Cell membrane & transport

Functions of biological membranes

• an essential feature of every cell is the presence of membranes that define the boundaries of the cell and its various internal compartments

• biological membranes protect and organise cells

• all cells have an outer plasma membrane, eukaryotic cells also possess internal membranes that encase their granules and control the exchange of essential cell components

1. Definition of cell’s boundaries

• cell surface membrane keeps the interior of the cell physically separated from the surrounding environment

• phospholipid bilayer is selectively permeable and allows for desirable substances to be kept within and undesirable substances kept out of the cell

2. Organisation & localisation of function

• molecules or structures with specific functions are embedded in membranes or localised within organelles

• for example

  • many electron carriers are embedded in the thylakoid membranes of chloroplasts and inner mitochondrial membrane

  • hydrolytic enzymes are found localised within lysosomes

• serve to organise and compartmentalise functions within eukaryotic cells

3. Regulation of cell’s contents

• proteins and other components of the membrane help to regulate the transport of substances into and out of the cell and its organelles

• for example

  • to take up and accumulate useful substances such as water, ions and small molecular weight metabolites such as glucose into various compartments

  • to remove various metabolic waste products

  • to confine certain chemicals within specific regions of the cell

4. signal transduction

• specific protein receptors on the outer surface of the cell membrane plays a key role in the detection of specific signals (after binding to the receptors) and thus triggering- specific responses within the cell, such as drug or hormone induced responses

5. cell-to-cell communication

• cell membrane has membrane proteins that bind the extra cellular matrix or cell surface constituents to mediate adhesion and communication between adjacent cells

Structure of cell membrane

• Fluid Mosaic model

• ‘mosaic’ —> proteins randomly distributed in or loosely attached to

• fluid phospholipid bilaterally is free to move about laterally (due to weak hydrophobic interactions)

Characteristics of the fluid mosaic model

1. the fluid layer is asymmetrical

• the two lipid bilayers may differ in composition and arrangement of proteins and lipids

• three major types of membrane lipids are phospholipids, cholesterol and glycolipids

2. the phospholipid bilayer is fluid or mobile i.e. lateral movement of phospholipids is possible

3. the unit membrane is a dynamic structure, where the embedded proteins can float, some moving freely while others are fixed in positions by micro filament on the cytoplasmic face

4. membranes are amphipathic

• hydrophilic phosphate head of the phospholipids face outwards into the aqueous environment both inside and outside of the cell

• hydrophobic hydrocarbon tails face inwards and create a hydrophobic core

Evidence for the mosaic structure

• from the use of electron microscopy and freeze fracturing

Freeze-fracture:

• a cell membrane can be split into its two layers, revealing the ultra structure of the membrane’s interior

• technique: a cell is frozen and fractured with a knife

  • the fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilge into two separated layers

  • membrane proteins go wholly with one of the layers

  • SEMs show membrane proteins (bumps) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer

Components of cell membranes

1. Phospholipids

2. Cholesterol

3. Proteins

4. Carbohydrates

Phospholipids

• responsible for the formation of bilayers in an aqueous environment

• each phospholipid molecule consists of

  • glycerol backbone (three-carbon molecule) with 3 hydroxyl (-OH) groups

    • 2 fatty acid chains - hydrophobic hydrocarbon tails

    • negatively charged phosphate group

    • additional small, charged molecules which may be linked to the phosphate group

• phosphate group and small, charged molecules - contribute to the hydrophilic head

• hydrophobic tail + hydrophilic head = phospholipid molecule is termed as amphipathic —> phospholipid bilayer

• long hydrocarbon chains of FA form an effective hydrophobic barrier against polar and charged solutes

membrane fluidity

• membrane comprises of phospholipid molecules which are held together primarily by hydrophobic interactions between the hydrophobic fatty acid tails

• these are weak interactions —> phospholipid molecules are free to move about laterally in the plane of the membrane

• rare for molecules to flip transversely across the membrane (hydrophilic part must cross hydrophobic core)

Factors affecting membrane fluidity

1. Temperature

• As temperature increases, membrane fluidity increases

• Membranes remain fluid as temperature decreases, until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies —> phase transition (change in state)

• temperature at which it occurs —> phase transition temperature

• at low temp

  • kinetic energy of the hydrocarbon chains decreases

  • hydrocarbon chains are tightly packed

  • increased hydrophobic interactions between phospholipids molecules and thus their motion is restricted

  • this bilayer exists in a semisolid state i.e. membrane is less fluid

• high temp

  • kinetic energy and motion of the hydrocarbon chains increases

  • increased lateral movements of individual molecules, flexing of the chains and transverse flipping

  • overcoming hydrophobic interactions between phospholipids, resulting in increased space between adjacent phospholipid molecules

  • bilayer exists in a fluid state (more fluid)

2. length of fatty acid chains

• as length of FA chains increases, membrane fluidity decreases

• longer hydrocarbon chains, higher melting point due to increased hydrophobic interactions between hydrocarbon chains

3. degree of saturation of fatty acid chains

• as degree of saturation of FA chains increases, membrane fluidity decreases and vice versa

• saturated lipids have long, straight hydrocarbon chains, which allows for close packing and thus enhances membrane solidification

• unsaturated lipids have kinks, which prevents the hydrocarbon chains from packing closely together thus enhances membrane fluidity

4. amount of cholesterol

• cholesterol increases the stability and regulates the fluidity of membranes in animal cells

Cholesterol

• cholesterols are steroids commonly found wedged between phospholipid molecules in the cell membranes of animal cells

Effect of cholesterol on cell membranes

1. Membrane stability

• cholesterol molecules are usually found in both layers of the cell membrane, intercalated into the lipid monolayers

• its rigid steroid ring interferes with the motions of the hydrocarbon chains of phospholipids, thus enhancing the mechanical stability of the membrane

2. Membrane fluidity

• at high temp

  • cholesterol restrains the movements of phospholipids by interfering with the motions of the hydrocarbon chains

  • decreased membrane fluidity

• at low temp

  • cholesterol prevents the hydrocarbon chains from packing closely together, thus decreasing the tendency of the membrane to freeze upon

  • increased membrane fluidity

• cholesterol has dual effects on the fluidity of the membrane - resisting changes in membrane fluidity that can be caused by changes in temperature, acting as a temperature buffer for the membrane

3. membrane permeability

• presence of cholesterol molecules decreases the permeability of a lipid bilayer to ions and small polar molecules

• fills in spaces between hydrocarbon chains of phospholipids, thereby plugging transient gaps through which ions and small molecules might otherwise pass

• membrane stability = fluidity + permeability

Proteins

• 2 categories of membrane proteins

1. integral (intrinsic) proteins

2. peripheral (extrinsic) proteins

• classified according to their degree of association with the membrane and their structures

Characteristics of integral and peripheral proteins

1. Location

• Integral proteins

  • deeply embedded in the hydrophobic interior of the lipid bilayer

  • 2 types exist

    • unilateral, reaching only a mono layer

    • transmembrane, spanning the entire bilayer

• Peripheral proteins

  • not embedded but loosely bound to membrane surface, often to exposed parts of integral proteins

  • found on both sides of the membrane

    • cytoplasmic side: may be held by network proteins (filaments of cytoskeleton) thus cannot move far

    • exterior side: may be attached to fibres of extracellular matrix

2. Structure

• Integral/intrinsic proteins

  • contain both hydrophilic (charged and polar amino acids) and hydrophobic (non polar amino acids) regions i.e. amphipathic

  • held in place by extensive hydrophobic interactions with the hydrocarbon portions of phospholipids

• Peripheral/extrinsic proteins

  • rich in hydrophilic amino acids so as to allow for interaction with surrounding water and polar surface of the phospholipid bilayer

3. solubility

• integral proteins

  • usually insoluble in aqueous media

• peripheral proteins

  • usually soluble in aqueous media

4. how they can be released from membranes

• integral proteins

  • release only through use of detergents or non polar solvents

• peripheral proteins

  • easy release by relatively mild treatment, such as adjustment of ionic strength or pH of the suspending medium

Functions of membrane proteins

• enzyme activity

• transport

• intercellular junctions

• cell-to-cell recognition

• signal transduction

• attachment of the cytoskeleton and extracellular matrix for anchorage

Function of membrane proteins - (1) anchorage

1. Anchorage

• anchoring proteins attach the cell membrane to other substances, stabilise the position of the cell membrane and can help maintain cell shape

• anchoring proteins attached to the extracellular matrix can coordinate extracellular and intracellular changes

  • on the cytoplasmic side, they are bound to microfilaments of the cytoskeleton

  • on the exterior side, they may attach the cell to fibres of the extracellular matrix

• disruptions in cell-cell adhesion can contribute to metastasis stage of cancer

Function of membrane proteins - (2) transport

2. Transport

• Carrier proteins

  • bind solutes and transport them across the membrane

  • involves a conformational change of the protein when solute binding occurs, and a return to its original form when the solute is released

  • energy in the form of ATP may or may not be required

    • facilitated diffusion (no ATP required): solute moves down concentration gradient

    • active transport (ATP required): solute moves against a concentration gradient

• Channel proteins

  • some integral proteins contain a water-filled central pore, or hydrophilic channel that forms a passageway to permit the movement (down the conc gradient) of water, ions and small hydrophilic solutes across the cell membrane

  • 2 major kinds of channels

1. leak channels

• permit movement of water at all times e.g. aquaporins

• permit movement of ions at all times (though the rate may vary) e.g. Na+ or K+ leak channels

2. gated channels, which can open or close to regulate ion passage e.g. voltage-gated Na+ or K+ channels

Figure of leak and gated channels

Function of membrane protein - (3) enzymatic activity

3. enzymatic activity

• these enzymes catalyse reactions in the extracellular fluid or within the cytoskeleton, depending on the location of the active site

• in some instances, several enzymes can be grouped together to carry out sequential steps in a metabolic pathway

Function of membrane protein - (4) signal transduction

4. signal transduction

• these proteins have very specific 3D conformations, making them ideal as receptor molecules for chemical signalling between cells

• chemical signalling works by the binding of a ligand to the receptor protein which triggers changes in the cell

• cell membranes differ in the type and number of receptor proteins they contain

Function of membrane protein - (5) cell-to-cell recognition

5. cell-to-cell recognition

• recognition proteins are usually glycoproteins

• wide array of possible shapes to the carbohydrate side chains, hence each cell type has its own specific markers

• enables cells to recognise other cells, and provides a means for foreign markers to be recognised and attacked by the immune system

Function of membrane protein - (6) Intercellular joining

6. Intercellular joining

• membrane proteins of adjacent cells may adhere together in various kinds of intercellular junctions, such as gap junctions and tight junctions

Carbohydrates

• membrane carbohydrates are usually short, branched chains of fewer than 15 units

• some of these are covalently bonded to polar ends of phospholipids molecules in the outer lipid layer forming glycolipids

• some are covalently bonded to membrane proteins, forming glycoproteins

• carbohydrate groups aid in maintaining the orientation of glycoproteins and glycolipids

• carbohydrates are highly hydrophilic, glycolipids and glycoproteins are kept in contact with external aqueous environment and are unlikely to rotate towards the interior to diffuse transversely

• they are importantly recognition components

1. sorting of cells into tissues and organs in animal embryos

2. binding extracellular signal molecules in antibody-antigen reactions

3. intercellular adhesion to form tissues

4. cell-to-cell recognition, ability of a cell to distinguish one cell from another e.g. the immune system identifies and acts upon foreign cells

Transport across membrane

Important for reasons

1. maintain a suitable pH and ionic concentration within the cell for enzyme activity

2. obtain food supplies for energy and raw materials

3. excrete toxic substances

4. secrete useful substances

5. generate ionic gradients essential for nervous and muscular activity

2 categories of processes for entry into, or exit from cells

1. Passive processes

2. Active processes

Differences between passive and active transport

1. Concentration gradient

• Passive processes occur down a concentration gradient - substances move from a region of higher concentration to a region of lower concentration

• Active processes occur against a concentration gradient - region of lower concentration to a region of higher concentration

2. Energy requirement

• Passive: no cellular energy expenditure (ATP) required

• Active: cellular energy expenditure requried usually in the form of ATP

3. Examples of processes

• Passive: simple diffusion, facilitated diffusion, osmosis

• Active: active transport, endocytosis, exocytosis

Passive transport

Diffusion: net movement of a a substance from a region of higher concentration to a region of lower concentration down a concentration gradient

• molecules have intrinsic kinetic (or thermal motion) energy, which is the tendency for them to spread out evenly into the available space (wrong to assume that passive tpt has completely no energy involved)

• diffusion continues until dynamic equilibrium is reached

• simple and facilitated diffusion

Simple diffusion

• molecules that are able to cross the phospholipid bilayer directly i.e. molecules that have a small molecular weight and/or are readily soluble in the lipid bilayer i.e. hydrophobic molecules

• diffusion occurs directly across the plasma membrane without any need for aid of channel or carrier proteins

• dynamic equilibrium is reached when concentrations of the diffusing substances are equal n both sides of the membrane = no net movement of substances occurs

Facilitated diffusion

• transport of substances down a concentration gradient without the use of ATP (i.e. passive0 until equilibrium is reached

• transport protein is used to enhance/increase the rate of transport of the substances across the membrane

  • transport ptn is specific to the substance being transported i.e. it only transports that substance

• is for larger hydrophilic substances e.g. glucose, amino acids and ions

• channel or carrier proteins

a. carrier proteins

—> possesses a binding site for solute molecules

—> undergo conformational changes to transport a substance across the membrane

b. channel proteins

—> possesses a central hydrophilic pore that allows free movement of the transported substance across the membrane

—> does not undergo conformational changes to transport a substance across the membrane medium

Factors affecting rate of diffusion

1. Concentration gradient: steeper conc. gradient = faster diffusion

2. Distance over which diffusion occurs: shorter distance = faster diffusion (thinner cell membrane = greater no. of molecules diffuse across per unit time)

3. Area across which diffusion occurs: larger surface area = greater no. of molecules diffuse across per unit time = faster diffusion

4. Structure through which diffusion occurs: presence of transient gaps in cell membrane may enhance diffusion

• type and number of tpt ptn present per unit surface area of the membrane will also affect the diffusion rate

5. size and type of diffusing molecule

• smaller molecules = faster diffusion

6. temperature

• higher temp = faster diffusion

Osmosis

• osmosis is the net movement of freely moving water molecules from a region of less negative water potential to a region of more negative water potential through a selectively permeable membrane

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Water potential

• Water potential: measure of the tendency for water to move from one region to another

• pure water has a water potential of 0 (highest possible value)

• presence of solutes will make the water potential more negative i.e. any solution will have a negative water potential

• water molecules always move from a region of less negative wp to a region of more negative wp

For plant cells:

• 2 factors affecting wp

1. solute concentration - solute potential (which is negative), Ψcell = Ψs

2. pressure exerted by cell wall on its contents, which is generated when water enters the cell = pressure potential Ψp (which is positive

water potential of plant cell = solute potential + pressure potential

For animal cells:

• water potential of animal cell is determined primarily by its solute potential cos no cell wall

Ψcell = Ψs

Solute potential

• Solute potential Ψs: measure of the ability of a solute to make the water potential more negative

• dissolving solute molecules in pure water reduces the number of free