Nucleus
Strucutre
- a jelly-filled sac surrounded by a double membrane called the nuclear envelope
- nuclear envelope has nuclear pores
- nucleoplasm: granular, jelly-like material
Function
- stores the cell’s DNA in the form of chromatin (DNA + histone proteins)
- chromosomes in the nucleus are linear and protein-bound
- site of DNA replication and transcription
- pores allow substances to move between nucleus and cytoplasm
Ribosomes
Structure
- small spherical organelles with no membrane
- made up of two sub-units of protein and ribosomal RNA
- 80S: large found in eukaryotic
- 70S: smaller, found in prokaryotic, mitochondria, chloroplasts
- some are free-floating, while others are attached to the rough endoplasmic reticulum (RER)
Function
- ribosomes attached to the RER make proteins that are secreted
- free ribosomes make proteins to be used within the cell
- where proteins are made
Mitochondrion
Strucutre
- it has two membranes
- the inner membrane is highly folded into cristae to increase the surface area over which chemical reactions occur
- contains an enzyme-rich liquid called the matrix
- contains a loop of mitochondria DNA
- 70S ribosomes
Function
- site of aerobic respiration where ATP is produced
- the more energy demanding a cell is the more mitochondria it has
- DNA to code for enzymes needed in respiration
Nucleolus
- smaller sphere inside the nucleus
- no membrane
- site rRNA production and where ribosomes are made
- made of RNA
Endoplasmic reticulum
- network of membranes connected to the nuclear membrane
- cisternae: flattened membrane sacs filled with fluid
- may be smooth: synthesizes and stores lipids, cholesterol, steroid hormones and carbs
- may be rough: attached ribosomes make proteins that the RER then passes onto the Golgi body
Golgi apparatus and vesicles
Structure
- a stack of folded membranes that make cisternae
- takes in sacs of raw material from ER
- secretary vesicles pinch off from the cisternae
Function
- modifies, packages and transports proteins ready for the cell to use or to be secreted
- material enters and leaves Golgi by vesicles
- Golgi makes lysosymes
- add carbohydrates to proteins to form glycoproteins
- produce secretary enzymes
- molecules are ‘labelled’ with their destination
Lysosomes
Strucutre
- sacs filled with hydrolytic (digestive) enzymes
- surrounded by a membrane to keep enzymes separate from the cytoplasm of the cell
Function
- digest worn out cell parts and ingested pathogens
- hydrolyse pathogens in phagosomes
- digest food absorbed by cell
- exocytosis: release enzymes to the outside of the cell to destroy material
Cell-surface membrane
Structure
- mainly made up of lipids and proteins
- phospholipid bilayer
Function:
- controls movement of substances into and out of the cell, the membrane is partially permeable
- has receptor molecules which allow it to respond to chemicals
Chloroplast
Structure
- contains the green pigment chlorophyll
- enclosed by a double membrane (chloroplast envelope), it is very selective
- Internal thylakoids membranes arranged in stacks to form grana which is where chlorophyll is contained.
- grana stacks surrounded by a fluid- filled matrix called stroma and linked by lamallae
Function
- site of photosynthesis
- First stage (light absorption) occurs in grana
- Second stage (sugar synthesis) occurs in stroma
Vacuoles
Structure
- huge fluid-filled sac filled with water, dissolved solutes and amino acids (cell sap)
- single membrane around it is called the tonoplast
Functions
- helps to maintain pressure inside the cell and keep the cells rigid, this stops plants wilting
Cell wall
Strucutre
- made from cellulose in plants and algae and chitin in fungi
- freely permeable (does not control what enters and leaves the cell like the partially permeable plasma membrane
- middle lamella: thin layer between two adjacent cell walls and cements adjacent cells together
Functions
- provides cell with mechanical strength, to prevent the cell bursting under pressure created by osmotic entry of water
- give mechanical strength to the whole plant
- to allow water to pass along it and so contribute to movement through the plant
Comparison of prokaryotic and eukaryotic cells
Prokaryotic cells:
- no membrane-bound organelles
- cell wall made of murein
- Divide by binary fission
- 0.1 - 5um
- naked DNA (DNA is not associated with proteins
- 70S ribosomes
- simple cytoskeleton
- whip-like structures called flagella
- slimy outer coating for protection called capsule
- much smaller than eukaryotic
- unicellular but can live as colonies
- circular DNA and small loops of DNA called plasmids
Eukaryotic cells:
- membrane-bound organelles
- cell wall made of cellulose (or chitin in fungi)
- divide by mitosis
- 10-100um
- DNA is associated with proteins called histones
- 80S ribosomes
- complex cytoskeleton
- whip-like structures called cilia and undulipodia
- no capsule
- bigger than prokaryotic cells
- multicellular
- contians nucleus and no plasmids
Viruses
- known as virus particles
- acellular, non-living particles
- they contain nucleic acids such as DNA or RNA, which is enclosed within a protein coat (capsid)
- smaller than prokaryotic cells
- some viruses are surrounded by a lipid coat
- have to invade host cells and replicate using the host cell’s machinery
- attachment proteins on the surface of the lipid coat or capsid allow the virus to bind to a host cell
