Cell Structure - Topic A2.2 - Ib Biology HL

light microscopes feature ( advantages and disadvantages

• cheap

• images have colour

• can examine live cells

• up to 2000x magnification

• for specimens over 200mn

• easy to carry

• if structure is smaller than the wavelength of the light, it will not be visible (mitochondria, RER, ribosomes…)

• simple sample preparation

Electron microscopes advantages and disadvantages

• can magnify until x500 000

• for specimens over 0.5mn

• very expensive

• very large and unpractical

• specimen needs to be dead

• complex sample preparation

• needs a vaccum to function

two types of electron microscopes

• scanning electron microscope

• transmission electron microscope

transmission electron microscope

• older

• focuses a beam of electrons thought specimen: denser parts of specimen absorb more electrons, so appear darker

• only works with very thin specimen

• complex sample preparation can introduce artefacts into specimen, leading to faulty conclusions

scanning electron microscope

• produces a 3D image

• external structure and thick speciments can be observed

• electrons bounce of the surface of the specimen

• however give lower resolutions than TEM

new technologies in electron microscopes

• cryogenic electron microscopy

• freeze capture

cryogenic electron microscopy

for proteins/ other biomolecules. flash freezing the protein so they retain their shape, and then allows for a 3D representation of proteins after going in SEM

freeze capture

freeze very quickly a specimen, for the structure to be maintained, and then fractured in a vacuum for planar view of organelles

methods for studying living samples

• fluorescent stains

• immunofluorescence

both with light microscope

fluorescent stains

•Florescent dyes absorb light at 1 wavelength and emit it at another, longer wavelength (e.g. some absorb uv and re-emit as blue light)

Fluorescent microscopy uses a much higher intensity light to illuminate the sample, which then excites flourescently stained specimen. This emits light at a longer wavelength.

immunofluorescence

dyes coupled withs specific antibody molecules, so they bind to certain structures and make them more easily recognisable

How to prepare a sample type question

• put cells on slide in a layer no larger than one cell thick (haha)

• add a drop of stain/ water

• put cover slip on gently

• avoid trapping air bubbles

• remove excess water using a paper towel

how to view a sample type question

• place slide on microscope stage

• focus using the lowest power objective lens

• do that using the larger, coarse focusing knob

• use fine focusing knob to focus on specific parts

• increase magnifications using a higher power obejctive lens

• then use only the fine focusing knob

main points of cell theory

• cells are the building blocks of life

• cells are the smallest units of life

• all cells derive from other cells

differences between prokaryotes and eukaryotes

• no membrane bound organelles so no nucleus

• ribosomes in cytoplasm in prokaryotes

• ribosomes in prokaryotes are smaller (70s) while in Eukaryotes they are larger (80s)

• prokaryotic cells between 0.1 and 5 μm, Eukariotic between 10 and 100 μm

• prokariotic cells have both DNA plasmids, like cicles around the cytoplams, and naked DNA in the nucloid. Eukariotes haves DNA exclusively in the nucleus

• prokariotes have no mitochondria

• overall, Eukaryotes have compartmentalisation thanks to their use of membrane bound organelles

advantages of compartmentalisation

• enzymes and substrates for certain metabolic reactions can be localised to have a higher concentration inside a cell

• the ability to separate toxins and potentially damaging substances from the rest of the cell. For example,

  hydrolytic enzymes can be stored in structures called lysosomes, away from the cell cytoplasm

• control over conditions inside organelles (such as pH) to maintain the optimal conditions for the enzymes that function in those parts of the cell

parts of an eukaryotic cell (write)

• nucleus

• mitochondria

• rough endoplasmic reticulum

• smooth endoplasmic reticulum

• plasma membrane

• cytoplasm

• 80s ribosomes

• vesicles

• golgi body

• vacuoles

• cytoskeleton

chromatin

uncondensed genetic material inside of a nucleus

plasma membrane

the plasma membrane separates the cell’s interior from its external environment and controls what can enter and exit the cell.

Has a bilayer

Has a really thin structure 7nm

cytoplasm

a water-based jelly-like fluid that fills the cell, suspends ions, organic molecules, organelles and ribosomes, and is the site of metabolic reactions.

mitochondrion ( singular)

double-membrane-bound organelles that convert glucose into ATP (the cell’s energy currency) in the process of aerobic respiration.

has double membrane. many cells can have a lot of mitochondria if they require a lot of energy

80s ribosomes

where translation (protein synthesis) occurs.

Both attached to endoplasmic reticulum and free-floating eukaryotic ribosomes are larger and have a higher mass than prokaryotic ribosomes.

some are also found in mitochondria and in chloroplast, but they are 70S.

they produce intracellular proteins

role of ribosomes

• perform translation

• protein synthesis

Nucleus

contains the DNA which is associated with histone proteins and is organised into chromosomes.

The nucleus contains the nucleolus, which is involved in the production of ribosomes. The nucleus has a double membrane which contains pores through which certain molecules can pass, including glucose, RNA and ions.

has a nuclear enveloppe for comparentalisation, nuclear pores for messenger RNA to pass

mitochondria role

• site of aerobic respiration

• to make atp/ release energy

nucleolus

suborganelle: inside of nucleus, no membrane

produces ribosomes

Smooth endoplasmic reticulum

produces and stores lipids, including steroids. doesn’t have ribosomes attached to it

rough endoplasmic reticulum

has ribosomes attached to its surface which produce proteins that are usually destined for use outside the cell. have ribosomes attached to it.

vesicles transport the proteins to the golgi apparatus

produces proteins for extracellular uses

vesicles

small sac that transports and releases substances produced within the cell by fusing with the cell membrane. Transports proteins from endoplasmic reticulum to golgi apparatus

golgi body

folds proteins into usable shapes, closer to plasma membranes, mostly for extracellular uses ( enzymes…)

vacuole

• helps to maintain the osmotic balance of the cell. It may also be used to store substances and sometimes has hydrolytic functions similar to lysosomes.

• single membrane

• used to store nutrients in plant cells ( carbohydrates, proteins…)

cytoskeleton

a system of protein fibres called microtubules and microfilaments. The cytoskeleton helps to hold organelles in place and maintain the structure and shape of the cell.

has 3 types

• microtubules

• microfilaments

• intermediate filaments

microtubules

thickest of all skeletal fibers

are the core of cillia and flagellan

can be dismantled and reassembled quickly

microfilaments

thinest of all skeletal fibers

resist tension effectively

are important in muscle contraction

in heart cells, important in cytoplasm streaming ( distribution of chemical substances)

Fibriae

•Hairlike structures that are shorter, straighter and thinner than flagella

•More numerous than pilli

-used for attachment to surfaces/other cells (not movement)

only in prokariotic

capsule

•Layer of viscous, gelatinous polysaccharides which protects the bacterial cell (Glycocalyx).

•If firmly attached – capsule.

-If loosely attached – slime layer

•Serves as a barrier against phagocytosis (pathogenic bacteria)

•Capsules may contribute to virulence in pathogenic species

only in bacteria

lysosomes

• formed in golgi apparatus

• contains concentration of digestive enzyes

• break down excess/ worn out parts

centrioles

• hollow cilinder form

• are central in mitosis

• before replication, they duplicate and grow spindle fibers. they are responsible for arranging chromosomes appropriately and separating them

glycoproteins

proteins on outside of cell which allows cells to recognise each other

chloroplast

• one of larger organelles

• contains chlorophyll, does photosynthesis, produce glucose

nucleoid

•Central region of the cytoplasm containing naked (not wrapped around a protein), single chromosomal DNA

•DNA in prokaryotes is circular

•Not surrounded by a membrane

plasmid

•Small, circular double strand of DNA

•Copy number and length/size of plasmids can vary greatly inside the cell

types of cells

• eukariotic

• prokariotic

• archaea

prokariotic cell example

•Example: Staphylococcos sp. And Bacillus sp.

•Gram-positive bacteria

•Spherical and rodshape

•0.5 – 6 µm

•Often cause throat or skin infections

fungal cells

• eukariotic

• saprotrophic

• largest organisms are funghi

difference between funghi, animal and plant cells

plant cells don’t have centrioles

only animal cells have lymosomes and cillia

all eukaryotes can have vacuoles

endosymbiosis

The theory of endosymbiosis posits that eukaryotic organisms evolved when this common ancestor endocytosed a prokaryotic cell capable of generating energy from oxygen

evidence for endosymbiosis

• mitochondria measure around 8 μm in length, the same size as many prokaryotic organisms

• have double membranes. It is thought that the inner membrane was formed from plasma membrane of the endocytosed prokaryotic cell, and the outer membrane is thought to have formed from the vesicle in which the cell was taken up into the ancestor of eukaryotic organisms

• have circular naked DNA, as is found in prokaryotes

• mitochondria and chloroplast do binary fission, which is what bacteria do

• have 70S ribosomes, the same size as the ribosomes in prokaryotes, rather than the 80S ribosomes in eukaryotic cells

• divide by binary fission like prokaryotic cells, unlike eukaryotic cells, which divide by mitosis

• are susceptible to some antibiotics, compounds that target prokaryotic structures and metabolic processes.

cell differenciation

• what happens for cells to become specialised

• happens by the expression of specific genes in by proteins called growth factors in embryo

• or in changes of environment of cell

evolution of multicellular organims

• occured by cell aggregation to have an efficient sharing of nutrients and protection from predators

cells with atypical structures

• skeletal muscle

• fungal hyphae

• phloem selve tube

• red blood cells

striated muscle cells

• very large

• fuse together to form long fibers

• act as a group

aseptate fungal hyphae

• aseptate: used to be separate cells separated by septa, but eventually they fused together, essentially appearing as one cell and being multinucleate

phloem sieve tube

• metabolic function are regulated by neighbouring companion cells in plants ( no nucleus )

• During development of the cell the nucleus and other cell organelles break down. Interconnected by plasmodesmata

• To allow for little resistance between adjacent cells the neighboring walls are perforated with pores.

  

red blood cells

• During development in the bone marrow the nucleus is pinched off and digested by cells of the immune system.

• This makes the cell smaller and more flexible, but it cannot renew itself and has a limited life span (ca. 120 days)

• Enables them to contain more haemoglobin + biconcave shape

Plasmodesma

pore in the plasma membrane of plant cell; phloem cells have a lot of them