2.1.5 biological membranes

what are biological membranes

-all cell and organelle membranes are composed of a phospholipid bilayer

-provide a partially permeable membrane, are the site of chemical reactions and have a role in cell communication (at surface)

-they compartmentalise fro different conditions, separate nucleus, protects cell form contents of lysosomes (within cell)

fluid mosaic model

-it's the mixture and movement of molecules like phospholipids, proteins (can be extrinsic or intrinsic), glycoproteins, glycolipids that the membrane is made of

-phospholipids align as a bilayer due to hydrophilic heads being attracted to water, FACE OUTWARDS and hydrophobic tails being repelled by water

-fluid as components can move around within it

-7-9nm thick

role of channel proteins

-they form tubes filled with water than enable water soluble ions, that CANT diffuse through phospholipid bilayer, to diffuse through 

-open in centre, can be gated

role of carrier proteins

-they bind to other ions and larger molecules, like glucose and amino acids, and change shape to transport them to other side of membrane 

role of cholesterol

-it's a lipid, regulates membrane fluidity

-present in some membranes and restricts lateral movement of other molecules in the membrane 

-it's useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions leaking out of the cell

-makes membrane fluid when temperature gets too low

-binds hydrophobic tails

How does temperature affect membrane structure and permeability?

-high temperatures increase kinetic energy of phospholipids, making them move more

-this increases fluidity of membrane , increasing permeability so structure starts to break

-meaning its easier for particles to cross membrane

-high temperatures can denature carrier and channel proteins 

How do solvents affect membrane structure and permeability?

-organic solvents, like alcohol, can dissolve phospholipid bilayer 

-organic solvents are less polar than water
-disrupt cell

-this damage causes the fluidity and permeability of the membrane to increase 

What are the modes of transport in and out of cells?

-simple diffusion

-facilitated diffusion

-osmosis

-active transport

-endocytosis

-exocytoais

simple diffusion

-the net movement of molecules from an area higher concentration to an area of lower concentration, down a concentration gradient, until equilibrium is reached. 
-no ATP required

-molecules must be small and lipid soluble and non-polar to diffuse across a membrane 

-e.g. O2 and CO2

fascilitated diffussion

-a passive process, down a concentration gradient, through proteins 

-the movement of ions and polar molecules can be transported across a membrane by channel and carrier proteins 

osmosis

-the movement of water from an area of higher water potential to an area of lower water potential across a PARTIALLY PERMEABLE membrane 

active transport definition

-the movement of molecules and ions from an area of lower concentration to an area of higher concentration, against the  concentration gradient, using ATP and carrier proteins

-this is selective, only certain molecules can bind to the receptor site on carrier proteins 

endocytosis

-active transport/bulk transport of molecules into cells

-cell surface membrane bends inwards to surround molecule, forming a vesicle

-the vesicle pinches off and moves within the cytoplasm

-endocytosis can be classed as phagocytosis, when a solid moves in, or pinocytosis, when a liquid particle moves in

-this requires energy from ATP for cell to engulf and change shape around material

exocytosis

-active transport/bulk transport of molecules out of cell

-vesicles moves towards cell surface membrane along cytoskeleton (requires energy), fuses with membrane and the content of the vesicle is released outside of cell

-requires energy from ATP for vesicle to move along cytoskeleton

What are the role of membranes within and at the surface of cells?

-site for biochemical reactions

-site of cell communication/cell signalling

-isolating organelles from cytoplasm (compartmentalisation)

-allows cell to change shape

-cover surfaces of cell and surround organelles

-a partially permeable barrier

-forms vesicles

what makes phospholipids amphipathic

-polar phosphate head, hydrophilic

-non-polar fatty acid tail, hydrophobic

what makes cholesterol amphipathic

-hydroxyl group makes head polar and attracted to phospahte head on phospholipid

-non-polar tail attracted to fatty acid tail

-it has carbon rings, meaning cholesterol is a steroid

role of phospholipids

-outer surface interacts with water

-barrier to large polar molecules

-allows entry of small non-polar molcules 

role of intrinsic/integral proteins

-carrier /channels for ions, sugars, amino acids that can't diffuse across a membrane (polar molecules)

role of extrinsic/peripheral proteins

-found on extracellular side of membrane

-act as receptors in cell recognition (membrane-bound receptors), sites where complementary hormones and drugs can bind

-some are glycoproteins/glycolipids

role of glycoproteins

-extrinsic proteins with attached carbohydrate chains

-involved in cell adhesion and act as receptors for chemical signals, elicits response in cell

-involved in cell signalling and recognition

role of glycolipids

-lipids with attached carbohydrate chains

-they are called antigens, can be recognised by cell's immune system as self/non-self

G-coupled receptor proteins

- neurotransmitter/hormone binds to extracellular domain

-G-protein releases

ligand-gated channel proteins

-as ligand/neurotransmitter binds to receptor, a gate opens so ion can move into cell

cotransporter proteins

-such as those moving sucrose into companion cell

importance of membrane fluidity for cell

-diffusion of substances across a membrane 

-membranes to fuse

-cells to move and change shape

how do typed of fatty acids affect membrane fluidity

-longer fatty acids chains decrease fluidity

-saturated fatty acids decrease fluidity, they create a well-structured arrangement of hydrocarbons

-unsaturated are spaced apart

what particles can’t diffuse through membrane by simple diffussion?

-ions because they're too ionic

-large polar molecules (glucose and amino acids)

passive and active transport

-passive is when no metabolic energy is expended, e.g. diffusion (energy comes from kinetic energy of particles)

-active requires ATP, e.g. active transport and bulk transport

what increases rate of diffusion

-higher temperature, increases kinetic energy of particles

-higher surface area

-increase in concentration gradient

-shorter diffusion distance

-number of channel proteins present (only for fasciliated diffusion)`

steps of active transport

-molecule binds to receptor complementary in shape on carrier protein

-ATP will bind to the carrier protein on the inside of the membrane and is hydrolysed into ADP and Pi

-this causes the protein to change shape and open towards the inside of the membrane 

-this causes the molecule to be released on the other side of the membrane 

-the Pi molecule is then released from the protein, causing it to revert to its original shape

receptor mediated endocytosis

water potential

-pure water has the highest water potential (0)

-the tendency of free water molecules to move in a system

water potential effect on plant cells

hypotonic, higher water potential outside cell, water moves in, vacuole swells, pushes against cell wall, cell is turgid

-hypertonic solution: membrane pulls away form wall, cell becomes flaccid (plasmolysis)

water potential effect on animal cells

-higher water potential outside cell (hypotonic solution), cell swells and bursts

-hypertonic solution, lower water potential, cel shrinks, net movement of water out of cell