chapter chapter 222 structure of the cell - usp · chapter chapter 222 structure of the cell....

General Biology

Mohammed Al-Gayyar - 10 - Autumn 2012

Chapter Chapter Chapter Chapter 2222

Structure of the CellStructure of the CellStructure of the CellStructure of the Cell

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Introduction to the cellsIntroduction to the cellsIntroduction to the cellsIntroduction to the cells The word cell comes from the Latin cellula, meaning "a small room". The cell is the basic

structural and functional unit of all known living organisms. It is the smallest unit of life that is

classified as a living thing, and is often called the building block of life. Some notes about

cells should be kept in mind:

§ Nothing less than cell can be called living: The vital functions of an organism occur

within cells. All cells come from preexisting. Like ourselves, the individual cells that form

our bodies can grow, reproduce, process information, respond to stimuli and carry out an

amazing array of chemical reactions. These abilities define life. Even simple unicellular

organisms exhibit all the hallmark properties of life, indicating that the cell is the

fundamental unit of life.

§ The Diversity: Organisms can be classified as unicellular (consisting of a single cell;

including most bacteria) or multicellular (including plants and animals). Cells come in an

amazing variety of sizes and shapes. Some move rapidly and have fast-changing

structures. Others are largely stationary and structurally stable. Oxygen kills some cells

but is an absolute requirement for others.

§ Similar basic chemistry: Despite the extraordinary diversity of plants and animals, all

living things are fundamentally similar inside. Cells resemble one another to an

astonishing degree in the details of their chemistry and sharing the same machinery for

the most basic functions. All cells are composed of the same sorts of molecules that

participate in the same types of chemical reactions.

§ Invention of the light microscope led to the discovery of cells: The descriptive term

for the smallest living biological structure was coined by Robert Hooke in a book he in

1665 when he compared the cork cells he saw through his microscope to the small

rooms. The name cell stuck even though the structures Hooke described were only the

cell walls that remained after the living plant cells inside them had died.

Types of cells:

The biological universe consists of two types of cells:

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§ Prokaryotic cells consist of a single closed compartment that is surrounded by the

plasma membrane, lacks a defined nucleus, and has a relatively simple internal

organization. Examples: Bacteria and Blue-green algae.

§ Eukaryotic cells: unlike prokaryotic cells, contain a defined membrane-bound nucleus

and extensive internal membranes that enclose other compartments. The region of the cell

lying between the plasma membrane and the nucleus is the cytoplasm, comprising the

cytosol (aqueous phase) and the organelles. Eukaryotes comprise all members of the

plant and animal kingdoms, including the fungi, which exist in both multicellular forms

(molds) and unicellular forms (yeasts).

Eukaryotes Prokaryotes

Complex in structure, with nuclei and

membrane-bound organelles

more complex in structure, with nuclei and

membrane-bound organelles

Large (100 - 1000 µm) Small (1-10 µm)

DNA in nucleus, bounded by membrane DNA circular, unbounded

Genome consists of several chromosomes Genome consists of single chromosome

Sexual reproduction common, by mitosis and

meiosis

Asexual reproduction common, not by mitosis

or meiosis

Mitochondria and other organelles present No general organelles

Most forms are multicellular Most forms are singular

Aerobic Anaerobic

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Biological Biological Biological Biological MembraneMembraneMembraneMembranessss

Membranes are the outer boundary of individual cells and of certain organelles. Plasma

membranes are the selectively permeable outermost structures of cells that separate the

interior of the cell from the environment. Certain molecules are permitted to enter and exit the

cell through transport across the plasma membrane.

Components of biological membranes:

All cell membranes are composed of the same materials:

1. Lipids

Lipids are the most abundant type of macromolecule present. Plasma and organelle

membranes contain between 40% and 80% lipid. There are three types of lipids are found:

§ Phospholipids: The most abundant of the membrane lipids are the phospholipids. They

are polar, ionic compounds that are amphipathic (have both hydrophilic and hydrophobic

components). The hydrophilic or polar portion is in the “head group”. Within the head group

is the phosphate and an alcohol that is attached to it. The hydrophobic portion of the

phospholipid is a long, hydrocarbon (structure of carbons and hydrogens) fatty acid tail.

While the polar head groups of the outer leaflet extend outward toward the environment,

the fatty acid tails extend inward.

§ Cholesterol: Another major component of cell membranes

is cholesterol. An amphipathic molecule, cholesterol

contains a polar hydroxyl group as well as a hydrophobic

steroid ring and attached hydrocarbon. Cholesterol is

dispersed throughout cell membranes, intercalating

between phospholipids.

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Its polarhydroxyl group is near the polar head groups of the

phospholipidswhile the steroid ring and hydrocarbon tails of

cholesterol areoriented parallel to those of the

phospholipids. Cholesterolfits into the spaces created by

the kinks of the unsaturatedfatty acid tails, decreasing the

ability of the fatty acids to undergomotion and therefore

causing stiffening and strengthening of themembrane.

§ Glycolipids: Lipids with attached carbohydrate (sugars), glycolipidsare found in cell

membranes in lower concentration than phospholipids and cholesterol. The carbohydrate

portion is always oriented toward the outside of the cell, projecting into the environment.

Glycolipids help to form the carbohydrate coat observed on cells and are involved in cell-

to-cell interactions.

2. Proteins

While lipids form the main structure of the membrane, proteins arelargely responsible for

many biological functions of the membrane. The types of proteins within a plasma membrane

vary depending on the cell type. However, all membrane proteins are associated with

membrane in one of three main ways:

§ Transmembrane proteins: They are embedded within the lipid bilayer of the membrane

with structures that extend from the environment into the cytosol. All trans membrane

proteins contain both hydrophilic and hydrophobic components. These proteins are

oriented with their hydrophilic portions in contact with the aqueous exterior environment

and with the cytosol and their hydrophobic portions in contact with the fatty acid tails of the

phospholipids.

§ Lipid-anchored proteins: They are attached covalently to a portion of a lipid without

entering the core portion of the bilayer of the membrane. Both trans membrane and lipid-

anchored proteins are integral membrane proteins since they can only be removed from a

membrane by disrupting the entire membrane structure.

§ Peripheral membrane proteins: These proteins are located on the cytosolic side of the

membrane and are only indirectly attached to the lipid of the membrane; they bind to other

proteins that are attached to the lipids.

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Structure of biological membranes:

The proteins and lipids of a cellular membrane are arranged in a

certain way to form a stable outer structure of the cell. The

membrane components, including lipids and proteins, are not

fixed rigidly into a particular location. Both can exhibit several

types of motions. Membrane proteins can also move laterally and

can rotate.

Despite its fluidity, the membrane structure is very stable and supportive for the cell. The

arrangement of the phospholipids provides the basic structure which is then augmented by

cholesterol, with functional roles played by proteins. The biological membranes have the

following characters:

§ Bilayer arrangement: Membrane phospholipids are oriented with their hydrophobic fatty

acid tails facing away from the polar, aqueous fluids of both the cytosol and the

environment. The hydrophilic portions of the phospholipids are oriented toward the polar

environment. Two layers of phospholipids are required to achieve this structure.

§ Asymmetry: The fatty acid tails of all the phospholipids are structurally very similar to

each other. Some phospholipids are found on the outer leaflet while others are more

commonly seen on the inner leaflet. In addition, glycolipids are differentially arranged as

well and are always on the outer leaflet with their attached carbohydrate projecting away

from the cell.

§ Fluid mosaic model: The membrane is described as a fluid, owing to the ability of lipids to

diffuse laterally. The overall structure is likened to a flowing sea. Membrane proteins are

dispersed throughout the membrane. Many of the membrane proteins retain the ability to

undergo lateral motion and are likened to icebergs floating within the sea of lipids.

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OrganellesOrganellesOrganellesOrganelles Organelles are complex intracellular locations where processes necessary for eukaryotic

cellular life occur. Most organelles are membrane-enclosed structures. Their membranes are

composed of the same components asplasma membranes that form the outer boundaries of

cells. Together with the cytosol (liquid portion of the cytoskeleton), the organelles help to form

the cytoplasm, composed of all materials contained within the boundaries of the plasma

membrane. Organelles do not float freely within the cytosol but are interconnected and joined

by the framework established by proteins of the cytoskeleton. Each organelle carries out a

specific function.

Nucleus (plural = nuclei):

All eukaryotic cells except mature erythrocytes (red

blood cells) contain a nucleus where the cell’s

genomic DNA resides. The outermost structure of

the nucleus is the nuclear envelope. This is a

double-layered phospholipid membrane with nuclear

pores to permit transfer of materials between the

nucleus and the cytosol. The interior of the nucleus

contains the nucleoplasm (the fluid in which the

DNAs are found). Within the nucleus there is a

suborganelle called the nucleolus. The nucleolus is

the site of ribosome production.

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Ribosomes:

Ribosomes are the cellular machinery for protein synthesis. They are composed of proteins

and ribosomal RNA (rRNA) with approximately 40% being protein and 60% rRNA.

Ribosomes are found within the cytosol either free or bound to the endoplasmic reticulum.

Endoplasmic reticulum (ER):

ER is often observed to surround the nucleus. The outer

layer of the nuclear envelope is actually contiguous with the

ER. The ER forms a maze of membrane-enclosed,

interconnected spaces that constitute the ER lumen

Regions of ER where ribosomes are bound to the outer

membrane are called rough endoplasmic reticulum

(rER). Bound ribosomes and the associated ER are

involved in the production and modification of proteins.

Smooth endoplasmic reticulum (sER) refers to the regions of ER without attached

ribosomes. Both rER and sER function in the glycosylation (addition of carbohydrate) of

proteins and in the synthesis of lipids.

Golgi complex:

It appears as flat, stacked, membranous sacs. Three

regions are described within the Golgi complex: the cis,

which is closest to the ER; the medial; and the trans Golgi,

which is near the plasma membrane. Each region is

responsible for performing distinct modifications to the

newly synthesized proteins, such as:

§ Glycosylations (addition of carbohydrate)

§ Phosphorylations (addition of phosphate)

§ Proteolysis (enzyme-mediated breakdown of protein)

Mitochondrion (plural = mitochondria):

Complex organelles, mitochondria have several important functions in eukaryotic cells. Their

unique membranes are used to generate ATP (greatly increasing the energy yield from the

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breakdown of carbohydrates and lipids). The very survival of individual cells depends on the

integrity of their mitochondria.

One characteristic feature of mitochondria is the double

phospholipidbilayer membranes that form the outer

boundary of the organelle. The inner mitochondrial

membrane forms folded structures called cristae that

protrude into the mitochondrial lumen known as the

mitochondrial matrix.

Lysosomes:

Lysosomes are membrane-enclosed organelles of various sizes that havean acidic internal

pH (pH 5). Lysosomes containpotent enzymes known collectively as acid hydrolases.

They function within the acidic environment of

lysosomes to hydrolyze or break down macromolecules

(proteins, nucleic acids, carbohydrates and lipids).

Nonfunctional macromoleculesbuild up to toxic levels if

they are not degraded within lysosomes andproperly

recycled for reuse within the cell.In addition, lysosomal

enzymes also degrade materials that have beentaken

up by the cell through endocytosis or phagocytosis.

Peroxisomes:

Peroxisomes resemble lysosomes in size and in structure. They havesingle membranes

enclosing them and contain hydrolytic enzymes. It helps in:

§ Break down of fatty acids and purines (AMP and GMP).

§ Detoxification of hydrogen peroxide (a toxic by-product of many metabolic reactions).

§ Synthesis of myelin (the substance that forms a protective sheath around many neurons).

§ In liver cells, peroxisomes participate in cholesterol and bile acid synthesis.

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CytoskeletonCytoskeletonCytoskeletonCytoskeleton The cytoskeleton is a complex network of protein filaments that establish a supportive

scaffolding system within the cell. Cytoskeletal proteins are located throughout the interior of

the cell, anchored to the plasma membrane and traversing the cytoplasm. Organelles reside

within the framework established by the cytoskeleton. The cytoskeleton is not simply a

passive internal skeleton but is a dynamic regulatory feature of the cell. These components of

the cytoskeleton work together as an integrated network of support within the cytoplasm.

Actin:

Actin helps to establish a cytoplasmic protein framework known as microfilaments visualized

radiating out from the nucleus to the lipid bilayer of the plasma membrane. Some forms of

actin are found only in muscle cells, while other forms of actin are found within the cytoplasm

of most cell types. Functions of actin in the cytoplasm of non muscle cells include:

§ Regulation of the physical state of the cytosol

§ Cell movement

§ Formation of contractile rings in cell division

§ Within the nucleus, actin is involved in the regulation of gene transcription.

§ Regulators of the gel/sol of the cytosol: One characteristic of a cell is the physical

nature of its cytosol. It can be described either as gel, a more firm state, or sol, a more

soluble state. The more structured the actin, the firmer (gel) the cytosol. The less

structured (more fragmented) the actin, the more soluble (sol) the cytosol. Actin is

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continuously tread-milling in both the gel and sol states, contributing to the character of

the cytoplasm.

Intermediate filaments:

They are larger than actin microfilaments and

smaller than microtubules. Most intermediate

filaments are located in the cytosol between

the nuclear envelope and the plasma

membrane. They provide structural stability

to the cytoplasm, somewhat reminiscent of

the way that steel rods can reinforce

concrete. There are six categories of

intermediate filaments, grouped by their

location. Examples include keratins, vimentin

and neuroflaments

Microtubules:

Microtubules are the last type of predominant structure

observed in the cytoskeleton. The structure of a

microtubule resembles a hollow cylindrical tube. They

are involved in:

§ Chromosomal movements during nuclear divisions

§ Formation of cilia and flagella in certain cell types

§ Intracellular transport

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Extracellular MatrixExtracellular MatrixExtracellular MatrixExtracellular Matrix A substantial part of tissue volume is an extracellular space largely filled by the intricate

network of macromolecules of the extracellular matrix (ECM). The ECM is specialized to

perform different functions in different tissues. For example, the ECM adds strength to

tendons and is involved in filtration in the kidney and attachment in skin.

The physical nature of the ECM also varies from tissue to tissue. Blood is fluid, while cartilage

has a spongy characteristic owing to the nature of extracellular materials in those tissues.

Three categories of extra cellular macromolecules make up the ECM:

1. Proteoglycans:

Proteoglycans are aggregates of glycosaminoglycans (GAGs) and proteins. GAGs arealso

known are composed of repeating disaccharide chains where one of the sugars is an amino

sugar and the other is an acidic sugar. They are organized in long, unbranched chains. GAGs

contain multiple negative charges and are extended in solution. The most prevalent GAG is

chondroitin sulfate. Other GAGs include hyaluronic acid, keratin sulfate, dermatan sulfate,

heparin and heparan sulfate.

Because of their net negative surface charges, GAGs

repel each other. In solution, GAGs tend to slide past

each other, producing the slippery consistency we

associate with mucous secretions. The bones of the joint

are cushioned by the water balloon–like structure of the

hydrated GAGs in the cartilage. When compressive

forces are exerted on it, the water is forced out and the

GAGs occupy a smaller volume. When the force of

compression is released, water floods back in,

rehydrating the GAGs, much like a dried sponge rapidly

soaking up water.

2. Fibrous proteins:

Fibrous proteins are extended molecules that serve structural functions in tissues. There are

two types of fibrous proteins:

§ Collagen: The most abundant protein in the human body, collagen forms tough protein

fibers that are resistant to shearing forces. Collagen is the main type of protein in bone,

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tendon and skin. In the ECM, collagen is dispersed as a gel-like substance and provides

support and strength. Collagen is a family of proteins, with 28 distinct types. However,

over 90% of collagen in the human body is in collagen types I, II, III, and IV. Together the

collagens constitute 25% of total body protein mass.

§ Elastin: The other major fibrous protein in the ECM is elastin. Elastic fibers formed by

elastin enable skin, arteries and lungs to stretch and recoil without tearing. The structure

of elastin is that of an interconnected rubbery network that can impart stretchiness to the

tissue that contains it. This structure resembles a collection of rubber bands that have

been knotted together.

3. Adhesive proteins:

The last category of ECM components consists of proteins that join together and organize the

ECM and also link cells to the ECM. Fibronectin and laminin are adhesive glycoproteins

secreted by cells into the extracellular space. Both are considered multifunctional proteins

because they contain three different binding domains that link them to cell surfaces and to

other components of the ECM, including proteoglycans and collagen. Through their

interactions with fibronectin or laminin, proteoglycans and collagen are linked to each other

and to a cell’s surface. Thus, adhesive proteins join ECM components to each other and link

cells to the ECM.

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Cell AdhesionCell AdhesionCell AdhesionCell Adhesion A growing tissue is able to form because the member cells remain attached and do not travel

elsewhere. Cell junctions are important in maintaining the structure of a tissue as well as its

integrity. They are composed of a collection of individual cell adhesion molecules.

Cell adhesion molecules mediate selective cell-to-cell and cell-to-ECM adhesion. These are

all transmembrane proteins that are embedded within the plasma membranes of cells. They

extend from the cytoplasm through the plasma membrane to the extracellular space. In the

extracellular space, they bind specifically to their ligands, which may be:

§ Cell adhesion molecules on other cells

§ Certain molecules on the surface of other cells

§ Components of the ECM

Four families of adhesion molecules function in cell-cell adhesion: the cadherins, the

selectins, the immunoglobulin super family, and the integrins. The integrins also function in

cell-to-ECM adhesion.

Cadherins:

The cell adhesion molecules that are important in holding

cells together to maintain the integrity of a tissue are

called the cadherins. These transmembrane linker

proteins contain extracellular domains that bind to a

cadherin on another cell. Cadherins also have intracellular

domains that bind to the actin cytoskeleton.

Therefore, when two cells are linked together via cadherins, their actin cytoskeletons are

indirectly linked as well. Calcium is required for cadherin binding to another cadherin.

Adhesion mediated by cadherins is long-lasting and important in maintaining the tissue

structure.

Selectins:

Selectins mediate more transient cell-to-cell adhesions.

They are particularly important in the immune system in

mediating white blood cell migration to sites of

inflammation.

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Selectins are named for their “lectin” or carbohydrate-binding domain in the extracellular

portionof their structure. A selectin on one cell interacts with a carbohydrate-containing ligand

on another cell.

Immunoglobulin superfamily:

They are named because they share certain structural

characteristics of immunoglobulins (antibodies). These

members of the immunoglobulin superfamily of adhesion

molecules regulate cell-to-cell adhesions. Some

immunoglobulin superfamily members facilitate adhesion

of leukocytes to endothelial cells lining the blood vessels

during injury and stress.

Integrins:

Both cell-to-cell and cell-to-ECM adhesions are mediated

by integrins. Members of this family of transmembrane

proteins bind to their ligands with relatively low affinity;

multiple weak adhesive interactions characterize integrin

binding and function.

Integrins consist of two transmembrane chains, α and β. When integrins mediate cell-to-cell

adhesions, their ligands are members of the immunoglobulin superfamily. When integrins join

a cell to the ECM, collagen and fibronectin commonly serve as their ligands. Therefore,

Integrins mediate interactions between the cytoskeleton within the cell and the ECM

surrounding the cell.

chapter chapter 222 structure of the cell - usp · chapter chapter 222 structure of the cell....

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