Transport across cell membranes Flashcards
Structure of cell membrane
The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER
Membranes do not only separate different areas but also control the exchange of material across them, as well as acting as an interface for communication
Membranes are partially permeable
Substances can cross membranes by diffusion, osmosis and active transport
The phospholipid bilayers that make up cell membranes also contain proteins
The proteins can either be intrinsic (or integral) or extrinsic (peripheral)
Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
Extrinsic proteins are found on the outer or inner surface of the membrane
Phospholipid
Phospholipids structurally contain two distinct regions: a polar head and two nonpolar tails
The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water
The lipid tail is non-polar (hydrophobic) and insoluble in water
If phospholipids are spread over the surface of water they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water
This is called a phospholipid monolayer
Micelle
If phospholipids are mixed/shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other
Fluid mosaic model
Describes cell membranes as ‘fluid’ because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
The fluid mosaic model describes cell membranes as ‘mosaics’ because:
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
Simple diffusion
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
Facilitated diffusion
Facilitated diffusion describes the process of passive transport of molecules across a membrane, with the help of transmembrane proteins
Osmosis
Osmosis is a term describing the movement of water from across a selectively permeable membrane as a result of a concentration gradient
I.e. it is a special type of diffusion concerned only with water
The water moves towards a high concentration of a solute, with the effect of equalising the solute concentration across a permeable membrane
From the point of view of the water, it undergoes diffusion, moving from a high concentration of a lower concentration of itself
Osmosis is also a form of passive transport, as it does not require the expense of energy
Active transport
Active transport across a cell membrane requires a transporter protein and a supply of energy for the transport of molecules the membrane
Its requirement for energy distinguishes it from passive transport
This process is very important to transport molecules across the cell membrane which are present at a very low concentration in the extracellular environment
Co-Transport
Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/electrochemical gradient
Substances bind to complimentary intrinsic proteins:
Symport: Transports substances in the same direction
Antiport: Transports substances in opposite direction
e.g. Sodium-potassium pump
Specialised cells for simple diffusion
Root hair cells and epithelial cells of the small intestine are examples of cells that are adapted for the rapid transport of molecules across their membranes
Root hair cells:
Are adapted for the absorption of water and mineral ions from soil
Have a specialised shape (the root ‘hair’) that increases the cell’s surface area so the rate of water uptake by osmosis is greater (can absorb more water and mineral ions than if the surface area was lower)
Have thinner walls than other plant cells so that water can move through easily (due to shorter diffusion distance)
Have a permanent vacuole containing cell sap, which is more concentrated than soil water. This ensures a high water potential gradient is maintained
Epithelial cells of the small intestine:
Have microvilli (highly folded sections of the cell membrane), which increases the cell’s surface area so the rate of diffusion of the products of digestion is greater (more particles can be exchanged in the same amount of time)
Each villus of the small intestine has a constant blood supply, which continually transports the products of digestion away from the epithelial cells. This maintains a high concentration gradient across the epithelial cell exchange surface (between the lumen of the small intestine and the interior of the epithelial cell)
Specialised cells for facilitated diffusion
Neurones, muscle cells and some kidney cells are examples of cells that are adapted for the rapid transport of molecules across their membranes via facilitated diffusion
Certain kidney cells:
Have cell membranes that contain a very high number of aquaporins
Aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes
The aquaporins allow these kidney cells to reabsorb water, stopping it from being unnecessarily excreted by the body
Neurones and muscle cells:
Are involved in the transmission of electrical impulses around the body
They have cell membranes that contain channel proteins for sodium, potassium and calcium ions
The opening and closing of these channel proteins (and the resulting facilitated diffusion of these different ions), as well as the number of these channel proteins, plays an important role in the speed of electrical transmission, both along the membranes of neurones (during nerve impulses) and in muscle cells (during muscle contraction)
