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