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The left part of this figure is a transmission electron micrograph of microvilli, which appear as long, slender stalks extending from the plasma membrane. The right side illustrates cells containing microvilli. The microvilli cover the surface of the cell facing the interior of the small intestine.
Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard)

The cytoplasm

The cytoplasm    is the entire region of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol    , the cytoskeleton, and various chemicals ( [link] ). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The nucleus

Typically, the nucleus is the most prominent organelle in a cell ( [link] ). The nucleus    (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail ( [link] ).

The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.

The nuclear envelope

The nuclear envelope    is a double-membrane structure that constitutes the outermost portion of the nucleus ( [link] ). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm    is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.

Chromatin and chromosomes

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nuclei of its body’s cells. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin    ( [link] ); chromatin describes the material that makes up the chromosomes both when condensed and decondensed.

Questions & Answers

who is the father of biology
Sean Reply
what are prokaryotic cells
Kusubira Reply
having cell that lack membrane - bound nuclei.
Please each person should suggest ur answers
Franklina Reply
differentiate between autelogical and synecology
Christiana Reply
differentiate between autelogical and synecology
Christiana Reply
answer please
Autecology is the study of individual organism or individual species. It is also known as population ecology. Synecology is the study of group of organisms of different species which are associated together as a unit in form of a community.
Autecology is the study of the environment in relation to only one species in contrast while synecology is the study of the environment affecting groups of species coexisting in an area.
Describe the factors affecting homotasis
Grace Reply
Fluids and electrolytes Energy and nutrition Immune response mediators
prove starch in a mango leaf
Ruth Reply
prove starch in a leaf
draw and name the plant cell
wat is classification
levels of classification
classification of what
Iodine solution is used to test leaves for the presence of starch.
Identify the caste of termites or Honey bees
Mensah Reply
Soldier Winged reproductive Worker King and queen
what is biology
Jackson Reply
biology is a study of living organism
what is the myelin sheath?
Myelin sheath is an insulting layer that forms around nerves, including those in the brain and spinal cord.
description on how an enzyme such as pepsin breaks down a substrate
linister Reply
The specific reaction catalyzed by pepsin is the acid hydrolysis of the peptide bond. This reaction will break down protein into smaller units to enable the digestive process
what is biology
Kanzy Reply
study of life
the study of life..meaning studying all aspects of life in humans,animals and everything inside the global
study of life
is the branch of science which deal with the study of living and non living things
Is the study of living organisms or living things
draw the taxonomic hierarchy of classification
KC Reply
Kingdom Class Order Family Genus Species
explain the following bush burning, deforestation, over grazing
Thomas Reply
what is biodiversity
No idea
Please what are alleles
Deforestation: this is the cutting down of trees without planting new ones
over grazing is the continuously planting of crops in one particular land.
what is afforestation
Bush burning is the removal of the natural vegetation cover, that protects the soil surface through the use of fire. Deforestation is the actor process of establishing a forest especially on land not previously forested.
Over grazing is the practice of grazing toi many livestock for too long a period on a land unable to recover its vegetation.
what is physiology
Joseph Reply
the study of physical things
dealing with those things which you can see by your neck eyes
Further explanation please
the study of things tt interact with time,energy
Is the scientific study of chemistry and physics of the structure of the body

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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