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3.2 Cells - Coggle Diagram
3.2
Cells
3.2.1.1 Structure of Eukaryotic Cells
Cell Surface Membrane
Phospholipid bilayer with embedded proteins etc
Selectively permeable (enables control of passage of substances in and out of cell)
Barrier between internal and external environment of cell
Nucleus
Nuclear envelope, nuclear pores, nucleolus, DNA/chromatin
Controls the cell's activity through transcription of mRNA
Nuclear pores allow substances (e.g. mRNA) to move between the nucleoplasm and cytoplasm
Nucleolus makes ribosomes which are made up of proteins and ribosomal RNA
Mitochondria
Double membrane - inner membrane folded to form cristae. Matrix containing small 70S ribosomes, small circular DNA and enzymes involved in aerobic respiration (glycolysis)
Site of aerobic respiration producing ATP for energy release
Golgi Apparatus
3 or more fluid filled membrane bound sacs with vesicles at edge
Receives protein from RER
Modifies/processes protein (e.g. add carbohydrates/sugars)
Packages into vesicles (e.g. for transport to cell surface membrane for exocytosis)
Also makes lysosomes
Lysosomes
Type of golgi vesicle containing lysozymes (hydrolytic enzymes)
Release of lysozymes to break down/hydrolyse pathogens or worn out cell components
RER (Rough Endoplasmic Reticulum)
Ribosomes bound by a system of membranes
Folds polypeptides to secondary/tertiary structure
Packages to vesicles, transport to the golgi apparatus etc.
SER (Smooth Endoplasmic Reticulum)
Similar to RER but without ribosomes (system of membranes)
Synthesises and processes lipids
Ribosomes
Float free in cytoplasm or bound to RER
Not membrane bound
Made from 1 large and 1 small subunit
Site of protein synthesis (specifically translation)
Chloroplasts (plant/algal)
Thylakoid membranes are stacked up in some parts to form grana, which are linked by lamellae. They sit in the stroma (fluid) and are surrounded by a double membrane.
Also contains starch granules and circular DNA
Chlorophyll absorbs light for photosynthesis to produce organic substances
Cell Wall (plant/algal/fungal)
Mainly made of cellulose (plants/algae) and chitin (fungi)
Rigid structure surrounding cells in plants, algae and fungi.
Prevents the cell changing shape and bursting (lysis)
Cell Vacuole (plant)
Contains cell sap (a weak solution of sugars and salts - surrounding membrane is called the tonoplast)
Maintains pressure in the cell (stops wilting)
Stores/isolates unwanted chemicals in the cell
Organisation of specialised cells in complex multicellular organisms...
Specialised cell = the most basic structural/functional subunit in all living organisms, specialised for a particular function
Tissue = group of organised specialised cells joined and working together to perform a particular function (often with the same origin)
Organ = group of organised different tissues joined and working together to perform a particular function
Organ system = group of organised organs working together to perform a particular function
E.g. Epithelial cells in the small intestine are specialised for efficient absorption. Villi and microvilli increase surface area. Lots of mitochondria to provide energy (for active transport).
3.2.1.3 Methods of Studying Cells
Optical Microscope...
Limitations -
2D image
Only used on thin specimens
Low resolution; can't see internal structures of organelles or organelles smaller than 200nm (e.g. ribosomes)
Low magnification
Principles
Use light to form a 2D image
Visible light longer wavelength so lower resolution 200nm
Low magnification x1500
Positives +
Can see living organisms
Transmission Electron Microscope (TEM)...
Limitations -
2D image
Vacuum; can't see living organisms
Only used on thin specimens
Principles
Use electrons to form a 3D image
Electromagnets focus beam of electrons onto specimen, transmitted, more dense = more absorbed = darker appearance
Electrons shorter wavelength (so higher resolution 0.2nm)
High magnification x150000
Positives +
High resolution; see internal structures of organelles
High magnification
Scanning Electron Microscope (SEM)...
Limitations -
Vacuum; can't see living organisms
Lower resolution than TEM
Principles
Use electrons to form a 2D image
Beams of electrons scan surface, knocking off electrons from the specimen, which gathered in a cathode ray tube to form an image
Electrons shorter wavelength (so higher resolution 0.2nm)
High magnification x1500000
Positives +
3D image
High resolution; can see internal structures of organelles
High magnification
Used on thick specimens
Magnification = how much bigger the image of a sample is compared to the size, measured by MAGNIFICATION = IMAGE SIZE / ACTUAL SIZE
Resolution = how well distinguished an image is between 2 points (shows the amount of detail). It is limited by the wavelength of radiation (e.g. light)
Cell Fractionation
STEP 1...
Homogenise the tissue using a blender
Disrupts the cell membrane (breaks the cell open)
Releases organelles
STEP 2...
Place in a cold, isotonic, buffered solution
Cold = reduces enzyme activity so organelles aren't broken down
Isotonic = so water doesn't move in or out of the organelles by osmosis (so they don't burst or shrivel)
Buffered = keeps pH constant so enzymes don't denature
STEP 3...
Filter homogenate
Remove large, unwanted debris (e.g. whole cells, connective tissues)
STEP 4...
Ultracentrifugation
a) Centrifuge homogenate in a tube at a low speed
b) Remove pellet of heaviest organelle and spin supernatant at a higher speed
c) Repeat at higher and higher speeds until organelles are separated out (each time a pellet is made out of lighter organelles)
d) Separated in order of mass/density:
NUCLEI :arrow_right:CHLOROPLASTS :arrow_right: MITOCHONDRIA :arrow_right: LYSOSOMES :arrow_right: ENDOPLASMIC RETICULUM :arrow_right: RIBOSOMES
There was a considerable period of time during with the scientific community distinguished between artifacts and cell organelles. They repeatedly prepared specimens in different ways, and if an object could be seen with one preparation technique but not another it was more likely to be an artefact than an organelle.
Sizes of objects viewed by an optical microscope...
Line up eyepiece graticule with stage micrometer
Use stage micrometer to calculate the size of divisions on eyepiece graticule at a particular magnification
Take the micrometer away and use the graticule to measure how many divisions make up the object
Calculate the size of the object by multiplying the number of divisions by the size of division
Recalibrate eyepiece graticule at different magnifications
Preparing a 'temporary mount' of a specimen on a slide...
Use tweezers to place a thin section of specimen (e.g. tissue) on a water drop on a microscope slide
Add a drop of stain (e.g. iodine in potassium iodide solution used to stain starch grains in plant cells)
Add a cover slip by carefully tilting and lowering it, trying not to get any air bubbles
3.2.1.2 Structure of Prokaryotic Cells and Viruses
Organelles of a typical prokaryote:
Cell wall (murein)
(Slime) capsule
Plasmids
Circular DNA
Cell-surface membrane
Flagellum
Smaller ribosomes
Cytoplasm
How do prokaryotes differ from eukaryotes?
Prokaryotes: cytoplasm contains no membrane bound organelles (e.g. mitochondria)
Eukaryotes: cytoplasm contains membrane bound organelles
Prokaryotes: have no nucleus (contains free floating DNA)
Eukaryotes: have a nucleus containing DNA
Prokaryotes: circular DNA, not associated with proteins
Eukaryotes: linear DNA, associated with proteins (histones)
Prokaryotes: cell walls contain murein and peptidoglycan
Eukaryotes: cell walls contain cellulose
Prokaryotes: smaller ribosomes
Eukaryotes: larger ribosomes
Prokaryotes may have one or more plasmids, a capsule and one or more flagella
Viruses
Acellular = not made of or able to be divided into cells
Non-living = unable to exist and reproduce without a host cell
Features of a viral particle:
Attachment proteins
Capsid
DNA or RNA
3.2.2 All Cells Arise from Other Cells
3.2.3 Transport Across Cell Membranes
3.2.4 Cell Recognition and the Immune System