(2) Introduction to CELLS and types of cells

(a) Why is cell biology important?

Introduction to cells and types of cells.

Cell biology is very important t understand the structure and function of cells e.g. what do the various sub-cellular structures do? and how does a cell function as a living unit.

Knowledge of cells helps us understand how organisms develop and interact with other organisms e.g. sexual reproduction or our bodies own defences in fighting bacteria.

Knowledge of cell chemistry is also important in the diagnosis of disease and developing drugs to counteract adverse medical conditions e.g. anti-cancer drugs.

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(b) What is a cell? what defines a cell?

A cell is the smallest unit of life able to control its own activities, BUT, it relies on the rest of the organism (if multicellular) or the surroundings (if unicellular) to provide it with raw materials i.e. nutrients and or place to remove waste material.

The different parts of a cell are referred to as subcellular structures.

The term organelle refers to specialized sub-cellular structure within a cell that perform a specific function (e.g., mitochondria, ribosomes, chloroplasts).

Organelles in unicellular organisms are the equivalent of organs in multicellular organisms.

You should know and understand  that the structures of different types of cells are related to their functions.

 You should know and understand the similarities and differences between animal cells, bacteria and plant cells.

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(c) The two main groups of cells

The majority of living things are made up of cells, the building blocks of life.

You can think of most cells as a unit of life surrounded and enclosed by an outer membrane of some sort.

Cells can be either eukaryotic (eukarya) or prokaryotic (archaea and bacteria) in character, see 'tree of life' diagram above..

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(d) Eukaryote cells (eukarya)

Eukaryotes are organisms made of eukaryotic cells, which are complex cells, and all plants and animals are made up of such cells.

Eukaryotes are usually multi-cellular organisms, but can consist of one cell e.g. yeast or algae (unicellular).

Plant and animal cells (eukaryotic cells) all have a cell membrane, cytoplasm and genetic material enclosed in a true nucleus in the cytoplasm (compared to prokaryotic cells described below) and other sub-cellular structures called organelles e.g. mitochondria, chloroplasts, ribosomes etc.

However there are significant differences between eukaryotic plant and animal cells (see later).

Watch out for adaptations in both unicellular and multicellular organisms that help them survive and reproduce.

See also Classification - domain, kingdom, phylum, class, order, family, genus, species, Linnaeus naming

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(e) Prokaryote cells (archaea and bacteria)

Prokaryotes, are smaller and simpler single celled organisms (unicellular prokaryotic) eg bacteria and archaea are prokaryotic cells.

Prokaryotic cells do NOT have a true nucleus containing the DNA - the DNA is in free floating loops/strands in the cytoplasm.

Archaea are now considered as a separate domain of single celled organisms (see 'tree of life' diagram below) even though they are like bacteria

See classification and evolutionary trees gcse biology revision notes

Prokaryotic cells are more primitive than eukaryotic cells and the oldest fossil evidence suggests that bacteria were evolving as much as 3.5 billion years ago.

Planet Earth is reckoned to be about 4.5 billion years old (4.5 x 10⁹ years from radiometric data - using the half-lives of elements in rocks - the calculations are VERY complex).

The early bacteria probably existed as thin purple or green coverings on shorelines.

These early bacteria used photosynthesis but produced sulfur instead of oxygen as a waste product.

See also Classification - domain, kingdom, phylum, class, order, family, genus, species, Linnaeus naming of organisms including the three domain system

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(f) Similarities of, and differences between animal and plant cells.

All plant and animal cells have similarities in basic structure, BUT, there are important differences between them.

Both eukaryotic and prokaryotic cells contain various subcellular structures, of some are referred to as organelles - a subcellular structure performing a particular function, on a larger scale, rather a like an organ in an animal.

There are also important differences between prokaryotic cells (single celled archaea and bacteria) and (usually) multicellular) eukaryotic cells (plants and animals)

Using diagrams and explanatory notes, their similarities and differences in sub-cellular structures will be described and their functions explained in further sections.

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(g) Organelles - sub-structures of cells.

Note that an organelle is a specialised sub-cellular unit that performs a specific function rather like a larger structure like an organ does.

It has its own membrane and examples include mitochondria (organelle of energy production molecules) in both plants and animals which have their own DNA and in plant cells, chloroplasts, the organelle of photosynthesis.

Examples of organelles

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Some typical learning objectives for this section on cell structure and function

All of these learning objectives are explored further in parts 3 to 9

Know the differences between eukaryotes and prokaryotes : 

Know that plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

Know that bacterial cells (prokaryotic cells) are much smaller and simpler in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

Know the similarities and differences between animal, plant and bacterial cells.

You should be able to explain how the main sub-cellular structures, including the nucleus, cell membranes, mitochondria, chloroplasts in plant cells and plasmids in bacterial cells are related to their functions.

Know that most animal cells have the following parts: a nucleus, which controls the activities of the cell, cytoplasm, in which most of the chemical reactions take place, a cell membrane, which controls the passage of substances into and out of the cell, mitochondria, which is where aerobic respiration takes place, ribosomes, which are where protein synthesis occurs.

Be able to recognise, draw and interpret images of cells.

Know the parts found in animal cells (listed above)

Know that plant cells often have: chloroplasts, which absorb light to make food by photosynthesis a permanent vacuole filled with cell sap.

Know that plant and algal cells have a more rigid cell wall made of cellulose, which strengthens the cell.

The diagram above illustrates some of the basic features to recognise, without the key below!

More detailed diagrams of typical animal cells and plants cells are in Parts 3 and 4.

L = mitochondria, N = chloroplasts, O = cilia hairs, P = cell wall, R = cell membrane, T = vacuole, W = nucleus, Z = cytoplasm

Mitochondria, nucleus and chloroplasts are examples of organelles.

Cell 1: Ciliated cell - mucous membrane cell with cilia hairs to move mucous along, found in nose, throat, stomach and lungs.

Cell 2: Plant cell - nucleus, chloroplasts, cell membrane, cell wall, large vacuole, nucleus, mitochondria

Cell 3: Red blood cell - has no nucleus, but important carrier of oxygen for respiration in all cells with mitochondria.

Cell 4: Non-specific cell - just showing cytoplasm, cell membrane, nucleus and mitochondria.

Cell 5: Sperm cell - showing swimming tail and nucleus.

Cell 6: Plant root hair cell - cytoplasm, nucleus, mitochondria, cell wall, cell membrane, (NO chloroplasts!)

All of these learning objectives are explored further in parts 3 to 9

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Key points - Summary of ideas and extra examples

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

What Defines a Cell?

A cell is the basic unit of life in all living organisms.

It is the smallest structural and functional unit capable of sustaining life.

Cells can be unicellular (single-celled, e.g., bacteria) or multicellular (many cells working together, e.g., humans).

Cell Theory:

1. All living things are made up of cells.

2. Cells are the basic units of life.

3. All cells arise from pre-existing cells.

Cells contain essential structures that support life, including genetic material (DNA), cytoplasm, and a plasma membrane that controls the movement of substances in and out of the cell.

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What Defines an Organelle?

Organelles are specialized structures within a cell that perform specific functions, like organs in the human body.

Each organelle plays a crucial role in maintaining cell processes.

Examples of Organelles:

• Nucleus – Contains genetic material (DNA); controls cell activities.

• Mitochondria – The powerhouse of the cell, generating ATP for energy.

• Ribosomes – Responsible for protein synthesis.

• Endoplasmic Reticulum (ER) – Transports materials; smooth ER synthesizes lipids, and rough ER is involved in protein processing.

• Golgi Apparatus – Modifies, sorts, and packages proteins.

• Lysosomes – Contain enzymes for breaking down waste.

• Chloroplasts (in plants) – Site of photosynthesis.

• Vacuole – Stores nutrients and waste (larger in plant cells).

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Types of Cells

Cells are classified into two main types:

1. Prokaryotic Cells (e.g., Bacteria)

• Simple structure; no nucleus.

• DNA floats freely in the cytoplasm.

• Smaller in size (~1-10 µm).

• Few organelles; ribosomes are the only organelle present.

• Cell wall is not made of cellulose (peptidoglycan in bacteria).

• May have flagella for movement.

2. Eukaryotic Cells (e.g., Plant, Animal, Fungi, Protists)

• Complex structure with a nucleus.

• DNA is enclosed within the nuclear membrane.

• Larger in size (~10-100 µm).

• Contains many organelles (e.g., mitochondria, ER, Golgi).

• Plant cells have cell walls made of cellulose; animal cells do not.

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Characteristics of Eukaryotes versus Prokaryotes

The Tree of Life

The Tree of Life shows how all organisms are related through evolution.

It categorizes life into three main domains:

1. Bacteria

• Prokaryotic

• Unicellular

• Peptidoglycan cell walls

• Includes common bacteria (e.g., E. coli)

2. Archaea

• Prokaryotic

• Unicellular

• Found in extreme environments (e.g., deep sea vents)

• Cell membranes have unique lipids

3. Eukarya

• Eukaryotic cells

• Includes plants, animals, fungi, and protists

• More complex than prokaryotes

These domains are further divided into kingdoms (Animalia, Plantae, Fungi, Protista, etc.), each representing major groups of life.

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Summary of learning objectives and key words or phrases for types of cells and organisms

What is a cell?

What are the main types of cells?  eukaryotic and prokaryotic

Know the different types of organisms that are eukaryotes or prokaryotes

In cell structure and function, what is an organelle?

Understand a simple diagram of evolutionary tree showing the common ancestor of eukarya, archaea and bacteria

Appreciate an evolutionary tree diagram showing the common ancestor of eukarya archaea bacteria

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