Cell Membrane Structure and Transport

Cell Membrane Structure and Transport

• The cell membrane is a protective layer surrounding all cells
• It's composed of lipids, proteins, and carbohydrates
• The membrane is selectively permeable, allowing some molecules to pass through but not others

Cell Membrane Composition

• Lipids (40%):
  • Phospholipids and cholesterol form a bilayer
• Proteins (50%):
  • Embedded in the bilayer, performing various functions
• Carbohydrates (10%):
  • Attached to proteins (glycoproteins) and lipids (glycolipids)
  • Extend out of the membrane surface

Organelle Membranes

• Intracellular organelles have membranes too
• These membranes have similar lipid bilayer structures, but slightly different compositions than the cell membrane
• Nucleus and mitochondria have two lipid bilayers. Other organelles like ER, Golgi, and lysosomes have one.

Lipid Bilayer Structure

• Phospholipids have hydrophilic heads and hydrophobic tails
• Hydrophilic heads interact with water, while hydrophobic tails cluster together, forming a bilayer

Phospholipids and Cholesterol

• Phospholipids have a polar head (hydrophilic) and nonpolar tails (hydrophobic).
• Cholesterol is interspersed within the bilayer, modulating fluidity.

Cell Membrane Lipid Bilayer

• Membrane lipids are exposed to two opposing forces: attraction to water (hydrophilic heads) and repulsion from water (hydrophobic tails).
• The bilayer forms due to these forces, with hydrophilic heads facing water on both surfaces, and hydrophobic tails clustered within the bilayer's interior

Cell Membrane Fluidity

• Membrane lipids and proteins can move freely within the membrane
• This movement creates a fluid structure

Cell Membrane Phospholipid Movement

• Phospholipids move and change positions within the same monolayer (lateral diffusion)
• This movement is catalyzed by enzymes
• Lipid molecules rarely move from one monolayer to the other (flip-flop)
• Fluidity depends on the tightness of packing of hydrocarbon tails

Factors Determining Cell Membrane Fluidity

• Temperature: Increased temperature increases fluidity, decreasing temperature decreases it.
• Chemical structure of phospholipid tails: Unsaturated phospholipids increase fluidity while saturated phospholipids decrease fluidity
• Cholesterol levels: Cholesterol acts as a buffer, maintaining fluidity at both high and low temperatures

Effect of Temperature on Cell Membrane Fluidity

• High temperatures: Increased energy allows more movement between phospholipids - thus increasing fluidity
• Low temperatures: Decreased energy results in less movement, tighter packing, and decreased fluidity.

Saturated vs Unsaturated Fatty Acids

• Saturated fatty acids have only single bonds between their carbon atoms, resulting in a straight, tightly packed structure at room temperature.
• Unsaturated fatty acids have one or more double bonds between carbon atoms, leading to bent structures and a lower packing density; more liquid-like at room temperature

Phospholipid tails

• Consist of two fatty acid tails
• Can be saturated or unsaturated

Effect of Phospholipid Structure on Cell Membrane Fluidity

• Length of hydrocarbon chains:
  • Shorter hydrocarbon tails less interact with each other, which increases fluidity
• Double bonds:
  • Hydrocarbon tails with double bonds are bent, increasing the space between hydrocarbon tails and increasing fluidity.

Effect of Cholesterol on Cell Membrane Fluidity

• Cholesterol fits in the spaces between phospholipid molecules and regulates membrane fluidity.
• Acts as a buffer to prevent extreme changes in membrane fluidity at both low and high temperatures

Importance of Cell Membrane Fluidity

• Allows cells to adapt their shape and movement to different conditions
• Enables membrane proteins to diffuse and interact with each other for cell signaling
• Ensures membrane molecules are evenly distributed between daughter cells during cell division
• Allows membranes to fuse together

Membrane Proteins and Carbohydrates

• Proteins are embedded within the membrane (integral proteins) or attached to its surface (peripheral proteins)
• Carbohydrates are attached to proteins and lipids on the cell's surface, forming glycoproteins and glycolipids.

Membrane Protein Functions

• Transporters:
  • Move ions and macromolecules in and out the cell
• Receptors: Detect chemical signals from the environment
• Enzymes: Catalyze chemical reactions
• Anchors: Connect the cell membrane to the cytoskeleton

Membrane Carbohydrates

• Some membrane lipids have sugars attached (glycolipids).
• Many proteins also have sugars attached (glycoproteins).
• Polysaccharides in chains form proteoglycans.
• Important for protection, lubrication, and cell-cell recognition

Membrane Transport Proteins

• Membrane transport proteins are needed in cells to allow the passage of certain hydrophilic molecules
• Channels allow particular ions to pass based on size and charge
• Transporters move molecules, which fit in specific areas of the protein (similar to enzyme-substrate binding)

The Cell Membrane is Semi-permeable

• The hydrophobic interior of the lipid bilayer prevents the passage of most hydrophilic molecules.
• The lipid bilayer is selective:
  • Small, non-polar molecules cross rapidly (e.g., O2, CO2, N2, steroid hormones)
  • Small polar molecules cross slowly (e.g., H2O, ethanol)
  • Large polar molecules and ions require transporter proteins

Transport Proteins

• Cells and organelles need transport proteins to move hydrophilic molecules across membranes.
• Examples of transport proteins include channels and transporters

Types of Ion Channels

• Voltage-gated channels open in response to membrane potential changes.
• Ligand-gated channels open when a molecule binds to them (such as neurotransmitters at synapses)
• Mechanically-gated channels open in response to mechanical forces (like auditory hair cells)

Resting Membrane Potential

• The cell membrane is not permeable to ions
• Differences in ion concentrations inside and outside the cell create a voltage difference (resting membrane potential)
• The inside of the cell is usually slightly more negative than outside.

Mechanisms for Membrane Transport:

• Concentration Gradient: The difference in concentration of a substance across a membrane.
• Diffusion: Molecules move from high to low concentration.
• Passive Transport:
  • Simple Diffusion: Small, nonpolar molecules cross the lipid bilayer directly
  • Facilitated Diffusion: Large, polar molecules use transporter proteins to cross.
• Active Transport:
  • Molecules move against their concentration gradient, requiring energy (ATP).

Osmosis

• Crucial for water movement into and out of cells
• Water moves from areas of low solute concentration to high solute concentration
• This process is facilitated by aquaporins

Explore the intricate details of cell membranes, including their composition, structure, and the functional significance of lipids, proteins, and carbohydrates. This quiz covers the selectively permeable nature of cell membranes and the membranes of various organelles. Test your knowledge on key concepts related to cell membrane structure.