Biology 1000: Chapter 6 - Cell Membranes

• Biology 1 – Cells, Molecular Biology and Genetics (Biol 1000) is taught by Professor Dr. Michael Cardinal-Aucoin in Winter 2025.

Cell Membranes (Chapter 6)

• The topics covered within this chapter are: Membrane Structure, Phospholipids, Cholesterol, Proteins, and Carbohydrates.
• The plasma membrane separates the living cell from its surroundings.
• Cell membranes are one of the few characteristics prokaryotic and eukaryotic cells have in common.
• In eukaryotes, membranes also form internal compartments.

Cell Membrane Functions

• Cell membranes serve to separate the inside of a cell from the outside, maintaining different environments.
• They act as a barrier to water-soluble (polar) molecules, charged molecules, and large molecules.
• Cell membranes regulate transport in and out of organelles or cells.
• They help maintain ion gradients for energy conservation and signaling.
• Cell membranes are involved in signaling, relaying signals from outside to inside the cell.
• The plasma membrane is 5-6 nm thick.

Cell Membranes Composed Primarily of Phospholipids

• Cell membranes consist primarily of phospholipids.
• Phospholipids are amphipathic, possessing a hydrophilic head and a hydrophobic tail.
• In an aqueous solution, phospholipids arrange in a bilayer with hydrophilic heads interacting with water on the outside and hydrophobic tails interacting inside.
• The types of phospholipids differ between monolayers, and these different types have different functions.
• The head of a phospholipid consists of glycerol + phosphate (+ other).
• The tails of a phospholipid consist of fatty acids.

Fatty Acids and Phospholipids

• Fatty acids are hydrocarbon chains ending in COOH (carboxylic acid).
• Saturated fatty acids have only single bonds between carbons and form straight chains.
• Unsaturated fatty acids have at least one double bond, causing a bend in the chain.

Phospholipids Structure and Function

• The major constituents of cell membranes are phospholipids.
• The structure of phospholipids include 2 fatty acid molecules, 1 glycerol molecule and 1 phosphate molecule and 1 additional component such as choline or serine

Membrane Mosaic

• Cell membranes are a mosaic, with other components embedded within the phospholipid bilayer.
• These components include cholesterol, proteins, and carbohydrates.

Cholesterol Influence

• Cholesterol is embedded within the membrane.
• The hydrophilic hydroxyl group (-OH) interacts with the hydrophilic heads of the phospholipids.
• It influences the fluidity of the membrane.

Membrane Proteins

• Membrane proteins can be be divided into two categories by association: peripheral and integral.
• Peripheral membrane proteins interact with the side (periphery) of the membrane.
• Integral membrane proteins penetrate into (integrated with) the membrane.

Protein functions

• Peripheral proteins interact with the phospholipid bilayer or integral membrane proteins by weak non-covalent bonds such as H bonds.
• They have temporary association with the membrane.
• They are generally used in internal cell signaling.
• Integral membrane proteins are permanently associated with the cell membrane.
• They are composed of three regions: 2 hydrophilic and one hydrophobic region.
• A large number of non-polar amino acids interact with non-polar fatty acid tails.
• Polar amino acids interact with a polar head and water.
• There are 6 major functions of inter-membrane proteins: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining and attachment.

Membrane Carbohydrates

• Carbohydrates can be attached to lipids or proteins associated with the membrane.
• Glycolipids are carbohydrates covalently bonded to lipids.
• Glycoproteins are carbohydrates covalently bonded to proteins.
• Membrane bound carbohydrates are involved in cell-cell recognition.
• Cells recognize other cells by binding to molecules that contain carbohydrates, located on the extracellular surface of the plasma membrane.
• Carbohydrates are important for immune reactions.
• The ABO blood groups, discovered by Karl Landsteiner in the early 1900s, are differentiated based on glycoprotein markers.
• If you receive blood from a donor who is not your blood type, your immune system will attack those blood cells as non-self.

Membrane Movement

• Cell membranes are fluid, not static, flexible and move.
• There are 3 types of movements of phospholipids in bi-layer: rotation, lateral movement (occurs 107 times per second) and flip-flopping across the membrane (is rare, once per month).
• Cell membranes are thus: flexible, repairable and expandable.

Membrane Fluidity

• Fluidity is influenced by length of the hydrocarbon tails, level of saturation of the hydrocarbon tails with respect to hydrogen and presence of the sterol cholesterol.
• Shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer.
• Lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions.
• Saturated fats pack tightly together.
• They are usually solid at room temperature and dominate animal fats.
• Unsaturated fats pack loosely.
• They are usually liquid at room temperature and dominate vegetable fats (e.g. oils).
• Cholesterol stiffens the bilayer and makes it less fluid.
• Cholesterol inserts into the membrane with its polar hydroxyl group close to the polar head groups of the phospholipids.
• The rigid hydrocarbon rings of cholesterol interact with and partly immobilize the regions of the fatty acid chains that are adjacent to the phospholipid head groups.
• Cholesterol makes membranes less fluid at high temperature.
• Cholesterol makes membranes more fluid at low temperature.
• Despite this, cholesterol makes the membrane less permeable overall.

Homeostasis

• Organisms adjust fatty acid composition and cholesterol concentration at different temperatures in order to maintain optimum fluidity for the function of that membrane.

Fluid Mosaic Model

• The cell membrane is a mosaic composed of a variety of lipids, proteins, and carbohydrates that are dynamic and fluid.
• This model is thus named "Fluid Mosaic Model".

Influences on Membrane Fluidity

• Increasing the amount of unsaturated fatty acids will increase the fluidity of the membrane

Cell Walls

• Cell walls are structural layers next to membranes in some cells.
• Bacteria's cell wall = peptidoglycan.
• Fungi cell wall = chitin.
• Plant cell wall = cellulose.
• Archaea cell wall lack peptidoglycan.

Membrane Composition

• Membrane composition varies depending on the organism, cell, organelle and regions of the same membrane.
• It is complex and dynamic.
• Composition is essential to function

Summary

• The following topics are summarized: Phospholipids, Cholesterol, Proteins (integral and peripheral), Carbohydrates (glycolipids and glycoproteins) Fluidity and Fluid mosaic model