Transport across cell membranes Flashcards
Why is the fluid mosaic model used to describe the structure of a cell membrane?
Fluid - Phospholipids form a bilayer in which the phospholipid molecules are constantly moving
Mosaic - Proteins of different shapes and sizes are embedded into the phospholipid bilayer
What are the 5 key components of a cell membrane?
Glycolipids
Glycoproteins
Proteins
Cholesterol
Phospholipid
Function of phospholipids in a cell surface membrane?
Form the phospholipid bi layer
Their hydrophilic heads and hydrophobic tails arrange themselves so only lipid soluble substances and small uncharged molecules can pass through.
Prevents water soluble substances entering and leaving the cell
Makes the membrane flexible and self sealing
Functions of proteins in the cell surface membrane
Extrinsic - Give mechanical support and work in conjunction with glycoproteins and glycolipids to act as receptors for molecules such as hormones.
Intrinsic - Act as protein channels allowing water soluble substances to pass. Others are carrier proteins which bind to ions or molecules and change shape to allow them to pass.
Helps cell adhesion
Function of cholesterol in the cell surface membrane
Reduce lateral movement of other molecules
Prevent water and dissolved ions escaping - cholesterol is very hydrophobic
Makes the membrane less fluid at high temperatures
Function of glycolipids in the cell surface membrane
Structure of a glycolipid?
Carbohydrate covalently bonded to with a lipid and extends out of the phospholipid bilayer
Acts as recognition sites
Helps maintain stability of the membrane
Helps cells to attach to one another and so forms tissues
Function of the glycoproteins in the cell surface membrane
Structure?
Carbohydrate chain is covalently bonded to a protein.
Sticks out of the membrane
Acts as recognition sites
Help cells to attach to one another and so form tissues
Allows cell to recognise one another. eg. lymphocytes can identify its organisms own cells.
What are the 5 functions of membranes within cells?
Control the entry and exit of materials in organelles.
Separate organelles from the cytoplasm so specific metabolic reactions can take place within them.
Provide an internal transport system eg. endoplasmic reticulum
Isolate enzymes which might damage cells eg. lysosymes
Provide surfaces for which reactions can occur on eg. protein synthesis on RER
Define difussion
The net movement of particles from an area of high concentration to an area of low concentration. It does not require any extra input of energy and relies on inbuilt kinetic energy of the particles themselves.
What is a concentration gradient?
The change in concentration between two areas
What are the two types of proteins used in facilitated diffusion?
Carrier and channel
How do carrier proteins work?
Large molecules attaches to the protein
Causing it to change shape
It is then released on the other side of the membrane
Factors affecting facilitated diffusion?
Temperature
Concentration gradient
Diffusion distance
Surface area
Number of channel or carrier proteins
Define osmosis
The net movement water from an area of higher water potential to an area of lower water potential through a selectively permeable membrane
What is a hypertonic solution
One which has a higher solute concentration relative to inside the cell
(More concentrated)
What is a hypotonic solution
One which has a lower solute concentration relative to inside the cell
What is water potential measure in?
Kilopascals
What is active transport?
Movement of molecules or ions into out out of a cell from a region lower concentration to a region of higher concentration using ATP and carrier proteins.
How are carrier proteins used in Active Transport
Molecule or ion binds to the carrier proteins
ATP binds to the carrier protein
Hydrolysis of ATP to ADP and phosphate causes the carrier protein to change shape.
Releasing the ion or molecule on the opposite side
ADP is released causing the protein to revert to its original shape.
How does glucose move from the lume to the epithelial cell without using Active Transport?
Glucose molecules are moved against their concentration gradient due to the Na+ concentration gradient, rather than by using ATP, so they move by co-transport rather than active transport.
What is a cotransporter?
A carrier protein that can bind to two molecules at once.
It moves one molecule down its concentration gradient whilst simultaneously moving another against its concentration gradient.
What molecules can diffuse across membranes?
Small, non polar molecules
How do protein channels allow facilitated diffusion?
Form water filled hydrophilic channels across the channels
Allow specific water soluble ions to pass through
Ions bind with one side causing it to change shape in a way that closes one side of the membrane and opens the other
How do carrier proteins allow facilitated diffusion?
Molecule that is specific to the protein binds
Causing it to change shape so the molecule is released to the inside of the membrane
