Cell Structure and Function

1. all living organisms are composed of one or more cells

2. the cell is the basic unit of structure and organisation

3. all cells arise only from pre-existing cells

• universal similarities bewteen cells:

  1. DNA as the heritable material, RNA as a messenger and proteins as the workers

  2. major cellular organelles - functions and arrangements within the cell

  3. ATP as an energy source

• the central dogma:

  • DNA → RNA → PROTEIN

what is cell theory

• both have:

  • plasma membrane

  • cytosol

  • DNA

  • RNA

  • protein and ribosomes

• eukaryotic cells have membrane-bound organelles and are much larger

• prokaryote cells lack a membrane-bound nucleus

prokaryote cell versus eukaryote cell

• the cytoplasm is everything inside the plasma membrane except the nucleus

• the fluid portion of the cytoplasm is the cytosol

  • water plus dissolved and suspended substances (e.g. ions, ATP, proteins, lipids)

• major organelles include:

  • nucleus

  • endoplasmic reticulum (smooth and rough)

  • golgi apparatus

  • vesicles

    • these four make up the endomembrane system (along with plasma membrane, they work together to package, label and ship molecules)

  • mitochondria

  • ribosomes

what is the cytoplasm

• the plasma membrane is a selectively permeable barrier controlling the passage of substances in and out of the cell

• made up of a double layer of phospholipids with embedded proteins:

  • hydrophilic polar heads (phosphate)

  • hydrophobic lipid tails (fatty acids)

    • arranged as a double layer, tail to tail

    • much of our body is hydrophobic or ‘water loving’

    • fats are hydrophobic (‘water hating’)

    • fats in cell membrane provide a barrier to water

plasma membrane

• membrane proteins mediate movement of hydrophilic substances

• are often amphipathic, meaning they have both hydrophilic and hydrophobic regions

• integral proteins:

  • embedded (partially or fully) into the membrane

    • e.g. transmembrane proteins are integral membrane proteins that fully span the entire membrane, contracting both extracellular and cytoplasmic areas

• peripheral membrane proteins:

  • are associated with the membrane, but not actually embedded within it

plasma membrane proteins

• transport

  • e.g. channels, transporters

  • may be general or selective, gated or not

• enzymatic activity

  • carry out chemical reaction, may or may not be a part of a team of enzymes

• signal transduction

  • external signaling molecule causing communication of information to the inside of the cell

• cell-cell recognition

  • use of glycoproteins (carbohydrate + protein) as molecular signature of the extracellular side of the cell

• intercellular joining

  • e.g. gap junctions or tight junctions

• attachment to the cytoskeleton and extracellular matrix (ECM)

  • e.g. fibronectin mediates contact between cell surface integrins and ECM (e.g. collagen)

  • can facilitate movement

what do the plasma membrane proteins do

• membranes are not static

• the membrane is a mosaic of molecules bobbing in a fluid bilayer of phospholipids

• cell specific and dynamic repertoire of membrane-bound proteins present as required

describe the movement of membranes

• largest distinct structure inside the cell

• enclosed by double lipid bilayer called nuclear envelope, continuous with rough ER

• entry and exit through nuclear pores

• nucleolus: rRNA production, assembly of small and large subunits of ribosomes

• functions:

  • to house/protect DNA

  • make RNA

  • pores regulate movement of substances (e.g. protein and mRNA) in and out

  • molecule segregation to allow temporal and spatial control of cell function

nucleus

• DNA wrapped 2x around group of 8 histones, to form nucleosomes - collectively known as chromatin

• as the cell prepares for cell division, chromatin condenses to form chromatin fibres then condenses further into loops and then stacks as fully condensed chromosomes

• most of the time, our DNA is present in our cells as chromatin and chromatin fibres

• chromosome — comprises many genes, usually >1000

• gene — a DNA segment that contributes to a phenotype/function

in the nucleus: deoxyribonucleic acid (DNA)

• two subunits, small and large made of ribosomal RNA (rRNA) in complex with many proteins

• rRNA is made in the nucleolus

• subunits assemble in the nucleolus and leave through nuclear pores

• function: protein production (translation), found in two places within the cell:

  • free in the cytoplasm — making proteins to be used in cytosol (non-endomembrane destinations)

  • attached to the RER — making non-cytosolic proteins/endomembrane

ribosomes

• the ER is an extensive network of tubes and tubules, stretching out from the nuclear membrane

• two types: rough ER and smooth ER

endoplasmic reticulum

• continuous with nuclear envelope dotted with attached ribosomes

• proteins enter lumen within the rough ER for folding

• rough ER membrane surrounds the protein to form transport vesicles destined for the Golgi

• major function is production of:

  • secreted proteins

  • membrane proteins

  • organelle proteins

rough endoplasmic reticulum

• extends from the rough ER

• lacks ribosomes: doesn’t make proteins

• synthesises lipids, including steroids and phospholipids

• stores cell-specific molecules

• functions of smooth ER vary greatly from cell to cell

  • very cell/tissue-type specific

• examples:

  • liver: houses enzymes for detoxification and for glucose release

  • muscle: calcium ions

smooth endoplasmic reticulum

• the ‘warehouse’ of the cell

• this complex is made up of 3 to 20 flattened membranous sacs called cisternae, stacked on top of one another (like ‘pita bread’)

• functions:

  • modify, sort, package, and transport proteins received from the rough ER using enzymes in each cisternae

• formation of:

  • secretory vesicles (proteins for exocytosis)

  • membrane vesicles (PM molecules)

  • transport vesicles (molecules to lysosome)

Golgi apparatus - receiving and modifying

• each sac or cisternae contains enzymes of different functions

• proteins move cis to trans from sac to sac

• mature at the exit cisternae

• travel to destination within vesicles

• modifications occur within each sac (formation of glycoproteins, glycolipids, and lipoproteins)

Golgi apparatus: to destination

• main function: generation of ATP through cellular respiration

• mitochondria are made up of:

  • outer mitochondrial membrane

  • inner mitochondrial membrane, with folds called cristae f

  • fluid filled interior cavity, called the mitochondrial matrix

• despite all of these membranes, mitochondria are not part of the endomembrane system

• the more energy a cell requires, the more ATP it must take, and the greater the number of mitochondria present

• mitochondria carry a separate small (37 genes) genome encoding mitochondrial-specific products

mitochondria

• structural support system of the cell

• fibres or filaments that help to maintain the size, shape, and integrity of the cell:

  • act as scaffolding across the cell

  • involving in intracellular transportation and cell movement

• three types of fibres (from smallest to largest):

  • microfilaments

  • intermediate filaments

  • microtubules

cytoskeleton

• diameter: ~7nm

  • comprised of actin molecules assembled in two long chains, twisted around each other

  • found around the periphery and lining the interior of cell

• function:

  • bear tension and weight by anchoring cytoskeleton to plasma membrane proteins, and promote amoeboid mobility if required (e.g. macrophage)

  • assembled and disassembled as required — they are dynamic

cytoskeleton: microfilaments

• diameter: 8-12nm

• comprised of diverse range of different materials; one example: keratin

• found in the cytoplasm of the cell

• function:

  • bear tension and weight throughout cell, e.g. during cell anchoring

  • acts as a scaffold for cellular organelles, e.g. the nucleus

• usually the most permanent of cytoskeletal structures — they are less dynamic

cytoskeleton: intermediate filaments

• diameter: tubular structure, 25nm with central lumen of 15nm diameter

• comprised of tubulin dimers (alpha and beta), coiled, to form a tube

• extends from centriole into cytoplasm/nucleus

• functions:

  • support cell shape and size

  • guide for movement of organelles

    • e.g. vesicles from Golgi to membrane

  • chromosome organisation — cell division

  • support and movement of cilia/flagella

• assembled and disassembled as required — are dynamic

cytoskeleton: microtubules