cell structure

• eukaryotic cells have a more complex ultrastructure than prokaryotic cells
• animal and plant cells both have membrane-bound organelles including a nucleus and larger ribosomes (80S ribosomes)
• animal cells contain centrioles and some have microvilli while plant cells do not

  microvilli are folded regions of the cell surface membrane that increase cell surface area for absorption e.g small intestine
• plant cells have a cellulose cell wall, large permanent vacuoles, and chloroplasts while animal cells do not
• plasma membrane

  structure: bilayer of phospholipids → fluid mosaic model

  function: controls entry and exit of substances between internal cell environment and external environment, through faciliated diffusion + simple diffusion, channel proteins + active transport, carrier proteins + endo/exocytosis

  
• nucleus

  structure: surrounded by a double bilayer membrane, contains channel proteins/pores

  function: contains chromosomes and is the site of transcription

  
• nuclear membrane

  structure: double bilayer membrane, containing channel proteins and nuclear pores

  function: allows mRNA to leave the nucleus and move to the ribosome
• nucleolus

  structure: small spherical structure that appears dense within the nucleus, contins rRNA

  function: involved in synthesis of ribosomes
• mitochondria

  structure: surrounded by a double membrane w the inner membrane folded to form structures called cristae

  function: site of aerobic respiration, matrix of mitochondria contains enzymes needed for aerobic respiration, producing ATP

  small circular pieces of DNA (mitochondrial DNA) and ribosomes found in matrix, needed for replication of mitochondria before cell division

  
• ribosomes

  can be found as free organelles in cytoplasm or part of rough endoplasmic reticulum

  structure: not surrounded by a membrane, complex of ribosomal RNA and proteins, 80S in eukaryotic, 70S in prokaryotes, mitochondria, chloroplasts

  function: reads codons on mRNA molecule, translates them into a primary protein sequence, produces proteins for inside the cell

  
• smooth endoplasmic reticulum

  structure: formed from folds of membrane

  function: involved in production, processing and storage of lipids, carbs and steroids

  
• rough endoplasmic reticulum:

  structure: formed from folds of membrane continuous with the nuclear envelope, surface covered in ribosomes

  function: process proteins made on ribosomes
• vesicle

  structure: single membrane

  function: transport substances within the cell to/from the cell for endo/exocytosis
• lysosome

  structure: surrounded by a single bilayer, specialist forms of vesicle

  function: contains hydrolytic enzymes, breaks down waste materials (worn-out organelles), used by cells of immune system and in programmed cell death (apoptosis)

  
• centriole

  structure: made of microtubules (filaments of protein used to move substances around inside a cell, supports shape of a cell from inside)

  function: two centrioles at right angles to each other form a centrosome which organises the spindle fibres during cell division

  
• cell wall

  • formed outside of the cell membrane, offer structural support to the cell
  • structural support provided by cellulose
• middle lamella

  • forms outermost layer of plant cell, acts like glue to stick adjacent plant cells together
  • provides stability

  
• plasmodesmata

  • narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells
  • allows substances to be transported between plant cells and facilitates cell to cell communication
• pits

  • thin regions of the cell wall
  • pits in adjacent plant cells lined up in pairs
  • facilitates transport of substances between cells

  
• golgi apparatus

  structure: consists of flattened sacs of membrane, regular, stacked appearance

  function: modify proteins and lipids before packaging them into golgi vesicles the vesicles then transport the proteins and lipids to their required destination

  
• chloroplasts

  structure: larger than mitochondria, surrounded by double membrane, membrane bound compartments (thylakoids) containing chlorophyll stack to form grana, grana joined together by lamellae (thin and flat thylakoid membranes)

  function: site of photosynthesis, certain parts of photosynthesis occurs in thylakoid membranes, other parts happen in stroma, small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

  
• amyloplasts

  structure: small, membrane bound organelle containing starch granules

  function: responsible for storing starch in plants and converting it back to glucose when the plant needs it

  
• vacuole and tonoplast

  vacuole is a sac in plant cells surrounded by the tonoplast (selectively permeable membrane)

  contains cell sap (made up of water, minerals, waste, enzymes)

  vacuoles keep cells turgid, stops the plant from wilting

  breaks down and isolates unwanted chemicals in plant cells

  tonoplast controls what can enter and leave

  
• prokaryotic cells are much smaller than eukaryotic cells
• prokaryotic cells differ from eukaryotic cells having

  • cytoplasm that lacks membrane-bound organelles
  • ribosomes are smaller (70S)
  • no nucleus, has single circular bacterial chromosome that is free in the cytoplasm and not associated with proteins
  • cell wall contains murein/peptidoglycan
• plasmids

  small loops of DNA separate from main circular DNA molecule

  contains genes that can be passed between prokaryotes (e.g genes for antibiotic resistance)

  not present in all prokaryotes
• capsule

  some prokaryotes (bacteria) are surrounded by a final outer layer known as a capsule

  helps protect bacteria from drying out and from attack by cells of the immune system of the host organism

  not present in all prokaryotes
• flagellum

  long hair-like structure that rotates, enabling the prokaryote to move

  some prokaryotes have more than one

  not present in all prokaryotes
• pilus (pili)

  thread like structures on the surface of some bacteria that enable the bacteria to attach to other cells or surfaces
• mesosomes

  infolded regions in the plasma membrane of some prokaryotic cells

  aerobic respiration, cell wall formation, DNA replication
• circular DNA

  genetic material of prokaryotic cells mainly consists of a single circular strand of DNA that is not contained in a membrane-bound nucleus

  the area in the prokaryotic cell where this circular DNA molecule is found is known as the nucleoid
• ribosomes

  prokaryotic ribosomes are 70S

  site of protein synthesis in prokaryotic cells
• prokaryotic cells differ from eukaryotic cells having

  • cytoplasm that lacks membrane-bound organelles
  • ribosomes are smaller (70S)
  • no nucleus, has single circular bacterial chromosome that is free in the cytoplasm and not associated with proteins
  • cell wall contains murein/peptidoglycan