Topic 2A- Cell structure and division

What are the two types of cell?

Eukaryotic and prokaryotic

What cells are eukaryotes?

Animal, plant, algae and fungi

What kind of organisms are prokaryotes?

Bacteria

What are organelles?

Subcellular structures that carry out a function

What are all the organelles within both animal cells and plant cells?

• Cell surface membrane

• Rough endoplasmic reticulum

• Smooth endoplasmic reticulum

• Nucleus

• Nucleolus

• Ribosome

• Golgi apparatus

• Cytoplasm

• Mitochondria

• Animals can also have a small vacuole to isolate waste products and get rid of them

What are all the organelles within only within plant cells and not animal cells?

• Cell wall with plasmodesma- channels connecting two plant cells

• Chloroplast

• Plastids- store pigments

• Permanent vacuole

How do algal and fungal cells compare to plant cells?

• Algal cells all have a cell wall and chloroplasts

• Fungal cells are also similar to plant cells except:

  • Their cell walls are made of chitin, not cellulose

  • They don’t have chloroplasts because they don’t photosynthesise

What is the structure of the cell surface membrane and what is it’s function?

• The membrane found on the surface of animal cells and inside the cell walls of other organisms

• Made of phospholipid bilayer and protein

• Regulates the movement of substances in and out of the cells

• Also contains receptor molecules which allows it to respond to chemicals like hormones

• Partially permeable to allow the movement of water by osmosis

What is the structure of the nucleus and what is it’s function?

• A large organelle surrounded by a nuclear envelope (double membrane) which contains many nuclear pores

• The nucleus contains chromosomes which are made from protein bound linear DNA

• The nucleus controls cell activities and contains instructions to make proteins

• The nuclear pores allows substances to move between the nucleus and the cytoplasm

• The nucleolus manufactures rRNA and assembles ribosomes

• Nucleoplasm- jelly substance making up the nucleus

• Contains chromatin- a complex of DNA and histone molecules which make up the genetic info of the cell

  • DNA wrapped around histones

What is the structure of mitochondria and what is their function?

• Oval-shaped

• Has a double membrane

  • The inner membrane is folded to form structures called cristae

• Cristae provide a large surface area for the attachment of proteins and other enzymes involved in respiration

• Inside is the matrix, which contains enzymes involved in respiration and well as lipids, ribosomes and DNA

• This is the site of aerobic respiration where ATP is produced

• Found in large numbers on cells that are very active and require a lot of energy

What is the structure of chloroplasts and what is their function?

• A small, flattened structure found in plant and algal cells

• Surrounded by a double membrane (chloroplast envelope) and has membrane inside called thylakoid membranes

  • These membranes are stacked up on some parts of the chloroplast to form grana which are linked together by lamellae- thin, flat pieces of thylakoid membrane

  • Thylakoids contain chlorophyll, where light absorption takes place

• This is the site where photosynthesis takes place

• Some parts of photosynthesis happen in the grana, while others happen in the stroma (a thick fluid found in chloroplasts)

• The stroma is a matrix in which the synthesis of sugars takes place

What is the structure of the golgi apparatus and what is it’s function?

• Consists of a stack of membrane making up flattened, fluid filled sacs called cisternae

• Vesicles seen at the end of sacs

• This modifies and packages new lipids and proteins than have passed through here from the endoplasmic reticulum.

• It also makes lysosomes

What is the structure of Golgi vesicles and what is their function?

• A small, fluid-filled sac in the cytoplasm

• Surrounded by the membrane and produced by the Golgi apparatus

• Stores lipids and proteins made by the Golgi Apparatus and transports them out of the cells

• They transport proteins out of the cell by fusing to the cell membrane

What is the structure of lysosomes and what is their function?

• A round organelle surrounded by a membrane with no clear internal structure

• It is a type of Golgi vesicle

• Contains digestive enzymes called lysozymes

  • Can be used to ingest invading cells such as bacteria by hydrolysing their cells walls or to break down worn out components of the cell

What is the structure of ribosomes and what is their function?

• Very small

• Floats free in the cytoplasm or is attached to the rough endoplasmic reticulum

• Made up of proteins and RNA

• Not surrounded by a membrane

• Site of protein synthesis

What is the structure of the rough endoplasmic reticulum and what is it’s function?

• Made of cisternae- A system of membranes enclosing a fluid filled space

• Surface is covered with ribosomes

• Folds and processes proteins that have been made at the ribosomes

• Provides a large surface area for the synthesis of proteins

• Provides a pathway for transport of materials like proteins throughout the cell

What is the structure of the smooth endoplasmic reticulum and what is it’s function?

• Made of cisternae- membranes enclosing a fluid filled space

• Doesn’t contain ribosomes like the rough endoplasmic reticulum

• Synthesises, processes and transports lipids and carbohydrates

• Processes proteins made by ribosomes

What is the structure of the cell wall and what is it’s function?

• A rigid structure surrounding the cell

• Supports cells and prevents them from changing shape

• Prevents cell from bursting because of the movement of water inside the cell due to osmosis

• Strengthened by microfibrils

• Allows water to pass along it and contributes to the movement of water throughout a plant

What is the structure of the permanent vacuole and what is it’s function?

• Membrane bound organelle found in the cytoplasm of plant cells

• Contains sell sap- a weak solution of sugars and salts

• The surrounding membrane is called the tonoplast

• Helps to maintain pressure inside the cell and prevent it from collapsing

• This stops plants wilting

• In animals who have a temporary vacuole as well as plants this can be involved in the isolation of toxins inside of the cell

What are the different types of ribosomes found in eukaryotes and prokaryotes

• Eukaryotes- 80S

• Prokaryotes- 70S

What cell organelles are found in all prokaryotes?

• Ribosomes

• Cell surface membrane

• Cell wall made of murein/peptidoglycan

• Cytoplasm

• Circular DNA

What organelles are only found in some prokaryotes?

• Pili- used to attach to surfaces

• Flagellum- used to swim

• Plasmids

• Capsule

• Folding of cell membrane

By what process do prokaryotes divide?

Binary fission

How does binary fission work?

• Replication of circular DNA

• Replication of plasmids

• Division of cytoplasm/cytokinesis

How do viruses replicate?

• Attachment- Virus attaches to host cell receptor proteins

• Entry- Genetic material is released into the host cell

• Synthesis- Genetic material and proteins are replicated by host cell organelles

• Assembly- Viral components are assembled

• Release- Replicated viruses released from host cell by lysis, destroying the cell

What is the calculation for magnification?

Magnification = Image size/ Actual size

What are the features of a light microscope

• Uses light to form an image

• Maximum resolution is about 0.2 micrometers

• Staining can produce coloured images

• Specimens can be living or dead

How would you prepare a temporary mount?

• Take off a thin layer of the specimen

• Place a drop of water on your slide

• Place specimen on water droplet

• Stain cells with iodine in potassium iodide

• Protect specimen with a coverslip using a mounting needle, pressing gently to remove air bubbles

Why does the specimen need to be thin (one cell thick) for optical microscopes?

So the light from the light microscope can penetrate the specimen

Why are light microscopes less detailed than electron microscopes?

• The resolution is lower

• This is because the wavelength of light is too long whereas a beam of electrons has a shorter wavelength and therefore higher resolution

What is magnification?

How many times enlarged an image is

What does resolution mean?

The smallest distance between two points that can still be seen as two points

What are the features electron microscopes?

• Uses electrons to form images- electron beams have a shorter wavelength than light

• Maximum resolution is 0.0002 micrometres

• Maximum magnification is x150,000

• Produced black and white images that can be coloured by computers

• Specimen is placed in a vacuum and therefore is dead

How does a scanning electron microscope work?

• Directs a beam of electrons onto the surface of the specimen from above rather than penetrating from below

• Beam passed back and forth through a portion of the specimen in a regular pattern

• Electrons are scattered by the specimen and this scattering depends on the contours of the specimen’s surface

• A 3D image is build up by analysis of scattered electrons and secondary electrons produced

How does a transmission electron microscope work?

• An electron gun produces a beam of electrons

• The beam passes through a thin section of the specimen

• Some parts of the specimen absorb electrons so they appear darker while others let electrons pass through and are lighter

• 2D Image produced on screen and photographed to give a photomicrograph

What are the advantages of transmission electron microscopes?

• Detailed image of cell organelles

• 2D- allows us to observe organelles from above

• Shows internal structure

• Higher resolution and magnification than both an optical (light) microscope and a scanning electron microscope

Limitations of a transmission electron microscope?

• Difficulties with preparing the specimen can limit resolution

• High energy electron beam can destroy the specimen

• Since the specimen has to be in a vacuum, it will be dead/non-living so we cannot observe living organisms

• Complex staining process but no colour anyway

• Specimen has to be extremely thin

• Image may contain artefact

What are the advantages of the scanning electron microscope?

• Detailed 3D structure

• Shows external structure of cells

• Specimen doesn’t need to be thin as electrons don’t penetrate

• Higher resolution and magnification than a light microscope

What are the limitations of a scanning electron microscope?

• Lower resolving power that a transmission electron microscope

• Cannot observe internal structures

• No colour

• Only used on non-living specimens

What is an artefact and how can they be produced?

• Things you can see down the microscope that aren’t part of the cell or specimen you are looking at

• They are often made during the preparation of the slide and are common in electron micrographs due to the complex nature of specimen preparation

How did scientists work out how what they were looking at down a microscope was an artefact or organelle?

• By repeatedly preparing the sample in different ways

• If an object was seen with one preparation method and not another, it is more likely that it is an artefact

What is cell fractionation?

A process used to gain a sample of organelles within some tissue cells

What happens in the homogenisation stage of cell fractionation?

• The sample is put in a homogeniser (essentially just a blender)

  • This is done to break open cell walls and cell membranes to release organelles

• The sample needs to be placed in a specific solution in order to prevent damage to cell organelles:

  • A buffer solution is used as it stops changes in pH, which would denature proteins in the membranes of organelles

  • The solution is isotonic to prevent osmotic lysis

  • The solution is kept ice-cold to prevent the hydrolysis/digestion of organelles by enzymes (specifically lysosomes) by reducing kinetic energy, lowering rate of reaction

What tissue is normally used in cell fractionation and why?

• Liver cells

• Because they have a high metabolic rate

What happens after the sample is homogenised and what do we call our sample after it is homogenised?

• The homogenate is filtered

• This is done to remove cellular debris

• Organelles are far smaller than the debris so they can pass through the material used to filtrate

What happens in the differential centrifugation stage of cell fractionation?

• The homogenate is spun slowly first to produce a pellet

• This pellet would be removed then then the homogenate would be spun again but at a higher speed to produce another pellet

• The pellets produced are different organelles

• This would be repeated, with the speed repeated each time until all organelles had been separated

• The reason why organelles separate is because they are of varying densities- the organelles with the highest density will form a pellet first

What is another name for a pellet?

Sediment

What is the solution above the sediment called?

Supernatent

In what order are organelles separated out?

Slowest

• Nuclei

• Chloroplasts

• Mitochondria

• Lysosomes

• Endoplasmic reticulum/golgi apparatus

• Ribosomes

Fastest

What tissue may we use to obtain a sample of chloroplasts?

Leaves from lettuce or spinach for example

What is mitosis?

Cell division

What is mitosis needed for?

• Growth and repair

• Development

What happens in interphase?

This is split into 3 stages:

• Gap phase 1

  • Cell grows new organelles and proteins made

• Synthesis stage

  • DNA replicates

• Gap phase 2

  • Cells keep growing and proteins needed for cell division are made

What happens in prophase?

• Chromosomes condense and become visible

• Nuclear envelope and nucleolus break down

• Centrioles move to opposite poles of the cells

• Spindle fibres develop from centrioles

What happens in metaphase?

• Chromosomes line up across the equator of the cell

• The centromeres of chromatids attach to spindle fibres

What happens in anaphase?

• Spindle fibres contract and centromeres divide

• Chromatids move to opposite poles of the cell

What happens in telophase?

• Nuclear envelope reforms and surrounds chromosomes at opposite ends of the cell

• Chromosomes become indistinct

• Spindle fibres disappear

What happens in cytokinesis

• Division of cytoplasm forming genetically identical daughter cells

• Organelles and cytoplasm equally distributed between daughter cells

How would you describe a virus?

They are acellular- not cells

What is the structure of viruses?

• They are nucleic acids (DNA or RNA) surrounded by protein

• No plasma membrane, cytoplasm or ribosomes

• Protein coat around the core is called the capsid

• Attachment proteins tick out from the edge of the capsid, allowing the virus to cling onto a suitable host cell

How small are viruses compared to bacteria?

Much smaller

What is cancer?

A group of diseases caused by uncontrolled cell division which leads to a malignant tumour

How does cancer develop?

• It is the result of damage to genes that regulate mitosis and the cell cycle

• Mutations in these genes lead to uncontrolled cell division

• When the cells grow out of control a tumour is the result- this tumour can be benign or malignant

What is the difference between benign and malignant tumours?

Benign

• Grows slowly

• More compact

• Not life threatening

Malignant

• Grows rapidly

• Less compact

• More likely to be life threatening- it metastasizes meaning that the cancer moves to other parts of the body

What is the problem with cancer treatments?

• Many treatments attempt to kill dividing cells by disrupting the cell cycle

• However, these treatments cannot differentiate between tumour cells and healthy cells such as hair, skin, gut and blood cells

• As tumour cells divide more rapidly, they are more likely to kill tumour cells

How do cancer treatments target the G¹ phase of mitosis?

• Uses chemical drugs (chemotherapy)

• Prevents the synthesis of enzymes needed for DNA replication

• The cell is unable to enter the S phase

• The cell is forced to destroy itself (apoptosis)

• Inhibits spindle formation

How do cancer treatments target the S phase of mitosis?

• Some drug and radiation damage DNA (radiotherapy)

• At various points in the cell cycle, DNA is checked for damage, if it is damaged then the cell destroys itself

• This prevents tumour growth

What are loci?

The physical position of genes on a chromosome