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Cells (Eukaryotic cells (Components
Cell/plasma membrane
…
Cells
Eukaryotic cells
ComponentsCell/plasma membrane
- Protection/barrier, controls movements in/out the cell
Nucleus
- Controls cells activities, contains DNA and chromosomes
Cytoplasm
- Fluid containing cytosol and cell organelles
Endoplasmic reticulum (smooth + rough)
- Rough (with ribosomes) synthesises protein for export
- Smooth (without ribosomes) lipid and steroid production, and detoxification
Golgi-body/apparatus
- Modifies proteins, packages membrane bound vesicles and exports proteins
Lysosome
- Destruction of damaged cellular components and ingested foreign material by degrative enzymes
Mitochondria
- Site of aerobic cellular respiration and production of ATP
Cytoskeleton
- Strengthens and maintains cell shape, moves organelles within the cell - microtubules and microfilaments
Cell membrane structures
Membrane proteinsIntegral
- Embedded within the whole membrane. Transmembrane or monotopic proteins
Peripheral
- Inner or outer surface of the membrane
Anchored
- Prenylated proteins, fatty acylated proteins, and glycosylphosphatidylinositol-linked proteins
- Glycoprotein - protein with carbohydrate attached
- Glycolipid - lipid with carbohydrate attached
Lipid bilayer
- Contains phospholipid bilayer
- Phosphate group on head
- Phospholipids arranged into phospholipid bilayer
- Proteins embedded in the membrane
- Proteins involved in transport of molecules or cell-cell communication
- Cholesterol is present in the cell membrane and regulates fluidity
- All components known as the fluid mosaic model
Cell Transport
Active
Requires energy (ATP) for movement of substances into and out of the cell
- Pumps for ions or molecules to be moved from an area of low concentration to an area of high concentrated. Up the concentration gradient
- Sodium-potassium pump - moves sodium ions out of the cell and potassium ions into the cell
Passive
Movement of substances into & out of cell with energy expenditure
- Diffusion - moves across the membrane from an area of high concentration to low concentration, down a concentration gradient
- Facilitated diffusion - When an ion or molecule diffuses across a membrane faster than expected by channel proteins or carrier proteins that change shape as they pass through
- Osmosis - diffusion of water across the membrane
Aquaporin proteins allow more rapid transport of water molecules
Bulk transport
Movement of larger substances into and out of the cell
- Vesicles take the substances into or out of the cell
- Plasma membrane surrounds and engulfs the particle (endocytosis)
3 types of endocytosis
- Phagocytosis
Large bacteria or food particle
- Pinocytosis
When vesicles form around a liquid or small particles
- Bulk transport
Molecules bind to specific receptor proteins embedded in a coated pit within the plasma membrane, when enough molecules accumulate in the coated pit, it deepens and seals and is incorporated into the cell as a vesicle
Exocytosis - opposite of endocytosis, moves vesicle out of the cell
Cell signalling
Main reasons
- Replication
- Differentiation
- Apoptosis
Signal delivery
- Neurotransmitters
- Hormones
- Cytokines (growth factors)
- Derivatives of vitamin A & D
Enabled by cell receptors
- Extracellular - tyrosine kinases and guanine nucleotide-binding proteins (G-protein)
- Intracellular - glucocorticoid specific
- Transmembrane and conformational changes (change in the shape of the macromolecule, induced by environmental factors, temperature, pH, voltage, ion concentration
Signal molecules
- Not involved in chemical reaction
- Enable reactions by their conformation and binding properties
Extracellular
- First messenger
- Hormone or neurotransmitter binds to receptors on the target tissue
Intracellular
- Second messenger
- Hormone-receptor interaction directing the cellular response to the presence of the hormone or transmitter
Protein kinases
- Enzymes that transfer phosphate groups to proteins
- Phosphorylation and dephosphorylation are universal ways of activating and deactivating proteins
Protein kinases can:
- Activate/deactivate enzymes
- Open/close ion channels in membrane
- Enable/disable signalling pathways
- Protein phosphatase reverses the action of protein kinase
- Reversible regulatory mechanisms to avoid aberrant cell signalling
G-protein signalling and calcium
- Ligand binds to receptor site on outside of cell membrane
- G-protein (made up of y, b, a subunits) changes conformation
- Guanosine triphosphate replaces guanosine diphosphate on the alpha subunit of the G-protein
- alpha subunit separates from y and b subunits
- alpha subunit with guanosine triphosphate attached, binds to calcium ion channel
- Calcium ion channel opens
- Calcium ions diffuse into the cell and bind with calmodulin
- Phosphorylation removes a phosphate from the guanosine triphosphate to make guanosine diphosphate bound to the alpha subunit
- Inactivated alpha subunit separates from calcium ion channel causing the channel to close
- Alpha subunit recombines with y & b subunits
- G-protein then recombines with the receptor
Enzymes
- Bring about chemical reactions but remain unchanged
- Cannot effect equilibrium or direction of chemical reaction but only promote the reaction
- Requires an active site on the surface that is 3D shaped into which compounds fit
- Formed by 3D folding of protein and dictated by interactions between amino acids
- Protein structures: primary, secondary, tertiary, quaternary
Properties
-ase suffix with root indicating nature of reaction with which it catalyses or the substrateIsoenzymes/isozymes - similar structure but different function
- Can be located in different tissues or sub-cellular organelles
- Have different regulatory mechanisms
- Have different affinity values
- Have different enzymes specificity
Require cofactor for activity
- Metal ions
- Vitamin derivatives - A B D K
- Coenzymes to couple enzymatic reactions
Enzyme propertiesActivity influenced by:
- pH
- Temperature
- Concentration of enzymes
- Concentrations of substrate
- Modification of enzyme
Activators and inhibitors - drug targets
- Competitive inhibitors mimic substrate and compete for active sites and binding sites
- Reversible inhibitors are metabolised and excreted so may be required several times per day
- Irreversible inhibitors inactivate a molecule of enzyme permanently so dose required only daily or less often
Enzymatic binding
Lock & key
Induced fit
- Dynamic mechanism
- Flexible confrontational change
Enzyme catalysed reaction
- Binding of substrate to an enzyme to form enzyme-substrate complex
- Reaction of enzyme-substrate complex to form enzyme-product complex
- Breakdown of enzyme-product complex and release of product
Homeostasis
- The physiological process to maintain the internal environment at a constant optimal state
- Positive and negative feedback loops
- Normal cell function relies on the appropriate environmental conditions
- Many of the body's homeostatic control mechanisms operate on the principle of negative feedback
- Occurs when a change in a controlled variable triggers a response that opposes the change, driving the variable in the opposite direction of the initial change
3 components of homeostasisReceptors/sensors
- Detect deviation from the set point
Control centre/system
- Receives messages from the receptor and to direct a response
Effectors
- Bring about changes to drive the deviation back towards the set point
Homeostatic control
- Temperature
- Blood glucose
- Concentration of iron, copper, calcium, sodium, potassium
- Blood oxygen, CO2, gasses, pH, pressure
- Fluid balance
- Energy balance
Calcium homeostasisStimulus = Falling blood calcium level
- Parathyroid gland secretes parathyroid hormone
- Stimulates calcium release of bones, and calcium uptake by kidneys
- Ca+ blood levels rise
Stimulus = High blood calcium
- Thyroid gland releases calcitonin
- Calcitonin stimulate calcium storage in bones
- Decreases calcium uptake in kidneys
- Calcium in blood return to normal
AngiogenesisIs when tumors develop there own blood supply
- Tumour cells secrete angiogenic growth factors
- Angiogenic growth factors stimulate endothelial cells lining near by vessels
- Produces blood supply to the tumour
Some tumour require activation by growth factor receptors such as epidermal growth factor receptor (EGFR) to grow and spread throughout the body