Animal Cells

Membranes and Organelles

Despite their microscopic size, cells contain numerous internal structures called organelles. The structure and function of organelles is often highly complex, and nearly all organelles are involved in intricate biochemical reactions that maintain the living cell. The major organelles in animal cells are outlined below.

1. 1. The cell membrane, or plasma membrane, is the thin (0.005 micrometer wide) membrane that encloses all components within the cell and controls the passage of materials into and out of the cell. Cell membranes are composed of phospholipids (both hydrophyllic and hydrophobic) and sterols (mainly cholesterol). The hydrophobic “tails” attached to these molecules are clustered on the inner part of the membrane. The exterior of the membrane may contain chains of carbohydrates, as well as transport proteins, or permeases, which regulate the transport of materials across the cell membrane. Some proteins also occur on the inner membrane.
   2. Internal membranes, which are present only in eukaryotic cells, are membranes that surround organelles. They have the same basic structure as the plasma membrane, often with modifications, and also regulate transport into and out of the organelle. Because the various functions organelles perform in the cell may require different conditions (pH for example), internal membranes maintain these distinct conditions by isolating organelles from each other.
   3. The cell nucleus contains its genetic material, or DNA (deoxyribonucleic acid). The nucleus is, therefore, the control center of the cell and is separated from the rest of the cell by the nuclear envelope (a membrane whose presence distinguishes eukaryotes from prokaryotes, the latter of which lack a nuclear envelope). The minute pores in the nuclear envelope permit substances to pass from the nucleus to the cytoplasm. As cellular conditions warrant, RNA acts as the intermediary between the DNA in the nucleus and the protein-building apparatus, usually ribosomes, outside the nucleus in the clear cell cytoplasm. The cell nucleus contains the nucleolus, which is the site of ribosome production.
   4. Mitochondria are the metabolic engines of the cell, occurring in both plant and animal cells. Mitochondria are surrounded by a permeable two-layer lipid membrane similar to the plasma membrane. The mitochondrion’s inner membrane is shaped to contain a huge number of folds, called cristae. The inner part of the organelle, called the matrix, is a space filled with enzymes and other metabolic substances.Mitochondria oxidize the products of metabolism from the cytoplasm and convert this energy into ATP (adenosine triphosphate), which fuels cell function. The essence of ATP production, which takes place in the matrix, lies in the transfer of electrons to and from various compounds, with ATP as the final product. A mitochondrion converts fats, amino acids, and sugars into acetyl coenzyme A. The acetyl part of this compound is then oxidized to yield carbon dioxide and hydrogen atoms, which are transferred to the hydrogen acceptors, coenzymes NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). Adding hydrogen produces NADH and FADH2. Oxidation of these compounds occurs gradually, controlled by proteins in the cristae, and eventually yields ATP.

A diagram of a cross-section of a mitochondrion, with labels

1. Lysosomes, which have a single phospholipid bilayer membrane, are vesicles, or large, liquid-filled sacs containing enzymes, including hydrolases, which break down and process some cell products, such as proteins and lipids. Their low pH (~ 5) assists in the degradation of cell materials. Once broken down, or lysed, some of these products are transported across the lysosome membrane and are used to make organic compounds required by the cell. The remaining waste usually remains inside the lysosome and is termed a residual body.
2. Peroxisomes are also vesicles that contain lysing enzymes. Peroxisomes specialize in degrading purines—nitrogenous compounds in cells. The degradation of purines produces hydrogen peroxide, a substance toxic to cells. So, peroxisomes use other enzymes (catalase) to break down hydrogen peroxide into harmless water and oxygen.
3. The Golgi apparatus, or Golgi body, is made up of a stack of five to eight flat, dish-shaped membrane-bound sacs. The stack of sacs is surrounded by tubules and vesicles. Golgi bodies are like factories and redistribution centers. They receive the proteins and carbohydrates that occur on the surfaces of cell membranes, even their own, and then reassemble them. The Golgi apparatus gets vesicles from the endoplasmic reticulum, alters the vesicle membranes, and then processes and distributes their contents to the surfaces of other cell components, including the plasma membrane.
4. The endoplasmic reticulum (ER), which occurs only in eukaryotic cells, is a membranous network of connected flattened sacs, tubes, and channels that may make up as much as half the total membrane in the cell. There are two types of ER, which are continuous with each other, though each has a distinct function. Smooth endoplasmic reticulum (SER) is a mesh of membranous tubules that synthesize phospholipids and cholesterol. The SER accomplishes this with enzymes bound to its membrane. Some of these products remain in the ER; others are transported to other organelles. SER is called “smooth” because it lacks ribosomes, cell particles made of RNA and proteins that synthesize other proteins from amino acids. The membrane of rough endoplasmic reticulum (RER) is made up of flattened sacs and vesicles and is covered with nubby ribosomes. RER is, therefore, key in making and releasing important cell proteins and glycoproteins. Enzymes that are necessary to lysosome function, and proteins required by the Golgi apparatus, the ER, and the plasma membrane are all synthesized and distributed in the RER. A freshly synthesized protein moves from the RER through the SER and then to the Golgi apparatus. The protein is processed and slightly altered at each step, sometimes by splitting its amino acids, and sometimes by adding carbohydrates.

A diagram of organelles involved in protein sythesis and transport

Staying in Shape

Cytoplasm is the living contents inside a cell, excluding the nucleus and the large vesicles. The cytoplasm contains a clear, colorless liquid termed the hydroplasm in which the organelles occur. Cytoplasm is about 90 percent water, but that water is a rich solution of ions, especially potassium, sodium, and chloride. Of course, the cytoplasm is bounded and contained within the plasma membrane.

A cell’s cytoskeleton helps support the cell and keep it in shape. The cytoskeleton consists of interconnected protein filaments in the cytoplasm. The cytoskeleton not only maintains the cell’s three-dimensional shape, it also anchors its organelles and helps the cell move.

Three basic types of structures make up the cytoskeleton:

1. Microtubules, often only 20 nanometers in diameter, are long, hollow strands of the protein tubulin. Microtubules extend from the center of the cell to its plasma membrane. They are a “scaffold” for the building of other cell structures; they also play a key role in cell division. Many microtubules are constantly being built or taken apart, as required by the cell. In unicellular organisms, they are a permanent part of the flagella and cilia and thus are key to locomotion (as in the tail of a sperm cell). Microtubules are the main organizers of the cytoskeleton and may control the position and function of both actin filaments and intermediate fibers, as well as the movement of certain organelles inside the cell.
2. Actin filaments are far smaller than microtubules and are made from the cell protein actin. In non-muscle cells, actin filaments link the plasma membrane to the cytoplasm and are involved in some types of cellular movement. In muscle cells, actin filaments have a wider role. Arranged in an orderly fashion, they interact with filaments made from another protein, myosin, which with actin is crucial in muscle contraction.
3. The size of intermediate fibers falls between that of the microtubules and actin filaments. Intermediate fibers are ropelike and made of fibrous proteins that are not easily disassembled. They are found primarily in cells under mechanical stress.

Rites of Passage

The plasma membrane encloses and protects the contents of the cell. As with all living things, however, cells do interact with their environment. This means that material must be able to pass into and out of the cell through the plasma membrane. The thin, seemingly simple plasma membrane is actually a formidable, complex structure that serves as an effective barrier that keeps unwanted material out of the cell while permitting the entry of useful substances. This makes the plasma membrane selectively permeable—only allowing in needed substances and keeping out unwanted material.

The plasma membrane selectively allows transport of substances into and out of the cell. Intrinsic proteins on the membrane enable passage of needed large, water-soluble molecules and ions across the plasma membrane and into the cell. Some of these proteins produce channels, like tiny pores, through which molecules can pass. Some channels are akin to gatekeepers that open only when a particular chemical signals its “intent” to enter the cell. Gap junctions are protein channels that permit communication between cells.

Other intrinsic proteins simulate ferries that carry a particular molecule across the plasma membrane. Some intrinsic proteins act on the molecule chemically to make it appear more “friendly” to the membrane. Still other proteins pump in the needed molecules.

Diffusion

Most molecules pass through the plasma membrane via diffusion, in which a molecule or substance moves, without any input of energy, from a region where it occurs in high concentration to a region where it occurs in low concentration. Molecules move via diffusion until the entire region reaches equilibrium in terms of the concentration of the particular molecule. (The spread of an aroma through a room is an example of diffusion). The concentrations for most substances inside and outside the cell differ and materials diffuse through channels via passive transport until equilibrium is reached. In some cases, intrinsic proteins assist in moving molecules along the concentration gradient; this process is called facilitated diffusion.

Active Transport

There are times, however, when a molecule must be moved into or out of a cell against the concentration gradient; that is, it must be moved from a region of low concentration or equilibrium to a region of high concentration. An input of energy is needed to accomplish this. Membrane proteins provide this energy boost to move molecules via the process of active transport.

The sodium-potassium pump is a typical active transport system in cells. In most cells, sodium is maintained in low concentrations inside the cell, though its concentration is higher outside the cell. The reverse is true of potassium. ATP (adenosine triphosphate) fuels the active transport of these ions across the plasma membrane to maintain the requisite cell concentrations. Both sodium and potassium are carried by a transport protein, which occurs in two forms—one for each ion. The pump acts as follows: A sodium ion binds to form 1 of the protein while ATP attaches a phosphate group. This causes the protein to change into form 2 and, while shape-shifting, to deposit the sodium ion outside the plasma membrane. The protein now grabs a potassium ion and releases its phosphate group, which transfigures the protein back to form 1. The protein then releases the potassium ion inside the cell.

Diffusion, facilitated diffusion, and active transport are used to move substances across the membranes of organelles, as well as across the plasma membrane.

A diagram of Na-K pump

Particular Problems

Proteins and amino acids are large particles, and they’re too big to cross the plasma membrane via diffusion or by active transport. Yet cells need these substances. What to do? Cells use vacuoles, or vesicles, to get these necessary particles inside.

Endocytosis is a transport process that carries large particles into the cell. In endocytosis, a protein particle, for example, attaches itself to a special area on the outside of the plasma membrane. Attachment triggers the membrane to bulge inward to form a kind of pouch, or vesicle. The pouch enlarges until the particle is completely enclosed in it. Then the vacuole and its contents are released into the cytoplasm.

Energetic Cells

You have read that cells get their energy from ATP (adenosine triphosphate). But what is ATP and where does it come from? ATP is the molecule of choice for energy transfer in all cells. ATP stores the energy that is used in cellular processes in the high-energy chemical bonds between its three phosphates. The breakdown of ATP breaks the phosphate bonds, releasing energy and making it available to the cell. The formula for this reaction, which works in reverse when phosphate is added to ADP to make ATP, is

ATP + H2O –> ADP + Pi + energy

Often, the breakdown of ATP does not release inorganic phosphate, but instead transfers it, with the aid of an enzyme, to another molecule. This process is called phosphorylation. Thus, the phosphorylation of ADP creates ATP. Getting ATP from glucose is a multi-step process. The first step is glycolysis, which breaks down glucose; the second step is respiration, which itself consists of two steps: the Krebs cycle and electron transport chain. Both oxidation (the loss of an electron) and reduction (the addition of an electron) may be used to create energy from glucose.

Airless Energy: Glycolysis

Glycolysis, which occurs in the cell cytoplasm, is an anaerobic process that entails the splitting, or lysing, of glucose. There is a specific sequence of nine chemical reactions in glycolysis, each catalyzed by a particular enzyme. The overall purpose of glycolysis is to break down the carbon bonds in glucose and to use the released energy to produce fuel, or energy, for the cell (in the form of ATP):

glucose + oxygen –> carbon dioxide + water + energy

C₆H₁₂O₆ + 6 O₂ –> 6 CO₂ + 6 H₂O + energy

In glycolysis, a phosphate group is transferred from an ATP molecule to a glucose molecule. The glucose molecule then splits apart, and energy is produced as NAD (nicotinamide adenine dinucleotide—a hydrogen carrier). NAD is reduced to NADH (hydrogen having been obtained from PGAL, phosphoglyceraldehyde). Finally, molecules of ADP are phosphorylated to become ATP.

A full schematic of Glycolysis

Glycolysis yields 2 ATP plus 2 molecules of pyruvic acid. ATP do not move easily across the inner mitochondrial membrane, so its electrons must be carried across. Once this occurs, more ATP molecules are produced. In total, glycolysis yields 8 ATP.

1. glucose + ATP ⇒ enzyme ⇒ glucose-6-phosphate + ADP
2. glucose-6-phosphate ⇒ enzyme ⇒ fructose-6-phosphate
3. fructose-6-phosphate + ATP ⇒ enzyme ⇒ fructose-1,6-diphosphate + ADP
4. fructose-1,6-diphosphate ⇒ enzyme ⇒ 2 PGAL (PGAL = phosphoglyceraldehyde, which readies the fuel for oxidation)
5. 2 PGAL + 2 NAD + 2 P ⇒ enzyme ⇒ 2 3-phosphoglycerolyl phosphate + 2 NADH (hydrogen from PGAL)
6. 2 3-phosphoglyceroyl phosphate + 2 ADP ⇒ enzyme ⇒ 1 3-phosphoglycerate + 2 ATP
7. 2 3-phosphoglycerate ⇒ enzyme ⇒ 2 2-phosphoglycerate
8. 2 2-phosphoglycerate ⇒ enzyme ⇒ 2 phosphoenolpyruvate + 2 H₂O
9. 2 phosphoenolpyruvate + 2 ADP ⇒ enzyme ⇒ 2 pyruvate + 2 ATP

Respiration

Cellular respiration is the oxidation of food (glucose) by cells. Cellular respiration entails the further breakdown of glucose to fuel cell function. In cellular respiration, a sequence of reactions oxidizes pyruvic acid, which is produced in the later steps of glycolysis, to yield energy, carbon dioxide, and water. Cellular respiration occurs in the mitochondria and involves two steps: the Krebs cycle and electron transport chain.

The prelude to the Krebs cycle is often called the transition reaction. The first step in cellular respiration entails the oxidation of pyruvic acid. The carbon is removed from the three-carbon pyruvic acid and forms 2 CO2. Two two-carbon acetyl groups are left (2 CH₃CO). The pyruvic acid’s hydrogen atoms are transferred to hydrogen-carrying molecules of NAD to form 4 NADH. Each acetyl group bonds with coenzyme A (a compound made up of nucleotides and forms of vitamin B) to form the substance acetyl coenzyme A, the key compound that links glycolysis with the Krebs cycle. Converting pyruvic acid to coenzyme A yields 6 ATP.

A schematic of acetyl coenzyme A production

Krebs Cycle

In the Krebs cycle, carbons from the acetyl group (in acetyl coenzyme A) are oxidized to create carbon dioxide, and hydrogen atoms are transferred in an electron transport process. Coenzyme A is involved both in the oxidation of pyruvic acid and in the Krebs cycle.

When the two-carbon acetyl group enters the Krebs cycle, it is combined with a four-carbon compound (oxaloacetic acid) to form the six-carbon compound citric acid. During this process, two of the six carbons are oxidized to carbon dioxide. Some of the energy released during oxidation and breaking of the carbon-carbon and carbon-hydrogen bonds is used to change ADP to ATP, and some of the energy is used to transform NAD to NADH. Remaining energy is used to reduce another electron carrier, FAD (flavin dinucleotide) into FADH₂. Oxygen is not used during the Krebs cycle. All the electrons and protons released are picked up by the NAD+ and FAD. The total number of ATP molecules produced in the Krebs cycle is 24 per molecule of glucose.

The formula for Krebs cycle is:

oxaloacetic acid + acetyl Coenzyme A + ADP + P + 3NAD + FAD ⇒ oxaloacetic acid + 2CO2 + Coenzyme A + ATP + 3NADH + FADH + 3H+ + H2O

Electron Transport Chain

The main carriers in the electron transport chain are known as cytochromes, which are made up of protein and a porphyrin ring. At each step, a different cytochrome, designed specifically to carry an electron at a particular energy level, carries the electron to the next lower level, ending finally with low-energy oxygen. When, at the end of the downward slide, the electrons link up with oxygen, it then combines with protons (hydrogen ions) to yield water.

At each lower step in the chain, the energy released by a pair of electrons is sufficient to transform, or phosphorylate one ADP to one ATP molecule. Once created, an ATP molecule is moved across the mitochondrial membranes. Simultaneously, an ADP molecule moves into the mitochondrion to begin its transformation by phosphorylation into ATP. You can see that the transformation of ADP into ATP, and vice versa, is a perpetual cycle in which a cell creates and uses the fuel that keeps it going.

In summary, glycolysis and cellular respiration yield 38 molecules of ATP to energize the cell.

Summary

• Cells contain internal structures called organelles that maintain all life functions.
• Cytoplasm is the fluid internal component of the cell.
• A cell’s cytoskeleton provides support and keeps it in shape.
• A cell’s plasma membrane is a phospholipid bilayer that is selectively permeable and encloses and protects the cell’s contents.
• Most small molecules required by the cell are able to pass through the plasma membrane via diffusion.
• Phosphorylation is the process of transferring ATP with the aid of an enzyme to another molecule.
• Glycolysis is an anaerobic process that entails the splitting of glucose to release energy.
• Cellular respiration is the oxidation of glucose by cells to release energy.
• The Krebs cycle is the process of oxidizing carbons from the acetyl group to create carbon dioxide, and then hydrogen atoms are transferred in an electron transport process to release energy.
• The electron transport chain is the process of transferring electrons to lower energy levels, and the energy given up is used to created ATP from ADP.