Cell Biology: An Overview of Cellular Biology

cell biology

Cells are the smallest functioning units of living things. They are the basic units of life. Cells were first observed by the scientist Robert Hooke in 1665 after the invention of the microscope. Hooke published a book called Micrographia containing drawings and descriptions of his observations under the microscope. Hooke’s discovery led to the development of the cell theory.

Cell Theory

Cell theory states that every living thing is made up of at least one cell, the cell is the basic unit of life, and all new cells arise from pre-existing cells. This theory is useful in explaining living things and their function.

There are 5 scientists who contributed to the development of the cell theory:

      • Antony van Leeuwenhoek
      • Robert Hooke
      • Matthias Schleiden
      • Theodor Schwann and
      • Rudolf Virchow

Before the development of cell theory, people believed that life arose spontaneously from non-living things. Scientists developed this theory after observing cells with the help of microscopes.

The cell theory has been expanded over the years as more discoveries are made. Today, cell theory also states that all cells have a similar chemical composition, they carry genetic material and pass it on to other cells during cell division, and that energy flow occurs within cells.

Structure and Function of Cells

Cells are found in all the parts of a living organism. Their structure consists of three main parts: the cell membrane, cytoplasm, and nucleus. The cell membrane separates the cell from the external environment. The cytoplasm is the fluid in the cell where most activities of the cell occur. These activities are controlled by the nucleus, which is a structure within the cell that contains genetic material. The cytoplasm has cell organelles, which are the structures that carry out the different functions of the cell. There are many organelles within a cell, each carrying out different functions.

Cell Organelles and their Functions Table

Cell membraneSurrounds the cell organelles and contains them within the cell.

Separates the cell from the external environment

Performs selective permeability to allow the movement of specific materials in an and out of the cell
Cell wallRigid structure that provides additional support surrounding the cell

Protects the cell from the external environment

Shapes the cell
NucleusContains deoxyribonucleic acid (DNA), which directs the functions of the cell
Nuclear membraneDouble membrane layer surrounding the nucleus

Enables movement in an out of the nucleus
ChromatinGranular DNA material in the nucleus bound to protein

Forms chromosome during cell division
NucleolusA region within the nucleus where ribosomes assemble
ChromosomeThreadlike structures that contain genetic information

They pass genetic information to other cells during cell division
Endoplasmic reticulumHighly folded membrane cell structures that move materials within the cell

Rough endoplasmic reticulum have ribosomes and synthesize proteins

Smooth endoplasmic reticulum lacks ribosomes. It synthesizes and stores lipids
RibosomesFound on rough endoplasmic reticulum

Protein synthesis
Golgi apparatusStacks of membrane that sort, pack, and transport protein
LysosomeHas digestive enzymes to digest food, worn out organelles, and unwanted materials
MitochondriaProduces energy for use by the cell
ChloroplastContains chlorophyll, which is a green pigment that enables photosynthesis in plants
CytoplasmClear fluid inside the cell in which chemical reactions take place

Cell Membrane

how a cell membrane looks like

The cell membrane, also known as the plasma membrane is an important part of the cell. In addition to separating the cell from the external environment, it carries out many other roles. The cell membrane is made up of two fluid phospholipid layers. It is also made of other compounds including proteins, cholesterol, glycolipids, and glycoproteins.

The proteins enable transport in the cell membrane by regulating the movement of molecules in and out of the cell. The membrane is selectively permeable, thus only specific molecules are allowed movement within it. The cell membrane, therefore, functions as an entrance and a selective barrier. The proteins in the cell membrane also initiate cell activities by responding to chemical signals. They also anchor cells to other cells in the external environment. The lipids strengthen the cell membrane.

Types of Transport across the Cell Membrane

There are two main types of transport across the cell membrane: passive transport and active transport.

Passive Transport

This is the movement of molecules from regions of higher concentration to regions of lower concentration. Passive transport does not require energy to move molecules through the cell membrane. There are 4 types of passive transport:

      • simple diffusion
      • facilitated diffusion
      • osmosis, and
      • Filtration

Simple diffusion is the movement of molecules down the concentration gradient, without the need for assistance by membrane proteins.

Facilitated diffusion, on the other hand, involves membrane proteins that assist the movement of the molecules through the cell membrane.

Osmosis is the movement of molecules contained within a solvent down a concentration gradient through the cell membrane.

Filtration is different from the other forms of passive transport in that it relies on pressure gradients instead of concentration gradients. Molecules move from a region of higher pressure to a region of lower pressure through the cell membrane. The cell membrane acts as a filter that has tiny pores through which molecules can pass through. The pores are so small that pressure is needed to push the molecules through them.

Active Transport

Active transport is the movement of molecules against the concentration gradient. The molecules move from a region of lower concentration to a region of higher concentration. This form of transport requires makes use of energy. Cells use Adenosine triphosphate (ATP) to enable the movement of molecules through the cell membrane. ATP is obtained through cellular respiration in the mitochondria. The process of active transport maintains optimal concentrations of ions and molecules within the cell at all times.

Cell Division

Cell division is the process through which cells multiply by dividing themselves. The parent cell goes through several stages during the cell division process to make new cells.

Types of Cell Division

Cell division can be in the form of mitosis or meiosis depending on the type of cell. Mitosis is the division of body cells resulting in two identical daughter cells. The cell duplicates its contents and splits to produce two similar cells.

Meiosis, on the other hand, is the division of gamete cells resulting in four daughter cells. This process ensures that the offspring are genetically identical to their parents. It reduces the chromosome number by half to form a gamete cell that can pair with another during sexual reproduction and produce an embryo with the usual number of chromosomes.

Cell Division Stages


S phaseThe genetic material is duplicated
ProphaseChromosomes condense
MetaphaseChromosomes align at the middle
AnaphaseSister chromatids separate to different sides of the cell
TelophaseThe cell separates through the middle forming two daughter cells.


S phase IDuplication of genetic material

Separation of the DNA double strand
Prophase IChromosome condensation
Metaphase ITetrad alignment at the center of the cell
Anaphase IMovement of homologous chromosomes to opposite sides of the cell with the sister chromatids attached
Telophase IDe-condensation of the chromatids
Prophase IIChromosomes re-condensate but are not duplicated
Metaphase IISister chromatids align at the center of the cell
Anaphase IISister chromatids separate to opposite sides of the cell
Telophase IIChromosomes de-condensate
Four new daughter cells are formed

Cell Communication

What is Cell Communication?

Cell communication, also known as cell signaling, is the method used by cells to receive, process, and transmit information. This process is important in cells because it enables them to obtain signals from themselves and the external environment.

What is the Purpose of Cell Communication?

Cells need to send and receive messages frequently to enable their function. They use these messages to control their internal activities and the activities in distant cells. Cell communication is, therefore, an important property of cells. Cells should be able to communicate in a fast and efficient manner at all times to enable their function.

Types of Cell Communication

There are four basic types of cell communication:

  • Paracrine signaling – Cells produce signals to effect change in the nearby cells. The signals lead to altered behavior in the neighboring cells.
  • Autocrine signaling – Cells produce hormones known as autocrine agents that cause changes within the same cell. The hormones bind to autocrine receptors in the cell.
  • Endocrine signaling – Cells use signaling molecules to target other distant cells via the bloodstream. The molecules travel through the bloodstream and reach the receptors in distant cells causing change.
  • Direct signaling –It involves the transfer of signals between cells that are in direct contact. In animal cells, direct signaling occurs through gap junctions, while in plant cells it occurs through plasmodesmata.

Microscope Techniques

Microscopes are instruments used to observe small objects that cannot be observed using the naked eye. Scientists use different microscope techniques to enable the observation of cells and their organelles. These different techniques are achieved by varying the type of lenses and light sources.


Bright-field microscopy is the standard method used to view living cells in biology. The cells are fixed on microscope slides, where they are stained with basic stains to achieve contrast for observation. It is the simplest illumination technique used in microscopy.


Phase-contrast microscopy is an illumination technique that uses phase shifts in light to observe specimens. The phase shifts are invisible but they cause brightness variations in the image.

Differential interference contrast

Differential interference contrast (DIC) is a method used to enhance the image of transparent, unstained specimens. The technique involves introducing contrast to images that cannot be viewed using the basic bright-field method. The DIC technique gives high-resolution images that have good contrast for observation.


Fluorescence microscopy is the use of fluorescent molecules to enhance the visibility of structures. The fluorescent materials emit light when irradiated, thus enabling the observation of structures that are not usually visible.

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