Cells

Definition

Cells are the smallest unit or the building block of all living things.

Anatomy of a Cell

Basic parts of a cell consist of:

1. Nucleus
2. Cytoplasm
3. Plasma membrane

The nucleus is located at the most central part of a cell which is surrounded by a semifluid component of a cell called a cytoplasm. The cytoplasm is enclosed by the outer cell boundary called the plasma membrane.

The Headquarters: Nucleus

The control center of a cell is the nucleus. This part of a cell contains the genetic material called DNA (deoxyribonucleic acid) which is encloses all data of instruction for building a person’s protein and cell production.

Parts of a nucleus

1. Nuclear Membrane (nuclear envelope) – a double membrane barrier that binds the nucleus. It has a selective permeability.
2. Nucleoplasm – a jelly-like fluid that is enclosed by the nuclear membrane. It is in this part where the nuclear components are floating.
3. Nucleoli – small, dark-staining round bodies. It is in this area where ribosomes are brought together. Ribosomes are the actual site of protein synthesis.
4. Chromatin – loose network of bumpy threads that is dispersed throughout the nucleus. When cell division occurs, the chromatin forms the chromosomes.

The Factory Area: Cytoplasm

The cytoplasm is the area where cellular activities take place. It is located outside the nucleus and inside the plasma membrane.

Major elements of a cytoplasm

1. Cytosol – suspends the other elements in cytoplasm.
2. Organelles – the metabolic machinery of a cell.
3. Inclusions – stored nutrients or stored cell products.

Cytoplasmic Organelles

• Mitochondria – referred as the “powerhouse” of the cell as it supplies ATP. It is a sausage-shaped organelle that is composed of a double membrane situated in each side.
• Ribosomes – actual site of protein synthesis.
• Endoplasmic Reticulum – it carries substances (mostly protein) from one part of the cell to another.
  1. Rough Endoplasmic Reticulum – an organelle that builds materials of cellular membranes. This structure is also referred as the cell’s membrane factory and it is studded with ribosomes.
  2. Smooth Endoplasmic Reticulum – basically functions in lipid metabolism and the clearing of drugs and pesticides. It is essential in cholesterol and fat synthesis and breakdown. Liver cells contain a large amount of smooth endoplasmic reticulum.

• Golgi apparatus – situated close to the nucleus. It is involved in the modification and packaging of the proteins from the rough ER through the transport vehicles.
• Lysosomes – contains digestive enzymes. These enzymes are formed by the ribosomes and packaged by the Golgi apparatus.
• Peroxisomes – contains oxidase enzymes that utilize molecular oxygen to cleanse harmful and poisonous substances (e.g. alcohol and formaldehyde). More importantly, it is responsible for converting free radicals to hydrogen peroxide then to water with the presence of the enzyme catalase.
• Cytoskeleton – determines the shape of the cell and provides support to other organelles.
• Centrioles – direct the formation of the mitotic spindle during cell division.

Plasma Membrane or Cell Membrane

The cell membrane separates the cells from the surrounding and contains all the cellular components or materials. Its structure consists of the following:

• Phospolipids
• Cholesterol
• Polar heads of phospholipid molecules
• Bimolecular lipid containing proteins – the proteins are responsible for the specialized function of the cell membrane. Proteins in plasma membrane could be:
  1. Enzymes
  2. Glycoprotein or sugar-proteins = determines blood type, serves as receptors of certain bacteria, viruses and toxins. It also plays a role in cell-to-cell communication or interaction.

Most proteins that are found on the area of plasma membrane have to do with transport functions. Water or small water-soluble molecules or ions can move through cells as proteins gather together to form tiny pores from protein channels. Other proteins are responsible for attaching to a substance and transport it towards the cell through the membrane.

• Nonpolar tails of phospholipids molecules

A plasma membrane has two common specializations namely:

• Microvilli
• Membrane junctions

Microvilli are the minute finger-like projections of the plasma membrane that is responsible for increasing the cell’s surface area for absorption making the process occurring more quickly. The membrane junctions, on the other hand, vary structurally depending on their roles:

• Tight junctions – these are resistant junctions that fuse cells together into impermeable structure that will prevent the substances from passing through the extracellular space between the cells.
• Desmosomes – prevent cells from mechanical stress. For instance, skin cells are prevented from being pulled apart (mechanical stress) because of the presence of desmosomes.
• Gap junctions – allow communication between cells. Neighboring cells are connected to each other by the presence of CONNEXONS.

Types of CELLS according to function or role

Cells that connect body parts

• Fibroblasts – elongated cells that contain a large amount of rough endoplasmic reticulum (ER) and large Golgi apparatus, to make and secrete the protein building blocks of these fibers.
• Erythrocytes – also called red blood cells (RBCs). It is a concave disk shaped cell which is responsible for carrying oxygen in the bloodstream. The shape of this cell is helpful in providing an extra surface area for oxygen uptake.

Cells that coats and lines the body organs

• Epithelial Cells – a hexagonal shape cell. The shape of this cell allows these fibers to pack together in sheets and covers a body organ.

Cells that mover organs and body parts

• Skeletal muscle cells
• Smooth muscle cells

Cells that stores nutrients

• Fat cells

Cells that fight disease

• Phagocytic cell (macrophage)

Cells that gather information and controls body functions

• Nerve cells (neurons)

Cells involve in reproduction

• Oocyte
• Sperm cell

Physiology of a Cell

The cell’s internal structure performs various functions such as:

• Metabolism (using nutrients to build new cell material, break down substances and make ATP)
• Digestion
• Excretion (dispose wastes)
• Reproduction
• Growth
• Movement
• Irritability (response to a stimulus)

Two major period of cell cycle:

• Interphase or Metabolic phase
• Cell division

Cell Division

Functions of cell division:

1. Promote growth by producing more cells.
2. Repair processes

Before cell division takes place the genetic materials are duplicated precisely. DNA replication occurs towards the end of interphase period. The following processes take place chronologically:

• DNA helix uncoils and slowly divides its nucleotide chains. Individual nucleotide contains a set of instruction or serves as template for building another nucleotide strand.
• Nucleotides unites in complementary ways such as the following:
  1. Adenine (A) ALWAYS bonds to thymine (T)
  2. Guanine (G) ALWAYS bonds to cytosine (C)

• Identical DNA molecules are formed from the original DNA helix.
• Each DNA consists one old and one newly constructed nucleotide strand.

Events of Cell Division:

1. Mitosis: division of the nucleus
2. Cytokinesis: division of cytoplasm

MITOSIS

• The division of the nucleus is termed as mitosis. This event occurs after DNA replication takes place. When the nucleus divides, the daughter cells contain the same genetic information as the original mother cell.

Stages during mitosis are as follows:

• Prophase – stage where nuclear envelope and nucleoli have broken down and disappeared. Chromosomes in during this time have joined the spindle fibers through the centromeres.

Detail of event:

• During this period, the chromosomes appear due to the coiling and shortening of the chromatin threads. Each chromosome is made up of a pair of chromatid which is held together by a centromere (small button like body). Mitotic spindle is brought together by separation and moving of the centrioles to the opposite side of the cell.

• Metaphase – chromosomes have gathered together during this period and are lined up at the center of the spindle midway between the centrioles (metaphase plate).
• Anaphase – chromatids that are held together split (now called chromosome again) and begins to move gradually apart from each other to the opposite ends of the cells. This phase ends when the chromosome movement ends.
• Telophase – the reverse of prophase.

CYTOKINESIS

• The division of the cytoplasm begins during the late anaphase period and ends or is already completed during the telophase stage.

image from peacethought.com, biologie.uni-hamburg.de, sciencecity.oupchina.com.hk