Biological Membranes

Cell membranes

• All cell membranes are composed of a phospholipid bilayer

• Partially permeable and the sight of chemical reactions - involved in cell communication

• Control the exchange of materials

• Seperate the internal and external environments

• Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport

Fluid mosaic model

• The mixture of movement of phospholipids, proteins, glycolipids, glycoprotein the membrane is composed of

• Very fluid - phospholipid molecules can move

• Mosaic - proteins scattered vary in shape and size

What does the model help explain

• Passive and active movement between cells and their surroundings

• Cell-to-cell interactions

• Cell signalling

Phospholipid role in model

• Align as a bilayer ensuring sugars, amino acids and proteins cannot leak out (due to hydrophobic heads and hydrophilic tails)

• Stops unwanted molecules getting in

• Composed of two sheet - like layers

• About 7nm width

Cholesterol role in model

• restricts lateral movement of other molecules in the membrane.

• Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together.

• Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)

• Increases mechanical strength and stability so cell does not burst

Glycolipids and Glycoproteins role in model?

• They contain very hydrophilic carbohydrate chains on the surface of proteins and lipids - attract water with dissolved solutes

• They act as receptor molecule for hormone's and drugs

• Signalling receptors, endocytosis receptors, cell adhesion receptors (C - H20 H bond)

• Act as antigens, for cell recognition of self and non self cells

• Hydrogen bonds can form between water molecules and glycoproteins - thus maintaining cell membrane stability

Transport proteins

• Channel and carrier proteins - INTEGRAL proteins (completely penetrate layer)

• Channel - Create hydrophilic channels with wayer to allow water soluble polar molecules to facilitated diffusion through membrane

• Carrier - bind with other ions and large molecules such as glucose and amino acids, change shape to transport them specific shape for particular molecule

• Other proteins may be attached to the carrier proteins and function as enzymes, antigens or

  receptor sites PERIPHERAL

Membranes in neurons

• Neurons have a myelin sheath formed by flattened cells giving several layers of membrane

Membranes in white blood cells

• The plasma membranes of white blood cells contain special protein receptors that enable them

  to recognise the antigens on foreign cells

Membranes in root hair cells

• Root hair cells in plants have many carrier proteins to actively transport nitrate ions from the soil into the cells.

Membrane of mitochondria

• The inner membranes of mitochondria are mostly protein

• Inner membranes contain many protein - electron carriers and hydrogen ion channels associated with ATP synthase enzymes.

Factors affecting membrane structure and permeability

• High temp increases kinetic energy of phospholipids - increase fluidity - larger gaps - more permeable easier for particles to cross

• High temp can denature channel and carrier proteins - wider - more molecules can move across membrane

• Solvents - dissolve lipids - this damage increase fluidity and permeability

Simple diffusion

• Net movement of high conc to low conc until equilibrium

• Must be lipids soluble and small - through bilayer until no gradient

Facillitated diffusion

• Move through embedded and carrier proteins and channel proteins as too big or not lipid soluble

• Movement of ions and and polar molecules

Isotonic

• Same water potential in solution and in cell - no net movement of water

Hypotonic

Positive WP in solution - more water in solution than solute - water moves from solution to cell causing it swell and sometimes burst

Hypertonic

Water potential is more negative in solution than cell so moves out of cell making it - shrivelled, animal cells (cremated) plant cells (plasmlysed)

Why does adding solute make water molecules less free

• Adding solute lowers the water potential as solute molecules associate with some of the water molecules.

• The more solute the more negative the Water potential

• water always moves from a less negative to more negative water potential, meaning it has to move less

Active transport

• Requires ATP - energy and carrier proteins

• Selective process - only certain molecules are complementary

• ATP binds to protein and is hydrolysed to ADP and Pi - causing protein to change shape and open up to molecule on other side

• Pi molecule is then released and protein reverts

• Faster than diffusion

Where does active transport take place?

• Absorption of amino acids from the gut

• Absorption of mineral ions by the plant root

• Excretion of urea and hydrogen ions by the kidney

• Loading of sucrose from the leaf into the phloem of plants

• Exchange of sodium out and potassium ions in nerve cells

Similarities and differences between of carrier proteins in active and passive movement

Similarities

• In  active and passive movement  carrier

  Proteins and integrated inside the cell.

• Both selective

• Change shape to allow passage of substances

  Carries large molecules and charged ions

Differences

• Passive goes down diffusion gradient,

  No energy or ATP used

• Active Active transport

  Against concentration gradient

  Requires energy from ATP

Endocytosis

• Bulk movement of materials into a cell by active means

• Membrane folds around material outside ---> formation of a sac- like vesicle containing external material

• Brings external material into cell

• Requires ATP and the cytoskeleton

Exoctyisis

• Bulk movement of materials into a cell by active means

• Vesicle moves through cytoplasm to membrane

• Fuses with membrane

• releases internal contents outside cell

• Requires ATP and the cytoskeleton

Phagocytosis

• Transport of solid

Pinocytosis

• transport of liquid