General Biology 11 p1 (more details)

Flashcards covering key concepts of membrane biology, lipid synthesis, dynamics, and functions.

Where are newly synthesized phospholipids deposited in the ER bilayer?

Into the cytosolic half.

Why doesn’t the ER membrane grow unevenly if phospholipids are added only to one side?

Because of scramblases, which redistribute phospholipids.

Do lipids spontaneously move between bilayer halves?

Rarely.

What do scramblases do?

Randomly remove phospholipids from one half of the bilayer and insert them into the other.

What is the result of scramblase activity in the ER?

Newly made phospholipids are equally distributed between both monolayers.

What do flippases do differently from scramblases?

They move specific phospholipids from the exterior-facing side to the cytosolic side.

How does flippase action affect the membrane?

It promotes membrane asymmetry.

What factors increase membrane fluidity?

Shorter fatty acid chains and unsaturated fatty acids.

Why do shorter chains and unsaturated fatty acids increase fluidity?

They reduce interactions between adjacent phospholipids.

How does increased membrane fluidity affect cellular respiration?

It increases respiration by allowing greater diffusion of electron transport chain carriers.

How does temperature affect membrane fluidity?

Higher temperatures increase fluidity and Lower temperature decrease fluidity

How do cells adapt to high temperatures to maintain proper fluidity?

They produce longer chains with fewer double bonds (less unsaturated).

How does the lipid bilayer behave?

Like a two-dimensional fluid.

How do phospholipids move once inserted into the bilayer?

They diffuse rapidly and continuously within the same monolayer plane.

Do phospholipids flip between monolayers easily?

Very rarely, unless aided by transporter proteins.

Why don’t phospholipids pop in and out of the bilayer spontaneously?

Their hydrophobic tails make exposure to water unfavorable.

How can phospholipids be added or removed from a bilayer?

Via lipid vesicles.

Where are new phospholipids made in eukaryotic cells?

In the endoplasmic reticulum (ER).

Which organelle produces both new phospholipids and membrane-bound proteins?

The ER.

Where are the enzymes that synthesize phospholipids located?

On the cytosolic face of the ER.

Why are enzymes on the cytosolic face of the ER?

They need access to free fatty acids (substrates).

Into which half of the bilayer are new phospholipids deposited?

The cytosolic half.

What does 'amphipathic' mean for membrane lipids?

They have hydrophilic heads and hydrophobic tails.

How are the conflicting forces in amphipathic molecules resolved?

By forming a lipid bilayer, which is energetically favorable.

Which way do hydrophilic heads face?

Toward water (both outside and inside surfaces).

Where are hydrophobic tails located in the bilayer?

Inside, shielded from water.

Why do lipids rarely flip-flop across the bilayer?

The polar head would have to pass through the hydrophobic interior.

What enables lipids to flip between faces of the bilayer?

Transporter proteins (flippases, scramblases).

What evidence showed lipids form bilayers?

Extracted lipids spread into a monolayer covering twice the area of intact cells.

Why are lipids not very soluble in water?

Their hydrophobic tails resist contact with water.

Why do phospholipids form bilayers spontaneously?

Hydrophilic heads face water, hydrophobic tails hide inside.

What does amphipathic mean?

Molecules with hydrophilic heads + hydrophobic tails.

Are membranes symmetric or asymmetric?

Asymmetric — two halves have different phospholipids.

How is asymmetry preserved?

During budding and fusion, cytosolic monolayer always faces cytosol.

Why is asymmetry important?

Certain phospholipids/glycolipids must stay on one side to perform their functions.

Which phospholipids are on the noncytosolic half?

Phosphatidylcholine & sphingomyelin.

Which are on the cytosolic half?

Phosphatidylserine & phosphatidylethanolamine.

What happens to phosphatidylserine in apoptosis?

It flips to the cell surface, signaling phagocytes.

Which enzymes regulate phosphatidylserine distribution?

Scramblases & flippases.

What signals a healthy cell vs apoptotic cell?

A healthy cell has active flippases, keeping phosphatidylserine (PS) on the cytosolic (inner) leaflet. An apoptotic cell inactivates flippases and activates scramblases, causing PS to move to the outer (extracellular) leaflet and signal phagocytes for removal.

How does cold affect membranes?

Slows motion, makes membranes more rigid.

How do organisms adapt to cold?

More unsaturated fatty acids & shorter tails.

Example of cold adaptation?

Antarctic fish: high % unsaturated phospholipids.

How do double bonds affect packing?

Cis double bonds kink tails, reducing tight packing.

What % of animal cell plasma membrane is cholesterol?

Up to 20% (by weight).

What does cholesterol do in the bilayer?

Fills gaps left by unsaturated tails, making membrane stiffer.

Does cholesterol affect bilayer thickness?

No, thickness is unchanged.

Is cholesterol a phospholipid?

No — lacks phosphate; it’s a lipid with small hydrophilic –OH head.

Do membrane proteins/lipids flip orientation during trafficking?

No — cytosolic side always faces cytosol.

Example: LDL receptor extracellular domain → where is it in a vesicle?

Inside the vesicle lumen

When vesicles fuse with plasma membrane, how do lipids orient?

The cytosolic leaflet stays cytosolic, and the vesicle lumenal leaflet becomes extracellular. Thus glyco-lipids/proteins on the lumenal side end up outside the cell. Lipids and proteins then mix laterally in the membrane; flip-flop between leaflets is rare unless scramblases/flippases are active.

Which membrane molecules have sugars attached?

Glycoproteins, proteoglycans, glycolipids.

Where are carbohydrates located?

Only on the outside of the plasma membrane.

What do carbohydrate they form collectively?

The carbohydrate layer (glycocalyx).

What determines bilayer fluidity?

Hydrocarbon tails — length & saturation.

Shorter tails = ?

More fluid.

Unsaturated tails = ?

More fluid (due to kinks).

At higher temps, how do cells adjust tails?

Make longer, less unsaturated tails to prevent too much fluidity.

Can bacteria & yeast use cholesterol?

No, only animal cells do.

What do phospholipids, glycolipids, and triacylglycerols all share?

A glycerol backbone + fatty acids.

What differs on the 3rd carbon of glycerol?

For a phospholipid, the 3rd carbon is esterified to a phosphate group, which typically also links to a polar head group. For a glycolipid, the 3rd carbon is esterified to a sugar moiety. For a triacylglycerol, the 3rd carbon is esterified to a third fatty acid, meaning all three carbons of the glycerol backbone are linked to fatty acids.

Plant vs animal fats at room temp?

Plant fats (like vegetable oils) are generally unsaturated, meaning their fatty acid tails have double bonds that cause kinks. These kinks prevent tight packing, making them liquid at room temperature. Animal fats (like butter or lard) are typically saturated, with straight fatty acid tails that pack tightly together, making them solid at room temperature.

How is margarine made from vegetable oil?

Margarine is made from vegetable oil through a process called hydrogenation. This chemical process involves adding hydrogen atoms to the unsaturated fatty acids in the vegetable oil,. his addition converts some or all of the carbon-carbon double bonds (C=C) into single bonds (C-C), effectively 'saturating' the fatty acid

Where are phospholipids inserted into the ER?

Into the cytoplasmic monolayer.

What is the role of scramblase in the ER?

Transfers phospholipids randomly between the two leaflets.

Where are specific flippases found?

In the Golgi and plasma membrane.

What does the flippase for phosphatidylethanolamine do?

Transfers it to the cytoplasmic monolayer.

Where does glycosylation of membrane lipids occur?

In the Golgi lumen.

Which bilayer face ends up with glycolipids at the plasma membrane?

The extracellular bilayer.

How is membrane orientation maintained during transport?

Orientation of lipids and proteins is preserved during vesicle budding and fusion.

Why is membrane fluidity important for proteins?

It allows proteins to move within the membrane and to interact for signaling.

Where does new membrane lipid synthesis occur?

On enzymes on the cytosolic surface of the ER.

Where are large amounts of new lipids transported after the ER?

In vesicles to the Golgi and other organelles.

How does fatty acid tail length affect membrane fluidity?

Longer tails interact more strongly, reducing fluidity.

How does packing affect membrane fluidity?

More tightly packed tails = less fluid.

Why do saturated fatty acid tails reduce fluidity?

They lack double bonds, so they pack closely together.

What is the basic structure and permeability of cellular membranes?

Phospholipid bilayer is the main structure; semi-permeable.

Why is SA:V crucial and how do cells manage it?

Diffusion is rate-limiting; larger surface area and smaller volume manage efficient transport.

What are the main components and their roles?

Phospholipids: structural bilayer; Proteins: transport and signaling.

How do saturated and unsaturated tails differ?

Saturated: single C–C bonds; Unsaturated: at least one C=C double bond.

What is the fluid mosaic model?

Components move; it’s flexible and not falling apart.

How is membrane orientation maintained and why is asymmetry important?

Two leaflets: asymmetry allows distinct functions.

What determines membrane fluidity?

Temperature, fatty acid saturation, and tail length affect fluidity.

Where are lipids made and how are they distributed?

Synthesis occurs at Smooth ER; vesicles carry to Golgi.

Differentiate semipermeable and selectively permeable.

Semipermeable: passive based on size; Selectively permeable: regulated via proteins.

Why does diffusion limit cell size and how do membranes help?

Diffusion limits speed; small volumes keep distances short.

List main parts & overall functions of the plasma membrane.

Parts: phospholipids, proteins, cholesterol; Functions: barrier, regulated transport.

How do you label a phospholipid diagram?

Head: polar/charged, hydrophilic; Tails: fatty acids, hydrophobic.

Why do saturated and unsaturated tails look different?

Saturated: straight chain; Unsaturated: kinks due to double bonds.

How does amphipathicity build bilayers?

Hydrophobic tails cluster inward; heads face water.

What is a liposome and how is it used?

Spherical bilayer vesicle with an aqueous core; used for drug delivery.

How do lipids move in the membrane?

Lateral diffusion, rotation, flexion; flip-flop is rare without enzymes.

How did hybrid cells demonstrate protein mobility?

Mouse + human cells fused; labeled proteins mixed over time.

Define flippase and floppase.

Flippase: specific, ATP-dependent transporter proteins that move phospholipids from the outer (non-cytosolic) leaflet to the inner (cytosolic) leaflet of the membrane, thereby promoting membrane asymmetry.

Floppase: transporter proteins that move phospholipids from the inner (cytosolic) leaflet to the outer (non-cytosolic) leaflet of the membrane.

How is orientation preserved during trafficking?

Cytosolic side remains cytosolic; extracellular side becomes lumen.

What does temperature do to membranes?

Cold decreases fluidity; warm increases risk of instability.

How do tail length and saturation change fluidity?

Shorter tails = more fluid; unsaturated = more fluid.

What do the numbers in fatty acid designation '20:2' mean?

20 carbons, 2 double bonds in the tail.

Define transition temperature.

The temperature at which a membrane shifts from gel-like to fluid state.

How does cholesterol regulate fluidity?

Stiffens at high temperatures; disrupts packing at low temperatures.