Organelles & Compartmentalization

IB Biology: Form & Function

Compartmentalization

Refers to the division of cells into different regions with one or two membranes, causing separation

Organelles

A (membrane-bound) sub-cellular structure that carries out specific functions

• Compartmentalized:

  • ribosomes, vesicles, nucleus, chloroplasts, mitochondria

• Non-Compartmentalized:

  • cell wall, cytoplasm & cytoskeleton

Purpose of Compartmentalization

• allows enzymes & substrates to be localised and available at higher concentrations

• damaging substances to be kept separated

  • e.g. digestive enzymes for lysosomes

• optimal conditions to be maintained for certain processes

  • e.g. optimal pH for digestive enzymes

• numbers and location of organelles to be altered depending on rthe equirements of the cell

Mitochondria: Membrane

• has two phospholipid membranes

Outer

• smooth & permeable to small molecules

Inner:

• folded(cristae) & less permeable

• the site of the electron transport chain(oxidative phosphorylation)

• location of ATP synthase

Mitochondria:Structure

Intermembrane space:

• has a low pH due to a high concentration of protons

• The concentration gradient across the inner membrane

Matrix:

• aqueous solution w/in the inner membrane

• contains ribosomes, enzymes and circular mitochondrial DNA for functioning

Mitochondria: Function

• intermembrane space & double membrane has space for the concentration build up of H+ ions required for respiration reactions

• large surface area - cristae enables membrane to hold electron transport chain & ATP synthase enzymes

• compartmentalization of enzymes and substrates occurs in the matrix — can happen more efficiently

  • e.g. Krebs cycle

Chloroplasts: Structure

• where photosynthesis occurs

• surrounded by a double-membrane envelope

• outer membrane is permeable to ions & small molecules

• inner membrane contains semipermeable transport proteins

• membrane system provides a large number of pigment molecules

  • ensures lots of light is absorbed

Photosystems

arrangement of pigment moelcules in light-harvesting clusters

• each pigment pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre

Stroma

• gel-like fluid contains enzymes that catalyze reactions of the light-independent stage(Calvin Cycle)

• enzymes & substrates are compartmentalized

• surrounds the grana and membranes — makes transport rapid

Grana

• granal stacks create a large surface area for photosystems —allows maximum light absorption

• provides more membrane space for electron carriers & ATP synthase enzymes

Chloroplasts: DNA & Ribosomes

• chloroplast DNA contains genes that code for proteins & enzymes in photosynthesis

• ribosomes allows for translation of proteins

Chloroplasts: Inner Membrane & Thylakoid space

Inner Membrane

• selectively permeable transport proteins control the flow of molecules between in the cytoplasm of the cell

Thylakoid Space:

• proton gradient develops(to generate ATP)

• space has a small volume so a proton gradient can develop quickly

Nuclear Membrane

• separated from the cytoplasm by a double membrane and nuclear envelope

  • separates reactions and functions of DNA from the rest of the cell

• outer membrane forms a continuous structure that links to the ER

Nuclear Pores

• apart of the nuclear envelope

• important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes & signalling molecules to travel in

  • e.g. DNA polymerase

Ribosomes

• site of protein synthesis

• consists of a large and a small subunit composed of protein & ribosomal RNA

  • Protein provides structure

• rRNA facilitates binding of mRNA & tRNA and catalyses the formation of peptide bonds between amino acids

Golgi Apparatus

• consists of flattened sacs of membrane — cisternae

• modifies proteins and lipids before packaging them into Golgi vesicles

• Vesicles transport the proteins & lipids to their required destination

  • Proteins can be exported(e.g. hormones like insulin)

  • put into lysosomes(e.g. hydrolytic enzymes)

  • delivered to membrane-bound organelles

Cis-side

• receives protein or lipid-filled vesicles

Trans-side

• sends out modified proteins

Vesicles

• Vesicles are membrane-bound sacs used for transport & storage

  • Peroxisomes — contain enzymes that digest fatty acids

  • Lysosomes — contain lytic enzymes which digest cellular waste or harmful substances

  • Transport vesicles—used to move various molecules within the cell

  • Secretory vesicles—these are responsible for transporting substances out of the cell via exocytosis

Clathrin

• Proteins that help with the formation of vesicles

  • anchor certain proteins to specific sites on the exterior plasma membrane in receptor-mediated endocytosis

• Stabilizes vesicles and the phospholipid bilayer

Vesicle formation

1. a clathrin coated pit is formed on the surface of the cell membrane

2. receptor proteins on the cell surface bind to the target molecules

3. target molecules attatch, cytoskeleton proteins help the clarithin pit to deepen and eventually seal off, trapping the target molecules inside

4. A vesicle is now formed

Ultracentrifugation

• involves breaking up a suitable sample of tissue and then centrifuging the mixture at different speeds

• a centrifuge is a machine that separates materials by spinning —> separates based on weight

Cell Fractionation:

• Homogenisation — the cell is broken up using a homogeniser which is a blender-like machine

• Filtration - the homogenate(containing the homogenised cells) is then filtered throughout gauze

• Ultracentrifugation— the filtrate is placed into a tube and the tube is placed in a centrifuge