IB biology: B2.1: membranes and membrane transport

where can one find membranes

• organelles

• cells

main unit of cell membranes and it’s properties

• phospholipids: they are amphipatic

• both hydrophobic and hydrophillic

• head is hydrophillic and tail is hydrophobic

what can pass through lipid bilayers

• nonpolar molecules such as oxygen or co2

• very small polar molecules such as water or ethanol

• lipid soluble molecules such as steroids ( so testosterone and aestrogen)

• not glucose cuz too polar

what organelles have double membranes

• mitochondria

• nucleus

• chloroplast

what makes up membranes

• phospholipids

• glycolipids

• glycoproteins

• proteins

• small quantities of carbohydrates

peripheral proteins

• found on surface of membrane

• are hydrophillic son only interact with the hydrophliic heads of the bilayer

• because of that they are easier to take remove

integral proteins

• they are proteins embedded in the lipid bilayer: they are amphipatic and thus the inside part of the protein is hydrophobic to be able to stick to the chains

• can be transmembrane or on only one side

membrane proteins

• carrier proteins

• channel proteins

• aquaporin

functions of membrane proteins

• Transport proteins

• Recognition – Membrane proteins help in cell–cell recognition acting as ‘name tags’ for the cells. Helps to distinguish between self and non-self cells. 

• Receptors – Membrane proteins act as receptors for chemical signals and are binding sites for molecules like hormones and neurotransmitters. Often, binding of these molecules triggers a chain of intracellular reactions.

• Enzymes – Membrane proteins show enzymatic activity and catalyse reactions. For example, glucose-6-phosphatase is a membrane-bound enzyme found in the endoplasmic reticulum.

• They can help in cell adhesion to other cells or to the environment and play a role in cell motility.

Aquaporins

• a tetrameric integral protein with 4 identical water channels

• their insides are filled with hydrophillic side chains which allow passage inside/ outside the cell ( bidirectional )

facilitated diffusion

when larger/ polar molecules pass through cell membranes and move down the concentration gradient with the help of carrier/ channel proteins. doesn’t require energy

carrier proteins

• transmembrane protein

• carrier protein binds to solute molecules

• undergoes a conformational change

• transfers molecules to the other side of the membrane

• sites are specific for each molecule that needs to be transported ( eg. GLUT/ glucose transporter )

channel proteins

• transmembrane proteins which form channels for the passage of polar molecules (eg. ion channels, porins )

• they are gated : they can open and close

why are channel proteins so selective

• the binding sites of the hydrophilic amino acid side chains lining the channel being highly ion-specific

• the size of the pore acting as a size filter.

what causes channel proteins to open or close

specific stimuli such as

• changes in voltage across the membrane or voltage-gated channels

• binding of small molecules to the channel proteins or

  ligand- gated channels

• mechanical forces like pressure.

active transport purpose in cells

• take up essential nutrients

• remove secretory waste materials from the cell into the extracellular fluid

• maintain the right concentration of ions in the cells

types of active transport

• direct active transport: where energy is released by an exergonic reaction ( breakdown of ATP) is used to transport molecules across a membrane

• indirect active transport: where the movement of one solute down it’s concentration gradient drives the movement of the second solute against it’s concentration gradient

factors which increase membrane fluidity

• higher proportion of unsaturated fatty acids

• weak hydrogen bonds with water; can be affected rather easily

• how much of the plasma membrane is phospholipids

cholesterol

• head of cholesterol binds to head of phospholip

• one part is stiffened, another part is more fluid

• when high temperatures; the harder part will help the plasma membrane hold together: decreases fluidity

• when low temperatures: increases fluidity: prevent packing of phospholipid heads.

why does more unsaturated fatty acids create more fluidity

• the kinks make it so that the fatty acids have more irregularities; some of them might be tilted and have less opportunities for them to settle in a regular manner

Functions of glycolipids and glycoproteins

• Cell recognition

• Cell adhesion

• Cell signalling

receptor molecules; what makes blood types

Glycocalyx

• sticky layer formed by the carbohydrate groups of the glycolipids and glycoproteins that protrude from the cell surface. The glycocalyx in addition to its roles in cell signalling, cell adhesion and cell–cell recognition, helps in protecting the cell surface.

Fluid mosaic model

• the lipid bilayer is fluid

• the proteins (both integral and peripheral) are embedded in the fluid bilayer which resembles a mosaic.

saturated vs unsaturated fatty acids

• unsaturated have kinds in their chains while saturated do not

• that makes it so the saturated can fit together more snugly, making the membrane more rigid and denser

• that makes it so the unsaturated do not fit together so closely, so they maintain fluidity - allows for when temperatures drop of the membranes maintain some fludity

adaptations to cold (eg hibernation, frogs…)

• increase ratio of unsaturated fatty acids in membranes to maintain fluidity

role of glycolipids and glycoproteins in cell recognition

• Cell recognition: Glycolipids and glycoproteins play an important role in cell recognition. They act as ‘markers’ on the cell surface and help cells of the body recognise each other. They also help cells of the immune system to recognise foreign cells.

role of glycolipids and glycoproteins in cell adhesion

Cell adhesion: Both glycolipids and glycoproteins help cells to attach and bind to other cells to form tissues. Cell-adhesion molecules or CAMs are cell-surface glycoproteins that play an important role in cell adhesion.

role of glycolipids and glycoproteins cell signalling

Cell signalling: They act as receptors for enzymes and other molecules helping in cell signalling, i.e. receiving and transmitting chemical signals.

what can pass through lipid bilayers ?

• nonpolar, hydrophobic lipid soluble molecules;

• that is because the inside part of the membrane is hydrophobic, so it repels anything hydrophobic

Exocytosis

• vesicle is formed in golgi apparatus

• vesicle goes to plasma membrane

• vesicle fuses with plasma membrane

• the material is released in extracellular matric

endocytosis

• the plasma membrane invaginates forming a cavity filled with extracellular fluid

• it folds in on itself and traps the fluid in a vesicle

• this process requires energy and is a form of active transport

pinocytosis

endocytosis but with liquids. the vesicles are smaller than with phagocytosis

voltage gated sodium potassium ion channels

• the sodium potassium pump functions, making the inside of the cell more negative; eventually the charge reaches a threshold; it becomes a stimulus, the cell is depolarised

• sodium ion channels open, allowing sodium to diffuse inside, creating an “action potential”; now the inside of the cell is more positive. the channel closes

• potassium channels open, potassium ions can then move from the inside to the outside of the cell as per their concentration gradient, and the channel closes

• now, through the action of the sodium potassium pump, the cell will again become more negative and repolarise

types of gates ion channels

• voltage-gated channels

• ligand-gated channels

• mechanically gated channels (which respond to mechanical cues such as sound waves and vibrations).

Ligand-gated channels

are ion channels that open when a ligand binds to the transmembrane protein (of the ion channel

nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAchR) are ligand-gated ion channels present at skeletal neuromuscular junctions which can bond to acetylcholine.

• The binding of acetylcholine molecules results in a conformational change ( shape change ) that opens the channel

• sodium ions can now diffuse inside the cells and the cell becomes more positive ( depolarise )

• really quickly the enzyme cholinesterase breaks down acetylcholine, leading to closure of the ion channels.

• nicotine can also activate those receptors

• Depolarisation is followed by opening of voltage-gated potassium channels and exit of potassium ions resulting in repolarisation.

cell adhesion molecules

they create the binding of cells with other adjacent cell/ extracellular matrix

are glycoproteins

Desmosomes

• binds cells together: allows cells to stretch but still stay together

• “tension reducing network of fibers”

• type of adhesive junction

cell junctions

• connect cells to each other

• allow intercellular transport and communication

• Use cell adhesion molecules ( a type of glycoprotein)

main types of cell junctions

• adhesive junctions

• tight junctions

• gap junctions

tight junctions

tight junctions create tight seals between cells; impermeable

present mostly in epitheal cells and prevents uregulated movement of molecules between cells

Gap junctions

intracellular channels connecting neighbour and allowing movement of molecules ( eg companion cell phloem ). allowing for sharing of resources. can be called communicating junctions

adhesive/anchoring junctions

• are present in epithelial cells and cardiac cells.

• used in organs where tissues muct stretch ( bladder, stomach…)

• They facilitate cell–cell adhesion in tissues to ensure structural stability and allow the cells to withstand mechanical stress.

plasmodesma

• fusing of cell wall; they are tubular structures which connect each other

• they allow for the sharing of materials such as water and small solutes

What is the Na^+/K^+ pump

• it is an enzyme that generates energy though the hydrolisis ofn ATP.

• that energy is used to drive the transport of sodium and potassium ions against their concentration gradients

• there is an active transport of potassium into the cells and sodium out of the cell

how does the sodium potassium ion pump work ?

1. Initially, the Na⁺/K⁺ pump is open to the inside of the cell in a way that the sodium ions bind to all three of its binding sites.

2. The binding of sodium ions triggers the hydrolysis of ATP to ADP and a phosphate group. The latter attaches to the pump resulting in a conformational change. The pump now opens to the exterior releasing the sodium ions.

3. At the same time, potassium ions attach to both binding sites. This causes the phosphate group to detach from the pump.

4. The pump undergoes a conformational change to regain its original form and once again opens to the interior of the cell

Indirect active transport explain principle

• one solute is transported down its concentration gradient

• the other is transported against its concentration gradient.

• The favourable movement (down the concentration gradient) is thereby coupled with an unfavourable movement (against the concentration gradient) and drives the latter.

indirect active transport example

glucose sodium

• Sodium ions bind to binding sites on the outer surface of the cotransporter.

• Simultaneously, a molecule of glucose also binds to its binding site on the cotransporter.

• This results in a conformational change that transports both the sodium ions and the glucose molecule to the inside of the cell.

• since the sodium is transported down it’s concentration gradient, it is an energy releasing exergonic process

importance of sodium potassium pump in nerve cells

• because it brings out 3 sodium ions and in 2 potassium ions, it creates a voltage difference

process of endocitosis through golgi >4 marks

1. Protein produced by the ribosomes of the rough ER enters the lumen (inner space) of the ER. The protein is packed into a vesicle.

2. The vesicle carrying the protein fuses with the cis side of the Golgi apparatus.

3. As the protein moves through the Golgi apparatus, it is modified and exits on the trans face inside another vesicle.

4. The vesicle with the modified protein inside moves towards and fuses with the plasma membrane, resulting in the secretion of the contents from the cell.