Cell membrane structure The membrane is composed of both proteins and lipids and the proportion of it ranges from 40:60 accordingly. The lipid content ismainly composed of phospholipids (Lecithin, Cephalin, & Sphingomyelin) and glycolipids and sterols as well as some polysaccharides. Thiscomposition of plasma membrane gives rigidity and stability to the cell. Apart from all these, it is also made up of fats or other triglycerides. Hence this portion of cell plays a vital role in the influx and outflux of materials through this membrane.

All the life processes take place in a cell. A cell itself is made of certain parts. Plant and animal cells are not exactly alike. Various kinds of cells show special differences, yet they all show some basic structural plan, which may be expressed in the term "generalized cell". A generalized cell consists of three essential parts: the cell membrane (plasma membrane), the cytoplasm and the nucleus.

Most parts of a cell have a definite shape, a definite structure and a definite function. Such parts are called organelles.In a very simple sense, a biological cell is a membrane-bound sack filled with molecular shapes interacting in an aqueous fluid. As a result of the magnificent internal organization of cells, mnay complex processes in an organism begin when a molecular "key" fits into a correspondingly shaped molecular "lock" (Study the details of understanding and predicting molecular shape in Chemistry). The organelles have the same status in a cell as the organs have in the entire body of an animal or plant. Cell organelles are living parts.

Cell membrane

Cell membrane (plasma membrane or plasmalemma or phospholipid bilayer) is present in both plant and animal cells. Each cell is surrounded by a cell membrane. The cell membrane has fine pores through which substances may enter or leave the cell. It is living, thin, delicate, elastic and made of proteins and lipids (fats). Its function is to provide a mechanical barrier for the protection of the inner cell contents and to regulate the movement of molecules in and out of the cell. The permeability of the cell membrane is selective. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane.

Cell membrane is a lipid bilayer, which contains a wide variety of biological molecules (primarily proteins and lipids) involved in a vast array of cellular processes. It also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. The cell wall is present only in plant cells. It is made up of a complex polysaccharide (carbohydrate) called cellulose. Its function is to give strength and rigidity to the cell. It is non−living.

Cell membrane fluidity Unsaturated and saturated hydrocarbon tails of phospholipids together enhance membrane fluidity.

Diffusion is a process in which molecules spontaneously move from a region of high concentration to a region of lower concentration leading finally to uniform concentration. Much of the traffic across cell membranes occurs by diffusion. The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion.

Membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. Each substance diffuses down its own concentration gradient, unaffected by the concentration differences of other substances. Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion.

Cellular metabolic processes produce carbon dioxide. Since the source is inside the cell, the concentration gradient is constantly being replenished/re‐elevated, thus the net flow of CO₂ is out of the cell. Cellular respiration requires oxygen, which is in lower concentration inside the cell, thus the net flow of oxygen is into the cell. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O₂ as it enters, diffusion into the cell will continue, because the concentration gradient favors movement in that direction.

Nucleus holds the cell's genetic material It is the membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. The main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle.

Two types of nuclear structures called the nucleolus (plural: nucleoli) and chromatin material are embedded within the nucleoplasm. The nucleus may contain one nucleolus or more nucleoli.

The nucleolus contains ribosomes, RNA, DNA, and proteins. The nucleolus synthesizes protein‐producing macromolecular assemblies called ribosomes. Nucleolus is known as factory of ribosomes.

The chromatin material is a thin, thread‐like intertwined mass of chromosomal material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins. DNA stores all the information necessary for the cell to function (i.e., metabolism), to grow and to reproduce further cells of the next generation. Segments of DNA are called genes. The chromatin is condensed into two or more thick ribbon like chromosomes during the division of cell.

Each eukaryotic species has a characteristic number of chromosomes. A typical human cell, for example, has 46 chromosomes in its nucleus. Erythrocytes in mammals are anucleate when mature, meaning they lack a cell nucleus.

A network of cytoplasmic filaments The cytoskeleton is a cellular scaffolding or skeleton contained within a cell's cytoplasm. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It forms structures such as flagella, cilia and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division.

Cytosol contains a system of protein fibers called cytoskeleton – a network of cytoplasmic filaments that are responsible for the movement of the cell and give the cell its shape. Cytoskeleton is connected to most organelles within the cytoplasm. The three types of proteins, which make up the cytoskeleton are: microtubles, intermediate filaments and microfilaments.

The microfilaments (actin protein) are twisted double strands, which helps in muscle contraction (involves number of muscle fibers) and changes in cell shape. The intermediate filaments (keratin and other types of proteins) consist of eight subunits, which maintains cell shape. The microtubules (tubulin protein) are tubes consisting of spiraling two-part protein subunits, whose function is the movement of chromosomes during cell division.

The cytoplasm helps materials move around the cell by moving and churning, through a process called cytoplasmic streaming. The nucleus often flows with the cytoplasm changing the shape as it moves. The cytoplasm is the site where most cellular activities are done. The cytoplasm holds and protects organelles such as the vacuole, mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and in plant cells - chloroplasts. The functions for cell expansion, growth and replication are carried out in the cytoplasm of the cell.

Cytoplasm has three main functions – energy, storage and manufacturing. It contains other organelles, which store and produce energy. Synthesis of fatty acids, nucleotides and some amino acids also take place in the cytosol. The organelles are the metabolic machinery of the cell and are like little organs themselves.

Ribosomes play a pivotal role in protein synthesis Ribosomes are the spherical, granular organelles that are attached to the outer surfaces of endoplasmic reticulum. These organelles play an important role in the synthesis of proteins, a process called translation.

Ribosomes are dense, spherical and granular particles, which occur freely in the cytosol or remain attached to the outer surfaces of endoplasmic reticulum (RER). These are the particles of 200 Angstroms and are found in all living cells. Chemically, the major constituents of ribosomes are the ribonucleic acid (RNA) and proteins. Lipids are virtually absent in ribosomes. Ribosomes, unlike most other organelles, are not enclosed in membrane.

Ribosomes play an important part in the synthesis of proteins. This process is called as translation. The weight of ribosomes is determined by ultracentrifugation. The sedimentation rate here is expressed as Sedimentation coefficient (S). Hence the weight of such macromolecules are expressed as Svedberg units or S units. Based on the sedimentation rate they are classified in to three groups: 80 S size animal ribosomes; 80 S size plant ribosomes; 70 S size eukaryotic and prokaryotic ribosomes.

Lysosomes act as digestive bags Lysosomes are the vesicles of 400-800μ formed as a result of budding of the Golgi bodies. They are large, blue, rounded & pear-shaped vesicles bound by a single membrane. They contain a concentrated mixture of digestive/hydrolytic enzymes that act like hydrolase, an enzyme that speeds up the process of Hydrolysis. These enzymes are used to destroy redundant cell organelles or damaged molecules from within or outside the cell. The organelle or molecule becomes enclosed in a membrane of the lysosome and gets digested.

Lysosomes are simple tiny spherical sac‐like structures evenly distributed in the cytoplasm. Lysosomes are built in the Golgi apparatus. Each lysosome is a small vesicle surrounded by a single membrane and contains powerful enzymes. They digest excess or worn‐out organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell.

Lysosomes serve as intracellular digesting system, hence, called digestive bags. The food vacuole formed during phagocytosis in Amoebas and many other Protists, fuses with a lysosome, whose enzymes digest the food. Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. Some human cells also carry out phagocytosis.

In humans, macrophages, a type of white blood cell helps defend the body by engulfing and destroying bacteria and other invaders as lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria. Lysosomes also remove the worn out and poorly working cellular organelles by digesting them to make way for their new replacements. In this way, they remove the cell debris and are also known as demolition squads, scavengers and cellular housekeepers.

Thus, lysosomes form a kind of garbage disposal system of the cell. During breakdown of cell structure, when the cell gets damaged, lysosomes may burst and the enzymes eat up their own cells (autophagy). Therefore, lysosomes are also known as suicide bags of a cell. With the help of lysosomes, the cell continually renews itself. A human liver cell, for example, recycles half of its macromolecules each week.

Anatomy of peroxisomes Peroxisomes contain at least 50 different enzymes, which are involved in a variety of biochemical pathways in different types of cells. Peroxisomes originally were defined as organelles that carry out oxidation reactions leading to the production of hydrogen peroxide. Because hydrogen peroxide is harmful to the cell, peroxisomes also contain the enzyme catalase, which decomposes hydrogen peroxide either by converting it to water or by using it to oxidize another organic compound.

Peroxisomes are small and spherical organelles containing powerful oxidative enzymes. They are bounded by a single membrane. Peroxisomes are mostly found in kidney and liver cells. Inner contents of peroxisomes are finely granular, but sometimes a crystalline core is visible by electron microscope in the center of peroxisomes.

This crystalline core is a crystallized protein, called catalase enzyme. Peroxisomes contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide (H₂O₂) as a by‐product, from which the organelle derives its name. The H₂O₂ formed by peroxisome metabolism is itself toxic, but the organelle contains a variety of enzymes, which primarily function together to rid the cell of toxic substance H₂O₂ by converting it to water.

Some peroxisomes use oxygen to break fatty acids down into smaller molecules that can then be transported to mitochondria, where they are used as fuel for cellular respiration. These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell.

Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen (a process termed oxidation). Others are more important for their ability to initiate the production of phospholipids, which are typically used in the formation of membranes.

Source of energy Sun is the ultimate source of energy for almost all cells because photosynthetic plant cells harness solar energy and use it to make complex organic food molecules and other organisms rely on them.

The living cell is a chemical factory in miniature, where thousands of reactions occur within a microscopic space. Sugars can be converted to amino acids that are linked together into proteins when needed, and proteins are dismantled into amino acids that can be converted to sugars when food is digested. Small molecules are assembled into polymers, which may be hydrolyzed later as the needs of the cell change. In multicellular organisms, many cells export chemical products that are used in other parts of the organism.

Cells manage a wide range of functions in their tiny package – growing, moving, housekeeping, and so on. Most of these functions require energy. Cells get this energy and use it in the most efficient manner. Cells, like humans, cannot generate energy without locating a source in their environment. However, whereas humans search for substances like fossil fuels to power their homes and businesses, cells seek their energy in the form of food molecules or sunlight. In fact, the Sun is the ultimate source of energy for almost all cells, because photosynthetic prokaryotes, algae, and plant cells harness solar energy and use it to make complex organic food molecules. Other cells rely on these food molecules for the energy required to sustain growth, metabolism, and reproduction.

An example of catabolism – Glucose breakdown In catabolism, large molecules such as polysaccharides, fat, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids etc to release energy.

It is the set of metabolic pathways, which break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, fat, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids(Cholesterol is a type of fatty acid and an important constituent of cells. It plays a crucial role in the synthesis of hormones and bile salts. It also helps in transporting fats in the bloodstream to tissues throughout the body), nucleotides and amino acids, respectively.

These include breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the energy. Catabolic reactions give out energy. They are exergonic. A simple example of a catabolic reaction that occurs in cells is the decomposition of hydrogen peroxide into water and oxygen:

2H₂O₂ → 2H₂O + O₂

The conversion of glucose during respiration to produce carbon dioxide and water is another common example:

C₆H₁₂O₆ + 6O₂ → 6CO₂+ 6H₂O

The most common set of catabolic reactions in animals can be separated into different stages. In the first, large organic molecules such as proteins, polysaccharides or lipids (Lipoproteins are the major carriers of cholesterol in the blood and are mainly composed of lipids) are digested into their smaller components outside the cells. Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, releasing the energy that is stored.

Pathways involved in Anaerobic Respiration The respiration occurring without oxygen, is termed as anaerobic respiration. Glucose is broken down and the products generated from this are energy and either lactic acid or ethanol and CO₂. This process is termed as fermentation.

Anaerobic respiration or fermentation occurs when oxygen is unavailable or cannot be used by the organism. Two common types of fermentation are alcohol fermentation and lactic acid fermentation. In alcohol fermentation, pyruvate gives off carbon dioxide and is converted to ethyl alcohol (ethanol) in a two–step process. In lactic acid fermentation, pyruvate is converted to lactate (lactic acid).

Humans capitalize on both of these fermentation processes. Yeast undergo alcohol fermentation in the production of beer and wine. Certain bacteria and fungi undergo lactic acid fermentation, and are used to make cheese and yogurt. Thus, metabolism and respiration balance is augmented by the control of other enzymes at different key locations in cellular respiration.

Cells are thrifty, expedient, and responsive in their metabolism. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, cells make energy–rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.

Any excess energy is then stored in larger, energy–rich molecules such as polysaccharides (starch and glycogen) and lipids. Thus, from glycolysis (lysis of glucose) to aerobic respiration (or through the anaerobic process of fermentation), ATP is produced as the prized power molecule.

State of entropy Entropy, is a measure of the "disorder" of a system. Disorder refers to, the number of different microscopic states a system can be in, given that the system has a particular fixed composition, volume, energy, pressure, and temperature.

If energy cannot be destroyed, why can't organisms simply recycle their energy over and over again? It turns out that during every energy transfer or transformation, some energy becomes unusable energy, unavailable to do work.

In most energy transformations, more usable forms of energy are at least partly converted to heat, which is the energy associated with the random motion of atoms or molecules. A logical consequence of the loss of usable energy during energy transfer or transformation is that each such event makes the universe more disordered.

Scientists use a quantity called entropy as a measure of disorder, or randomness. The more randomly arranged a collection of matter is, the greater its entropy (Entropy and enthalpy are two important properties of a thermodynamic system. Enthalpy is the total heat content in a system whereas entropy is the degree of the disorder of the same). We can now state the second law of thermodynamics as follows:

Every energy transfer or transformation increases the entropy of the universe.

For example, in the adjacent figure the intact egg is in a highly ordered state. The picture depicts the state of entropy that defines the second law of thermodynamics, as the scrambled egg is in a state of entropy.

Adenosine triphosphate It is the coenzyme used as an energy carrier in the cells of all known living organisms.

In order to function, every machine requires specific parts such as screws, springs, cams, gears, and pulleys. Likewise, all biological machines must have many well–engineered parts to work. A critically important macromolecule – arguably “second in importance only to DNA” – is ATP (an abbreviation for Adenosine triphosphate). ATP is a complex nanomachine that serves as the primary energy currency of the cell.

ATP is the “most widely distributed high–energy compound within the human body”. This ubiquitous molecule is used to build complex molecules, contract muscles, generate electricity in nerves. All fuel sources of nature, all foodstuffs of living things produce ATP, which in turn powers virtually every activity of the cell and organism. Imagine the metabolic confusion if this were not so!

ATP is a complex molecule that contains the nucleoside adenosine and a tail consisting of three phosphates. ATP is a multifunctional nucleoside triphosphate with three phosphate groups and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP). It is one of the end products of photophosphorylation and cellular respiration. The three main pathways for ATP synthesis is substrate level phosphorylation and oxidative phosphorylation in cellular respiration, and photophosphorylation in photosynthesis.

As far as known, all organisms from the simplest bacteria to humans use ATP as their primary energy currency. The energy level it carries is just the right amount for most biological reactions. Nutrients contain energy in low–energy covalent bonds which are not very useful to do most of kinds of work in the cells. These low energy bonds must be translated to high energy bonds, and this is a role of ATP.

Enzymes present in the outer membrane of mitochondria Some enzymes and enzyme complexes have fixed locations within the cell, and act as structural components of particular membranes.

The cell is not just a bag of chemicals with thousands of different kinds of enzymes and substrates in a random mix. Structures within the cell help bring order to metabolic pathways. In some cases, a team of enzymes for several steps of a metabolic pathway is assembled into a multi–enzyme complex. The arrangement controls and speeds up the sequence of reactions, as the product from the first enzyme becomes the substrate for an adjacent enzyme in the complex, and so on, until the end product is released.

Some enzymes and enzyme complexes have fixed locations within the cell, and act as structural components of particular membranes. Others are in solution within specific membrane–enclosed eukaryotic organelles, each with its own internal chemical environment. For example, in eukaryotic cells, the enzymes for cellular respiration reside in specific locations within mitochondria. Thus, we can say the metabolism is the intersecting set of chemical pathways. It is a choreographed interplay of thousands of different kinds of cellular molecules.

Thus, cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation. Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides (starch and glycogen) and lipids.