Biology - cell structure

Use of microscopy 

allow us to view structures too small for the naked eye 

Max resolution and magnification of Light microscopy

200nm

x1500

Max resolution and magnification of TEM

0.1 nm

x 1,000,000

Max resolution and magnification of SEM

1-10 nm

x500,000

Describe light microscopy

light is used to illuminate the sample. Depending on the particular type of microscope, the image can be produced by light that is transmitted through the sample, or by light that is reflected (or fluoresced) by the sample, or by a combination of these.

Describe TEM microscopy

a beam of electrons is fired at the sample. The image is produced by electrons that are transmitted through the sample

Describe SEM microscopy

a beam of electrons is fired at the sample. The image is produced by electrons that are emitted by the sample

What are the characteristics of TEM

Better resolution + higher magnification than light microscopy 

Sample must be dead and enclosed in vacuum

Sample must be ultra-thin

No colour information

Provides 2D image of the cell’s ultrastructure

Meaning of ultrastructure

the internal details of cell organelles

What are characteristics of SEM

Better resolution + magnification than light microscopy

Sample must be dead and enclosed in vacuum

Each image shows 3D structure of sample 

What are characteristics of light microscopy

Can be used to image live or dead/fixed sample

Natural colour can be observed

Sample preparation is simpler than electron microscopy 

Define resolution

the minimum distance apart that two objects can be in order for them to be seen as distinct objects

Define magnification 

the amount of times bigger an image is, compared to the object

Linear magnification 

the size increase is in both length and width

Why does electron microscopy provide better resolution than light microscopy?

Electrons have a much shorter wavelength than visible light

Means that electrons transmitted through/emitted by a sample can be very close to each other without interfering with each other

Order of quality (sem, tem, light)

Light<SEM<TEM

Steps for preparing a slide: stage of biopsy

1. Fixation = add a chemical that instantly stops all chemical reactions within the cells

2. Dehydration = remove all water from the cells

3. Embedding = to coat the sample with wax

4. Sectioning = to produce very thin cuts, using microtome machine

5. Differential staining = add a stain that binds to some structures and not others

Why do we fixate the cells

In order to stop decomposition - we want the slides to look like what they originally were when alive

How do we dehydrate?

we CANT evaporate off since the cells will denature

we add alcohol at increasing concentrations to water to diffuse

Why do we dehydrate

in order to prevent decomposition

Why do we embed the sample

It makes it easier to section the sample into ultra-thin pieces 

What are the steps of preparation for SEM sample

Fixation

Dehydrate

Embed 

Cover in thin layer of gold 

Why is the sample covered in a thin layer of gold

To ‘reflect’ the electrons, so that they can be detected and create an image

What are the steps of preparation for TEM sample

Fixation

Dehydrate

Embed

Section => ultra thin so the electrons are able to transmit through the cell

Add stain => heavy metals e.g. uranium

TEM diagram

Property of water

A universal solvent, cells need to it survive

Scale on micrometer

1mm = 100 divisions

1 division = 10 micrometers

Definition of chemical stain

coloured chemicals that bind to molecules in or on the specimen which make the specimen easy to see 

What is differential staining

Stains that bind to specific cell structures

5 different types of stain and their use

• Methylene blue – stains DNA and RNA

• Iodine solution – stains starch blue-black

• Eosin – stains cytoplasm pink

• Haematoxylin – stains nuclei purple-blue

• Acetic orcein - binds to DNA, red

Equation for magnification

Magnification = image/real

Rules of drawing diagrams

½ the page in size 

Simple

Title 

Draw rule line

Labels + annotations 

What is in the ultrastructure of eukaryotic cells (14)

Nucleus

Nucleolus

Nuclear envelope 

RER

SER

Golgi apparatus

Ribosomes

Mitochondria

Lysosomes

Chloroplasts 

Plasma membrane

Centrioles

Cell wall

Flagella and cilia 

What is the nuclear envelope and it’s function

the double membrane that surrounds the nucleus 

Has pores to allow mRNA to exit 

Nucleus STRUCTURE

surrounded by nuclear envelope

contains DNA wrapped in histone proteins

What is the chromatin 

DNA + histone proteins 

Nuclear FUNCTION

contains DNA; site of transcription 

Nucleolus STRUCTURE

found in nucleus 

very dense region = ‘dark’

not membrane-bound

Nucleolus FUNCTION

Site of ribosome production

transcription of DNA (genes) to produce RNA

What are cisternae

the collection of membranous sacs

Rough endoplasmic reticulum STRUCTURE

collection of membranous sacs - cisternae

continuous with nuclear membrane

membrane covered in ribosomes

RER FUNCTION

if mRNA is translated at a ribosome bound to RER, the new protein passes into the RER lumen (inside it)

Smooth endoplasmic reticulum STRUCTURE

continuous with RER

contain cisternae

SER FUNCTION

synthesise lipids and steroids (e.g. cholesterol)

SER is very large in cells which make lots of lipids → ovary/testes cells that need steroid hormones 

NOT TO DO WITH PROTEIN SYNTHESIS 

Golgi apparatus STRUCTURE

stack of cisternae (stack of pancakes)

inside of cisternae = ‘lumen’

Golgi apparatus FUNCTION

modifies, packages and transports proteins

Ribosomes STRUCTURE 

made of ribosomal DNA and protein

not membrane-bound

70s = prokaryotes

80s = eukaryotes

Ribosomes FUNCTION

Translation = protein synthesis 

Mitochondrion STRUCTURE

contains their own DNA

bacterial-sized ribosomes (70s) - smaller in cytoplasm

same size as bacterium

double-membrane bound organelle

inner membrane highly feld into ‘cristae’

fluid inside called ‘matrix’

How to mitochondria and chloroplast divide

through binary fission

Mitochondria FUNCTION

site of aerobic respiration - contains enzymes that can release energy from food using oxygen 

Lysosome STRUCTURE

Membrane-bound organelle

Contain digestive/hydrolytic enzymes

Lysosome FUNCTION

Used when cells need to break down large structures

Example of 'compartmentalisation' - a benefit to keeping these enzymes out of the cytoplasm, as otherwise they would break down all cellular organelles

Example of lysosome

e.g.

• White blood cells (e.g. neutrophils/phagocytes), after engulfing a pathogen will fuse their lysosomes with the pathogen to digest it

• Use lysosomes to break down/recycle old organelles

Chloroplast STRUCTURE

contain their own DNA

bacterial-sized ribosomes (70s) - smaller in cytoplasm

same size as bacterium

double-membrane bound organelle

third set of interna membranes - ‘thykaloid membrane system’ - highly folded 

thykaloids can be stacked into grana, or some connect different grana (lamellae)

Chloroplast FUNCTION

Site of photosynthesis

Plasma membrane STRUCTURE

cell surface membrane

‘fluid mosaic model’ - lipid bilayers 

Plasma membrane FUNCTION

NOT to do with strength

Control what enters and what exits the cell

Provide a surface for receptor proteins (e.g. for hormones)

Centrioles STRUCTURE 

Made of microtubules

Centrioles FUNCTION

Produce 'spindle' in mitosis by creating more microtubules

At the base of cilia/undulipodia - make the microtubules

Cellulose cell wall - STRUCTURE

Plants cells - have a cell wall made of cellulose

Fungal cells - cell wall made of chitin

Bacteria - cell wall made of peptidoglycan

Cell wall FUNCTION

Strength/support to the cell

Hold a cell's shape - lets them store large volumes of water inside, creating a pressure ('turgor pressure)

(Fully permeable)

Undulipodium STRUCTURE + FUNCTION

• Sperm cell have an undulipodium ('tail' for eukaryotic cells). In humans, only sperm cells have an undulipodium

• Bacterial cells (prokaryotes) have a flagellum ('tail)

Made of microtubules - 9 pairs of microtubules around the circumference with one pair in the centre ('9+2 arrangement)

Motor proteins pull microtubules up and down to move the undulipodium

Cilia STRUCTURE

Made of microtubules - 9 pairs of microtubules around the circumference with one pair in the centre ('9+2 arrangement)

Motor proteins pull microtubules up and down to move the undulipodium

Cilia FUNCTION

Move substances outside of cells

• Mucus in the trachea

• Move the egg along the oviduct, from the ovaries towards the uterus

Vacuole STRUCTURE

Membrane-bound organelle (membrane = 'tonoplast membrane')

Found in plants, and some fungi - can fill nearly all of a cell's volume

Vacuole FUNCTION

Store 'cell sap' - solution of water, ions and toxins

Regulate cell's 'turgidity' - the internal pressure caused by water volumes

How to measure the nucleus of cell using light microscope

1. use eyepiece graticule

2. calibrate graticule using stage micrometre → calculate length of one epu

3. measure diameter of nucleus in epu

4. take repeat measurements and calculate a mean diameter

5. use calibrated epu to calculate diameter of nucleus (in micrometres) 

Path of protein secretion

RER

Golgi

Vesicle

Cell surface membrane

What are the similarities between protists, bacteria, fungi

Protists = Prokaryotes 

Bacteria & Fungi = Eukaryotes 

Structure of microscope

Cytoskeleton STRUCTURE

Network of protein structured

rod-like microfilaments made of subunits of protein actin

allows of hydrolysis of ATP

Importance of cytoskeleton FUNCTION

transport organelles around the cell

to provide strength to the cell

hold organelles and nucleus in place

form mitotic/meiotic spindle

movement of chromatids/chromosomes

cytokinesis

Which of these are membrane bound organelle

(RER, SER, Ribosome, Mitochondrion)

RER, SER, Mitochondrion

Which of these are found in animal and plant cells

(RER, SER, Ribosome, Mitochondrion)

RER, SER, Ribosome, Mitochondrion

Which of these has a role of lipid production

(RER, SER, Ribosome, Mitochondrion)

SER