Cell MembraneCell Membrane

Transport systemsTransport systems

OverviewOverview Cell membrane Cell membrane separatesseparates living cell from nonliving living cell from nonliving

surroundingssurroundings– thin barrier = 8nm thickthin barrier = 8nm thick

Controls traffic in & out of the cellControls traffic in & out of the cell– selectively permeableselectively permeable– allows some substances to cross more easily than allows some substances to cross more easily than

othersothers Made of Made of phospholipidsphospholipids, , proteinsproteins & other & other

macromoleculesmacromolecules Provide:Provide:

1. Fluidity1. Fluidity

2. Asymmetry2. Asymmetry

3. Formed closed compartments3. Formed closed compartments

Chemical compositionChemical composition

Lipids – about 60%Lipids – about 60% Proteins – about 40%Proteins – about 40% Carbohydrates – about 1-3%Carbohydrates – about 1-3% Water and minerals – very small Water and minerals – very small

amountamount

Membrane fat Membrane fat composition variescomposition varies Fat composition affects flexibilityFat composition affects flexibility

– membrane must be fluid & flexiblemembrane must be fluid & flexible about as fluid as thick salad oilabout as fluid as thick salad oil

– % unsaturated fatty acids in phospholipids% unsaturated fatty acids in phospholipids keep membrane less viscouskeep membrane less viscous cold-adapted organisms, like winter wheat cold-adapted organisms, like winter wheat

– increase % in autumnincrease % in autumn

– cholesterol in membranecholesterol in membrane

Membrane ProteinsMembrane Proteins Proteins determine membrane’s specific Proteins determine membrane’s specific

functionsfunctions– cell membrane & organelle membranes each have cell membrane & organelle membranes each have

unique collections of proteinsunique collections of proteins Membrane proteins:Membrane proteins:

– peripheral proteinsperipheral proteins loosely bound to surface of membraneloosely bound to surface of membrane cell surface identity marker (cell surface identity marker (antigensantigens))

– integral proteinsintegral proteins penetrate lipid bilayer, usually across whole membrane penetrate lipid bilayer, usually across whole membrane transmembranetransmembrane protein protein transport proteinstransport proteins

– channels, permeases (pumps)channels, permeases (pumps)

Membrane ProteinsMembrane Proteins

Membrane phospholipids act as a Membrane phospholipids act as a solvent for membrane proteinssolvent for membrane proteins

Integral (intrinsic)Integral (intrinsic)– Most membrane proteins are Most membrane proteins are

integral components of the integral components of the membrane (70%)membrane (70%)

– Penetrate the matrix from the outer Penetrate the matrix from the outer surface to the inner surface surface to the inner surface

AP Biology

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

Functions of membrane Functions of membrane proteinsproteins TransportTransport StructureStructure ReceptorsReceptors EnzymesEnzymes AntigensAntigens

Many Functions of Membrane ProteinsMany Functions of Membrane Proteins

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

Membrane Membrane carbohydrates carbohydrates Play a key role in Play a key role in cell-cell cell-cell recognitionrecognition– ability of a cell to distinguish one cell ability of a cell to distinguish one cell

from anotherfrom another antigensantigens

– important in organ & important in organ & tissue developmenttissue development

– basis for rejection of basis for rejection of foreign cells by foreign cells by immune systemimmune system

Transport Across Transport Across MembranesMembranes 11. Macrotransport. Macrotransport

2. Microtransport2. Microtransport

AP Biology

Macrotransport Moving large molecules into & out of cell

through vesicles & vacuoles endocytosis

phagocytosis = “cellular eating” pinocytosis = “cellular drinking”

exocytosis

exocytosis

AP Biology

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specificprocess

triggered bymolecular signal

MicrotransportMicrotransport

1. Passive1. Passive 2. Active2. Active

Passive transportPassive transport 1. Simple diffusion1. Simple diffusion

2. Facilitated diffusion2. Facilitated diffusion

Simple diffusionSimple diffusion Movement of molecules in response to a Movement of molecules in response to a

concentration gradient.concentration gradient. It doesn’t need energyIt doesn’t need energy

AP Biology

Facilitated Diffusion Diffusion through protein channels (permeases)

channels move specific molecules across cell membrane

no energy needed Movable Immovable

“The Bouncer”“The Bouncer”

open channel = fast transport

facilitated = with help

high

low

Mechanism of the Facilitated Mechanism of the Facilitated diffusiondiffusion

Active transportActive transport

Occurs against a Occurs against a concentration gradient and so concentration gradient and so it requires energy.it requires energy.

1. Primary1. Primary

2. Secondary2. Secondary

Primary active transportPrimary active transport

This is transportation of the This is transportation of the ions by the help of special ions by the help of special enzymatic transport systems enzymatic transport systems (ATP-ases)(ATP-ases)

Na K – ATP-aseNa K – ATP-ase

Secondary active Secondary active transporttransport This is transportation of the molecules This is transportation of the molecules

by the help of electro- chemical by the help of electro- chemical potential of some ions (Na, H).potential of some ions (Na, H).

1. Symport systems move 2 molecules 1. Symport systems move 2 molecules in the same directionin the same direction

2. Antiport systems move 2 molecules 2. Antiport systems move 2 molecules in the opposite directionin the opposite direction

Highest level – nervous system

Intermediate -hormonal regulation

Intracellular(enzymes)

Levels of the homeostasis regulation

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SYNTHESIS OF SIGNALLING MOLECULES

RELEASE OF SIGNALLING MOLECULES

TRANSPORT OF SIGNAL TO TARGET CELLS

DETECTION & BINDING OF SIGNAL BY SPECIFIC RECEPTOR

CHANGES DUE TO RECEPTOR-SIGNAL COMPLEX

SIGNAL REMOVAL & RESPNOSE TERMINATION

STEPS IN CELL SIGNALINGSTEPS IN CELL SIGNALING

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CLASSIFICATION OF INTERCELLULAR COMMUNICATION

Intercellular signaling is subdivided into the following classifications:

Autocrine signals target the cell itself. Sometimes autocrine cells can target cells close by if they are the same type of cell as the emitting cell. An example of this are immune cells.

Paracrine signals target cells in the vicinity of the emitting cell. neurotransmitters represent an example.

Endocrine signals target distant cells. Endocrine cells produce hormones that travel through the blood to reach all parts of the body.

Juxtacrine signals target adjacent (touching) cells. These signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent.

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JUXTACRINE SIGNALINGJUXTACRINE SIGNALING

MEMBRANE-ANCHORED LIGANDSMEMBRANE-ANCHORED LIGANDS

NEIGHBOURING CELLS

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 e.g.*Notch signalling.

Hormones – organic biologically active compounds of different chemical nature that are produced by the endocrine glands, enter directly into blood and accomplish humoral regulation of the metabolism of compounds and functions on the organism level.

Hormonoids (tissue hormones) – compounds that are produced not in glands but in different tissues and regulate metabolic processes on the local level, but some of them (serotonin, acetylcholine) enters blood and regulate processes on the organism level.

Specific stimulus for hormones secretion is:

-nervous impulse

-concentration of the certain compound in blood passing through the endocrine gland

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Membrane receptors

To receive and pass on chemical or physical signals, cells are equipped with receptor proteins.

Membrane-located receptors can be divided into three parts, which have different tasks.

1. The receptor domain reacts specifically to a given signal

2. The effector domain of the receptor is usually separated by a membrane, a mechanism for signal transfer between the domains is needed.

3. By binding or interconversion, many receptors activate special mediator proteins, which then trigger a signal cascadeOther receptors function as ion channels. This is particularly widespread in receptors for neurotransmitters

Metabotropic receptor’s

action

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Ion channels

Ion channels facilitate the diffusion of ionsthrough biological membranes. Some ionchannels open and close depending on themembrane potential (voltage-gated channels, A) in response to specific ligands (ligandgated channels, B).