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Topic 1: Cells Pt. 1 - Coggle Diagram
Topic 1: Cells Pt. 1
Cell size
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Cells need to exchange materials with the environment in order to produce the chemical energy required for survival:
- The rate of metabolism is a function of cell's volume
- The rate of material exchange is a function of a cell's surface area
As a cell grows, volume increases faster than surface area:
- if metabolic requirements exceed material exchange, a cell will die
- Hence, cells must stay small or increase their SA:Volume ratio to survive
Differentiation
All cells of an organism contain an identical genome - each cell contains the entire set of genetic instructions for that organism
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The activation of different genes within a given cell will cause it to develop differently from other cells
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Organelles
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Examples of eukaryotic organelles include:
- 80S ribosomes - responsible for protein synthesis
- Nucleus - Stones genetic information
- Mitochondria - Site of aerobic respiration
- Endoplasmic reticulum - transports materials between organelles
- Golgi complex - Sorts, stores, modifies and exports secretory products
- Centrosomes - involved in cell division
Organelles found only in specific cell types are:
- Chloroplasts - site of photosynthesis
- Lysosomes - Breakdown of macromolecules
Phospholipid bilayer
Structure of phospholipids:
- Contain a polar (hydrophilic) head composed of phosphate ( + glycerol)
- Contain 2 non-polar (hydrophobic) tails; each composed of a fatty acid chain
- Hence, phospholipids are amphipathic (have hydrophilllic and hydrophobic parts)
Arrangement in membranes:
- Phospholipids spontaneously arrange into a bilayer
- The hydrophilic phosphate heads face out into the surrounding solution, while the hydrophobic fatty acid tails face inwards and are shielded from the polar fluids
Properties of the phosphlipid bilayer:
- The bilayer is held together by weak hydrophobic interactions between the tails
- Individual phospholipids can move within the bilayer
- Amphipathic properties restrict passage of certain substances (semi-permeable)
Membrane models
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This model was falsified based on the following findings:
- Fluorescent tagging showed the proteins are mobile
- Not all membranes have a constant lipid:protein ratio
- Freeze fracturing identified transmembrane proteins
Cell theory
Cell theory:
- Living organisms are composed of cells
- The cell is the smallest unit of independent life
- Cells can only arise from pre-existing cells
Exceptions include:
- Striated muscle - composed of fused cells that are multinucleated
- Giant algae - unicellular organisms that are very large in size
- aseptate hyphae - lack partitioning and have a continious cytoplasm
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Microscopes
Light microscopes use lenses to bend light:
- Can view living specimens in natural colour
- Have lower magnification and resolution
Electron microscopes use electromagnets to focus electrons:
- Can only view dead specimens in monochrome
- Have higher magnification and resolution
- Can show cross-sections or surface renderings
Emergent properties
An emergent property is a function that is present in multicellular organisms but is not present in it's individual component cells
Emergent properties arise from synergistic interactions between the individual cells to produce entirely new aggregate functions
Stem cells
Stemcells are unspecialised cells that have 2 key qualities:
- Self-renewal - They can continously divide and replicate
- Potency - They have the capacity to differentiate
There are 4 main types of stem cells during human development:
- Totipotent - Can form any cell type, as well as extra-embryonic tissue
- Pluripotent - Can form any cell type
Multipotent- Can differentiate into closely related cell types
Unipotent - Cannoy differentiate, but are capable of self-renewal
Stem cell therapy
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The therapeutic use of stem cells involves:
- harvesting stem cells from appropriate sources
- Using biochemical solutions to trigger cell differentiation
- Surgically implanting new cells into patient's own tissue
-Suppressing the host immune system to prevent rejection
- Monitoring new cells to ensure they do not become cancerous
Gene packaging
Within the nuclei of eukaryotic cells, gene instructions (DNA) are packaged with proteins as chromatin
- Active genes are loosely packed as euchromatin
- Inactive genes are packed tight as heterchromatin
Bacterial cell division
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In this process:
- The circular DNA is copied
- The DNA loops attach to the membrane
- The cell elongates, separating the loops
- Cytokinesis occurs to form 2 cells
Animal VS Plant cells
Animal cells -
- No chloroplast
- No cell wall
- No plasmodesmata
- Temporary vacuoles
- Cholesterol present in the cell membrane
- Glucose to glycogen
Plant cells:
- Have choloroplast
- Cell wall
- Plasmodesmata
- Large central vacuole
- No cholesterol in the cell membrane
- Glucose to Starch
Cholesterol
Cholesterol is a fundamental component of animal cell membranes, it is not present in plant cell membranes
Cholesterol reduces membrane fluidity and permeability to some solutes, it also anchors certain peripheral proteins and prevents crystallization
Membrane proteins
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Membrane proteins serve many functions:
- Junctions
- Enzymes
- Transport
- Recognition
- Anchorage
- Transduction
Fluid mosaic model
Cell membranes are represented as a fluid-mosaic model
- Fluid - membrane components can move position
- Mosaic - phospholipid bilayer is embedded with protein
This model was proposed by Singer-nicolson in 1972, following the falsification of the Davson-Danielli model
Functions of life
Organisms consisting of only one cell carry out all the life functions in that single cell:
- Metabolism
-Reproduction
- Sensitivity
- Homeostasis
- Excretion
- Nutrition
- Growth
Cellular organisation
In multicellular organisms:
- Cells may be grouped together to form tissues
- Tissues may interact to form functional organs
- Organs may combine to form body systems
Therapeutic examples
Examples:
- Stargardt's disease - macular degeneration - replace defective retinal cells
- Parkinson's disease - death of nerve tissue - replace damaged nerve cells
- Leukemia - cancer of the blood - replacement of bone marrow
Ethics of stem cell use
Source - Growth potential - Tumour risk - Harvesting - Disadvantages
Embryo - high (plutipotent) - higher risk - can be generated artificially by SCNT - requires destruction of the embryo
umbilical cord blood - low (multipotent) - lower risk - easily obtained and stored - cells must be stored from birth at cost
Adult tissue - low (multipotent) - lower risk - invasive to extract - may be restriction in availability