BIOLOGY NOTES

Cell Structure and

Organisation
 A cell is the smallest unit that can carry on all the processes of life.
All organisms are made of cells, organisms are made of several organ systems, each
organ system contains several organs, each organ contains several tissues; each tissue
is made of cells. Cells are very tiny they could be seen only through a microscope. We
have two types of cells:

1.Prokaryotic cells
2.Eukaryotic cells\

-A prokaryotic cell is a simple, single celled organism that does not have a nucleus or any other
membrane bound organelles.
-Bacteria are the most common type of prokaryotic cell.
-A Eukaryotic cells on the other hand are much more complex and have a true nucleus as well as
other membrane bound organelles.
-Examples of eukaryotic cells include plant cells and animal cells.
  As you can see from the diagram, there are some features found in plant cells but not in
animal cells.

Features found in both plant and animal cells:

 Cell surface membrane: This is a partially permeable membrane separating the cell
from the environment it’s made of lipid and protein, it controls movement of substances in
and out, and it’s strong but flexible.

 Cytoplasm: This is a jelly like substance; it’s made of mostly water and protein.
Metabolic reactions occur in it.

 Nucleus: This determines how the cell behaves and it contains chromosomes made of
strings of DNA which also determines which proteins the cell should make etc.

Features found in only plant cells:

  Cell Wall: This is a rigid layer surrounding the cell made of cellulose, it gives the plant its
shape and prevents it from bursting.

 Chloroplasts: They are sacs which contain chlorophyll which is a green pigment that
traps sunlight for photosynthesis.

 Vacuole: This is a large room in the centre of the cell, it stores sugars and salts and
controls movement of water in and out of the cell.
 Animal cells store sugars in glycogen form but plant cells store it as starch. Animal cells
have an irregular shape but plant cells have a regular shape.
 Both types of cells contain Mitochondria these are structures that convert chemical
energy in foods to energy that could be used in moving, dividing, etc., it is evidence that
the cell is an Active Cell.

Specialised Cells

Red Blood Cells:

 Red blood cells are

found in the blood of
animals, its function is
to transport oxygen
from the lungs to all
the body cells, and
carbon dioxide from
the body cells to the
lungs.

They are adapted by four ways:

 They have a biconcave disc shape that gives it a large surface area to carry more
oxygen.
 They contain chemical called haemoglobin that combines with oxygen and carbon
dioxide.
 They have no nuclease to carry more oxygen and CO2
 They are tiny enough to squeeze through capillaries.
MUSCLE CELLS

They are cells found in

muscles in animals,
they contract and relax
together to move the
organisms.
Their function is to
contract to support
and move the body.

They are adapted by two ways, First, Is that they are made of contractile filament to help in
contraction. Second is it contains lots of mitochondria to supply the cell with energy.

Ciliated Cells

 Ciliated cells are present in the trachea and bronchi of out respiratory system.
 Their function is to use their cilia to move the mucus up the trachea to the throat. The
mucus traps bacteria and dust particles. When it reaches the throat, mucus is swallowed
to the stomach where the acid kills the bacteria.
 They are
adapted
by the
tiny hair
like
projection
s called
cilia
which
sweeps
the
contamin
ated
mucus
upwards.

 The mucus is secreted by goblet cells which are next to ciliated cells.
Root Hair Cells

 These are cells situated

in the roots of plants.
They contain no
chloroplasts.
 Their function is to
absorb water and
minerals from the soil.
And to anchor the plant
in the soil.
 They are adapted by 3
ways. One, they have an
extension that increases
the surface area for
more water intake. Two,
they have a large
number of mitochondria
for respiration to become
more active. Three a
concentrated vacuole to
help absorbing water by
osmosis.

Xylem Vessels

 These are dead lignified cells that exist in the stem of a plant.

Their function is to transport water and minerals from the roots to the leaves and the rest
of the plant through the stem and to support the plant.
 They are adapted by 2 ways. Firstly, they are hollow to allow water and minerals to pass
through them with no resistance. Secondly they are strong and lignified to support the
plant.
Movement In And Out Of Cells

Substance move in and out of cells by three ways:

 Diffusion: The net movement of particles from a region of their higher concentration to a
region of their lower concentration down a concentration gradient, as a result of random
movement.

 Osmosis: The diffusion of water molecules from a region of their higher concentration
(dilute solution) to a region of their lower concentration (concentrated solution) through a
partially permeable membrane.

Active Transport: The movement of ions, in and out of a cell, through a cell membrane, from a region of
their lower concentration to a region of their higher concentration, against the concentration gradient,
using the energy released by respiration.

Diffusion:

 Diffusion is the process by which oxygen enters the blood from the lungs, and by which
carbon dioxide enters the leaf from the atmosphere. There are many more examples of
diffusion in biology.
 Diffusion always takes place down a concentration gradient that means that the particles
that diffuse try to spread evenly in all spaces, so it moves from where it’s very
concentrated to where it’s not concentrated.
 There are some factors affecting the rate of diffusion, like the steepness of the
concentration gradient. The steeper the gradient the faster the particles diffuse.

1. The surface area of the exchange membrane also affects the rate of diffusion. The
larger the surface area of the exchange membrane the faster particles diffuse.
2. Thickness of exchange membrane too determines the diffusion rate, the thinner it is,
and the easier it will be for particles to go through it, the faster the diffusion rate.
3. Temperature is another factor affecting the diffusion rate, increasing the temperature will
give particles more kinetic energy, making them move faster, thus increasing the rate of
diffusion.

Osmosis

 Osmosis is the diffusion of water molecules. When we speak about osmosis, we don’t
say water concentration; instead we use the term water potential. A dilute solution
means it has lots of water molecules, and a high water potential. A concentrated solution
has few water molecules and low water potential. Osmosis has to take place through a
partially permeable membrane (or Semi permeable) this means that the Water molecules
move from a place of their high concentration to a place of their low concentration
through a membrane with pores in it that lets some molecules through but not others.
 ows two solutions, one dilute and one concentrated, separated by a semi permeable
membrane. The solution on the right is diluted while the concentration on the left is
concentrated. The water molecules will move from the right hand side solution where
they are very concentrated to the left hand side solution where they are of a very low
concentration, osmosis took place.
 Osmosis happens all the time in cells. If you
place an animal cell in distilled water. Osmosis
will result in the water molecules moving from
the distilled water where they are very
concentrated to the Cell Where they are of low
concentration through the cell surface
membrane. The cell becomes fat. As more
Water molecules enter the cell, the cell will
eventually burst and die.

 If we do the opposite, and place a red blood cell in a concentrated salt solution, the water
in the cell has a higher water potential that the concentrated salt solution. Water
molecules will move from the cell to the salt solution causing the cell to become shrunken
and shrivel as in the diagram.
 Hypotonic Solution Isotonic Solution Hypertonic Solution

Anima
l Cell

Plant
Cell

In plant cells, if a plant cell is placed in distilled water, water molecules will move from the
distilled water to the cell, the cell swells up and becomes turgid but it will never burst
because plant cells are surrounded by cell walls, which are made of cellulose and is
elastic, it will stretch but never break, the cell becomes turgid.
 If we place a plant cell in a concentrated salt solution with low water potential, water will
move from the cell to the solution causing the cell to become plasmolysed as in the
diagram.

Active Transport

 Active transport occurs in cells, it is basically the movement of molecules or ions from a
region of their low concentration to a region of their high concentration (against the
concentration gradient) using energy of respiration. Active transport occurs in living,
active cells only because it needs energy, these cells usually have a structure called
mitochondria which respires producing energy to be used in active transport.
 Active transport happens in roots to absorb mineral salts from the soil. It
also occurs in the digestive system of mammals.
 If oxygen is absent, respiration won’t take place, active transport will stop.
Molecules are taken into the cell by protein carriers within the cell
membrane.