# Human Physiology/Cell physiology

## Cell Structure and Function

### What is a Cell?

A **cell** is a structure as well as a functional unit of life. Every
living thing has cells: **bacteria, protozoans, fungi, plants, and
animals** are the main group of living things. Some organisms are made
up of just one cell are called **unicellular**. (e.g. bacteria and
protozoans), but animals, including human beings, are
**multi-cellular.** An adult human body is composed of about
100,000,000,000,000 cells! Each cell has basic requirements to sustain
it, and the body\'s organ systems are largely built around providing the
many trillions of cells with those basic needs (such as oxygen, food,
and waste removal).

There are about 200 different kinds of specialized cells in the human
body. When many identical cells are organized together it is called a
tissue (such as muscle tissue, nervous tissue, etc). Various tissues
organized together for a common purpose are called organs (e.g. the
stomach is an organ, and so is the skin, the brain, and the uterus).

Ideas about cell structure have changed considerably over the years.
Early biologists saw cells as simple membranous sacs containing fluid
and a few floating particles. Today\'s biologists know that cells are
inconceivably more complex than this. Therefore, a strong knowledge of
the various cellular organelles and their functions is important to any
physiologist. If a person\'s cells are healthy, then that person is
healthy. All physiological processes, disease, growth and development
can be described at the cellular level.

### Specialized Cells of the Human Body

Although there are specialized cells - both in structure and function -
within the body, all cells have similarities in their structural
organization and metabolic needs (such as maintaining energy levels via
conversion of carbohydrate to ATP and using genes to create and maintain
proteins).

Here are some of the different types of specialized cells within the
human body.

-   **Nerve Cells**: Also called neurons, these cells are in the nervous
    system and function to process and transmit information (it is
    hypothesized). They are the core components of the brain, spinal
    cord, and peripheral nerves. They use chemical synapses that can
    evoke electrical signals, called action potentials, to relay signals
    throughout the body.
-   **Epithelial cells**: Functions of epithelial cells include
    secretion, absorption, protection, transcellular transport,
    sensation detection, and selective permeability. Epithelium lines
    both the outside (skin) and the inside cavities and lumen of bodies.
-   **Exocrine cells**: These cells secrete products through ducts, such
    as mucus, sweat, or digestive enzymes. The products of these cells
    go directly to the target organ through the ducts. For example, the
    bile from the gallbladder is carried directly into the duodenum via
    the bile duct.
-   **Endocrine cells**: These cells are similar to exocrine cells, but
    secrete their products directly into the bloodstream instead of
    through a duct. Endocrine cells are found throughout the body but
    are concentrated in hormone-secreting glands such as the pituitary.
    The products of the endocrine cells go throughout the body in the
    bloodstream but act on specific organs by receptors on the cells of
    the target organs. For example, the hormone estrogen acts
    specifically on the uterus and breasts of females because there are
    estrogen receptors in the cells of these target organs.
-   **Blood Cells**: The most common types of blood cells are:
    -   **red blood cells (erythrocytes)**. The main function of red
        blood cells is to collect oxygen in the lungs and deliver it
        through the blood to the body tissues. Gas exchange is carried
        out by simple diffusion.
    -   various types of **white blood cells (leukocytes)**. They are
        produced in the bone marrow and help the body to fight
        infectious disease and foreign objects in the immune system.
        White cells are found in the circulatory system, lymphatic
        system, spleen, and other body tissues.

### Cell Size

Cells are the smallest structural & functional living units within our
body, but play a big role in making our body function properly. Many
cells never have a large increase in size like eggs, after they are
first formed from a parental cell. Typical stem cells reproduce, double
in size, then reproduce again. Most Cytosolic contents such as the
endomembrane system and the cytoplasm easily scale to larger sizes in
larger cells. If a cell becomes too large, the normal cellular amount of
DNA may not be adequate to keep the cell supplied with RNA. Large cells
often replicate their chromosomes to an abnormally high amount or become
multinucleated. Large cells that are primarily for nutrient storage can
have a smooth surface membrane, but metabolically active large cells
often have some sort of folding of the cell surface membrane in order to
increase the surface area available for transport functions.

### Cellular Organization

Several different molecules interact to form organelles within our body.
Each type of organelle has a specific function. Organelles perform the
vital functions that keep our cells alive.

#### Cell Membranes

The boundary of the cell, sometimes called the plasma membrane,
separates internal metabolic events from the external environment and
controls the movement of materials into and out of the cell. This
membrane is very selective about what it allows to pass through; this
characteristic is referred to as \"selective permeability.\" For
example, it allows oxygen and nutrients to enter the cell while keeping
toxins and waste products out. The plasma membrane is a double
phospholipid membrane, or a lipid bilayer, with the nonpolar hydrophobic
tails pointing toward the inside of the membrane and the polar
hydrophilic heads forming the inner and outer surfaces of the membrane.
center\|framed\|The molecular structure of the cell
membrane.22

#### Protein and Cholesterol

Proteins and cholesterol molecules are scattered throughout the flexible
phospholipid membrane. Peripheral proteins attach loosely to the inner
or outer surface of the plasma membrane. Integral proteins lie across
the membrane, extending from inside to outside. A variety of proteins
are scattered throughout the flexible matrix of phospholipid molecules,
somewhat like icebergs floating in the ocean, and this is termed the
*fluid mosaic model* of the cell membrane.

The phospholipid bilayer is selectively permeable. Only small, uncharged
polar molecules can pass freely across the membrane. Some of these
molecules are H~2~O and CO~2~, hydrophobic (nonpolar) molecules like
O~2~, and lipid soluble molecules such as hydrocarbons. Other molecules
need the help of a membrane protein to get across. There are a variety
of membrane proteins that serve various functions:

-   **Channel proteins**: Proteins that provide passageways through the
    membranes for certain hydrophilic or water-soluble substances such
    as polar and charged molecules. No energy is used during transport,
    hence this type of movement is called facilitated diffusion.
-   **Transport proteins**: Proteins that spend energy (ATP) to transfer
    materials across the membrane. When energy is used to provide
    passageway for materials, the process is called active transport.
-   **Recognition proteins**: Proteins that distinguish the identity of
    neighboring cells. These proteins have oligosaccharide or short
    polysaccharide chains extending out from their cell surface.
-   **Adhesion proteins**: Proteins that attach cells to neighboring
    cells or provide anchors for the internal filaments and tubules that
    give stability to the cell.
-   **Receptor proteins**: Proteins that initiate specific cell
    responses once hormones or other trigger molecules bind to them.
-   **Electron transfer proteins**: Proteins that are involved in moving
    electrons from one molecule to another during chemical reactions.

#### Passive Transport Across the Cell Membrane

**Passive transport** describes the movement of substances down a
concentration gradient and does not require energy use.

-   **Bulk flow** is the collective movement of substances in the same
    direction in response to a force, such as pressure. Blood moving
    through a vessel is an example of bulk flow.
-   **Simple diffusion**, or diffusion, is the net movement of
    substances from an area of higher concentration to an area of lower
    concentration. This movement occurs as a result of the random and
    constant motion characteristic of all molecules, (atoms or ions) and
    is independent from the motion of other molecules. Since, at any one
    time, some molecules may be moving against the gradient and some
    molecules may be moving down the gradient, although the motion is
    random, the word \"net\" is used to indicate the overall, eventual
    end result of the movement.
-   **Facilitated diffusion** is the diffusion of solutes through
    channel proteins in the plasma membrane. Water can pass freely
    through the plasma membrane without the aid of specialized proteins
    (though facilitated by aquaporins).
-   **Osmosis** is the diffusion of water molecules across a selectively
    permeable membrane. When water moves into a body by osmosis,
    hydrostatic pressure or osmotic pressure may build up inside the
    body.
-   **Dialysis** is the diffusion of solutes across a selectively
    permeable membrane.

#### Active Transport Across the Cell Membrane

**Active transport** is the movement of solutes against a gradient and
requires the expenditure of energy, usually in the form of ATP. Active
transport is achieved through one of these two mechanisms:

##### Protein Pumps

-   Transport proteins in the plasma membrane transfer solutes such as
    small ions (Na^+^, K^+^, Cl^-^, H^+^), amino acids, and
    monosaccharides.
-   The proteins involved with active transport are also known as **ion
    pumps**.
-   The protein binds to a molecule of the substance to be transported
    on one side of the membrane, then it uses the released energy (ATP)
    to change its shape, and releases it on the other side.
-   The protein pumps are specific, there is a different pump for each
    molecule to be transported.
-   Protein pumps are catalysts in the splitting of ATP → ADP +
    phosphate, so they are called **ATPase enzymes**.
    -   The sodium-potassium pump (also called the Na^+^/K^+^-ATPase
        enzyme) actively moves sodium out of the cell and potassium into
        the cell. These pumps are found in the membrane of virtually
        every cell, and are essential in transmission of nerve impulses
        and in muscular contractions.

**Cystic fibrosis** is a genetic disorder that results in a mutated
chloride ion channel. By not regulating chloride secretion properly,
water flow across the airway surface is reduced and the mucus becomes
dehydrated and thick.

##### Vesicular Transport

-   Vesicles or other bodies in the cytoplasm move macromolecules or
    large particles across the plasma membrane. Types of **vesicular**
    **transport** include:

1.  **Exocytosis**, which describes the process of vesicles fusing with
    the plasma membrane and releasing their contents to the outside of
    the cell. This process is common when a cell produces substances for
    export.
2.  **Endocytosis**, which describes the capture of a substance outside
    the cell when the plasma membrane merges to engulf it. The substance
    subsequently enters the cytoplasm enclosed in a vesicle.

:   There are three kinds of endocytosis:
    -   **Phagocytosis** or cellular eating, occurs when the dissolved
        materials enter the cell. The plasma membrane engulfs the solid
        material, forming a phagocytic vesicle.
    -   **Pinocytosis** or cellular drinking occurs when the plasma
        membrane folds inward to form a channel allowing dissolved
        substances to enter the cell. When the channel is closed, the
        liquid is encircled within a pinocytic vesicle.
    -   **Receptor-mediated endocytosis** occurs when specific molecules
        in the fluid surrounding the cell bind to specialized receptors
        in the plasma membrane. As in pinocytosis, the plasma membrane
        folds inward and the formation of a vesicle follows.

    :   **Note:** Certain hormones are able to target specific cells by
        receptor-mediated endocytosis.

## Parts of the Cell

![](Animal_cell_structure_en.svg "Animal_cell_structure_en.svg")

### Cytoplasm

The gel-like material within the cell membrane is referred to as the
cytoplasm. It is a fluid matrix, the cytosol, which consists of 80% to
90% water, salts, organic molecules and many enzymes that catalyze
reactions, along with dissolved substances such as proteins and
nutrients. The cytoplasm plays an important role in a cell, serving as a
\"molecular soup\" in which organelles are suspended and held together
by a fatty membrane.

Within the plasma membrane of a cell, the cytoplasm surrounds the
nuclear envelope and the cytoplasmic organelles. It plays a mechanical
role by moving around inside the membrane and pushing against the cell
membrane helping to maintain the shape and consistency of the cell and
again, to provide suspension to the organelles. It is also a storage
space for chemical substances indispensable to life, which are involved
in vital metabolic reactions, such as anaerobic glycolysis and protein
synthesis.

The cell membrane keeps the cytoplasm from leaking out. It contains many
different organelles which are considered the insoluble constituents of
the cytoplasm, such as the mitochondria, lysosomes, peroxysomes,
ribosomes, several vacuoles and cytoskeletons, as well as complex cell
membrane structures such as the endoplasmic reticulum and the Golgi
apparatus that each have specific functions within the cell.

-   **Cytoskeleton**

Threadlike proteins that make up the cytoskeleton continually
reconstruct to adapt to the cell\'s constantly changing needs. It helps
cells maintain their shape and allows cells and their contents to move.
The cytoskeleton allows certain cells such as neutrophils and
macrophages to make amoeboid movements.

The network is composed of three elements:
_microtubules_, actin filaments, and intermediate
fibers.

-   **Microtubules**

Microtubules function as the framework along which organelles and
vesicles move within a cell. They are the thickest of the cytoskeleton
structures. They are long hollow cylinders, composed of protein
subunits, called tubulin. Microtubules form mitotic spindles, the
machinery that partitions chromosomes between two cells in the process
of cell division. Without mitotic spindles cells could not reproduce.

Microtubules, intermediate filaments, and microfilaments are three
protein fibers of decreasing diameter, respectively. All are involved in
establishing the shape or movements of the cytoskeleton, the internal
structure of the cell. !A photograph of
microfilaments.

-   **Microfilaments**

Microfilaments provide mechanical support for the cell, determine the
cell shape, and in some cases enable cell movements. They have an
arrow-like appearance, with a fast growing plus or barbed end and a slow
growing minus or pointed end. They are made of the protein actin and are
involved in cell motility. They are found in almost every cell, but are
predominant in muscle cells and in the cells that move by changing
shape, such as phagocytes (white blood cells that scour the body for
bacteria and other foreign invaders).

### Organelles

Organelles are bodies embedded in the cytoplasm that serve to physically
separate the various metabolic activities that occur within cells. The
organelles are each like separate little factories, each organelle is
responsible for producing a certain product that is used elsewhere in
the cell or body.

Cells of all living things are divided into two broad categories:
prokaryotes and eukaryotes. Bacteria (and archea) are prokaryotes, which
means they lack a nucleus or other membrane-bound organelles. Eukaryotes
include all protozoans, fungi, plants, and animals (including humans),
and these cells are characterized by a nucleus (which houses the
chromosomes) as well as a variety of other organelles. Human cells vary
considerably (consider the differences between a bone cell, a blood
cell, and a nerve cell), but most cells have the features described
below.

center\|framed\|A comparison of Eukaryote and Prokaryote
cells.

#### Nucleus

Controls the cell; houses the genetic material (DNA). The nucleus is the
largest of the cells organelles. Cells can have more than one nucleus or
lack a nucleus all together. Skeletal muscle cells contain more than one
nucleus whereas red blood cells do not contain a nucleus at all. The
nucleus is bounded by the nuclear envelope, a phospholipid bilayer
similar to the plasma membrane. The space between these two layers is
the nucleolemma Cisterna.

The nucleus contains the DNA, as mentioned above, the hereditary
information in the cell. Normally the DNA is spread out within the
nucleus as a threadlike matrix called chromatin. When the cell begins to
divide, the chromatin condenses into rod-shaped bodies called
chromosomes, each of which, before dividing, is made up of two long DNA
molecules and various histone molecules. The histones serve to organize
the lengthy DNA, coiling it into bundles called nucleosomes. Also
visible within the nucleus are one or more nucleoli, each consisting of
DNA in the process of manufacturing the components of ribosomes.
Ribosomes are shipped to the cytoplasm where they assemble amino acids
into proteins. The nucleus also serves as the site for the separation of
the chromosomes during cell division.

!A cross-sectional diagram of a
nucleus.

-   **Chromosomes**

right\|framed\|A rough sketch of a
chromosome.

Inside each cell nucleus are chromosomes. Chromosomes are made up of
chromatin, which is made up of protein and deoxyribonucleic acid
strands. Deoxyribonucleic acid is DNA, the genetic material that is in
the shape of a twisted ladder, also called the double helix. Humans have
23 pairs of chromosomes. Down Syndrome and Cri du Chat Syndrome result
from having an abnormal number of chromosomes.

#### Centrioles

Centrioles are rod like structures composed of 9 bundles which contain
three microtubules each. Two perpendicularly placed centrioles
surrounded by proteins make up the centrosome. Centrioles are very
important in cellular division, where they arrange the mitotic spindles
that pull the chromosome apart.

Centrioles and basal bodies act as microtubule organizing centers. A
pair of centrioles (enclosed in a centrosome) located outside the
nuclear envelope gives rise to the microtubules that make up the spindle
apparatus used during cell division. Basal bodies are at the base of
each flagellum and cilium and appear to organize their development.

#### Ribosomes

center\|framed Ribosomes play
an active role in the complex process of protein synthesis, where they
serve as the structures that facilitate the joining of amino acids. Each
ribosome is composed of a large and small subunit which are made up of
ribosomal proteins and ribosomal RNAs. They can either be found in
groups called polyribosomes within the cytoplasm or found alone.
Occasionally they are attached to the endoplasmic reticulum.

!A cutaway view inside a
mitochondria.

#### Mitochondria

Mitochondria are the organelles that function as the cell
\"powerhouse\", generating ATP, the universal form of energy used by all
cells. It converts food nutrients such as glucose, to a fuel (ATP) that
the cells of the body can use. Mitochondria are tiny sac-like structures
found near the nucleus. Little shelves called cristae are formed from
folds in the inner membrane. Cells that are metabolically active such as
muscle, liver and kidney cells have high energy requirements and
therefore have more mitochondria.

Mitochondria are unique in that they have their own mitochondrial DNA
(separate from the DNA that is in the nucleus). It is believed that
eukaryotes evolved from one cell living inside another cell, and
mitochondria share many traits with free-living bacteria (similar
chromosome, similar ribosomes, etc).

#### Endoplasmic Reticulum

:   **Endoplasmic** means \"within the plasm\" and **reticulum** means
    \"network\".

A complex three dimensional internal membrane system of flattened
sheets, sacs and tubes, that play an important role in making proteins
and shuttling cellular products; also involved in metabolisms of fats,
and the production of various materials. In cross-section, they appear
as a series of maze-like channels, often closely associated with the
nucleus. When ribosomes are present, the rough ER connects
polysaccharide groups to the polypeptides as they are assembled by the
ribosomes. Smooth ER, without ribosomes, is responsible for various
activities, including the synthesis of lipids and hormones, especially
in cells that produce these substances for export from the cell.

Rough endoplasmic reticulum has characteristic bumpy appearance due to
the multitude of ribosomes coating it. It is the site where proteins not
destined for the cytoplasm are synthesized.

Smooth endoplasmic reticulum provides a variety of functions, including
lipid synthesis and degradation, and calcium ion storage. In liver
cells, the smooth ER is involved in the breakdown of toxins, drugs, and
toxic byproducts from cellular reactions.

#### Golgi Apparatus

\"Packages\" cellular products in sacs called vesicles so that the
products can cross the cell membrane and exit the cell. The **Golgi
apparatus** is the central delivery system for the cell. It is a group
of flattened sacs arranged much like a stack of bowls. They function to
modify and package proteins and lipids into vesicles, small spherically
shaped sacs that bud from the ends of a Golgi apparatus. Vesicles often
migrate to and merge with the plasma membrane, releasing their contents
outside the cell. The Golgi apparatus also transports lipids and creates
lysosomes and organelles involved in digestion.

#### Vacuoles

Spaces in the cytoplasm that sometimes serve to carry materials to the
cell membrane for discharge to the outside of the cell. **Vacuoles** are
formed during endocytosis when portions of the cell membrane are pinched
off.

#### Lysosomes

Lysosomes are sac-like compartments that contain a number of powerful
degradative enzymes. They are built in the Golgi apparatus. They break
down harmful cell products and waste materials, cellular debris, and
foreign invaders such as bacteria, and then force them out of the cell.
Tay-Sachs disease and Pompe\'s disease are just two of the malfunctions
of lysosomes or their digestive proteins.

#### Peroxisomes

Organelles in which oxygen is used to oxidize substances, breaking down
lipids and detoxifying certain chemicals. Peroxisomes self replicate by
enlarging and then dividing. They are common in liver and kidney cells
that break down potentially harmful substances. Peroxisomes can convert
hydrogen peroxide, a toxin made of H~2~O~2~ to H~2~O.

### Extracellular structures

-   **Extracellular matrix**

Human cells, like other animal cells, do not have a rigid cell wall.
Human cells do have an important and variable structure outside of their
cell membrane called the extracellular matrix. Sometimes this matrix can
be extensive and solid (examples = calcified bone matrix, cartilage
matrix), while other times it consists of a layer of extracellular
proteins and carbohydrates. This matrix is responsible for cells binding
to each other and is incredibly important in how cells physically and
physiologically interact with each other.

-   **Flagella**

Many prokaryotes have flagella, allowing, for example, an *E. coli*
bacteria to propel its way up the urethra to cause a UTI (Urinary Tract
Infection). Human cells, however (and in fact most eukaryotic cells)
lack flagella. This makes sense since humans are multicellular, and
individual cells do not need to swim around. The obvious exception to
this is with sperm, and indeed each sperm is propelled by a single
flagellum. The flagellum of sperm is composed of microtubules.

-   **Cilia**

Cilia are especially notable on the single-celled protozoans, where they
beat in synchrony to move the cells nimbly through the water. They are
composed of extensions of the cell membrane that contain microtubules.
When present in humans they are typically found in large numbers on a
single surface of the cells, where rather than moving cells, they move
materials. The *mucociliary escalator* of the respiratory system
consists of mucus-secreting cells lining the trachea and bronchi, and
ciliated epithelial cells that move the mucus ever-upward. In this
manner mold spores, bacteria, and debris are caught in the mucus,
removed from the trachea, and pushed into the esophagus (to be swallowed
into a pit of acid). In the oviducts cilia move the ovum from the ovary
to the uterus, a journey which takes a few days.

right\|framed\|A magnified view of several cells, with visible
cilia.

## Cell Junctions

The plasma membranes of adjacent cells are usually separated by
extracellular fluids that allow transport of nutrients and wastes to and
from the bloodstream. In certain tissues, however, the membranes of
adjacent cells may join and form a junction. Three kinds of cell
junctions are recognized:

-   **Desmosomes** are protein attachments between adjacent cells.
    Inside the plasma membrane, a desmosome bears a disk shaped
    structure from which protein fibers extend into the cytoplasm.
    Desmosomes act like spot welds to hold together tissues that undergo
    considerable stress, such as our skin or heart muscle.

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-   **Tight junctions** are tightly stitched seams between cells. The
    junction completely encircles each cell, preventing the movement of
    material between the cell. Tight junctions are characteristic of
    cells lining the digestive tract, where materials are required to
    pass through cells,rather than intercellular spaces, to penetrate
    the bloodstream.

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-   **Gap junctions** are narrow tunnels that directly connect the
    cytoplasm of two neighbouring cells, consisting of proteins called
    connexons. These proteins allow only the passage of ions and small
    molecules. In this manner, gap junctions allow communication between
    cells through the exchange of materials or the transmission of
    electrical impulses.

## Cell Metabolism

**Cell metabolism** is the total energy released and consumed by a cell.
Metabolism describes all of the chemical reactions that are happening in
the body. Some reactions, called anabolic reactions, create needed
products. Other reactions, called catabolic reactions, break down
products. Your body is performing both anabolic and catabolic reactions
at the same time and around the clock, twenty four hours a day, to keep
your body alive and functioning. Even while you sleep, your cells are
busy metabolizing.

-   **Catabolism**: The energy releasing process in which a chemical or
    food is used (broken down) by degradation or decomposition, into
    smaller pieces.

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-   **Anabolism**: Anabolism is just the opposite of catabolism. In this
    portion of metabolism, the cell consumes energy to produce larger
    molecules via smaller ones.

### Energy Rich Molecules

#### Adenosine Triphosphate (ATP)

!Chemical diagram of an ATP
molecule.
ATP is the currency of the cell. When the cell needs to use energy such
as when it needs to move substances across the cell membrane via the
active transport system, it \"pays\" with molecules of ATP. The total
quantity of ATP in the human body at any one time is about 0.1 Mole. The
energy used by human cells requires the hydrolysis of 200 to 300 moles
of ATP daily. This means that each ATP molecule is recycled 2000 to 3000
times during a single day. ATP cannot be stored, hence its consumption
must closely follow its synthesis. On a per-hour basis, 1 kilogram of
ATP is created, processed and then recycled in the body. Looking at it
another way, a single cell uses about 10 million ATP molecules per
second to meet its metabolic needs, and recycles all of its ATP
molecules about every 20-30 seconds.

#### Flavin Adenine Dinucleotide (FAD)

When two hydrogen atoms are bonded, FAD is reduced to FADH~2~ and is
turned into an energy-carrying molecule. FAD accommodates two
equivalents of Hydrogen; both the hydride and the proton ions. This is
used by organisms to carry out energy requiring processes. FAD is
reduced in the citric acid cycle during aerobic respiration

#### Nicotinamide Adenine Dinucleotide (NADH)

Nicotinamide adenine dinucleotide (NAD^+^) and nicotinamide adenine
dinucleotide phosphate (NADP) are two important cofactors found in
cells. NADH is the reduced form of NAD^+^, and NAD^+^ is the oxidized
form of NADH. It forms NADP with the addition of a phosphate group to
the 2\' position of the adenosyl nucleotide through an ester linkage.

NAD is used extensively in glycolysis and the citric acid cycle of
cellular respiration. The reducing potential stored in NADH can be
converted to ATP through the electron transport chain or used for
anabolic metabolism. ATP \"energy\" is necessary for an organism to
live. Green plants obtain ATP through photosynthesis, while other
organisms obtain it by cellular respiration.

NADP is used in anabolic reactions, such as fat acid and nucleic acid
synthesis, that require NADPH as a reducing agent. In chloroplasts, NADP
is an oxidising agent important in the preliminary reactions of
photosynthesis. The NADPH produced by photosynthesis is then used as
reducing power for the biosynthetic reactions in the Calvin cycle of
photosynthesis.

!Chemical diagram of an NADH
molecule.

MH~2~ + NAD^+^ → NADH + H^+^ + M: + energy, where M is a metabolite. Two
hydrogen ions (a hydride ion and an H^+^ ion) are transferred from the
metabolite. One electron is transferred to the positively-charged
nitrogen, and one hydrogen attaches to the carbon atom opposite to the
nitrogen.

The human body synthesizes NAD from the vitamin niacin in the form of
nicotinic acid or nicotinamide.

#### Cellular Respiration

Cellular respiration is the energy releasing process by which sugar
molecules are broken down by a series of reactions and the chemical
energy gets converted to energy stored in ATP molecules. The reactions
that convert the fuel (glucose) to usable cellular energy (ATP) are
glycolysis, the Krebs cycle (sometimes called the citric acid cycle),
and the electron transport chain. Altogether these reactions are
referred to as \"cellular respiration\" or \"aerobic respiration.\"
Oxygen is needed as the final electron acceptor, and carrying out
cellular respiration is the very reason we breathe and the reason we
eat.

center\|framed\|Flowchart of cellular
respiration..svg "wikilink")

### Glycolysis

The glycolytic pathway (glycolysis) is where glucose, the smallest
molecule that a carbohydrate can be broken into during digestion, gets
oxidized and broken into two 3-carbon molecules (pyruvates), which are
then fed into the Kreb\'s Cycle. Glycolysis is the beginning of cellular
respiration and takes place in the cytoplasm. Two molecules of ATP are
required for glycolysis, but four are produced so there is a net gain of
two ATP per glucose molecule. Two NADH molecules transfer electrons (in
the form of hydrogen ions) to the electron transport chain in the
mitochondria, where they will be used to generate additional ATP. During
physical exertion when the mitochondria are already producing the
maximum ATP possible with the amount of oxygen available, glycolysis can
continue to produce an additional 2 ATP per glucose molecule without
sending the electrons to the mitochondria. However, during this
*anaerobic respiration* lactic acid is produced, which may accumulate
and lead to temporary muscle cramping.

### Krebs Cycle

The Krebs cycle was named after Sir Hans
Krebs (1900-1981), who proposed
the key elements of this pathway in 1937 and was awarded the Nobel Prize
in Medicine for its discovery in 1953.

Two molecules of pyruvate enter the Krebs cycle, which is called the
aerobic pathway because it requires the presence of oxygen in order to
occur. This cycle is a major biological pathway that occurs in humans
and every plant and animal.

After glycolysis takes place in the cell\'s cytoplasm, the pyruvic acid
molecules travel into the interior of the mitochondrion. Once the
pyruvic acid is inside, carbon dioxide is enzymatically removed from
each three-carbon pyruvic acid molecule to form acetic acid. The enzyme
then combines the acetic acid with an enzyme, coenzyme A, to produce
acetyl coenzyme A, also known as acetyl CoA.

Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split
into eight steps, each of which will be explained below.

-   Step 1: The acetic acid subunit of acetyl CoA is combined with
    oxaloacetate to form a molecule of citrate. The acetyl coenzyme A
    acts only as a transporter of acetic acid from one enzyme to
    another. After Step 1, the coenzyme is released by hydrolysis so
    that it may combine with another acetic acid molecule to begin the
    Krebs cycle again.
-   Step 2: The citric acid molecule undergoes an isomerization. A
    hydroxyl group and a hydrogen molecule are removed from the citrate
    structure in the form of water. The two carbons form a double bond
    until the water molecule is added back. Only now, the hydroxyl group
    and hydrogen molecule are reversed with respect to the original
    structure of the citrate molecule. Thus, isocitrate is formed.
-   Step 3: In this step, the isocitrate molecule is oxidized by a NAD
    molecule. The NAD molecule is reduced by the hydrogen atom and the
    hydroxyl group. The NAD binds with a hydrogen atom and carries off
    the other hydrogen atom leaving a carbonyl group. This structure is
    very unstable, so a molecule of CO~2~ is released creating
    alpha-ketoglutarate.
-   Step 4: In this step, our friend, coenzyme A, returns to oxidize the
    alpha-ketoglutarate molecule. A molecule of NAD is reduced again to
    form NADH and leaves with another hydrogen. This instability causes
    a carbonyl group to be released as carbon dioxide and a thioester
    bond is formed in its place between the former alpha-ketoglutarate
    and coenzyme A to create a molecule of succinyl-coenzyme A complex.
-   Step 5: A water molecule sheds its hydrogen atoms to coenzyme A.
    Then, a free-floating phosphate group displaces coenzyme A and forms
    a bond with the succinyl complex. The phosphate is then transferred
    to a molecule of GDP to produce an energy molecule of GTP. It leaves
    behind a molecule of succinate.
-   Step 6: In this step, succinate is oxidized by a molecule of FAD
    (Flavin adenine dinucleotide). The FAD removes two hydrogen atoms
    from the succinate and forces a double bond to form between the two
    carbon atoms, thus creating fumarate.
-   Step 7: An enzyme adds water to the fumarate molecule to form
    malate. The malate is created by adding one hydrogen atom to a
    carbon atom and then adding a hydroxyl group to a carbon next to a
    terminal carbonyl group.
-   Step 8: In this final step, the malate molecule is oxidized by a NAD
    molecule. The carbon that carried the hydroxyl group is now
    converted into a carbonyl group. The end product is oxaloacetate
    which can then combine with acetyl-coenzyme A and begin the Krebs
    cycle all over again.
-   Summary: In summary, three major events occur during the Krebs
    cycle. One GTP (guanosine triphosphate) is produced which eventually
    donates a phosphate group to ADP to form one ATP; three molecules of
    NAD are reduced; and one molecule of FAD is reduced. Although one
    molecule of GTP leads to the production of one ATP, the production
    of the reduced NAD and FAD are far more significant in the cell\'s
    energy-generating process. This is because NADH and FADH~2~ donate
    their electrons to an electron transport system that generates large
    amounts of energy by forming many molecules of ATP.

To see a visual summary of \"Kreb Cycle\" please click
here.

### Electron Transport System

The most complicated system of all. In the respiration chain, oxidation
and reduction reactions occur repeatedly as a way of transporting
energy. The respiratory chain is also called the electron transport
chain. At the end of the chain, oxygen accepts the electron and water is
produced.

#### Redox Reaction

This is a simultaneous oxidation-reduction process whereby cellular
metabolism occurs, such as the oxidation of sugar in the human body,
through a series of very complex electron transfer processes.

The chemical way to look at redox processes is that the substance being
oxidized transfers electrons to the substance being reduced. Thus, in
the reaction, the substance being oxidized (aka. the reducing agent)
loses electrons, while the substance being reduced (aka. the oxidizing
agent) gains electrons. Remember: LEO (Losing Electrons is Oxidation)
the lion says GER (Gaining Electrons is Reduction); or alternatively:
OIL (Oxidation is Loss) RIG (Reduction is Gain).

The term redox state is often used to describe the balance of
NAD^+^/NADH and NADP^+^/NADPH in a biological system such as a cell or
organ. The redox state is reflected in the balance of several sets of
metabolites (e.g., lactate and pyruvate, β-hydroxybutyrate and
acetoacetate) whose interconversion is dependent on these ratios. An
abnormal redox state can develop in a variety of deleterious situations,
such as hypoxia, shock, and sepsis.

## Cell Building Blocks

What major classes of molecules are found within cells?

### Lipids

The term is more-specifically used to refer to fatty-acids and their
derivatives (including tri-, di-, and mono-glycerides and phospholipids)
as well as other fat-soluble sterol-containing metabolites such as
cholesterol. Lipids serve many functions in living organisms including
energy storage, serve as structural components of cell membranes, and
constitute important signaling molecules. Although the term lipid is
sometimes used as a synonym for fat, the latter is in fact a subgroup of
lipids called triglycerides and should not be confused with the term
fatty acid.

### Carbohydrates

Carbohydrate molecules consist of carbon, hydrogen, and oxygen. They
have a general formula C~n~(H~2~O)~n~. There are several sub-families
based on molecular size.

Carbohydrates are chemical compounds that contain oxygen, hydrogen, and
carbon atoms, and no other elements. They consist of monosaccharide
sugars of varying chain lengths.

Certain carbohydrates are an important storage and transport form of
energy in most organisms, including plants and animals. Carbohydrates
are classified by their number of sugar units: monosaccharides (such as
glucose and fructose), disaccharides (such as sucrose and lactose),
oligosaccharides, and polysaccharides (such as starch, glycogen, and
cellulose).

The simplest carbohydrates are monosaccharides, which are small
straight-chain aldehydes and ketones with many hydroxyl groups added,
usually one on each carbon except the functional group. Other
carbohydrates are composed of monosaccharide units and break down under
hydrolysis. These may be classified as disaccharides, oligosaccharides,
or polysaccharides, depending on whether they have two, several, or many
monosaccharide units.

![](Protein-structure.png‎ "Protein-structure.png‎")

### Proteins

All proteins contain carbon, hydrogen, oxygen and nitrogen. Some also
contain phosphorus and sulfur. The building blocks of proteins are amino
acids. There are 20 different kinds of amino acids used by the human
body. They unite by peptide bonds to form long molecules called
polypeptides. Polypeptides are assembled into proteins. Proteins have
four levels of structure

-   **Primary**

Primary structure is the sequence of amino acids bonded in the
polypeptide.

-   **Secondary**

The secondary structure is formed by hydrogen bonds between amino acids.
The polypeptide can coil into a helix or form a pleated sheet.

-   **Tertiary**

The tertiary structure refers to the three-dimensional folding of the
helix or pleated sheet.

-   **Quaternary**

The quaternary structure refers to the spatial relationship among the
polypeptide in the protein.

-   **Hexagonary**

The hexagonary structure refers to the carpal relationship among the
bipeptide in the person.

### Enzymes

A biological molecule that catalyzes a chemical reaction. Enzymes are
essential for life because most chemical reactions in living cells would
occur too slowly or would lead to different products without enzymes.
Most enzymes are proteins and the word \"enzyme\" is often used to mean
a protein enzyme. Some RNA molecules also have a catalytic activity, and
to differentiate them from protein enzymes, they are referred to as RNA
enzymes or ribozymes.

## Review Questions

Answers for these questions can be found here

1\. List 2 functions of the cell membrane:

Questions 2 - 6 Match the following organelles with their function: 2.
Mitochondria 3. Vacuoles 4. Cilia 5. Smooth ER 6. Golgi Apparatus

:   A. Movement of the cell
:   B. Lipid synthesis and transport
:   C. \"Powerhouse\" of the cell, makes ATP
:   D. Storage areas, mainly found in plant cells
:   E. Packages and distributes cellular products

7\. The diffusion of H~2~O across a semi permeable or selectively
permeable membrane is termed

:   A. Active transport
:   B. Diffusion
:   C. Osmosis
:   D. Endocytosis

8\. Oxygen enters a cell via?

:   a\. Diffusion
:   b\. Filtration
:   c\. Osmosis
:   d\. Active transport

9\. The term used to describe, \"cell eating\" is?

:   a\. Exocytosis
:   b\. Phagocytosis
:   c\. Pinocytosis
:   d\. Diffusion

10\. Which of the following requires energy?

:   a\. Diffusion
:   b\. Osmosis
:   c\. Active transport
:   d\. Facilitated diffusion

11\. Protein synthesis occurs at the

:   a\. Mitochondria
:   b\. Lysosomes
:   c\. Within the nucleus
:   d\. Ribosomes

12\. Which of the following is not found in the cell membrane?

:   a\. Cholesterol
:   b\. Phospholipids
:   c\. Proteins
:   d\. Galactose
:   e\. Nucleic acids

13\. What is a cell?

:   a\. The largest living units within our bodies.
:   b\. Enzymes that \"eat\" bacteria
:   c\. Microscopic fundamental units of all living things.
:   d\. All of the above.

## Glossary

**Active Transport**: the movement of solutes against a gradient and
requires the expenditure of energy

**Adenosine Triphosphate (ATP):** a cell's source of energy

**Bulk Flow:** the collective movement of substances in the same
direction in response to a force

**Cells:** the microscopic fundamental unit that makes up all living
things

**Cell Membrane:** boundary of the cell, sometimes called the plasma
membrane

**Cytoplasm:** a water-like substance that fills cells. The cytoplasm
consists of cytosol and the cellular organelles, except the cell
nucleus. The cytosol is made up of water, salts, organic molecules and
many enzymes that catalyze reactions. The cytoplasm holds all of the
cellular organelles outside of the nucleus, maintains the shape and
consistency of the cell, and serves as a storage place for chemical
substances.

**Cytoskeleton:** made of threadlike proteins, helps cells maintain
their shape and allows cells and their contents to move

**Dialysis:** the diffusion of solutes across a selectively permeable
membrane. Most commonly heard of when a patient has had renal failure.
In medicine, dialysis is a type of renal replacement therapy which is
used to provide an artificial replacement for lost kidney function due
to renal failure. It is a life support treatment and does not treat any
kidney diseases.

**Endocrine cells:** similar to exocrine cells, but secrete their
products directly into the bloodstream instead of through a duct

**Endocytosis:** the capture of a substance outside the cell when the
plasma membrane merges to engulf it

**Endoplasmic Reticulum:** organelle that play an important role in
making proteins and shuttling cellular products; also involved in
metabolisms of fats, and the production of various materials

**Epithelial Cells:** cells that aid in secretion, absorption,
protection, trans-cellular transport, sensation detection, and selective
permeability

**Exocrine Cells:** cells that secrete products through ducts, such as
mucus, sweat, or digestive enzymes

**Exocytosis:** the process of vesicles fusing with the plasma membrane
and releasing their contents to the outside of the cell

**Facilitated Diffusion:** the diffusion of solutes through channel
proteins in the plasma membrane

**Golgi Apparatus:** \"packages\" cellular products in sacs called
vesicles so that the products can cross the cell membrane and exit the
cell

**Glycolysis:** process in which sugars (glucose) are converted to acid

**Lysosomes:** sac-like compartments that contain a number of powerful
degradative enzymes

**Microfilaments:** provide mechanical support for the cell, determine
the cell shape, and in some cases enable cell movements

**Microtubules:** function as the framework along which organelles and
vesicles move within a cell

**Mitochondria:** the organelles that function as the cell
\"powerhouse\", generating ATP

**Nucleus:** controls the cell; houses the genetic material

**Organelles:** bodies embedded in the cytoplasm that serve to
physically separate the various metabolic activities that occur within
cells

**Osmosis:** the diffusion of water molecules across a selectively
permeable membrane from an area of high solute concentration to an area
of low solute concentration.

**Passive Transport:** the movement of substances down a concentration
gradient and does not require energy use

**Peroxisomes:** organelles in which oxygen is used to oxidize
substances, breaking down lipids and detoxifying certain chemicals

**Phagocytosis:** a form of endocytosis wherein large particles are
enveloped by the cell membrane of a (usually larger) cell and
internalized to form a phagosome, or \"food vacuole.\" In animals,
phagocytosis is performed by specialized cells called phagocytes, which
serve to remove foreign bodies and thus fight infection. In vertebrates,
these include larger macrophages and smaller granulocytes, types of
blood cells. Bacteria, dead tissue cells, and small mineral particles
are all examples of objects that may be phagocytosed.

**Pinocytosis:** also called cellular drinking, is a form of
endocytosis, a process in which small particles are taken in by a cell
by splitting into smaller particles. The particles then form small
vesicles which subsequently fuse with lysosomes to hydrolyze, or to
break down, the particles. This process requires adenosine triphosphate
(ATP).

**Receptor-mediated Endocytosis:** occurs when specific molecules in the
fluid surrounding the cell bind to specialized receptors in the plasma
membrane

**Red Blood Cells (erythrocytes):** cells that collect oxygen in the
lungs and deliver it through the blood to the body tissues

**Ribosomes:** play an active role in the complex process of protein
synthesis, where they serve as the structures that facilitate the
joining of amino acids

**Simple Diffusion:** the net movement of substances from an area of
higher concentration to an area of lower concentration

**Vacuoles:** spaces in the cytoplasm that sometimes serve to carry
materials to the cell membrane for discharge to the outside of the cell

**White Blood Cells (leukocytes):** produced in the bone marrow and help
the body to fight infectious disease and foreign objects in the immune
system
