Cell structure

Introduction

Different living organisms can be found in air, water, land and surface rocks. This life- sustaining part of Earth is called the biosphere.

A living organism is a group of molecules, organelles or body parts that work together and influence each other, creating an independent form of life. All living organisms have one thing in common - they are made of cells.

This chapter looks at different cell types and their structures.

Cell types

All biological cells are similar to each other but they are never identical, even within the same tissue or organism. Plants and animals have different cellular structures and so do different tissues and organs within the same organism.

There are different components within the cellular cytoplasm called organelles. They all have different jobs and vary depending on the cell's function.

All cells have:

DNA - a cell's genetic information carrier. In some cells DNA is in a separate cell structure called a nucleus and in others it just 'floats' in the cell's cytoplasm in the form of a nucleoid;
Membrane - the outer, thin, protective layer;
Cytoplasm - jelly-like matter inside the membrane.

See animation 1.

A cell's nucleus is a very important organelle. It controls all processes within the cell. Some cells have a nucleus and some do not. So, all cells are divided into two large groups: cells with a nucleus, called eukaryotic cells, or simply eukaryotes; and cells without a nucleus, called prokaryotic cells, or prokaryotes.

The table below will help you to better understand differences between prokaryotes and eukaryotes.

organelles	prokaryotes	eukaryotes
organelles	function
nucleus	none	contains cell's genetic material - chromosomes and controls all main chemical reactions within the cell
cell membrane	cell's boundary layer, prevents cytoplasm from "spilling out", controls what goes in and out of cell, holds cells together
cell wall	found only in plants and some bacteria cells; rigid outer wall of cell made of non-living secretion; protects and binds cells together
cytoplasm	the material inside the cell; contains cell's organelles and other chemical compounds; sometimes is called "cell's chemical factory"
vacuoles	none	storage areas of the cell; contain water, fluid, air or waste
mitochondria	none	cell's main energy producers
endoplasmic reticulum	none	network of membranes, connected to the cell's nucleus, responsible for the transportation of chemicals between and within cells
ribosome	responsible for the protein synthesis

The table above gives only a general and very simplified picture of a cell's structure. The organelles of all cells are very complex and vary enormously between eukaryotes and prokaryotes, plants and animals, and different body tissues and organs.

Prokaryotes

The first living cells on Earth were prokaryotes. Prokaryotes are much simpler in structure than eukaryotes. Prokaryotes were well adapted to the harsh environment of the young planet Earth. They were anaerobic type of organisms, meaning that oxygen was poisonous to them. Because at that time there was no oxygen, they thrived. Some of these early organisms started to use sunlight as a source of energy in a process called photosynthesis. Oxygen is one of the 'waste' products of photosynthesis. The photosynthesizing prokaryotes released oxygen which eventually turned Earth into a place suitable for other living organisms. The blue-green cyano-bacteria were one of the first prokaryotic cells. They can still be found today. See image 1.

Prokaryotes reproduce by cell division which is also called asexual reproduction.

Eukaryotes

Eukaryotic cells are also microscopic in size but they are much bigger than the prokaryotic ones. Eukaryotes are much more complex in structure and have more cellular organelles. Scientists believe that some organelles are the evolutionary result of mutual coexistence between prokaryotic and eukaryotic cells. In other words, millions of years ago some organelles used to be prokaryotic cells living on their own. Organelles like mitochondria and chloroplasts still have their own DNA and divide in the same way they did when they were free living organisms. Eukaryotic cells protect them from oxygen and provide a stable environment with all the necessary nutrients. See image 2.

Eukaryotes can reproduce by asexual or sexual reproduction. Sexual reproduction means that the new organism is produced by the mating of two cells.

Coenocytes

Some cells have multiple nuclei. They are actually healthy normal cells called coenocytes. When a cell nucleus divides but the cell itself does not, a coenocytic cell is formed. These types of cells are found in some fungi, algae and some unicellular organisms from the protozoa group. Plant tissue, called endosperm, is also made from cells containing a triploid set of chromosomes. The seeds of flowering plants contain endosperm.