Chapter 2 cellular physiology

Cell : building block & function unit

The cell is the basic unit of the body to carry out and control the functional processes of life.

The cell is contained within a limiting membrane, and it consists of various organelles suspended in cytoplasm.

Schematic three-dimensional illustration of cell structures visible under an electron microscope

• A cell is made up of three major parts:

• A plasma membrane: enclose the cell• The nucleus: houses the cells genetic

material• Cytoplasm: contain highly specialized

organelles

Cell membrane (plasma membrane)

• Functions: 1. Selective barrier2. Containing enzyme system3. Containing transport system4. Containing specific recognition sites

membrane structure “Fluid mosaic model”

• LM: too thin to be seen• EM: tri-laminar structure two dark layer one light middle layer

• Sandwich appearance

Structure of cell membrane

Composition of cell membrane

• Lipid (more) phospholipids+cholesterol(lesser)

• Protein

• Carbohydrates (lesser)

Phospholipids molecule

Lipid Bilayer(phospholipids and cholesterols )

• polar head-negatively charged;hydrophilic

• Nonpolar fatty acid tails;hydrophobic

Lipid Bilayer(phospholipids and cholesterols )

Lipid Bilayer

Cell membrane separating ICF from

ECF

When in contact with water self-assembled

Characteristics of phospholipids

-- Lipid bilayer ’s nature: not a rigid but fluid The phospholipids are not held together by chemical bonds

-- Consistency: liquid cooking oil-- Twirl around;move about-- Flexible: The cell can change the shape

e.g. the RBC

Cholesterol

Contribute to the fluidity as well as the stability of the membrane

prevent the fatty acid chains form packing together

Membrane proteinintegral (intrinsic) protein : ion channels

peripheral (extrinsic) protein

Membrane protein function(P53)

• Channels• Carrier molecules• Receptor sites• Docking-marker acceptors• Membrane-bound enzymes• Cell adhesion molecules (CAMs)

……

Carbohydrates(located only at the outer surface

and bond to protein and lipids)

Fluid mosaic model

• This view of membrane structure is known as the fluid mosaic model :

membrane fluidity the ever-changing mosaic pattern of the

proteins embedded within the lipid bilayer.

Fluid mosaic model

• The phospholipids , which are not held together by chemical bonds, are able to twirl around rapidly (more)

• The membrane protein stud or attach on the membrane (lesser)

• The carbohydrate was binding with lipid or protein

Membrane transport

Lipid bilayer :

- primary barrier ,

-selectively permeable

Permeable or impermeable

• Relative solubility in lipid uncharged or nonpolar

molecules -highly lipid-soluble: O2 CO2 fatty acid

• Size of the particle Water-soluble ions less than 0.8nm in diameter: H2O glycerol ethanol

Force

• Passive force: don’t require energy(ATP)

• Active force:require energy(ATP)

Categories of Transport Across the Plasma Membrane

--Active Transport

Primary Active Transport

Secondary Active Transport

--Endocytosis and exocytosis

Simple Diffusion

Facilitated Diffusion

--Passive transport No need ATP

need ATP

Passive transport

Diffusion: Physical process that occurs whenever there is a

concentration difference across the membrane and the membrane is permeable to the diffusing substance

A B

A B

Diffusion from A to B

Diffusion from B to A

Net diffusion

Diffusion from A to B

Diffusion from B to A

No net diffusionDown a concentration gradient

Simple diffusion

Related to the concentration gradient Movement is DOWN the concentration gradient ONLY (higher

concentration to lower concentration)

Rate of diffusion depends on (Fick’law of diffusion)

factor Rate of diffusion

Concentration gradient of substance (ΔC)

Permeability (P)

Surface area of membrane (A)

Molecular weight of substance (MW)Distance (thickness) (ΔX)

Simple diffusion

• Substances:– Non-polar molecules (02).

– Lipid soluble molecules (steroids).

– Small polar covalent bonds (C02).

– H20 (small size, lack charge).

• Osmosis is the net diffusion of water down its own concentration gradient

• The solute concentration increases, the water concentration decreases

correspondingly

Different cases

• If the membrane is permeable to the solute as well as to water

the solute is able to move down its own concentration gradient in the opposite direction of the net water movement.

• If the membrane is impermeable to the solute …

Water movement alone till the concentrations of water and solute on the two sides of the membrane become equal.

• If a nonpenetrating solute is present on side A and pure water is present on side B?

• The concentrations between the two compartments can never become equal.

• Tonicity: refers to the effect on cell volume of the concentration of nonpenetrating solutes in the solution surrounding the cell.

• Isotonic solution:has the same concentration of nonpenetrating solutes as normal body cells.

hypotonic solution hypertonic solution

Facilitated diffusion

• Definition: the diffusion of lipid insoluble or water soluble

substance across the membrane down their concentration gradients by aid of membrane proteins (carrier or channel)

• Substances: K+, Na+, Ca 2+ , glucose, amino acid,

urea

Carrier-mediated

Substance: glucose, amino acid

Concept: Diffusion carried out by carrier protein

Mechanism: a “ferry” or “shuttle” process

Characteristics of carrier-mediated diffusion

• Net movement depend on concentration gradient

--specificity: with specific molecule only.

-- competition : Molecules with similar chemical

structures compete for carrier site.

-- saturation: Carrier binding sites is limited

Simple diffusion

Carrier-mediated

Facilitated diffusion

Rate

of tra

nsp

orta

tion in

to

cell

Concentration transported molecules in

ECF

Channel-mediated

Characteristic of ion channel

• Specificity• Gating

ClosedOutside

Inside

Activated Inactivated

Channel type

• Voltage-gated channel• Chemical-gated channel• Mechanically-gated channel• Water channel• etc.

Voltage-gated channel

The molecular conformation of the gate responds to the electrical potential across the cell membrane, this type of channel is called voltage-gated (or dependent) channel.

Voltage-gated Na+ channel

Na+ channel conformation

ClosedOutside

Inside

Activated Inactivated

Chemical-gated channel

Some protein channel gates are opened by the binding of another molecule with the protein; this causes a conformational change in the protein molecule that opens or closes the gate.

This is called chemical gating. This type of channel is called chemically-gated (or dependent) channel

N2-Ach receptor channel

Mechanically-gated channel

Outside

Inside

membrane

Inside

membrane

Outside

stretch

Mechanically-gated channel

When sound waves move the basilar membrane it moves the hair cells that are connected to it, but the tips of the hair cells are connected to the tectorial membrane so the they get bent .There are little mechanical gates on each hair cell that open when they are bent. K+ goes into the cell and Depolarizes the hair cell. (concentration of K+ in the endolymph is very high)

Water channels

The structure of aquaporin

(AQP)

Aquaporin

• Aquaporin are water channel that exclude ions

• Aquaporin are found in essentially all organisms, and have biological and medical importance

Water transportation through the membrane

• Simple diffusion• Ion channel• Water channel

Active transport

• Primary active transport

• Second active transport

(Involving the use of a protein carrier and transport substance against its concentration gradient)

Primary active transport

• Iodine -- thyroid gland cells

the energy required is derived directly from the breakdown of ATP or some other high-energy phosphate compound and moves a substance uphill

Primary active transport

• ATP is required in active transport to vary the affinity of the binding site when exposed on opposite sides of the plasma membrane.

• These active transport mechanisms are frequently called pumps.

H+ pump; Ca++ pump Na+-K+ ATPase pump

Concentration gradient of Na+ and K+

ECF (m mol/L) ICF (m mol/L)

Na+

K+

140.0

150.0

15.0

4.0

Na+-K+ ATPase (Na+ pump , Na+-K+

pump)--electrogenic pump

Physiological role of Na+-K+

pump• Maintaining Na+ and K+ concentration gradients

across the membrane (transport Na+ out of cell and pick up K+ from the outside).

• Partly responsible for establishing negative electrical potential inside the cell( moves 3 Na+ outside and 2 K+ inside)

• Controlling cell volume ( by controlling the concentration of solutes )

• Providing energy for secondary active transport

Secondary Active Transport

• the energy required is derived indirectly from the breakdown of ATP . Rather it use “second hand” energy stored in the form of and ion concentration gradient

• The established by primary active transport permits the transport of other substance against their concentration gradient

Secondary Active Transport

Na+

Low X

out in

Na+

Low H+

out in

Na+

co-transport counter-transport (symport) (antiport)

e.g. glucose, in the same direction as the Na+.

e.g. H+, in the opposite direction to the Na+.

high low Na+

high X

high

high H+

low Na+

Secondary Active Transport

Endocytosis and Exocytosis

Exocytosis

Endocytosis

Summary

• Term : - Voltage-gated channel - Ligand-gated channel - co-transport and counter-transport - Primary Active Transport and Secondary Active Transport

• Describe the ways of membrane transport

• Describe the physiological role of sodium pump