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It involves the movement of material along a concentration gradient (high concentration low concentration)
Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis
There are three main types of passive transport:
Is the net movement of molecules from a region of high concentration to a region of low concentration
can be influenced by a number of factors, including:
is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)
Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions
At a higher solute concentration there are less free water molecules in solution as water is associated with the solute
Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration
Uncontrolled osmosis will have negative effects with regards to cell viability:
ANIMALS
PLANTS
Is the passive movement of molecules across the cell membrane via a membrane protein
It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins
Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
Channel proteins have a much faster rate of transport than carrier proteins
It involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)
Materials destined for secretion are transported around the cell in membranous containers called vesicles
(TYPE OF ACTIVE TRANSPORT)
The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials
Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use
Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism
Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material
The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex
Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face
While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)
Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome
Vesicles containing materials destined for extracellular use will be transported to the plasma membrane
The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid
Materials sorted by the Golgi apparatus may be either:
The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane
Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis
The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane
An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the material
Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)