# Exercise - Surface:Volume

Compare the surface to volume ratio (surface:volume) of a cube that is 1 cm $$\times$$ 1 cm $$\times$$ 1 cm with that of a cube that is 10 cm $$\times$$ 10 cm $$\times$$ 10 cm.

### Smaller cube (1 cm $$\times$$ 1 cm $$\times$$ 1 cm)

The surface area of one side = 1 cm $$\times$$ 1 cm = 1 square cm (or 1 cm2).

There are 6 sides, so the total surface area = 6 $$\times$$  cm2 = 6 cm2.

Volume = 1 cm $$\times$$ 1 cm $$\times$$ 1 cm = 1 cubic cm (or 1 cm3)

$$\mathrm{Surface:Volume = \dfrac{6\: cm^2}{1\: cm^3} = 6\: cm^2/cm^3}$$ (or 6 square cm of surface area for each cubic cm of volume)

### Larger cube (10 cm $$\times$$ 10 cm $$\times$$ 10 cm)

The surface area of one side = 10 cm $$\times$$ 10 cm = 100 square cm (or 100 cm2).

There are 6 sides, so the total surface area = 600 $$\times$$  cm2 = 600 cm2.

Volume = 10 cm $$\times$$ 10 cm $$\times$$ 10 cm = 1000 cubic cm (or 1000 cm3)

$$\mathrm{Surface:Volume = \dfrac{600\: cm^2}{1000\: cm^3} = 0.6\: cm^2/cm^3}$$ (or 0.6 square cm of surface area for each cubic cm of volume).

The larger cube has more surface area and more volume but less surface area for each cubic centimeter of volume.

### Nonmathematical Example

A nonmathematical example may help explain why the smaller objects have more surface for each unit of volume.

Imagine that a knife is used to cut the large cube three times to produce the eight smaller cubes shown above. In order to produce the smaller cubes, new surfaces were created when the knife cut the cube. The total surface area of the eight smaller cubes is greater even though the total volume has not changed.

### Cells

For any given geometric object (cubes, spheres, etc.), smaller objects have a greater surface to volume ratio (surface:volume) than larger objects of the same shape.

Every cell is surrounded by a plasma membrane (discussed below and in the next chapter). Most cells are very small and therefore have a high ratio of plasma membrane surface to cell volume.

# Cell Theory

All organisms are composed of cells, and a cell is the smallest unit of living matter.

Cells come only from preexisting cells.

# Major Kinds of Cells

### Prokaryotic Cells

Bacteria and archaea are prokaryotes. Their cells are very small and very simple. They will be discussed later.

Chapter on prokaryotes

### Eukaryotic cells

All other cells are eukaryotic cells. These include protists, fungi, plants, and animals.

The diagram below shows evolutionary relationships between Bacteria, Archaea, and Eukarya

# Eukaryotic Cells

Cells contain structures called organelles. The structure and function of the major organelles found in eukaryotic cells are described below.

## Plasma membrane

All cells are surrounded by a plasma membrane. It separates the contents of the cell from its environment and regulates the passage of molecules into and out of the cell.

The membrane contains proteins that have a variety of functions. For example, some proteins are receptors which can detect the presence of certain kinds of molecules in the surrounding fluids. The function of membrane proteins will be discussed in more detail in the chapter on membranes.

An actively metabolizing cell needs a large surface area. Cells are limited in size because larger cells have a smaller surface to volume ratio.

Cells that are specialized for absorption (ex: intestinal cells) have folds in the plasma membrane called microvilli that increase the surface area.

Pseudopodia are temporary extensions of the plasma membrane used for movement or to engulf particles. Pseudopodia can be seen in the Amoeba below.

The type of crawling movement where pseudopods are extended is called amoeboid movement. These words can be used for any kind of cell that exhibits this kind of movement.

The video linked below shows Amoeba engulfing another microorganism for food. The process where material is brought into a cell is called phagocytosis. During phagocytosis, pseudopods surround the  material being phagocytized. The material becomes surrounded with plasma membrane as it is engulfed and it pinches inward.

VIDEO - Amoeba movement and phagocytosis

## Cell Wall

The cell wall functions to support and protect the cell.

Plants have cell walls composed of cellulose; fungi have walls composed of chitin.

The cell walls of these onion skin cells can be easily seen.

## Nucleus

The nuclei can be seen in the photograph of human cheek cells below.

The nucleus contains DNA. Recall that DNA contains instructions needed to produce proteins that control  metabolism and other cell functions.

One nucleus can serve a limited amount of cytoplasm, so large cells are often multinucleate, that is, they contain more than one nucleus.

 Teased skeletal muscle X 200 Note the many nuclei visible in the cell on the left.

The nucleus of eukaryotic cells contain a complex of DNA and proteins called chromatin. It functions to package DNA so that it fits within the nucleus and it compacts the DNA during cell division. It also plays a regulatory role in the expression of some genes. The chromatin contains several discrete pieces called chromosomes. The individual chromosomes are normally not visible but during cell division, the DNA becomes more condensed and the chromosomes become visible using light microscopy.

The material within the nucleus is referred to as the nucleoplasm.

A double membrane (nuclear envelope) surrounds the nucleus.

Nuclear pores are openings in the nuclear envelope that allow materials to pass into and out of the nucleus. The passage of RNAs, proteins, and other large molecules can be regulated by proteins associated with the pore called a pore complex.

## Cytoplasm

Cytoplasm is the material enclosed by the plasma membrane, excluding the nucleus.

## Ribosomes

Ribosomes read the code in mRNA and synthesize protein accordingly.

 The symbols to the left are used in the drawings of protein synthesis below.

 The ribosome attaches to the mRNA.

 As ribosomes move along messenger RNA (mRNA), the amino acids are added to a growing chain to form a particular protein. In these drawings, the ribosome moves from left to right.

 In this drawing, the protein is nearly complete. When the ribosome reaches the end of the genetic message, it will become detached from the mRNA.

Several ribosomes may be attached to a strand of mRNA forming a unit called a polysome.

A ribosome is composed of 2 subunits. In eukaryotic cells, the subunits are synthesized in the nucleolus and move into the cytoplasm. During the process of protein synthesis, two subunits will come together along with mRNA..

Ribosomes are composed of both RNA (called ribosomal RNA or rRNA) and protein.

Some ribosomes are attached to the rough endoplasmic reticulum or the nuclear envelope and others are unattached. Ribosomes attached to the rough endoplasmic reticulum synthesize proteins directly into the space enclosed by the rough endoplasmic reticulum (the ER lumen).

Ribosomes in eukaryotes about 1/3 larger than those in prokaryotes.

## Nucleolus

The nucleolus is a structure within the nucleus where the ribosomal subunits are produced.

In cells that have been stained, it appears darker than the nucleus.

## Endomembrane System

The endomembrane system is a network of membranes that extend throughout the cell. It includes organelles and parts of organelles that function together in the production and secretion of lipids and proteins.The following structures are part of the endomembane system and are discussed below.

• Nuclear envelope
• Rough endoplasmic reticulum
• Smooth endoplasmic reticulum
• Golgi apparatus
• Transport vesicles
• Lysosomes
• Plasma membrane

## Endoplasmic Reticulum

The endoplasmic reticulum is a membranous network that extends throughout the cell.

It is continuous with the nuclear envelope and the plasma membrane.

### Rough Endoplasmic Reticulum

The rough appearance of rough endoplasmic reticulum is due to the presence of ribosomes on the membrane.

The rough ER functions in protein synthesis. Proteins are synthesized by ribosomes attached to the rough ER and enter the lumen (interior) of the endoplasmic reticulum while being synthesized.

The rough endoplasmic reticulum also functions in the modification of newly formed proteins. For example, some enzymes may add carbohydrate chains forming glycoproteins. Molecular chaperones are enzymes that function to fold the newly-synthesized proteins into their proper shape.

Transport vesicles are small sacs that pinch off of the endoplasmic reticulum or the Golgi apparatus (discussed below) and transport molecules to other parts of the cell.

Many of the proteins produced by the rough ER and packaged into transport vesicles are destined for secretion by the cell.

Some proteins produced by the rough ER will become packaged in organelles called lysosomes. They function in intracellular digestion.

The rough endoplasmic reticulum also functions to synthesize new membrane, including the phospholipids and embedded proteins. Transport vesicles that originate from the rough endoplasmic reticulum fuse with other membranes in the cell becoming part of the membrane.

Summary- function of the rough ER:

1) Protein synthesis and subsequent modification

secreted proteins
lysosomes
membrane proteins

2) Membrane synthesis

### Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum contains passages that appear to be tubular in structure. The outer surface does not contain ribosomes so it appears to be smooth.

Smooth endoplasmic reticula are connected to rough endoplasmic reticula and, in most cells, the endoplasmic reticula are continuous with the nuclear envelope.

The smooth endoplasmic reticula have a variety of different functions. One important function is to produce lipid compounds such as phospholipids, steroids, and fatty acids.

Certain kinds of cells have smooth endoplasmic reticulum with a specialized function. The following are some examples:

• Smooth endoplasmic reticulum is abundant in the adrenal cortex and the testes where it produces steroid hormones.
• The smooth endoplasmic reticulum of liver cells helps detoxify drugs in the blood.
• Calcium ions needed for contraction are stored in the smooth endoplasmic reticulum of muscle cells.

Vesicles pinch off the smooth endoplasmic reticulum and carry materials to other parts of the cell such as the plasma membrane or Golgi apparatus.

## Golgi Apparatus (also Golgi Complex or Golgi Body)

The Golgi complex is a stack of 3 to 20 flattened, slightly curved saccules which appear like a stack of pancakes.

Vesicles containing molecules from the endoplasmic reticulum arrive at the cis face of the golgi complex.

Chemical reactions within the Golgi complex modify the molecules. For example, the carbohydrate chains that were added to glycoproteins within the lumen of the rough endoplasmic reticulum may be modified.

The Golgi apparatus also manufactures macromolecules such as polysaccharides.

Golgi products are packaged into vesicles that pinch off from the trans face of the golgi complex.

## Lysosomes

Lysosomes are membrane-bound vesicles containing hydrolytic (digestive) enzymes produced by the Golgi complex.

They fuse with other vesicles formed around material that has entered the cell, allowing the digestion of the vesicle contents. For example, bacteria that are engulfed by white blood cells are destroyed by enzymes contained within the lysosomes.

Lysosomes are used to break down parts of the cell such as worn out organelles. The part of the cell to be broken down is surrounded by a double membrane and then it fuses with a lysosome.

## Cellular Secretion

Ribosomes attached to the rough endoplasmic reticulum function to produce proteins. Various chemical reactions may occur within the rough endoplasmic reticulum which modify the proteins. Vesicles pinch off of the rough endoplasmic reticulum, carrying the protein molecules to the golgi apparatus for further modification. The completed molecules are then packaged into vesicles by the golgi apparatus and move to the plasma membrane where they fuse with the plasma membrane, emptying their contents. Some vesicles such as lysosomes remain within the cell.

## Peroxisomes

Some metabolic processes within cells require the removal of hydrogen (oxidation) from certain molecules. For example, the breakdown of fatty acids often involves the removal of hydrogen.

Peroxisomes are vesicles that contain enzymes which remove hydrogen from (oxidize) a variety of different compounds and pass the hydrogen to oxygen, producing hydrogen peroxide (H2O2). Hydrogen peroxide is toxic but the enzyme catalase converts it to water and oxygen.

$\mathrm{2H_2O_2 → 2H_2O + O_2}$

Peroxysomes within the liver are involved in detoxifying certain drugs such as alcohol.

## Vacuoles

Vacuoles are membranous sacs similar to, but larger than

Food vacuoles are produced when the cell takes in food particles by phagocytosis.

Most of the center of a plant cell is occupied by a central vacuole that stores water and dissolved substances. Water pressure within the central vacuole makes the cell rigid (turgid) while the cell wall prevents the cell from bursting.

Some protists have specialized contractile vacuoles for eliminating excess water.

## Chloroplasts

The two diagrams below summarize how energy from sunlight is used for the energy requirements of cells. The chloroplast is involved in storing light energy in glucose. The mitochondrion uses energy stored in glucose to produce ATP.

Photosynthesis is a process by which light energy is used to make sugar from CO2 and H2O. The equation that summarizes these reactions is:

$\mathrm{Energy + 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}$

In eukaryotes, photosynthesis occurs in chloroplasts. Photosynthetic prokaryotes do not have chloroplasts.

The photograph below is an Elodea leaf (X 400). The numerous green structures are chloroplasts.

 Click on the photograph to view an enlargement.

A double membrane surrounds the chloroplast.

Chloroplasts contain membranous disk-like structures called thylakoids that are stacked together in larger structures (grana) that resemble stacks of coins (see diagram below). Molecules that absorb light energy (called photosynthetic pigments) are located in the thylakoid membranes.

The fluid-filled space surrounding the grana is the stroma. Many enzymes needed in photosynthesis are found embedded in the thylakoid membranes and in the stroma.

Click here to view the chapter on photosynthesis

The diagram below shows a chloroplast that has been cut lengthwise to reveal the interior.

In addition to the chloroplast, plant cells have other kinds of plastids that serve a variety of functions such as storing starch or storing pigment molecules that give the cell a certain color.

## Cellular Respiration

Cellular respiration refers to the chemical reactions that break down glucose to CO2 and H2O, releasing the energy stored within its bonds.

The energy is temporarily stored in the bonds of ATP (adenosine triphosphate).

$\mathrm{ADP + P_i + energy \rightarrow ATP}$

This process requires oxygen in aerobic organisms. Anaerobic organisms do not require oxygen, but produce much less ATP per glucose molecule.

Aerobic cellular respiration occurs in the mitochondria.

Prokaryotes do not have mitochondria.

Click here to view the chapter on cellular respiration.

## Mitochondria

Mitochondria have an external membrane and an inner membrane with numerous folds called cristae.

The cristae that project into the gel-like matrix. Enzymes involved in cellular respiration are found in the matrix and embedded in the membrane of the cristae.

The diagram below summarizes the relationship between photosynthesis and cellular respiration.

## Cytoskeleton

The cytoskeleton is a network of protein elements that extend through the cytoplasm in eukaryotic cells.

It provides for the distinctive shape of cells such as red blood cells, muscle cells, and nerve cells (neurons). It also produces movement of cells and is associated with movement of materials within cells.

The cytoskeleton is composed of three types of protein fibers: microtubules, actin filaments, and intermediate filaments. The general function of each of these is listed in the table below.

 Cytoskeleton Element General Function Microtubules Move materials within the cell  Move the cilia and flagella Actin Filaments Move the cell Intermediate Filaments Provides mechanical support

## Microtubules

Microtubules are small cylindrical fibers that change in length by assembling (polymerizing) and disassembling (depolymerizing).

They are made of a protein called tubulin. Tubulin dimers are arranged to form a long hollow cylinder.

The fibers are lengthened and shortened as tubulin dimers assemble or disassemble from one or both ends of the filament.

The assembly of microtubules in animal cells is controlled by an area called the microtubule organizing center. In animal cells, the microtubule organizing center is called a centrosome and contains 2 centrioles. Plant and fungal cells contain microtubules but do not have centrosomes or centrioles.

Like other components of the cytoskeleton, microtubules support and maintain the shape of the cell. Microtubules are particularly effective in resisting compression.

Microtubules act as tracts along which organelles can move. For example, they are associated with movement of vesicles from the Golgi complex to the plasma membrane.

Microtubules are responsible for the movement of cilia and flagella.

They move the chromosomes during cell division.

### Cilia and Flagella

Cilia and flagella are hairlike structures projecting from the cell that function to move the cell by their movements. They contain cytoplasm and are enclosed by the plasma membrane.

Cells that contain cilia are ciliated.

Cilia are shorter than flagella but are similar in construction.

Examples:

• Sperm use flagella to move.
• Many kinds of single-celled organisms such as the Paramecium in the photograph below move by cilia or flagella. The cilia can be seen covering the cell in the photograph.

• Cells lining the human upper respiratory tract are ciliated (have cilia). The cilia move mucous and debris upward to the mouth where it is swallowed. The photograph below is a cross section of a human trachea (400 X). Note the cilia on the upper surface.

Eukaryotes have 9 doublets (pairs) of microtubules arranged in a circle around 2 central microtubules. This 9 + 2 pattern is characteristic of all eukaryotic cilia and flagella but not those of prokaryotes.

The pairs of microtubules are cross-linked. The shifting positions of the cross-links move the cilia or flagella.

### Basal Body

Each cilium or flagellum has a basal body located at its base.

Basal bodies anchor the cilia or flagella and are thought to be responsible for their formation.

Basal bodies contain triplets of microtubules along the periphery but do not have central microtubules (9 + 0).

They look like centrioles (discussed below) and are believed to be derived from them.

### Centrioles and Centrosome

The structure of centrioles is similar to that of basal bodies in that they have 9 triplets of microtubules. Centrioles occur in pairs; each one oriented at a right angle to the other.

Centrioles are contained within a structure called a centrosome.   The centrosome and centrioles are involved in the formation of the microtubules.

## Actin Filaments (Microfilaments)

Actin filaments are long, thin fibers composed of 2 chains of protein wrapped around each other.

The filaments may be branched, producing networks which provide mechanical support and determine the shape of the cell.

Because they can assemble and disassemble quickly, the shape of a cell can change rapidly.

### Movement in eukaryotic cells

Actin filaments assist in the movement of nearly all eukaryotic cells.

Microvilli and pseudopodia move by the action of actin filaments.

Actin filaments are important in muscle contraction.

During cell division a ring of actin filaments that surrounds the cell constricts, pinching the cell into two.

In plant cells, actin filaments are involved in moving the cytoplasm to help circulate the chloroplasts and other materials within the cell. This movement is called cytoplasmic streaming.

## Intermediate Filaments

Intermediate filaments are composed of long, threadlike protein molecules wrapped around one another like the strands of a cable.

As the name suggests, they are intermediate in size. Actin filaments are smallest and microtubules are largest.

Intermediate filaments are important in maintaining the cell's shape, providing mechanical support; preventing excessive stretching, and supporting other organelles. For example, some intermediate filaments support the plasma membrane and others support the nuclear membrane. Skin cells contain intermediate filaments that provide mechanical strength. They also function to attach cells together (desmosomes).

## Extracellular Matrix (ECM)

The cell wall of plants was discussed above.

The extracellular matrix of animal cells is composed of polysaccharide gels, glycoprotein fibers, and other molecules that surround the exterior of the cell. It supports the cell, facilitates the attachment and spacing of cells, facilitates communication between cells, and transmits information to the interior of the cell.

Collagen fibers are embedded within the polysaccharide gel and are capable of providing structural support. Collagen fibers are connected to membrane proteins by a glycoprotein called fibronectin. The membrane proteins, called integrins, span the membrane and are attached to microfiliments on the interior of the cell.

Integrins span the plasma membrane and are capable of transmitting signals from the extracellular matrix to the interior of the cell. They trigger biochemical pathways within the cell.

# Activity

Draw a typical plant cell. Use arrows to label each structure. Next to each label, write a brief description of the function of that structure. Include the following structures in your diagrams.

# Prokaryotic Cells

Prokaryotic cells are small; eukaryotic cells are typically 10 times bigger in diameter and 100 to 1000 times bigger in volume.

Prokaryotic cells do not have a true nucleus. They have few organelles, and have no membrane-bound organelles.  In cyanobacteria, the cell membrane folds inward in a number of places allowing for the attachment of enzymes.

The DNA of prokaryotes is a single, circular chromosome located in a region called the nucleoid. There may be small rings of accessory DNA called plasmids.

Some prokaryotic cells are photosynthetic (example:

The cells have a cell wall and some contain a gelatinous sheath outside the cell wall.

Motile bacteria have flagella.

Prokaryote ribosomes are smaller than those in eukaryotes.

Cell reproduction is by binary fission, not mitosis. By this process, a second chromosome is produced that is an identical copy of the first. The cell elongates and the chromosomes separate so that each new cell receives a chromosome. The elongated cell pinches into two, forming two cells each with one chromosome.