Membrane or Membrane transport

Membrane structure:

  • Glycoproteins (cell-cell recognition)

  • Integral proteins (control the entry and removal of specific molecules from the cell)

  • Phospholipid bilayer

  • Cholesterol (helps regulate membrane fluidity and important for membrane stability)

  • Peripheral protein

  • Glycolipd

  • Fatty acid tailds

The bias of cell membranes

  • Structures of biological membranes are the same

  • Bilayer of phospholipids: hydrophobic tails, hydrophilic head

  • The membrane controls movement in and out of the cell 

Phospholipids

  • Structure is amphipathic because it contains both a polar head (hydophilic) and two non-polar tails (hydrophobic) 

  • Arrangement in membranes: 

-phosphoslipids spontaneously arrange into a bilayer 

-hydrophobic tails face inward

-hydrophilic head regions cytosolic and extracellular fluids respectively 

  • phospholipids are held together by a week hydrophobic reaction between the tails 

  • layers restrict the passage of many substances 

  • individual phospholipids can move with the bilayer allowing for membrane fluidity and flexibility  

  • the fluidity allows for the spontaneous braking and reforming of membranes (endocytosis, exocytosis)

Bilayers as barriers 

  • hydrophobic tails have low permeability to all hydrophilic particles including ions and polar molecules 

  • the core are more attracted to each other then they are towards the solutes 

  • larger molecules have a harder time passing through the membrane 

Membrane transport 

  • cellular membranes possess two key qualities: semi-permeable and selective 

  • materials for essential cell metabolism are imported 

  • waste products that are toxic to the cell are excreated 

  • useful substances that need to be secreted such as  enzymes and hormones are exported 

  • other reasons for transporting substances across the cell membrane include cell deference cellular homeostasis 

  

Diffusion 

  • movement of particles from a high concentration region to a low concentration region and is the result of random motion of particles 

  • It is affected by: 

  • temperature - molecules diffuse faster in high temp 

  • surface area of membrane - increase in surface area allows more molecules to diffuse 

  • size of particles - smaller molecules diffuse faster 

  • concentration gradient of diffusing particles - bigger gradient = faster diffusion 

Simple diffusion: 

  • gas or liquid medium

  • requires a concentration gradient 

  • living and non-living systems 

  • uniformly distributed 

  • doesn’t work for particles that are too big or charged 

Integral and Peripheral proteins

  • diverse in terms of function, position in the membrane and function

  • either permanently or temporarily attached to the membrane 

  • integral proteins are permanently attached to the membrane and are typically tansmembrane  

  • peripheral proteins temporary attached by non-covalent interactions and associate with one surface of the membrane  

Membrane proteins

junctions- connect and join two cells

enzymes- fixing to membranes localises metabolic pathways

transport - responsible for facilitated diffusion and active transport

recognition- may function as markers for cellular identification

anchorage - attachment points for cytoskeleton and extracellular matrix

transduction - function as receptors for peptide hormones

Facilitated diffusion :

  • requires channel proteins or carrier proteins

  • specific to the molecules being transported

  • size of protein carries and channels determine what substance

Osmosis and aquaporins

  • osmosis is passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane.

  • special form of diffusion that involves water molecules only

  • soludes determine the direction in which the water molecules move

-hypertonic -water from the cell moves into the solution

-isotonic - as much on the inside as the outside

-hypotonic - to much into the cell

  • aquaporins - channels of water

Increases permeability to water

Water molecules pass in a single file (as wide as a water molecule)

Active transport and pump proteins

  • energy is used to move substances against a concentration gradient from low to high

  • passive is with the gradients(without energy)

  • active against the gradient (with energy)

Pump proteins: moves in one direction, uses energy, against the gradient

Channel proteins: moves in multiple directions, does’t use energy, with the gradients

Glycoprotins and Glycolipid

  • glycoprotein - membrane protein that is covalently attached to a carbohydrate (cell to cell recognition)

  • glycolipids - carbohydrates linked to lipids(cell recognition and help immune system to distinguish between self and non-self)

  • they form the glycocalyx - a carbohydrate rich layer on the outside of the cell membrane(bind cells together to prevent tissue from falling apart)

Phospholipids and fatty acids

-saturated fatty acid -unsaturated fatty acid

  • unsaturated fatty acids leads to more fluidity and flexibility

  • amount of saturated and unsaturated are regulated to give the properties that each membrane requires

  • ratio depends on temp that the cell is exposed to

  • cold temp leads to more saturated

  • higher ratio of saturated fatty acid gives thicker membrane

Cholesterol and membrane in animals

  • cholesterol in animal cell membranes helps maintain mechanical stability and shape

  • absent in plant cells

  • amphipathic molecule: hydroxyl group is hydrophilic ,the remainder is hydrophobic

  • chlesteral helps maintain the correct fluidity

  • cholesteral interacts with the fatty acid tails of phospolipids to modirate the properties of the membrane

Endocytosis and Exocytosis

  • bulk transport

  • endocytosis- a cellular process where cells take in molecules or substances from outside of the cell by engulfing them in the cell membrane

  • exocytosis - the opposite of endocytosis and involves the ejection of waste products or useful substances from inside of the cell

Endocytosis:

-Pinocytosis is taking in liquid

-Phagocytosis involves the absorption of solids

Exocytosis:

-excreation : waste removal (undigested remains of the microb that are useless )

-secretion: removal of hormones that are needed somewhere else proteinessynthesised ribosomes

Gated ion channels:

  • specialized channels allow ions to pass through membranes

  • many are controlled by electrical impulses or by channels -gated

  • controls the electrical potential across membranes

Neurotransmitter(transmitter)-gated- chemically gated

  • first cell releases neurotransmitters who then attach to receptors in the other cell so that the message can be transmitted when the receptor opens the gate so positive ions can pass through

  • channeled are gated by neurotransmitters or ions

  • neuron connected to muscle contain acetylcholine

Voltage gated:

  • opens by changes in membrane polarity

  • electrical stimulus opens and closes the proteins

  • opens for a very short amount of time allowing rapid movement of ions

  • Sodium potassium pump:

Contently going on

  • move 3 sodium +out and take 2 potassium -in so that on the inside the charge is - while the outside is +

When a signal arrives:

  • the sodium channels opens and sodium rushes in (inside goes from - to positive) triggering the next sodium channel (chain reaction)

  • sodium channels open then when the next opens that one closes then potassium channel opens then rushes out , so it can even out so the inside becomes negative

Indirect active transport

  • uses energy from transporting one molecule down a concentration gradient to transport another molecule

  • the same proteins that transport glucose also transport sodium and potassium this is called co-transport

Cell adhesion

  • multicellular organisms are dependent on cells sticking together

  • between plasma membrane can be stable or temporary

  • a cell-adhesion molecule (CAM)

  • different types of CAM dependent on the cell junction required

  • Animal cells: Desmosomes (flexible strong sheets of cells)

  • Plant cells: Plasmodesmata (tubes, connects cytoplasms)