2.5- Biological Membranes

roles of cell surface membranes

• Control exchange of material

• Compartmentalisation

• sites of chemical reactions

• cell signalling→ interface for communication between cells

fluid mosaic model of membranes

• fluid:

  • proteins and phospholipids can move around via diffusion

  • phospholipids move sideways within their layers

  • proteins interspersed throughout bilayer and move about with it

  • some proteins may be in a fixed position

• mosaic due to scattered pattern produced by proteins

components of the fluid mosaic model of membranes

• phospholipids

• cholesterol

• glycolipids and glycoproteins

• transport proteins

phospholipids in the cell membrane

• forms phospholipid bilayer

• acts as a barrier to most water-soluble substances→ non-polar hydrophilic tail

  • ensures water soluble molecules cannot leak out of the cell and unwanted water cannot get in

• can act as signalling molecules by:

  • moving within bilayer to activate other molecules

  • being hydrolysed , releasing small water soluble molecules that bind to specific receptors in cytoplasm

cholesterol in the cell membrane

• increases fluidity of cell membrane

• stops phospholipid tails packing too closely together

• at high temps:

  • cholesterol bind to hydrophobic tails of phospholipids, causing them to pack more closely together

• Increases mechanical strength and stability of membranes

glycolipids and glycoproteins in the cell membrane

• contain carbohydrate chains that exist on the surface→ allows them to act as receptor molecules

  • bind with substances at cell’s surface

• some act as cell markers or antigens for cell-to-cell recognition

types of glycolipid/protein receptors

• signalling→hormones, neurotransmitters

• Endocytosis

• Cell adhesion and stabilisation→ carbohydrate forms H-bonds

transport proteins

• create hydrophilic channels to allow ions and polar molecules to travel through the membrane.

• Two types:

  • channel proteins

  • carrier proteins

• specific to a particular ion or molecule

factors effecting cell membrane permeability

• temperature

• solvent concentration

affect of temperature on cell membrane permeability below 0C

• phospholipids have less kinetic energy= membrane is less fluid=very rigid

• proteins become denatured= more permeable

• ice crystals form in the membrane=membrane can fracture=increased permeability

affect of temperature on cell membrane permeability between 0C and 45C

• membranes are fluid→ phospholipids move easily

• semi permeable→ as temp increases, kinetic energy of phospholipids increases=more movement=more gaps in membrane so increased permeability

affect of temperature on membrane permeability above 45C

• bilayer breaks down→ increased E_k=phospholipids move far away from one another

• membrane becomes freely permeable→proteins denature- increased membrane permeability

• membrane may burst→ heat causes water inside the cells to expand- puts pressure on membrane causing

effect of solvents on cell membranes

• increase permeability→ lipids dissolve in alcohol so cell membrane will break down in solvents= cell membrane more fluid and permeable as it breaks down

what substances can and can’t cross phospholipid bilayer

• CAN:

  • lipid soluble/ non-polar

  • small

  • e.g. steroids, oxygen, carbon dioxide

• CAN’T:

  • water soluble/ non polar

  • large molecules

  • e.g. glucose, amino acids

how are proteins embedded in the membrane

• intrinsic:

  • span full plasma membrane→ act to transport water soluble/ charged molecules across the membrane

• extrinsic:

  • located on the surface/ partially embedded- never fully extend across entire cell surface membrane

what is diffusion

• the movement of particles from an area of high concentration to an area of low concentration

• passive process→ energy that particles have comes from their natural inbuilt motion rather than from an external source

factors affecting rate of diffusion

• temp→ higher temp=higher E_k=higher rate of diffusion

• diffusion distance→ longer distance= lower rate

• surface area→ larger surface area= more area over which diffusion can take place

• size of diffusing molecules→ smaller ions= higher rate of diffusion

• concentration gradient→ steeper gradient= higher rate of diffusion

Fick’s Law

 

facilitated diffusion and the role of channel and carrier proteins

• only occurs at specific points on plasma membrane where there are specific protein molecules

channel proteins

• form water filled channels across membrane and allow water soluble molecules to cross e.g. ions

• channels are specific→ will only allow certain molecules to cross- will remain closed if molecule is not there

carrier proteins

• carriers that span the membrane

• when the specific molecule e.g. glucose, is present, it binds to the carrier protein→ carrier changes shape so molecule can cross

water potential

• represented using psi (𝛙)

• measured in kPa

• created by the pressure of water particles

• under standard conditions pure water has a WP of 0kPa

• if solute is added, WP is less than 0→ more solute= more negative

what is osmosis

the movement of water molecules from an area of high water potential to a low water potential through a partially permeable membrane

hypotonic solution

• more water

• less solute

hypertonic solution

• less water

• more solute

• more negative WP

isotonic solution

• same solute concentration and water concentration compared to body fluids

animal cells in hypertonic solution

• water leaves cell through partially permeable membrane by osmosis

• cell will shrink and shrivel up→ crenation

animal cells in hypotonic solution

• water enters cell through partially permeable membrane by osmosis

• cell will continue to gain water until membrane is stretched too far and cell bursts→ cytolysis

animal cells in isotonic solution

movement of water molecules into and out of the cell occurs at the same rate→ no net movement

plant cells in hypertonic solution

• water leaves plant cell through partially permeable membrane by osmosis

• water leaves vacuole of plant cell→ volume of plant cell decreases

• protoplast shrinks and no longer exerts pressure on cell wall→ plasmolysis

plant cells in hypotonic solution

• water enters plant cell through partially permeable membrane by osmosis

• water enters vacuole of plant cell→ volume increases

• expanding protoplast pushes against cell wall and pressure builds up inside cell

• when plant cell is fully inflated with water→ turgid

active transport

movement of molecules and ions through a cell membrane from a region of lower concentration to region of higher concentration using energy from respiration

what does active transport require

• energy→ provided by hydrolysis of ATP

• carrier proteins→ require energy to change shape allowing it to transfer the molecules/ ions across the cell membrane

where is active transport important

• reabsorption of useful molecules and ions into the blood after filtration into kidney tubules

• absorption of some products of digestion from digestive tract

• loading of sugar from photosynthesising cells of leaves into phloem tissue for transport around plant

• loading of inorganic ions from the soil into root hairs

endocytosis and exocytosis

• endocytosis→ bulk transport into cells:

  • segment of plasma membrane surrounds and encloses particle and brings it into the cell, enclosed in a vesicle

• exocytosis→ bulk transport out of cells

  • vesicle containing substances is moved towards and then fuses with plasma membrane

types of endocytosis

• phagocytosis:

  • bulk intake of solid material by cell

  • carried out by phagocytes

  • vacuoles formed are phagocytic vacuoles

• pinocytosis:

  • bulk intake of liquids

  • if the vacuole that is formed is extremely small then the process is called micropinocytosis

exocytosis

• substances packages into secretory vesicles formed from Golgi body

• vesicles travel to cell surface membrane→ fuse with membrane and release contents outside the cell

antiport vs symport

• antiport→ carries two different types of ions in different directions

• symport→ transporting ions in one direction

the sodium potassium pump

• carrier proteins have sites that combine reversibly with only certain solute molecules or ions

• also have regions that binds to ATP, allowing for its hydrolysis to release energy→ helps protein change shape and carries ion from one side of membrane to other

• the sodium potassium pump has a repeating cycle of conformational (shape) changes to transport 3 sodium ions out of and 2 potassium ions into the cell

co transport of glucose (indirect active transport)

1. sodium ions are actively transported out of the epithelial cell, via sodium potassium pump, into the blood→ creates a lower concentration of Na+ in epithelial cell than in the lumen

2. Na+ diffuse into epithelial cells down conc. gradient via facilitated diffusion→ as sodium ions diffuse, they carry either glucose or amino acid molecules with them→ co-transport

3. glucose/AA molecule pass into blood via facilitated diffusion through separate carrier protein→ moving against their concentration gradient