Transport & Trafficking

Key Terms

Compartmentalisation - The separation of the cell into different sections / organelles based on their functions.


Peroxisomes - Break down lipids etc 


Autophagosomes - Autophagy which means self eating - so the cell eat its self and then recycles these. 


Endosomes - Sorts and transports substances taken in from the outside of the cell to their correct destinations


Microtubules - Supports shape and separates chromosomes during division.


Actin filaments - Protein strands that support shape and cell movement


Leptocyte - A metabolic active liver cell


Amphiphilic - have hydrophobic and hydrophobic properties.


Exocytosis - The process by which a cell releases substances by transporting them in vesicles that fuse with the cell membrane.


Constitutive Pathway: Continuous secretion of soluble proteins and plasma membrane lipids. - makes it and then immediately secretes it 


Regulated Pathway: Secretion of proteins triggered by specific signals (e.g., hormones or neurotransmitters).


Endocytosis - where the cell takes substances into cytoplasm by surrounding them in its cell membrane.


Phagocytosis: Ingestion of large particles


Pinocytosis: Ingestion of fluids and small molecules.


Myosin: Acts along actin microfilaments.


Kinesin: Moves along microtubules in one direction.


Dynein: Moves in the opposite direction along microtubules.

Dividing cells Compartments

Compartmentalisation - The separation of the cell into different sections / organelles based on their functions.

  • It is important for organising and increasing the speed of cellular activities and functions.


Based on organelles:

  • size


Cellular Compartments include

  • Nucleus - Contains the genetic material.

  • Plasma membrane - Control what enters and leaves the cell

  • Endosomes - Sorts and transports substances taken in from the outside of the cell to their correct destinations

  • Actin filaments - Protein strands that support shape and cell movement

  • Microtubules - Supports shape and separates chromosomes during division.

  • Mitochondria - Creates ATP and is the site of respiration

  • Lysosomes - Contains digestive enzymes

  • Golgi apparatus - Modifies, sorts and packages proteins and lipids for transport.

  • Autophagosomes - Autophagy which means self eating - so the cell eat its self and then recycles these. 

  • Endoplasmic Reticulum (E.R.) - RER synthesis and transports proteins. SER makes lipids and removes toxins

  • Peroxisomes - Break down lipids

  • Membranes - Enclose organelles and are important for cellular functions.


Membranes

Functions:

  1. Controlling transport across the cell

  2. Concentrating enzyme activity in the membranes

  • To speed up the process

  1. Controlling cell communication

  2. Connecting cells

  3. Recognising cells

  • Self and non Self


Structure & Composition

  • Lipid Bilayer - Thin double-layered sheet of lipids that are amphiphilic

  • They self assemble

  • 2 sides of the bilayer are asymmetrical

  • to 10 nm in thickness.


Types of lipids in lipid membranes

  • Phosphoglycerides - Phosphate + Glycerol + 2 fatty acids, is it is specific type of phospholipid

  • Sphingolipids - Contains a long chain of amino acids called sphingosine

  • Sterols - Regulates membrane fluidity and stability. (Cholesterol)

  • Glycolipids


Membrane Proteins

  • Transmembrane - cross the bilayer and are usually α-helical

  • Channel proteins are made up of large channels called β barrels

  • Attach to one side

  • Anchored by a lipid molecule


Membrane Protein Functions

  • Transporter and Channels for movement

  • Receptors fro receiving external signals

  • The cytoskeleton (the cell cortex of proteins) binds to the membrane, providing strength.


Transport between compartments

Three primary methods of protein transport include:

  1. Gated Transport - Movement of molecules between the nucleus and cytoplasm through nuclear pores complexes that allow certain substances to pass.

  2. Transmembrane Transport: Is the movement of molecules across the cell membrane through the lipid bilayer or transport proteins.

  3. Vesicular Transport: Exocytosis & Endocytosis

Transmembrane Transport Mechanisms

Types of Transport

  • Passive Transport

  • Active Transport

  • Simple diffusion

  • Facilitated diffusion

  • Signal sequences


Passive Transport - The movement of substance from a high concentration to a low concentration with out the use of energy in the from of ATP


Simple Diffusion - The Movement of small lipid solubles across the lipid bilayer, along their concentration gradient.


Facilitated Diffusion - The Movement of water soluble and other molecules, across the lipid bilayer, along their concentration gradients. Using a carrier or channel protein.


Active Transport - The movement of substances against their concentration gradient, via ATP.

  • E.g. The Na+/K+ pump maintains ionic balance by pumping Na+ out of the cell.

    • Counteracting intracellular solute concentrations and thereby preventing cellular lysis due to osmotic pressure.


Signalling Processes - The specific amino aids sequences that direct proteins to their correct destinations within the cell.


Vesicular Transport


Endocytosis - where the cell takes substances into cytoplasm by surrounding them in its cell membrane.


Types

  1. Phagocytosis: Ingestion of large particles (e.g., food vacuoles). - cell eating 

  2. Pinocytosis: Ingestion of fluids and small molecules. - cell drinking 

  1. Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor-ligand interactions.


Exocytosis - The process by which a cell releases substances by transporting them in vesicles that fuse with the cell membrane.


Process

  • proteins are secreted

  • they are transported in vesicles to the plasma membrane

  • they fuse and release their contents externally

  • Proteins can either be soluble in the vesicle or integrated within the vesicle membrane.


  • Constitutive Pathway: Continuous secretion of soluble proteins and plasma membrane lipids. - makes it and then immediately secretes it 

  • Regulated Pathway: Secretion of proteins triggered by specific signals (e.g., hormones or neurotransmitters).

Nuclear Pores

Nuclear pore complexes (NPCs) - specialised structures that act as the primary gateway for the transport of molecules between the cell’s nucleus and the cytosol.


Structure & location

  • embedded in the nuclear envelope

  • 3000-4000 NPCs

  • Cytoplasmic filaments

  • Cytoplasmic ring

  • Spoke-ring assembly (scaffold)

  • Central transporter (central channel)

  • Nuclear ring

  • Nuclear basket


Cytoplasmic filaments

  • Project into the cytoplasm.

  • Function: First point of contact for proteins entering the nucleus.

  • Bind transport receptors (importins/exportins) carrying cargo.


Cytoplasmic ring

  • Ring on the cytoplasmic side.

  • Anchors the cytoplasmic filaments and helps stabilize the pore.


Spoke-ring assembly (scaffold)

  • Forms the main structural framework of the NPC.

  • Gives the pore its shape and anchors it in the nuclear envelope.


Central transporter (central channel)

  • The actual passageway through the pore.

  • Lined with FG nucleoporins (rich in phenylalanine-glycine repeats).

  • Acts as a selective barrier:

    • Small molecules (< ~40–60 kDa) diffuse freely.

    • Larger proteins and RNAs require active transport via importins/exportins and Ran-GTP.


Nuclear ring

  • Ring on the nuclear side.

  • Supports the nuclear basket.


Nuclear basket

  • Basket-like filaments extending into the nucleus.

  • Functions:

    • Helps export mRNA.

    • Participates in gene regulation.

    • Quality control of RNA before export.

Motor proteins

Motor Proteins - A protein that uses energy (ATP) to move materials around the cell along cytoskeletal filaments.


3 Types of motor proteins

  • Myosin: Acts along actin microfilaments.

  • Kinesin: Moves along microtubules in one direction.

  • Dynein: Moves in the opposite direction along microtubules.


Myosin

Acts along actin microfilaments.

  • Transports vesicles and organelles


Structure

  • Head

    • Binds to actin filaments.

    • Contains an ATP-binding site that provides energy for movement.

    • Also called the motor domain.

  • Neck/stalk

    • Connects the head to the tail.

    • Acts as a lever arm to generate movement.

  • Tail

    • Attaches to cargo (such as vesicles or organelles) or to other myosin molecules.

    • Determines the specific function of the myosin.


How Myosin Motor Proteins Moves

  1. One head binds the microtubule.

  2. ATP binds to the attached head.

  3. The neck linker swings forward.

  4. The second head steps ~8 nm to the next binding site.

  5. ATP is hydrolysed and the rear head detaches.

  6. The cycle repeats.


Direction of Kinesin Transport

  • Direction - Plus (+) end

  • Movement - From the centre of the cell (near the centrosome/MTOC) towards the cell periphery (plasma membrane).

  • This is called anterograde transport.

Kinesin

  • moves organelles and vesicles throughout the cytoplasm


Structure

  • 2 motor (globular) head

    • Each containing ATP-binding sites (ATPase)

    • Microtubule-binding sites

  • Neck Linker

    • Short flexible region behind each head

    • changes position when ATP binds

  • Coiled-coil stalk

    • Long α-helical region where the two heavy chains wrap around each other.

    • Provides structural support.

    • Connects the motor heads to the cargo-binding region

  • Tail domain

    • Located at the opposite end from the heads.

    • Often associated with light chains.

    • Binds cargo such as:

      • Vesicles

      • Mitochondria

      • Lysosomes

      • Protein complexes


How Kinesin Moves

  1. One head binds the microtubule.

  2. ATP binds to the attached head.

  3. The neck linker swings forward.

  4. The second head steps ~8 nm to the next binding site.

  5. ATP is hydrolysed and the rear head detaches.

  6. The cycle repeats.


Direction of Kinesin Transport

  • Direction - Plus (+) end

  • Movement - From the centre of the cell (near the centrosome/MTOC) towards the cell periphery (plasma membrane).

  • This is called anterograde transport.


Colour Changes in Animals

Zebra Fish

  • Motor proteins transport pigment molecules along microtubules to change the colour of some organisms.

  • light - dark in this case


Cephalopods

  • contains pigments called chromatophores that change shape due to action of muscle fibers