Chapter 4 Transport Across Membranes Flashcards
What is a plasma membrane?
The name given to all membranes in and around all cells and membrane bound organelles. They all have the same basic structure.
Describe the structure and give 5 functions of the plasma membrane.
Plasma membranes are made up of a phospholipid bilayer, cholesterol, proteins, glycoproteins and glycolipids. Membranes are flexible and self healing.
Functions
1. Separates cell contents from the external environment.
- Controls what enters and leaves the cell.
- Identifies the cell as self.
- Acts as a receptor for various chemicals.
- Site of chemical reactions.
What is the cell-surface membrane?
The cell surface membrane refers specifically to the plasma membrane that surrounds cells and forms the boundary between the cytoplasm and the environment.
How do phospholipids arrange themselves to form the plasma membrane?
The hydrophilic heads point out and are attracted to water on both sides of the membrane. The hydrophobic tails point into the membrane, repelled by water on both sides. This forms a bilayer.
Describe the movement of lipid soluble and water soluble materials into and out of the cell.
Lipid soluble material is able to move into and out of the cell. Water soluble material is unable to move in or out of the cell.
How are proteins embedded in the plasma membrane?
Proteins either occur on the surface only, and don’t cross the membrane, or completely span the bilayer from one side to the other.
What is the role of proteins which are embedded on the surface of a plasma membrane?
Proteins embedded on the surface of a plasma membrane either give mechanical support, or work as cell receptors for molecules such as hormones.
What is the role of proteins which span the width of the plasma membrane? What names are used to describe these proteins?
Transmembrane or integral proteins can act as protein channels, water filled tubes allowing water soluble ions to diffuse across the membrane. Others are carrier proteins that bind to molecules and change shape to move them across the membrane.
Give 6 roles of proteins in the membrane.
1- Structural support
2- Channels for water soluble substances
3- allow active transport
4- cell-surface receptors for identifying cells
5- cell adhesion
6- act as receptors e.g. for hormones.
How does cholesterol interact with water?
Cholesterol is very hydrophobic.
How does cholesterol affect fluidity?
At low temperatures cholesterol gets in between phospholipids and increases fluidity and at high temperatures it reduces the space between molecules and lowers fluidity.
How does solvent concentration affect permeability? How does temperature affect permeability?
As solvent concentration increases the permeability of the cell membrane increases.
At subzero temperatures permeability is very high due to water crystals damaging the membrane. As temperature increases above zero permeability increases as phospholipid fluidity increases until the proteins in the membrane denature resulting in full permeability.
What is the difference between passive and active transport?
Passive uses only the energy of the motion of particles. Active transport uses an external energy source, such as ATP.
Describe the movement of particles.
1 - Particles are constantly in motion due to kinetic energy.
2 - Particle motion is random.
3 - Particles are constantly bouncing off each other and their surroundings.
What is the definition of diffusion?
The net movement of molecules or ions from a region where they are highly concentrated to one where their concentration is lower until evenly distributed.
What kind of molecules can diffuse through the cell-surface membrane? Give examples.
Small, non-polar molecules can diffuse through cell surface membranes.. E.g. oxygen and carbon dioxide.
What does polar mean?
A polar molecule has a positive charge on one side and a negative charge on the other.
Which molecules do not diffuse easily across the plasma membranes. Why?
Charged ions and polar molecules do not diffuse easily across the plasma membrane. This is due to the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane.
Define the term facilitated diffusion
Movement of molecules from a high concentration to a low concentration, across a partially permeable membrane, via specific channel or carrier proteins. It is passive, does NOT require ATP
Where does the energy for facilitated diffusion come from?
The inbuilt kinetic energy of the molecules.
What is a channel protein?
A protein which creates a water filled hydrophilic channel through the cell membrane through which ions can pass. It is selective to a specific ion.
What is a carrier protein?
A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport it requires ATP.
State 4 factors that affect the rate of diffusion
- Surface area
- Number of channel or carrier proteins
- Temperature
- Concentration gradient.
What is the definition of osmosis?
The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.
How can the water potential of a cell or tissue be determined?
Water potential can be determined by placing the substance in various solutions of different water potentials. Where there is no net change of mass, (no gain or loss of water), the water potential must be the same inside the cell/tissue as in the external solution.
Describe what would happen to a red blood cell placed in a solution with a more negative water potential than that of its cytoplasm
Water would move OUT of the red blood cell, down a water potential gradient, shrinking and shrivelling (also known as crenation)
Describe what would happen to a guard cell placed in a solution of less negative water potential than its own cytoplasm
Water would move IN to the guard cell, down a water potential gradient, causing the cell to swell and become turgid.
What is the protoplasm?
The protoplasm refers to the cell surface membrane, nucleus, cytoplasm and vacuole membrane.
What does turgid mean?
When the protoplasm pushes against the cell wall we describe the cell as turgid.
What does plasmolysed mean?
When water has left the cell its volume therefore decreases, the protoplasm no longer pushing against the cellulose cell wall. The cell surface membrane pulls away from the cell wall.
What are the two methods of active transport?
- Directly move molecules. (uniport)
- Co-transport. (antiport and symport)
Sometimes 2 types of molecules or ions are moved at once by active transport, one type in and one out type out of the cell, give an example of this.
The sodium-potassium pump. Sodium ions are actively removed from the cell, whilst potassium ions are actively taken in from the surroundings.
What is the process by which amino acids and glucose are absorbed from the ileum?
Co-transport.
What is the basic principle of co-transport of glucose or amino acids from the ileum?
The molecules are taken into the cells along with sodium ions which have been actively transported out by the sodium potassium pump.
What are the stages involved in the cotransport of glucose from the ileum?
1 - Sodium ions are actively pumped out of the epithelial cells via a sodium-potassium pump, into the blood.
2 - This creates a low concentration of sodium inside the cell compared to the lumen of the ileum.
3 - Sodium ions diffuse into the epithelial cells through a carrier protein. as they do they carry glucose (or amino acid) molecules with them.
4 - This increases the concentration of glucose in the epithelial cell, so it moves via facilitated diffusion into the blood plasma.
Sodium ions and glucose/amino acid molecules move from the ileum to the epithelial cells together, in which direction compared to their concentrations?
Sodium moves down the concentration gradient. Glucose/amino acids move up the concentration gradient.
Describe endocytosis.
A substance is engulfed by the cell creating a vesicle.
Describe exocytosis.
A vesicle in the cell fuses with the membrane and releases its contents out of the cell.
