Exchange across membranes

• DIFFUSION= The movement of molecules from a high concentration to a low concentration across a partially permeable membrane down a concentration gradient
• If there is a high concentration of, for example oxygen and carbon dioxide, the molecules with kinetic energy will bump into each other as they randomly move across the membrane.
• More molecules will move down the concentration gradient, until each side of the membrane has reached equilibrium
• MOLECULES THAT USE THIS TRANSPORT: Oxygen, Carbon Dioxide, Fat-soluble molecules e.g steroid hormones
• Oxygen diffusing into the cytoplasm will then diffuse into mitcohondria to use for aerobic respiration, and carbon dioxide diffusing into the palisade mesophyll cells then diffuse into chloroplasts to use for photosynthesis
• FACTORS AFFECTING RATE OF DIFFUSION: Temperature, Diffusion distance, Nature of molecules, Concentration gradient, Surface Area

• FACILITATED DIFFUSION= The movement of molecules from a high concentration to a low concentration across a partially permeable membrane down a concentration gradient using channels and carrier proteins
• Ions that have electrical charge are insoluble in lipid because they are hydrophilic, as opposed to the hydrophobic tails of bilayer
• Ions with electrical charge need to pass through pores embedded in the membrane (0.8nm in diameter)
• Cholesterol molecules reduce the permeability of the membranes to water-soluble molecules
• There are special carrier proteins for different types of molecule including glucose: neurone plasma membranes have many channels specific to either sodium or potassium ions; membranes of epithelial cells that line the airways have chloride ion channels

• OSMOSIS= The net movement of water molecules from a high water potential to a low water potential across a aprtially permeable membrane down a potential gradient
• Water molecules diffuse directly through the phopholipid bilayer
• Higher water potential outside the cell than inside- water molecules will difuse to the region with a lower water potential (vice versa)
• HYPOTONIC= Greater water potential in solution (dilute)
• HYPERTONIC=Lower water potential in solution (concentrated)
• ISOTONIC= Equal water potential in solution and cell (equilibrium)
• Pure water has the highest water potential; most negative= lower water potential
• Measured in kPa
• Animal cell- lots of water- swell and burst (CYTOLYSIS)
  
• Animal cell- too little water- shrivel (CRENATED)
• Plant cell- water enters the cell- membrane expands- pressure on the cell wall- cell wall adds pressure against the membrane=turgid
• Plant cell- water leaves the cell- membrane pulls away from the cell wall- plasmolysed and flaccid
• The membrane in the plant cell may become torn, which will cause the cell to die

• ACTIVE TRANSPORT= The movement of molecules from a low concentration to a high concentration across a partially permeable membrane against their concentration gradient using energy from ATP
• Energy is provided by the hydrolysis of ATP
• Molecules will need more energy than the membrane as it travels against the concentration gradient
• Carrier proteins act as enzymes- they have a region that binds to and allows the hydrolysis of a molecule of ATP to release energy. This helps to change its shape to carry the ion from one side to another

• TEMPERATURE AND KINETIC ENERGY - Increased tmperature=more kinetic energy=molecules move faster
• PHOPHOLIPIDS AND CHANGING TEMPERATURE - Temperature varies with enivronment
• When temperature drops= unsaturated fatty acids making up bilayer become compressed, pushing phopholipid molecules away, maintaining membrane fluidity
• Cholesterol bufferes the effect of lowered temperature, to prevent reduction in fluidity= prevents phophlolipid molecules being packed too closely.
• When temperature increases= phopholipids gain more kinetic energy and moe around more= permeability increases
• Increase in temperature may change ability of cell signalling
• Presence of cholesterol molecules buffers the effects of increasing heat as it reduces the membrane fluidity
• PROTEINS AND TEMPERATURE - High temperatures cause larger molecule atoms to vibrate, breaking hydrogen and ionic bonds; proteins denature; plasma membrane begins to fall apart=more permeable 
• Solvents e.g ethanol and detergents damage cell membranes as they dissolve lipids

• Partially permeable membrane
• Involved in cell signalling
• Site for chemical reactions e.g proteinsynthesis, respiration
• Communication with external environment
• Contain cell organelles and compartmentalise individual processes
• Allow cell to change shape
• Transport and packaging within a cell

PROTEINSYNTHESIS: (follow as chain of events)

DNA- amino acids=mRNA- translated to tRNA- released into cytoplasm and into ribosomes- attached to RER- Golgi body- packaged and processed by vesicles- fuses with full outer cell

• Organelles with own membrane: Nucleus; Ribosomes; Mitochondria; Vacuole (aka tonoplast); RER; Lysosomes; Chloroplasts; Golgi body; SER; Vesicles
• SER is involved in lipid synthesis, and RER is involved in protein synthesis.
• Lysosomes contain digestive enzymes and membrane acts as a barrier; optimum pH=4.8

Non-polar molecules:

• Oxygen, Carbon dioxide, nitrogen, ethane
• Neither side is slightly negative
• Electrons are equally shared
• Water insoluble- HYDRPHOBIC
• Pass through the membrane via simple diffusion

Polar molecules:

• Water (but very small), Glucose, Ions with electrical charge
• One part of molecule is slightly negative than the other
• Electrons are not equally shared
• Water soluble- HYDROPHILIC
• Large molecules use carrier proteins
• Oxygen=very small; simple diffusion
• Carbon dioxide=small; passes out= diffusion / into membrane=photosynthesis
• Water= very polar; very small; osmosis
• Ions= polar and have charge; transported across cell membranes- protein channels and carrier proteins

• Important in multicellular organisms for communication
• Need to signal: COORDINATION, CELL DIVISION, STIMULUS + RESPONSE, GROWTH + DEVELOPMENT
• Proteins in membrane=receptors- they have sensors for detecting signals and changes in environment
• Glycocalyx- for cell recognition so cells group together to form tissues
• Integral proteins are TRANSMEMBRANE proteins

• NEUROTRANSMITTERS = released by nerve endings of synapses/muscle junction; transmitter diffuses across gap and binds with receptors on motor neurone; nerve impulse is generated or muscle contracts- water soluble
• HORMONES = Chemical messengers made by endocrine glands; released from glands directly into bloodstream; hormones diffuse from blood into target cell and either bind with target/pass through membrane- steroid hormones= insoluble, but peptide hormones=soluble
• LOCAL CELL MEDIATORS = Influence cells only in immediate environment; either taken up by cells rapidly/destroyed rapidly
• Water soluble signalling molecules bind to surface receptors; release intracellular/second messenger= CYCLIC AMP

GLYCOPROTEINS:

• Form hydrogen bonds with water molecules surrounding the cell, which help to stabilise membrane structure
• Used as receptor molecules, binding with hormones and neurotransmitters to trigger chemical reactions 
• Act as chemical pathways that allow ions and molecules to move into and out of a cell
• Act as antigens as they are involed in cell signalling that they are 'self'; foreign particles will be recognised as 'non-self' so they will attack with phagoctyes or antibodies

GLYCOLIPIDS:

• Same as bulletpoint 1 of glycoproteins
• Same as bulletpoint 2 of glycoproteins
• Same as bulletpoint 4 of glycoproteins

ROLE OF DIFFERENT PROTEINS:

• CARRIER PROTEINS= Transport molecules and ions across cell membrane via active transport
• HORMONES= Receptors for molecules in cell signalling

Bulk Transport:

• Some cells need to transport large molecules that are too large to pass through the membrane in or out. Bulk transport requires energy from ATP
• ENDOCYTOSIS= Bulk transport of molecules, too large to pass through the membrane, into the cell
• Segment of the membrane surounds and engulfs the particle to bring into the cell= PHAGOCYTOSIS e.g amoeba
• Cells ingest liquids by endocytosis= PINOCYTOSIS
• EXOCYTOSIS= Bulk transport of molecules, too large to pass through the membrane, out of the cell
• Vesicles are moved towards and then fused with the cell membrane; the fused site opens to released secretory vesicle contents; ATP needed to fuse membranes together as well as move vesicles