1. A2.2 Cell structures (cellular units)

Nucleus

• Largest organelle (10-20µm in diameter)

• Surrounded by a nuclear envelope

• Double-membrane bound

• Contains nuclear pores (100nm in diameter)

• Contains chromosomes (during cell division)

  • made of DNA bound to histone proteins

  • histones pack DNA into condensed structures for movement

• Contains chromatin (diffuse network) at other times

• Nucleolus (singular) / nucleolus (plural)

• Function

  • Site of ribosome subunits synthesis

  • Control cell activities and division

• Exception

  • Red blood cells

  • Phloem sieve tubes

Mitochondria

• bean shaped (0.5-1.5µm wide, 3.0-10µm long)

• present in all cells

• more abundant in

  • metabolically active cells

  • muscles and liver cells

• Double-membrane bound

• Outer membrane is smooth

• Inner membrane is folded to form CRISTAE (inner foldings)

• Matrix (cytoplasm of mitochondrion)

  • aqueous solution of metabolites and enzyme

• Functions: site of aerobic respiration

Ribosomes

• Non-membrane bound (25nm)

• Made up of 2 subunits

• Chemically consist of protein & RNA

• 2 forms — bound ER & free floating in cytoplasm

• Can also occur in mitochondrion & chloroplasts (in size 70S)

• The size recorded in Svedberg units (S) (rate of sedimentation in centrifugation

• Function: Site of protein synthesis (translation)

Endoplasmic reticulum

• A network of interconnected folded membranes into sheets, tubes or sacs

• Contiguous with the nuclear envelope

• Abundant in metabolically active cells

• 2 types - rough and smooth ER

Smooth ER

• Consists of a branched network of tubular membranes

• Not studded with ribosomes

• Function: manufacture of lipids, phospholipids & steroids

• In muscles cells, it functions store calcium ions

Rough ER

• consists of cisternae (flattened membrane sacs) NOT CRISTAE

• attached with ribosomes

• vesicles are formed at its margin and pinched off

• tiny sacs store and transport proteins around the cells

• Function: Site of synthesis, folding and post-translational modification of secretory proteins

Golgi Apparatus (GA)

• stack-like collection of flattened membranous sacs (cisternae)

• CIS face of the membranes is formed by fusion of membranes of vesicles from ER

• TRANS faces is the opposite side formed from swellings at the margin that pinches off (closer to the PLASMA MEMBRANE)

• Abundant in metabolically active cells i.e. secretory cells

• Function: Synthesise hormones and enzymes which are packaged into vesicles

Lysosomes ‘garbage collector’

• temporary vacuoles derived from GA

• tiny single-membrane bound spherical vesicles

• contains hydrolytic enzymes breakdown

  • foreign particles (phagocytosis)

  • worn out organelles (auto-phagy)

• these enzymes may escape into the cytoplasm, causing self-digestion (autolysis - random release, uncontrolled, passive, consequence of injury, disease)

• also involved in programmed cell death (apoptosis - highly regulated, energy dependent, driven by enzymes)

Plasma membrane

• extremely thin (7nm thick)

• phospholipid bilayer embedded with proteins

• forms barrier across the cell

• Function: regulate movement of substances in and out of the cell

Cytoskeleton (NOT AN ORGANELLE)

• a network of fibres extending throughout the cytoplasm

  • microtubules

  • microfilaments

Microtubules

• Straight, unbranched, thickest, hollow cylindrical fibres (25nm in diameter)

• Made of tubulin (globular protein)

• Capable of reassembling and dismantling

• Functions: Control /guide movement of

  • chromosomes in cell division

  • secretory vesicles from the GA to the plasma membrane

• Provide mechanical support, maintains cell shape

Microfilaments / Actin filaments

• thinnest fibres (7nm in diameter)

• Each filament consists of twisted double chain of actin (globular proteins)

• Function:

  • resist tension to maintain and change cell shape

  • involved in cleavage furrow formation during cell division

  • regulates cell motility particularly during muscle contraction

Centrioles

• 9 sets of 3 microtubules each arranged in a short, hollow, cylinder.

• 2 centrioles occur at right angles outside the nucleus forming a centrosome

• Function: replicate and grow the spindle fibres to organise the movement of chromosomes during nuclear division

Cilia and flagella

Compare:

• Extensions of the cell membrane

• Structurally similar cytoskeleton

• Both are made up of microtubules

  • a ring of 9 microtubules doublets (9×2)

  • that surrounds 2 central single microtubules

• Both contain a basal body at the base that anchors the cilia and flagella

  • 9 triplets of microtubules in a ring (like a centriole)

Contrast

Length

• Cilia is shorter (10µm in length, coconut shell like

• Flagella longer (100µm in length) party strings

Movement

• Cilia has a wave-like movement

• Flagella rotational movement (propeller-like)

Function & occurence

• Cilia is occurs numerously

• Locomotion (unicellular eukaryotes):

  • found in certain protists such as the euglena and paramecium

  
  

• Moving substances across its surface (multicellular eukaryote)

  • in the epithelium (thin layer that lines hollow. strcutures) of the respiratory system to move mucus across the cell surface

  • in the oviduct to help move the ovum from the ovary to the uterus

• The flagella occurs singly on small motile cells for locomotion (whole cell move)

  • sperm —> aid movement

Chloroplasts

• double membrane-bound plastids

• biconvex in shape

Contains

• Stroma (aqueous solution containing starch granules “cytoplasm”)

• Thylakoids (contains chlorophyll and other photosynthetic pigments)

• pl. grana sing., granum stack of thylakoids

Other plastids: leucoplasts (starch storage), chromoplasts (non-photosynthetic pigments, found in flower petals, carrot tissues etc.)

Permanent vacuole

Only found in plant cells

• fluid filled space

• surrounded by tonoplast

• when filled with water, it pushes the cytoplasm against the cell wall, creating turgor pressure, maintaining the plant tissue’s turgidity

• chemicals such as pigments, carbohydrates, fats and proteins can be stored

• interior is slightly acidic (pH 5) and contains hydrolase enzymes to break down waste material

Animal cells have temporary small vacuoles

• used for temporary storage, waste disposal, or endocytosis/exocytosis.

Cell wall (not an organelle)

• bcus exterior of plant cells

• made up of cellulose arranged in bundles called MICROFIBRILS

• allows the cell to develop high internal pressure due to water influx and remain turgid

Glycoproteins

• Large molecules of proteins to which sugar molecules, oligosaccharides (smiple carbohydrates) are bonded together

• Enable animal cells to adhere together and to a basement membrane to form tissues and organs

Fungal cells

• eukaryotic

• obtain food by absorbing nutrients from the external environment

• do not have chloroplasts

• have a cell wall made of chitin

• mostly multicellular

• unicellular organisms include yeast

• reproduces asexually (offspring is genetically identical but not equal in size/shape) through budding

• has

  • nucleus w/ nuclear membrane

  • mitochondrion

  • vacuoles (permanent and highly dynamic, cycles of fusion and fission)

  • plasma membrane