2 Cells- Transport across cell membrane Flashcards
What model is the structure of the plasma membrane described by and why?
The fluid mosaic model
‘fluid’ because the phospholipids are constantly moving around
‘mosaic’ because protein molecules scattered throughout phospholipids like a mosaic
What is the function of the plasma membrane?
-Defines the borders of cells and most organelles
-Selectively permeable (some materials can enter/leave freely, others cannot)
What is the structure of phospholipids?
-Molecule containing glycerol, two fatty acids and a phosphate-linked head group
-Arrange themselves in a bilayer, 5-10nm thickness
-Hydrophilic phosphate head faces outwards, hydrophobic fatty acid face inwards
What is the structure of cholesterol?
-Lipid, sits with phospholipids in core of membrane
-Not found in bacterial cell membranes
-Make membrane more rigid
Which factors affect the permeability of the cell membrane and how?
-Temperature
higher= increased fluidity
water baths at a constant temp. can control this
-pH
affects protein structure in cell membrane
a buffer can control this
-Solvent concentration
the more easily the bilayer is dissolved the more permeable it is
can be controlled by using same solvent at the same concentration
What is diffusion?
The net movement of particles from an area of high concentration to an area of lower concentration. e.g. O2, CO2 and urea
It is the passive movement of particles in fluids and molecules move randomly until they are evenly distributed
Goes down a concentration gradient
What is facilitated diffusion?
Molecules, which are either too large to move through the membrane or polar ones repelled by the hydrophobic part of the membrane, diffuse through carrier proteins or channel proteins spanning across the membrane, down a concentration gradient. It is a passive process.
How do channel and carrier proteins allow facilitated diffusion?
Channel proteins allow polar compounds to avoid the non-polar central layer of the plasma membrane , eg. aquaporins
Carrier proteins bind to substances, making the protein change shape- this change moves the substance from the outside of the cell to the inside
What are the factors affecting the rate of diffusion?
-Concentration gradient
greater= faster diffusion
-Temperature
higher=faster diffusion
-Membrane surface area
larger= faster diffusion
-Thickness of membrane
thicker= slower
-Channel or carrier proteins
more=faster
What is osmosis?
The diffusion of water across a partially permeable membrane from a dilute solution (high water potential ) to a concentrated solution (low water potential)
What factors affect the rate of osmosis?
Water potential gradient- lower= slower rate of osmosis
Membrane thickness- the thicker, the slower rate of osmosis
Surface area- the smaller, the slower rate of osmosis
What is active transport?
The net movement of ions and molecules across a membrane against the concentration gradient, using energy and specific carrier proteins to facilitate movement. e.g. sugar absorption in the gut, mineral absorption in plants
What factors affect the rate of active transport?
-Surface area of cell membranes
-Number of carrier proteins in cell membranes
What is co-transport?
The mechanism of transport of glucose and amino acid into the epithelial cells in the ileum. Sodium binds to co-transport proteins to allow glucose and amino acids to be transported
How is glucose and sodium absorbed through co-transport?
-Sodium ions are actively transported into the blood by sodium-potassium ATPase, from the inside of the epithelial cells lining the ileum
-The concentration of Na+ is now lower inside of the cell than outside
-2 Na+ ions from the lumen of the ileum bind to a transmembrane protein known as the sodium-glucose cotransporter
-At the same time, the cotransporter binds with a molecule of glucose from the ileum and moves into the cell
-Sodium ions diffuse into the cell, moving the glucose in at the same time
