BIOLS 102
Dr. Tariq Alalwan
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 membrane.
membrane
 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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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 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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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
 Membrane phospholipid tail length –
b
h
h li id il l
h shorter hydrocarbon h
h d
b 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.)
With Cholesterol
Phospholipid Movement
Unsaturated/Saturated
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Carbohydrate Chains  Membrane contain carbohydrate chains linked to phospholipds “Glycolipids” and proteins “Glycoprotein” on the extracellular surface
 Glycocalyx – a a ‘sugar coat’ in animal cells that 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
 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 transmembrane channels
b
h
l
 H+ ions across mitochondrial inner membrane during ATP production
 Faulty Cl‐ channel causing cystic fibrosis  Channel proteins are only responsible for passive transport
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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)  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 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  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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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 beat at the same time i  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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Types of Transport:
Active vs. Passive
 Passive Transport
 No ATP requirement; includes diffusion and facilitated transport
 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 concentration gradient
i di
 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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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 from a “down” their own concentration gradient
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)
 Molecule size (smaller molecules often move more M l
l i (
ll l
l f 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
 O2
 Small lipid‐soluble molecules
 Passive transport (carrier proteins):
 H2O (aquaporin)
 Glucose
 Many small ions
 Some amino acids
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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 osmotic ti 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) Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Types of solutions: Isotonic
 Isotonic Solution
 Solute and water concentrations are equal on both sides of membrane
l b th id f b
 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 Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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 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 lipids
Can t get through membrane lipids
 Combine with carrier proteins
 Follow concentration gradient (i.e. no ATP)
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
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
 Exocytosis – vesicles formed from Golgi apparatus fuse l f
d f
G l f 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)
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Exocytosis
Bulk Transport: Endocytosis
 Endocytosis ‐ substances that enter the cell by vesicle formation
 There are three mechanisms of endocytosis:
 Phagocytosis
Ph
t i – large, solid particles into vesicle, such l
lid ti l i t i l h 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
Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Pinocytosis
 Pinocytosis (“cell drinking”)
 Cell takes in dissolved materials
 Droplets of fluid are trapped by folds iin the plasma membrane, which h l
b
hi h 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 vesicles
esicles 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 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 Chapter 5: Plasma Membrane:
Structure and Function
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BIOLS 102
Dr. Tariq Alalwan
Receptor‐Mediated Endocytosis Chapter 5: Plasma Membrane:
Structure and Function
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