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B1: Humans as Organisms
Cell activity (H)
Cell activity
All living things are made of cells.
Some organisms, for example, bacteria, are composed of only one cell, but
humans are composed of millions of cells, most of which are specialised for a
particular job.
Cytoplasm.
In which most of the
cell’s chemical
processes take place
Nucleus
Plasma membrane.
which regulates the
movements of substances
in and out of the cell
Regulates the activities
of the cell and carries
the genetic material
This is a white blood cell. (This cell can engulf and digest any bacteria
which infect the body).
Cell activity
The cell membrane is differentially permeable. It allows small molecules such as
oxygen, carbon dioxide, water, salts, sugars and amino acids to pass through it, but will
not allow large molecules such as proteins to cross it. So it regulates what leaves or
enters the cell.
The nucleus contains the chromosomes and genes which are involved in inheritance, cell
division, growth and development.
The cytoplasm. Contains food reserves, for example oil droplets or glycogen granules (in
liver cells). It also contains mitochondria which are responsible for energy release by aerobic
respiration, also ribosomes involved in making proteins.
Below is an animal cell as seen under the light microscope.
(ribosomes can only be seen under the electron microscope).
Carbon dioxide, water, salts
Mitochondria
Nuclear membrane
Oxygen, water, salts,
sugars, amino acids
Chromosomes
Cell activity
• One of the commonest methods of getting across cell or body surfaces is
by diffusion.
• Diffusion is the spreading of the molecules of a gas, or the molecules
of a substance in solution, resulting in a net movement from a region
where they are at a higher concentration to a region where they are at
a lower concentration.
• The greater the difference in concentration, the faster the rate of
diffusion.
• Oxygen required for respiration passes through cell membranes and
through gas exchange surfaces by diffusion. Other substances such as
sugars and ions can also pass cell membranes.
Cell activity
The diagram below illustrates diffusion.
High concentration
e.g. oxygen in
alveolus
When might diffusion stop?
Low concentration
Diffusion
Oxygen molecules
diffuse down the
concentration
gradient. More
molecules move from
high to low
concentration than
move from low to
high concentration.
Blood capillary has lower oxygen
concentration.
When the concentrations have equalised. This would happen at
death, when the blood flow and breathing would stop.
Cell activity
Before you can understand osmosis you need to understand
diffusion. Can you remember the definition of diffusion? Click for
the correct answer.
Diffusion is the net movement of molecules from an area of high
concentration to an area of low concentration.
Osmosis is the diffusion of water molecules from a dilute solution
to a more concentrated solution through a partially permeable
membrane, that allows the passage of water molecules but stops
the passage of solute molecules.
The concentration of water molecules (the solvent) will be highest in the
dilute solution and least in the concentrated solution.
A partially permeable membrane will allow small molecules, for
example, water,to pass through it but will not allow larger molecules, for
example, sugar or starch, to pass through it.
Cell activity
The diagram below illustrates the process of osmosis.
Dilute sugar solution
Concentrated sugar solution
High concentration of water.
Low concentration of water
Water molecule
Sugar molecules
When would osmosis
stop? (Click).
Only when the two
solutions were of equal
concentration.
Cell membrane is partially
permeable
For example,
leaf vein
Osmosis
Leaf cell
Cell activity
Cells may be specialised to carry out a particular function. A group of cells
with similar structure and a particular function is called a tissue. The four main
classes of tissue are shown in the table.
Epithelial
tissues
Covering
surfaces in the
body, also form
glands.
Examples:
Pavement epithelium
(lines inside of blood
vessels).
Ciliated epithelium
(lines some airways in
the lung).
Mucus secreting
epithelium (lines the
inside of the stomach).
Connective
tissues
Join parts of
the body
together.
Examples:
Blood (connects by
flowing between
parts of the body).
Ligaments (join bone
to bone).
Tendons (join muscle
to bone).
Cartilage (gristle - acts
as support).
Bone (for muscle
attachment, body
framework).
.
Muscular
tissues
Enable
movements.
.
Nervous tissues
Carry nerve
impulses enabling
control.
Examples:
Examples:
Cardiac muscle
(found in the heart,
responsible for the
heartbeat).
Neurones (make up
nerves and nerve
pathways in brain and
spinal cord. Carry
nerve impulses).
Voluntary striated
muscle (attached to
the skeleton and
enables movement).
Involuntary smooth
muscle (found in
organs such as the
stomach – allows
churning movements).
Cell activity
• Diffusion allows absorption of substances along a
gradient. However, substances are sometimes absorbed
against the concentration gradient. The process is called
active transport.
• Active transport requires the use of energy from
respiration.
• The advantage of active transport is that it enable
substances to be absorbed from very dilute solutions.
For example, in plants it allows the root hairs to absorb
ions when the soil solution is more dilute than the cell
sap.
Cell activity
Active transport of ions:
Low concentration of mineral
ions
High concentration of mineral ions
Mineral ion
Water molecule
Soil water
Cell membrane
Root hair cell
Active transport
State two main features of active transport. (Click).
It moves molecules/ions against the concentration gradient.
It requires the use of energy from respiration.
Cell activity
The structure of many human cells and tissues is related to their specific
functions.
A simple pavement epithelium.
Flattened cells on a basement
membrane.
• In this type of epithelium the cells are flattened like paving slabs.
• This gives a protective but thin structure for covering certain body surfaces,
for example, renal capsules in the kidney, alveoli in the lungs and capillary
walls.
Suggest an advantage given by the thinness of this tissue.
It enables easy diffusion of substances across it, in places in the body
where efficient exchange is required.
Cell activity
The structure of many human cells and tissues is related to their specific
functions.
A ciliated columnar epithelium.
Cilia in a layer of mucus
Mucous goblet cell
Columnar cell
Nucleus
Basement membrane
• Mucous goblet cells secrete mucus in which the cilia (small hairs) beat.
• This epithelium is found in parts of the respiratory passages.
• The mucus traps dust and bacteria in the airflow and the cilia waft the trapped
material away from the lungs, to be swallowed into the stomach.
Suggest what will happen to the swallowed bacteria in the stomach.
They will be killed (disinfected) by the strong acid in the stomach.
Cell activity
Tissues are organised into organs. An organ is a functional unit made up of several
different tissues, the individual functions of which contribute to the functions of the
organ as a whole.
Organs are grouped together to form systems. The functions of the organs work together
to perform the overall functions of the system.
Example of system
Digestive system
Respiratory (breathing) system
Examples of organs
Stomach, small and large intestines, liver, pancreas.
Lungs, trachea, intercostal muscles, diaphragm.
Self-check questions:
1.Why is a muscle an organ?
Because it contains several tissues, for example, muscular, nervous and blood tissues.
2.Why is a red blood cell said to be ‘specialised’?
Because it has lost its nucleus to make more room for haemoglobin to carry more
oxygen. It is flattened so has a large surface area relative to volume for efficient gas
exchange.
Cell activity
Summary:
Specialised cells e.g. Nerve cells
Similar cells with similar functions grouped together.
Tissue e.g. Nervous tissue
Several tissues grouped together to give a
structure with a special function.
Organ e.g. Brain
Several organs work together.
System e.g. Nervous System
Systems grouped together.
Organism e.g. Human
Cell activity
The drawing shows the structure of a typical motor neurone. Motor neurones carry nerve
impulses from the brain or spinal cord to a muscle or gland.
Dendrites
Cell body
Nucleus
Cytoplasm
Schwann cell
sheath
Myelin sheath
Axon (nerve fibre)
Much omitted
Impulse direction
Motor end plate
(in muscle)
Synaptic knobs
Cell activity
The job of any neurone is to transmit impulses and the following structures enable it
to do this efficiently:
• the cell body has numerous dendrites which synapse with, (make contact with),
other neurones;
• the dendrites receive incoming impulses from the other neurones, which enables
impulses to be set up in the neurone;
• the axon can be long which enables impulses to be carried to all parts of the body;
• the fatty myelin sheath is laid down in sections leaving short lengths of the axon
with little or no myelin, (called nodes), – this speeds up the impulses which jump
from node to node;
• the Schwann cell sheath produces the myelin as the neurone develops;
• the motor end plate and synaptic knobs enable the impulses to reach the
muscle fibres which are caused to contract.