B2.1 - Membranes and Membrane Proteins

(B2.1.1) State the primary molecule that makes up the cell membrane

- Phospholipids. Which are a class of lipids and are the main component of all cell membranes.

Define amphiphilic

(Of a molecule, especially a protein) having both hydrophilic and hydrophobic parts. Phospholipids are amphiphillic

Explain why phospholipids form bilayers in water (with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails)

- There is water on either side of the cell membrane. Extracellular fluid on the outside and cytoplasm on the inside

- Phospholipids will arrange themselves as a bilayer so: the hydrophilic head associates with water on either side of the cell membrane and the hydrophobic tails face each other within the membrane

(B2.1.2) Describe the properties of phospholipids that make them good barriers

- Low permeability to large molecules

- Low permeability to hydrophilic particles (including small stuff)

- Fluidity, they can move/stretch/bend and enable other embedded molecules.

(B2.1.3) Define diffusion

The net movement of molecules from an area of high concentration to an area of low concentration

Describe one examples of simple diffusion of molecules into / out of cells

Gas exchange by diffusion in lung alveoli cells.

- Alveoli in the lungs is where blood cells drop off waste CO₂ and "fresh" O₂

- Because both CO₂ and O₂ are very small and non-polar, they can both diffuse passively across the cell membrane

- (This process happens without any energy input required, this is an example of simple diffusion.)

(B2.1.4) Define membrane protein

A protein molecule that is attached to, or associated with, the membrane of the cell.

Contrast the structure of integral and peripheral proteins

Integral membrane proteins: Embedded in the phospholipid bilayer (go completely through the membrane layers)

Peripheral membrane proteins: Indirectly or loosely attached to the surface of the cell membrane, but may dip slightly into the lipid bilayer.

(B2.1.5) Define solute

The substance that dissolves in a solvent to produce a mixture.

Define solvent

The substance that is capable of dissolving one or several substances.

Define solution

The resulting mixture of solute + solvent.

Define osmosis

The movement of water molecules across a semi-permeable membrane from a high to low concentration.

Explain why osmosis occurs without the input of energy

- Molecules are always moving. In a solution, both the water and the solute molecules move around randomly. -Therefore, osmosis takes place naturally, and without input of energy.

Explain the movement of water during osmosis

Osmosis is the movement of water molecules from an area of higher water potential to an area of lower water potential until equilibrium is reached.

Define aquaporins

Aquaporins are specialised integral proteins that function as a channel for water to enter/exit the cell.

State the purpose of aquaporins

Without aquaporins, water would have a hard time passing through the hydrophobic center of the phospholipid bilayer.

(B2.1.6) Define facilitated diffusion

- The passive movement of molecules across the cell membrane via the aid of a membrane protein.

- (It is utilised by molecules that are unable to freely cross the phospholipid bilayer.)

Define channel protein and carrier protein

- A channel protein is a protein which allows the transport of specific substances across a cell membrane.

- A carrier protein is a membrane protein that moves solutes across the membrane by creating conformational changes in the protein. (They are much slower than channel proteins.)

Explain how the structure of channel proteins makes membranes selectively permeable

The structure of channel proteins allows them to act as selective gates, enabling the passage of specific substances while blocking others.

Channel proteins can be selective based on charge, size, the binding of a solute, voltage and more.

Outline an example of a selective channel protein

- An example of selective channel proteins are potassium channels found in nerve cells.

- These are an example of voltage-gated channel - they open and shut in response to transmembrane voltage.

(B2.1.7) Define active transport

Active transport uses energy to move molecules against a concentration gradient (from low to high)

Explain one example of active transport of molecules into and out of cells through protein pumps

The sodium potassium pump in nerve cells allows for signals to be sent within the cells.

(B2.1.8) Define selective permeability

The property of cell membranes that only allows certain molecules to enter or exit the cell.

Outline the selective permeability of membrane proteins

- Simple diffusion is not selective and depends only on the size and hydrophilic/hydrophobic properties of the particles.

- Both facilitated diffusion and active transport can be selectively permeable.

(B2.1.9) Define glycosylation

The process in which phospholipids and membrane proteins can have carbohydrate chains attached.

Outline the functions of glycolipids and glycoproteins

- Glycosylation of a phospholipid results in a glycolipid.

- Glycosylation of a membrane protein produces a glycoprotein.

(B2.1.10) Explain the term "fluid mosaic model"

The membrane is fluid, meaning it can change shape and flow. Mosaic refers to the diverse collection of molecules that make up the membrane

Draw and label the structure of a cell membrane

(Check notebook)

(B2.1.11) (AHL) Outline how temperature affects cell membrane fluidity

The higher the temperature is, the lower the viscosity is. The lower the temperature is, the higher the viscosity is.

Explain how the fluidity of a membrane is affected by fatty acid composition

Unsaturated fatty acids have double bonds in their lipid chains, thus they have kinks in their hydrocarbon tails. This mean the lipids are harder to pack together, increasing their fluidity.

Saturated fatty acids have no double bonds in their lipid chains which results in straight hydrocarbon tails. This means that the lipids will be easier to pack together, lowering their fluidity.

Give an example of adaptations in membrane composition in relation to habitat

The ratios can vary:

- For species living in colder environments, too many saturated fatty acids in the phospholipid bilayer will pack too closely together becoming rigid or solid. This would freeze proteins in the membrane, limiting their functions.

- However, there is an increase in saturated fatty acids at higher temperatures because these will not freeze in more arid temperatures.

(B2.1.12) Identify the structure of cholesterol in molecular diagrams

(Check notebook)

Describe the function of cholesterol molecules in the cell membrane

- At high temperatures, cholesterol restrains the movement of the phospholipid fatty acid chains

- This makes the membrane less fluid and reduces its permeability to small molecules, this helps stabilise the membrane.

- At low temperatures cholesterol prevents the stiffening which maintains membrane fluidity

(Check notebook)

(B2.1.13) Outline how the phospholipid bilayer can break away to form vesicles

- The membrane is held together by weak hydrophobic associations between the fatty tails of phospholipids.

- This allows for the membrane to "break" and reform around the material.

Describe the processes of exocytosis and endocytosis

- Endocytosis is the process by which certain substances enter the cell. (endo = inside) (e.g a hormone telling the brain that you've had enough to eat.)

- Exocytosis is the process by which certain substances exit the cell. (exo = outside) (e.g insulin made in the pancreas reaching other places in the stomach.)

(Both of them form a vesicle to diffuse substances in and out)

State the two different types of endocytosis

- Phagocytosis: Intake of solid substances, (to eat)

- Pinocytosis: Intake of liquid or dissolved substances (to drink)

Give an example of endocytosis

- A common example of endocytosis is the process by which cells uptake cholesterol from their external environment.

Give an example of exocytosis

- A common example of exocytosis is the release of neurotransmitters in nerve cells.

- (Neurotransmitters are chemical messengers released from nerve cells that can trigger or prevent a signal in connecting nerve cells.)

(B2.1.14) Define ion channel

- Ion channels are integral membrane proteins which contain a hydrophilic inner pore through which ions may pass.

- (This allows ions to either enter or exit a cell according to the concentration gradient) (facilitated diffusion.)

Outline the importance of ion channels to nerve cell function

They are important because they're used to establish charge differentials across a membrane.

Outline the function of voltage-gated channels and ligand-gated channels

- Voltage-gated: Only allows passage when a threshold voltage is reached

- Ligand-gated: Only open when a specific chemical - called a ligand - is present.

Outline acetylcholine receptors as an example of a neurotransmitter-gated ion channel

- In neurons, acetylcholine is a neurotransmitter released from the nerve cells to stimulate adjacent cells.

- Muscles contain acetylcholine receptors that will trigger the opening of an ion channel when activated.

Describe the function of sodium/potassium channels as examples of a voltage-gated channel

In neurons, voltage-gated sodium channels are used to transport sodium ions into the neuron during the "firing" phase of a neuron.

Describe the structure and function of the sodium-potassium pump

The sodium-potassium pump expels 3 sodium ions out of the cell and moves 2 potassium ions into the cell.

Describe the role of the sodium-potassium pump in maintaining neuronal resting potential

- The end result of the sodium-potassium pump is an unequal distribution of ions on different sides of the membrane.

- This is called the resting potential

Describe the action of the "voltage gate" of the potassium channel

When the neuron fires, these ions swap locations via facilitated diffusion via separate sodium and potassium channels. These channels are called voltage gates.

(B2.1.16) Define carrier protein

Transmembrane transporters which undergo a conformational change to translocate a material across the cell membrane. They use energy either in the form of primary or secondary active transport.

Compare and contrast cotransport, symport, and antiport

Carrier proteins move substances simultaneously in a process called contransport.

- Cotransport can be either in the same direction (symport) or opposite direction (antiport)

Explain the absorption of glucose as an example of cotransport

- Sodium-glucose transporters symport glucose alongside sodium ions.

- Sodium-glucose transporters do not directly utilise ATP to transport glucose against its concentration gradient; instead, they rely on the sodium concentration gradient.

- This is a form of secondary active transport, as the electrochemical gradient is used as an energy source,

(B2.1.17) Define cell adhesion

Cell adhesion describes the attachment of cells to other surfaces via specialised membrane proteins called cell adhesion molecules (CAM's)

Outline the two primary mechanisms by which cells adhere to each other

- CAM's can either directly attach cells to one another.

- CAM's can also indirectly anchor cells to the extracellular matrix.

Outline the function of CAMs (cell adhesion molecules)

Cell adhesion molecules can play important roles in a variety of cellular processes - including growth, apoptosis (killing cells), signal transduction, migration and tissue development.

Describe the different types of cell-cell junctions

Anchoring junctions: hold cells together to strengthen contact within tissues.

Tight junctions: Create tight seals that result in an impermeable barrier to diffusion.

Gap junctions: Link cells together with molecular tunnels to allow the movement of material between them.