Cellular and Lipid Membranes: Chapter 5

• Chapter 5 is about membranes including cellular membrane and lipid membrane
• Cellular membranes are barriers that separate one area from another
• Cellular membranes are selectively permeable, allowing only certain substances to pass through
• These membranes are fluid mosaic and phospholipid-based.
• They're mainly made of a phospholipid bilayer, which has embedded proteins and other biomolecules
• The plasma membrane is a barrier which separates a living cell from its surroundings
• The plasma membrane manages what enters/exits the cell, receives external signals, initiates intracellular responses, and enables connections to nearby cells

Phospholipid Structure

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) carbon chain tails

• Phospholipids form a bilayer membrane

• The structure of a phospholipid includes:

  • Two nonpolar fatty acid chains
  • A glycerol molecule
  • A polar phosphate group

• Heads and tails align uniformly to make "flat" bilayers.

• On a larger scale they make spheres but in a close up they make large flat layers

Fluid Mosaic Model

• Molecules in the membrane are in constant motion, like boats on a lake.
• The membrane has various lipids and proteins called:
  • Integral membrane proteins
  • Peripheral membrane proteins
  • Glycoproteins
  • Glycolipids
• These components attach to structures, help anchor and stabilize the cell, and aid communication with other cells
• The two membrane layers are isolated and each layer has a different aqueous environment and composition of proteins

Lipid Membranes

• In a phospholipid bilayer polar heads face outward on each side and hydrophobic tails face each other
• Lateral movement of lipids and proteins is quick
• Lipids and proteins rarely flip across the lipid bilayer

Additional Membrane Proteins

• Integral membrane proteins extend into the "tails" section.
• Integral membrane proteins function in transport or communication across the membrane
• Peripheral membrane proteins are only on the "heads" side.
• Peripheral membrane proteins connect to the internal cytoskeleton
• Surface markers facilitate cell-cell recognition and attachment

Membrane Protein Functions

• Transporters move ions and molecules through channels or pumps.

• Enzymes are anchored in the membrane where needed.

• Receptors receive information or bind and bring inside, such as identifying bacteria to destroy them.

• Signal transduction activates pathways and passes signals to activate other pathways like cell division

• Structural proteins attach a cell to something else (another cell, cytoskeleton, etc.)

• Identity markers identify self from nonself to prevent the immune system attacking healthy cells

Membrane Transport

• The plasma membrane is selectively permeable
• This allows cytosol solutions to differ from the extracellular fluid because not everything can pass through
• All cells maintain an imbalance of sodium and potassium ions by regulating their movement
• Transport across a membrane can be passive (no energy) or active (requires ATP)

Selective Permeability

• Biological membranes are selectively permeable
• Small molecules and lipids are able to pass through the membrane
• Larger and charged/polar molecules cannot pass through the membrane
• Proteins are required to move charged and polar molecules through channels or to carry them across

Passive Transport

• Passive transport includes diffusion
• Diffusion happens when a substance moves from high to low concentration
• Net movement stops once equilibrium is reached, however particles travel back and forth
• When membranes are involved, only small nonpolar molecules diffuse directly

What is Diffusion

• A concentration gradient is a difference in the concentration of a substance over a distance
• Particles move randomly
• Net movement goes from high to low concentration.
• Diffusion is the spreading of atoms or molecules from areas of high concentration to areas of low concentration

Diffusion Speed

• The speed of diffusion depends on concentration gradient strength, size/mass of molecules, temperature, and the state of matter
• The state of atoms/molecules affects diffusion speed:
  • Diffusion is slow in solids (like rock)
  • Faster in liquids
  • Fastest in gases (100 mm/min/10cm/min)
• When a concentration gradient disappears, particles are distributed uniformly
• Diffusion stops at equilibrium, also called dynamic equilibrium, because although there is no net movement, particles are always moving

Describing Diffusion

• Areas are always described in terms of the solute

• Hyper- sides have a higher solute concentration

• Hypo- sides have a lower solute concentration

• Iso- means the concentration on both sides is equal

• Other definitions:

  • A solution is a mix of two or more substances
  • The solute is the substance in a lesser quantity
  • The solvent dissolves/mixes with the solute

Diffusion & Membranes

• All things diffuse if there is a concentration gradient

• Solute diffusion across a membrane equates to dialysis

• Solvent diffusion across a membrane equates to osmosis

• If able to fit through a membrane they go from higher concentration to lower concentration

• Water diffuses from areas of higher concentration to lower concentration.

• Since water concentrations aren't typically measured, its location is determined by solute concentrations

Passive Transport: Help

• Dialysis is when some substances cross a membrane through direct diffusion
• Facilitated diffusion requires assistance from proteins, because:
  • Large molecules need help making large holes or openings
  • Ions or highly polar molecules need a hydrophilic channel (as they can't travel through a hydrophobic nonpolar tail)
  • Protein channels or Aquaporins help water diffuse rapidly

Facilitated Diffusion

• Uniports move a single solute in one direction down the gradient
• The energy comes from the concentration gradient, which makes it an exergonic process

Channel Proteins

• Channel proteins form a pore (tunnel) through the membrane
• The size and polarity determine what travels through it
• Some stay open and others are gated, which open/close based on electric/chemical signals
• They are essential for the nervous system
• Three conditions determine ion movement: concentration, voltage and gates:
  • Relative concentration on either side of the membrane
  • Voltage differences (because ions have electrical charge)
  • Gate-regulated signals

Examples of Channels

• Specific channels include:
  • Aquaporins which are specific for H2O
  • Gated ion channels, located in muscle cells to help with contraction

Carrier Proteins

• Carrier proteins bind to a molecule and change shape to push it across the membrane
• Usually each carrier protein is specific to a single substance
• Carrier proteins transport ions, sugars amino acids
• A concentration difference is required and these often allow movement in either direction as gradients change

Osmosis

• Osmosis is for diffusion of water.
• A hypertonic side has more hydrogen bonds to molecules, so doesn't have as many free water molecules
• A hypotonic side contains fewer hydrogen bonds, so has more free water molecules
• Water goes from the side with lower concentration (hypo) to the to higher concentration (hyper) because that's where the water has less restrictions
• NOTE: Solute concentration and diffusion direction should be considered when labeling a cell/material as hyper/hypo

Direction of Water

• In living membranes, size and polarity determine which substances can diffuse based on their gradients
• In artificial membranes, size is the only factor
• In an artificial cell with high solute on the right, it is hypertonic to the left.
• Effective water concentration is lower on the hypertonic side due to solute molecules
• Thus, water moves towards the right to dilute the solute

Tonicity

• Tonicity influences the volume of a cell by affecting osmosis
• Tonicity depends on a membrane
• osmotic pressure and solute amounts.
• When comparing tonicity, it helps to add a suffix to the comparisons (direction of solute and solvent diffusion are determined by total solute concentration):
  • Isotonic is when the solute concentration is equal on both sides
  • Hypotonic is when a side has less solute
  • Hypertonic is when a side is has more solute

Isotonic Environments

• The term environment always refers to the outside of the cell
• Living cells in an isotonic environment won't change in size
• There is no net water movement when solute concentrations are equal
• The inside of isotonic environments is isotonic, meaning cells are similar in size.
• Animal cells thrive in a 0.9% solute environment

Hypertonic Environments

• Living cells in hypertonic environments shrink, as a net water movement proceeds out of the cell

• The interior of hypertonic environments is hypotonic to the exterior

• Animal/plant cells shrink

• These cells exhibit different effects when shrunk;

  • Red blood cells appear spiky (crenate)
  • Plant cells undergo plasmolysis, pulling away from the wall

Hypotonic Environments

• Living cells in hypotonic environments swell because a net water movement goes into the cell
• When cells are in a hypotonic environment, the interior ends up hypertonic to the outside
• Both types of cells swell, however animal cells risk bursting
• Plant cells are supported against the osmotic pressure because of vacuoles and cell walls that regulate size

Balance

• Cells regulate and maintain balance to overcome osmosis' negative effects
• Extrusion involves cells like paramecium, which eject water through contractile vacuoles
• Isosmotic regulation keeps cells isotonic - matching concentrations, not transporting water

Marine creatures adjust salinity and aren't in freshwater.

Molecules

• In passive transport, molecules don't need energy because it is a constant form of diffusion
• During passive transport, the molecules are traveling along the down gradient
• Since free of energy there is a spontaneous -ΔG
• Passive transport includes dialysis, osmosis and facilitated diffusion
• In active transport, energy is required so molecules travel against gradient with support from protein carriers

Active Transport

• Active transport is used when cells require a large concentration of substances up against the membrane
• It requires an external source of energy
• Protein helpers cannot be channels because something must force solutes against the gradient
• The protein helpers are always carriers

Transport Types

• Primary transports with ATP as energy
• Secondary uses an electrochemical gradient

Carrier Proteins - Types

• Uniporter- Carries one molecule/ion
• Symporter- Carries two different molecules in same direction
• Antiporter- Carries two different molecules in different directions

Electrochemical Gradients

• Electrochemical gradients is when concentration gradients and electrical gradients combine
• An electrical gradient uses a crucial cytoplasm featuring charged molecules for cell function

Primary Active Transport

• Primary active transport moves a molecule up its concentration gradient and uses hydrolysis

• It important for cell types like neurons

• Gradients have opposite high concentration sides

• gradients are maintained and made through transport and one ATP

• Primary transport transports transports gradients through different means

  • Neurons
  • gradients that are high -1 ATP to pump 3 Na+ and for every two K+
  • Negative charge

• The Primary Example is Sodium/Potassium Pump (Na+-K+)

Secondary Active Transport

• Secondary active transport uses ATP energy indirectly through an electrochemical or primary transport
• Glucose and acids make their way due to this system

Glucose Transport

• Glucose is transferred to cells through pumps
• Intestinal cells maintain a good amount of concentration with a Na-K pump
• Glucose is transported through Glucose-Na + to create energy
• Once here a transport is passed down a gradient

Bulk Transport

• Active transport includes the movement of vesicles using ATP.
• It uses energy for organelles and effects of cytoplasm.
• Bulk transport moves the vesicles, so there is not a concentration in the transport.
• The 2 types are Endocytosis when importing inward -There are 3 types of bulk transport: -Phagocytosis
  • Pinocytosis
  • Receptor-Mediated Endocytosis

Endocytosis

• Phagocytosis happens when cells "eat" viruses, food or bacteria

• Food is then digested from fusion from lysosomes and vacuoles Example

• Blood Cells take wastes like bacterias and cells that can go into debris and go through the WBC

• Pinocytosis is when fluids known better as watery are transferred

• Pinocytosis transports from its source -The 3 types:

  • Specific or Targeted

Receptor-Mediated Endocytosis

Molecules uptake and selection.Receptors bind -Pit formation -Example: LDL receptor

Exocytosis

• Vesicles fuse and contain substances which go through plasma
• Particles and solutes like hormones fuse through cell waste.
• The final step takes pathway of carbs, proteins, and lipids for processes outside the cell