2.1.5

• plasma membranes create a barrier separating cytoplasm and environment and control movement in/ out via osmosis, diffusion or active transport as they are partially permeable. they also have cell signalling to recognise other cells or foreign cells
• membranes within cells compartmentalise for transport, hold necessary substances for chemical reactions as well as being the site of a chemical reaction and are partially permeable
• fluid mosaic model:

  A) glycolipid

  B) glycoprotein

  C) cholesterol

  D) channel protein

  E) hydrophilic

  F) hydrophobic

  G) carrier protein

  
• phospholipid function: forms bilayer with hydrophilic outside and hydrophobic inside, allows in small, lipid soluble molecules
• cholesterol function: adds to internal hydrophobic barrier of phospholipid bilayer, restricts phospholipid movement and increases thermal stability
• channel protein function: ion or water soluble molecule transport through pores
• carrier protein function: binds to and changes shape of large molecules allowing them to pass through to the other side
• glycolipids: act as receptors for cell signalling and as binding sites, forms hydrogen bonds to strengthen the membrane
• glycoproteins: act as receptors for cell signalling and binding sites, form hydrogen bonds to strengthen membrane and help cells join together to form a tissue
• intrinsic protein: embedded into the membrane. its place is determined by the hydrophobic or hydrophilic region, for example protein channels
• extrinsic proteins: loosely bound to the exterior of the cell membrane by weak hydrophilic interactions with the phosphate heads, for example enzymes for inside cell reactions
• solvent: a liquid that can dissolve other substances
• solvents dissolve the lipids in a membrane causing it to loose its structure and become more permeable. it depends on the solvent concentration or the type of solvent
• the affect of temperature on cell structure and permeability

  1 - proteins are denatured becoming unselective, ice crystals form fracturing the phospholipid bilayer which continues during thawing

  2 - the optimum temperature for protein function, the phospholipids have enough KE to act as an fluid

  3 - water expands creating pressure, proteins are denatured and non longer selective, there is too much KE so gaps form in the fluid

  
• Cells communicate via cell signalling by releasing messenger molecules
• Proteins on the cell membrane act as receptors they are called membrane - bound receptors
• The receptor protein has a specific shape so it is only complimentary to the receptor molecule to bind
• A cell that responds to a particular messenger molecule is called a target cell
• Osmosis: the diffusion of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane
• Water potential is the likelihood of water molecules to diffuse in or out a solution
• Pure water has the highest water potential of zero
• Water potential is shown by ψ
• Adding solutes to a solution lowers the water potential
• The more negative the water potential the more concentrated a solute
• Isotonic solution: A solution that has the same water potential as the cell in it. There is no net movement of water
• Your body wants your blood to be an isotonic solution
• Crenation is the dehydration of red blood cells causing them to shrivel
• Lysis - the bursting of an animal cell
• Hypotonic solution: when the solution has a high water potential than the cell in it causing the water to move into the cell. animal cells can burst. Plant cells become turgid
• Hypertonic solution : when a solution has a lower water potential than the cell within it causing water to move out the cell. cells will become flaccid
• Plasmolysed: when a plant cells membrane pulls away from its cell wall due to the shrinking of the vacuole
• Rate of movement in molecules is faster than in a real cell than a model cell as real cells use vesicles and the cytoskeleton to move fluid
• Facilitated diffusion is passive, uses carrier or channel proteins, for large molecules or ions, down concentration gradient
• Carrier protein example: Glut 1 for glucose
• Active transport : requires energy, against concentration gradient, low to high, ions
• Channel proteins only accept specific shapes to enter
• Active transport is found in root hair cells with mineral ions
• Endocytosis: entering the cell, requires energy and a vesicle
• Exocytosis: leaving a cell requires ATP and vesicle
• as temperature increases, cell membrane permeability increases causing percentage absorbance of light to increase