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BIOLS 102 Dr. Tariq Alalwan
Chapter 5: Plasma Membrane: Structure and Function 1
Biology, 10e
Sylvia S. Mader
Lectures by Tariq Alalwan, Ph.D.
Learning Objectives
Describe the structure of the plasma membrane and the diverse functions of the embedded proteins.
Describe what is meant by a semipermeable membranemembrane.
Predict the effect of osmotic conditions on animal versus plant cells.
Compare and contrast the passive means of crossing a plasma membrane.
Compare and contrast the active means of crossing a plasma membrane.
The Phospholipid Bilayer The plasma membrane is a phospholipid bilayer with partially or wholly embedded proteins
Phospholipids are amphipathic – molecules that have both hydrophilic and hydrophobic regions
Nonpolar tails (hydrophobic) are directed inward
Polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid
Cholesterol – a lipid found in animal plasma membranes that helps modify the fluidity of the membrane
The proteins are scattered throughout the membrane forming a mosaic pattern
BIOLS 102 Dr. Tariq Alalwan
Chapter 5: Plasma Membrane: Structure and Function 2
Plasma Membrane of an Animal Cell
Plasma Membrane Structure The plasma membrane is asymmetrical, how?
Membrane proteins may be integral (embedded) or peripheral
Integral proteins are found in the membrane and are Integral proteins are found in the membrane and are held in place by the cytoskeleton and the extracellular matrix (ECM)
Peripheral proteins are found on the inner membrane surface
ECM are only found in animals and their functions include supporting the plasma membrane and communicating between cells
BIOLS 102 Dr. Tariq Alalwan
Chapter 5: Plasma Membrane: Structure and Function 3
Fluid‐Mosaic Model
The fluid‐mosaic model describes the plasma membrane
The fluid component refers to the phospholipids bilayer of the plasma membrane (PM)p ( )
The mosaic component refers to the protein content in the PM
Fluidity of the plasma membrane allows cells to be pliable (flexible)
Protein movements are limited by interactions with the cytoskeleton and ECM
Membrane Fluidity
Four main factors contribute to membrane fluidity
Temperature – at body temperature, the phospholipid bilayer has the consistency of olive oil
b h h li id il l h h h d b Membrane phospholipid tail length – shorter hydrocarbon tails can move sideways (lateral) more easily; rarely flip‐flop, why?
The degree of unsaturation of membrane phospholipid tails
Amount of cholesterol ‐ keeps the hydrocarbon tails fluid at cold temperatures, and stabilizing them at high temperatures
Membrane Fluidity (cont.)
Phospholipid Movement With Cholesterol
Unsaturated/Saturated
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Chapter 5: Plasma Membrane: Structure and Function 4
Carbohydrate Chains Membrane contain carbohydrate chains linked to phospholipds “Glycolipids” and proteins “Glycoprotein” on the extracellular surface
Glycocalyx – a ‘sugar coat’ in animal cells that Glycocalyx a sugar coat in animal cells that facilitates cellular adhesion, protection, signal reception and cell‐cell recognition
Carbohydrate chains vary by number (from 15 to 100’s), sequence of sugars and whether the chain is branched (a “fingerprint”)
Carbohydrate chains are the basis for A, B and O blood groups in humans
Functions of Membrane Proteins The manner in which a protein associates with a membrane depends on its structure and can be categorized as follows
Channel proteins Channel proteins
Carrier proteins
Cell Recognition proteins
Receptor proteins
Enzymatic proteins
Junction proteins
Chanel Proteins
Allows passage of molecules or ions freely through membrane
They facilitate diffusion by forming hydrophilic b h ltransmembrane channels
H+ ions across mitochondrial inner membrane during ATP production
Faulty Cl‐ channel causing cystic fibrosis
Channel proteins are only responsible for passive transport
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Chapter 5: Plasma Membrane: Structure and Function 5
Carrier Proteins Selectively interact with a specific molecule so that it can cross the plasma membrane to enter or exit the cell
This process often requires energy (ATP) This process often requires energy (ATP)
When ATP is involved with actively moving molecules through the membrane the process is called active transport
Example: Na+‐K+ pump of nerve cells
Cell Recognition Protein Glycoproteins and some glycolipids serve as surface receptors for cell recognition and identification (cellular fingerprint)
Important in that the immune system cells can Important in that the immune system cells can distinguish between one’s own cells and foreign cells
The major histocompatibility complex (MHC) glycoprotiens are different in each individual
MHC determines organ transplant acceptance or rejection
Receptor Proteins Receptor proteins serve as binding or attachment sites
Protein has a specific shape so that specific molecules can bind to them molecules can bind to them
Binding of a molecule (e.g. insulin hormone) can influence the liver to store glucose
Pygmies are short due to their faulty PM hormone receptors that cannot interact with growth hormone
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Chapter 5: Plasma Membrane: Structure and Function 6
Enzymatic Proteins
Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface
Catalyzes a specific reaction
Adenylate cyclase is a membrane bound enzyme that is involved in ATP metabolism
Cholera toxin activates the adenylate cyclase enzyme in the intestinal cells
Results in the loss of H2O, Na+ and K+ from the intestinal cells (i.e. dehydration)
Junction Proteins
Form various types of junctions between animal cells
Signaling molecules that pass through gap junctions allow the cilia of cells lining the respiratory tract to b h i beat at the same time
Tight junctions joining animal cells in order to form a specific function
Example – nervous system in animal embryos
Permeability of the Plasma Membrane
Plasma membrane is differentially (selectively) permeable
Allows some material to pass freely
Inhibits (blocks) passage of other materials
Some materials enter or leave the cell only by the using cell energy
By regulating chemical traffic across its plasma membrane, a cell controls its volume and its internal ionic and molecular composition
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Chapter 5: Plasma Membrane: Structure and Function 7
Types of Transport:Active vs. Passive Passive Transport
No ATP requirement; includes diffusion and facilitated transporttransport
Molecules follow concentration gradient (i.e. from high to low concentration)
Concentration ‐ the number of molecules of a substance in a given volume
Gradient ‐ a physical difference between two regions so that molecules will tend to move from one of the regions toward the other (i.e. concentration, pressure & electrical charge)
Active vs. Passive (cont.) When the distribution of molecules is not equal, and we have a gradient, there is a net movement of molecules along “down” the gradient
Example: Cellular respiration
Concentration of O2 is lower inside a cell than outside
Concentration of CO2 is higher inside the cell than outside
Active vs. Passive (cont.)
Active Transport
Requires carrier protein
Molecules move through the membrane against the i diconcentration gradient
Requires energy in form of ATP
Movement out of the cell involving changes of the membranes & formation of vesicles is exocytosis
Movement of materials into the cell is endocytosis
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Chapter 5: Plasma Membrane: Structure and Function 8
Diffusion A solution consists of:
A solvent (liquid) and a solute (dissolved solid)
Diffusion – the net movement of solute molecules “down” their own concentration gradient from a down their own concentration gradient, from a region of higher concentration to one of lower concentration, until molecules are equally distributed
In terms of cellular activity, diffusion:
Requires no energy
However, the cell has no control over diffusion, and the rate of diffusion is quite slow
Diffusion (cont.)
The rate of diffusion can be affected by:
Temperature (higher temperature faster molecule movement)
M l l i ( ll l l f Molecule size (smaller molecules often move more easily)
Concentration (Initial rate faster with higher concentration)
Electrical & pressure gradients of the two regions (greater the gradient differential, the more rapid the diffusion)
Membrane Transport
Materials that may move through membranes freely by simple diffusion include:
CO2
O O2
Small lipid‐soluble molecules
Passive transport (carrier proteins):
H2O (aquaporin)
Glucose
Many small ions
Some amino acids
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Chapter 5: Plasma Membrane: Structure and Function 9
Osmosis Focuses on solvent (water) movement rather than solute
Osmosis – diffusion of water across a differentially (selectively) permeable membrane
Solute concentration on one side high, but water concentration is low
Solute concentration on other side low, but water concentration is high
Water diffuses both ways across membrane but solute can’t
Net movement of water is toward low water (high solute) concentration
Osmosis Demonstration
Osmotic pressure is the pressure that develops due to osmosis
The more solute particles present, the higher the
ti pressureosmotic pressure
Significance of Osmosis Absorption of water from the soil by plant roots
Turgidity is developed by the process of osmosis which provides mechanical strength in plants
Re‐absorption of water by the kidneys p y y
Absorption of water by the digestive tract (i.e. stomach, small intestine and the colon)
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Types of solutions: Isotonic Isotonic Solution
Solute and water concentrations are l b th id f bequal on both sides of membrane
This results in no net movement of water into or out of cells – the cell neither swells nor shrinks
Osmotically balanced
Physiological or normal saline consists of 0.9% NaCl in water, which is isotonic to red blood cells (RBCs)
Types of solutions: Hypotonic Hypotonic Solution
The solution surrounding the cell has a lower solute concentration (i.e. more water) than the cell
This results in a net movement of water into the cells
Cells placed in a hypotonic solution will swell
May cause animal cells to burst – lysis
Hypotonic Environments Cells which typically exist in hypotonic solutions (fresh water), use various mechanisms such as
The contractile vacuoles found in protists (e.g. paramecium) are used to expel excess water
Well‐developed kidneys in freshwater fish to excrete large volume of diluted urine
Plant cells use osmotic pressure to their advantage
When plant cells immersed in water, the vacuole (containing the stored molecules) gain water which increases the turgor pressure
This pressure forces the cytoplasm against the plasma membrane and cell wall, helping to keep the cell rigid
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Chapter 5: Plasma Membrane: Structure and Function 11
Types of solutions: Hypertonic Hypertonic Solution
The surrounding solution has a higher solute concentration (i e less water) solute concentration (i.e. less water) than the cell
Cells placed in a hypertonic solution will shrink – Plasmolysis
Antibiological activities used in food preservation (i.e. meats, fruits and vegetables are pickled, salted, or mixed with concentrated sugar solutions to prevent bacterial & fungal growth)
Hypertonic Environments Salt water is hypertonic to the cells of freshwater organisms
Central vacuole in plants lose water and the plasma membrane pulls away from the cell wallmembrane pulls away from the cell wall
Plasmolysis occurs in plants when the soil or water around them contains high concentrations of salts or fertilizers
Marine animals cope in various ways
Sharks increase/decrease urea in blood
Fishes excrete salts across their gills
Facilitated Transport:Carrier Proteins Facilitated Transport
Small molecules (i.e. glucose & amino acids)
Can’t get through membrane lipidsCan t get through membrane lipids
Combine with carrier proteins
Follow concentration gradient (i.e. no ATP)
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Chapter 5: Plasma Membrane: Structure and Function 12
Active Transport Across a Membrane
Active Transport
Small molecules (i.e. glucose & amino acids)
Move against concentration gradient
Requires a direct expenditure of energy
Requires two carrier protein active sites:
one to recognize the substance to be carried
one to release ATP to provide the energy for the protein carriers or "pumps“
The sodium‐potassium pump
The Na+‐ K+
Pump
Bulk Transport: Exocytosis
Macromolecules are transported into or out of the cell inside vesicles
Vesicle formation requires ATP
l f d f G l f Exocytosis – vesicles formed from Golgi apparatus fuse with plasma membrane and secrete contents
Hormones, neurotransmitters & digestive enzymes are secreted by exocytosis
Example: insulin, made in pancreatic cells, are secreted by exocytosis
Regulated secretion occurs when plasma membrane receives a signal (i.e. rise in blood sugar)
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Chapter 5: Plasma Membrane: Structure and Function 13
Exocytosis
Bulk Transport: Endocytosis Endocytosis ‐ substances that enter the cell by vesicle formation
There are three mechanisms of endocytosis:
Ph t i l lid ti l i t i l h Phagocytosis – large, solid particles into vesicle, such as a bacterium
Pinocytosis – liquid or very small particles, such as macromolecules, go into the vesicle
Receptor‐Mediated Endocytosis – specific form of pinocytosis using a receptor protein
Vesicle membrane is added to plasma membrane
Phagocytosis
Phagocytosis (“cell eating”)
Cell ingests large solid particles such as food or bacteria
Folds of plasma membrane enclose the cell or particle, forming a phagocytic vacuole
Vacuole may fuse with lysosomes, which degrade the ingested material
Examples‐ amoeba & macrophage
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Chapter 5: Plasma Membrane: Structure and Function 14
Pinocytosis
Pinocytosis (“cell drinking”)
Cell takes in dissolved materials
Droplets of fluid are trapped by folds i h l b hi h in the plasma membrane, which pinch off into the cytosol as vesicles
Vesicles become smaller as liquid in the vesicles is transferred into the cytosol
Examples – Blood cells & plant root cells
Receptor‐Mediated Endocytosis
A form of pinocytosis, occurs when specific macromolecules bind to plasma membrane receptors
The macromolecules are taken into the cell via coated esicles that pinch from the plasma membranevesicles that pinch from the plasma membrane
Receptors for specific molecules are concentrated in coated pits (i.e. layer of fibrous protein) on the plasma membrane
Coating detaches from vesicle, and uncoated vesicle fuses with a lysosome
Receptor‐Mediated Endocytosis (cont.) Pits are associated with exchange of substances between cells (e.g. maternal and fetal blood)
System is selective and more efficient than pinocytosisSystem is selective and more efficient than pinocytosis
Defects in receptor‐mediated endocytosis are responsible for certain diseases such as hypercholesterolemia
LDL receptors cannot bind to the coated pit, thus the cells are unable to take up cholesterol
Access cholesterol accumulates in the circulatory system
Will cause heart attacks & atherosclerosis
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Chapter 5: Plasma Membrane: Structure and Function 15
Receptor‐Mediated Endocytosis