2.1- Cell structure

Magnification

How many times larger an image appears, compared to the actual size of the object being observed

Resolution

The ability to distinguish two objects as separate - the clarity of an image

Qualities of light microscopes

• cheaper

• easy to use

• can be used to study whole living specimens

maximum magnification for light microscopes

x1500

Maximum resolution for light microscopes

0.2 micrometres

Why can’t ribosomes be examined under a light microscope?

They are very small organelles of about 20 micrometres, which makes them too small to be resolved by light microscopes

How to use a light microscope

• Clip prepared slide onto the stage

• select objective lens with lowest power

• use the coarse focus to bring the stage just below the objective lens

• look down the eyepiece and use the coarse focus to move the stage downwards until the image is roughly in focus

• use the fine focus to make the image clearer

• if a higher magnification is needed, swap to a more powerful objective lens and refocus

Magnification formula for light microscopes

total magnification = magnifying power of objective lens x magnifying power of eyepiece lens

How do laser scanning microscopes work?

• They use laser light to scan an object point by point and assemble, by computer, the pixel information into one image, displayed on a computer screen

• These microscopes have depth selectivity and can focus on structures at different depths within a specimen.

Why do electron microscopes have a higher resolution than light microscopes?

They have smaller wavelengths than visible light.

How do Transmission electron microscopes work?

They use electromagnets to transmit a beam of electrons through a specimen. The denser parts absorb more electrons, so appear darker in the image formed.

What type of images do TEMs produce?

2D images of the internal structures of a cell. The image produced is black and white

Magnification and resolution of TEMs

• Magnification- x1500000

• resolution- 0.5 nm

How do scanning electron microscopes work?

They scan a beam of electrons across the surface of a specimen. Reflected electrons are then used to form an image. 

Magnification and resolution of SEMs

• magnification- x1500000

• resolution- 5 nm

Conditions for a specimen to be viewed under a TEM

the specimen must be viewed in a vacuum, meaning only non-living or dead organisms can be observed. Also, the specimen must be thin to allow electrons to pass through

Conditions for a specimen to be viewed under a SEM

SEMs can only view non-living or dead specimens. However, SEMs can be used on thicker specimens

What type of stain is Methylene blue?

all-purpose stain

Differential staining

staining in which the stain only binds to specific cell structures, staining each structure differently so the structures can be easily identified within a single preparation

acetic orcein

binds to DNA and stains chromosomes dark red

Eosin

stains cytoplasm

Iodine in potassium iodide solution

stains cellulose yellow and starch granules blue/black

magnification formula

image size/ actual size

wet mount

• Use a pipette to place a small drop of water onto the centre of the glass slide.

• Use a pair of forceps to place a thin section of the specimen onto the drop of water. The specimen should be thin enough to allow light to pass through.

• Add a few drops of stain to the specimen. This increases contrast and allows cell components to become visible.

• Slowly add a cover slip onto the specimen.

Dry mount

The specimen is placed directly onto the slide and covered with a cover slip.

squash slides

A wet mount is prepared and the cover slip is pressed to squash the cells. 

smear slides

The edge of a slide is used to smear the sample to create a thin, even coating on a separate slide.

what is a stage micrometre used for?

to calibrate the eyepiece graticule

how is an eyepiece graticule calibrated?

1. Fix the stage micrometer into place on the stage.

2. Look through the eyepiece to line up the micrometer and the graticule. 

3. Count the number of graticule divisions that fit into one micrometer division.

4. graticule division= number of graticule divisions/ size of one micrometer division​

Which cells are known as eukaryotic?

animal, plant, fungal and protoctist

nucleolus

• contains RNA

• where chromosomes unwind

• involved in making ribosomes

• no membrane around it

cytoskeleton

• a network of protein filaments (actin or microtubules) within the cytoplasm that move organelles from place to place within the cell

• allow some cells to move

• allow muscle cells to contract

definition of membrane-bound

the organelle is covered in a membrane, which keeps it separate from the rest of cell

nucleus structure

• surrounded by a nuclear envelope

• the nucleolus does not have a membrane around it

• chromatin is the genetic material inside the nucleus, consisting of DNA wound around histone proteins

nucleus function

• nuclear envelope separates contents of nucleus from the rest of the cell

• nuclear pores enable larger substances, such as mRNA to leave the nucleus

• Controlling the cell's activities - It contains DNA with instructions to produce proteins. 

• Synthesis of ribosomes - The nucleolus makes ribosomal RNA.

structure of RER

• system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane

• It is covered with ribosomes

function of RER

• Synthesis and transport of proteins

• the proteins made actively pass through the membrane into the cisternae and are transported to the golgi apparatus for modification and packaging

structure of SER

system of membranes, containing fluid-filled cavities that are continuous with the nuclear membrane

function of SER

• Synthesis, storage, and transport of lipids and carbohydrates - e.g. cholesterol and steroid hormones.

structure of golgi apparatus

• Contain fluid-filled, membrane-bound sacs known as cisternae.

• Contain smaller vesicles.

function of golgi

• Process and package lipids and proteins - Carried out by the cisternae.

• Store and transport lipids and proteins - Carried out by the vesicles.

  • proteins are either stored in the cell or moved to the plasma membrane, either to be incorporated into the plasma membrane or exported out of the cell

• Synthesise lysosomes - Specialised vesicles. 
  

structure of mitochondria

• Contain an enzyme-rich liquid known as the matrix. 

• Surrounded by a double membrane in which the inner membrane is folded to form structures known as cristae.

• Contain their own DNA and ribosomes.

function of mitochondria

• site of ATP production during aerobic respiration

• they are abundant in cells where a lot of metabolic activity takes place

structure of chloroplasts

• surrounded by a double membrane

• Contain fluid-filled sacs known as thylakoids which are stacked up to form grana

• has fluid-filled matrix called stroma

• Contain their own DNA and ribosomes

function of chloroplasts

• site of photosynthesis- these reactions take place in the grana and stroma

structure of vacuole

• Contains cell sap

• Surrounded by a selectively permeable membrane known as a tonoplast

function of vacuole

• Helps to maintain pressure within the cell, which keeps the cell rigid and stops the plant from wilting

structure of lysosomes

• small bags formed by golgi

• Contain hydrolytic enzymes. 

• Surrounded by a membrane to keep enzymes separate from the cytoplasm of the cell

function of lysosomes

• engulf old cell organelles and foreign matter and digest them

• Digest pathogens using enzymes

• Break down waste material like old organelles and cells

structure of cilia and undulipodia

• protrusions from the cell that are covered by the cell surface membrane

• contains microtubules

• formed from centrioles

function of cilia and undulipodia

• cilia in airways to move the band of mucus

• cell signalling

structure of ribosome

• not membrane-bound

• around 20 nm in diameter

• made in nucleolus as two separate units, which pass through the nuclear envelope and then combine

function of ribosomes

• protein synthesis

structure of centrioles

• consist of two bundles microtubules perpendicular to each other

function of centrioles

• spindle fibres form from centrioles

• involved in the formation of cilia and undulipodia

Cytoskeleton

A network of protein structures, within the cytoplasm, that support the cell.

• includes: microfilament, microtubules and intermediate filaments

microfilaments

• Made of the protein actin

• Involved in cell movement and locomotion, such as crawling and muscle contraction

microtubules

• Made of the protein tubulin.

• Form a scaffold-like structure throughout the cell.

• Form the main component of the mitotic spindle, used for cell division.

• Act as tracks for intracellular transport of vesicles and organelles.

intermediate filaments

• Maintain the position of organelles within the cell.

• Responsible for the mechanical strength of certain tissues, such as the skin and hair.

structure of cell wall

• on the outside of the plasma membrane

• made of bundles of cellulose fibres

• Contains gaps known as plasmodesmata
  

function of cell wall

• provides strength and support

• Prevents the cell from bursting - The cell wall can withstand high osmotic pressure

• maintain cell’s shape

• permeable, so it allows solutions to pass through

making and secreting a protein

1. DNA is copied into mRNA and the mRNA passes through the nuclear pore and enters the cytoplasm

2. mRNA leave nucleus via nuclear pore

3. mRNA binds to ribosomes on the RER, which assemble the amino acid sequence to form a polypeptide

4. Polypeptide is folded into its secondary/tertiary structure inside the RER and transported via vesicles that are pinched off the RER

5. vesicle fuses with Golgi

6. golgi processes and modifies proteins

7. proteins are packaged into secretory vesicles

8. vesicles fuse with plasma membrane by exocytosis

9. protein is secreted out of the cell

What type of cells do bacteria have?

prokaryotic

similarities between eukaryotic cell and prokaryotic cell

• plasma membrane

• cytoplasm

• ribosomes for assembling amino acids into proteins

• DNA and RNA

how are prokaryotic cells different to eukaryotic cells?

• cells are much smaller

• have a less well-developed cytoskeleton with no centrioles

• no nucleus

• no membrane-bound organelles

• cell wall is made of peptidoglycan

• smaller ribosomes

• naked DNA

some prokaryotic cells have:

• protective waxy cuticle surrounding the cell wall

• small loops of DNA called plasmids

• flagella

• pili- small, hair-like structures that allow bacteria to adhere to each other or host cells

How do prokaryotes divide?

By binary fission- Before the cells divide, their DNA is copied so that each new cell receives a large loop of DNA and any smaller plasmids

what type of organelles do prokaryotes have?

non-membrane bound