structrue of prokaryotic and eukaryotic cells 2

endomembrane system: list organelles

• plasma membrane

• endoplasmic reticulum

• golgi body

• transport vesicles

• nuclear envelope

• microbodies

endoplasmic reticulum

• network of flattened sacs and cisternae

• connected to outer nuclear envelope

• synthesises membrane lipids

• export of molecules to the outside of cells

two components of endoplasmic reticulum

• rough ER

• ribosomes attached → gives bumpy appearance

• protein synthesis

• smooth ER

• no ribosomes attached

• lipid synthesis

golgi body

• stacks of cisternae

• cis face → points towards the endoplasmic reticulum

• trans face → points towards the plasma membrane

cis vs trans face

• cis face → where vesicles arrive from the ER

• trans face → where transport vesicles pinch off and carry proteins and other molecules towards the plasma membrane

proteins can be (?) as they travel through the golgi

glycosylated → sugar group added

exocytic pathways (unregulated vs regulated)

• unregulated

• used by all eukaryotic cells

• secrete soluble proteins

• supply new synthesised lipids and proteins to the plasma membrane

• regulated

• used by specialised eukaryotic cells

• store proteins in vesicles until a signal stimulates secretion

endocytic pathway

• part of the plasma membrane buds inwards to form a vesicle

• entrapping molecules from outside the plasma membrane

• molecules are taken up by cell

• ultimately molecules are digested in lysosome

endocytosis for viruses

• viruses can enter the cell

• binds to protein receptors on the outside of the cell membrane

• allows entry of viruses into the cell

microbodies

• single-membrane bound organelles

• e.g. peroxisomes, glyoxysomes, glycosomes, hydrogenosomes

• contains enzymes → detoxification (liver), photorespiration (carbon recycling in plants)

double membrane organelles

• nucleus

• mitochondria

• chloroplast

nucleus

• surrounded by a nuclear envelope

• outer membrane of nuclear envelope connected to the ER

• material exchange with the cytosol via nuclear pores

• contains most cellular DNA that becomes visible as condensed chromosomes during cell division

• transcription of DNA into RNA

• partial ribosome assembly in the nucleolus

mitochondria

• site of cellular respiration and energy (ATP) production

• outer membrane → permeable to ions and small molecules

• inner membrane → highly folded into cristae and is impermeable: requires transport proteins

• matrix → contains DNA that codes for RNA, mitochondrial ribosomes, and enzymes for the citric acid and krebs cycle

energy production in the mitochondria

1. pyruvate (from glycolysis) enters the mitochondrial matrix where it is oxidised to acetyl CoA

2. in the matrix: acetyl CoA is further oxidised in the matrix via the citric acid cycle → NADH and CO2

3. in the inner mitochondrial membrane: electron transport chain uses NADH in oxidative phosphorylation to create a pH gradient

• oxidation of NADH to NAD+

• reduction of O2 to H2O

4. pH gradient enables ATP synthase to produce lots of ATP: ADP + Pi → ATP

how do mitochondria and chloroplasts differ in their number of membranes?

• mitochondria → 2 membranes

• chloroplasts → 3 membranes

chloroplasts

• outer membrane → permeable

• inner membrane → impermeable: requires transport proteins

• stroma: surrounded by inner membrane

• contains chloroplast DNA, ribosomes, RNA, enzymes

• contains thylakoids stacked into grana → photosynthesis

mitochondria and chloroplasts similarities

• both have an inner and outer membrane

• both have a space surrounded by the inner membrane: mitochondrial matrix and chloroplast stroma

• matrix and stroma contain circular DNA

• ribosomes

• DNA replication and transcription enzymes

• enzymes for metabolism

• both have an electron transport chain and an ATP synthase to generate ATP

mitochondria and chloroplasts differences

• mitochondria uses NADH

• chloroplasts produce and use NADPH

• mitochondria use energy from food

• chloroplasts use energy from light

• chloroplasts can synthesise sugars whereas mitochondria cannot

photosynthesis stage 1

• thylakoid membranes contain two photosystems PSI and PSII

• these contain light harvesting pigments such as chlorophyll that absorb light energy (photons)

• light reaction: light energy is converted to reducing power (NADPH) and chemical energy (ATP)

• via a series of oxidation-reduction reactions

photosynthesis stage 2

• in the stroma: NADPH & ATP from the light reactions and atmospheric CO2 are used by the calvin cycle to produce sugars

• the enzyme rubisco catalyses the first reaction in the calvin cycle

• overall: light energy + CO2 + H2O → sugars + O2

evolution of membrane closed organelles

1. ancient archaeal cell → expansion of the plasma membrane forming protrusions to better exchange metabolites with environment

2. segments between protrusions pushed themselves inwards → gave rise to intracellular membrane surrounded spaces

3. this gave rise to organelles

• surrounding the genomic DNA → nucleus

• surrounding aerobic bacteria → mitochondria

• surrounding photosynthetic bacteria → chloroplasts

• forming endomembrane system

evolution of mitochondria and chloroplasts

• ancestral eukaryotic cell ingested an aerobic bacterium → over time evolved into a mitochondrion

• plant cells later ingested a photosynthetic bacterium → over time evolved into a chloroplast