3- Lipids, Membranes and Transport

What elements are lipids made from?

• carbon

• oxygen

• hydrogen

What are lipids?

large and varied group of non-polar molecules that are insoluble in water but dissolve easily in organic solvents

What is a triglyceride?

3 fatty acids and 1 glycerol molecule held together with ester bonds

What is a saturated fatty acid?

no double bonds between carbon atoms

What is an unsaturated fatty acid?

1 or more double bonds between carbon atoms

What is glycerol?

3 hydroxyl groups

How is a triglycerol formed?

Glycerol is bonded with 3 fatty acids and removes 3 water molecules during condensation reactions to get a triglyceride held with ester bonds between the OH of glycerol and COOH of fatty acids

Why are triglycerides not classed as polymers?

made from 2 different types of molecules

What are the biological roles of triglycerides?

• storage of energy

• insulation (electrical + heat)

• protection

What are the structures and properties of triglycerides and how do they relate to their function?

• High ratio of energy storing carbon-hydrogen bonds to carbon atoms= excellent source of energy

• Low mass to energy ratio= good storage molecules as much energy can be stored in a small volume

• Large, non-polar molecules and insoluble in water= no osmotic effect and do not affect the water potential in cells

What is the difference between a triglyceride and a phospholipid?

TRIGLYCERIDE:

• glycerol

• 3 fatty acids

PHOSPHOLIPID:

• phosphate head

• 2 fatty acid tails

What is the food test for lipids?

• add ethanol to sample

• shake

• add water

• shake gently

• milky white emulsion indicates presence of lipid

What are the 3 main functions of plasma membranes?

• to control transport of substances into/out of cell or organelles

• to act as a receptor site to recognise chemicals which need to enter the cell/organelle

• to compartmentalise, separate off the cell from environment, and the different reactions of the cell from each other by forming organelles

Why is control of transport of substances important for plasma membranes?

• allow certain molecules to pass through, but not others (partially permeable)—> O2, C6H12O6, H20, CO2

Why is acting as a receptor site important for plasma membranes?

• allows cells to communicate and react to their internal and external environments

Why is compartmentalisation important for plasma membranes?

• allows reactions to occur more efficiently

• different concs can be maintained on either side of the membrane

What is the membrane structure?

primarily made from a phospholipid bilayer

What are phospholipids?

• a polar/ hydrophilic (attracts water) head (glycerol + phosphate)

• non-polar hydrophobic (repels water) tails (fatty acids)

How is the phospholipid bilayer structured?

• hydrophobic tails all point towards each other

• hydrophilic heads point out into the water

What is the function of the phospholipid bilayer?

• separates the cytoplasm from the water outside

• acts as a barrier to polar/ charged/ water-soluble molecules

• allows non-polar/ uncharged/ lipid soluble molecules to pass through

What is the structure of the bilayer with proteins in it called?

fluid mosaic structure

What are the 2 types of proteins that are in the membrane called?

• Intrinsic

• Extrinsic

What is an intrinsic protein?

protein across the whole membrane

What is an extrinsic protein?

protein in 1 layer of the membrane

What is the function of the proteins in the membrane?

have hydrophilic channels to allow transport of some substances more easily

Which substances would be able to move through the hydrophilic channels of the proteins?

large, charged, polar, water soluble molecules

E.G.

• glucose

• sodium ions

• amino acids

etc

What are the functions of intrinsic proteins?

transport= channel + carriers

What are the functions of extrinsic proteins?

• enzymes

• receptors= for binding to hormones

• antigens= for cell recognition

What is a glycoprotein?

short, branching, carbohydrate chains attached to proteins in the membrane

What is the function of a glycoprotein?

act as receptors for hormones and neurotransmitters/ antigens

What is a glycolipid?

a carbohydrate covalently bonded to a lipid

What is the function of a glycolipid?

• act as recognition sites

• help maintain stability of membrane

• help cells attach to one another to form tissues

What is cholesterol?

• restricts movement of other molecules making up the membrane and increases strength and stability of membranes (makes them less flexible)

• helps prevent loss of water and dissolved ions

What are the components of the fluid mosaic model?

1. phospholipid bilayer

2. phospholipid (hydrophilic head + hydrophobic tail)

3. channel protein (intrinsic)

4. extrinsic protein

5. glycoprotein

6. glycolipid

7. carbohydrate branch

8. cholesterol

Why is the model described as fluid mosaic?

• FLUID= all different molecules can move around

• MOSAIC= different types of molecules (proteins + carbohydrates ‘float around’ within the phospholipid bilayer)

This fluidity means that cells are able to change their shape

Explain the significance of protein-lined pores for simple diffusion

allows polar, water soluble molecules to move through

Explain the significance of small gaps between phospholipids for simple diffusion

allows small, lipid soluble molecules to diffuse

Explain the significance of phospholipid bilayer for simple diffusion

repels water soluble, charged and polar molecules

Explain the significance of intrinsic proteins for facilitated diffusion

movement of specific polar/ water soluble/ charged/ large molecules down a conc gradient

How does high water potential affect movement of water?

many free water molecules

How does lower water potential affect movement of water?

fewer free water molecules as there is an increase in solute conc. Water will move to this side of the membrane by osmosis

What are the ways of transport across membranes?

• simple diffusion

• facilitated diffusion

• osmosis

• active transport

Which processes of transport are passive?

diffusion and osmosis as they don’t require energy in the form of ATP

Which processes of transport are active?

active transport and endo/exocytosis as they require energy in the form of ATP

What is simple diffusion?

the net movement of molecules down a concentration gradient until equilibrium is reached

What type of ions cannot diffuse across a membrane?

charged particles/ ions

What are the factors that affect rate of diffusion?

• surface area

• thickness of exchange surface

• concentration gradient

• size of diffusing molecule

• temperature

• water solubility/ polarity

How does surface area affect rate of diffusion?

greater the SA= faster rate of diffusion

How does thickness of exchange surface affect rate of diffusion?

thinner the surface= faster rate of diffusion

How does concentration gradient affect rate of diffusion?

steeper the gradient= faster rate of diffusion

How does size of diffusing molecule affect rate of diffusion?

smaller diffusing molecule= faster rate of diffusion

How does temperature affect rate of diffusion?

higher temperature= faster rate of diffusion

How does water solubility/ polarity affect rate of diffusion?

non-polar molecules diffuse faster than polar ones (polar molecules have to go through a protein)

What is Fick’s law?

rate of diffusion ∝ (surface area x difference in conc) /

thickness of exchange surface

How do we increase diffusion rate according to Fick’s law?

• increase SA + difference in conc

• decrease thickness of exchange surface

Which molecules will be able to move through he phospholipid bilayer by simple diffusion and why?

• Non-polar= diffuse rapidly across bilayer as are lipid soluble and not repelled by fatty acids in the hydrophobic tails

• Smaller molecules= can fit between phospholipid molecules

• Oxygen molecules= non-polar and small so diffuse rapidly through bilayer

What is facilitated diffusion?

simple diffusion with intrinsic proteins

• down conc gradient by may be limited by number of carrier/channel proteins (levels off above certain conc because all proteins are occupied/ saturated)

• passive

• moves specific charged/ polar/ hydrophilic substances (channels= ions, carriers= others)

Which molecules will move by facilitated diffusion?

specific charged/ polar/ hydrophilic substances

• fatty acids tails act as barrier to these molecules

e.g.

sodium ions, glucose, amino acids

What could increase rate of facilitated diffusion?

• increase conc of channel proteins

• steeper conc gradient

What is active transport?

Movement against a conc gradient which requires ATP and needs specific protein carriers

• any factor that reduces rate of respiration will reduce active transport e.g. lack of oxygen/ glucose, low temp etc

• roots take up ions by active transport

What can limit rate of active transport?

• any factor that reduces rate of respiration

• amount of carrier proteins

What is endocytosis?

taking products into cell

What is exocytosis?

releasing products out of cell

What are endocytosis + exocytosis used for?

transport of large amounts

• cells make vesicles to transport solids/ liquids across the plasma membrane into/out of cell

• importing material in vesicles= endocytosis

• exporting material in vesicles= exocytosis

• both require ATP

What is osmosis?

movement of water across a partially permeable membrane down a water potential gradient from higher (less negative) to lower (more negative) water potential

What is water potential measured in?

kPa

What is pure water’s water potential?

0 kPa

What water potential does any non pure water have?

negative

What water potential will a concentrated solution of solute (e.g. glucose) have?

lower water potential —> fewer water molecules free to move

Describe and explain the impact of additional solutes on water potential

the addition of solutes reduces the water potential of a solution by reducing the number of free water molecules (more are stuck/bound to the solute molecules)