Biology: Unit F211: Cells, Exchange and Transport
Module 1: Cells
1.1.3
– Cell Division, Cell Diversity and Cell Organisation
A human body cell has 46
chromosomes in 23 pairs.
One of each pair if from
mum, one is from dad.
Chromosomes are made
from 2 sister chromatids
which contain identical
genetic information. The
two chromatids are
joined by the centromere,
a protein glue. Each end
is protected by a cap
called a telomere.
DNA, deoxyribonucleic
acid, is the chemical from
which chromosomes are
made.
DNA is wrapped around
histone proteins to form
chromosomes.
Genome: All of the genes that carry instructions to make you. The human genome
is made up of around 30,000 genes.
Chromosome: Chromosomes are DNA molecules wrapped around protein
histones. The human genome is divided into 23 sections called chromosomes. Each
of these chromosomes is made from DNA. Each chromosome contains around 1000
genes.
Gene: Section of DNA which codes for the production of one protein/feature of a
person.
Allele: Alternative forms of the same genes, for example, for hair colour; the alleles
may be brown, blonde, black or red.
Homologous Chromosomes are chromosomes in a
diploid cell which have the same genes at the same loci.
Members of an homologous pair of chromosomes pair
up during mitosis. They may have different alleles.
The Human Genome is made from around 30,000
genes. The 23 chromosome pairs are numbered 1-22,
with the sex chromosomes being the 23rd pair.
Chromosome pairs 1-22 are the autosomes.
The Chromosomes of pair 23 are the sex
chromosomes. Males have XY chromosomes and
females have XX.
This is a diploid karyotype –
there are 2 copies of every
chromosome. Normal human
body cells are diploid. One of
each pair comes from mum,
one from dad.
This is a haploid
karyotype – there is
just one copy of every
chromosome. Human
gametes, egg and
sperm cells are
haploid.
Mitosis is the process of nuclear division where 2 genetically identical nuclei are formed
from one parent cell nucleus. Due to the accuracy of mitosis,
every cell in the human body is an exact genetic copy of the first
cell that formed you; when the sperm fertilised the egg. Mitosis
works to maintain genetic stability, keeping the genome
constant. This is vital to health, because any changes which
occur are mutations which could increase risk of developing
tumours.
Mitosis has 3 functions;
1. Growth of the organism, producing more cells
2. Repair of tissues and organs after injury
3. Asexual reproduction, increasing the population by division of cells
Mitosis is very tightly controlled: no cell
can undergo mitosis without receiving a
number of signals to check it is safe to divide
and desired by the rest of the body. If the
signals regulating mitosis breakdown, mitosis
goes on uncontrolled; this is cancer and the
result is a tumour.
Hormones or signal molecules must bind to the glycoprotein
receptors on the cell surface in order to release the ‘brakes’ on
the cell and allow it to undergo mitosis.
When a cell has passed all of the relevant
checkpoints and received all the necessary
signals, the cell is ready to undergo mitotic
cell division.
The Cell Cycle describes the events that take place as one parent cell divides to
produce two new daughter cells which then grow to full size. The cell cycle includes
interphase, the phase between divisions (split into G1, S and G2), and mitotic phase,
which is further divided into prophase, metaphase, anaphase, telophase and
cytokinesis. It is controlled by enzymes.
A cell spends 95% of its time in interphase. The
cell goes about its normal functions as well as
preparing itself for mitosis.
In G1
(biosynthesis),
organelles are
replicated and
new proteins
are made.
Mitotic phase involves prophase, metaphase,
anaphase, telophase and cytokinesis. This is
the nuclear division creating two genetically
identical daughter cells.
S phase is DNA
synthesis,
where
chromosomes
are replicated.
G2 is growth, from a small cell into a big
cell, by making more membrane and
taking up more water.
In every tissue, there is a small specialised subset of cells which can divide – these
are called stem cells. After mitosis, they must specialise, by switching on particular
genes. The process of becoming a particular type of cell is called differentiation.
Cells can differentiate by changing the number of a particular organelle, changing
the shape of the cell, or changing the contents of the cell.
Cells, tissues and organs are specialised to perform their specific function;
 A tissue is a collection of similar cells working together to perform a
specific function [xylem and phloem in plants; ciliated epithelial tissue in
animals]
 An organ is a collection of similar tissues working together to perform a
specific function [leaf in plants, lungs in animals]
 An organ system is a collection of similar organs working together to
perform a specific function [reproductive and respiratory systems]
Specialist Animal Cells
Red blood cells – erythrocytes
o Produced by stem cells in bone marrow
o They transport carbon dioxide and oxygen between
lungs and body tissues
Adaptations to their function
o Biconcave shape: large surface area: volume ratio
increases ability to carry oxygen and Carbon Dioxide
o Small: travel in capillaries to get close to body cells
and tissues
o No nucleus: more room for haemoglobin
o Lots of haemoglobin: to combine with oxygen and
carbon dioxide to carry around the body
White blood cells – leucocytes
o Produced in stem cells in bone marrow
o Circulate around blood stream identifying and helping to
destroy foreign material.
Adaptations to their function
o Glycoprotein receptors to identify self or non self
o Lysosomes in cytoplasm to destroy foreign material brought
into the cell by endocytosis.
Sperm cells – spermatozoan
Plasma Membrane: Contains receptors
which bind with the egg allowing
fertilisation to take place.
Flagellum: tail which
helps to propel the
sperm towards the egg.
Acrosome: a specialised
lysosome which digests
egg so that fertilisation
can occur.
Small, long and thin:
streamlined shape helps
them to move easily.
Mitochondria: produce
Modified Cytoskeleton:
made of microtubules which
use ATP to move and slide
over each other causing the
lashing movements of the
tail.
ATP by aerobic
respiration, providing
energy for the sperm to
swim.
o Used to transport male genetic information
o They are produced by meiosis, and so have nucleus containing 23
chromosomes. They are haploid so that when the sperm and egg gametes
come together at fertilisation, the zygote formed will have the full 46
chromosomes, and will be diploid.
Specialist Plant Cells
Palisade Cells
Palisade cells are the main photosynthetic cells of the plant.
They are found on the top side of the leaf, where they can
absorb light. They are packed with chloroplasts containing
chlorophyll so that they are able to photosynthesise.
Root Hair Cells
Root hair cells are found at the tips of roots. They are
designed to increase the surface area of the root to
help with the maximum absorption of water and
minerals.
Guard Cells
Guard cells are found in the lower epidermis. They
have unevenly thickened cell walls; the inner walls
contain spirals of cellulose. They contain
chloroplasts so that they can release energy in
order to take up water and minerals. When water
moves into these cells and they become turgid,
only the outer walls stretch, because of the
cellulose thickening at the inner wall, and so the
two guard cells create a pore between them. This is
known as a stoma.
Specialist Animal Tissue
Ciliated Epithelial Tissue
Cilia waft in
one direction
to move
substances
away.
Ciliated epithelial cells:
Lots of mitochondria
and a large cytoskeleton
to enable the cilia to
move.
Basement
membrane cells
have complex
cytoskeleton to
anchor goblet
and ciliated cells.
Found in the trachea
and the oviducts.
Goblet Cell: Lots of
rough endoplasmic
reticulum and large
Golgi body to synthesise
and secrete protein
mucus.
Squamous Epithelial Tissue
Squamous Cell
Lumen
Basement membrane
Nucleus
Underlying tissues
Squamous epithelial tissue is made
up of cells which are flattened and
very thin. The cells together make up
a smooth, flat surface, making them
ideal in environments where friction
can be reduced, such as blood
vessels, so that fluids can pass easily
over them. They also form thin walls
such as those of the alveoli in the
lungs. This provides a short diffusion
pathway for the exchange of gases.
Specialist Plant Tissue
Xylem and Phloem
Phloem – have perforated end ‘sieve plates’
which allow sap (sucrose dissolved in water).
Xylem – made from dead cells aligned end to
end to form a continuous column. The walls are
lined with extra lignin, which makes them
strong, and ideal for carrying water.
Cooperation - Leaves
Palisade Mesophyll: parenchyma
cells contain many chloroplasts
for photosynthesis
Vascular bundle:
midrib containing
xylem and phloem
Upper Epidermis: Thin
transparent layer
which allows light to
reach mesophyll.
Protective, covered in
waterproof cuticle to
reduce water loss.
Spongy Mesophyll:
Large air spaces to
allow carbon dioxide
for photosynthesis to
circulate
Phloem: Transports
organic solutes
(sugars) made by the
plant during
photosynthesis.
Lower Epidermis:
containing stoma and
guard cells.
Xylem: Transports
water and mineral
ions
Mitosis vs Meiosis
Where?
Why?
What?
How?
Mitosis
Meiosis
Everywhere else
Growth of tissue, repair, asexual
reproduction
Genetically identical cells - clones
Diploid parent cell produces 2
diploid daughter cells which are
genetically identical.
Ovaries, testes
Sexual reproduction
Gametes – genetically variable cells
Diploid parent cell produces 4
haploid non identical daughter cells.
Stem cells exist in every tissue, and their role is to undergo mitosis when an
organism needs to grow, asexually reproduce or repair themselves. Stem cells are
undifferentiated cells with no identity; they have no specific features like other cells.
They are able to undergo mitosis to produce daughter cells which are then able to
differentiate into specific and specialised cells.
Undifferentiated stem cell with no identity.
MITOSIS
Differentiated cells with an identity
The two types of stem cell are:
Totipotent: such as in an embryo. These are able to divide and
differentiate to form any type of cell in the body.
Pluripotent: such as in adults. These are able to divide and
differentiate to become just a few cell types in the body.
Plants are slightly different: they have meristem tissue
in the growing roots and shoots. Mitosis in these cells
drives the growth of the plant. The meristem tissues
are sensitive to auxin plant hormone. They undergo
mitosis and push the plant up and the root down,
elongating the cell.
Phloem
Meristem – Cambium
Xylem
Binary fission in Bacteria
In binary fission, the DNA is copied and then the cell grows
larger and larger, finally splitting into two. This is a type of
asexual reproduction, as the cells produced are genetically
identical clones.
Budding in Yeast
Yeast, a single celled eukaryotic organism, with
structures similar to our own, divide by a process
called budding. This is another form of asexual
reproduction. Yeast buds are created when a
mother cell grows to a critical size at a time
coinciding with DNA synthesis. There is a
weakening of a small area of the cell wall and this,
together with the turgor pressure of osmosis allows
swelling of the plasma membrane and cytoplasm to
form a bud. This process leaves a ring in the plasma
membrane of the mother cell known as a bud scar,
and a birth scar on the surface of the daughter cell.
The number of bud scars on the surface of a yeast
cell is a useful indicator of cellular age.