Biology | Foundations in Biology: Cells

define magnification

How much bigger the image is than the specimen

Define resolution

is the ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures)

Types of microscopes

• optical

• electron

Describe optical microscopes in terms of properties

-cheap -magnifies x2000 -uses a beam of light -was developed in the 17th century -used anywhere (portable) -can observe whole living specimen

describe how an optical microscopes work

-uses light to form an image (using visible light)

resolution : 200nm

magnification : x1500 (x2000)

what can optical microscopes observe

used to observe eukaryotic cells:

nuclei, cell wall, cell membrane

what can't optical microscopes observe

can't be used to observe smaller organelles eg ribosomes, endoplasmic reticulum or lysosomes

properties of electron microscope

-large -expensive -kept in special conditons uses a beam of electrons invented in 1930

how does an electron microscope work

It uses a beam of electrons which are focused using magnets. The electrons hit a fluorescent screen which emits visible light, producing an image.

Magnification : x2000000 Resolution: 0.2nm

difference between electron and optical

This greatly increases the resolution of electron microscopes compared to optical microscopes, giving a more detailed image.

This means electron microscopes can be used to observe small organellessuch as ribosomes, the endoplasmic reticulum or lysosomes

types of electron microscopes

transmission electron microscope (TEM)

scanning electron microscope (SEM)

Describe a transmission electron microscope

TEMs use electromagnets to focus a beam of electrons This beam of electrons is transmitted through the specimen Denser parts of the specimen absorb more electronsThis makes these denser parts appear darker on the final image produced (produces contrast between different parts of the object being observed)

what image is formed on a TEM

2D grey-scale image

shows ultrastructure and cell organelles

advantages of TEM

They give high-resolution images (more detail) This allows the internal structures within cells (or even within organelles) to be seen

disadvantages of TEM

They can only be used with very thin specimens or thin sections of the object being observed

They cannot be used to observe live specimens (as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed, meaning that specimens must be dead, unlike optical microscopes that can be used to observe live specimens)

The lengthy treatment required to prepare specimens means that artefacts can be introduced (artefacts look like real structures but are actually the results of preserving and staining)

They do not produce a colour image (unlike optical microscopes that produce a colour image)

describe SEM

SEMs scan a beam of electrons across the specimen This beam bounces off the surface of the specimen and the electrons are detected, forming an image

what image is formed in a SEM

three-dimensional images that show the cell surface of specimens

advantages of SEM

They can be used on thick or 3-D specimens They allow the external, 3-D structure of specimens to be observed

disadvantages of SEM

They give lower resolution images (less detail) than TEMs They cannot be used to observe live specimens (unlike optical microscopes that can be used to observe live specimens) They do not produce a colour image (unlike optical microscopes that produce a colour image)

resolution of electron

TEM : 0.02 SEM : 0.2

magnification of electron

TEM: 500,000 SEM: 100,000

How to prepare a light microscope

1. The specimen on a slide is placed here on the stage and clipped into place.

2. By rotating the nosepiece. the lowest power (smallest) objective lens is placed over the specimen

3. Adjust the coarse focus knob while looking into the eyepiece. until the image you see is clear and in focus.

4. Whilst viewing the image adjust the iris diaphragm for optimum light. ocular tube

5. Make sure that the object you wish to view is directly over the hole in the stage. Now rotate the nosepiece and bring the x10 objective into place over the specimen. Look down the ocular tube and use the fine focus knob to focus the image.

6. Repeat 5 using the next objective lens

light microscope diagram

why using staining

Many biological specimens are colourless when they have been cut into thin sections. Staining is a procedure to add false colour to make areas of biological material easier to see.

Different parts of a cell, or different types of cells take up (absorb) a stain more than others.

types of stains

methylene blue iodine solution

methylene blue

staining living cells

dark blue nucleus, light blue cytoplasm (bacteria the whole cell takes the cell)

iodine solution

staining living plant cells very dark blue starch grains

advantages of staining specimens

easier to clearly see certain organelles such as the nucleus

able to differentiate between cells

This provides contrast to distinguish between different organelles in the sample.

difference between eukaryotic and prokaryotic cells

p: nu nuclei more simpler

e: presence of nuceli has membrane bound organelles more complex can be multicellular and unicellular have chromosomes

ultrastructure of animal cell

Function of cell surface membrane

~ Regulates the movement of substances in and out of cells ~ Also Have receptor molecules on the surface so can respond to chemicals like hormones

-is partially permeable

function of cell wall

he cell wall surrounds the plasma membrane of plant cells and provides tensile strength and protection against mechanical and osmotic stress.

It provides protection to the cell and prevents from any physical damage. It provides structure to the cell. It prevents from osmotic bursting.

function of nucleus

chromatin (the material from which chromosomes are made)

what is the cell membrane made of

delicate lipid and protein skin around the cytoplasm

function of smooth endoplasmic reticulum

Synthesises and processes lipids and carbohydrates

function of lysosomes

is a membrane-bound cell organelle that contains digestive enzyme the vesicles are filled with these digest food absorbed by cells

specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down) Break down waste materials such as worn-out organelles, used extensively by cells of the immune system and in apoptosis (programmed cell death)

function of Golgi body

flattened sac like structure which packages and modifies RNA into a vesicle which fuses the protein to the membrane and sends it out for use

function of mitochondria

site of aerobic respiration, releases the cell's energy in the form ATP

function of ribosomes

protein synthesis

centrioles

have a role in mitosis. Responsible for the organisation of chromosomes by spindle fibres

pair of bundles tubes, organise cell division

function of rough endoplasmic reticulum

Folds and processes proteins that have been made at the ribosomes. Stores the proteins made at the attached ribosomes

function of nuclear envelope

separates the nucleus from the cytoplasm

nucleolus

located within the nucleus and is the site of ribosome production

function of DNA/chromosomes

to store the information needed to make proteins correctly

structures coding for genes

function of nucleoplasm

highly viscous liquid that includes chromosomes and nucleoli

many substances such as nucleotides and enzymes are dissolved in it

function of nuclear pore

allows substances in and out of the nucleus

function of chloroplast

Larger than mitochondria, also surrounded by a double-membrane

Chloroplasts are the site of photosynthesis:

Function of permanent vacuole

store nutrients and water on which a cell can rely for its survival. They also store the waste from the cell and prevents the cell from contamination.

production and secretion of proteins step by step

nucleus , contains gene (for protein) / site of transcription / produces mRNA1) 1)Protein is made at ribosomes- translation at ribosomes 2) Protein is transported to cis-face of golgi body by vesicle 3) Protein is modified by enzymes then packaged 4) secretary vesicles transport the protein out of the cell from the trans-face of the golgi body 5) protein fuses with cell surface membrane and exported out of cell for use

what happens between the ribosomes and RER

The ribosome 'reads' the genetic instructions contained within the mRNA and uses this code to synthesise a protein via a process known as translation This protein then passes into the lumen (the inside space) of the rough endoplasmic reticulumto be folded and processed

what happens when the proteins are released

released the proteins by the process of exocytosis

what happens before the protein is translated at the ribosome

The DNA from the nucleus is copied into a molecule of mRNA via a process known as transcription The mRNA strand leaves the nucleus through a nuclear pore and attaches to a ribosome on the rough endoplasmic reticulum

benefits of protein synthesis

-maintains integrity and structure of DNA to make only a copy it reduces possible damage

-allows copies of DNA to leave nucleus- DNA too large

-many proteins synthesised at once in different locations

-vesicles enable soluble proteins to be transported from organelle to organelles

Uses of cytoskeleton

structural support (mechanical strength) transport movement

Function is to give the cell its shape and mechanical resistance to deformation, involved in many cell signalling pathways, involved in cytokinesis, endocytosis and intracellular transport. ​

what do the protein fibres do

secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move.

What is the cytoskeleton?

a network of protein fibers extending throughout the cytoplasm

describe structural support

The cytoskeleton provides the cell with mechanical strength, forming a kind of 'scaffolding' that helps to maintain the shape of the cell It also supports the organelles, keeping them in position

describe transport

the cytoskeleton aids transport within cells by forming 'tracks' along which organelles can move Examples of this include the movement of vesicles and the movement of chromosomes to opposite ends of a cell during cell division

describe movement

The cytoskeleton enables cell movement via cilia and flagella These structures are both hair-like extensions that protrude from the cell surface and contain microtubules that are responsible for moving them

what is cytoskeleton composed of

microtubules, microfilaments, intermediate filaments

describe the microtubules

25nm

straight tubes eg cilia flagella

used for transport

describe microfilaments

7nm -movement

Describe intermediate filaments

10nm for structural support resisting mechanical stress

Describe prokaryotic cells

bacteria and archaea

smaller simpler older

single cells colony: forms a film or filaments

how are prokaryotic cells different

A cytoplasm that lacks membrane-bound organelles Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S) No nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins) A cell wall that contains murein (a glycoprotein)

other features of prokaryotic cells

pseudopods: fake feet, extension of cell membrane from microfilaments

cilia: microtubules attached to cell membrane which beat in unison

structure of prokaryotic cell

the cell wall, the plasma membrane, flagella, pili, ribosomes, the nucleoid

describe DNA in prokaryotic cells

Not contained within a nucleus. Packaged differently - no histones (chromatin) and usually 1 condensed chromosome with genes grouped into operons. Genes turned on or off as necessary.​

describe ribosomes

Smaller in prokaryotes (70s not 80s). Simple protein synthesis.​

describe cell wall

Peptidoglycan (aka murein). Made from amino acids and sugars. ​

describe flagella

Thinner, no 9 +2 arrangement. Energy supplied from chemiosmosis not ATP.​

energy supplied from chemiosmosis not ATP

A long, hair-like structure that rotates, enabling the prokaryote to move (a bit like a propeller). Some prokaryotes have more than one. Not present in all prokaryotes.

describe mesosome

Inner folding of cell surface (plasma) membrane which may be used for aerobic respiration.​

describe granules

Nutrients can be stored in cytoplasm in form of proteins​

describe slime capsule

• Protects bacteria from attack by cells of the immune system.

• Helps bacteria stick together capsule. It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism.

Additional protection (phagocytosis).​

describe folded membrane

For nitrogen fixation or photosynthesis.​

describe plasmids

Small loops of DNA that are separate from the main circular DNA molecule. Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance). Not present in all prokaryotes.quick synthesis of genes for antibiotic resistance

describe cytoplams

where anaerobic respiration occurs

ways in which prokaryotes release energy

-photosynthesis -nitrogrn fixing -feeding on dead matter -feeds of host for disease causing ones

advantages of binary fission

The advantages of asexual reproduction include: the population can increase rapidly when the conditions are favourable only one parent is needed it is more time and energy efficient as you don't need a mate it is faster than sexual reproduction.

advantages of being able to switch between aerobic and anaerobic

can survive in docntions where limited oxygen is present

How big are eukaryotic cells?

10-100 micrometers

how big are prokaryotic cells

0.5-10 um

What is the endosymbiotic theory?

The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes.

Many scientists theorise that eukaryotes evolved from prokaryote ancestors.​ in 1981, Lynn Margulis popularised the "endosymbiont theory."​ a prokaryote ancestor "ingests" a smaller prokaryote​ the smaller prokaryote evolves a way to avoid being digested, and lives inside its new "host" cell kind of like a pet.​

proof for endosymbiotic theory

Mitochondria has double membrane, their own DNA ( circular and naked ) and 70S ribosomes. resistant to certain antibiotics

Small ribosomes 70s​ Mitochondria and chloroplasts have their own DNA which is circular, not linear single stranded DNA (no histone proteins). Single stranded, circular DNA is found exclusively in prokaryotes.​ Similar in size to prokaryotes.​ Can divide by binary fission