Chapter 4 - Transport across cell membranes Flashcards
What are cell-surface membranes?
A partially permeable barrier between the cell and its environment, which controls what enters and leaves the cell
What is a plasma membrane?
A membrane anywhere in the cell, including those that surround the organelles
What three things is a cell membrane made of?
Lipids, proteins and carbohydrates
What are the two components of a phospholipid?
A hydrophobic tail and a hydrophilic head
How are phospholipids organised in the bilayer?
With the hydrophilic heads pointing out and the hydrophobic tails pointing in
What type of molecules CAN’T get through the bilayer?
Large or charged (e.g. ions or glucose)
Functions of phospholipids (3)
Allow lipid-soluble substances in and out of the cell
Prevent water-soluble substances from entering or leaving
Make the membrane flexible
What are the functions (2) of the proteins that only occur on the surface of the membrane?
Mechanical support to cell
Cell receptors
A function of protein channels
Water filled tubes which allow water-soluble ions to diffuse across the membrane
What are carrier proteins?
Proteins which bind to ions or molecules and change shape in order to move them across the membrane
Main functions of proteins in the cell membrane (3)
Structural support
Cell surface receptors
Allow substances to cross the membrane
Where are cholesterol molecules found?
Between the phospholipids
Functions of cholesterol
Structural support
Make the membrane less fluid
Prevent leakage of fluids from cell
How does cholesterol stop the membrane being so fluid?
It binds to the hydrophobic tails and causes them to pack closely together, which restricts the movement of the phospholipids
What are glycolipids?
A carbohydrate bonded to a lipid
Function of glycolipids (3)
Recognition sites
Keep the membrane stable
Help cells attach to eachother
What are glycoproteins?
Carbohydrate chains attached to extrinsic proteins
Functions of glycoproteins (2)
Recognition sites
Allow lymphocytes to distinguish between cells
What name is given to the current model of the cell membrane?
Fluid mosaic
Why is the current model of the cell membrane described as fluid?
The phospholipids can move relative to eachother
Why is the current model of the cell membrane described as a mosaic?
The proteins embedded in the structure fit together like tiles in a mosaic
Why does a higher temperature mean the cell membrane is more permeable?
The phospholipids move more and create gaps in the membrane
What is diffusion?
The passive net movement of particles from a high to a low concentration until they are evenly distributed
What is facilitated diffusion?
The passive movement of large or charged molecules across a cell membrane using channel or carrier proteins
How do channel proteins work?
These create pores in the membrane across which water-soluble ions can pass. A different channel is needed for each charged particle
How do carrier proteins work?
A large molecule attaches to a carrier protein in the membrane
The protein changes shape
The molecule is released on the other side of the membrane
Three things simple diffusion depends upon
A high concentration gradient
A thin exchange surface
A large surface area, perhaps provided by microvilli
Two things facilitated diffusion depends upon
A high concentration gradient
The number of channel or carrier proteins
What is osmosis?
The movement of water from an area of higher water potential to an area of lower water potential
What do water molecules have to pass through in osmosis?
A partially permeable membrane
3 things the rate of osmosis depends upon
The concentration gradient (higher = faster osmosis)
The thickness of the cell membrane
The surface area of the exchange surface
What is the water potential of pure water?
0kPa
From what water potential will water move?
From a less negative water potential to a more negative water potential
Explanation of osmosis:
The solution on one side has a low concentration of solute and a high concentration of water molecules. These both have kinetic energy and so move around rapidly. The partially permeable membrane only allows water molecules, not solute molecules, to cross. The water molecules diffuse from the side with the higher water potential to the side with the lowest water potential until an equilibrium is reached.
Why won’t plant cells burst?
The vacuole stores water and the cell wall provides extra support to prevent bursting
What is active transport?
The movement of molecules or ions from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
How does active transport work?
Carrier proteins span the membrane
The molecule attaches to the receptor site on the protein
On the inside of the cell, ATP attaches to the carrier protein, releasing the energy needed for it to change shape
The carrier protein opens the other side of the membrane
The molecule is allowed into or out of the cell
What is the difference between active transport and facilitated diffusion?
Facilitated diffusion doesn’t require an energy input from ATP
How does ATP release energy?
It undergoes a hydrolysis reaction to produce ADP and phosphate group, which also releases energy
What are co transporters?
A type of carrier protein which can bind to two molecules at a time
How does co transport of glucose work?
Sodium ions are actively transported out of the epithelial cells lining the ileum by a sodium-potassium pump
This creates a steep concentration gradient
Sodium ions attached to a glucose molecule diffuse into the lumen of the epithelial cells, down the concentration gradient via the co transporter proteins
The concentration of glucose inside the cell is high
Glucose diffuses out of the cell and into the blood by facilitated diffusion
3 things needed for efficient active transport
Fast carrier proteins
Lots of carrier proteins
The rate of respiration in the cell and the availability of ATP
Why is co transport described as indirect?
Rather than ATP fuelling the movement of glucose, it is the movement of sodium ions
What is Oral Rehydration Therapy?
A way of treating the dehydration caused by severe diarrhoea
What four things does an ORT sachet include?
Water
Sodium ions
Glucose
Potassium ions
Why will the length of a potato cylinder increase when you put it in distilled water?
The potato has a more negative water potential, so water will move into the potato by osmosis, which causes the cells to extend and become turgid
What is important to remember about the proteins associated with active transport?
Active transport only uses carrier proteins, not channel
How do sodium and glucose move into a cell by co-transport?
Facilitated diffusion
