lec 4 cell biochem foundatin

Cell Structure and Function

• Cells are the basic structural and functional units of life
• Most biochemical reactions take place within cells
• Cells are comprised of a variety of components, including organelles.

Cell Theory

• Developed by three scientists: Theodor Schwann, Matthias Schleiden, and Rudolf Virchow
• All organisms are made of one or more cells
• Cells are the basic building blocks of life
• All cells come from pre-existing cells

Cell Membrane Structure

• 50-80 Angstrom thick, lipid bilayer with polar heads outside and apolar hydrocarbon tails inside
• Highly fluid and dynamic structure
• Selective permeability (barrier between inside and outside of the cell)
• Asymmetric lipid bilayer with distinct inner and outer surfaces

Cell Membrane Structure - Components

• Phospholipid bilayer
• Hydrophobic hydrocarbon tail
• Hydrophilic phosphate head
• Cholesterol
• Glycoproteins
• Integral proteins
• Peripheral proteins

Davson-Danielli Model

• Saw two dark lines with a lighter band in between (on micrographs)
• Proteins typically look dark, lipids lighter

Falsification of Davson-Danielli Model (Freeze-Fracturing)

• This model is contrary to the Davson-Danielli model which only involves proteins coating the membrane's surface.
• A new model is needed that considers the presence of trans-membrane proteins.

Singer-Nicolson Fluid Mosaic Model

• Proteins occupy various positions within the membrane
• Proteins can move and change their location.
• Fluid-mosaic model describes the membrane like tiles in a mosaic.
• Phospholipids in membranes are in a fluid state, allowing the membrane to change shape.

Fluid Mosaic Model - Proportional to Lipids

• Temperature increase causes a change from order to disorder which results in membrane fluidity (transition temperature).
• Unsaturated fatty acids in cis form increase membrane fluidity.

Fluid Mosaic Model - "Cis" Conformation

• The "cis" conformation introduces kinks that influence membrane fluidity.

Fluid-Mosaic Membrane Model - Properties

• Membranes are not static and have fluid consistency.
• Most membrane lipids and proteins can drift about laterally within the membrane.
• Cholesterol enhances fluidity and reduces permeability to biological molecules.

Membrane Components

• Phospholipids, Cholesterol, Proteins, Glycocalyx

Phospholipids

• Amphipathic (have both hydrophilic and hydrophobic portions)
• Phospholipid phosphate groups are attracted to water, while hydrocarbon tails repel water and are attracted to each other.

Cell Membrane Lipids

• Phospholipids (e.g., phosphoglycerides, sphingomyelin, glycosphingolipids, cholesterol)
• Amphipathic lipids (e.g., lecithin, cephalin)
• Saturated fatty acids (straight tails)
• Unsaturated fatty acids (cis double bonds → kinks → fluidity)
• Lipid-soluble materials easily enter the cell through the membrane

Membrane Fat Composition and Flexibility

• Fat composition affects membrane flexibility and fluidity, which is about as fluid as thick salad oil.
• Unsaturated fatty acids in phospholipids keep the membrane less viscous.

Cholesterol in Membrane

• Contributes to membrane fluidity
• Tucked between phospholipid molecules
• Prevents packing and crystallization of fatty acids

Cholesterol in Membranes

• Found in animal cells
• A steroid type of lipid
• Mostly hydrophobic, attaches to lipid portion of membrane
• Has a small hydrophilic portion attached to the phosphate head
• Typically found between phospholipids

More Than Lipids

• Singer and Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer
• Hydrophilic regions of the protein are located on the exterior surface and hydrophobic regions are embedded within the bilayer.

Role of Cholesterol in Membranes

• Controls the fluidity of the membrane
• Too fluid, cannot control what enters/exits.
• Not fluid enough, restricts cell movement.
• Disrupts packing of hydrocarbon tails (not rigid).
• Reduces permeability of some ions.

Membrane Proteins

• Hormone Binding Sites
• Enzymes
• Cell Adhesion (tight junctions)
• Cell-to-Cell Communication receptors

Passive Transport Proteins

• Channels or passages through membrane proteins
• Channels are specific for certain substances.

Protein Pumps (Active Transport)

• Pumps release energy from ATP to move substances across the membrane

Cell Membrane Proteins

• Amphipathic
• Integral part of the membrane
• Interact with phospholipids (globular)
• Anchored to one leaflet/span the bilayer
• Peripheral Proteins
• Bound to hydrophilic regions of phospholipids or integral proteins.

Membrane Proteins Continued

• Proteins determine membrane functions.
• Cell membrane & organelle membranes each have unique protein collections
• Peripheral proteins are loosely bound to the surface of the membrane
• integral proteins penetrate the lipid bilayer, usually across the whole membrane
• Transport proteins (channels and permeases-pumps)

Membrane Proteins Domains

• Nonpolar amino acids within the membrane (hydrophobic) anchor proteins to the membrane
• Polar amino acids on the outer surfaces (hydrophilic), extend into the extracellular fluid and cytosol.

Signal Transduction

• Membrane proteins have many functions, including transporting substances (transporters), catalyzing reactions (enzyme activity), acting as receptors, adhering to other cells (cell adhesion), and attaching to the cytoskeleton.

Membrane Carbohydrates (Glycocalyx)

• Branched molecules (15 or less sugar units)
• Bound to lipids (glycolipids) or proteins(glycoproteins)
• Play a key role in cell-cell recognition (ability of a cell to distinguish one cell from another).
• Antigens important in organ and tissue development; basis for rejection of foreign cells.
• Involved in immune system and blood grouping.

Functions of the Plasma Membrane

• Forms a physical barrier between cell interior and exterior.
• Selectively permeable: allows some substances to pass but not others.
• Regulates movement of substances into and out of the cell
• Connects cells to other cells and surrounding structures.
• Involved in communication, coordination, and chemical reactions
• Cell recognition labels/identifies the cell

Transport Processes

• Passive transport: substances moving from high to low concentration; no energy required.
  • Simple diffusion (through a semipermeable membrane)
  • Facilitated diffusion (transport proteins)
  • Osmosis (water transport)
• Active transport: substances moving from low to high concentration; requires energy.
  • Protein-assisted
  • Endocytosis
  • Exocytosis

Types of Passive Transport

• Simple diffusion: molecules diffuse directly through the semipermeable bilayer without the aid of transport proteins (examples: oxygen, carbon dioxide).
• Facilitated diffusion: transport of polar substances or ions across a semipermeable membrane that requires transport proteins (examples: ions, salts, potassium, etc.)
• Osmosis: diffusion of water across a semipermeable membrane.

Transport Mechanisms

• Passive Diffusion (Simple)
• Moves small molecules across the membrane passively.
• No energy required.
• Diffusion proceeds down the concentration gradient (high to low).
• Passive Diffusion (Facilitated)
• Carrier-mediated process with carriers that can be saturated like enzymes.
• Bi-directional movement
• No energy required
• Faster than simple diffusion

Facilitated Diffusion

• Passive transport that does not require energy.
• Uses transport proteins to move molecules from high to low concentration.
• Examples: Glucose or amino acids moving from the blood into a cell.

Active Transport

• Requires the cell to use energy.
• Molecules move against the concentration gradient (low to high).
• Used for transporting large molecules, concentrating molecules inside the cell, and removing waste.

Types of Active Transport - Specific Examples

• Sodium Pump (antiport): extrudes sodium, imports potassium in cells actively (high intracellular potassium, low intracellular sodium).
• Calcium pump: regulates muscle contraction (transports calcium).
• Secretion of H+ by parietal cells in the stomach (proton pumps).
• Uptake of iodide by cells of the thyroid gland.
• Transportation of glucose in small intestine through glucose/sodium symport and glucose uniport

Receptor G-Protein

• Regulatory molecules
• Receptor
• G protein (alpha, beta, gamma)
• GDP/GTP
• Enzyme or ion channel

Receptor-G-Protein-Enzyme (Mechanism)

• Non-steroid hormone → Receptor → GDP → GTP → Second messenger → Effect on cellular activity .

Biochemical Reaction

• Location where electron transporters and ATP synthase take place

Exocytosis and Endocytosis

• Types of bulk transport
• Exocytosis: moves substances out of the cell
• Endocytosis: moves substances into the cell

Clinical Aspects of Endocytosis

• Receptor-mediated endocytosis is responsible for many diseases (viruses, hepatitis, poliovirus, AIDS).
• Iron toxicity can occur from excessive uptake.

Clinical Correlates of Cell Membrane Protein

• Hereditary spherocytosis
• Cystic Fibrosis

Hereditary Spherocytosis

• Abnormal erythrocytes (sphere-shaped, not biconcave disk shaped).
• Dysfunctional membrane proteins interfere with the cell's ability to be flexible.
• Increased destruction of RBCs (hemolytic anemia).

Hereditary Spherocytosis

• Causes abnormality of erythrocytes
• Mutations in genes relating to membrane proteins lead to erythrocytes shape change.
• Red blood cells are more prone to rupture
• Cells with damaged proteins are destroyed by the spleen leading to hemolytic anemia.

Cystic Fibrosis

• Inherited CF gene directs epithelial cells to produce a defective cystic fibrosis transmembrane conductance regulator (CFTR) protein.
• CFTR protein regulates the transport of ions in cells lining lungs, digestive tract, mucus glands, and genitourinary system.

Important Points

• Cell membrane function linked with cell theory.
• Membrane components and characteristics.
• Cell transport system (active and passive transports)

Questions

• Scientists proposed the Cell Theory
• Major components of cell membrane
• Main types of membrane proteins
• Meaning of amphipathic and peripheral proteins
• Different types of active/passive transport proteins
• Types of facilitated diffusion
• Examples of facilitated/active transport
• Viruses affected by endocytosis

Fill in the Blank(s)

• (Answers will depend on the specific blanks from the provided images)

Test your knowledge of cell theory and membrane structure with this quiz. You'll answer questions about key scientists, historical observations, and the significance of cellular components. Ideal for students studying basic biology concepts.