generate a cross-secCon o Scanning electron microscopes will render a 3D surface map Light (top) vs Electron (bo4om) MICROSCOPE DEVELOPMENTS The clarity of sub-cellular structures has been improved by a number of advancements in microscopy: Immunofluorescence involves using anCbodies that are conjugated to fluorescent probes to specifically target a cellular component of choice. The fluorescent probe can be conjugated to the targeCng anCbody (direct immunofluorescence) or a generic secondary anCbody that binds to the targeCng anCbody (indirect immunofluorescence). The cell component can be visualised under a light microscope using relevant filters. Cryogenic electron microscopy involves freezing biological specimens prior to visualisaCon with an electron microscope. This allows for the determinaCon of molecular structures at near atomic resoluCon without requiring the crystallisaCon of the specimen. If the frozen specimen is physically broken along a specific plane via freeze fracturing, then internal cellular structures can also be studied at high resoluCon. MAGNIFICATION To calculate the linear magnifica>on of a drawing or image, the following calculaCons may be used (mnemonic = MIA): MIA: MagnificaCon = Image size ÷ Actual size To calculate the actual size of a specimen within an image, the following calculaCons may be used (mnemonic = AIM): AIM: Actual size = Image size ÷ MagnificaCon Any calculaCon requires all sizes (image and actual) to be in the same units (e.g. both represented by micrometres) EXAMPLE: COMMON DUST MITE Image = 6.3 cm Actual = 350 µm Magnifica>on = ´ 180 (63,000 ÷ 350)