BIO101 Study Guide

Unit 4: Cells and Cell Membranes

4a. Describe the structure and function of a typical biological membrane

What is another name for the cell membrane?
What types of molecules make up a cell membrane?
Why is it necessary for biological membranes to form a lipid bilayer?
How does the chemistry of the molecules in a membrane explain why a cell membrane forms?

The cell is the functional unit of life. Every organism has at least one cell, and metabolism (the chemistry of life) occurs within cells. A membrane separates the cell from its surroundings.

Every cell features a cell membrane, also called the plasma membrane. The plasma membrane is a complex arrangement of several different types of molecules. The chief components are phospholipids. Each phospholipid molecule is an amphipathic molecule (polar at one end and non-polar at the other end). This explains why plasma (cell) membranes form.

In the presence of water, phospholipids self-assemble into a phospholipid bilayer, with the non-polar tails in each monolayer pointing toward the non-polar tails of the other monolayer and the polar heads of each monolayer pointing toward the watery solution on its side of the membrane (the water interior of the cell for one monolayer, and the water exterior of the cell for the other monolayer).

In addition to the phospholipid bilayer, the plasma (cell) membrane features various other macromolecules, including proteins, sterols, and polysaccharides.

The plasma (cell) membrane is fundamental to life, so be sure to review its structure (and the structure of an individual phospholipid).

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4b. Describe the characteristics of a membrane, solutes, and solvents

What does it mean for a membrane to be semipermeable?
How does water move through a membrane versus other molecules?
What is the difference between a solvent and a solute?
What does it mean to say that water is a "universal solvent"?
What is the fluid mosaic model, and what part do lipids play?

Membranes are called semipermeable because they allow some substances through, based on size. They are composed of a lipid bilayer with hydrophobic and hydrophilic ends. Pores exist through the membrane that allow water to move via osmosis (down a concentration gradient), and some molecules move through due to their small size. Larger molecules that can not diffuse through the membrane can be transported using embedded integral protein channels through passive transport or facilitated diffusion.

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4c. Predict where molecules will move and how the mass of a cell may change

What are the components of a solution?
What is the difference between a solvent and a solute?
What happens to cell volume when osmosis occurs?
What substances are insoluble in water?

A solution is a mixture that includes a solvent and a number of solutes. The solvent is the part of the solution that dissolves the solutes; the solutes are the parts the solvent has dissolved. In an aqueous solution, water is the solvent. A cell's plasma membrane forms a barrier between intracellular fluid and extracellular fluid (which are both aqueous solutions).

The plasma membrane is selectively permeable, which means some particles easily pass through the membrane while other particles cannot get through. Many solutes are effectively (although not perfectly) prevented from passing through the membrane, so we say the membrane is impermeable to these solutes. Water, on the other hand, can pass through to a certain degree.

Water passes through a plasma membrane using a mechanism called osmosis, a special type of passive diffusion process. The direction and rate of osmosis depend on the relative solute concentrations inside and outside the cell. Water always osmoses to the area that is less watery. This means water always moves away from the compartment that has a higher solute concentration. If the solute concentration of the extracellular fluid is higher than the solute concentration of the intracellular fluid, this means the extracellular fluid is less watery, so water will leave the cell by osmosis, and the cell volume will decrease.

If the reverse is true (the gradient is reversed), then water will enter the cell by osmosis, and cell volume will increase. In each case, notice that the water moves toward the less watery compartment (from high concentration to low, or "down" a concentration gradient). Organisms must regulate their osmotic conditions since changes in osmotic gradients can profoundly damage their cells.

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4d. Describe the characteristics of different cell types

What distinguishes a eukaryotic from a prokaryotic cell?
Are animal and plant cells eukaryotic or prokaryotic?
What do eukaryotic and prokaryotic cells have in common?
What types of cell division are possible for prokaryotic cells but not for eukaryotic cells?

Although all cells share specific characteristics (for example, every cell has a plasma membrane), biologists recognize two fundamentally different categories of cells: prokaryotic cells and eukaryotic cells.

A prokaryotic cell does not feature membrane-bounded organelles; a eukaryotic cell does feature membrane-bounded organelles. A membrane-bounded organelle is an organelle (a tiny organ-like structure within a cell) that is enclosed by its own membrane, separate from the plasma membrane that encloses the entire cell.

Membrane-bounded organelles include diverse structures such as the nucleus, endoplasmic reticulum, lysosomes, mitochondria, chloroplasts, and others. Only eukaryotic cells feature these membrane-bounded organelles, though a eukaryotic cell might feature only some (but not all) of them.

For example, an animal cell (like one in a human body) features most of the membrane-bounded organelles, but it does not feature chloroplasts.

A plant cell, on the other hand, typically includes the membrane-bounded organelles found in an animal cell, plus it also features chloroplasts.

A bacterium, which is a prokaryotic cell, does not feature any of these membrane-bounded organelles. Ensure that you appreciate the differences between these major categories of cell types.

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4e. Classify cells as prokaryotic or eukaryotic, unicellular or multicellular, animal or plant (or other)

What is the difference between unicellular and multicellular organisms in terms of complexity?
What do eukaryotes have that prokaryotes lack?
Which organisms belong in each category: eukaryote or prokaryote?
What functions of eukaryotes are impossible in prokaryotes?

The term prokaryotic refers to relatively simple cells that lack membrane-bound organelles (including the absence of a nucleus; hence, "pro" means "early", and "eukaryotic" refers to "having a nucleus"). Their DNA is in a simplistic circular form (not in well-defined chromosomes, as seen in eukaryotes) and is located in a "nuclear region" rather than confined and organized in a central location, also seen only in eukaryotes.

Bacteria are prokaryotic, and the rest of the organisms we have studied (plants, fungi, and animals) are eukaryotic ("eu" refers to having a "true" nucleus). Prokaryotes are always unicellular, but they can grow as colonies of individuals. For this reason, they are limited to a rudimentary type of cell division (binary fission or division into two parts) and cannot perform meiosis.

On the other hand, eukaryotic organisms have membrane-bound organelles and a membrane-bound central nucleus. Consequently, biologists consider them more complex. They can perform complex cellular functions, including meiosis. Eukaryotes differ slightly in their organelles due to specialized functions. For example, we only see chloroplasts in some algae and plants, allowing for photosynthesis.

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4f. Indicate the functions of the organelles of different cell types

What are some examples of organelles?
Are all organelles membrane-bound?
What types of cells feature these organelles?
Which organelles are common to plant and animal cells?
Do bacteria have organelles?

You should recognize several organelles in this course:

Ribosome – not membrane-bounded; in prokaryotic and eukaryotic cells
Plasma (cell) membrane – in prokaryotic and eukaryotic cells
Cell wall – in most prokaryotic and some eukaryotic cells (although not animal cells)
Nucleus – membrane-bounded; only in eukaryotic cells
Mitochondrion – membrane-bounded; only in most eukaryotic cells
Chloroplasts – membrane-bounded; only in photosynthetic eukaryotic cells (plants and algae)
Golgi body – membrane-bounded; only in eukaryotic cells
Central vacuole – membrane-bounded; only in some eukaryotic cells, including plants and some protists
Rough endoplasmic reticulum – membrane-bounded; only in eukaryotic cells
Smooth endoplasmic reticulum – membrane-bounded; only in eukaryotic cells
Lysosome – membrane-bounded; only in eukaryotic cells
Peroxisome – membrane-bounded; only in eukaryotic cells

Notice that most of these organelles are membrane-bound (wrapped in a membrane). Consequently, they only appear in eukaryotic cells, which are structurally more complex than the prokaryotic cells from which they evolved.

the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The pla

Figure 4.8 These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole – structures not in animal cells. Most cells do not have lysosomes or centrosomes.

What are the primary functions of the various types of organelles?
What advantage is gained by some organelles being membrane-bound?

One difference between the various organelles is their shapes. However, our primary reason for classifying organelles differently is because they perform different functions, just as different organs in our body perform different functions.

Ribosome – molecular machines that interpret codes in mRNA to build proteins
Plasma (cell) membrane – defines the cell and forms the boundary between the contents of the cell and its surroundings
Cell wall – thicker, more rigid than, and exterior to the plasma membrane; withstands pressure and prevents the cell from bursting
Nucleus – enclosed by two membranes; houses the DNA
Mitochondrion – enclosed by two membranes; site for cellular respiration
Chloroplast – enclosed by two membranes; site for photosynthesis
Golgi body – receives newly-formed proteins, modifies them, and packages them for transport to the plasma membrane or out of the cell
Central vacuole – a largely water-filled organelle that can also house pigments and wastes
Rough endoplasmic reticulum – the site for the synthesis of proteins the Golgi body will package
Smooth endoplasmic reticulum – the site for the synthesis of lipids and storage of calcium ions
Lysosome – digests materials by subjecting them to enzymes
Peroxisome – safely breaks down harmful chemicals in the cell

The organelles that are membrane-bound form sub-compartments so they can perform their functions in isolation from the rest of the cellular contents. Before proceeding, be sure you know which functions each organelle performs.

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4g. Explain how large signal molecules get their signal into the cell

What are signal modules?
What are receptors?
How do signal molecules travel to receptors?
Where are signal molecules produced in the body?

Signal molecules are examples of ligands because they must bind to other molecules. We call the molecules signal molecules that bind to receptors. When a signal binds to a receptor, that binding causes changes in the cell. These changes are the responses to the signal.

Some signal molecules are small and non-polar, so they pass easily through a cell's plasma membrane and bind to internal receptors. However, most signals are too large or too polar to pass through the plasma membrane and must bind to receptors on the cell's exterior surface. Although they cannot enter the cell, they still cause changes inside using three primary mechanisms – the difference lies in what kind of receptor receives the signals.

Ion-channel-linked receptors are transmembrane proteins that simultaneously serve as signal receptors and ion channels. When a signal molecule binds to this type of receptor, the ion channel either opens or closes its gate. This leads to changes in the flow of ions, which are charged particles. This redistribution of charge causes various responses.
G-protein-linked receptors are transmembrane receptors associated with special proteins (G proteins) situated on the part of the protein that is in contact with the interior surface of the membrane. The binding of a signal molecule to the receptor activates (and frees) the G protein, which causes various responses.
Enzyme-linked receptors are transmembrane proteins that simultaneously serve as signal receptors and enzymes. The binding of a signal molecule to the receptor activates the enzymatic portion of the receptor (which faces the interior of the cell). Once activated, the enzyme catalyzes various reactions, which causes various responses.

Be sure to understand the functional differences between these three classes of receptors; all three operate by binding to a signal molecule at the exterior surface.

To review, see:

Signaling Molecules and Cellular Receptors

4h. Describe the mechanisms of transport across biological membranes

What are the primary categories of transmembrane transport?
What is the fundamental difference between these primary categories?
What energy source is used for active transport?
What is the difference between osmosis and diffusion of molecules across a membrane?

Particles pass through biological membranes (including the plasma membrane) using various mechanisms, which we can classify into two categories. Transmembrane transport (transport of a particle through a biological membrane) can be active or passive. The requirement of an external source of energy distinguishes the two.

Active transport requires an additional (external) source of energy to drive it. ATP is the most common energy source, but other energy sources can be used. Since additional energy is applied, active transport can move particles against their gradient (see definition in next paragraph), which causes gradients to become even steeper.
Passive transport does not require additional (external) energy to occur. The energy that drives passive transport is in the gradient, a difference in magnitude. The gradient that drives passive transport can be a concentration gradient (the concentration of the particle type is higher on one side of the membrane than the other), an electrical gradient (when the charge distribution is different on one side of the membrane than the other), or both. In all cases of passive transport, the transport occurs down the gradient, from higher to lower concentration. Passive transport never occurs in the direction against the gradient.

There are important subcategories of passive transport:

Simple diffusion is the passive transport of solute particles down the gradient for that type of solute directly through the phospholipid bilayer of the biological membrane. This can only occur for particles small enough or non-polar enough to pass through the bilayer.
Facilitated diffusion is also diffusion, but it requires a transport protein's help (facilitation) to get the particle through the membrane. This occurs for particles that are too big or polar to cross the phospholipid bilayer directly.
Osmosis is the passive transport of solvent particles (not solute particles) down the gradient for solvent particles through a selectively permeable membrane. In biological systems, the solvent is always water, so biological osmosis is the movement of water.

These transmembrane transport processes are fundamental to life because organisms must continuously exchange materials with their surroundings to stay alive.

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Unit 4 Vocabulary

This vocabulary list includes terms you will need to know to successfully complete the final exam.

active transport
bacterium
binary fission
cell membrane
cell wall
central vacuole
chloroplast
concentration gradient
electrical gradient
enzyme-linked receptor
eukaryotic cell
facilitated diffusion
G-protein-linked receptor
Golgi body
ion-channel-linked receptor
lysosome
membrane-bound
metabolism
mitochondrion
nucleus
osmosis
passive transport
peroxisome
phospholipid bilayer
plant cell
prokaryotic cell
receptor
ribosome
rough endoplasmic reticulum
signal molecule
simple diffusion
smooth endoplasmic reticulum
solution
solvent
transmembrane transport