chapter 5: membrane structure and function (outline) plasma membrane structure and function...
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Chapter 5: Membrane Structure and Function (Outline)
Plasma Membrane Structure and Function Phospholipids and proteins Fluid-Mosaic Model Carbohydrate Chains
Functions of the Proteins Plasma Membrane Permeability
Diffusion Osmosis Active Transport Across a Membrane Bulk Transport
Structure and Function: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
Plasma Membrane of an Animal Cell
Plasma Membrane Structure 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
Peripheral proteins are found on the inner membrane surface
The plasma membrane is asymmetrical; the distribution of integral proteins can be analyzed by freeze-fracture technique
Fluid-Mosaic Model
The fluid-mosaic model describes the plasma membrane The fluid component refers to the phospholipids
bilayer of the plasma membrane (PM) 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 other proteins
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 – 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
Phospholipid Movement With Cholesterol
Unsaturated/Saturated
Carbohydrate Chains Membrane contain carbohydrate chains linked to
phospholipds “Glycolipids” & proteins “Glycoprotein” on the extracellular surface
Glycocalyx – a ‘sugar coat’ 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”)
The cell surface of blood cells is covered with glycoproteins – A, B, and O blood groups
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
Channel Proteins
Allows passage of molecules or ions freely through membrane
They facilitate diffusion by forming hydrophilic transmembrane channels H+ ions across mitochondrial inner
membrane during ATP production Faulty Cl- channel causing cystic fibrosis
Channel proteins are only responsible for passive transport
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: Sodium and potassium ions across the plasma membrane of nerve cells
Cell Recognition Proteins
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
plasma membrane hormone receptors that cannot interact with growth hormone
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 (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 beat at the same time Tight junctions joining cells in
order to form a specific function
Example – nervous system in animal embryos
Types of Transport:Active vs. Passive Plasma membrane is differentially (selectively) permeable
Allows some material to pass freely Inhibits passage of other materials Some materials enter or leave the cell only by the using
cell energy
Passive Transport: No ATP requirement Molecules follow concentration gradient
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 When the distribution of molecules is not equal, and
we have a gradient, there is a net movement of molecules along 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 Transport Requires carrier protein Molecules move through the membrane against the
concentration 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
Types of Passive Transport:Diffusion
A solution consists of: A solvent (liquid) , and A solute (dissolved solid)
Diffusion – the net movement of solute molecules from where there is more of it along a concentration gradient to where there is less of it, 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
The rate of diffusion can be affected by: Temperature (higher temperature, faster
molecule movement)
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 (carbon dioxide) O2 (oxygen) Small lipid-soluble molecules
Passive transport (carrier proteins): H2O (aquaporin) Glucose Many small ions Some amino acids
Types of Passive Transport:Osmosis Focuses on solvent (water) movement rather
than solute 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 pressure
Significance of Osmosis
Absorption of water from the soil by plant roots Turgidity is developed by the process of
osmosis which mechanical strength in plants Re-absorption of water by the kidneys Absorption of water by the digestive tract -
stomach, small intestine and the colon
Types of solutions: Isotonic
Isotonic Solution Solute and water concentrations
are equal on both sides of membrane
This results in no net movement of water into or out of cells
Isotonic solutions are osmotically balanced
Physiological or normal saline consists of 0.9% NaCl in water, which is isotonic to red blood cells
Types of solutions: Hypotonic
Hypotonic Solution The solution has a lower solute
concentration (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 cells (animal) to burst – lysis
Hypotonic Environments Cells which typically exist in hypotonic solutions
(fresh water), use various mechanisms to oppose this inevitable influx of water 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
Types of solutions: Hypertonic Hypertonic Solution
The solution has a higher solute concentration (less water) than the cell
Cells placed in a hypertonic solution will shrink – Plasmolysis
Antibiological activities used in food preservation (i.e., meats, fruits, & vegetables are pickled, salted, or mixed with concentrated sugar solutions to prevent bacterial & fungal growth)
Hypertonic Environments
Salt water, for example, is hypertonic to the cells of freshwater organisms Central vacuole in plants lose water and the
plasma membrane pulls away from the cell wall
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 Combine with carrier proteins Follow concentration gradient (i.e., no ATP)
Types of Transport:Carrier Proteins
Active Transport Small molecules (i.e., glucose, amino acids) Move against concentration gradient Requires 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 fuse with plasma membrane and secrete contents
Hormones, neurotransmitters and 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)
Exocytosis
Bulk Transport:Endocytosis
Endocytosis - substances that enter the cell by vesicle formation
There are a variety of endocytosis processes: Phagocytosis – Large, solid material 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 Endocytosis takes up large amounts of the plasma
membrane and is balanced by the return of membrane components to the plasma membrane by exocytosis
Methods of Endocytosis:Phagocytosis
Membrane surrounds and engulfs object, used for larger objects & pinches off placing the particle in a phagocytic vacuole
The phagocytic vacuole then fuses with lysosomes and the material is degraded
Examples: Unicellular organisms (amoebas) White blood cells (macrophage)
Methods of Endocytosis:Pinocytosis
The process by which a cell takes in extracellular fluid or very small particles by the invagination of the cell membrane
A pocket then forms and pinches off to form a vesicle
The liquid contents of the vesicle is then slowly transferred to the cytosol
Examples: Blood cells Plant root cells
Methods of Endocytosis:Receptor-Mediated Endocytosis A form of pinocytosis, occurs when specific
macromolecules (e.g. vitamin, lipoprotein, etc.) bind to plasma membrane receptors
The macromolecules are taken into the cell via coated vesicles that pinch from the plasma membrane
Molecules first bind to specific receptor proteins, which are found at one location in the plasma membrane
This location is a coated pit with a layer of fibrous protein on the cytoplasmic side; when the vesicle is uncoated, it may fuse with a lysosome
Receptor-Mediated Endocytosis Pits are associated with exchange of substances
between cells (e.g., maternal and fetal blood) This system is selective and more efficient than
pinocytosis, why? 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
Receptor-Mediated Endocytosis