Chapter 5: Plasma Membrane - جامعة...

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



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



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



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



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



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



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



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



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)



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



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)



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)



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



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



Receptor‐Mediated Endocytosis

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