3.2.3 Transport Across Membranes Flashcards
What is the cell-surface membrane?
The cell surface membrane separates the internal cell environment and the external environment, or the inside of organelles from the rest of the cell. The inside of the cell is separated from tissue fluid, and the inside of the organelles is separated from the cytoplasm and water.
What is the name of the model of the cell membrane?
Fluid-mosaic model
Why is the cell membrane model known as fluid?
Molecules are not fixed and are free to move relative to one another, which gives it a flexible shape to self-repair.
Why is the cell membrane model known as mosaic?
It is made up of many different molecules that vary in shape, size and pattern so is similar to a mosaic.
How do phospholipids orientate themselves in the cell membrane?
Phospholipids form a bilayer, with the polar hydrophilic head pointing outwards, and the non-polar hydrophobic tail pointing inwards.
What are the molecules that make up the cell membrane?
Phospholipids
Intrinsic (channel or carrier) proteins
Extrinsic proteins
Cholesterol
Glycoproteins
Glycolipids
What are the functions of the cell membrane?
Controls the entry and exit of materials in discrete organelles eg. Mitochondria
Separates organelles from the cytoplasm so specific metabolic reactions can take place there
Provides an internal transport system eg. Endoplasmic reticulum
Isolates enzymes that might damage cell
Provides surfaces on which reactions can occur
Why do most molecules not freely diffuse across the cell membrane?
They are not lipid soluble, so diffuse by facilitated diffusion.
They are too large to pass through the phospholipid bilayer, so diffuse by facilitated diffusion.
They are polar and so have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer, so diffuse by facilitated diffusion.
What is the structure of a phospholipid?
Polar hydrophilic head points outwards towards the surface of the membrane.
Non-polar hydrophobic tail points inwards towards the membrane interior.
What is the function of phospholipids?
Allow lipid-soluble substances to enter and leave the cell.
Prevent water-soluble substances from entering and leaving the cell.
Make the membrane flexible and self-repairing.
What is the structure of intrinsic and extrinsic proteins?
Intrinsic proteins span the bilayer and cross the whole membrane, including channel proteins and carrier proteins.
Extrinsic proteins occur on the inner and outer surface of the membrane and do not extend across it.
What is the function of intrinsic and extrinsic proteins?
Intrinsic proteins act as channels transporting water-soluble substances across the membrane.
They also allow active transport to take place across the membrane through carrier proteins.
Extrinsic proteins form the cell surface-receptors for identifying cells eg. Hormones
Both provide structural strength
Intrinsic proteins are specific to the molecule or ion they transport.
What is the structure of cholesterol?
It has a hydrophobic tail and a hydrophilic head, and fits in between the phospholipids in the bilayer.
What is the function of cholesterol?
Reduces the lateral movement of phospholipids by pulling the fatty acid tails together.
Makes the membrane less fluid in high temperatures
Prevents the leakage of water and dissolved ions from cells.
What is the structure of glycolipids?
Made up of a carbohydrate chain attached to a phospholipid. The carbohydrate chain extends out from the bilayer out of the cell.
What is the function of glycolipids?
Act as recognition sites
Help to maintain the stability of the membrane by preventing lateral movement
Attach different cells to one another to form tissues
Act as receptors for other substances eg. Hormones
What is the structure of glycoproteins?
Extrinsic proteins molecules with a carbohydrate chain attached to it.
The carbohydrate chain extends out from the bilayer outwards from the cell.
What is the function of glycoproteins?
Act as recognition sites
Attach different cells to one another to form tissues
Act as receptors for other substances eg. Hormones
Define diffusion
The net movement of molecules from an area of higher concentration of molecules to an area of lower concentration of molecules
What is simple diffusion?
When molecules move automatically, meaning the cell or organelle cannot control or change the rate of diffusion.
It is used to transfer molecules that are lipid-soluble or very small so that they can diffuse across the bilayer.
Does diffusion require energy?
The molecules move down a concentration gradient, so no energy is required from ATP.
However, the particles are moving so do have energy, but this is their own kinetic energy that enables them to flow.
What is facilitated diffusion?
Involves the use of channel and carrier proteins to transport molecules.
The molecules that can be transferred are ions and polar molecules, and those that are too large for simple diffusion.
How do channel proteins transfer molecules across membranes?
They form a water-filled channel that enables water-soluble ions to pass through the membrane.
They are selective, as channel proteins only open in the presence of certain ions when they bind to the protein.
How do carrier proteins transfer molecules across membranes?
They bind with a molecule which causes a change in the shape of the protein, enabling the molecule to be released at the other side of the membrane.
Define osmosis
The movement of water from an area of higher water potential to an area of lower water potential, across a partially permeable membrane.
What is water potential?
The pressure created by water molecules, measured in KPa.
What is the waste potential of pure water?
0
This means when solutes dissolve in it, the water potential will be negative.
The more negative the water potential, the more solutes must be dissolved in it.
There are no positive values of water potential.
Define isotonic
When the water potential is the same on both sides of a membrane, so there is no net movement of water.
Define hypotonic
When the water potential of a solution is higher than the water potential of the cell inside it, so water moves into the cell, causing it to become turgid or burst.
Define hypertonic
When the water potential of a solution is lower than the water potential of the cell inside it, so water moves out of the cells, so the cell becomes shrivelled.
What are the factors that affect the rate of diffusion?
Temperature
Surface area
Concentration gradient
Properties of molecules or ions
What is active transport?
The movement of particles from a low concentration of particles to a high concentration of particles, using energy from ATP and carrier proteins.
Why is ATP needed for active transport?
Substances move against the concentration gradient, so energy is required
What types of molecules are transported by active transport?
Large molecules and ions
Describe the process of active transport
The molecule binds to a receptor site on the carrier protein that is complementary in shape.
ATP is hydrolysed when it binds to the carrier protein from the inside, forming ADP and Pi.
This enables the carrier protein to change shape, so releases the molecule to the other side of the cell.
How is the small intestine adapted for efficient active transport?
Large surface area
Walls of the villi are one cell thick (short diffusion distance)
High concentration gradient in capillaries
Many mitochondria to produce ATP
Many channel and carrier proteins to transport molecules
What is ficks law?
The rate of diffusion is proportional to…
(Surface area x concentration difference) / distance between areas
This means that the rate of diffusion increases as the surface area increases, the concentration difference increases, and the distance decreases.
What is co-transport?
When one substance moves against a concentration gradient, as another moves with the concentration gradient.
Give an example of when co-transport is used and why?
To absorb glucose or amino acids from the lumen of the small intestine into the epithelial cells.
There must be a higher concentration of glucose in the lumen than in the epithelial cell for facilitated diffusion to take place.
Active transport and co-transport are required as this is not usually the case.
Describe the process by which glucose is absorbed into the epithelial cells from the lumen?
Sodium ions are actively transported out of epithelial cells by the Na+/K- pump (which is a specific protein) into the blood.
This also causes potassium ions to move into the cell, but means that there is a higher concentration of sodium ions in the lumen than in the epithelial cell.
Sodium ions diffuse down the concentration gradient through a different carrier protein. This is by binding to receptors on the carrier protein, which also encourages glucose molecules to bind, so they are transported together.
Glucose from the cell passes into the blood plasma by facilitated diffusion through a channel protein from the epithelial cell.
Why is glucose absorption in the small intestine an example of indirect active transport?
It is the sodium concentration gradient, rather than ATP directly, that powers the movement of glucose or amino acids.
Which two molecules are transported from the lumen into epithelial cells by co-transport with sodium ions?
Glucose molecules or amino acid molecules
