AP BIO 2.3-2.8

Fluid Mosaic Model

Structure of membranes where phospholipids form a bilayer which is like a “lake” in which a variety of proteins “float”

• has a “mosaic” of components, primarily phospholipids, cholesterol, and proteins

  • phospholipid bilayer embedded with proteins

  • polar hydrophilic phosphate head and non-polar hydrophobic tails

• constantly moves to fill in gaps and open up for substances 

Integral membrane proteins

Proteins that are permanently attached to a biological membrane.

• have hydrophobic mid-portions that hold the protein within the hydrophobic parts of the membrane

• Integral proteins found inside the membrane are hydrophibic, and ones exposed to the cytoplasm are hydrophilic hydrophobic

• Transmembrane proteins:

  • A type of integral membrane protein that spans the entire lipid bilayer.

  • may cross the membrane once, or as many as 12 different membrane-spanning sections

Membrane Fluidity

Cells must maintain membrane fluidity within a narrow range to maintain proper structure and function

Important factors:

• Lipid composition

• Temperature

What can pass:

• small,nonpolar substances — very quick and can squeeze through

• small, polar substances — can pass w/o proteins, but very slow

• large, nonpolar — can pass but is very slow

• large, polar substances or ions — to difficult to pass through without proteins

Proteins in membrane

• Proteins move across the membrane evidenced by when membrane proteins fuse

  • The higher the temperature, the more cells with mixed proteins

• Proteins inside the cell can restrict the movements of membrane proteins, as can attachments to the cytoskeleton 

• ROLE: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and the ECM

peripheral membrane proteins

proteins that are loosely attached to the outer or inner surface of a cell's membrane 

• attached either to integral proteins or phosphlipids 

• don’t attach to the hydrophobic core

Carbohydrate on membrane

Plasma membrane carbohydrates are located on the outer membrane and can serve as recognition sites

Restricted to exterior surface

• Glycolipid: a carbohydrate bonded to a lipid

• Glycoprotein: a carbohydrate bonded to a protein

Fatty acids and fluidity

Saturated fats:

• pack tightly together at low temperatures, making a dense and rigid membrane

Unsaturated fats:

• cannot pack tightly together, staying more fluid at lower temperatures

Membranes have a mix of unsaturated and saturated phospholipids

• example: fishes adjust to more unsaturated phospholipids in lower temperatures

Cholesterol in membranes

• in the core of membrane

• minimizes effects of temperature fluidity to maintain a healthy fluidity 

• in lower temperatures, cholesterol increases fluidity by keeping phospholipids from packing 

• at higher temperatures, cholesterol reduces fluidity 

Membranes are bifacial

the inside layer is different than the outside layer, since proteins have specific orientations and carbohydrates are found only on the outer surface.

• the outside of the plasma membrane is the same as the inner surface of the ER, golgi, and vesicle membranes

Gibb’s Free Energy and Diffusion

Diffusion is spontaneous because particles move down their chemical potential gradient, making the Gibbs free energy change negative until equilibrium (ΔG = 0) is reached.

• When diffusion happens, entropy increases and Gibb’s free energy decreases

Osmosis

Net movement of water across a semipermeable membrane from an area of low solute to high solute

3 types of tonicity

1. Hypertonic: increases in volume, when there is a higher solute concentration

2. Hypotonic: decreases in volume, when there is a lower solute concentration

3. Isotonic: stable volume, no net flow change, when there is equal solute concentration

   1. Red blood cells’ ideal condition

Passive Transport

• transportation that does not require ATP, whcih moves down a concentration gradient 

• Allows small molecules, nonpolar molecules, and uncharged molecules 

• Facilitated diffusion:

  • diffusion involving channel proteins that may be gated which open or close in response to a ligand

  • involves substances that have charged or are too big 

  • support from proteins in order to pass through hydrophobic parts 

  • may involve carrier proteins

  • Facilitated diffusion systems can be saturated

  • Carrier proteins: carry substances into the cell 

  • Channel protein: has holes to allow ions into the cell 

Electrochemical gradient 

the combined force of a chemical gradient and an electrical gradient that drives the movement of ions across a membrane

• the positive and negative charges are seperated by the membrane

• Inside the cell would have extra negative charges compared to the outside 

Active Transport

Requires input of energy to move substances AGAINST their concentration gradients

• can have secondary and primary active transport

Primary active transport

• uses chemical energy

• ex. sodium potassium pump

sodium-potassium pump

• has an integral membrane protein that pumps sodium out and potassium into the cell

• Involves ATP

• Typically, there is a higher concentration of sodium outside, and a lower concentration of potassium outside

• How it works:

1. The pump is open to the inside of the cell. The pump has a high affinity for sodium ions, so it takes 3 sodium ions

2. The sodium binding to the protein triggers hydrolysis of ATP → P-group is attached to get phosphorylated while ADP gets released

3. Phosphorylation changes the pump’s shape, re-orienting it so that it opens towards the extracellular space, where the pump no longer has a high affinity for sodium and releases it

4. The pump switches and now has a high affinity for potassium, which binds the 2 potassium ions and triggers the phosphate to be removed

5. Without the phosphate, the pump returns to its original shape, facing inward

6. Inward, they lose affinity for potassium and release it. The process begins from number 1 again.

Secondary active transport

• use electrochemical gradient as an energy source

• Does not directly require chemical energy 

Proton sucrose pump

• secondary active transport

• The proton pump pumps h+ across a membrane to establish a gradient 

• A cotransporter will then pump an H+ back into the cell, carrying a long a sucrose molecule

• The cotransport protein relies on the proton gradient created by the proton pump to function. If the pump stops, the gradient is lost and cotransport ceases. 

Symporter v.s Antiporter

The main difference is the direction of transport: symporters move two or more substances in the same direction across a membrane, while antiporters move two substances in opposite directions.

• antiporters have one thing going against the concentration gradient and the other going down concentration gradient

Endocytosis

• For things that are too large or too charged to pass directly through biological membranes and need to pass via vesicles

• Bulk transport, which transports large particles or large quantities

• Invaginates the molecule/s, forming a pocket which then pinches off with the help of proteins

  • Phagocytosis:

    • “cell eating”, transporting a large molecule to the lysosome for digestion

    • used to hunt pathogens

  • Pinocytosis:

    • “Cell-drinking”, taking in small amounts of extracellular fluid and molecules

    • held in smaller vesicles than phagocytosis

  • Receptor-mediated endocytosis:

    • Receptor proteins capture target molecules

    • The binding triggers endocytosis

    • may be used by harmful substances to gain access

Exocytosis

• Opposite of endocytosis

• The vesicle becomes the cellular membrane

Extracellular matrix of animal cells

• collagen: protein modified by carbohydrates that play a role in giving tissues strength and structural integrity

• proteoglycans: interwoven with collagen and is attached to a long polysaccharide backbone

• integrins: bridges between integrins and other proteins

• integrins: anchor cell to extracellular matrix, help sense environment by detecting cehmical and mechanical cues from the extracellular matrix

Cell wall

• can have a cubic structure

• cell wall → plasma membrane → cytoplasm

• combined with the vacuole’s pressure allows plants to stand up right

• cell walls have fibrous polysaccharides that gives it its structure

• in mature cells, once it stops growing, the layers of cellulose and molecules can be built to form a secondary cell wall layer

  • this is what allows wood to still have its rigidity and structure even without water 

• there are direct tunnels called plasmodesmata that allows small molecules to flow through the cell wall

Explain this

• Decreased FA chain length: Shorter tails → less interactions → more fluid, lipids spread into the hole quickly.

• Increased desaturation: More C=C “kinks” prevent tight packing → more fluid → faster recovery.

• Increased cholesterol: At typical physiological temps, cholesterol reduces lipid mobility → less fluid → slower recovery.

• Increased membrane proteins: Protein crowds up and create obstacles for the phospholipids to rearrange → least fluid → slowest fill.

Water potential

pressure potential + solute potential (-iCRT)

• water moves towards lower water potential

Volume and Surface Area formulas