BIOL125 Lecture Notes - Lecture 5: Electron Transport Chain, Nuclear Membrane, Membrane Transport Protein
8 Jun 2018
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Cells
• 2.1. What is a cell?
o Basic unit of life
• Smallest unit of life
• Self-sustaining
• Self-replicating
o Outer boundary of cell is a thin, photophilic bilayer membrane
o Body cells contain fluid (cytoplasm) and surrounded by extracellular fluid (interstitial
fluid, blood, lymph)
o Cytoplasm - cell organelles and structures in cytosol
• 2.2. The cell membrane
o 2.2.1. its structure
• Phospholipid bilayer
• Hydrophilic head
• Hydrophobic tail
• 6-10nm thick
• Fluid mosaic model - has fluidity and isn't going to snap or break
• Proteins interspersed in the membrane
• Some stick out and others act as channels
• Specific molecules will only go through their specific channels
• Cholesterol allows fluidity - membrane can distort slightly without breaking
• Lipid soluble molecules and small uncharged molecules easily move in and out of
the cell
• Integral/transmembrane proteins span bilayer; peripheral proteins attached to
surface
• Glycolipids and glycoproteins
• Act as enzymes, receptors, self-recognition molecules (MHC proteins)
• Lubricate and protect membrane surface
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o 2.2.2. its functions
• Physically isolates cell from external environment
• Conditions inside the cell different to outside
• Conditions must be maintained for homeostasis
• Cell must retain enzymes and structural proteins
• Regulates exchange with environment (selectively permeable)
• Controls ion entry and exit through channels
• Absorbs nutrients and releases wastes
• Sensitivity to environment
• Membrane receptors respond to specific molecules, e.g. hormones
• MHC proteins tell immune cells this cells is "self" not "foreign"
• Structural support
• Holds tissues together via connections between cell membranes e.g. tight
junctions in epithelial cells: or cell membranes and extracellular matrix in
dermis, connect with hypodermis
• 2.2.3. Different methods of transport across the cell membrane, with examples:
o 2.2.3.1. diffusion and osmosis
• Diffusion
• Net movement from areas of high concentration across a membrane to
areas of low concentration
• Molecules move (have kinetic energy) so bump into and bounce off other
molecules - even distribution of solutes through solution
• Most effective over short distances, with small molecules, and with a higher
temperature
• Osmosis
• Diffusion by water molecules across a membrane through water channels
(aquaporins), to the side with the higher concentration of dissolved solutes
• Solutes diffuse in an opposite direction to water
• Osmotic pressure and hydrostatic pressure determine which way water
moves
• Hydrostatic pressure opposes osmotic pressure
• Osmotic pressure - force with which water moves into a solution due
to its dissolved solute concentration
• Hydrostatic pressure - force generated by pushing against a fluid,
created by heart
o 2.2.3.2. carrier mediated: facilitated diffusion, active transport, secondary active
transport
• Requires specialised integral/transmembrane proteins
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• Passive or active process depending on substance transported, and the nature of
transport mechanism
• Integral proteins bind to specific ions or organic molecules, and transport them
across the membrane
• Receptor for an integral protein is typically a glycoprotein
• All carrier-mediated transport mechanisms have 3 things in common:
• Specificity - each integral protein only binds and transports certain
molecules e.g. glucose transporter will not transport other sugars
• Saturation limit - when all relevant carrier proteins are utilised, they are
saturated
• Regulation - binding of other molecules such as hormones can affect activity
• In cotransport/symport two molecules moved the same way simultaneously
• In countertransport/antiport the two molecules are moved in opposite ways
simultaneously
• Facilitate Diffusion
• Some important nutrients e.g. glucose and amino acids are too large to
diffuse and not lipid soluble
• Passively transported across membrane by a carrier protein
• Molecule to be transported binds to receptor site on carrier protein,
which now changes shape and the molecule moves across
• Molecules moved down concentration gradient until carrier protein
receptors are saturated
• Active transport
• ATP needed to move molecules across membrane.
• Requires cellular energy.
• Cell can import or export molecules regardless of their concentration
gradient.
• All cell membranes have ion pumps (carrier proteins) to actively transport
Na+, K+, Ca2+ and Mg2+ ions across the membrane
• Specialised cell membranes also transport Cl-, I- and Fe3+ ions.
• May move one ion in one direction only, or two ions at once.
• If an ion inside is swapped for an ion outside, it is an exchange pump e.g.
Sodium-potassium pump.
• Na+ at higher concentration in ISF than in cytoplasm - Na+ ions
constantly diffuse into cell
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Document Summary
What is a cell: basic unit of life. Self-replicating: outer boundary of cell is a thin, photophilic bilayer membrane, body cells contain fluid (cytoplasm) and surrounded by extracellular fluid (interstitial fluid, blood, lymph, cytoplasm - cell organelles and structures in cytosol, 2. 2. The cell membrane: 2. 2. 1. its structure, phospholipid bilayer, hydrophilic head, hydrophobic tail, 6-10nm thick, proteins interspersed in the membrane. Fluid mosaic model - has fluidity and isn"t going to snap or break. Some stick out and others act as channels. Specific molecules will only go through their specific channels: cholesterol allows fluidity - membrane can distort slightly without breaking. Lipid soluble molecules and small uncharged molecules easily move in and out of the cell. Integral/transmembrane proteins span bilayer; peripheral proteins attached to surface: glycolipids and glycoproteins, act as enzymes, receptors, self-recognition molecules (mhc proteins) Specificity - each integral protein only binds and transports certain molecules e. g. glucose transporter will not transport other sugars.
