Cell Structures and Organelles

Cell Surface Membrane (plasma membrane)

All cells possess a cell membrane (both prokaryotic and eukaryotic). This is the outermost structure of the cell which separates the cell from its environment. The cell membrane is a thin partially permeable bilayer structure (2 layers) which has protein embedded within the layers. The layers are made of lipids, especially phospholipids and function not only to separate the cell from its outer environment but also filters the entrance and exit of molecules.

Cytoplasm

This is a semi-liquid material enclosed within the cell membrane. The cell cytoplasm, often described as jelly-like, essentially acts as a form of support for the cell structures located within the semi-liquid medium. About 70% of the cytoplasm is water. The entire content of the cell is called protoplasm.

Nucleus

This is the largest cell organelle that is present in both the plant and animal cell however it is absent in prokaryotic cells.  The nucleus is composed mostly of protein and deoxyribonucleic acid (DNA). This DNA is the genetic material of the cell and controls its structure and function.  DNA is organized into linear units called chromosomes. The functional parts of each chromosome are called genes. It is through these genes that genetic material is passed during reproduction.

The nucleus is enclosed by an envelope of two membranes called nuclear membrane (similar to the cell membrane). Though prokaryotic cells do not have a nucleus, they do have DNA. This DNA is found freely in the cytoplasm of the cell, most times wrapped in a tight coil.

Endoplasmic Reticulum

The endoplasmic reticulum is a very important organelle in the cell and is found in both animal and plants cells. It is responsible for protein and lipid synthesis.  There are 2 types of endoplasmic reticulum: rough and smooth. The rough endoplasmic reticulum has ribosomes present and is responsible for the synthesis of proteins. The smooth endoplasmic reticulum does not have ribosomes present and is responsible for the synthesis of lipids.  In addition to protein synthesis the endoplasmic reticulum is also used to modify proteins.

The Mitochondrion (plural mitochondria)

The mitochondrion appears as dark spots when viewed under the microscope. The mitochondrion is elongated and shaped like a sausage. It is referred to as the “power house of the cell” because it provides energy in the form of ATP through the process of respiration. Cells release energy from foods during respiration by way of the mitochondrion. This organelle is present in all eukaryotic cells.

Chloroplast

These are large plastid structures containing chlorophyll. They have large internal surface areas and conduct photosynthesis on their thylakoid membranes. The large surface area increases the amount of photosynthesis taking place within a given area thus increasing efficiency. Starch grains may be found within the chloroplast, which have been made as a result of photosynthesis.

Vacuoles

These are small sacs bounded by a single membrane called the vacuolar membrane. Vacuoles contain cell sap, which is made up of variable amounts of water, mineral salts, sugars, pigments and enzymes depending on the cell. Vacuoles are present in both the plant and animal cell, but they are much smaller in the animal cell and are not permanent.  The presence of a large vacuole increases the osmotic pressure of the cell. It is the presence of a large vacuole in a plant cell that causes it to become turgid (swollen). Plants need turgid cells to aid in support as it does not have a bone structure.

Cell wall

This structure is only found in plant cells. This rigid cell structure gives the cell a regular shape and prevents it from bursting when the plant cells become swollen. The cell wall is made of cellulose, which is a polysaccharide. The cell wall encloses/ covers the cell membrane and its contents (protoplasm). Due to its structural make up its primary function is to provide protection and support to the cell.

There are many other organelles present in both the animal and plant cells which we have not discussed. However, at this stage in your studies only these will really be asked for. It would not hurt however to familiarize yourself with other organelles.

These include:

-Golgi body/apparatus

-Ribosomes

-Lysosome