Module 3: Cells

Question 1

What is a secretory vesicle?

Answer 1

Secretory vesicles are produced by the Golgi apparatus and are used to transport various types of proteins out of the cell for use in other parts of the body. The process by which the cell releases proteins through the membrane into the extracellular environment is called secretion?

Question 2

What is the nucleolus?

Answer 2

The nucleolus is a dense body within the cell nucleus which contains the specific DNA that produces the RNA found in ribosomes.

Question 3

What are the centrioles?

Answer 3

Centrioles are cylindrical bundles of microtubules that are responsible for directing the movement of DNA strands during the process of cell division.

Question 4

What are structural proteins associated with the membrane?

Answer 4

Structural proteins generally attach to the inside surface of the cell membrane. Structural proteins can support and strengthen the membrane while others may anchor some cell organs to the intracellular side of the membrane.

Question 5

What are membrane spanning proteins?

Answer 5

Certain proteins are embedded in the phospholipid bilayer such that they span the entire width of the membrane. These membrane spanning proteins can act as gates or channels that control the movement of certain substances into and out of the cell.

Question 6

What are carbohydrate molecules in the membrane?

Answer 6

Groups of carbohydrate molecules can be found associated with extracellular membrane proteins or lipids. They form a protective layer called a glycocalyx which plays a key role in the immune response of the cell and in recognition of other cells in the body.

Question 7

What are enzymes in the membrane?

Answer 7

Associated proteins can be attached to either the intracellular or extracellular surface of the membrane. Enzymes are a form of associated protein which act as catalysts for certain reactions immediately inside or outside the membrane.

Question 8

What are cholesterol molecules?

Answer 8

Cholesterol molecules are found inserted into the non-polar lipid layer of the membrane. This cholesterol helps make the membrane impermeable to some water soluble molecules and also helps to keep the membrane flexible over a wider temperature range.

Question 9

What are the factors influencing the rate of diffusion through protein channels?

Answer 9

• The size of protein channels, which is ~ 0.8 nm, will limit the size of the molecule. For example, sugar molecules are too large to diffuse through protein pores.
• The charge of the molecule will affect the rate of movement through channels because the proteins that make up the channels also have charges on them. Therefore, a positive ion will not go through a channel with a positive charge.
• The greater the electrochemical gradient of a molecule, the greater its rate of movement through the channels. Substances move down both their concentration and electrical gradients.
• The number of channels in the membrane affects the rate. Even if there is a large concentration gradient for an ion, that ion will not move across the membrane unless there are channels for it. The more channels that exist, the more ions that will diffuse across the membrane.

Question 10

What is a solute, solvent and solution?

Answer 10

• A solute is the substance that is being dissolved in a liquid
• A solvent is the liquid that is doing the dissolving
• A solution is what you get when you dissolve a solute in a solvent

Question 11

What are excitable cells?

Answer 11

Excitable cells can use the membrane potential to do work and then spontaneously regenerate electrical potentials at their membranes. There are two types of excitable cells in the body - nerves and muscles.

Question 12

What are the functions of the sodium potassium pump?

Answer 12

• Since sodium leaks into the cell and K+ leaks out of the cell, the pump acts to maintain the concentration gradients of these two ions
• Since three sodium ions are removed for every two potassium ions pumped in, the pump causes the cell to become electronegative on the inside, which contributes a small amount to the resting membrane potential.

Without the sodium potassium pump, most cells would swell until they burst.

• Cells contain large numbers of proteins and other organic compounds to which the cell membrane is impermeable. Many of these carry negative charges and attract large numbers of positive ions around them. This increases the number of particles inside the cell, which would cause substantial osmosis of water into the cell.
• Since the sodium/potassium pump removes three Na+ for every 2 K+ it pumps in, this causes a reduction in the number of particles inside the cell. This causes osmosis of water out of the cell and offsets the osmosis into the cell - this keeps the cell volume constant.

Question 13

What is the sodium potassium pump?

Answer 13

It is an integral membrane protein that pumps 3 Na+ ions out and 2 K+ ions in. Since it contributes to the resting membrane potential by making the inside of the cell more negative (by removing more Na+ from the cell than it replaces with K+), it is called an electrogenic pump. It pumps both of these ions against their concentration gradient, so it requires ATP for active transport.

Question 14

What are equilibrium potentials?

Answer 14

The equilibrium potential for a particular ion is the electrical potential that must be applied to the inside of the cell in order to stop the movement of that ion down its concentration gradient. The larger the concentration gradient, the larger the equilibrium potential needed. The equilibrium potential of an ion varies depending on the animal species and the type of cell. Even nerve cells in the same animal may have different equilibrium potentials for the same ions.

• E(K+) = -90 mV
• E(Na+) = +60 mV
• E(Cl-) = -70 mV

Since resting membrane potential is -70 mV, some K+ will leave the cell and some Na+ will enter. The Na+/K+ pump balances the leakage of these ions.

Question 15

What is the resting membrane potential?

Answer 15

The fluids inside and outside of cells are electrolytic (containing ions). Generally, there is a small excess of anions that accumulate immediately inside the cell membrane along its inner surface. There is an equal number of cations that accumulate immediately outside the membrane. The resting membrane potential is the electrical potential difference across the membrane (the inside is negative with respect to the outside). All cells of the body have a resting membrane potential.

Resting membrane potential is ~ -70 mV in most cases.

Question 16

What are the general permeabilities of key ions?

Answer 16

The following are generalizations for most cells in the body:

• Most cells are not very permeable to Na+. These ions cannot move into the cell because there are few channels for them in the membrane.
• Cl- and Ca++ have similar low permeabilities as Na+. Both have large concentration gradients trying to move the ions into the cell, but each one has few channels through which they can diffuse.
• The membrane is more permeable to K+, so some will leak out down its concentration gradient.

When other channels open, ions will be allowed to move and relative permeabilities will change.

Question 17

What is tonicity?

Answer 17

Tonicity describes the ability of a solution to cause osmosis across a biological cell membrane. The fluid inside a typical human cell has a concentration of ~ 300 mOsm/kg of water.

An isotonic solution has the same concentration as cellular body fluids. A hypotonic solution has a lower concentration relative to cellular fluids and would cause osmosis into the cell - swelling would occur. A hypertonic solution has a high concentration compared to the cell and would cause osmosis out of the cell - the cell would shrink.

Question 18

What are the units of osmosis?

Answer 18

An individual particle that causes osmosis is called an osmotically active particle. The unit used to describe the number of osmotically active particles in a solution is called an osmole..

• Osmolarity = #osmoles / kg of water
• Osmolarity = # osmoles / L of water

Question 19

What are the factors affecting osmosis across the cell membrane?

Answer 19

• The permeability of the membrane to the solutes in the intracellular and interstitial fluid
• The concentration gradients of the solutes in the intracellular and interstitial fluids
• The pressure gradient across the cell membrane

Question 20

What is osmosis?

Answer 20

The diffusion of water requires special pores, since it cannot diffuse through the hydrophobic portion of the lipid membrane. Normally the amount of water that diffuses into the cells is exactly equal to the amount of water that diffuses out; hence, the volume of the cell usually remains constant. Under certain conditions, however, it is possible for a concentration difference for water to develop across a membrane and then there is a net movement of water down its concentration gradient (osmosis).

A solution that has a high concentration of a solute will have a low concentration of water. Pure water will have a high water concentration. If a cell with a high intracellular concentration of glucose were placed in a pure water solution, water would move into the cell, causing it to swell.

Question 21

What is the most abundant substance to diffuse through cell membranes?

Answer 21

Water

Question 22

What is active transport and how does it compare to facilitated diffusion?

Answer 22

Like facilitated diffusion, active transport requires protein carriers that span the cell membrane. This transport mechanism can also be saturated, show chemical specificity, and be competitively inhibited. Unlike facilitated diffusion, active transport involves the use of energy because it moves molecules up their concentration gradients. Energy comes from splitting ATP into ADP and attaching inorganic phosphate (PE). The consequent release of energy powers the carrier movement.

Question 23

How does facilitated diffusion differ from simple diffusion?

Answer 23

The rate of transport in facilitated diffusion is limited by the number of available proteins. Once the carriers are all occupied, the system becomes saturated and cannot operate any faster. The speed at which the carrier can change shape or configuration is also limited; once all the carriers are working and occupied, they are said to be saturated. Facilitated diffusion shows chemical specificity and may be competitively inhibited by molecules that are very similar in shape.

Question 24

What is diffusion?

Answer 24

Diffusion is the movement of molecules from an area of high concentration to low concentration due to the molecules’ random thermal motion, until a chemical equilibrium is reached and the net diffusion rate becomes zero (even though molecules are still randomly moving about). Lipid-soluble substances like oxygen, carbon dioxide, fatty acids and some steroid hormones can diffuse right through the bilayer and are not stopped by the hydrophobic fatty acid chains.

Electrically charged molecules like ions tend to move towards areas of opposite charge, down their electrical gradient. If the chemical and electrical gradients are in opposite directions, the movement of the ion will depend on the balance of the two gradients and will stop moving when the molecules reach electrochemical equilibrium (when the electrical force is equal to and in the opposite direction to the chemical force). Substances that are water soluble may require special protein channels or pores that are specific to generally one type of ion.

Other water-soluble molecules (such as sugars) that cannot diffuse through the lipid bilayer and are too large to pass through protein channels will still cross the membrane at a relatively fast rate. These molecules attach to specific protein carriers on the membrane and cause a change in the protein’s shape. The result is either an opening of the protein channel through which the molecule passes, or the protein rotates the molecule to the inner surface of the membrane where it is released.

Question 25

What are the forms of membrane transport?

Answer 25

• Endocytosis/exocytosis (pinocytosis for small molecules)
• Diffusion through the lipid bilayer (in the case of fat-soluble molecules)
• Diffusion through protein channels (in the case of water and water-soluble molecules)
• Facilitated diffusion
• Active transport

Question 26

What do endocytosis, exocytosis and pinocytosis have in common?

Answer 26

The contents packaged inside of vesicles are able to enter or leave the cell without actually crossing the plasma membrane, due to merging of the membranes.

Question 27

What are some examples of membrane proteins?

Answer 27

• Receptors for the attachment of chemical hormones and neurotransmitters
• Enzymes that help with chemical reactions or break down molecules
• Ion channels or pores that allow water-soluble substances into the cell
• Membrane-transport carriers that transport molecules across the membrane (this may include gated channels)
• Cell identity markers, like antigens or glycoproteins. Antigens are foreign particles that can stimulate the immune system.

Question 28

What is the hydrophobic tail of a phospholipid molecule?

Answer 28

The hydrophobic tails are oriented into the cell membrane - away from the aqueous and extra/intracellular solutions.

The fatty acid tails are the major barrier to water and water-soluble substances such as ions, glucose, urea and most other polar molecules found in living organisms. Fat-soluble substances like oxygen, carbon dioxide, and steroid hormones can penetrate this portion of the membrane with ease since they can dissolve through the lipid region of the membrane.

Question 29

What is the hydrophilic head of a phospholipid molecule?

Answer 29

The hydrophilic heads of the phospholipid molecules that make up the cell membrane face out into the water base solutions inside and outside of the cell.

Question 30

What is the cell membrane?

Answer 30

The primary function of the plasma membrane is to regulate the passage of substances into and out of the cell - allowing certain molecules to cross the membrane while excluding other molecules. The structure of the plasma membrane and the mechanism by which the membrane regulates transport are important. The membrane also plays a role in detecting chemical signals from other cells and in forming physical links with adjacent cells.

The cell membrane separates the intracellular and extracellular environments. Proteins, nucleotides and other large molecules needed for the structure and function of the cell cannot penetrate the membrane. The cell membrane is selectively permeable- providing two-way traffic for nutrients and waste needed to sustain metabolism, while it prevents the passage of other substances between the intracellular and extracellular compartments.

The cell membrane is made up of proteins that form channels and pores, carbohydrate molecules for cell recognition and cholesterol for stability. The most abundant components of the cell membrane are the phospholipid molecules.

Question 31

What is the endoplasmic reticulum?

Answer 31

The ER is a continuation of the cell’s nuclear membrane and is the site for the synthesis, storage and transport of proteins and lipid molecules. There are two types of ER; rough or granular ER which is covered with rows of ribosomes, and smooth or agranular ER which lacks ribosomes. Rough ER is the site of protein synthesis and smooth ER is the site of lipid/fatty acid synthesis. Proteins manufactured in the rough ER are packaged into vesicles that transport them to the Golgi apparatus.

Question 32

What is the mitochondrion?

Answer 32

The mitochondrion is the membranous organelle where most of the body’s ATP is generated. Since ATP is the primary cellular mechanism for energy storage and transfer, the mitochondrion is referred to as the powerhouse of the cell. The number of mitochondrion in a cell is determined by that particular cell’s energy needs. The mitochondrion can replicate themselves even if the cell is not undergoing cell division. This will occur when a cell has increased energy demands over a period of time, such as muscle cells that are regularly exercised.

Question 33

What are lysosomes?

Answer 33

Lysosomes are one type of storage vesicle produced by the Golgi apparatus. Lysosomes act as the digestive system of the cell. They contain different kinds of enzymes that are used by the cell to destroy damaged organelles, kill bacteria, and break down other kinds of biomolecules.

Question 34

What are free ribosomes?

Answer 34

Ribosomes are dense granules of RNA and protein. They are responsible for manufacturing proteins from amino acids under the control of the cell’s DNA. There are two types of ribosomes in the cell: fixed ribosomes, which are attached to the ER; and free ribosomes, which float in the cytoplasm. Free ribosomes often form in groups of 10-20 known as polyribosomes.

Question 35

What is the Golgi Apparatus?

Answer 35

The Golgi apparatus is responsible for packaging proteins from the rough endoplasmic reticulum into membrane-bound vesicles. Two types of vesicles are produced by the Golgi bodies: Secretory vesicles, which transport proteins to the cell membrane for release into the extra-cellular environment; and storage vesicles, such as lysosome, whose contents are stored within the cell.

Question 36

What are equilibrium potentials?

Answer 36

The equilibrium potential for a particular ion is the electrical potential that must be applied to the inside of the cell in order to stop the movement of that ion down its concentration gradient. The larger the concentration gradient, the larger the equilibrium potential needed. The equilibrium potential of an ion varies depending on the animal species and the type of cell. Even nerve cells in the same animal may have different equilibrium potentials for the same ions.

• E(K+) = -90 mV
• E(Na+) = +60 mV
• E(Cl-) = -70 mV

Since resting membrane potential is -70 mV, some K+ will leave the cell and some Na+ will enter. The Na+/K+ pump balances the leakage of these ions.