Cells

• Specialised cells

  • stem cells = undifferentiated cells that differentiate into specialised cells/make copies of themselves
  • differentiation = cells develop specific adaptations allowing it carry out a specific function

  Categories of differentiation

  • Increase/Decrease in no. of specific organelle
  • Change in shape
  • Both
• SA:V single celled organisms

  • large SA:V -> all parts of cell exposed to environment
  • whole cell-surface can act as exchange surface
  • each cell performs all functions

  SA:V Complex/multicellular organisms

  • small SA:V -. not all cells in contact with external environment
  • requires specialised cells to perform specific functions
• Nucleus

  • structure: nucleoplasm with chromatin & nucleolus enclosed by nuclear envelope with nuclear pores & is spherical
  • function (overall) = contain genetic information
  • envelope = encloses & protects DNA
  • pores = allow entry & exit of substances (e.g. mRNA, ribosomes)
  • chromatin = uncondensed DNA & related proteins
  • nucleolus = site of ribosome & RNA production

  A) nuclear pore

  B) chromatin

  C) nucleolus

  D) nucleoplasm

  E) nuclear envelope

  
• Mitochondria

  • Structure: matric enclosed by folded inner membrane (cristae) & surrounded by outer membrane
  • function (overall) = site of aerobic respiration
  • outer membrane = isolates Kreb's cycle & ETC from cytoplasm

  - maintains high conc. of enzymes & substrates to increase rate of respiration

  • inner membrane = allows entry of pyrovic acid & O2 + exit of ATP & CO2 (selectively permeable membrane)
  • matrix contains enzymes for aerobic respiration & small circular DNA & ribosomes for replication

  A) inner membrane

  B) outer membrane

  C) cristae

  D) matrix

  E) circular DNA

  
• Chloroplast

  • structure: stroma containing thylakoid, lamella, granum & starch enclosed by double membrane
  • function (overall) = site of photosynthesis
  • double membrane = isolates photosynthesis
  • thylakoid = contain chlorophyll & absorb energy (site of light-dependent cycle)
  • grana = stacked thylakoids & increases surface area for attachment of enzymes in light dependent cycle
  • lipid = accumulate when membrane breaks down
  • lamellae = join grana together to form granum
• Chloroplast diagram

  A) thylakoid

  B) lamella

  C) granum

  D) starch granule

  E) chloroplast DNA

  F) outer membrane

  G) intermediate membrane

  H) inner membrane

  I) lipid drops

  J) stroma

  K) ribosome

  
• RER & SER

  RER (rough endoplasmic reticulum)

  • structure: interconnected flattened sacs covered in ribosomes
  • function:

  - ribosomes = make proteins

  - RER = folds & processes proteins

  SER (smooth endoplasmic reticulum)

  • structure: interconnected flattened sacs without ribosomes
  • function: synthesises & processes lipids & carbohydrates + processes drugs
• Golgi apparatus & vesicles

  Golgi apparatus

  • structure: fluid-filled membrane bound sacs
  • function = allows internal transport

  - modifies & packages proteins & lipids into vesicles from SER & RER

  - forms vesicles

  - adds carbs to proteins to form glycoproteins

  Vesicles

  • structure: end of golgi body
  • function = protect molecules sent from golgi body when transported across cytoplasm to plasma membrane
• Lysosomes & Centrioles

  Lysosomes

  • structure: sphere of plasma membrane
  • function = contain lysozymes or acid/strong alkali

  - lysozymes hydrolyse pathogens/break down old cell components

  Centrioles

  • structure: hollow fibres made of microtubules & only permanent in animal cells
  • function = form spindle fibres to move chromosomes + organise microtubules to move organelles around cell
• Ribosomes & Plasma/cell-surface membrane

  Ribosomes

  • structure: made of a large subunit & a small subunit made of rRNA & protein
  • found freely in cytoplasm
  • 8OS ribosomes found in eukaryotes
  • function: site of protein synthesis (translation)

  Plasma/cell-surface membrane

  • structure: phospolipid bilayer & is partially permeable
  • function = controls exchange between external & internal cell environment

  - molecules embed within bilayer & attach on outside
• Cell wall & vacuole

  Cell Wall

  • structure: made of cellulose fibres (in plants) & has holes called plasmodesmata
  • plasmodesmata allow transport between nearby cells
  • in fungi -> cell wall made of chitin (N-containing polysaccharide)
  • function = provide structural support + prevent cell bursting from turgor pressure

  Vacuole

  • structure: sac containing water & dissolved salts/sugars surrounded by tonoplast membrane
  • function = maintains turgor pressure in cell & keeps it rigid
• Prokaryotic cells basics

  • 'prokaryo' = before the nucleus
  • no membrane-bound organelles
  • Only bacteria & archea can be prokaryotes
  • 5 structures always present: cell wall, plasma membrane, cytoplasm, (smaller) ribosomes, circular DNA
• Differences between prokaryotes & eukaryotes (1)

  • cell wall made of peptidoglycan
  • Only large circular DNA in nucleoid (area of cell where circular DNA is)
  • slime capsule coats outside of cell wall

  - protects bacteria from other cells by allowing them to stick together

  - prevents cell from drying out
• Differences between prokaryotes & eukaryotes (2)

  • respiration occurs on mesosomes

  - mesosome = inner extensions of cell-surface membrane

  • plasmids are separate, smaller rings of DNA that often code for antibiotic resistance
  • some bacteria have thin fibres called flagella to propel bacteria in different directions
• Viruses

  • considered to be acellular (not a cell) as they don't have usual organelles

  - also considered to not be living

  A) lipid envelope

  B) capsid

  C) attachment proteins

  D) viral genetic information

  E) matrix

  
• Viral replication

  A) attachment

  B) entry

  C) capsid comes apart

  D) replication & gene expression

  E) viral proteins

  F) genome copies

  G) assembly

  H) release

  
• Magnification & resolution

  • magnification = how much bigger the image produced is compared to the real thing

  size of image / size of object

  • resolution = the ability to distinguish 2 points as being separate. This allows you to see detail
• How an optical microscope works

  • use light to form an image
  • max. resolution = 200nm

  - can't see organisms smaller than 0.2 micrometres

  • max. useful magnification = approx. x1500
  • light must be able to pass through specimen
• Limitations of optical resolution

  • sectioning -> some material distorts when cutting into thin sections

  - specimens embedded in wax to support tissue

  • staining -> lot of biological material is colourless

  - stains added to bind to specimen

  - some stains specific to certain cell structures

  - e.g. acetic orcein stains DNA dark red
• Electron microscope basics

  • uses electrons to form images
  • electron beam has shorter wavelength -> greater resolving power -> more detail
  • max. resolution = 0.1nm
  • max. magnification = approx. x1500000
• How a transmission electron microscope (TEM) works

  1. Electron beam focussed onto specimen by electromagnets
  2. beam transmitted through specimen

  certain parts/denser parts of specimen absorb more electrons & appear darker on final image
• Limitations of TEM

  • High energy may destroy specimen
  • system in a vacuum -> can't have living organisms
  • specimen must be extremely thin
  • Artefacts = things resulting from how the specimen was prepared (may show up on photomicrograph -> unreliable)
  • flat, colourless, 2D image (taking multiple photos is a slow & complicated process)
• How a scanning electron microscope works

  1. electron beam scanned across specimen
  2. electrons bounce off surface of specimen & scattered electrons are detected

  produces a 3D image
• Limitations of SEM

  • mostly same as TEM except specimen doesn't need to be thin
  • lower resolution (approx. 20nm)
• Cell fractionation

  • process where cells are broken up & the different organelles they contain are separated out
  • used to study cell structure & function (2 stage process)
• Before cell fractionation

  tissue placed in cold, buffered, isotonic solution

  • cold -> reduces enzyme activity so it doesn't hydrolyse organelles
  • isotonic -> prevents cells bursting/shrinking due to turgor pressure
  • buffered -> pH doesn't alter & denature enzymes/reduces enzyme activity
• Stage 1 of cell fractionation: Homogenisation

  1. Cells broken up by homogeniser (blender) & released from cell membrane
  2. Resultant fluid (homogenate) filtered through gauze to filter out large cellular debris

  • debris may have same density as other organisms & make results unreliable
• Stage 2 of cell fractionation: Ultracentrifugation

  • process where fragments of homogenate are seperated in machine called centrifuge
  • Also called differential centrifugation

  1. Tube of filterate spun at low speed in centrifuge
  2. Heaviest organelles forced to the bottom & form thin sediment/pellet
  3. Fluid at top (supernatant) is removed & transferred & spun in centrifuge at faster speed than before
  4. next heaviest organelle forms pellet
  5. process repeated at increasing speeds until all organelles have separated out

  order: nuclei, (chloroplasts), mitochondria, lysosomes, endoplasmic reticulum & ribosomes
• Cell division

  • Reasons for cell division

  - for growth

  - for replacement

  - for asexual reproduction

  - to produce gametes

  • mitosis = cell division that produces 2 genetically identical daughter cells (for growth, repair & asexual reproduction)
  • meiosis = cell division that produce 4 genetically different cells (producing gametes)
  • mitosis & meiosis result in division of nucleus & cytoplasm (cytokinesis)
  • interphase = phase between divisions including G1, S, G2
• Stages of interphase

  1. G1 (G = gap) phase
  2. cell grows
  3. new organelles form
  4. proteins synthesised
  5. CHECKPOINT 1: checks cell does correct function
  6. 2. S (S = synthesis) phase
  7. A. DNA synthesis
  8. B. DNA is replicated (amount of DNA doubles)
  9. CHECKPOINT 2: checks DNA has replicated/doubled
  10. 3. G2 phase
  11. A. protein synthesis
  12. 4. Mitosis -> cytokinesis
• Cancer

  tumours = result of uncontrolled division of genetically abnormal cells

  • 'genetically abnormal' as they have mutations
  • mutations in genes regulating cell division may lead to tumour forming
  • Cancer genes:

  tumour suppressor genes = genes that stop inappropriate cell growth

  oncogenes = genes that promote cell growth
• Cancer treatment

  • Often involves blocking a part of the cell cycle

  - preventing DNA replicating

  - inhibit metaphase of mitosis by interfering with spindle formation

  • problem: also disrupts cell cycle of normal cells

  - HOWEVER drugs are more effective against rapidly dividing cells as cancer cells have fast dividing rate typically & so are damaged to a greater degree

  - HOWEVER normal cells that are rapidly dividing are also vunerable to damage (e.g. hair-producing cells)
• Chromosomes

  • chromatin = loosely coiled DNA and associated histone proteins
  • during cell division -> chromatin supercoils & condenses & become visible
  • occur in homologous pairs -> humans have 23 pairs (2 versions of each chromosome -> 1 maternal and 1 paternal)
  • sex chromosomes are non-homologous

  Mitotic index = (no. of cells with condensed chromosomes/total no. of cells) x 100
• Mitosis: Interphase

  • Organelles multiply
  • Chromosomes are copied (DNA replication)

  Mitosis: prophase

  • Chromosomes condense
  • nuclear membrane breaks down
  • centrioles move to poles & form spindle fibres

  Mitosis: Metaphase

  • Chromosomes line up along the equator of the cell
  • chromosomes attach to spindle fibres via the centromere
• Mitosis: Anaphase

  • Spindle fibres shorten & separate sister chromatids
  • each chromosome moves to an opposite pole of the cell

  Mitosis: Telophase

  • Chromosomes reach poles of cell
  • Nuclear envelopes start to form
  • Chromosomes begin to decondense
  • Spindle fibres break down

  Mitosis: cytokinesis

  • Cytoplasm & cell membrane divide
  • 2 genetically identical daughter cells are formed
• Structure of membranes: phospholipids

  • most organelles have single membrane except mitochondria, chloroplasts & nucleus + ribosomes have no membrane
  • made of phospholipids

  - allow lipid-soluble substance to diffuse in/out of cell

  - prevent water-soluble molecules leaving -> hydrophobic tails

  - makes membrane flexible (fluid) & self-healing

  • membrane always ~7nm
• Structure of membrane: proteins

  • extrinsic proteins = occur in surface of lipid bilayer & act as receptors with glycolipids or mechanical support
  • intrinsic proteins = span from one side of the phospholipid bilayer to other (are vertical)

  - can act as channel proteins = water filled tubes allowing water-soluble ions to diffuse across membrane

  - can act as carrier proteins = bind to ions/molecules & change shape to move molecules across
• Function of proteins in membrane:

  • structural support
  • channel & carrier proteins -> allow active transport
  • cell-surface receptors + for cell identification
• Structure of membrane: cholestrol

  • randomly distributed across bilayer between phospholipids
  • prevents water loss -> very hydrophobic molecule
  • helps regulate fluidity of membrane

  - pulls fatty acid tails together -> limits movement without making membrane rigid

  - stabilises phospholipids
• Glycolipids/glycoproteins

  • carb chains added to lipids/extrinsic proteins
  • act as cell-surface receptors
  • help cells attach to each other
  • allows cells to recognise each other