Biological Membranes

What are the roles of cell membranes?

• partially permeable barriers

• isolates cell contents from external environment 

• form vesicles

• site for attachment (of enzymes)

• regulating the transport of materials into or out the cell

What is the structure of plasma membrane? (7-10nm)

• formed of a bilayer of phospholipids with hydrophillic heads facing out the bilayer and hydrophobic tails facing into the bilayer

• contains different types of proteins within

• has cholestrol which regulates fluidity in the membrane

What is the fluid mosaic model?

• fluid: phospholipids in bilayer can move freely within the membrane

• mosaic: different proteins scattered randomly in the bilayer (patchwork effect)

What is permeable through the plasma membrane?

• non-polar molecules diffuse rapidly across bilayer if theyre small enough eg O2

• lipid soluble molecules

• large charged polar molecules hardly permeate the bilayer eg glucose, NaCl

Explain why phospholipids form a bilayer in the membranes (3) 

• has a polar phosphate head which is hydrophillic and faces the aqueous solutions

• has non-polar fatty acid tails which are hydrophobic and face away from the aqueous environment 

• creates a barrier between the aqueous environment and the cell 

What are the functions of glycoproteins

• act as antigens for cell recognition

• act as receptor sites; allowing hormones to bind

• act as recognition sites

• form H bonds with H2O to stabilise membrane structure

• act as enzymes

• form protein channels and carrier proteins

What are the functions of glycolipids

• provide energy

• support the structure

• act as recognition sites

What are the functions of cholestrol

• regulates fluidity

• gives membrane mechanical stability

• fits between fatty acid tails to help make the barrier more complete

What are the mechanisms of cell signalling

• receptor acts as an ion channel

• receptor activates a glycoprotein

• receptor acts as an enzyme 

What is cytokines (signal molecule) 

• small proteins; peptides; glycoproteins

• secreted by specific cells of the immune system and glial cells

• carry signals locally between cells + have an effect on other cells 

Explain the ion channel process

• in the plasma membrane the G-protein is acting as an ion channel

• the signal (may be ligand) attaches to the receptor - they are complementary / specific to each other

• the channel opens and this allows the ions to enter the cell

• this brings about a response

What is a ligand

a signal molecule which is specific and binds to a receptor

Explain how cell surface membrane contribute to cell signalling process (4)

• signal molecule released by exocytosis

• glycoproteins act as receptors

• a signal molecule which is specific binds to a receptor, complimentary

• this attachment causes a change on the cell surface

• surface membrane allows entry of some signal molecules

What are the functions of channel proteins

• form a hydrophillic channel through membrane which ions can pass through by diffusion (down conc gradient)

• each channel formed by a protein will only allow a specific ion/molecule to pass through

• channel can change shape

• act as PORES

What are the functions of carrier proteins

• shaped so that a specific molecule can fit onto them at membrane surface 

• when specific molecule fits, the protein changes shape to allow the molecule through the other side of the membrane  

• act as PUMPS

What are passive processes

• simple diffusion

• facilitated diffusion

• osmosis

the purpose of diffusion is to reach an equilibrium

What happens to the membrane at higher temperatures

• phospholipids have more KE so move more freely

• proteins in membrane denatured

• this causes gaps in the plasma membrane

• membrane become more fluid as structure is damaged

What is the role of intrinsic proteins

• span the entire membrane

• involved in transport by acting as carriers or channels for ions

What is the role of extrinsic proteins

• on the surface of the plasma membrane

• involved in cell recognition, structural support by interacting with the cytoskeleton

What is cell signalling

• the communication between cells

• to trigger a response / cause a change in another cell

Define simple diffusion

• the net movement of molecules from an area of high conc to low conc

• where non-polar, small molecules pass directly through the phospholipid bilayer

• eg O2 entering RBCs

Define facilitated diffusion

• the net movement of molecules from an area of high conc to low conc

• where large, polar molecules pass through the cell surface membrane with the aid of a channel/carrier protein

• eg glucose, amino acids, Ca2+ entering nerve cell

How does a protein channel act in faciliited diffusion

• ions and smaller molecules pass through

• protein channel acts like a pore in the membrane

• can often be opened and closed to change the flow

How does a carrier protein act in facilitated diffusion

• larger molecules are transported and attach to the protein

• protein changes shape to transfer the molecule across the membrane

What factors increase the rate of diffusion

• higher concentration gradient

• increased temperature (more KE)

• greater SA

• thinner membrane (short diffusion pathway)

What are the adaptations of alveoli to maximise gas exchange

• thin membrane → short diffusion pathway

• large surface area → increases rate of diffusion

• large capillary network → good ventilation & maintains conc gradient

• moist → gases like O2 dissolve in water before diffusing across membranes

Explain the differences + similarities between simple and facilitated diffusion

• both passive processes

• both move substances down the conc gradient (high-low)

• simple diffusion transports small non-polar molecules eg O2 directly through the phospholipid bilayer

• facilitated diffusion involves the use of channel/carrier protein to transport larger polar molecules eg glucose and ions across the surface membrane

Formula for rate of diffusion

SA X Concentration difference / distance

Define osmosis

The net movement of water molecules across a partially permeable membrane from an area which is more dilute to an area that is less dilute (more concentrated) 

What is water potential

• the pressure exerted on water molecules as they collide with a membrane or container

• measure of tendency of water molecules to diffuse from one place to another

What is the pattern between water potential and solution concentration

• pure water: 0kPa

• dilute solution: ~ -10kpa

• concentrated solution: ~ -500 kPa

The more concentrated, the lower the water potential

Define iso ,hypo & hypertonic solutions

• isotonic: a solution which has equal concentration to the cell

• hypotonic: a solution which has lower concentration than the cell

• hypertonic: a solution which has a higher concentration than the cell

What is the effect of a hypertonic solution on an animal cell

• water potential is lower outside the cell, so water moves out of the cell

• cell shrivels

• CRENATION

What is the effect of an isotonic solution on animal & plant cells

• water potential inside and outside the cell are equal

• no net movement of water

• equilibrium reached

What is the effect of a hypotonic solution on an animal cell

• water potential is higher outside the cell, so water moves into the cell 

• cell bursts

• LYSIS 

What is the effect of a hypertonic solution on a plant cell

• water potential is lower outside the cell, so water moves out the cell

• cytoplasm pulls away from the cell wall

• cell is PLASMOLYSED 

What is the equation for water potential

water potential = solute potential + pressure potential 

Define active transport

movement of molecules from a region of lower concentration to a region of higher concentration which requires energy in the form of ATP

What is the active transport mechanism using carrier protein pumps

• solute molecule is recognised by carrier protein and grabs it

• protein rotates in the membrane and releases it inside the cell by using energy

• protein rotates back again using energy

Why does active transport require energy in the form of ATP

• ATP is hydrolysed into ADP and a phosphate

• phosphate binds to the carrier protein which changes its shape

• phosphate is released and original shape is restored

What are the functions of the protein pumps (carriers)

• carries specific molecules one way across the membrane

• uses energy in the form of ATP to carry molecules across membrane

• carries molecules against conc gradient

• carries molecules at a faster rate than by diffusion

What is exocytosis

• when a substance is released from the plasma membrane

• along microtubules

• in a vesicle which fuses with plasma membrane

What is endocytosis

• cells engulfing materials into the cell to allow larger molecules to enter (vesicle formed at membrane)

  • phagocytosis; cells engulf solid particles

  • pinocytosis: cells take in liquids & small molecules