Cells and Control
Cell Structure and Organisation
The cell is the smallest significant unit of all living organisms; it is the basic building block of all living
things. Every tissue and organ of the body is composed of cells. Each cell contains specialised
structures inside called organelles, which carry out particular tasks that help the cell function as a
whole.
Types of cells
Scientists usually categorize cells by how their genetic material is packaged.
Prokaryotic cells – If the DNA within a cell is not separated from the cytoplasm (not membrane
bound), then that cell is a prokaryote. These cells have cytoplasm, an outer cell membrane and a cell
wall. All organelles, including the genetic material in the cell, are not enclosed by a membrane.
Prokaryotic cells have a single DNA loop in the cytoplasm and there may be small rings of DNA
floating in the cytoplasm, called plasmids. Currently, all known prokaryotic organisms are single-celled
e.g. bacteria and archaea.
Eukaryotic cells – These cells contain both membrane bound and non-membrane bound organelles.
The genetic information (DNA) is enclosed within a nucleus. All animal and plant cells are eukaryotic.
Animal cells – These cells are the basic unit of life in organisms of the kingdom Animalia. These cells
have a nucleus, mitochondria, ribosomes and other organelles (discussed below). Animal cells are
also enclosed by a cell membrane. Unlike the eukaryotic cells of plants and fungi, animal cells do not
have a cell wall.
Plant cells – These cells have all the same sub-cellular organelles as animal cells. However, they
also have chloroplasts, a vacuole and cell wall made of cellulose. There are several types of plant
cells e.g. xylem cells and phloem cells.
,Definitions
Organelle – A specialised structure within a living cell which carries out specific functions e.g.
Mitochondria or Cilia (hairs which project from the cells lining your windpipe.). Membrane-bound
organelles are only found in eukaryotic cells. Prokaryotic cells e.g. bacteria only contain non-
membrane bound organelles like ribosomes (You will learn about these organelles later!).
Cells – The smallest building blocks of all living things. Some cells are organisms unto themselves;
others are part of a larger multicellular organism. An example of a cell is a neurone (a nerve cell that
transmits electrical signals in the body).
Tissue – A collection of similar cells that work together to carry out a particular function e.g. the
Mucosa tissue is a tissue that lines the inside of the stomach.
Organ – A collection of several tissues that work together to carry out a particular function e.g. the
stomach (in an animal) or the flower which produces the seeds for reproduction (in a plant).
Organ system - A collection of several organs that work together to carry out a particular function e.g.
the Digestive system or Circulatory system (this moves material around the organism from organ to
organ in animals).
Cell cycle- The ordered sequence of stages by which the organelles and genetic material in a cell are
duplicated in preparation for cell division. This is a cycle with four stages (G1, S, G2, and M) which
cells must go through to divide and produce new cells.
Organelles and their functions
Nucleus –Contains the genetic information of the cell in the form of DNA. A gene is a functional unit
of DNA. Information about every feature of the cell and the whole organism can be found in the
genes. The nucleus also controls the activities that occur within the cell e.g. the substances the cell
creates, and what goes in and out of the cell.
Cytoplasm – A thick liquid where most of the chemical reactions in the cell occur. The organelles of
the cell move around in this substance and important molecules, such as enzymes (which catalyse
many metabolic reactions), are also found in the cytoplasm.
Mitochondria – Sausage-shaped organelles found within the cytoplasm which are responsible for
providing energy by carrying out aerobic respiration.
Ribosomes – There are millions of these located in the cell, each responsible for the synthesis
(production) of proteins such as enzymes.
Cell membrane – Every cell has a cell membrane which controls what moves in and out of the cell.
This is the boundary between the cell cytoplasm and the cells surroundings, keeping the cell
cytoplasm and cell organelles locked within the cell. Substances may move across this membrane via
different methods e.g. diffusion or active transport.
Chloroplast – These are found primarily in plant cells (but also in some bacteria and protoctists).
These green disc-shaped organelles are the site of photosynthesis by absorption of light. Chloroplasts
are only found in the photosynthetic cells of the plant egg not in the plant cells of the roots.
Permanent vacuole – A large section in the centre of the cell which is filled with cell sap (a watery
fluid). This helps keep the cell turgid. It does this by pushing the cytoplasm outwards against the cell
wall. These large permanent vacuoles are only found in plant cells but sometimes animal cells have
small vacuoles.
Cell wall – Plant cells are all surrounded by this tough cell well. It is composed of cellulose, providing
strength, shape and protection to the cells. Cell walls make it easy to spot a plant cell under a light
, microscope. The cell walls of fungi are made of chitin and the cell walls of bacteria are made of a
polymer of sugars and amino acids.
Stem cells and differentiation
A stem cell is an undifferentiated cell that can become any type of specialised cell.
All multicellular organisms begin life as a single cell. A zygote (fertilised egg cell) divides by mitosis
over and over again forming identical daughter cells until an individual organism is made up of billions
of differentiated (specialised) cells.
How does this work?
As new cells are being produced (undifferentiated), specific genes are switched on or off within
each cell. This happens in response to the chemical hormones surrounding the cell. This pattern
causes a cell to differentiate into one of the 250 possible types of specialised cell in the adult
human body.
Totipotent stem cell – At this stage, the cell has the potential to differentiate into any type of
specialised cell.
Eventually, the cell becomes stuck in a particular pathway of development and is committed to
becoming a specific type of cell e.g. a red blood cell or guard cell.
Stem cells are mostly found in growing embryos but can also be found in adults e.g. in the brain, bone
marrow, blood vessels, skin, teeth, liver, and other (although not all) organs and tissues. They divide
to replenish dying cells and revive damaged tissues. However, in adults, there are a limited
number of specialised cells that stem cells can become. Plants contain stem cells, called meristem
tissue, in the roots, shoot tips and the flowering parts of the plant. These are areas where cells are
rapidly dividing and where growth takes place e.g. elongation, differentiation of cells and cell division.
What are the functions of stem cells?
They replace cells in the particular organ of the body in which they are found. This happens when new
cells are required in tissues that been destroyed by accidents or disease. Stem cells found in bone
marrow, in particular, can differentiate into many types of cells e.g. red blood cells. Stem cells in
embryos differentiate to give us ALL the specialised cells found in the adult body. Meristem tissue
(found in the shoot tips and root tips of plants) can become specialised cells required for a plant to
grow.
Cell Structure and Organisation
The cell is the smallest significant unit of all living organisms; it is the basic building block of all living
things. Every tissue and organ of the body is composed of cells. Each cell contains specialised
structures inside called organelles, which carry out particular tasks that help the cell function as a
whole.
Types of cells
Scientists usually categorize cells by how their genetic material is packaged.
Prokaryotic cells – If the DNA within a cell is not separated from the cytoplasm (not membrane
bound), then that cell is a prokaryote. These cells have cytoplasm, an outer cell membrane and a cell
wall. All organelles, including the genetic material in the cell, are not enclosed by a membrane.
Prokaryotic cells have a single DNA loop in the cytoplasm and there may be small rings of DNA
floating in the cytoplasm, called plasmids. Currently, all known prokaryotic organisms are single-celled
e.g. bacteria and archaea.
Eukaryotic cells – These cells contain both membrane bound and non-membrane bound organelles.
The genetic information (DNA) is enclosed within a nucleus. All animal and plant cells are eukaryotic.
Animal cells – These cells are the basic unit of life in organisms of the kingdom Animalia. These cells
have a nucleus, mitochondria, ribosomes and other organelles (discussed below). Animal cells are
also enclosed by a cell membrane. Unlike the eukaryotic cells of plants and fungi, animal cells do not
have a cell wall.
Plant cells – These cells have all the same sub-cellular organelles as animal cells. However, they
also have chloroplasts, a vacuole and cell wall made of cellulose. There are several types of plant
cells e.g. xylem cells and phloem cells.
,Definitions
Organelle – A specialised structure within a living cell which carries out specific functions e.g.
Mitochondria or Cilia (hairs which project from the cells lining your windpipe.). Membrane-bound
organelles are only found in eukaryotic cells. Prokaryotic cells e.g. bacteria only contain non-
membrane bound organelles like ribosomes (You will learn about these organelles later!).
Cells – The smallest building blocks of all living things. Some cells are organisms unto themselves;
others are part of a larger multicellular organism. An example of a cell is a neurone (a nerve cell that
transmits electrical signals in the body).
Tissue – A collection of similar cells that work together to carry out a particular function e.g. the
Mucosa tissue is a tissue that lines the inside of the stomach.
Organ – A collection of several tissues that work together to carry out a particular function e.g. the
stomach (in an animal) or the flower which produces the seeds for reproduction (in a plant).
Organ system - A collection of several organs that work together to carry out a particular function e.g.
the Digestive system or Circulatory system (this moves material around the organism from organ to
organ in animals).
Cell cycle- The ordered sequence of stages by which the organelles and genetic material in a cell are
duplicated in preparation for cell division. This is a cycle with four stages (G1, S, G2, and M) which
cells must go through to divide and produce new cells.
Organelles and their functions
Nucleus –Contains the genetic information of the cell in the form of DNA. A gene is a functional unit
of DNA. Information about every feature of the cell and the whole organism can be found in the
genes. The nucleus also controls the activities that occur within the cell e.g. the substances the cell
creates, and what goes in and out of the cell.
Cytoplasm – A thick liquid where most of the chemical reactions in the cell occur. The organelles of
the cell move around in this substance and important molecules, such as enzymes (which catalyse
many metabolic reactions), are also found in the cytoplasm.
Mitochondria – Sausage-shaped organelles found within the cytoplasm which are responsible for
providing energy by carrying out aerobic respiration.
Ribosomes – There are millions of these located in the cell, each responsible for the synthesis
(production) of proteins such as enzymes.
Cell membrane – Every cell has a cell membrane which controls what moves in and out of the cell.
This is the boundary between the cell cytoplasm and the cells surroundings, keeping the cell
cytoplasm and cell organelles locked within the cell. Substances may move across this membrane via
different methods e.g. diffusion or active transport.
Chloroplast – These are found primarily in plant cells (but also in some bacteria and protoctists).
These green disc-shaped organelles are the site of photosynthesis by absorption of light. Chloroplasts
are only found in the photosynthetic cells of the plant egg not in the plant cells of the roots.
Permanent vacuole – A large section in the centre of the cell which is filled with cell sap (a watery
fluid). This helps keep the cell turgid. It does this by pushing the cytoplasm outwards against the cell
wall. These large permanent vacuoles are only found in plant cells but sometimes animal cells have
small vacuoles.
Cell wall – Plant cells are all surrounded by this tough cell well. It is composed of cellulose, providing
strength, shape and protection to the cells. Cell walls make it easy to spot a plant cell under a light
, microscope. The cell walls of fungi are made of chitin and the cell walls of bacteria are made of a
polymer of sugars and amino acids.
Stem cells and differentiation
A stem cell is an undifferentiated cell that can become any type of specialised cell.
All multicellular organisms begin life as a single cell. A zygote (fertilised egg cell) divides by mitosis
over and over again forming identical daughter cells until an individual organism is made up of billions
of differentiated (specialised) cells.
How does this work?
As new cells are being produced (undifferentiated), specific genes are switched on or off within
each cell. This happens in response to the chemical hormones surrounding the cell. This pattern
causes a cell to differentiate into one of the 250 possible types of specialised cell in the adult
human body.
Totipotent stem cell – At this stage, the cell has the potential to differentiate into any type of
specialised cell.
Eventually, the cell becomes stuck in a particular pathway of development and is committed to
becoming a specific type of cell e.g. a red blood cell or guard cell.
Stem cells are mostly found in growing embryos but can also be found in adults e.g. in the brain, bone
marrow, blood vessels, skin, teeth, liver, and other (although not all) organs and tissues. They divide
to replenish dying cells and revive damaged tissues. However, in adults, there are a limited
number of specialised cells that stem cells can become. Plants contain stem cells, called meristem
tissue, in the roots, shoot tips and the flowering parts of the plant. These are areas where cells are
rapidly dividing and where growth takes place e.g. elongation, differentiation of cells and cell division.
What are the functions of stem cells?
They replace cells in the particular organ of the body in which they are found. This happens when new
cells are required in tissues that been destroyed by accidents or disease. Stem cells found in bone
marrow, in particular, can differentiate into many types of cells e.g. red blood cells. Stem cells in
embryos differentiate to give us ALL the specialised cells found in the adult body. Meristem tissue
(found in the shoot tips and root tips of plants) can become specialised cells required for a plant to
grow.
