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CHAPTER 5 THE STRUCTURE AND FUNCTION OF MACROMOLECULES. CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION. How things get into and out of the cell. The plasma membrane separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell.

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How things get into and out of the cell

Section a membrane structure introduction

The plasma membrane separates the living cell from its nonliving surroundings.

This thin barrier, 8 nm thick, controls traffic into and out of the cell.

Like other membranes, the plasma membrane is selectively permeable مُنفِذإختياريا, allowing some substances to cross more easily than others.

The most abundant lipids cell membrane are phospholipids.

Phospholipids and most other membrane constituents are amphipathic molecules.

Amphipathic molecules have both hydrophobic regions and hydrophilic regions.

The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model.

A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids.

Section A: Membrane Structure (Introduction)

The molecules in the bilayer nonliving surroundings.طبقة مزدوجة are arranged as hydrophobic fatty acid tails are sheltered محمية from water while the hydrophilic phosphate groups interact with water.

Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions.

If at the surface, the hydrophilic regions would be in contact with water.

In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in contact with water and the hydrophobic regions are in a nonaqueous لا مائىenvironment.

Fig. 8.1b

Fig. 8.2b

Membranes are mosaics of structure and function

A membrane is a collage nonliving surroundings.تـَجَمُّع of different proteins embedded مُـنْغَمِس in the fluid matrix of the lipid bilayer.

Membranes are mosaics of structure and function

To work properly with permeability, membrane must be fluid, about as fluid as oil.

Fig. 8.6, Page 142

A)- The plasma membrane has a unique collection nonliving surroundings.تجمع مميَـز of proteins.

There are two populations of membrane proteins.

Peripheral proteinsطرفى are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins.

Integral proteinsمُندَمج penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).

Where they contact the core, they have hydrophobic regions with nonpolar amino acids, often coiled into alpha helices.

Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids.

Peripheral proteins

Integral proteins

Peripheral proteins

Fig. 8.7, Page 143

The proteins in the plasma membrane may provide a variety of major cell functions.

Fig. 8.9, Page 144

Functions of cell membrane plasma membrane 1 selective permeability

A steady traffic major cell functions.العبور المنتطم of small molecules and ions moves across the plasma membrane in both directions.

For example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave it.

The cell absorbs O2 and expels CO2.

It also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by passing them across the membrane.

However, substances do not move across the barrier indiscriminately عشوائيا as membrane is selectively permeable.

Hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.

Ions and polar molecules pass through with difficulty.

This includes small molecules, like water, and larger critical molecules, like glucose and other sugars.

Proteins can assist and regulate يساعد و ينظم the transport of ions and polar molecules.

Functions of cell membrane (Plasma membrane) 1- Selective permeability النفاذية الإختيارية

Selective Permeability major cell functions.






The cell is able to take upتـنـتـقى particular molecules and exclude تـتجنبothers

2 passive transport is diffusion across a membrane

Diffusion: major cell functions.الإنتشار

Is the tendency إستعداد of molecules of any substance to spread out للإنتشار in the available space randomly عشوائيا.

For example, a permeable membrane غشاء منفز separating a solution with dyemolecules from pure water, dye molecules will cross the barrier randomly.

The dye will cross the membrane until both solutions have equal concentrations of the dye (dynamic equilibriumالتعادل الديناميكى).

2- Passive transport الإنتقال السلبى is diffusion across a membrane

Fig. 8.10a, Page 145

A substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient مـُنحدرالتركيز.

Fig. 8.10b, Page 145

  • Passive transport:الإنتقال السلبى

  • is the diffusion of a substance across a biological membrane whichrequires no energy from the cell to make it happen.

    • The concentration gradient represents potential energy and drives diffusion.

CD Unit 2, Chapter 8.

2 osmosis is the passive transport of water

Differences in concentration of to where it is less concentrated, dissolved materials in two solutions can lead to the movement of ions from one to the other.

The solution with the higher concentration of solutes is hypertonic.

The solution with the lower concentration of solutes is hypotonic.

Solutions with equal solute concentrations are isotonic.


Is a passive transport in which water diffuses across a selectively permeable membrane from the hypotonic solution to the hypertonic solution until the solutions become isotonic.

2. Osmosis الأسموزية:is the passive transport of water

Fig. 8.11, page 146

Osmosis to where it is less concentrated,


Selectively permeable membrane

Low conc. of sugar

High conc. of sugar

Principal of water movement




Hypertonic to where it is less concentrated,



Passive transport of water (Osmosis)

  • Hypertonic solution:عالى التركيزcontains high concentration of solute مُذابmolecules.

  • Hypotonic solution:منخفض التركيزcontains low concentration of solute molecules.

  • Isotonic solution:متعادلcontains equal concentrations of solute molecules



The cell in a hypertonic environment will loose water, shrivel تنكمش, and die.

A cell in a hypotonic solution will gain تسحب water, swell, and burst.

Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain للحفاظ على their internal environment.

Example, Paramecium have a specialized organelle (the contractile vacuole), that functions as a pump to force يطرد water out of the cell.

Osmoregulationالتوازن الأسموزى

Fig. 8.12, Page 147

Fig. 8.13 & 8.12, Page 147

Specific proteins facilitate passive transport

Many polar molecules and ions diffuse passively through the lipid bilayer with the help of transport proteins (gated channels قنوات مُبَوبة).

The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion.

Fig. 8.14a & b, Page 148

Specific proteins facilitate تـُسَهـِّل passive transport

  • Many transport proteins simply provide channels allowing a specific molecule or ion to cross the membrane.

  • Other transport proteins translocateينقل the molecules across the membrane as the protein changes shape (active transport)