Cell structure A2.2

cell definition

the basic structural unit of all living organisms.

principles of cell theory

• all living things are made out of cells.

• unicellular cells are made from one cell

• Multicellular organisms have a number of specialised cells

• cells are the smallest units of life

• cell components cannot survive alone

• organelles carryout metabolic functions in cell

• cells arise from pre-existing cells

• cells multiply by division (mitosis + meiosis)

• all cells descended from simpler common ancestors

Cell (Plasma) membrane significance + function

• outer boundary of cell + encloses all its contents

• controls entry + exit of substances. (can pump substances in even when concentration outside is low) (keeps unwanted substances outside)

• allows cell to maintain concentrations of substances that are different from those outside of the cell

• permeability of plasma relies on a structure based on lipids

what is lysis?

when the plasma membrane of a cell bursts. caused by excess pressure, viruses or the cell carries it out itself (autolysis).

lysis leads to the death of the cell

Genetic material significance + function

• contains information needed for a cell to carry out its functions

• Many genes hold the instructions to making a protein 

• DNA can copy and pasted on to daughter cells → information stored is inheritable

• DNA is stored in nucleus

• Bacteria don’t have a nucleus - DNA is stored in Cytoplasm

Cytoplasm Significance + function

• main component → water

• substances are dissolved in the water

• water allows enzymes to catalyse reactions → metabolism of cell

• cytoplasm must continuously break down and replace proteins due to proteins being easily damaged.

importance of metabolism in cytoplasm

provides cell with energy and produces proteins and other substances that make up the structure of the cell

label

• A - eyepiece lens

• B - objective lens

• C - stage

• D - aperture

• E - light source

• F - stage clips (where slide goes)

• G - coarse focus

• H - fine focus

• I - stage controls

• J - base

eyepiece graticule vs stage micrometer

• eyepiece graticule - scale in the microscope eyepiece. By itself, it has no fixed units (the "divisions" are arbitrary).

• stage micrometer - a slide with a precise scale, usually 0.01 mm or 10 µm divisions

how to determine the size of a division in a eye piece

• Place the stage micrometer on the microscope stage.

• Focus on the scale.

• Line up the eyepiece graticule scale with the stage micrometer scale.

• See how many eyepiece divisions match up with a known length on the stage micrometer.

• 1 eye piece division = 100/number of eyepiece divisions

• (⚠ Important: If you change objective lenses (magnification), you must recalibrate, since the apparent size changes.)

how to determine the size of a cell using an eye piece graticule

• Replace the stage micrometer with your specimen slide after finding the length of one division.

• Count how many eyepiece graticule divisions span across the cell (e.g., diameter of a nucleus, length of a cell).

• Convert using your calibration.

(e.g : Cell = 12 eyepiece divisions, calibration = 2.5 µm/division → 12 × 2.5 µm = 30 µm)

magnification triangle formula

Magnification = image size/actual size = measured length/scale bar length

what is resolution

the ability to distinguish between two objects very close together.

The higher the resolution of an image, the greater the detail that can be seen. ​

resolution is limited by the wavelength of the radiation used to view the sample

Light microscopes

• magnify images only up to x1000 due to long wavelength of light

• low resolution - 0.5µm (micrometers)

• cells viewed under microscope are alive

• images are coloured

• easy to use

Electron microscopes

• types - transmission + scanning

• use beams of electrons → have shorter wavelength than light

• high resolution - 2nm (nanometers)

• high magnification - x 1,000,000

• expensive

• not easy to use

• black + white pictures

transmission electron microscopes

• 2D images

• samples stained with heavy metals → samples are dead

• good imagery of structures in cells

• electrons are scattered as they pass through a thin section of the specimen, and then detected and projected onto an image on a fluorescent screen.

scanning electron microscopes

• 3D images

• electrons are reflected off the surface of the specimen.

• sample is stained using harsh chemicals → dead

staining with methylenblue

Molecules are colourless under electron microscopes and so stains such as methylenblue binds to DNA or RNA in order to be able to visualize the nucleus or cytoplasm. 

fluorescent staining

• uses a higher intensity light to illuminate the sample

• sample is stained with fluorescence dye which causes the light to emit at a longer wavelength

Immunofluorescent staining

• uses antibodies which are equipped with a fluorescent marker.

• Upon binding with an antigen a fluorescent image can be produced

cryogenic microscopy

• used for researching the structures of proteins

• protein is frozen and then placed in an electron microscope and patterns of many different proteins are produced

• using computer algorithms a 3D image is then produced

freeze-fracture electron microscopy

• used to produce images of surfaces within cells

• rapid freezing of cells + fracturing allows the cell to be broken along lines of weakness (center of membrane)

• any structures which appear globular are transmembrane proteins

Prokaryote cell structure - bacteria

cell wall features + functions in Prokaryote

• features : 

• semi rigid structure

• made from peptidoglycan

• function:

• maintains shape of the cell

• protects the cell

• prevents the cell from bursting

Cell membrane features + functions in Prokaryote

• features:

• thin

• partially permeable layer of phospholipids

• function:

• controls the entry + exit of substances

• pumps substances in and out by active transport

cytoplasm features + function in Prokaryote

• features:

• fluid (mostly water) that fills space inside the plasma membrane

• contains many enzymes + ribosomes

• does not contain any membrane bound organelles

• functions:

• carries out chemical reactions of metabolism using enzymes and biochemical molecules

Ribosomes features + functions in Prokaryote

• features:

• 70S (smaller than eukaryotic ribosomes)​

• granular appearance in the EM

• functions:

• Synthesize (make or manufacture) proteins through transcription & translation

Nucleoid features + functions in Prokaryote

• features:

• Central region of the cytoplasm containing naked (not wrapped around a protein) single chromosomal DNA​

• DNA in prokaryotes is circular ​

• Not surrounded by a membrane

• functions:

• essential for controlling the activity of the cell and reproduction.

• where transcription and replication of DNA take place​

Eukaryotic cells

• animal, plant, fungi 

• more complex + bigger in size

• Nucleus with genetic material surrounded by a membrane​

• Membrane bound organelles​

• Unicellular or multicellular​

• Cell structure is compartmentalised.​

plant cells and structure : plastids, cell wall, vacuole, centrioles, Undulipodia

• Plastids : plastids of varied types such as chloroplasts (for photosynthesis) and amyloplasts (to store starch)

• Cell wall: have walls composed of cellulose

• Vacuole: have a large permanent vacuole used for storage of substances and pressurising the cell

• Centrioles: Absent

• Undulipodia: Absent

Animal cells and structure - plastids, cell wall, vcuole, centrioles, undulipodia

• Plastids : none

• Cell wall: none

• Vacuole: small + temporary, used to expel water and digest food or pathogen

• Centrioles: used to construct spindle that moves chromosomes in mitosis

• Undulipodia: cilia + flagella present in many animal cells

Fungal cells and structure — plastids, cell wall, vacuole, centrioles, undulipodia,

• Plastids : none

• Cell wall: present + composed of chitin

• Vacuole: large permanent vacuole used for storage of substances + pressuring cell

• Centrioles: Absent

• Undulipodia: Absent

• eukaryotic

Nucleus + Nucleolus features + functions

• features

• spherical with double membrane

• have holes in membrane

• chromatin - uncoiled chromosomes

• Nucleolus consists of RNA and

  proteins, makes up 25% of nucleus

• functions:

• Stores genetic information in form of chromosomes (DNA and associated histones)​

• Nucleolus produces rRNA (ribosomal) which combine with proteins for use outside the cell to form ribosomes.​

Free ribosomes features + functions

• features

• 80S (larger than in prokaryotes), ca.20nm​ 

• No exterior membrane​

• Free in the cytoplasm or bound to ER​

• Composed of ribosomal RNA and protein

• produced in the nucleolus of the nucleus​

• Appear as dark granules

• functions

• Produces proteins to function in the cytoplasm for use within the cell (enzymes)​

Mitochondrion features + functions

• features : 

• Has a double membrane​

• Outer membrane is smooth, inner membrane is folded​

• The folds are called “cristae”​

• Variable in shape and number (spherical or ovoid) ​

• functions : 

• Site of ATP production by (aerobic) cell respiration.​

• Fat digestion if it is used as an energy source in the cell​

rough endoplasmic reticulum (rER) features + functions

• features:

• Made of flattened membrane sacs called cisternae, attached to the outside of the cisternae are ribosomes (rER)

• Extensive network of tubules or channels that extends almost everywhere in the cell from the nucleus to the plasma​

• functions

• responsible for the production of proteins which are then transported by vesicles to the Golgi apparatus for modification.​

Golgi apparatus features + functions

• features

• Consists of flattened sacs called cisternae, which are stacked on top of one another​

• Has a two sides: cis-side (receives products at that site), and a trans-side (discharges products)​

• Transport vesicles bud off​

• Most of these are packaged into vesicles for secretion through the plasma membrane​

• Difference to rER:​

• No attached ribosomes​

• Often sited close to the plasma membrane​

• The cisternae are shorter and more curved than those of rER​

• functions 

• Processes proteins that arrive from the rER. ​

• functions in collection, packaging, modification and distribution and transportation of materials synthesised in the cell​. 

chloroplasts  features + functions

• features

• Double membrane surrounding the chloroplast​

• Stacks of thylakoids inside​

• Each thylakoid is a disc composed of a flattened membrane.​

• Variable shape (spherical or ovoid)​

• functions

• Production of glucose and other organic compounds by photosynthesis

lysosomes  features + functions

• features 

• Formed from Golgi vesicles which bud off​

• spherical with single membrane​

• High concentration of enzymes (proteins)

• cause this organelle to stain heavily and hence appears dark​

• Only in animal cells (plants use vacuoles)​

• functions

• Used for the breakdown of food or

  unwanted, damaged substances

  organelles using enzymes.

vacuoles + vesicles features + functions

• features

• Single membrane with fluid inside​

• Plant cells: vacuoles are large and permanent, often occupying the majority or the cell​

• Animal cells: Small and temporary – typically referred to as vesicles.

• functions

• Vacuoles: In plant cells: Used for maintenance of water balance and internal pressure.​

• Vesicles: Used for transport of substances within the cell – often from rER to Golgi apparatus.

flagellum + cilia features + functions

• features

• Whip-like structures projecting from the cell surface​

• Contain a ring of nine double microtubules + 2 central ones​

• Flagella are larger, and only one is present, cilia are smaller and many are present​

• Have microtubules inside

• functions

• Cilia move liquid over surfaces (e.g. particle-laden mucus towards throat)

• For movement (sperm cells)

microtubules + centrioles features + functions

• features

• Microtubules:

• Small cylindrical fibres ​

• Form core inside flagella or cilia

• Composed of the polymer tubulin​

• centrioles:

• Consist of 2 groups of 9 triple microtubules​

• Only in animal cells

• functions

• Microtubules move chromosomes to opposite sides of a cell during cell division and help to construct cell walls.

• In animal cells, centrioles move towards the poles of a cell and serve as anchor points for microtubules during cell division. ​

cytoskeleton features + functions

• features

• Constructed from protein fibers like

  tubulin and actin, which are used to

  make microtubules and microfilaments.

• functions

• Cytoskeleton microfilaments help animal cells to maintain shape.

cell wall features + functions

• features

• extracellular component, not an organelle​

• All plant cells have a cell wall, but also fungi and some protists​

• Consists of the polysaccharide cellulose

• functions

• Permeable – does not affect transport in and out of the cell ​

• Strong – gives support to the cell and prevents plasma membrane bursting when under pressure

prokaryotes vs eukaryotes Genetic material

• Prokaryotes - DNA often circular (plasmids) and nucleoid without proteins (histones)

• Eukaryotes - DNA is linear and associated with proteins (histones) to form chromatin​

prokaryotes vs eukaryotes size

• prokaryotes - small (0.2-0.3 micrometers)

• eukaryotes - large (10-100 micrometers)

prokaryotes vs eukaryotes membrane enclosed organelles

• prokaryotes - no nucleus or any membrane bound organelles, such as mitochondria. Have nucleoid instead of nucleus.

• eukaryotes - Always have membrane-surrounded nucleus and other membrane-bound organelles​

prokaryotes vs eukaryotes uni/multicellular

• prokaryotes - always unicellular

• eukaryotes - often multicellular

prokaryotes vs eukaryotes examples

• prokaryotes - bacterial cells

• eukaryotes - plant cells, animal cells, fungal cells

prokaryotes vs eukaryotes Genetic

• prokaryotes -

• eukaryotes -

prokaryotes vs eukaryotes ribosomes

• prokaryotes - small 70S

• eukaryotes - large 80S

characteristics + functions of life

• Metabolism - sum of all biochemical reactions that occur in a living organism

• Reproduction - production of offspring, sexually/asexually

• Homeostasis - maintenance of a constant internal environment in an organism

• Growth - an increase in size or number of cells

• Response - perception of stimuli and carrying out appropriate reactions in response

• Excretion - removal of waste products of metabolism from an organism

• Nutrition - supplying the nutrients required for energy, growth and repair in an organism

why are Striated muscle cells atypical

have multiple nuclei (multinucleated)

why are aseptate fungal hyphae atypical

have multiple nuclei (multinucleated)

why are red blood cells atypical

dont have a nucleus

why are phloem sieve cells atypical

• dont have a nucleus

• have holes in cell walls

• hardly have any cytoplasm