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What is a cell?

Cells are the basic unit of life. They are small membrane-bound structures containing several smaller structures called organelles.

eukaryotic cell, including the cells of animals and plants

prokaryotic cell, including bacterial cells.

There are two main categories of cell, each of which have important different structural properties:


A brief history


The cell membrane and cytoplasm

All cells, and all true organelles, are contained within a membrane, based on a phospholipid bilayer.

The cell membrane holds acell together and controlswhat enters and leaves thecytoplasm, as it is a selectively permeable barrier.

The cell membrane and cytoplasm are universal features of the living cell.

The cytoplasm comprises a liquid called cytosol, and all the organelles suspended in it (except the nucleus in eukaryotes).


The universal energy carrier

Cells need a constant supply of energy to carry out vital processes such as protein synthesis, DNA replication and cell division.

This energy originally comes from fuel molecules, such as glucose, consumed by the organism. These are broken down during aerobic or anaerobic respiration, and the energy released is used to make adenosine triphosphate (ATP).

ATP diffuses throughout the cell, and breaks down into adenosine diphosphate (ADP), releasing chemical energy wherever it is needed.

ATP

ADP


What is a eukaryote?

A eukaryote is any organism consisting of one or more cells that contain DNA in a membrane-bound nucleus, separate from the cytoplasm.

plants

a diverse group known as the protists (or protoctists).

All eukaryotic cells contain a large number of specialized, membrane-bound organelles.

Eukaryotes include:

fungi

animals


The organelles of protein synthesis


Mitochondria

The mitochondrion is an energy-generating organelle.

The inner membrane is coated in enzymes, which catalyze the reactions of aerobic respiration to produce ATP.

It is surrounded by two membranes. The inner layer folds inwards to form the cristae. The cristae project into a liquid called the matrix.

outer membrane

matrixinner membrane

cristae


Microtubules and the cytoskeleton


Which organelle?


Plant cells

Plant cells share all the common features of animal cells, but also contain some additional organelles.

Plants gain all their energy from sunlight; cells in their leaves contain many chloroplasts to convert this into a useful form.

Every plant cell is surrounded by a cell wall, and contains one or more permanent vacuoles.

chloroplast

vacuole

cell wall


Chloroplasts

Chloroplasts use carbon dioxide, water and light energy to build sugars. They are present in all green plants.

The chloroplast is surrounded by a double membrane. It is filled with a liquid called the stroma, and contains stacks of thylakoid membranes called grana.

stroma

thylakoid membrane

The thylakoid membranes are the site of photosynthesis.

grana


Permanent vacuoles only exist in plant cells. Animal cells can contain temporary vacuoles but they are not common features.

A vacuole consists of a membrane called the tonoplast, filled with cell sap – a watery solution of different substances, including sugars, enzymes and pigments.

The vacuole is important in keeping the cell firm. When the vacuole is full of sap the cell is said to be turgid.

Vacuoles


The cell wall does not seal off a cell completely from its neighbours. There are pores within the walls called plasmodesmata. These connect two cells together by their cytoplasm, enabling the exchange and transport of substances.

The cell wall

The cell wall of a plant cell gives it support and structure. It is made of the polysaccharide cellulose, and can function as a carbohydrate store by varying the amount of cellulose it holds.


Eukaryotic organelles


What is a prokaryote?

A prokaryote is any organism – usually single-celled – whose DNA is suspended freely in the cytoplasm. The word means ‘before the nucleus’.

bacteria archaea.

Prokaryotes have simpler structure than eukaryotes, lacking organelles such as the nucleus, ER and Golgi.

Prokaryotes can be divided into two groups:


Features of the bacteria and archaea


Bacterial capsules

Many pathogenic bacteria are surrounded by a mucous-like protective layer called a capsule.

The capsule is usually composed of polysaccharides, and also contains water to protect against desiccation (drying out).

capsule cell wall

The capsule protects bacteria from viruses, or attack from a host organism’s immune system, by hiding antigens on the cell surface.


Some prokaryotic cells feature one or more flagella. These are long helical tubes extending out of the cell wall, which rotate to provide locomotion.

Flagella and pili

Many bacteria also feature pili. These are hollow protein structures used during bacterial conjugation – the transfer of genetic material from one bacterium to another.

Flagella are powered by protein motors and can propel bacteria at a rate of more than 50 lengths per second.


Plasmids

Bacterial cells often contain several plasmids – small continuous loops of DNA.

Plasmids are replicated independently of a bacterium’s genophore (e.g. during bacterial conjugation), and may confer an advantage, such as antibiotic resistance.

Plasmids are commonly used in genetic engineering to make copies of genes or large quantities of proteins or hormones.

pilus drawsbacteria together

replication of plasmid


Structure of a bacterium


How small is a cell?


Light microscopes

Light (or optical) microscopes use lenses to project a magnified image of an object onto the eye.

Light microscopes are limited to a magnification of 1500× by their resolving power (resolution). This is a measure of their ability to distinguish between two separate points. A light microscope cannot resolve two points that are closer than half a wavelength of visible light (250 nm).

Magnification is a measure of how many times bigger the image is than the object:

size of image

actual size of the objectmagnification =


Virtual microscope


Electron microscopes


Preparing a specimen for microscopy

Electron microscopes contain a vacuum as air particles would interfere with the beam of electrons.

Optical microscopes can be used to view living specimens. Coloured dyes (stains) can be used to make specific structures more visible under a light microscope.

Water boils at room temperature in a vacuum, so the specimen must be dried out completely (dead).


Extracting organelles from cells


Microscope characteristics


Glossary


What’s the keyword?


Eukaryotes vs. prokaryotes


Multiple-choice quiz

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