B2.1 - Membranes and membrane transport

B2.1.1—What are phospholipids? What are their properties? What do they form

Cell membranes are primarily composed of phospholipids;

they are Amphipathic;

meaning they have both hydrophilic and hydrophobic;

phospholipds are made from a polar head, which is hydrophilic;

it contains phosphate and glycerol;

also contains two non-polar fatty acid tails;

which are hydrophobic;

B2.1.1—What structures do phospholipids spontaneously form in water?

Phospholipids spontaneously arrange into a bilayer;

Hydrophobic tail regions face inwards and are shielded from the surrounding polar water/fluid;

the hydrophilic phosphate and glycerol in the head region attracts to the water outside and inside the cell;

Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations) of the fatty acid tails;

B2.1.2—How do lipid bilayers serve as barrier around cells? What is the basis of this?

The lipid bilayers in cell membranes act as selective barriers;

which are impermeable to and block the entry of large molecules, ions, and polar substances;

due to the hydrophobic region in the middle made of fatty acid tails;

This selective permeability is essential for maintaining the internal environment of the cell;

B2.1.3—What molecules can use simple diffusion to cross membranes?

an example of simple diffusion is the exchange of oxygen and carbon dioxide across cell membranes;

these molecules are small and non-polar and can cross the cell plasma membrane easily; without using energy;

following their concentration gradients;

This process is vital for cellular respiration, where oxygen is required for energy production, and carbon dioxide is a waste product to be expelled;

B2.1.4—What are Integral and peripheral proteins in membranes?

Integral proteins are embedded in one or both of the lipid layers of a membrane;

Peripheral proteins are attached to one or other surface of the bilayer.

they have diverse structures, locations and functions; including transport channels (integral proteins only) and receptors (usually inegral as well);

What are some of the functions of proteins in the membrane?

Transport Proteins: Facilitate molecule movement in and out of cells, including channel and carrier proteins.

Channel Proteins: Form pores for molecule passage.

Carrier Proteins: Change shape to transfer molecules across the membrane.

Recognition: Act as cellular 'name tags' for cell-cell recognition, crucial in immune system functioning.

Receptors: Bind to chemical signals like hormones, triggering intracellular reactions.

Enzymes: Catalyse reactions, e.g., glucose-6-phosphatase in the endoplasmic reticulum.

Cell Adhesion & Motility: Aid in cell adherence and movement.

B2.1.5—How do water molecules move across membranes?

Water moves across membranes always via osmosis:

This due the random movement of water molecules;

water moves from areas of lower solute concentration to areas of higher solute concentration;

solutes cannot pass the through the membrane easily as it is not very permeable to them;

water can move through aquaporins;

which are specialized channel proteins facilitating water movement;

B2.1.6—What are channel proteins what process of membrane transport uses them? Give examples

Channel proteins are proteins which cross the plasma membrane;

they allow specific ions to diffuse through when open;

due to the amino acids which make up the inside of the channels, so they only attract certain ions/molecules;

e.g. glucose; through the GLUT channels;

contributing to selective permeability;

B2.1.7—What are pump proteins? What process uses them? Give examples

Active transport is the passage of materials against a concentration gradient (from low to high);

This process requires the use of protein pumps which use the energy from ATP to translocate the molecules against the concentration gradient;

The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance;

Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients);

e.g. Na+/K+ pump which is involved in the generation of nerve impulses;

3 sodium pumped out for every 2 potassium pumped in to the axon;

B2.1.8—What is permeability? How are membranes selectively permeable?

Permeability is ability of a membrane to allow molecules to pass through;

selective permeability is when a membrane does not allow the free movement of all molecules and is permeable only to certain molecules;

due to specific channel proteins;

pump proteins;

which only allow specific molecules to pass, e.g. Calcium or sodium ions;

B.2.1.9—What is the structure, function and location of glycoproteins and glycolipids?

Glyoproteins are carbohydrate structures linked to proteins in membranes;

glycolipids are carbohydrate structures linked to lipids in membranes;

They are both exclusively on the extracellular side;

they are used in crucial for cell adhesion;

and cell recognition;

e.g. as receptors;

B2.1.10—What is the fluid mosaic model of membrane structure?

the Fluid mosaic model was proposed by Singer and Nicolson;

it says that both integral and peripheral proteins are embedded in the fluid bilayer, forming a mosaic pattern;

Lipids and proteins can move laterally within the membrane; meaning it is fluid;

Fluidity depends on fatty acid types in phospholipids and the cholesterol content;

Draw a diagram to represent the fluid mosaic structure of the membrane

AHL Only - B2.1.11—What is the relationships between fatty acid composition of lipid bilayers and their fluidity?

The fatty acid composition of the membrane can affect its fluidity;

if there are more unsaturated fatty acids;

which contain double bonds, leading to lower melting points;

this makes the lipid bilayer more fluid and flexible;

if there are more saturated fatty acids which do not have double bonds;

this results in higher melting points;

AHL Only - B2.1.12—What is cholesterol? What is the impact of cholesterol on membrane fluidity in animal cells?

Cholesterol is hydrophobic found embedded within the lipid bilayer; between hydrophobic fatty acid tails; as it has a hydrophilic region as well; making it amphipathic;

Function: it acts as a fluidity regulator;

cholesterol reduces fluidity;

making membranes more stable at higher temperatures;

it also prevents crystallisation at lower temperatures;

AHL Only - B2.1.13— What is membrane fluidity?

Fluidity is the ability of the membrane to move in a flexible way;

It also describes the way that membranes can fuse;

and the way membranes can form smaller regions of membrane without breaking;

AHL Only - B2.1.13— What is endocytosis? What is an example?

endocytosis is a process where large amounts of substances can enter the cell;

during endocytosis the membrane can wrap around;

and pinch off;

forming a vesicle;

due to fluidity of membrane; it can remain unbroken;

e.g. phagocytosis of bacteria by phagocytes;

AHL Only - B2.1.13— What is exocytosis? What is an example?

exocytosis is a process where large amounts of proteins; synthesised by rough endoplasmic reticulum;

are packaged into vesicles;

which pinch-off or bud-off; from the rough endoplasmic reticulum;

and are carried to the golgi apparatus;

vesicles fuse with the flattened-sac membranes of the golgi;

modification and processing of proteins to put them in their final form takes place;

vesicles bud-off again;

travel to the plasma membrane;

or other locations in cell;

fuse with the membrane to secrete contents outside the cell;

AHL Only - B2.1.14—How are gated ion channels used in neurons?

voltage-gated channels open and close in response to electrical charge; they are carrier proteins;

if there is a change in voltage around the channel causes it to open;

potassium channel open;

when there are more positive charges inside the cell than outside;

K+ can flow through;

down the concentration gradient;

aids in repolarisation of axon as positive potassium flow down concentration gradient out of cell;

AHL Only - B2.1.15—What is the sodium-potassium pump? How is it an example of as an example of an exchange transporter?

Active transport of sodium and potassium uses energy from ATP to pump;

the Sodium potassium pump transports 3 sodium ions OUT of cell for every 2 potassium ions IN;

sodium ions bind to interior of pump on inside of axon;

ATP hydrolysis allow phosphate to bind to pump;

causes a conformational change (change in shape) of pump;

releasing sodium outside the cell;

2 potassium bind to pump outside of the cell;

causing the release of phosphate;

causing a conformational change in the pump;

releasing potassium inside the cell

AHL Only - B2.1.16—What are sodium-dependent glucose co-transporters? How are they an an example of indirect active transport?

Sodium-dependent glucose co-transporters facilitate glucose transport into cells;

alongside sodium ions;

it is a form of indirect active transport;

sodium ions are pumped out of cells;

leading to a concentration gradient;

as they flow back down their gradient the energy can be used to transport glucose into cells;

AHL Only - B2.1.17—How do cells adhere to form tissues? What are CAMs?

Cell-Adhesion Molecules (CAMs) are proteins that allow cells to adhere to each other, forming stable tissues;

Different forms of CAMs are used in different types of cell-cell junctions;