Transport across Membranes Flashcards
What is the structure of a cell membrane?
A phospholipid bilayer (two phospholipid molecules thick)
What is a phospholipid?
Two fatty acid chains linked to two of the three carbons of a glycerol molecule
Properties of fatty acid chains and the head (glycerol + phosphate)
2 fatty acid chains: non-polar, hydrophobic (avoids water, faces inwards)
Head: polar, hydrophilic (likes water, faces outward)
Why are phospholipids amphipathic?
Due to their distinct hydrophobic (fatty acid chains) and hydrophilic (glycerol + phosphate head) regions
What is the fluid mosaic model
It describes the structure of the cell membrane
Fluid: phospholipids move freely and laterally within the phospholipid bilayer
Mosaic: protein molecules randomly scattered and embedded within the phospholipid bilayer
What are glycolipids and glycoproteins?
Glycolipids: carbohydrate molecules covalently bonded to lipids
Glycoproteins: carbohydrate molecules covalently bonded to proteins
What is membrane fluidity influenced by?
Unsaturated fatty acids increase membrane fluidity and decreases the temperature the membrane solidifies at
Saturated fatty acids decreases membrane fluidity and increase the temperature the membrane solidifies at
Cholesterol (only in animal but not plant cell membranes) stabilise membrane fluidity, increasing fluidity at low temperature and decreasing fluidity at high temperature
Describe cell membranes
They are partially permeable and allow some but not all substances to pass through them
What passes through the cell membrane via diffusion?
Small molecules and lipid-soluble substances are able to pass directly through the hydrophobic interior of the phospholipid bilayer via simple diffusion.
Non-polar molecules that can pass: oxygen, carbon dioxide, nitrogen
Polar but small: water, glycerol
Large hydrophobic molecules: hydrocarbons
Polar molecules have a slight positive and negative charge and cannot pass directly across the lipid bilayer
What are transport proteins?
(Carrier and channel proteins) allow useful molecules like ions, amino acids, sugars (like glucose) a and other water-soluble substances to move across membranes, moving down a concentration gradient. This process is facilitated diffusion, a passive process.
How do water molecules move across?
Either by osmosis or with the aid of channel proteins known as aquaporins, which create hydrophilic pathways.
Define diffusion
Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration, down a concentration gradient.
It is a passive process.
Facilitated diffusion requires the aid of transport proteins, while simple diffusion does not
Factors affecting diffusion rate
Steepness of concentration gradient (increase) Temperature (increase) Diffusion distance (decrease) Size of molecules (decrease) Surface area to volume ratio (increase)
(Change to increase diffusion rate)
What is equilibrium?
When there is no NET movement of molecules, molecules are uniformly distributed
Define osmosis
Osmosis is the net movement of water molecules from an area of less negative water potential to an area of more negative water potential, across a partially permeable membrane.
It is a passive process
When water molecules enter the cell via aquaporins, it is still osmosis (with the cell membrane being the partially permeable membrane)
What is water potential?
The tendency of water to move from one place to another
Pure water’s water potential = 0
Less negative means less concentrated, more negative means more concentrated
What are isotonic, hypotonic, hypertonic solutions?
Isotonic: same water potential as the cell
Hypotonic: less negative water potential (more water)
Hypertonic: more negative water potential (less water)
Animal cells in isotonic, hypotonic, hypertonic solutions
Isotonic: no change, no net movement of water molecules since water potential is the same
Hypotonic: cell wall will swell and burst
Due to absence of cell wall and net movement of water into the cell
Hypertonic: cell will shrink and crenate
Due to net movement of water out of the cell
Plant cells in isotonic, hypotonic, hypertonic solutions
Isotonic: no change, no net movement of water molecules since water potential is the same
Hypotonic: cell increases in size and becomes turgid
Due to absence of cell wall and net movement of water into the cell
Hypertonic: cell will plasmolyse, where cell membrane pulls away from the cell wall
Due to net movement of water out of the cell
Definition of active transport
Active transport is the net movement of molecules from an area of lower concentration to an area of higher concentration, up a concentration gradient, which is an active process requiring energy.
Traits of cells that do active transport
High energy demand, large numbers of mitochondria. Protein transporters are involved.
How are water and mineral salts absorbed at root hair cells?
Concentrated cell sap has more negative water potential than soil. Water molecules are absorbed via osmosis, and mineral ions are absorbed via active transport
What are exocytosis and endocytosis?
The bulk transport of substances (large molecules) across membranes in transport vesicles. Energy is required
The amount of cell membrane is kept relatively constant (in non-growing cells), by allowing the addition and loss of membranes to be approximately equal.
What is exocytosis?
When a cell removes substances by the fusion of a vesicle with the cell membrane
The contents of the vesicle exit the cell, and the vesicle becomes a part of the cell membrane
Example: secretion of substances like enzymes or hormones out of secretory cells
What is endocytosis?
When substances are taken into the cell by forming new vesicles in the cell membrane
Small area of the cell membrane sinks inwards to form a pocket, which deepens and pinches to form a vesicle
Example: phagocytosis
How does a virus enter a cell?
Through endocytosis
