Cells,Discovery of cells & Cell theory

Much of the diversity of forms and functions in living organisms results from small atoms being combinedin different ways to form a number of molecules and molecules form macromolecules. Eventually, thesemacromolecules build cells, tissues, organs and finally, an entire organism.

Cells
A cell is the smallest unit of life that can survive and reproduce on its own, given
information in DNA, energy, and raw materials. Some cells live and reproduce
independently. Others do so as part of a multicelled organism.

Discovery of cells
In the middle of the 17th century, one of the pioneers of microscopy, Robert
Hooke (1635–1703), decided to examine a piece of cork tissue with his home-built
microscope. He saw numerous box shaped structures that he thought resembled
row of empty boxes or rooms, so he called them ‘cells’.

Cell theory
Matthias Schleiden and Theodor Schwann, hypothesized that a plant cell is an independent living unit even
when it is part of a plant and both concluded that the tissues of animals as well as plants are composed of
cells and their products. Together, the two scientists recognized that cells have a life of their own even
when they are part of a multicelled body.
Later, physiologist Rudolf Virchow realized that all cells he studied descended from another living cell.
These and many other observations yielded three generalizations that today constitute the cell theory:
1) Every organism is composed of one or more cells
2) Cell is smallest unit having properties of life
3) Continuity of life arises from growth and division of single cells
Thus, Cell theory is that all organisms consist of one or more cells, which are the basic unit of life.

Cell
A cell is the smallest unit that shows the properties of life.
These properties include -
• Can survive on its own or has potential to do so
• Is highly organized for metabolism
• Senses and responds to environment
• Has potential to reproduce

Structure of Cells

Despite their differences, however, all cells share certain organizational and functional features. Every cellhas a plasma membrane. A plasma membrane is selectively permeable, allows only certain materials tocross. All cell membranes, including the plasma membrane, consist mainly of lipids. The plasmamembrane encloses a fluid or jellylike mixture of water, sugars, ions, and proteins called cytoplasm. Some or all of a cell’s metabolism occurs in the cytoplasm, and the cell’s internal components are suspended in it. All cells start out life with DNA, although a few types of cells lose it as they mature.

Cell type
Biologists have categorized cells into two general types: eukaryotic and prokaryotic cells.
The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the metabolic function it performs. In general, they are large in comparison to Prokaryotic cells. These cell types do not have a nuclear membrane; therefore they lack a cellular nucleus. In addition, they display unique chemical and metabolic characteristics but do not have the variety and number of organelles seen in eukaryotes.

Lipid Bilayer
Lipids—mainly phospholipids—make up the bulk of a cell membrane. A phospholipid consists of a phosphate containing head and two fatty acid tails. The polar head is hydrophilic, which means that it interacts with water molecules. The nonpolar tails are hydrophobic, so they do not interact with water molecules, but they do interact with the tails of other phospholipids. Lipid bilayers are the basic structural and functional framework of all cell membranes, gives membrane it's fluidity.



Fluid mosaic

Other molecules, including steroids and proteins, are embedded in or associated with the lipid
bilayer of every cell membrane. Most of these molecules move around the membrane more or less freely. A cell membrane behaves like a two dimensional liquid of mixed composition, so we
describe it as a fluid mosaic. The “mosaic” part of the name comes from a cell membrane’s mixed composition of lipids and proteins. The fluidity occurs because the phospholipids in a cell membrane are not bonded to one another. They stay organized as a bilayer as a result of collective hydrophobic and hydrophilic attractions.

Membrane proteins Separate

Many types of proteins are associated with a cell membrane, and each type adds a specific function to it, different cell membranes can have different characteristics depending on which proteins are associated with them. For example, a plasma membrane has certain proteins that no internal cell membrane has. Many plasma membrane proteins are enzymes. Others are adhesion proteins, which fasten cells together in animal tissues. Recognition proteins function as identity tags for a cell type, individual, or species. Being able to recognize “self” means that foreign cells (harmful ones, in particular) can also be recognized. Receptor proteins bind to a particular substance outside of the cell, such as a hormone or toxin. Binding triggers a change in the cell’s activities that may involve metabolism, movement, division, or even cell death. Receptors for different types of substances occur on different cells, but all are critical for homeostasis. Additional proteins occur on all cell membranes. Transport proteins move specific substances across a membrane, typically by forming a channel through it. These proteins are important because lipid bilayers are impermeable to most substances, including ions and polar molecules. Some transport proteins are open channels through which a substance moves on its own across a membrane.

Movement of Molecules Across the Membrane

Cells must continuously receive nutrients and rid themselves of waste products—one of the characteristics of life. Many of the proteins that are associated with the plasma membrane are involved in moving molecules across the membrane. Some proteins are capable of moving from one side of the plasma membrane to the other and shuttle certain molecules across the membrane. Others extend from one side of the membrane to the other and form channels through which substances can travel. Some of these channels operate like border checkpoints, which open and close when circumstances dictate. Some molecules pass through the membrane passively, whereas others are assisted by metabolic activities within the membrane.

 

Microscopes

Microscopes allow us to study cells in detail. The ones that use visible light to illuminate objects are called light microscopes. There are two types: Simple and Compound. A more powerful microscope is the Electron microscopes use electrons instead of visible light to illuminate samples. Because electrons travel in wavelengths that are much shorter than those of visible light, electron microscopes can resolve details that are much smaller than you can see with light microscopes. Electron microscopes use magnetic fields to focus beams of electrons onto a sample.

Limitations of Light
• Wavelengths of light are 400-750 nm
• If a structure is less than one-half of a wavelength long, it will not be visible
• Light microscopes can resolve objects down to about 200 nm in size

Electron Microscopy
• Uses streams of accelerated electrons rather than light
• Electrons are focused by magnets rather than glass lenses
• Can resolve structures down to 0.5 nm

Cell size

Almost all cells are too small to see with the naked eye. Why? The answer begins with the processes that keep a cell alive. A living cell must exchange substances with its environment at a rate that keeps pace with its metabolism. These exchanges occur across the plasma membrane, which can handle only so many exchanges at a time. Thus, cell size is limited by a physical relationship called the surface-to-volume ratio. By this ratio, an object’s volume increases with the cube of its diameter, but its surface area increases only with the square. If the cell gets too big, the inward flow of nutrients and the outward flow of wastes across that membrane will not be fast enough to keep the cell alive.

Two Major Cell Types

According to their structure, cells can be of two types:
• Prokaryotes eg. Bacteria
• Eukaryotes eg. Fungi, Plants, Animals

Prokaryotic
Prokaryotic cells are so called because they have no nucleus (‘prokaryote’ comes from the Greek,
meaning ‘before the nucleus’). They also have no organelles (internal structures), so there is little
compartmentalization of function within them. From the mid-20th century, when the electron microscope
was developed, it became possible to study the internal detail of cells.
• The cell wall surrounds the cell. It protects the cell from bursting and is composed of peptidoglycan,
which is a mixture of carbohydrate and amino acids.
• The plasma membrane controls the movement of materials into and out of the cell. Some substances
are pumped in and out using active transport.
• Cytoplasm inside the membrane
contains all the enzymes for the
chemical reactions of the cell. It also
contains the genetic material.
• The chromosome is found in a
region of the cytoplasm called the
nucleoid. The DNA is not contained
in a nuclear envelope and also it is
‘naked’ – that is, not associated with
any proteins. Bacteria also contain
additional small circles of DNA called plasmids. Plasmids replicate
independently and may be passed
from one cell to another.
• Ribosomes are found in all prokaryotic cells, where they synthesize proteins. They can be seen in very
large numbers in cells that are actively producing protein.
• A fagellum is present in some prokaryotic cells. A flagellum, which projects from the cell wall, enables a cell to move.
• Some bacteria have pili (singular pilus). These structures, found on the cell wall, can connect to other bacterial cells, drawing them together so that genetic material can be exchanged between them.
Prokaryotic cells are usually much smaller in volume than more complex cells because they have no
nucleus. Their means of division is also simple. As they grow, their DNA replicates and separates into two different areas of the cytoplasm, which then divides into two. This is called binary fission. It differs slightly from mitosis (a type of cell division) in eukaryotic cells.

Eukaryotes
The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the metabolic function it performs. In general, they are large in comparison to prokaryotic cells.

Animal cells
• Plasma membrane
• Nucleus
• Ribosomes
• Endoplasmic reticulum
• Golgi body
• Vesicles
• Mitochondria
• Cytoskeleton

Plant cells
• Plasma membrane
• Nucleus
• Ribosomes
• Endoplasmic reticulum
• Golgi body
• Vesicles

Functions & Components of Nucleus

Functions of Nucleus
• Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm
• Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells

Components of Nucleus
– Nuclear envelope
– Nucleoplasm
– Nucleolus
– Chromosome
– Chromatin

Nucleus:
The nucleus is the defining organelle of eukaryotic cells. The nucleus is separated from the cytoplasm by a double membrane (two phospholipid bilayers); known as the nuclear envelope. The nuclear envelope controls the passage of molecules between the nucleus and cytoplasm. The nucleus contains the DNA, the stored genetic instructions of each cell. In addition, important reactions for interpreting the genetic instructions occur in the nucleus.

• In the nucleus, DNA is organized into discrete units called chromosomes
• Each chromosome is composed of a single DNA molecule associated with proteins
• The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

 

Nucleolus

•  Dense mass of material in nucleus
•  May be one or more
•  Cluster of DNA and proteins
•  Materials (mostly rRNA) from which ribosomal subunits are built
• Subunits must pass through nuclear pores to reach cytoplasm

Chromatin

Chromatin is composed of long molecules of DNA, along with proteins. Most of the time, the chromatin is arranged as a long, tangled mass of threads in the nucleus. However, during cell division, the chromatin becomes tightly coiled into short, dense structures called chromosomes (chromo=color; some=body). Chromatin and chromosomes are really the same molecules, but they differ in structural arrangement. In addition to chromosomes, the nucleus may also contain one, two, or several nucleoli. A nucleolus is the site of ribosome manufacture. Specific parts of the DNA become organized within the nucleus to produce ribosomes. A nucleolus is composed of this DNA, specific granules, and partially completed ribosomes.

1.  The DNA and proteins of chromosomes are together called chromatin
2. Chromatin condenses to form discrete chromosomes as a cell prepares to divide
3. Chromosome is one DNA molecule and its associated proteins
4. Appearance changes as cell divides

Mitochondria

The mitochondrion (plural, mitochondria) is a type of organelle that specializes in making ATP (molecule used by cells as main energy source). They have various enzymes to catalyze cellular respiration. Bacteria have no mitochondria; they make ATP in their cell walls and cytoplasm. Cells that have a very high demand for energy tend to have many mitochondria e.g. liver needs more because needs more energy. Mitochondria, like most organelles, can move within the cell and they grow and divide independently. Each has two membranes, one highly folded inside the other. Double-membrane system: Smooth outer membrane (lipid bilayer) faces cytoplasm and permeable to small solutes; blocks macromolecules where as Inner Membrane (cristae) folds back on itself to enlarge surface area for chemical reactions to take place. Membranes form two distinct compartments. ATP-making machinery is embedded in the inner mitochondrial membrane.

• Mitochondria and chloroplasts have similarities with bacteria,
• Enveloped by a double membrane
• Contain free ribosomes and circular DNA molecules
• Grow and reproduce somewhat independently in cells

They may have evolved from ancient bacteria that were engulfed but not digested. Mitochondria and chloroplasts developed because as a prokaryote it gained protection by living inside the eukaryote and in turn produced energy for the eukaryote (symbiotic relationship).

Chloroplasts: Capture of Light Energy

Plastids are a category of membrane-enclosed organelles that function in photosynthesis or storage in
plant and algal cells. Plastids called chloroplasts are organelles specialized for photosynthesis. Chloroplastscontain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis. Chloroplasts are found in leaves and other green organs of plants and in algae.

Chloroplast structure includes 

Stroma: Each has two outer membranes enclosing a semifluid interior, the stroma, that contains
enzymes and the chloroplast’s own DNA.

 

Thylakoids: Inside the stroma, a third, highly folded membrane forms a single, continuous compartment. The folded membrane resembles stacks of flattened disks. The stacks are called grana (singular, granum). Photosynthesis takes place at this membrane, which is called the thylakoid membrane. The abundance of chlorophylls in thylakoids is the reason most plants are green. By the process of photosynthesis, chlorophylls and other molecules in the thylakoid membrane harness the
energy in sunlight to drive the synthesis of ATP. The ATP is then used inside the stroma to build carbohydrates from carbon dioxide and water.

Ribosomes: Protein Factories

Ribosomes are nonmembranous organelles responsible for the synthesis of proteins from amino acids. They are composed of RNA and protein. Each ribosome is composed of two subunits—a large one and a small one. As mentioned before, they are constructed in the Nucleolus. Ribosomes carry out protein synthesis in two locations

Bound ribosomes: Many ribosomes are attached to the endoplasmic reticulum. Because ER that has attached ribosomes appears rough when viewed through an electron microscope it is called rough ER. Areas of rough ER are active sites of protein production.

Free ribosomes: Many ribosomes are also found floating freely in the cytoplasm wherever proteins are being assembled. Cells that are actively producing protein (e.g., liver cells) have great numbers of free and attached ribosomes. Ribosomes are not surrounded by membrane (found in prokaryotic cells too)

Cytomembrane System

The cytomembrane system is a series of interacting organelles between the nucleus and the plasma
membrane. Its main function is to make lipids, enzymes, and proteins for secretion, or for insertion into cell membranes. It also destroys toxins, recycles wastes, and has other specialized functions. The
system’s components vary among different types of cells, but here we present the most common ones:
Components of Cytomembrane System
– Endoplasmic reticulum
– Golgi bodies
– Vesicles

Endoplasmic Reticulum

Part of the cytomembrane system is an extension of the nuclear envelope called endoplasmic reticulum, or ER. ER forms a continuous compartment that folds into flattened sacs and tubes. The space inside the compartment is the site where many new polypeptide chains are modified. Two kinds of ER, rough and smooth, are named for their appearance in electron micrographs. Thousands of ribosomes are attached to the outer surface of rough ER.

Rough ER

•  Arranged into flattened sacs
•  Ribosomes on surface give it a rough appearance
•  Some polypeptide chains enter rough ER and are modified
•  Cells that specialize in secreting proteins have lots of rough ER

Smooth ER

•  A series of interconnected tubules
•  No ribosomes on surface
•  Lipids assembled inside tubules
•  Smooth ER of liver inactivates wastes, drugs
•  Sarcoplasmic reticulum of muscle is a specialized form that stores calcium

Functions of Smooth & Rough ER

• The smooth ER
1. Synthesizes lipids
2. Metabolizes carbohydrates
3. Detoxifies drugs and poisons
4. Stores calcium ions

• The rough ER
1. Has bound ribosomes
2. Distributes transport vesicles,
proteins surrounded by membranes
3. Is a membrane factory for the cell

Golgi Bodies

Golgi : The Golgi is a series of flattened membrane compartments, whose purpose is to process and
package proteins produced in-the rough endoplasmic reticulum. The processed molecules are packaged into membrane vesicles, then targeted and transported to-their final destinations.
Functions of the Golgi apparatus
• Modifies products of the ER
• Manufactures certain macromolecules
• Sorts and packages materials into transport vesicles

Vesicles

Small, membrane-enclosed, saclike vesicles form in great numbers, in a variety of types, either on their own or by budding. There are many types but two main are:

 Lysosomes: Digestion & recycling centers

Lysosomes that bud from Golgi bodies take part in intracellular digestion. They contain powerful enzymes that can break down carbohydrates, proteins, nucleic acids, and lipids. Vesicles inside white blood cells or amoebas deliver ingested bacteria, cell parts, and other debris to lysosomes for destruction. The enzymes work best in the acidic environment inside the lysosome. Lysosomes break down worn out cell parts or molecules so they can be used to build new cellular structures. Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole. A lysosome fuses with the food vacuole and digests the molecules Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy

Peroxisomes: In plants and animals, vesicles called peroxisomes form and divide on their own, so they are not part of the endomembrane system. Peroxisomes contain enzymes that digest fatty acids and amino acids. They also break down hydrogen peroxide, a toxic byproduct of fatty acid metabolism. Peroxisome enzymes convert hydrogen peroxide to water and oxygen, or use it in reactions that break down alcohol and other toxins.

Organic Molecules of Life

Living systems are composed of various types of molecules. There are two main types: Organic and Inorganic molecules. All organic molecules contain carbon and those that don't are classified as Inorganic molecules. Organisms maintain reserves of small organic molecules that they can assemble into complex Macromolecules such as carbohydrates, lipids, proteins, and nucleic acids. These are the building blocks of the living organisms.

How are macromolecules formed?

Small molecules common to all organisms are ordered into unique macromolecules.
Many macromolecules consist of polymers. A polymer is a large molecule built up from smaller building block molecules, called monomers. Monomers (subunits) are the building block molecules. The inherent differences between human siblings reflect variations in polymers, particularly DNA and proteins. Macromolecules that make up living organisms are formed via polymerization.
Polymerization is the linking together of monomers to form polymers. Large organic molecules are often built from smaller ones by condensation, a process in which an enzyme covalently bonds two molecules together. A condensation reaction occurs via the loss of a small molecule, usually from two different substances, resulting in the formation of a bond. Polymerization in biological systems typical occurs via dehydration synthesis. Dehydration reaction is synonymous with condensation reaction except that dehydration reaction is limited to those condensations in which the small molecule is water. Dehydration synthesis is synonymous with dehydration reaction.

 

 Figure: Condensation reaction. Three boxes represent molecules attached to each other that have free hydrogen and hydroxyl group. These groups can be utilized to form a bond via dehydration reaction, removing H2O.

Energy is expended to polymerize so all condensation/dehydration reactions require an input of energy in order to move forward!!! Energy is expended to make polymers!

How are macromolecules broken or digested?

Hydrolysis, which is the reverse of condensation, breaks apart large organic molecules into smaller ones. Hydrolysis enzymes break apart polymers into monomers. By breaking the bonds between monomers, Hydrolysis liberates the energy that polymers contained during dehydration synthesis; thus, some of the energy required to polymerize is returned upon hydrolysis. Hydrolysis plays a very important role in the liberation of usable energy (ATP) within cells. Enzymes are employed in biological systems to effect most hydrolysis reactions. Example: Digestion of food involves numerous hydrolysis reactions.

Figure: Hydrolysis reaction requires water to break the bond between two molecules and give back the free hydrogen and hydroxyl groups.

Major Macromolecules Carbohydrates

Carbohydrates are organic compounds that consist of carbon, hydrogen, and oxygen in a 1:2:1 ratio. Cells use different kinds of carbohydrates as structural materials, for fuel, and for storing and transporting energy. The three main types of carbohydrates in living systems are monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides (one sugar unit) are the simplest type of carbohydrate, but they have extremely important roles as monomers of larger molecules. The molecular formula of monosaccharides is (CH2O)n. The number of carbons (n in the formula above) varies between monosaccharide types, but for every carbon in a monosaccharide, there is also one water-molecule equivalent (H2O in the formula). Glucose is the main “fuel” for bacteria, plants and animal cells.
Monosaccharides are the building blocks of more complex carbohydrates. For example, two monosaccharides can bond to form a disaccharide (two sugar unit).
Examples: Sucrose (glucose+fructose) (Table Sugar)
Lactose (glucose+galactose) (Milk Sugar)
Maltose (glucose+glucose) (figure Right)
Before disaccharides can be used by organisms, they must be broken down into their monosaccharide units. The disaccharides have the molecular formula C12H22O11.
A disaccharide is formed upon the formation of a glycosidic linkage (a type of bond) between monosaccharides. This glycosidic linkage forms via a dehydration synthesis reaction.
The “complex” carbohydrates, or polysaccharides, are straight or branched chains of many sugar monomers, often hundreds or thousands of them. There may be one type or many types of monomers in a polysaccharide (many sugar units). Most macromolecular carbohydrates are polysaccharides. Polysaccharides typically serve as (i) carbon and energy storage molecules (starch, glycogen) or (ii) as structural material (e.g., in plants, insects, and fungi).
Most plants make much more glucose than they can use. The excess is stored as starch inside cells that make up roots, stems, and leaves. Some starches are made of thousands of monosaccharide (glucose) units.
Potatoes, beans, and grains such as rice, corn, and wheat are examples of plants that store large quantities of starch. When sugars (fuel for energy) are in short supply, hydrolysis enzymes break the bonds between starch’s monomers to release glucose subunits. Some common Polysaccharides and their characteristics are given below:

Functions of Carbohydrates

1. Providing energy and regulation of blood glucose
2. Sparing the use of proteins for energy
3. Breakdown of fatty acids and preventing ketosis
4.  Biological recognition processes
5. Flavor and Sweeteners
6. Dietary fiber, which is also a form of carbohydrate, is essential for the elimination of waste materials and toxins from the body

Major Macromolecules Proteins

Of all biological molecules, proteins are the most diverse in both structure and function. A tremendous number of different proteins, including some structural types, actively participate in all processes that sustain life. Amazingly, cells can make all of the thousands of different kinds of proteins they need from only twenty kinds of monomers called amino acids. Proteins are polymers of amino acids.

Functions of Proteins

Main functions of Protein

1.  Protein's main function is to build, maintain and repair all our body tissues, such as muscles, organs, skin and hair.
2. Protein can also be used as energy source by body, but this usually only happens when carbohydrate and fat stores are in short supply.

Biological function of Protein
1. Protein acts as storage material of food and energy.
2. Many proteins are enzymes that catalyze biochemical reactions, and are vital to metabolism.
3. Proteins are molecular instrument through which genetic information is expressed.
4. They act as antibodies to prevent disease.
5. The milk proteins help the growth of infant mammals.
6. Like other biological macromolecules such as polysaccharides, lipids and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells.
7. Many proteins are enzymes that catalyze biochemical reactions and are vital to metabolism.
8. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape.
9. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle.
10. Proteins are also necessary in animals’ diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food. Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism.

Amino Acids and Important Facts

An amino acid (AA) is a small organic compound with an amine group(NH3), a carboxyl group (COOH) (the acid), and one or more atoms called an “R group.” In most amino acids, all three groups are attached to the same carbon atom. Amine group acts like a base, tends to be positive. Carboxyl group acts like an acid, tends to be negative. Side chain “R” group is variable, from 1 to 20. During protein synthesis, the amine group of one amino acid becomes bonded to the carboxyl group of the next to make a polypeptide chain.

Amino acids contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S)
There are 20 different kinds of amino acids (AA)

Amino acids are divided into two groups- 1. Essential AA
                                                                    2. Non-essential AA
1. An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized by the organism (usually referring to humans), and therefore must be supplied in the diet.
2. A non-essential amino acid is an amino acid that can be synthesized by the organism (usually referring to humans).So there is no deficiency of this AA in the body if they are not supplied in the diet.
Protein synthesis involves covalently bonding amino acids into a chain. The bond that forms between two amino acids is called a peptide bond. Enzymes repeat this bonding process hundreds or thousands of times, so a long chain of amino acids (a polypeptide) forms.

 

 Important Fact

1. Most microorganisms and plants can biosynthesize all 20 standard amino acids, while animals (including humans) must obtain some of the amino acids from the diet. The amino acids that an organism cannot synthesize on its own are referred to as essential amino acids
2. In animals, amino acids are obtained through the consumption of foods containing protein. Ingested proteins are then broken down into amino acids through digestion, which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases. Some ingested amino acids are used for protein biosynthesis, while others are converted to glucose through gluconeogenesis, or fed into the citric acid cycle. This use of protein as a fuel is particularly important under starvation conditions as it allows the body's own proteins to be used to support life, particularly those found in muscle. Amino acids are also an important dietary source of nitrogen.

Lipids and Examples of lipids

Lipids are fatty, oily, or waxy organic compounds. Lipids are a structurally heterogeneous class of biological molecules that are, as their common characteristic, hydrophobic. Which means that they are insoluble in water. The building blocks of lipids are fatty acids and glycerol. Lipids posses numerous C-H bonds (i.e., they are very hydrocarbon-like). Examples of lipids include: (i) Fats, (ii) Oils, (iii) Waxes, (iv) Phospholipids, and (v) Steroids, etc.
Lipids are similar to carbohydrates in that they contain only carbon, hydrogen, and oxygen. They differ from carbohydrates in one important way: no specific ratio (C:H:O). Many also serve as source of energy. In fact, a gram of fat can produce over twice as much energy as a gram of carbohydrate. Lipids are also a storage form of energy. The proportion of hydrogen to oxygen in carbohydrates is two to one. In lipid it is much higher.

Fats and oils

Fats are lipids with one, two, or three long chain fatty acids bonded (called an ester linkage) to a small alcohol called glycerol. When three fatty acids attach to a glycerol, the resulting molecule, which is called a triglyceride, is entirely hydrophobic.
Fatty

Fatty acids are long-chain hydrocarbons with a carboxyl group (-COOH) at one end.
Fatty acids can be saturated or unsaturated. Saturated types have only single bonds in their tails. In other words, their carbon chains are fully saturated with hydrogen atoms. Saturated fatty acids have no C=C double bonds. Unsaturated fatty acids have one or more C=C double bonds. The tails of unsaturated fatty acids have one or more double bonds that limit their flexibility. Increasing the unsaturation of a fatty acid results in a decreasing melting point.
Fats and oils possess more energy per molecule and less hydration compared with carbohydrates, resulting in fats or oils possessing much more energy stored per unit mass or volume. During digestion, the fat or oil is broken down into these simple molecules (monomers). Fats and oils function in biological systems as energy storage molecules (e.g., nuts, seeds, and animals).

1.Saturated fatty acids have no C=C double bonds. eg. Octanoic acid

2. Unsaturated fatty acids have one or more C=C double bonds. eg. 3-octanoic acid

3. Increasing the unsaturation of a fatty acid results in a decreasing melting point. eg, 3,6-octanoic
acid

Fats
Fats are solid at ordinary temperatures. Generally, fats are produced by animals. In animals, fats are stored in adipose cells. Fats are also important as cushions for body organs and as an insulating layer beneath skin.

Oils
Oils are liquid at ordinary temperatures. Generally, oils are produced by plants. Some common vegetable oils are peanut, soybean, and corn oil.

Waxes
Both plants and animals produce waxes. The waxy coating on some plants leaves is an example of plant waxes. Beeswax is an example of a wax produced by an animal.

Phospholipids & Steroids

Phospholipids
Phospholipids differ from triacylglycerol in the sense that, one fatty acid (out of three) is replaced with a phosphate group, which in turn is bound to additional functional groups.
Structurally and functionally, the important thing about phospholipids is that these molecules are simultaneously hydrophobic (at one end, thefatty acid end) and hydrophilic (at the other end, the phosphate end). Phospholipids are the most abundant lipids in cell membranes, which have two layers of lipids.

Steroids
Steroids are lipids with a rigid backbone of four carbon rings and no fatty acid tails. All steroids possess a common ring structure. These ring structures vary by attached functional groups. Cholesterol is example of a steroid; cholesterol is a membrane component. The common steroid structure is the basis of sterol hormones including the human sex hormones (the estrogens and the androgens, including testosterone).

Nutrition and Health facts regarding lipid consumption

• Most of the lipid found in food is in the form of triacylglycerols, cholesterol and phospholipids.
•  A minimum amount of dietary fat is necessary to facilitate absorption of fat-soluble vitamins (A, D, E and K) and carotenoids.

• Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet. Both of these fatty acids are 18-carbon polyunsaturated fatty acids differing in the number and position of the double bonds.

• Most vegetable oils are rich in linoleic acid (safflower, sunflower, and corn oils). Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts and legumes (particularly rapeseed, walnut and soy).

•  Fish oils are particularly rich in the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
• A large number of studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses, such as depression, attention-deficit hyperactivity disorder, and dementia. In contrast, it is now well-established that consumption of trans fats, such as those present in partially hydrogenated vegetable oils, are a risk factor for cardiovascular disease.

The cytoskeleton is a network of fibers that organizes structures and activities in the cell

Between the nucleus and plasma membrane of all eukaryotic cells is a system of interconnected protein filaments collectively called the cytoskeleton. The cytoskeleton is a network of fibers extending throughout the cytoplasm. Elements of the cytoskeleton reinforce, organize, and move cell structures, anchoring many organelles.

Microtubules

Microtubules are long, hollow cylinders that consist of subunits of the protein tubulin. They form a dynamic scaffolding for many cellular processes, rapidly assembling when they are needed and then disassembling when they are not. For example, before a eukaryotic cell divides, microtubules assemble, separate the cell’s duplicated chromosomes, then disassemble. As another example, microtubules that form in the growing end of a young nerve cell support and guide its lengthening in a particular direction.

Microfilaments

Microfilaments are fibers that consist primarily of subunits of the globular protein actin. They strengthen or change the shape of eukaryotic cells. Crosslinked, bundled, or gel-like arrays of them make up the cell cortex, which is a reinforcing mesh under the plasma membrane. Actin microfilaments that form at the edge of a cell drag or extend it in a certain direction. Myosin and Actin microfilaments interact to bring about contraction of muscle cells.

Intermediate filaments

Intermediate filaments that support cells and tissues are the most stable elements of the cytoskeleton. These filaments form a framework that lends structure and resilience to cells and tissues. Some kinds underlie and reinforce membranes. The nuclear envelope, for example, is supported by an inner layer of intermediate filaments called lamins. Other kinds connect to structures that lock cell membranes together in tissues.

Microtubules control the beating of cilia and flagella, locomotor appendages of some cells. Cilia and flagella differ in their beating patterns Cilia – Cilia (singular, cilium) are short, hairlike structures that project from the surface of some cells. Cilia are usually more profuse than flagella. The coordinated waving of many cilia propels cells through fluid, and stirs fluid around stationary cells. Flagella – Eukaryotic flagella are structures that whip back and forth to propel cells such as sperm through fluid. They have a different internal structure and type of motion than flagella of bacteria.

Plant Cells

Plants are eukaryotes and have the typical eukaryotic cell organization, consisting of nucleus and cytoplasm. The cytoplasm is enclosed by a plasma membrane and contains numerous membrane-enclosed organelles, including plastids, mitochondria, microbodies, oleosomes, and a large central vacuole. Chloroplasts and mitochondria are semiautonomous organelles that contain their own DNA.

 The main Characteristics are given below: 

Cell wall – rigid, support & protect plant, Cellulose fiber embedded

Vacuoles – fluid-filled; store enzymes & metabolic wastes

Plastids – contain DNA surrounded by 2 membranes. Store starch/fats

Absorb visible light – pigments

Chloroplast – site where photosynthesis takes place

Thylakoids – membranous sacs contains chlorophyll

Plants have & we don’t:

·         Cell wall

·         Vacuoles

·         Plastids (where photosynthesis takes place)

Cell Walls of Plants

·         The cell wall is an extracellularstructure that distinguishes plant cells from animal cells, made of cellulose fibers embedded in other polysaccharides and protein.

·         Prokaryotes, fungi, and some protists also have cell walls.

·         The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water.