3.2.1 Cell Structure: 3.2.1.1 Structure of eukaryotic cells Flashcards
What do eukaryotic cells in complex multicellular organisms do?
Specialise to their specific function
Levels of organisation
Specialised cells -> tissues -> organs -> organ system
Organelles in plant cells
- Cell-surface membrane
- Cell wall
- Smooth endoplasmic reticulum
- Rough endoplasmic reticulum
- Nucleus
- Nucleolus
- Nuclear envelope
- mitochondria
- ribosomes
- Golgi apparatus
- Vacuole
- Chloroplast
- Cytoplasm
- Plasmodesma
Organelles in animal cells
- Cell-surface membrane
- Smooth endoplasmic reticulum
- Rough endoplasmic reticulum
- Nucleus
- Nucleolus
- Nuclear envelope
- mitochondria
- ribosomes
- Golgi apparatus
- Lysosome
- Cytoplasm
Nucleus structure
10-20 μm diameter
Nucleus function
- Acts as a control centre of the cell through the production of mRNA and tRNA
- retains genetic material in the cell as DNA + chromosomes
- manufactures rRNA and ribosomes
Nucleolus
- small spherical region within the cytoplasm
- manufactures RNA and assembles ribosomes
- may be more than 1 in a nucleus
Nucleoplasm
Granular + jelly like material which makes up bulk of the nucleus
Nuclear membrane/envelope
- Double membrane surrounding the nucleus
- Outer membrane is continuous with RER
- Controls entry + exit of materials in + out of the nucleus
Nuclear pores
- Allow the passage of large molecules (e.g. mRNA) out of the nucleus
Diameter of nuclear pore
40-100μm
Mitochondrion structure
- Rod-shaped
*1-10 μm diameter - Has a double membrane which controls movement of substances in + out of the mitochondria
- Inner membrane is folded to form extensions called cristae
Mitochondrion function
- Site of aerobic respiration
- where energy is released/ATP is produced
- high in number as lots of ATP required for metabolic reactions
Cristae structure
- extension of inner membrane of mitochondrion
Cristae function
provides a large S.A. for attachment of enzymes + other proteins involved in the reaction
Matrix
inner fluids which contains lipids, proteins, ribosomes, DNA
Chloroplasts structure
- disc-shaped
- chloroplasts envelope
- grana
- stroma
Chloroplasts function
carry out photosynthesis
Chloroplast envelope
- Double plasma membrane that surrounds the chloroplast
- Highly selective
Grana structure
- stacks of thylakoids
- where 1st stage of photosynthesis takes place
Thylakoids
Disc-like structures containing chlorophyll
Adaptations of grana
- Large S.A. for attachment of chlorophyll
- fluid in the stroma contains all enzymes needed to make sugars in 2nd stage of photosynthesis
- chloroplasts contain both DNA + ribosomes so proteins for photosynthesis can be easily be manufactured
Rough endoplasmic reticulum structure
- Continuous with outer nucleur membrane
- Has ribosomes on the outer membrane
Rough endoplasmic reticulum: function
- provide large S.A or synthesis of proteins + glycoproteins
- provide a pathway for transport of materials (mainly proteins) out of the cell
Smooth endoplasmic reticulum: structure
no ribosomes on surface and more tubular
Golgi apparatus: structure
made up of cisternae (flattened sacs) with vesicles (small rounded hollow structures)
Golgi apparatus: process
- Proteins + lipids produced by the ER are passed through the Golgi apparatus in strict sequence
- Golgi apparatus modifies these (by adding non-protein components e.g. carbohydrates to them)
- Golgi apparatus labels them -> allows them to be accurately sorted + sent to their correct destination
- These modified + labelled proteins + lipids are transported in Golgi vesicles (which are regularly pinched off the ends of the Golgi cisternae)
- These vesicles (phagosomes formed in phagocytosis) may move to the cell surface, where they fuse with the membrane + release their contents to the outside
Golgi apparatus: functions
- add carbohydrates to proteins to form glycoproteins
- produce secretory enzymes (e.g. those secreted by the pancreas)
- secrete carbohydrates (e.g. those in the plant cell wall)
- form lysosomes
Smooth endoplasmic reticulum: function
synthesise, store and transport lipids + carbohydrates
Lysosomes: structure
- Formed when vesicles produced by the Golgi apparatus contain enzymes (e.g. protease, lipase, lyzozymes)
- 50 μm diameter
- isolate the enzymes from the rest of the cell before releasing to the outside or into a phagocytic vesicle in the cell wall
Lysosymes
enzymes that hydrolyse the cell walls of certain bacteria
Lysosomes process
- Primary lysosymes formed by the Golgi apparatus contain hydrolytic enzymes
- These enxymes hydrolyse the particle in the vesicle
- Soluble products are absorbed into cytoplasm. Insoluble debris is egested
Lysosomes: functions
- hydrolyse material ingested by phagocytic cells (e.g. white blood cells)
- release enzymes in the outside of the cell (exocytosis) to destroy material around the cell
- digest worn out organelles so that useful chemicals that they are made of can be recycled
- completely break down dead cells (autolysis)
Number of lysosomes in phagocytic cells
High
Ribosomes: structure
- Small cytoplasmic granules in cytoplasm or in RER
- 2 subunits: 1 large, 1 small -> both containing ribosomal RNA and protein
Types of ribosomes
- 80S = in eukaryotic cells (bigger)
- 70S = in prokaryotic cells, mitochondria, chloroplasts (smaller)
Cell wall: structure
contain microfibrilis
Cell wall: function
- provides mechanical strength to prevent the cell from bursting due to osmotic entry of water
- gives mechanical strength to plant
- allows water to pass through it
Vacuole: structure
- fluid filled sac bound by a single membrane (tonoplast)
- contains mineral salts, sugars, amino acids, wastes
Vacuole: function
- support herbaceous plant
- sugars + amino acids act as temporary food store
Microscopes
instruments that produce a magnified image of an object
Image
the appearance of the material under the microscope
Object
material under microscope
Magnification
how many times bigger the image is when compared to an object
Magnification equation
size of image/size of real object
- must be the same units
Resolution/resolving power
- minimum distance apart that 2 objects can be in order for them to appear as separate items
- dependant on the wavelength or radiation used
- each microscope has a limit of resolution
Relationship between resolution and clarity
Higher resolution, higher clarity
Cell fractionation
Process where cells are broken up and the different organelles in the cell are separated out
Before cell fractionation:
The tissue is placed in a solution which is:
* Cold
* Buffered
* Isotonic
Solution for cell fractionation: cold
To reduce enzyme activity that might break down organelles
Solution for cell fractionation: buffered
To prevent pH changes which could alter the organelle’s structure or affect enzyme functionality
Solution for cell fractionation: isotonic
same water potential as tissue, to prevent osmotic gain or loss of water causing bursting or shrinking of organelle
Stages of cell fractionation
- Homogenization
- Ultracentrifugation
Stages of cell fractionation: Homogenisation
- Cells are broken up in a homogeniser (blender) to release organelles
- The homogenate (resultant fluid) is filtered to remove any complete cells or large pieces of debris
Ultracentrifugation definition
Process by which fragments in the filtered homogenate are separated in a centrifuge machine
Centrifuge
- Spins tubes of homogenate at high speed to create a centrifugal force
- Tubes must be places at opposite ends of the centrifuge to balance out the force
Stages of cell fractionation: Ultracentrifugation
- Tube of filtered homogenate is placed in a centrifuge and spun at low speed
- The heaviest organelles (nuclei) go to the bottom and form a sediment
- The fluid at the top (supernatant) is removed and transferred to a different tube
this process is repeated with an increase in speed each time and for a longer duration
Light microscope
- Uses wavelength of light (long wavelength) to form an image
- Can see organelles
What organelles can be seen under a light microscope
Cell wall
Nucleus
Cell membrane
Cytoplasm
Electron microscope
- Uses wavelength of electron beams (short wavelength) to form an image
- As electrons have a negative charge, they can be focussed using electromagnets
- object is put in a vacuum - as electrons can be absorbed or deflected by air
2 types of electron microscope
- Transmission electron microscope (TEM)
- Scanning electron microscope (SEM)
Transmission electron microscope
- produces a beam of electrons which is focussed on a specimen by a condenser magnet
- the beam passes through thin parts of the specimen = bright, and the beam is absorbed by thick parts of the specimen = dark
Photomicrograph (TEM)
photograph of image produced on screen
Reasons why highest resolution cannot always be achieved in transmission electron microscope
- difficulties preparing the specimen
- high electron beam required may destroy the specimen
Limitations of transmission electron microscope
- Whole system must be in a vacuum
- Specimen must be thin
- Image not in colour
- Image may contain artefacts due to specimen preparation but do not exist in original sample
- Initial image is 2D but can be built into 3D by taking a series of sections through a specimen
Limitations of scanning electron microscope (SEM)
Same as TEM, but specimens do not need to be thin
Scanning electron microscope
- produces a beam of electrons which is directed on the surface of the specimen from above
- beam is passed back + forward across the specimen in a regular pattern
- the electrons are scattered by the specimen (depending on the contours in the specimen -> can be built into 3D image on computer
Resolving power of TEM vs SEM
SEM has a lower resolving power
Eyepiece graticule
used to measure the size of objects using a light microscope
Stage micrometer
microscope slide used to calibrate the eyepiece graticule
