chapt 02

GENERAL BIOLOGY

SCHOOL OF MLTFACULTY OF HEALTH SCIENCE

PREPARED BY:MANEGA

HDL 121CELL COMPONENTS

CELL COMPONENTS

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Chapter 2 : Cell Components

Course Purpose

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This course introduces animal cells based on structure, characteristics, functions and cellular division. It also emphasises the theory of inheritance including the structure

and role gene, DNA, and chromosome.

CELL COMPONENTS

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Topic Outlines

2.1 Fluid Mosaic Model

2.1.1 Proposed by Singer & Nicholson2.1.2 Functions of cell membrane

2.1.3 Connection between cells 2.1.4 Special Structure

2.2 Connection between cells

2.2.1 Tight Junction2.2.2 Desmosome

2.2.3 Gap Junction

2.3 Special structure

2.3.1 Microvilli

© 2010 Cosmopoint

CELL COMPONENTS

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Learning Outcomes

© 2010 Cosmopoint

After completing this lecture, students will be able to:(a) Describe the structure of cell membrane – recall the fluid mosaic model(b) List the functions of cell membrane(c) Outline the membrane junctions – tight junction, gap junction & desmosome(d) Explain special structures of cell membrane, the microvilli

CELL COMPONENTS

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Chapter 2 : Cell Componenets

Sub Topic 2.1 : Fluid Mosaic Model

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Lipoprotein layer that surrounds the cell & organelles eg. nucleus, mitochondria, chloroplast, vacuole & lysosome.

The structure of the membrane is based on Singer & Nicolson’s fluid-mosaic model

CELL COMPONENTS

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Sub Topic 2.1.1 : Proposed by Singer & Nicholson

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The basic structure of membrane consists of a bimolecular phospholipid fluid layer (2 layers of phospholipids/phospholipid bilayer) with globular protein units floating in it forming a mosaic pattern

The heads of the phospholipid are hydrophilic pointing outwards into the aqueous medium on both sides of the membrane

The tails of phospholipid are hydrophobic facing each other & forming a non-polar interior in the middle of the membrane

The structure is dynamic, each lipid molecule can move within its own monolayer and so is each of the protein unit.

Some protein units are immobilised by microfilament with the interior of the cell

CELL COMPONENTS

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CELL COMPONENTS

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The fluidity of the membrane depends on the length of the fatty acid chains, their saturation & the amount of cholesterol among them.

Fluidity affects permeability, membrane enzyme activities, reception to molecules & ease with which membranes fuse

Cholesterol with its hydrophilic head & hydrophobic tail fits neatly within the phospholipid layer

It functions to control mechanical stability, flexibility & permeability of membrane, particularly reduced leakage of small polar molecules

The proteins are embedded in the phospholipid layer like mosaic, only in one monolayer or span the whole membrane.

These are integral or intrinsic proteins, fitted neatly because of their charged properties on their surfaces.

CELL COMPONENTS

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The peripheral or extrinsic ones are attached on the outer polar layer of phospholipid

The proteins function as carriers or channels for molecules to cross the membrane, as structural components, enzymes, receptors & electron carriers for respiratory or photosynthetic phosphorylations

The carbohydrates exists as branched short chain of sugars on the outer surface of membrane

i. glycoproteins (CHO + proteins)ii.glycolipids (CHO + lipids)Function: i. as receptors for chemical like hormonesii.adhesion to neighbouring cells & for immune responses

CELL COMPONENTS

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CELL COMPONENTS

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CELL COMPONENTS

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CELL COMPONENTS

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Sub Topic 2.1.2 : Functions of cell membrane

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1.Barrier/Boundary – physically separates the intracellular components from the extracellular environment (substances outside do not affect reactions taking place within the cell)

2.Anchor – peripheral proteins anchor the cytoskeleton to provide shape to the cell & in attaching to the extracellular matrix to help group cells together in the formation of tissue

3.Transport /regulates or controls the passage – phospholipid bilayer is selectively permeable, allows only certain molecules to pass through protein channels allow only specific polar molecules to go in or out

CELL COMPONENTS

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4.Receptor sites – some integral proteins as receptor/binding sites for hormones (for recognising external stimuli i.e hormone & antigen molecules– enables cells to recognise other cells & to behave in an organised manner during formation of tissue in the embryo

5.Identification – Glycoproteins as marker, recognized by other cells

6.Protection – any chemical/reaction that happened outside would not harm the cell

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7. Compartmentalisation – Within cells, membranes allow compartmentalisation & division of labour to occur especially within membrane-bound organelles.

8. Special functions: light reaction in the membrane of chloroplast & oxidative phosphorylation in the inner membrane of mitochondria

9. Cell Mobility (eg. WBC) & communication between cells

CELL COMPONENTS

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Sub Topic : 2.2 Connection between cells

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3 major types:(a) Tight junction

(b) Gap junction

(c) Desmosome

CELL COMPONENTS

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Sub Topic 2.2.1 :Tight Junction

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Composed of protein fibres that seal adjacent cells to prevent leakage

Forms impermeable junction

Eg. Bladder & the lining of the digestive tract

CELL COMPONENTS

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Sub Topic 2.2.1 :Tight Junction

© 2010 Cosmopoint

CELL COMPONENTS

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Sub Topic 2.2.2: Desmosome

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On inner surface of cell membrane are specialized integral proteins called plaques

Plaques from adjacent cells connected by thin linker filament (special protein called cadherins)

Similar to rivets or staples that attach to components of the cytoskeleton

Many epithelial cells must adhere to adjacent membranes to prevent free passage or free movement & to not break apart under stress

CELL COMPONENTS

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CELL COMPONENTS

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CELL COMPONENTS

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Chapter 2 : Cell Membrane

Sub Topic 2.2.3: Gap Junction

© 2010 Cosmopoint

Also known as nexus

At gap junction, the cell membranes of adjacent cells are very close

Transmembrane proteins form specialized structures of hollow cylinder (protein channel) called connexons

Connexons allow transfer of chemical / small molecules (nutrient monomers) between the cells

Common in brain cells, forming the synapse, in many glands & in cells in the heart muscle that coordinate contraction for heartbeat.

CELL COMPONENTS

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Sub Topic 2.2.3: Gap Junction

© 2010 Cosmopoint

CELL COMPONENTS

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Sub Topic 2.3 : Special Structure

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Found in absorptive cells

Eg. Cells lining the small intestines (microvilli), cells in Loops of Henle.

Microvilli are formed as cell extensions from the plasma membrane surface.

Function: to increase the surface area for absorption

CELL COMPONENTS

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Sub Topic 2.3.1 : Microvilli

© 2010 Cosmopoint

MICROVILLI

CELL COMPONENTS

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1.1. Cytoskeleton

= cell inclusionsDefinition: cellular ‘scaffolding’ or ‘skeleton’ contained

within the cytoplasmA dynamic structure that maintains cell shape,

structural integrity & cell & organelle motilityDetermines the 3 dimensional shape of the animal cells

& give a certain firmness in the plant cells3 elements of cytoskeleton:

(a) microfilament(b) microtubule(c) intermediate filament

Topics

CELL COMPONENTS

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1.1.1:Definition, elements of cytoskeleton (microfilament, microtubule, intermediate filament)

Topics

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1.1.1:Definition, elements of cytoskeleton (microfilament, microtubule, intermediate filament)

Topics

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1.2 : Microfilament

Structure

- fine filaments made of protein with a diameter of 7 nm & a length of several µm

- composed of one / two types of protein including actin & myosin

- dynamic can change their length very quickly depending on their locations & functions

- each type of protein forms subunits that are arranged helically

- the subunits can slide over one another causing the microfilament to contract

Topics

CELL COMPONENTS

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1.2.1 Structure, position in cells, functions

Position in cells

- exist in bundles & normally found in layer in cytoplasm

Functions

- cause invagination & evagination of membrane during endocytosis & exocytosis

- cause protrusion of pseudopodium during amoeboid movement in white blood cells

- assist in the cleavage process during cytokinesis of animal cells after nuclear division

Topics

CELL COMPONENTS

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Illustrated in Figure 2 is a fluorescence digital image of an Indian Muntjac deer skin fibroblast cell stained with fluorescent probes targeting the nucleus (blue) and the

actin cytoskeletal network (green).

CELL COMPONENTS

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1.3 Microtubule

Structure

- fine unbranched tubules with diameter of 25 nm, a wall of 5 nm thick & vary in length

- the wall composes 13 rows of globular protein subunits called tubulin, which arranged helically in the wall

- tubulin can grow from a certain organisation centre, which is made of dense protein.

- tubulin can be added at the base or at one end of microtubule causing to increase in length / removed, causing it to decrease in length

CELL COMPONENTS

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1.3.1 Structure, position in cells, functions, centrosome & centriole (structure, position in cells, functions)

CELL COMPONENTS

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Functions

- form the cytoskeleton that determines the shape of the cell

- divide the cytoplasm into compartments specialized enzyme system can be isolated from others to function better

- can contract causing movement in the cilia & flagella

- can pull chromosomes / chromatids during mitosis or meiosis

- cause the movement of organelles including mitochondria, lysosomes & vesicles along them like railway tracks

CELL COMPONENTS

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Centrosome & Centriolemicrotubule organizing centre is an area in the cell where microtubles are produced. Within an animal cell centrosome there is a pair of small

organelles, the centrioles, each made up of a ring of nine groups of microtubules.

There are three fused microtubules in each group.The two centrioles are arranged such that one is

perpendicular (90°) to the other.

CELL COMPONENTS

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During animal cell division, the centrosome divides and the centrioles replicate (make new copies).

The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and

from each centrosome, microtubules grow into a

"spindle" which is responsible

for separating replicated

chromosomes into the

two daughter cells.

CELL COMPONENTS

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1.4 :Intermediate filament

Structure- filament bigger than the microfilament but smaller than the microtubule, diameter between 8 to 12 nm, only found in animal cellsmade up of 4 long stands α-helix coiled fibrous proteins, each consists of only secondary coiled polypeptide- several types; each composes of only one type of protein, including one with keratin- very stable

CELL COMPONENTS

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CELL COMPONENTS

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Position of cells

- branched & formed a network of cytoskeleton in the cytoplasm & nucleus

Functions

- maintain the shape of the cell including nucleus

- distribute the organelles & support them in the cytoplasm

- help some specialised cells to perform their functions.

Eg. The nail producing cells to form the nail & neuron to transmit impulse

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1.5. Cytoskeletal projection

Cilia & flagella are projections form the cell, have the same internal structure but with the different length

They are made up of microtubules

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1.5.1 Definition, example – cilia, flagella, basic structure of cilia & flagella

Flagellum A flagellum (plural: flagella) is a long, slender projection from the cell

body The main differences among these three types of flagella:

(a) Bacterial flagella: helical filaments that rotate like screws. They provide bacterial motility

(b) Archaeal flagella are superficially similar to bacterial flagella, but are different in many details & considered non-homologous

(c) Eukaryotic flagella: those of animal, plant & protist cells are complex cellular projections that lash black & forth

CELL COMPONENTS

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An eukaryotic flagellum is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules

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Cilium

An organelle found in eukaryotic cellsMade up of microtubulesTail-like projections extending approximately 5-10 µm

outwards from the cell bodyTwo types of cilia

(a) motile: constantly beat in a single direction

(b) non-motile: typically serve as sensory organelles

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http://upload.wikimedia.org/wikipedia/commons/f/fd/Eukaryotic_cilium_diagram_en.svg

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The primary purpose of cilia in mammalian cells is to move fluid, mucous or cells over their surface

In humans, ie. Motile cilia are found in the lining of the trachea (windpipe), where they sweep mucus & dirt out of the lungs

In female mammals, the beating of cilia in the Fallopian tubes moves the ovum from the ovary to the uterus

CELL COMPONENTS

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Cytosol

Definition: the liquid medium of the cytoplasm which is viscous & transparent / soluble part of cytoplasm

= ground substanceConsists of approximately 75 – 90% water plus ions &

many types of organic molecules eg. EnzymesCytoplasm minus organelles & insoluble components

e.g mitochondria

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CELL COMPONENTS

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Functions:

- stores vital chemicals including fats

- as the site for certain metabolic pathways (eg. glycolysis, synthesis of fatty acids & amino acids)

- as the medium in which many intracellular chemical reactions occur

- enables organelles (mitochondria, chloroplasts, ribosomes, lysosomes & vacuole) to move about in it.

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CELL COMPONENTS

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Organelles

Organelles in cytoplasm include

(a) mitochodria

(b) ribosomes

(c) chloroplasts

(d) lysosomes

(e) vacuoles

(f) Golgi apparatus

(g) endoplasmic reticulum

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CELL COMPONENTS

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Definition: structures within the cell that are specialized for particular functions

Basic structure

- most organelles have membranes that are very similar to the plasma membrane

- the membrane effectively separate the organelle from the cytosol

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CELL COMPONENTS

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Mitochondria

Basic structure:

- spherical, filamentous or rod-shaped bodies which are bound by a double membrane (outer & inner)

- Size: 2 – 8 µm in length

- The outer membrane is smooth in texture & surrounds the mitochondrion itself

- The inner membrane is arranged in a series of folds known as cristae

- The central cavity of the mitochondria which is enclosed by the inner membrane & the cristae is called the matrix

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CELL COMPONENTS

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Distribution:

- found in every eukaryotic cell

- the location inside the cell in not fix, they can move

- protozoa & yeasts have only one mitochondrion per cell.

- In liver cell 500 – 1400 per cell

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CELL COMPONENTS

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Functions of mitochondria

(a) carry out Krebs cycle part of cellular respiration within their matrise

(b) carry out oxidation of fatty acids & amino acids

(c) carry out oxidative phosphorylation, which produces ATP from ADP & phosphate

principal sites for the generation of cellular energy (ATP) during cellular respiration; ‘power houses’

(d) produce their own proteins from DNA with the aid of RNA

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RibosomesDefinition: small granules where synthesis of proteins occurs

Distribution:

- found in all cells particularly cells that produce a lot of proteins (eg. Glandular cells of the pancreas & liver cells).

- found in the nucleus, cytoplasm freely or attached to ER, mitochondria & chloroplasts.

- their numbers is not fixed

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Basic Structure:

- spheroid in shape, consisting of two subunits, one is larger than the other

- Very small, ~ 20 nm in diameter for 80S (eukaryotic) & smaller for 70S (prokaryotic)

- The subunits can be attached to form bigger functional units in the presence of magnesium ion.

Eukaryote: 60S + 40S 80S

Prokaryote: 50S + 30S 70S

- made up of RNA & protein synthesized in the nucleolus

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Functions:

- provide site for the formation of peptide bonds

- amino acids are joined together to form polypeptide / protein.

- the subunits can form complex with mRNA

- two sites are provided on the surface where tRNA would bring two amino acids to the sites matching the codon of mRNA to that of anti-codon of the tRNA.

- ribosomes can ‘read’ the codes on the mRNA & join specific sequence of amino acids to form specific protein

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CELL COMPONENTS

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Endoplasmic Reticulum (ER)

Basic structure:- formed by a complex system of membranes forming tubes & branching channels through the cytoplasm- can be divided into two types (a) Rough ER – with ribosomal attachments on its outer surface, found in glandular cells that produce a lot of protein for secretion (eg. Digestive system pacrease, stomach) (b) Smooth ER – lacks ribosomal attachments,

embedded on the inner surface of the membrane, there are a lot of enzymes catalysing the

synthesis of CHO & lipids

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Functions:

- differentiated largely based on the presence or absence of attached ribosomes.

- Rough ER

produce proteins for secretion & also for internal use. Eg. Digestive enzyme, hormones & antibody.

transports proteins to smooth ER / Golgi apparatus through sacs pinched off from its surface membrane. Eg. Protein like mucus has its CHO component added in the smooth ER or Golgi apparatus.

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- Smooth ER

synthesis, secrete & store lipids, CHO & other non-protein product

participates in the detoxification process of harmful chemicals

forms lysosomes; vesicles that are used for internal transport & reactions

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Golgi Apparatus

= Golgi body or Golgi complex & found universally in both plant & animal cells

Basic structure:

- consists of flat vesicular discs structures placed one on top of the other, which can produce vesicles full of secretion for internal & external uses

- proteins made within rough ER bud off in vesicles will be transported to the Golgi where the vesicle fuse with the membrane

- the components are then modified & packaged by the time they bud off as vesicle

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Distribution

- found in large no. in glandular cells, neurones * muscle cells.

- locations within the cell are not fix, can move & formed form ER- usually one per cell

Functions

- modifies, packages & distributes all proteins (form glycoprotein) & lipids (form glycolipids) for export & storage

- forms lysosomes through the budding of larger vesicle or fusion of several smaller ones.

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Lysosome

Basic structure:

- spherical membrane bound vesicles containing hydrolytic enzyme that can digest most biological macromolecules

- size: 0.05 to 0.50 µm in length

- the limiting membrane keeps the digestive enzymes separated from the cytoplasm (Eg. Protease, lipase)

- contain digestive hydrolases

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- These enzymes function optimally at pH 5 & inactive at cytosol pH (pH 7.2)

- together with the limiting membrane, this will protects the cell from digesting itself

Distribution

- found in cells that carry out endocytosis (eg. WBC & protozoa)

- found in animal cells, absent in plant cells

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Functions:

- Carry out intracellular digestion (phagocytosis) in which plasma membrane engulf substances & pinch off to form a particle-containing vacuole. Lysosome fuse with the vacuole & digest with their hydrolytic enzyme

- Carry out program cell destruction (autolysis)

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< Electron micrograph of lysosomes

Lysosome in the process

of destroying a membrane

bound mitochondrion >

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1. Vesicles containing materials from outside the cell are taken into the cell

2. The vesicle is pinched off from the plasma membrane & becomes a separate vesicle

3. A lysosome approaches the vesicle

4. The lysosome fuses with the vesicles

5. The enzyme from the lysosome mix with the material in the vesicle & the enzymes digest the material

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Nucleus

Largest membrane-enclosed organelle in the eukaryoric cell, diameter 10 -20 µm

Normally it is spherical or oval in shape; may be cylindrical or lobed in the WBC. The shape can be changed

Distribution: found in the centre of the cell but in matured plant cells, it is pushed one side of the protoplast by the big sap vacuole. Normally one per cell

Topics

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1.1.1: Basic structure & Functions

Topics

It is found in all cells, except in the red blood cells & sieve tubes of phloem

Contains most of the cell’s genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins (eg. histones) to form chromosomes

Function: to control all the activities of the cell by regulating gene expression and to maintain the integrity of these gene

CELL COMPONENTS

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1.2. Nucleus parts or components

Topics

The nucleus can be divided into(a) nuclear envelope(b) nucleoplasm(c) nucleolus(d) chromosome

http://universe-review.ca/I10-04-cellnucleus.jpg

CELL COMPONENTS

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1.2.1 Structure and functions (nuclear envelope, nucleolus, nucleoplasm, chromatin / chromosome, DNA)

Nuclear envelope

Double lipoprotein membrane that encloses the entire organelle & keeps its contents separated from the cellular cytoplasm

Consists of two cellular membranes, an inner & an outer membrane, arranged parallel to one another & separated by 10 – 50 nm

The inner membrane is smooth; no ribosome is attached to it & is not folded. This envelope disappears at prophase of cell division & reappears at the end of it.

Topics

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The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER) & is similarly studded with ribosomes

Sometimes, it may be continuous right to the plasma membrane

The space between the membranes (about 10 – 40 nm) is called the perinuclear space & is continuous with the RER lumen

Topics

CELL COMPONENTS

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Nuclear pores, which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleoporins

The pores are relatively big, 40 -150 nm & covered a surface of 8% of the envelope

Passage of substances is very controlled allows the free passage of small water-soluble molecules while preventing larger molecules (eg. nucleic acids & proteins) from inappropriately entering or existing the nucleus

These large molecules must be actively transported into the nucleus instead.

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Nuclear pore

1. Nuclear envelope.2. Outer ring. 3. Spokes. 4. Basket. 5. Filaments. (Drawing is based on electron microscopy

images)

http://www.answers.com/main/Record2?a=NR&url=http://commons.wikimedia.org/wiki/Image:NuclearPore%20crop.svg.png

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Functions of nuclear envelope It protects the inner structures of the nucleus It separates the nucleus from the cytoplasm / separates the cell’s

genetic material from the surrounding cytoplasm reactions occur in the nucleus are not affected

It controls the shape of nucleus It controls the passage of substances from & to nucleus / serving as

a barrier to prevent macromolecules from diffusing freely between the nucleoplasm & the cytoplasm

CELL COMPONENTS

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Nucleoplasm

Nuclear sap / karyoplasmPart of protoplasm that is inside the nucleus Its composition – same as cytoplasm consisting mainly

water with crystalloids & colloids dissolved in itHas DNA, histone & pentoses that are not found in the

cytoplasmEasily stained with acidic eosin to form purple colour

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Crystalloids: monosaccharides, amino acids, organic acids, nucleotides & mineral ions

Colloids: DNA, RNA & proteins particularly histone that mixed with DNA forming chromatins

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Functions of nucleoplasm

Contains various enzymes for metabolism including that for glycolysis, Krebs cycle, phosphorylation & synthesis of NAD, replication & transcription of DNA

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Chromatin Describes nuclear material that contains the genetic code The code is stored in individual units called ‘chromosomes’ The complex of DNA & protein that makes up chromosomes It is found inside the nuclei of eukaryotic cells, & within the nucleoid

in prokaryotic cells The major proteins involved in chromatin are histone proteins,

although many other chromosomal proteins have prominent roles too

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Changes in chromatin structure are affected mainly by methylation (DNA & proteins) & acetylation (proteins)

The functions of chromatin are to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis & meiosis & to serve as a mechanism to control expression

Chromatin structure is also relevant to DNA replication & DNA repair

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Two types of chromatin can be describes:

(a) Heterochromatin

(b) Euchromatin

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Nucleolus

Spherical structure that is the site of ribosome synthesis in interphase nucleus

It is not surrounded by a membrane & is sometimes called suborganelle

It forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA)

The main roles of the nucleolus are to synthesize rRNA & assemble ribosomes

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The transcription, post-transcriptional processing & assembly of rRNA occurs in the nucleolus

The assembled ribosomal subunits are the largest structures passes through the nuclear pores

Nucleolus disappears during cell division but reappears in the final stage of mitosis

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Chromosome Functions:

- control the production of RNA & proteins in cells. Through these RNA & proteins (enzymes), chromosomes control all the activities of the cell & inheritable characters of an organism

- compact chromosomes enable mitosis & meiosis. Such forms can move easily compared to untidy long slender DNA enable genes to be passed down from one mother cell to daughter cells & one generation to the next generation

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DNA

A nucleic acid that contains the genetic instructions used in the development &

functioning of all known living organismsMain role: long term storage of informationLong polymere of simple units called

nucleotides, with a backbone made of

sugars & phosphate groups joined by

ester bondsAttached to each sugar is one of four types

of molecules calles bases

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It is the sequence of these four bases along the backbone that encodes information

This information is read using the genetic code, which specifies the sequence of the amino acids within proteins

The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription

Within cells, DNA is organized into structures called chromosomes

CELL COMPONENTS

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chapt 02

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