3.2.3 - transport across cell membranes Flashcards
what is the basis of all cell membranes (cell-surface membranes and the membranes around cell organelles of eukaryotes)?
a phospholipid bilayer (their basic structure is the same)
what are the functions of membranes in cells?
controlling the movement of substances in and out of the cell/organelle, acting as barriers, separating one part of an organelle from another, location for chemical reactions, contain receptors for molecules like hormones, allow adjacent cells to stick together
which molecules are able to pass through the cell-surface membrane?
lipid soluble molecules eg. steroid hormones - they can pass through the hydrophobic centre
which molecules can’t pass through the cell-surface membrane?
water soluble molecules - they are hydrophilic and polar, so they can’t pass through the non-polar region of the membrane (hydrophobic centre)
what its the current model of the cell membrane known as and why?
the fluid mosaic model
fluid - phospholipid molecules move around in each layer so the membrane is flexible
mosaic - membrane is studded with protein molecules and their arrangement varies
what are the main components of the cell membrane?
phospholipids, cholesterol, glycoproteins, glycolipids, proteins
what is the function of the cholesterol in the cell membrane?
increases the strength of the membrane so it is less likely to get damaged
reduces sideways movement of phospholipids which controls fluidity of membrane
packs the space between phospholipids to reduce movement of water-soluble chemicals across the cell-surface membrane
why is cholesterol able to provide these functions?
cholesterol molecules have a polar hydrophilic group at one end which attracts the polar heads on phospholipid molecules, however the rest is non-polar and hydrophobic and attracts the non-polar fatty acids
what are the two categories of proteins found in the cell membrane?
intrinsic/integral and extrinsic/peripheral
what do intrinsic proteins do?
they are fully embedded in the membrane from one side to the other and have hydrophobic amino acids on their outside surface - these interact with the hydrophobic fatty acids in the phospholipid bilayer
what are the two types of intrinsic protein?
channel proteins and carrier proteins
what is the function of channel proteins?
they have a channel lined with hydrophilic amino acids and filled with water molecules, which allows water soluble molecules to diffuse through
what is the function of carrier proteins?
to transfer molecules from one side of the membrane to the other by changing their shape or position
how are extrinsic proteins different to intrinsic proteins?
they are only found on one side of the membrane or the other, but may be attached to intrinsic proteins
what are the functions of extrinsic proteins?
they may play a structural role in the membrane, act as enzymes, or act as receptors for molecules like hormones
what are the functions of glycoproteins?
allowing cells to attach to each other to form tissues, working in the immune system, acting as receptors to hormones
what are the functions of glycolipids?
cell communication - glycolipids on one cell can be recognised by another cell to determine whether the cells come into contact
may act as antigens to determine blood group
what is 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
what are key features of simple diffusion?
molecules or ions move down the concentration gradient
random movement is caused by the natural kinetic energy of the molecules/ions
diffusion is passive so doesn’t require metabolic energy
what happens as a result of diffusion?
molecules or ions reach an equilibrium situation so they are evenly spread within a given volume of space
what factors affect the rate of a substance’s diffusion across a membrane?
concentration gradient, temperature, surface area, size of molecule, charge on molecule
factors affecting diffusion - concentration gradient?
a greater difference in concentration on two sides of a membrane means a greater difference in the number of molecules passing in the two directions, so a faster rate of diffusion
factors affecting diffusion - temperature?
at higher temperatures, molecules and ions have more kinetic energy and diffuse faster
factors affecting diffusion - surface area?
the greater the surface area across which diffusion occurs, the greater the number of molecules/ions that can cross it at a time, so diffusion occurs faster
as SA:V ratio decreases, diffusion slows down because the distance is too great (so cell membranes are thin)
factors affecting diffusion - size of molecule?
large molecules diffuse more slowly because they require more energy to move
factors affecting diffusion - charge on molecule?
uncharged and non-polar molecules diffuse directly across the non-polar phospholipid bilayer and more quickly than polar ones because they are soluble in it
which substances can’t diffuse through the phospholipid bilayer?
large polar molecules eg. glucose, and ions eg. sodium ions (hydrophilic)
what is facilitated diffusion?
when highly specific intrinsic proteins (channel proteins and carrier proteins) enable hydrophilic or charged substances to cross the phospholipid bilayer
how do channel proteins enable facilitated diffusion?
they are pores filled with water and lined with hydrophilic amino acids which allow charged substances to diffuse through the cell membrane
they are usually ‘gated’ to control the exchange of ions - part of the protein on the inside membrane can move to open or close the pore
how do carrier proteins enable facilitated diffusion?
they switch between two shapes - this means the binding site of the carrier protein is open to one side of the membrane first, and then open to the other side when it switches shape
how do you calculate rate of diffusion?
number of molecules or ions which diffuse/time (usually in seconds)
unit= s^-1
what is active transport?
when specific carrier proteins in the cell membrane transport a chemical from a region of lower concentration to a region of higher concentration (against the concentration gradient)
what is the key feature of active transport?
it requires ATP (Adenosine Tri-Phosphate), which is produced in the mitochondria during aerobic respiration
how does ATP enable active transport?
ATP is hydrolysed to ADP (Adenosine Di-Phosphate) which releases energy (from the bond which breaks when the third phosphate is released) and causes a change in the shape of the membrane protein to allow the molecule to pass through
what is the process of active transport occurring in a cell?
- the molecule/ion attaches to a receptor site on a carrier protein
- an ATP molecule binds to the carrier protein
- the ATP molecule undergoes hydrolysis to produce phosphate and an ADP molecule
- the phosphate attaches to the carrier protein so it changes shape, and the molecule/ion is transported to the other side of the membrane and released
- the phosphate leaves the carrier protein so it returns to its original shape
- the ADP and phosphate reform to ATP in respiration
where does co-transport occur?
in the mammalian ileum, part of the small intestine which absorbs molecules produced by digestion
how does glucose initially move from ileum into the bloodstream?
there is a high concentration in the lumen of the ileum after digestion , so molecules move via facilitated diffusion into the epithelial cells which line the ileum and then into the bloodstream
what is the problem with glucose moving into the bloodstream via facilitated diffusion?
the concentration gradient falls as glucose moves into the epithelial cells, so the rate of diffusion does too - this means not all available glucose can move in this way
how is remaining glucose absorbed into the bloodstream?
- a carrier protein called the sodium-potassium pump (antiport) uses ATP to transport sodium ions from the epithelial cells into the bloodstream via active transport (and potassium ions into the epithelial cells)
- the low concentration of sodium ions in the epithelial cells but high concentration in the lumen creates a concentration gradient
- sodium ions diffuse (facilitated diffusion) into the epithelial cell via a protein in the membrane called the sodium-glucose transporter (symport)
- simultaneously, glucose molecules are transported into the epithelial cell through the protein via active transport (against their conc gradient - energy comes indirectly from conc gradient of sodium ions)
how are epithelial cells adapted for their role in active transport?
contain lots of mitochondria to produce ATP for sodium potassium pump
cell membrane is folded into microvilli to increase the surface area so there is more space for membrane proteins
what is a uniport membrane protein?
transports one type of molecule/ion in one direction
what is an antiport membrane protein?
transports two types of molecule/ion - when one enters, the other leaves
what is a symport membrane protein?
transports two types of molecule/ion - both move in same direction
what is endocytosis?
when particles or molecules are transported into the cell including large molecules like proteins and cells like bacteria
what is exocytosis?
when material is moved out of the cell including secreted proteins like enzymes and hormones
what is osmosis?
the net movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential
what is water potential measured in and what does it mean?
measured in kPa - it relates to the pressure free water molecules exert on the walls of their container
what is the water potential of pure/distilled water?
0 kPa (highest)
what happens when a solute is added to pure water?
- some water molecules associate with the solute, so it dissolves and there are less free water molecules
- as there are fewer free water molecules in the container
, the pressure they collectively exert on the walls is lower - the more solute added, the lower the water potential becomes
what happens to cells in a hypertonic solution?
there is a lower water potential outside the cell so water moves out of the cell causing crenation
what happens to cells in a hypotonic solution?
there is a lower water potential inside the cell so water moves into the cell causing lysis
what happens to cells in an isotonic solution?
the water potential is the same inside and outside the cell so there is no net movement of water
how may cells be adapted for rapid transport across internal or external membranes?
increase in surface area of the membrane
increase in number of channel proteins in membrane
increase in number of carrier proteins in membrane
