cells

A2.2

Cells as basic structure for all organisms

Cells as basic structure for all organisms

while some organisms are single celled others are multicellular.

Multicellular organisms have many different types of cells,each with a specific function

Cell theory

• All living organisms are composed of one or more cells.

• A cell is the basic structural and functional unit of living organisms.

• All cells arise from pre-existing cells.

Suggested by theodore schwan and matthais jakob

and was developed by robert hooke from the 17th century

cells only arise from pre exisiting cells

Cells multiply through divison

mitosis results in genetically identical diploid daughter cells

and meosis leads to haploid gametes

Microscopy skills

Light microscope

Allowed discovery of bacteria and unicellular organisms, chromosomes and mitosis, mesois and gamete formation were socidoved

more than 400X magnification, difficult to see a focused image with light microscope, dots appear closer together, and eventually itll because impossible to see them seprately.

There is a limit to which a light microscope can see as there are distortions caused by the wavelength of light.

Produce image in color

Electron microscope

Uses beams of electrons instead of light, since their wavelength is much shorter than wavelength of visible light. Give magnifications up to 1,000,000X.

Electron miscroscopes have better resolution thus they have better magnificantion.

Allowed scientites to investigate detailed strucutre of cells

However they only give balck and white images, so color needs to be added artifically, and this method always kills the cells, however regardeless the cell would die inside an electron microscope becuase of beams of electrons and vaccum is destructive.

fluorescent stains and immunofluorescence

the staind which are used in microscopy bind to some chemicals but not others. Methylene blune binds with RNA and DNA, so it sairs the nucleus dark blue and cytoplasn light blue.

These stains are used since most chemicals in cells are white or colorless

fluoroscense is when substances absorb light and re emit it at a longer wavelength. some of these stains absorbs ultraviolet light and re emit it as blue light.

Special microscopes have been desinged, and these are built with intense light source such as LED and lasers which emit a single wavelength. This ligth is absorbed and re emitted thus creating bright images

Immunofluorescence is a developed of fluorscent staining, which bind to particular chemicals (antigens). The fluorescnet marks of different color link to the antibodies, thus allowing a multi coloured imaging and showing exactly where chemicals are location.

Freeze fracture electron microscopy

Produce images of surface within cells.

Using liquified propane at -190 degree celsius the sample is frozen instantly, and then a blade is used to fracture the same. The fracture goes through weakest points of the cells.

Some ice at fractured area is removed by vaporization to enhance texture of surface, this is called etching. vapour of platinum or carbon is fired on to the fractured surface at an angle to form a coating, allowing a replica of the fractured surface to be formed.

replica is then remove from frozn sample and is examined using eletron microsciped. generally about 2nanometers thick.

Impression of 3D imaging through shadowing.

*weakest point in cells is in the middle of membranes between two layers of phospholipid, the freeze fracture process gives a unique image of parts of cells

cryogenic electron microscopy

cryo-EM

• used to research about structure of protiens

• Thin layer of pure protien is applied to the grid, where the solution is flash frozen creating viterous ice and prevents the formation of water crystals. Liquid ethane just above its melting point of -182.6 degree celsius is used as a coolant.

• The frozen grid is put under an electron microscope and dectectors record pattern of electron transmitted by protien molecules.

• Cryo-EM analyses protiens at instant in time when water around them frose, allows scientites to research protiends which change from one form to another

• noow given resolution of 0.12nanometers, allowing individual atoms in protiends to create imaging

processes of life in unicellular organisms

• homeostasis: maintenance of constant internal environment in an organism

• metabolism all biochemical reaction that occur in an living organism

• Nutrition —> supplying nutrients needed for energy. growth and repair

• excretion- removal of waste products of metabolism from an organism

• growth - increase in size or no of cells

• response to stimuli- perception of stimuli and carrying out appropriate actions in response

• reproduction - production of offspring sexually or asexually.

common parts in cells of all organisms

• plasma membrane

• cytoplasm

• DNA

Plasma membrane

• It is the outer boundary of cell, controls entry and exit of substances, even if the the concentration outside is low.

• Preventing unwanted and toxic substances to enter the cell

• allows cell to maintain concentration of substances that are very different from those in surrounding environment.

• ability of liquids and gases to pass is based on structure of lipids

• burtsing of plama mebrane is known as lysis and is caused due to excess pressure, or at tiems it happens by itself called autolysis.

  • lysis always leads to death of the cells

Cytoplasm

• water is a nescessary component of cytoplasm and many substance are dissolved in this water.

• In the cytoplasm, several enzymes catalyse different chemical reactions —> metabolism

• metabolism mainly occurs in cytoplasm which provides cell with energy and produces all protiends which make up the structure of a cell. protiens are easily damaged, even when cell is not growing, the cytoplasm constantly breaks downs and replaces its protiens.

DNA

genes are made of dna, which carry information needs for cell to perform all its functions.

Genes hold information for making a protiens

DNA can be copied and pasted on to daughter cells, plants and animal cells have nucleus that constains all their DNA

If an organism doesn’t have a nucleus then the DNA is store in the cytoplasm itself, thus while the dna as a store for genetic information is common for all cells, its location in cells is not the same.

Cytosis

it is the movement of cells between parts of organism

endocytosis: substances enter the cell from outside and fuse with its plasma membrane

exocytosis: causes release of substances fro membrane, and these substances exit the cell.

Prokaryotes

They are single cells organisms which don’t have any membrane bound organelles

Do not have a nucleus

structure:

• nucleiod

• genophore

• plasmids

• ribosomes

• cell wall —> glycocalyx

• Pili

• flagella

  • cell membrane/plasma membrane

Binary fission

Prokaryotes reproduce asexually through binary fission, in this DNA is replicated, and two dna loops attach to the membrane, the membrane elongates and pinches off cytokinesis forming two seperate daughter cells which are gentically identical

Nucleiod region

performs similar function to that of an actual nucleus, however it is not membrane bound and it is a region, where all the genetic information is stored. distinct cytoplasmic structure which have bacterial chromosomes

There are single stranded DNA’s found here called genophore

Plasmid

Additional dna molecule exchanged through bacterial conjucation. these are nescessary to transfer information from one cell to another

They are small rings of DNA in cytoplasm which can be replicated individually

Ribosomes

mainly responsible for protien synthesis

Also responsible or elongation, termination and cytoplasmic organelle.

Prokaryotes have 70S ribosomes which have 50S large and 30 S small subunits

Ribisomes are the cytoplasmic organelle in prokaryotes

70S ribosomes are present in mitochondria as well as chloroplast

cell wall

Present outside the plasma membrane, which is thicker and stronger than membrane, it protects the cell and helps maintain its shpae and prevents the membrane from bursting. it contains peptidoglycan

Cell wall also has an additional covering called glycocalyx

Pili

hair like structures outside the cell wall, two types

Sex pili responsible for DNA exchange from one cell to another and attachment pili which allows cells to attach to surfaces

Flagella and cilia

responsible for locomotion of the cell, and cilia allows cells to move through water.

Cell membrane/plasma membrane

allows movement of substances in and out of the cell. protects cell from external environment and transmits cellular signals

Eukaryotes

All Eukaryotes have a nucleus however they are mainly divided into two, which each have sub parts

uni cellular and multicellular

only protists are uni celular

multicellular is further divided into with and without cell, animal cells dont have a cell wall, and with cell wall includes fungi and plantae

The cytoplasm of eukaryotes is compartmentalized, areas are seprated from rest of cytoplasm by single or double membrane

They are compartmentalized for

effciency of metabolism: enzymes and substrates can be localised and are more concentrated

pH and other factors can be kept optimal, optimal level of pH in one part of cell is easier

the toxic substances can be isolated eg, digestive enzymes(digest the cell itself) are stored in lysosomes

number and location of organelles can be changed depending on requirement

three main differences to prokaryotes include

• nucleus

• 80S ribosomes

• mitochondria

Nucleus

Compartment holds the cell’s chromosomes, has a double membrane with pores, each chromosome one long DNA molecule has protiens attached, except when a cell is preparing for mitosis

Protiens in nucleus are arranged in gobular groups with DNA wound around the outside

Double membrane have pores

uncoiled chromosomes are called chromatin, dark color and concerated at the edges of nucleus

RNA is transcribed in the nucleus, before protien synthesis in cytoplasm occurs

RNA leaves nucleus through the pores, however DNA is too large to move through the pores.

mitochondria

• double membrane

• smooth outer membrane and folded inner membrane, and these folds are called cristae

• Site of ATP production by aerobic respiration (if fat is source of energy then it is digested in the mitochondria)

• When folded inwards, the interior is called matrix, large number of enzyme reactions take place

• powerhouse of the cell

  • muscle cells have a greater number of mitchondria becuase htey require large amounts of energy

Ribosomes

80 S ribosomes

40S subunit has 18S RNA and 33 protiens

60S subunit has 25S RNA and 47 ribosomal protiens

No membrane

Dark granules in cytoplasm

synthesizes protiens to function in the cytoplasm for use within the cell

Rough endoplasmic reticulum

have flattened membrane sacs called cisternae, located near the nucleus

ribosmes are attached to the outside of cisternae

Synthesized protiends which ar transported by vesicles to golgi appratus for modification before secretion outside the cell

Smooth endoplasmic reticulum

Branched network of tubular membranes

smooth as no ribosomes are attached to it

has variety of functions

used to synthesize lipids, phospohlipids and steroids

golgi apparatus

• also has cisternae

• doesnt have ribosomes

• close to plasma membrane

• the cisternae are shorter and more curved than the ones in RER

• it processes protiends from the rER, and are then repackeged in vesicldd for secretion outside the cell

Vesicles

Single membrane with fluid inside

very small in size

used to transport material inside of a cell

Lysosomes

Spherical with single membrane

golgi vesicles

digestive ensymes for breakdown of indigested food, damaged organelles and the cell itself

High concetration of enzyme cause this organelle to stain a lot thus very dark on micrographs

Vacuoles

Single membrane with fluid inside

Vaculoes are large and permanent, occupy majority of cell volume

However in animal cells, vacuoles are smaller and temporary and are used for various reason, eg to absorb food and digest it

Flagellum

Thin projection from cell

contain microtubles

used to move the cell

found in animal cells only

Cilia

also found only in animal cells

used to mose the cell or the fluids around the cell

Microtubles

• small cylindrical fibres

  • variety of functions, part of the structure of flagella and role inn cell division

Centrioles

Two groups od nine triple microtubules

found in animal cells not in vascular plants or fungi

Chloroplast —> only in plants

Most plant cells contain cholroplast

double membraned

contain stacks of thylakoids, which have a flattened membrnae

generally in an ovoid shape

site of photosynthesis and glucose is produced here

starch grains present if photosynthisis is happening quikly

Cell wall

Extracellular componenet secreted by all plant cells

consisted of cellulose which is permeable thus does not affect transport in and out of the cell

Strong, gives support to the cell and prevents plasma membrane bursting when under pressure

hard to digest, can’t be broken down easily thus lasting a long time without maintence or replacement

Atypical cell structure in eukaryotes

Red blood cells

pholem sieve tube elements

skeletal muscles

asepatate fungal hyphae

Red blood cells

RBC’s in mammals don’t have a nucleus

In the development in their bone marrow, nucelus moved to the edge of cytoplasm and small part of cell which has it is pinched off and destroyed by phagocyte.

RBC’s are flexible and smaller, but they can’t repair themselves once they are damaged, thus their life span if of 100 to 120 days

Pholem sieve tube elements

don’t have a nucleus

sap of plants moves through tubular vessels made from columns of cylindrical cells. In xylems vessels, watery sap is conducted from roots to leaves, diving walls b/w adjacent cells are removed nd plasma membrane and all cell contents break down. Creates hollow tubes which doesn’t consist of cells.

Wheras in pholem, sugary sap from leaves to other parts is conductio in in vessels which are called sieve tubes. during their develipment, nucleus and other cell contents are broken down but plasma membrane remains essential fo rpholem transport

subunits of sieve tubes are called elements and not cells, and the sieve tubes are connected to companion cells which have a nucleus and mithondria. companion cells help sieve tubes to survive and carry out their function

Skeletal muscle

Large multinucleated structure when groups of cells fuse together, this structure is syncytium.

columns of cells each with a nucleus are formed by cell divisions and these cells fuse together to form long muscle fibers

aseptate fungal hyphae

In growing cells, nucleus divides repeatdely without any cell division, resulting in unusually large multinucleate structure called coenocyte. thread like hyphae of fungi develop in this way

walls divid the hyphae of other type sof fungi into uninucleate cells are called speta, thus hyphae without these disivion are aseptate.

coenocyte is when a nucleus divides repeatdly causing unusually lrge multinuclear to be formed

Gram positive bacteria

they have peptidoglyglycan and no outer lipid membrane, positive because they cause a positive results

hold dye —> purplish and violet

Gram negative bacteria

layer of peptidogylcan is thinner and there is an outer membrane

colorless or pink or red

harmful