Cell Membranes Flashcards
(43 cards)
What is a cell surface membrane?
These surround a cell and create a boundary. They control what enters and exits the cell and allows different conditions inside and outside. They are partially permeable - let only some molecules out.
What are membranes within cells?
The membranes around the organelles which control entry and exit of materials. They keep the organelle and the substances required (such as mitochondria and substances for respiration) separate from the cytoplasm. They are also partially permeable (in nucleus, RNA leaves but DNA is too big).
What is the basic membrane struture?
Composed of lipids (mostly phospholipids), proteins and carbohydrates.
What are the two parts of phospholipids and what do they do?
- A hydrophilic head - this attracts water and therefore points outside of the cell surface membrane.
- A hydrophobic tail - this repels water and therefore points inwards away from the cell surface membrane.
This creates a bilayer, forming a barrier to dissolved substances.
What are the functions of a phospholipid bilayer?
- Allows lipid-soluble substances (non-polar molecules and water) through the membrane.
- Prevents water-soluble substances (polar molecules and ions) through the membrane.
- Makes the membrane flexible and self-sealing.
What are two ways proteins are embedded in the phospholipid bilayer?
- Some are found at the surface and never extend all the way through. They act as either mechanical support or as receptors for chemicals sent by other cells.
- Some extend all the way across the phospholipid bilayer. These can either be protein channels (water filled tubes which allow water-soluble molecules through), whereas some are protein carriers, which change shape to allow ions (such as glucose/amino acids) through.
What are the functions of proteins in cell membranes?
- Provide structural support.
- Act as water channels to allow water-soluble substances.
- Allow active transport through carrier proteins.
- Form cell surface receptors to identify cells or for hormones.
- Help adhere cells.
What are glycoproteins?
Proteins which have carbohydrates attached to them.
They act as cell surface receptors, recognition sites, and help them attach to each other to form tissues.
What are glycolipids?
Lipids that have a carbohydrate attached to them.
These extend from the bilayer to the watery environment outside the cell. They act as cell surface receptors and recognition sites, as well as build stability and allow them to attach to each other to form tissues.
What is cholesterol?
Molecules found within the bilayer that add strength in all cells except bacteria cells.
They bind to the hydrophobic tails making them more compact together so they move less and be more rigid, providing support and help maintain the shape.
This is important for animal cells as they do not have a cell wall, but also for cells which do not have support form other cells, such as red blood cells floating freely.
Cholesterol also has a hydrophobic region therefore helps further prevent polar molecules from entering the bilayer.
What is the permeability of the cell surface membrane?
The cell surface membrane only allows a few substances through. Some cannot pass through as:
- They are not soluble in lipids.
- Too large to pass through channels.
- Same charge as the protein channel so are repelled.
- Polar molecules so cannot pass through non-polar hydrophobic tails in bilayer.
What is the fluid mosaic model and why is it called that?
The fluid mosaic model is how the molecules are arranged in the cell membrane.
Fluid- the individual phospholipid molecules can move within each other, making it flexible and fluid-like.
Mosaic - proteins embedded in the bilayer vary in shape, size and patterns just like stones in a mosaic pattern.
What happens to cell membranes in temperature bellow 0?
The phospholipids do not have much energy, so stay close together and rigid.
Carrier proteins and channels denature, increasing permeability as they cannot control what enters and leaves.
Ice crystals may pierce the membrane, making it highly permeable when it thaws.
What happens to cell membranes in temperatures between 0-45?
Phospholipids can move around, not tightly packed together, so partially permeable.
As temperature increases, they move more, increasing permeability.
What happens to cell membranes in temperatures above 45?
Phospholipids begin to melt, increasing permeability.
Water expands so puts pressure on membrane.
Protein channels and carriers denature, so cannot control what enters or exits, increasing permeability.
Describe the experiment to investigate the permeability of a cell membrane.
- Use a scalpel to cut 5 equal beetroot cylinders.
- Rinse them to get rid of any excess pigment.
- Add the five pieces into 5 test tubes and add even amounts of water.
- Place each one into a water bath at a different temperature for equal length of time.
- Remove the beetroot to just leave the colored liquid.
- Set up a colorimeter. Put distilled water (three quarters) into a cuvette and calibrate the machine to zero.
- Transfer some of the liquid into a clean cuvette and allow the colorimeter to measure the absorbance of the wavelength.
- Analyze the results - higher absorbance, more pigment, more permeability.
- Draw a graph.
What would happen if you increase a concentration of a solvent that a cell membrane is in?
The permeability would increase as the solvent would dissolve the lipids in the cell membrane, causing it to lose its structure and leak piment.
What is passive transport?
A type of substance exchange between cells and the environment that does not require metabolic energy or external energy such as ATP, but rather relies on its natural inbuilt motion of particles.
What is simple diffusion and some examples of substances that diffuse this way?
When particles move around randomly in constant motion due to the kinetic energy they possess, they end up bouncing off each other and other objects in the vessels. Due to this, they end up evenly distributing if they are in a high concentration vessel.
Simple diffusion is when particles diffuse directly through a cell membrane.
For example, oxygen and carbon dioxide diffuse through the cell membrane as they are small and can pass through the phospholipid bilayer. They are also non-polar so are soluble in lipids, so can dissolve and get through in the hydrophobic bilayer.
What is diffusion?
The net movement of particles from an area of high concentration to an area of low concentration until evenly distributed.
What are three factors affecting the rate of diffusion?
The concentration gradient - the higher the concentration gradient, the faster the rate of diffusion. This means diffusion slows down over time.
The thickness of the exchange surface - the thinner the surface, the less distance the particles have to travel and therefore.
Surface area - the more surface area, the faster the rate of diffusion.
What is facilitated diffusion?
Hydrophilic substances cannot get through the cell membrane.
Therefore, these instead get through the phospholipid bilayer through carrier and channel proteins.
It is a passive process - uses the inbuilt motion of the particles not external energy. It also moves down a concentration gradient but at the certain parts where the proteins are.
What are protein channels?
These proteins form hydrophilic water channels through the membrane, allowing water soluble molecules through.
They are very selective, only opening up to certain ions, or else they stay closed.
What are carrier proteins?
These move large molecules across a membrane. The ion, such as glucose, binds to it, causing it to change shape in a way that the protein is released on the other side. No external energy is required.
