Cell Membrane Structure and Transport

Cell Membrane Structure and Transport

• Cell membranes are protective layers surrounding all cells.
• Composed of lipids, proteins, and carbohydrates.
• Semi-permeable; some molecules pass through, others do not.

Lecture Objectives

• Understand the basic structure and composition of the cell membrane.
• Understand the lipid bilayer structure and factors influencing fluidity.
• Understand the different mechanisms of molecule transport across membranes.

Cell Membrane Composition

• Lipids (40%): Phospholipids and cholesterol arrange in a bilayer.
• Proteins (50%): Embedded in the bilayer, performing various functions.
• Carbohydrates (10%): Attached to proteins (glycoproteins) and lipids (glycolipids), extending from the membrane surface.

Organelle Membranes

• Intracellular organelles also have membranes with lipid bilayers.
• Compositions slightly differ from cell membranes.
• Nucleus and mitochondria have double lipid bilayers.
• Endoplasmic reticulum, Golgi, and lysosomes have single lipid bilayers.

Lipid Bilayer Structure

• Phospholipids and cholesterol are the main components.
• Hydrophilic heads face water; hydrophobic tails face away from water, aggregating together.
• This arrangement forms a bilayer.

Phospholipids and Cholesterol

• Phospholipids have a hydrophilic head and hydrophobic tails.
• Cholesterol regulates membrane fluidity.

Cell Membrane Lipid Bilayer

• Membrane lipids are exposed to two forces: attraction to water by the hydrophilic heads, and hydrophobic tails avoiding water.
• Formation of bilayer: Hydrophilic heads face water on both sides; hydrophobic tails stay within the bilayer interior.

Cell Membrane Fluidity

• Membrane lipids and proteins move freely within the membrane.
• This creates a fluid structure.
• Fluidity depends on factors such as temperature, phospholipid tail structure, and cholesterol levels.

Cell Membrane Phospholipid Movement

• Phospholipids move and change places within the same monolayer (lateral diffusion) catalyzed by enzymes.
• Lipid molecules flex their tails and rotate rapidly around their long axes.
• Flip-flop (movement from one monolayer to the other) is rare.
• Fluidity depends on how tightly hydrocarbon tails pack together.

Factors Determining Cell Membrane Fluidity

• Temperature.
• Chemical structure of phospholipid tails (saturated vs. unsaturated).
• Cholesterol levels.

Effect of Temperature

• High temperatures increase fluidity as phospholipids have more energy to move and create spaces between them.
• Low temperatures decrease fluidity as phospholipids have less energy, pack closely, and interact more strongly, reducing movement.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids have only single bonds and are usually solid at room temperature.
• Unsaturated fatty acids contain double bonds, creating bends, and are typically liquid at room temperature.
• Saturated fatty acids pack tightly; unsaturated fatty acids create spaces between them.

Effect of Phospholipid Structure

• Length of hydrocarbon tails.
• Short hydrocarbon tails interact less increasing fluidity.
• Double bonds in hydrocarbon chains cause bends increasing fluidity.

Effect of Cholesterol

• Cholesterol fits between phospholipid molecules.
• Regulates membrane fluidity.
• Acts as a buffer preventing extreme changes in fluidity at low and high temperatures.
• At high temperatures, cholesterol reduces fluidity and prevents melting.
• At low temperatures, cholesterol increases fluidity and prevents freezing.

Importance of Cell Membrane Fluidity

• Enables cells to adapt their shape and movement to different conditions.
• Enables membrane proteins to diffuse and interact.
• Ensures membrane molecules are distributed evenly between daughter cells when cells divide.
• Allows membranes to fuse, e.g., vesicles fusing with each other.

Membrane Proteins and Carbohydrates

• Proteins are embedded in the membrane (integral or peripheral).
• Integral proteins span the membrane, embedded in the lipid bilayer.
• Peripheral proteins are attached to the membrane surface.
• Proteins perform various functions: Transport, Receptors, Enzymes, Anchors

Membrane Carbohydrates

• Some lipids have sugars (glycolipids) attached to them.
• Most membrane proteins have sugars attached (glycoproteins).
• Important for protection, lubrication, and cell recognition.

Membrane Transport Proteins

• Cells and organelles must allow the passage of hydrophilic molecules such as inorganic ions, sugars, amino acids, and nucleotides.
• Vital for metabolism and function.

Types of Transport Proteins

• Channels: Allow ions of a particular size and charge to pass; need to be open.
• Transporters: Transport specific molecules that fit specific binding sites on the protein.
• These are similar to enzyme-substrate interactions.

The Cell Membrane is Semi-Permeable

• The hydrophobic interior of the lipid bilayer stops most hydrophilic molecules from passing.
• Lipid bilayer is selective.
• Small, non-polar molecules (O2, CO2, N2, steroid hormones) cross rapidly.
• Small polar molecules (H2O, ethanol) cross slowly.
• Large polar molecules (glucose, amino acids) and ions cross extremely slowly, if at all, and need transporter proteins.

Passive Transport

• Molecules move from high to low concentration regions without energy.
• Two types:
  • Simple diffusion: Small, non-polar molecules (e.g., gases) directly cross the lipid bilayer.
  • Facilitated diffusion: Large polar molecules and ions cross the lipid bilayer with the help of membrane transport proteins

Passive Transport of Charged Molecules

• For charged molecules, two forces determine the direction of transport:
  • Resting membrane potential: The inside of the cell is more negatively charged, attracting positively charged molecules.
  • Concentration gradient: Ions move from high to low concentration regions.

Active Transport

• Molecules move against their concentration gradient (from low to high concentration) requiring energy and pumps.
• Facilitated by specialized transporter proteins (pumps).
• Energy obtained from ATP hydrolysis or electrochemical gradients.

Summary of Membrane Transport

• Passive transport (simple diffusion, facilitated diffusion)
• Active Transport

Osmosis

• Crucial for water movement inside cells.
• Movement of water across the membrane is slow without special proteins called aquaporins.
• Water moves from low solute concentration to high solute concentration (across the membrane).

Explore the intricate structure of cell membranes, including their lipid and protein compositions. This quiz covers the mechanisms of transport and the unique features of organelle membranes. Test your knowledge on how these components function to protect and regulate cells.