Transport across cell membranes

Created by

Natalie Hollerer

Cards (50)

Why is the plasma membrane referred to as the fluid mosaic model?
Fluid - the phospholipid bilayer is viscous and individual phospholipids can move position.
Mosaic - the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components.
What are the components that make up the cell-surface membrane?
Phospholipids, proteins, cholesterol, glycolipid, glycoprotein.
Why are phospholipids important components of the cell-surface membrane?
They allow lipid-soluble substances to enter and leave the cell.
They prevent water-soluble substances entering and leaving the cell.
They make the membrane flexible and self-sealing
Why are proteins important in the cell-surface membrane?
They are interspersed throughout so provide structural support.
They act as cell receptors in conjunction with glycolipids for molecules like hormones.
Protein channels transport water soluble substances(ions) across the membrane.
Carrier proteins bind to large molecules allowing them to move across the cell surface membrane by active transport.
They help cells adhere together.
Why is cholesterol important in the cell-surface membrane?
Cholesterol molecules are very hydrophobic, prevent loss of water and dissolved ions.
They pull together the fatty acid tails of the phospholipid molecules, limiting their movement.
Make membrane less fluid at high temperatures.
Why are glycolipids important to the cell-surface important?
They are made up of a carbohydrate covalently bonded with a lipid.
They are responsible for the human ABO blood system.
They help maintain the stability of the membrane.
They act as recognition sites.
They help cells to attach to one another and so form tissues.
Why are glycoproteins important to the cell-surface membrane?
Carbohydrate chains are attached to many extrinsic proteins.
They act as recognition sites - for hormones and neurotransmitters.
They help cells attach to one another and form tissues.
They allow cells to recognise one another e.g. lymphocytes recognise an organism's own cells.
Draw a diagram of the structure of the cell-surface membrane.
As shown below as well as a carrier protein.
What are the functions of membranes within cells?
They control the entry and exit of materials in organelles.
They separate organelles from the cytoplasm so specific metabolic reactions can take place.
They provide an internal support system.
They isolate enzymes that mig by damage the cell.
They provide surfaces on which reactions can occur.
What types of molecules do not freely diffuse across the cell-surface membrane?
Large molecules, not soluble in lipids, same charge as charge on the protein channel, polar molecules and would have difficulty diffusing through the non-polar hydrophobic tails.
What is the definition of simple diffusion? 
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed(equilibrium).
What is the definition of a passive process?
When molecules move down a concentration gradient so does not require energy.
What are the important things to remember about passive processes?
All particles are constantly in motion due to kinetic energy that they possess, this motion is random. Particles are constantly bouncing off one another as well as off other objects.
What type of molecules can diffuse by simple diffusion?
Small, non-polar molecules e.g. O2 and CO2.
What is facilitated diffusion?
Facilitated diffusion is the passive process that speeds up the diffusion for large, polar or or charged molecules with the use of intrinsic transport proteins.
How do protein channels work?
Protein channels form water-filled hydrophilic channels, they allow specific water-soluble ions to pass through. They only open in the presence of specific ions. This is why there is control of what ions go in and out of the cell. The ion binds with the protein causing it to change shape in a way that closes it at one end and opens it at the other.
How do carrier proteins work?
When a large molecule such as glucose binds to the protein, this causes it to change shape so that the molecule is released to the inside of the membrane.
What factors affect both types of diffusion?
Distance, temperature, characteristics of solvent(dense could slow down), mass, characteristics of membrane(surface area + thickness), concentration gradient.
What factor only affects facilitated diffusion?
The number of intrinsic proteins - when all are in use the rate of diffusion plateaus.
What is Fick's law?
Rate of diffusion is proportional to $\frac{surafce\ area\ \times concentration\ gradient}{thickness\ of\ membrane}$
Rate of diffusion doubled if:
SA or concentration is doubled
Thickness of membrane is halved.
What is the definition of osmosis?
Osmosis is the movement of water from an area of high water potential to an area of low water potential through a selectively permeable membrane. It is a passive process.
What is the definition of hypertonic?
The solution has a higher solute concentration then the other solution it is being compared with.
What is the definition of hypotonic?
A hypotonic solution has a lower solute concentration compared to the other solution it is being compared with.
What is the definition of isotonic?
An isotonic solution has equal solute concentration to whatever solution it is compared with.
What is the definition of turgid?
A cell that is described as turgid, swells due to a gain of water by osmosis.
What is the definition of flaccid?
A cell that is described as flaccid, shrinks due to the loss of water by osmosis.
What is water potential?
Water potential is the pressure created by water molecules represented by the Greek letter psi (𝛙) and is measured in units of pressure, usually kPa. Under standard conditions of temperature and pressure (25 $^{\circ}$ C and 100kPa), pure water is said to have a water potential of zero.
How do you know which out of 2 solutions is more concentrated?
Solutions with a more negative water potential are more concentrated.
What is the motion of particles when in equilibrium?
Water molecules are always moving in a random motion due to kinetic energy, as the selectively permeable membrane only allows water across it.
What happens to the rate of reaction when temperature increases?
As temperature increases, kinetic energy increases, particles move faster and reach equilibrium faster where there is no net movement of water.
What happens to animal cells when placed in different types of solutions?
Hypotonic: water diffuses in, no cell wall so pressure will cause osmotic lysis(bursting).
Isotonic: water moving in random motion but no net movement of water due to equilibrium.
Hypertonic: water diffuses out, cell shrinks
What happens to plant cells when placed in solutions of different concentrations?
Hypotonic: water enters cell by osmosis, the protoplast swells, cell is turgid due to strength of cell wall.
Isotonic: water neither enters nor leaves, there is no change to the protoplast, the condition of the cell is said to be incipient plasmolysis.
Hypertonic: water leaves the cell by osmosis, the protoplast shrinks and pulls away from the cell wall, the plant cell is plasmolysed.
Describe the process of osmosis.
Both the solute and water molecules are in random motion due to kinetic energy, the selectively permeable membrane only allows water molecules across it. Water diffuses from an area of high water potential to an area of low water potential, until the point of equilibrium.
What is the definition of active transport?
Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
What are the vital uses of active transport?
Uses of active transport:
uptake of glucose and amino acids in small intestine(co-transport)
absorption of mineral ions by plant roots
excretion of hydrogen ions and urea by the kidneys
exchange of sodium and potassium ions in neurons and muscle cells
How does active transport differ from passive processes?
Differences:
metabolic energy in the form of ATP is needed
substances are moved against a concentration gradient
carrier protein molecules are involved
process is very selective, with specific substances being transported.
What are the similarities between passive processes and active transport?
Both involve the movement of substances across a cell membrane.
Describe the process of active transport for a single ion or molecule?
1.The carrier proteins bind to the molecule/ion.
2. The molecule/ion binds to receptor sites on the carrier protein.
3. On the inside of the cell/organelle, ATP binds to the protein causing it to split into ADP and Pi, releasing energy.
4. As a result the protein changes shape and opens to the opposite side of the membrane.
5. The molecule/ion is then released to the other side of the membrane.
6. The phosphate molecule is released from the proteins which causes the protein to revert to its original shape, ATP reforms.
What are the adaptations of epithelial cells?
Microvilli on cells lining the ileum, 0.6 $\mu m$ in length. It increases the surface area and more carrier proteins can be inserted through which facilitated diffusion and active transport takes place. High density of channel and carrier proteins in a given area. Good blood supply maintain concentration gradient between lumen of capillaries and cytosol of epithelial cell.
What is the role of diffusion in absorption?
Carbohydrates and proteins are being digested continuously, there is greater concentration of amino acids and glucose in ileum then the blood. These molecules diffuse into the blood by facilitated diffusion, glucose is constantly removed from blood into cells for respiration. This helps maintain the concentration gradient from the inside of the ileum and the blood.