Cell Structure

Cells are the basic building blocks of all living things. The stuff found inside them depends on whether the cell is eukaryotic — like our own— or prokaryotic. All cells start off as stem cells and become specialised through a process called differentiation.

Eukaryotes and prokaryotes

Eukaryotes are organisms whose cells have their genetic material enclosed in a nucleus. Animal cells and plant cells are both examples of eukaryotic cells. Eukaryotic cells also contain cytoplasm which is enclosed in a cell membrane.

Prokaryotes are single-celled organisms whose cells do not have a nucleus. Bacteria are examples of prokaryotic cells and their DNA floats freely in the cytoplasm. They also have extra pieces of smaller circular DNA, called plasmids. Bacterial cells also have a cytoplasm enclosed in a cell membrane. Outside of the cell membrane is a cell wall. Prokaryotic cells are much smaller than eukaryotic cells.

Animal and plant cells

Cells contain structures called organelles which each perform a different role in the cell. The organelles which can be found in both animal cells and plant cells are:

• Nucleus – this stores genetic material and controls the activities of the cell

• Cytoplasm – where metabolic reactions take place

• Cell membrane – holds the cell together and controls what enters and exits the cell

• Mitochondria – the site for aerobic respiration – energy is produced here

• Ribosomes – responsible for synthesising proteins

Plant cells have some extra features which are absent in animal cells:

• Chloroplasts – where photosynthesis takes place

• Vacuole – filled with cell sap, helps to keep the cell turgid

• Cell wall made of cellulose – this provides structure and support to the plant

Size of cells

Animal and plant cells range in size from 0.1 mm to 0.01 mm. Bacterial cells are even smaller and have an average size of approximately 0.001 mm. When referring to cell size, it is most appropriate to use the unit micrometres. To convert between millimetres and micrometres, you need to multiply by 1000. Therefore the size of a 0.001 mm bacterial cell is 1 um.

Cell specialisation

The cell is the ‘basic building block of life’ and is the smallest functioning part of an organism. A group of cells working together is called a tissue and a collection of tissues all performing a specific function is called an organ. Multiple organs which are connected together are referred to as an organ system. Examples of organs include the heart, the lungs, the liver, kidneys, the intestines and the stomach. Examples of organ systems include the respiratory system, circulatory system, reproductive system and digestive system.

Cells exist as a variety of different shapes with various features and will carry out different functions in the body. For example:

• Sperm cells are long and thin to make them streamlined for swimming to the egg cell. They have a flagellum which also facilitates swimming and lots of mitochondria to provide the energy for movement.

• Nerve cells contain branched endings called dendrites which lets them form connections to other cells. The nerve cell is surrounded by a fatty sheath, which insulates the nerve cell and speeds up the nerve impulse.

• Muscle cells possess a high number of mitochondria to provide the energy for muscle contraction.

• Root hair cells are long and thin, providing a large surface area for the movement of water and mineral ions into the root.

• Xylem vessels transport water through a plant. They have no cell walls at the top and bottom of the cell, allowing the xylem vessels to form one continuous tube when stacked on top of each other. They contain a substance called lignin in the cell walls which gives strength to the xylem vessel.

• Phloem vessels transport sugars through a plant. They have sieve plates at the end of each cell which contain pores, allowing sugars to move between the phloem cells. They contain few organelles which provides more space to transport sugar.

Cell differentiation

All cells start out as unspecialised cells, called stem cells. Stem cells have the ability to differentiate (specialise) into any type of cell. A developing embryo contains a lot of stem cells, which will differentiate to become all the cells of the human body, including heart cells, brain cells and muscle cells. Animal cells tend to differentiate at a very early stage in development. By the time our bodies are fully formed, cell differentiation is mostly finished. We still have some stem cells present in our bone marrow, but cell division is mainly restricted to repair and replacement. In contrast, plant cells retain the ability to differentiate into specialised cells throughout their life.

As a cell differentiates, it gains new features, or loses others which make it suited to its function. For example, during the differentiation of a stem cell into a red blood cell, it will start to produce haemoglobin (the protein which carries oxygen) and its nucleus will disappear (to provide more space for oxygen transport).

Microscopy

A light microscope is the type of microscope you’ll have used in school. It uses light to magnify objects up to 1,500x their actual size. They have a low resolution (they can’t produce very clear images). The magnification on a light microscope isn’t large enough to visualise any of the smaller organelles, such as ribosomes. They are more commonly used for visualising whole cells or tissues.

Electron microscopes are used whenever scientists want to visualise the structure of organelles and to study cells in finer detail. They have a much higher magnification (500,000x – 1,000,000x) and a higher resolution than light microscopes. The invention of electron microscopy has increased our understanding of the structure of cellular organelles.

You can calculate the magnification of an object using the following equation:

Culturing microorganisms