Lecture 10

The Cell Membrane

Outline

▪ Review of Cell Components

▪ Membrane Composition– Lipids

– Proteins

▪ Selective Permeability– Transport Proteins

– Passive Transport

– Active Transport

Overview of the Cell

▪ All organisms are made of cells

▪ The cell is the simplest collection of matter that can be alive

▪ Cell structure is correlated to cellular function

▪ All cells are related by their descent from earlier cells

Figure 6.8a

ENDOPLASMIC RETICULUM (ER)

RoughER

SmoothER

Nuclearenvelope

Nucleolus

Chromatin

Plasmamembrane

Ribosomes

Golgi apparatus

LysosomeMitochondrion

Peroxisome

Microvilli

MicrotubulesIntermediate filaments

Microfilaments

Centrosome

CYTOSKELETON:

Flagellum NUCLEUS

Figure 6.15-1

Smooth ER

Nucleus

Rough ER

Plasmamembrane

Figure 6.15-2

Smooth ER

Nucleus

Rough ER

Plasmamembrane

cis Golgi

trans Golgi

Figure 6.15-3

Smooth ER

Nucleus

Rough ER

Plasmamembrane

cis Golgi

trans Golgi

Figure 6.17aa

Outermembrane

Innermembrane

Cristae

Matrix0.1 m

Mitochondria

▪ Present in almost all Eukaryotic cells– RBCs don’t have a nucleus or mitochondria

▪ The steps of cellular respiration occur in the mitochondria– Glycolysis is in the cytoplasm

– Citric Acid Cycle & ETC in mitochondria

Figure 6.17a

Intermembrane space

Outer

DNA

Innermembrane

Cristae

Matrix

Freeribosomesin themitochondrialmatrix

(a) Diagram and TEM of mitochondrion0.1 m

membrane

Figure 6.18aa

Stroma

Inner and outermembranes

Granum

1 m

Chloroplasts

▪ Photosynthesis

▪ Contain Chlorophyll – green pigments

▪ A sub category of organelles called Plastids

Figure 6.18a

RibosomesStroma

Inner and outermembranes

Granum

1 mIntermembrane spaceThylakoid(a) Diagram and TEM of chloroplast

DNA

Peroxisomes

▪ Specialized metabolic compartment

▪ Produce Hydrogen Peroxide and convert it to water

▪ Perform many different reactions

Cell Membrane - Composition

▪ Made up of phospholipids and proteins

▪ Amphipathic molecules – have both hydrophobic and hydrophilic regions

▪ The membrane is a fluid (moving) structure with proteins embedded in it– Mosaic – made of many, varied pieces

Figure 7.5

Glyco-protein

Carbohydrate

Glycolipid

Microfilamentsof cytoskeleton

EXTRACELLULARSIDE OFMEMBRANE

CYTOPLASMIC SIDEOF MEMBRANE

Integralprotein

Peripheralproteins

Cholesterol

Fibers of extra-cellular matrix (ECM)

Figure 7.2

Hydrophilichead

Hydrophobictail

WATER

WATER

Membrane Composition – Phospholipid bilayer

Figure 7.3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Membrane Composition▪ Membrane bound proteins have hydrophobic regions

Membrane Composition - Fluidity▪ Phospholipids and some proteins can move

▪ Most drift laterally

▪ Sometimes (rarely) flip transversly

Figure 7.6

Membrane Composition - Fluidity▪ Must be fluid to work properly

▪ Fluidity depends on temperature– Lower temperatures – less fluid

▪ Phospholipid types vary– Unsaturated fatty acids are more fluid than saturated

▪ Cholesterol in animal cell membranes helps maintain fluidity– Steroid

– At warm temps (37 degrees) it slows movement of phospholipids

– As temp cools, it maintains fluidity by preventing tight packing

Membrane Composition – Fluidity & Cholesterol

Membrane Composition – Fluidity

▪ Lipid composition varies from species to species– They are adapted to specific environments

– Some species have the ability to change composition in response to temperature changes where they live.

Membrane Composition - Proteins

▪ Lots of different proteins– Embedded in the fluid matrix of the lipid bilayer

▪ Proteins determine most of the membrane’s specific function

Membrane Composition - Proteins

▪ Peripheral proteins – bound to the surface of the membrane

▪ Integral proteins – penetrate the hydrophobic core– Transmembrane proteins – span the membrane

– Hydrophobic regions of the protein are usually alpha helices made of nonpolar amino acids

Figure 7.9

N-terminus

helix

C-terminus

EXTRACELLULARSIDE

CYTOPLASMICSIDE

Membrane Composition – Proteins

▪ Transport

▪ Enzymatic activity

▪ Signal transduction

▪ Cell-cell recognition

▪ Intercellular joining

▪ Attachment to the cytoskeleton and Extracellular matrix (ECM)

Membrane Composition – Transport Proteins

Figure 7.10a

Enzymes

Signaling molecule

Receptor

Signal transductionATP

(a) Transport (b) Enzymatic activity(c) Signal transduction

Selective Permeability

▪ Cell must exchange materials with it’s surroundings– Controlled by the plasma membrane

– Nutrients, signaling molecules, Ions in; waste products out.

▪ Selectively permeable – the membrane selects what goes in and out, only allowing specific things

Selective PermiabilitySmall, nonpolar molecules can move through

Large, polar molecules can’t

Selective Permeability – Transport Proteins

▪ Allow hydrophilic substances to cross the membrane

▪ Two types:– Channel proteins

▪ Have a hydrophilic channel that allows polar molecules through

▪ Aquaporins are specific for the movement of water

– Carrier Proteins▪ Bind to molecules and move them across

▪ Once the molecule binds, the carrier protein changes shape to move it across

▪ They are specific for the substances they move

Selective Permeability – Transport Proteins; Channel proteins

Selective Permeability – Transport Proteins; Channel proteins

Selective Permeability – Passive transport; diffusion

▪ Diffusion – the tendency of molecules to spread out evenly into the available space

▪ Can be directional

▪ At dynamic equilibrium – equal number of molecules moving in both directions

Figure 7.13a

Molecules of dyeMembrane (cross section)

WATER

(a) Diffusion of one solute

Net diffusion Net diffusion Equilibrium

Figure 7.13b

(b) Diffusion of two solutes

Net diffusion Net diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium

Selective Permeability – Passive transport; diffusion

▪ Substances move down their concentration gradient– No work is done

– Diffusion across a membrane, down the concentration gradient is called passive transport▪ No energy is expended

Selective Permeability – Passive transport; Osmosis

▪ The diffusion of water across a selectively permeable membrane

▪ Water moves from low solute concentration to higher solute concentration– Diffuses until the solute concentration is equal on both sides of

the membrane

Figure 7.14

Lowerconcentrationof solute (sugar)

Higher concentrationof solute

Sugarmolecule

H2O

Same concentrationof solute

Selectivelypermeablemembrane

Osmosis

Selective Permeability – Passive transport; Osmosis

▪ Tonicity – the ability of a surrounding solution to cause a cell to gain or lose water

▪ Isotonic solution – Solute concentration is the same outside as it is inside the cell– No net movement of water across the membrane

▪ Hypertonic solution – solute concentration outside is greater than that inside the cell; cell loses water

▪ Hypotonic solution – solute concentration outside is less than that inside the cell; cell gains water

Figure 7.15

Hypotonicsolution

Osmosis

Isotonicsolution

Hypertonicsolution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2OCell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

Selective Permeability – Passive transport; Osmosis

▪ Osmoregulation – organisms need mechanisms for controlling water loss or gain. – Various organisms manage osmosis differently based on

adaptations to their specific environments

Selective Permeability – Passive transport; Facilitated diffusion▪ Facilitated diffusion – passive transport aided by

proteins– Transport proteins speed the passive movement of molecules

across the membrane

– Channel proteins▪ Aquaporins – faclilitated diffusion of water

▪ Ion channels – gates that open or close in response to a stimulus

Selective Permeability – Active transport

▪ Moving solutes against their concentration gradient requires energy– Usually provided in the form of ATP

– Specific proteins are required for specific substances

Figure 7.19Passive transport Active transport

Diffusion Facilitated diffusionATP

Selective Permeability – Active transport; sodium potassium pump

▪ ECM – High Sodium, Low potassium

▪ Cytoplasm – Low Sodium, High potassium

▪ Potassium moves out – Sodium moves in

ECM Cytoplasm

Figure 7.18-1

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1

Figure 7.18-2

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2

Na

Na

Na

PATP

ADP

Figure 7.18-3

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2 3

Na

Na

Na

Na

Na

Na

P PATP

ADP

Figure 7.18-4

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2 3

4

Na

Na

Na

Na

Na

Na

K

K

P P

PP i

ATP

ADP

Figure 7.18-5

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2 3

45

Na

Na

Na

Na

Na

Na

K

K

K

K

P P

PP i

ATP

ADP

Figure 7.18-6

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2 3

456

Na

Na

Na

Na

Na

Na

K

K

K

K

K

K

P P

PP i

ATP

ADP

Selective Permeability - Ion pumps

▪ Membrane potential – the voltage difference across a membrane– Positive and negative ions are separated on either side of a

membrane creating a voltage potential

Selective Permeability - Ion pumps

▪ Electrochemical Gradient – drives the diffusion of ions across a membrane– Two forces make up the electrochemical gradient

▪ Chemical force – the ion’s concentration gradient

▪ Electrical force – the effect of the membrane potential on the ion’s movement

▪ Electrogenic Pump – a transport protein that generates voltage across a membrane– Sodium-potassium pump in animals

– Proton pump in plants

Selective Permeability - Ion pumps; nerve impulse

▪ https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html