TL - IB HL BIO YR 1 UNIT FOUR

CELL MEMBRANE

• Cell Membrane

  • Fluid Mosaic Model: proposed by singer and nicolson (1972), “fluid” - bilayer = viscous, move around and flow past each other, “mosaic” - embedded with proteins, mosaic of components

• Phospholipid: amphipathic, form bilayer, major structural component of almost all membranes

  • Hydrophilic Head: hydrophilic, on either side of bilayer

  • Hydrophobic Tails: hydrophobic, on inside of bilayer, make bilayer selectively permeable

  • Amphipathic: both hydrophilic and hydrophobic

• Phospholipid Bilayer: heads on outside, tails on inside, almost all membranes

• Selective Permeability: only certain substances can cross bilayer without assistance, polarity/size/charge of substance determines permeability

  • Permeable: small non-polar molecules, “lipid soluble” molecules, ex: O2, CO2, N2, steriods

  • Mostly Permeable: small uncharged polar molecules, ex: H2O and glycerol

  • Mostly Impermeable: large uncharged polar molecules, ex: glucose, sucrose, other mono/disaccharides

  • Impermeable: ions and anything with a charge, ex: Na+, K+, Cl-, H+, Ca2+

• Membrane proteins: channel and carrier proteins, proteins embedded in membrane

  • Integral: membrane protein, embedded in bilayer, amphipathic , mostly transmembrane, difficult to isolate

    • Transmembrane: spans across entire membrane, parts outside cell, inside bilayer, and inside cell

    • Amphipathic: both hydrophilic and hydrophobic

  • Peripheral: membrane protein, located on surface of bilayer (either side), hydrophilic, easier to isolate

    • Hydrophilic: water-loving, attracted to water, polar

  • Functions of membrane proteins

    • Transport: transport substances across membrane

    • Receptors: generate different intracellular signals

    • Enzymes: can be embedded in/associated with bilayer

    • Recognition: different cells have different membrane proteins

    • Adhesion: cells with same tissue are joined by membrane proteins

  • Glycolipids and Glycoproteins

    • Extracellular Side: outer part of membrane

    • Function of glycolipids and glycoproteins

      • Recognition: carb chains = ID tags

      • Adhesion: carb chains bind to chains on other cells

      • Cell Signaling: receptor for chemical messengers

      • Cell protection: carb creates glycocalyx

  • Glycoprotein = conjugated protein: carb chain (oligosaccharide) covalently bonded to protein, protein with non-protein part

• Cholesterol: found in animal cell membranes

  • Steroid

  • Amphipathic: both hydrophilic and hydrophobic

  • Impact on membrane fluidity: prevents fatty acid chains from sticking together and freezing at low temperatures by increasing fluidity, stabilizes membrane and decreases fluidity at high temperatures

  • Location within the phospholipid bilayer: in between phospholipid heads

• Membrane fluidity: depends on the fatty acid composition of the phospholipids

  • Fatty acid length: longer fatty acids = lower fluidity & more rigid

  • Fatty acid saturation: saturated fatty acids provide stability (because of higher melting points)

  • Cholesterol: depends on cholesterol in animal cells (at high temperatures cholesterol reduces fluidity)

  • Temperature: lower temperatures = lower fluidity

• Skill: Draw & label a diagram of the cell membrane

MEMBRANE TRANSPORT

• Passive transport: does not require additional input of energy (ATP), ex: simple diffusion, facilitated diffusion, osmosis

  • Dynamic equilibrium: no net movement, equal concentration on both sides, constant movement back and forth, permeable & some mostly permeable molecules

  • Factors impacting diffusion rate

    • Concentration gradient: passive transport goes down gradient

    • Distance: greater distance = slower rate

    • Temperature: higher temperatures = faster rate

    • Size: smaller molecule = faster rate

    • Polarity: nonpolar molecules can diffuse into the cell membrane but polar ones cannot

  • Simple diffusion: movement of molecules of a substance down a concentration gradient, spontaneous, movement of molecules eventually results in equal concentration among both regions, passive transport

    • Concentration gradient: molecules move down gradient

    • Selective permeability - types of materials: permeable molecules

  • Facilitated diffusion: movement of molecules down gradient, passive transport, requires help from transport transmembrane integral proteins (channel or carrier protein)

    • Transmembrane integral protein: spans across entire membrane, parts outside cell, inside bilayer, and inside cell

      • Channel protein: transmembrane proteins, assemble to form channels for the passage of polar molecules, selectively permeable because of hydrophilic/phobic side chains inside channel, can be opened or closed (gated), many only allow one type of ion/molecule through, ex: ion channels

      • Carrier protein: transmembrane proteins, play important role in facilitated diffusion, bind to solute molecules, undergoes a conformational change when molecule binds to it, transfers molecules to other side of membrane, ex: glucose transporter

    • Concentration gradient: down the gradient

    • Selective permeability - types of materials: all permeability levels

    • Control of facilitated diffusion

      • Specificity: only certain molecules can travel through

      • Gated: proteins can be opened or closed

  • Osmosis: diffusion of water across a selectively permeable membrane, net movement of water into cell, water always moves from higher water concentration to lower and continues until concentration is same on both sides, type of diffusion, passive transport

    • Water: only thing moved by osmosis

    • Solution: solute + solvent

      • Solute: what is being dissolved (ex: salt, sugar, etc)

      • Solvent: what is doing the dissolving, almost always water

    • Concentration gradient: higher to lower solute concentration

    • Aquaporin: type of channel/integral protein, specific to H2O, bidirectional, permit rapid movement of water in and out of cells

    • Tonicity/Osmolarity

      • Hypertonic: more solute concentration in this solution than other solution, water flows in, other solution is hypotonic to this solution

      • Hypotonic: less solute concentration in this solution than other solution, water flows out, other solution is hypertonic to this solution

      • Isotonic: no net movement of water, dynamic equilibrium, both solutions have the same solute concentration

    • Animal Cells

      • Normal: isotonic solution

      • Crenated: hypertonic solution, shrink/shrivel

      • Lysed: hypotonic solution, swell/burst

    • Plant Cells

      • Turgid: hypotonic, normal

        • Turgor Pressure: pressure that is exerted by the fluid (e.g. water) against the cell wall

      • Flaccid: isotonic, wilted

      • Plasmolyze: hypertonic, membrane shrivels away from cell wall

      • Contractile Vacuole: collect and expell H2O to maintain internal osmoic balance (homeostasis)

      • Medical applications of osmosis

        • IV Fluids: must be isotonic to human body cells so that cells dont lyse or crenate

        • Organ transplant Fluids: must be isotonic to human body cells so that cells dont lyse or crenate

• Active transport

  • Pump Proteins

    • Transmembrane integral protein: moves molecules across phospholipid bilayer

    • ATP: needs ATP to move molecules

    • Concentration gradient: low to high concentration, “against the gradient”

    • Examples:

      • Proton Pump

      • Na⁺/K⁺ Pump

  • Bulk Transport

    • Exocytosis: bulk transport of material to be secreted/excreted out of cell via vesicles, vesicles fuse with cell membrane & contents are discharged to extracellular space, exo = exit

      • Secretory Vesicle: store and transport molecules (hormones, neurotransmitters, digestive enzymes) to be secreted outside the cell

      • Examples:

        • Secretion of glycolipids and neurotransmitters

        • Excretion of wastes

    • Endocytosis: bulk transport mechanism by which particles are moved into the cell, cell membrane progressively invaginates and engulfs particles, pinches off to form vesicle

      • Phagocytosis: ingestion of large solid particles, cellular eating, ex: white blood cells

        • Phagosome/Food vacuole: vacuole containing large solid particle to be phagocytized

      • Pinocytosis: ingestion of liquids, smaller vesicles than phagocytosis, cellular drinking

CELL SPECIALIZATION

• Prokaryote

  • Nucleoid: DNA-containing area of a prokaryotic cell

• Eukaryote

  • Compartmentalization: organization or different functions and processes within specific areas or structures within the cell that are separated by plasma membranes

    • Benefits of compartmentalization: allows for the development of specialized cell structures (chloroplasts and mitochondria), allows specific reactions to occur in specific places

• Cell Structures

  • Cell Wall: structure: external outer covering made of cellulose, function: provides support and mechanical strength, prevents excess water uptake, types of cells: bacterium and plant, not considered organelle

  • Cytoskeleton: structure: filamentous scaffolding within the cytoplasm, function: provides internal structure and mediates intracellular transport, types of cells: bacterium, plants, animals, not considered organelle

  • Cytoplasm/Cytosol: structure: mostly water, but full of enzymes and other molecules, function: hold organelles in place and catalyses reactions (like glycosis), types of cells: bacterium, plants, animals

• Organelles (Not bound by a membrane)

  • Ribosomes (70s v. 80s): structures that make proteins out of mRNA from nucleus, not membrane-bound but are considered organelles because they have a specific function, 70S in prokaryotes, 80S in eukaryotes

    • Bound ribosomes: attached to the rough endoplamic reticulum, what make it “rough”, more numerous

    • Free ribosomes: located in cytoplasm, less numerous

• Membrane bound organelles

  • Cell Membrane/Plasma Membrane: structure: phospholipid bilayer embedded with proteins, function: semi-permeable and selective barrier surrounding the cell, types of cells: bacterium, plants, and animals

  • Nucleus: stores genetic material (DNA) and chromatin, protects DNA from damage

    • Nucleolus: dense, darker region in the nucleus, site of ribosome synthesis, plant and animal cells

    • Nuclear envelope/membrane: double membrane that surrounds nucleus, controls movement in and out of nucleus, plant and animal cells

    • Nuclear pore: allow entry and exit of molecules into nucleus, integral proteins, serve as channel proteins that also regulate mRNA leaving nucleus for RER or free ribosomes

    • Double membrane: regulates gene expression, protects DNA

  • Vesicle: small membrane-bound cell structures, play a key role in transport and storage of protein and other products , act as delivery trucks, transport and release proteins, lipids, and RNA from one part of cell to another, plant and animal cells

    • Transport vesicles: transport materials from one part of a cell to another, intracellular

    • Secretory vesicles: store and transport molecules (hormones, neurotransmitters, digestive enzymes) to be secreted outside the cell

    • Lysosomes: membranous sacs filled with hydrolytic enzymes, breakdown/hydrolysis of macromolecules and waste, animal cells, requires enzymes

      • Hydrolytic enzymes: what fills up lysosomes, break down protein, lipids, nucleic acids, carbohydrate and fat molecules into their simplest units

    • Peroxisomes: membranous sac containing a variety of catabolic enzymes, catalyzes breakdown of toxic substances (eg H2O2) and other metabolites, plant and animal cells, similar to lysosomes but have a different set of enzymes that are involved in detoxification of harmful compounds and lipid metabolism

    • Clathrin coated vesicles (CCVs): vesicles made by clathrin, clathrin acts as a scaffold and protects vesicle

    • Clathrin coated pit (CCPs): forms as membrane begins to invaginate, not full vesicle yet, clathrin acts as scaffold and protects pit (which will later become vesicle)

  • Endoplasmic reticulum

    • Rough ER: folded membrane studded with bound ribosomes, forms series of flattened sacs and tubes, protein synthesis and modification for export, plant and animal cells

      • Bound Ribosomes: ribosome bound to cytosolic side of RER, proteins it produces ends up inside RER and will be exported outside of cell, tend to be more numerous

    • Smooth ER: folded membrane continuous with the nuclear envelope, production of lipids and metabolism of toxins, plant and animal cells

  • Mitochondria: site of aerobic respiration, produce ATP through the breakdown of molecules (glucose), plant and animal cells, contains transport proteins that assist in moving larger molecules to mitochondria

    • Double membrane: inner membrane highly folded into internal cristae, outer membrane is permeable to many small molecules and ions

  • Chloroplast: site of photosynthesis, manufactured organic molecules are stored in various plastids, plant cells

    • Double membrane: helps with compartmentalization and protection

  • Golgi: an assembly of vesicles and folded membranes located near the cell membrane, involved in the sorting, storing, modification, and export of secretory products(exocytosis), plant and animal cells, organized into cis, medial and trans compartments

  • Cell Size: most cells are small because small cells have high SA:V and are more efficient

    • SA:V Ratio: must be large in order for cell to be efficient, high SA:V = high efficiency, SA:V increases if SA increases or V decreases, high SA:V = shorter diffusion pathway because there is a shorter distance to the membrane