3A cell structure and organisation

cell theory

all living organisms are made of cells, sharing

• Cell surface membrane

• Cytoplasm

• DNA

• Ribosomes

organelle

a component in a cell that carries out a specific task

cells

basic functional and structural units in a living organism

tissues

group of similar cells of similar structure working together to perform a particular function

organ

made from a group of different tissues working together to perform a particular function

organ system

made from a group of organs w related functions, working together to perform body functions within organism

Eukaryotic cells diameter

10-100 μm

Prokaryotic cells diameter

0.1-5 μm

differences between eukaryotic and prokaryotic cells

• Eukaryotic cells have a more complex ultrastructure

• Eukaryotic cells are larger

• eukaryotic cells is divided up into membrane-bound compartments

Animal and plant cells are both types of eukaryotic cells that share key structures such as

• Membrane-bound organelles, including a nucleus

• Larger ribosomes known as 80S ribosomes

Key differences between animal and plant cells include

• Animal cells have centrioles and some have microvilli

• Plant cells have a cellulose cell wall, large permanent vacuoles, and chloroplasts

labelled animal cell

labelled plant cell

nucleus

• relatively large

• separated from the cytoplasm by a double membrane called the nuclear envelope

• contains nuclear pores - important channels for allowing mRNA/ribosomes/enzymes to travel out of the nucleus

• chromatin - the material from which chromosomes are made (Chromosomes are made of sections of linear DNA)

• nucleolus - makes ribosomes

ribosomes

• They aren’t surrounded by a membrane

• It’s made up of ribosomal RNA (rRNA) and proteins

• 80s ribosomes are found in eukaryotic cells

• 70s ribosomes are found in prokaryotes, mitochondria, and chloroplasts

• the site of translation

Rough Endoplasmic Reticulum (RER)

• RER is formed from folds of membrane continuous with the nuclear envelope

• surface is covered in ribosomes

• to process proteins made on the ribosomes

Smooth Endoplasmic Reticulum (SER)

• also formed from folds of membrane

• involved in the production, processing and storage of lipids, carbohydrates and steroids

• doesn’t have ribosomes on its surface 

Mitochondria

• The site of aerobic respiration within eukaryotic cells - visible with a light microscope

• surrounded by a double-membrane with the inner membrane folded to form structures called cristae

• matrix contains enzymes needed for aerobic respiration producing ATP - also contain DNA and ribosomes

Centrioles

• made of hollow fibres - microtubules

• used to move substances around inside a cell and to support the shape of a cell

• arranged at right angles

• involved in the separation of chromosomes during cell division

Lysosomes

• forms of vesicle which contain hydrolytic enzymes

• break down waste materials such as worn-out organelles

Golgi apparatus

• consists of flattened sacs of membrane

• modifies proteins and lipids before packaging them into Golgi vesicles

• The vesicles then transport the proteins and lipids to their required destination

organelles involved in protein production and transport

• proteins first go to the Golgi apparatus from the RER

• the Golgi apparatus modifies the protein

• then packages it into a secretory vesicle

• the secretory vesicle then goes to the cell membrane

• which then fuses w the membrane

• releases the proteins through exotyocisis

free ribosomes

found within the cytoplasm make proteins that stay within the cytoplasm

Proteins that go through the Golgi apparatus are usually

• Exported, e.g. extracellular enzymes

• Put into lysosomes, e.g. hydrolytic enzymes

• Delivered to other membrane-bound organelles

differences b/w prokaryotic and eukaryotic

• A cytoplasm that lacks membrane-bound organelles

• Ribosomes that are smaller (70 S)

• No nucleus, instead having a single circular bacterial chromosome that is free in the cytoplasm and is not associated with proteins

• A cell wall that contains the glycoprotein murein 

• Loops of DNA known as plasmids

• Capsules

• Flagella

• Pili

• A cell membrane that contains folds known as mesosomes

plasmid

• small loops of DNA

• contain genes that can be passed b/w prokaryotes

• not present in all prokaryotes

capsule

• surrounds prokaryote

• helps to protect from bacteria from drying out and from attack by cells of the immune system of the host organism

• not present in all

flagellum

• long hair like structure that rotates

• allows mobility

• sometimes theres more than one

• not present in all

pilli

• thread like structures on the surface of some bacteria

• enables bacteria to attack to other cells or surfaces

mesosomes

infoldinfgs of the inner membrane which contains enzymes required for respiration

ciruclar DNA

• the genetic material of prokaryotic cells consists of a single circular strand of DNA

• the area where this molecule is found is known as nucleoid

ribosomes

• 70S ribosomes which are smaller

• the site of protein synthesis

Transmission Electron Microscopes 

• use electromagnets to focus a beam of electrons

• which is transmitted through a thin specimen

• Denser parts of the specimen absorb more electrons; which appear darker on the image

• The internal structures can be seen as a 2D image

• very high resolution

Scanning Electron Microscopes

• scan a beam of electrons across a specimen

• This beam bounces off the surface of the specimen and the electrons are detected, forming an image

• can produce 3D images that show the surface of specimens

• specimen viewed does not have to be thin

• lower resolution than TEMs

magnification

how many times bigger the image of a specimen observed is in comparison to the actual size of the specimen

how to calculate the total magnification in light microscope

total magnification = eyepiece lens magnification (10) x objective lens magnification

Resolution

The ability to distinguish between two separate points

Relationship bw magnification and resolution

The resolution of a microscope limits the magnification that it is capable of

why do electron microscopes have a better resolution than light microscopes

• electrons have a much smaller wavelength than visible light

• The resolution of a light microscope is limited by the wavelength of light; the wavelength of light is too long to allow for high resolution

comparing light and electron microscopes

• vacuum is needed for EM while its not needed for LM

• EM has a magnification over x500000 and LM has up to x2000

• EM has a resolution of 0.5nm while LM has 200nm

• EM observes dead specimens while LM observes dead or living

why are specimens stained in microscopy

as the cytoplasm and other cell structures may be transparent or difficult to distinguish

light microscope stains

most of the colours seen in images taken using a light microscope are due to added stains. Except for chloroplasts which show up as their natural colour (green)