1. B1: Humans as Organisms
Cell activity (H)

2. 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
Plasma membrane.
which regulates the movements of substances in and out of the cell
Nucleus
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).

3. 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).
Mitochondria
Carbon dioxide, water, salts
Nuclear membrane
Oxygen, water, salts, sugars, amino acids
Chromosomes

4. 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.

5. Cell activity The diagram below illustrates diffusion.
High concentration
Low concentration
Oxygen molecules diffuse down the concentration gradient. More molecules move from high to low concentration than move from low to high concentration.
e.g. oxygen in alveolus
Blood capillary has lower oxygen concentration.
Diffusion
When might diffusion stop?
When the concentrations have equalised. This would happen at death, when the blood flow and breathing would stop.

6. 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.

7. Cell activity The diagram below illustrates the process of osmosis.
Water molecule
Dilute sugar solution
Concentrated sugar solution
Osmosis
Sugar molecules
High concentration of water.
Low concentration of water
For example, leaf vein
Leaf cell
Cell membrane is partially permeable
When would osmosis stop? (Click).
Only when the two solutions were of equal concentration.

8. 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.
Connective tissues
Join parts of the body together.
Muscular tissues
Enable movements.
Nervous tissues
Carry nerve impulses enabling control.
Examples:
Cardiac muscle (found in the heart, responsible for the heartbeat).
Examples:
Neurones (make up nerves and nerve pathways in brain and spinal cord. Carry nerve impulses).
Examples:
Blood (connects by flowing between parts of the body).
Examples:
Pavement epithelium (lines inside of blood vessels).
Voluntary striated muscle (attached to the skeleton and enables movement).
Ligaments (join bone to bone).
Ciliated epithelium (lines some airways in the lung).
Tendons (join muscle to bone).
Involuntary smooth muscle (found in organs such as the stomach – allows churning movements).
Cartilage (gristle - acts as support).
Mucus secreting epithelium (lines the inside of the stomach).
Bone (for muscle attachment, body framework).

9. 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.

10. Active transport of ions:
Cell activity
Active transport of ions:
Soil water
Root hair cell
Low concentration of mineral ions
High concentration of mineral ions
Mineral ion
Water molecule
Cell membrane
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.

11. A simple pavement epithelium.
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.

12. A ciliated columnar epithelium.
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.

13. 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
Examples of organs
Digestive system
Stomach, small and large intestines, liver, pancreas.
Lungs, trachea, intercostal muscles, diaphragm.
Respiratory (breathing) system
Self-check questions:
Why is a muscle an organ?
Why is a red blood cell said to be ‘specialised’?
Because it contains several tissues, for example, muscular, nervous and blood tissues.
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.

14. 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

15. 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
Nucleus
Cytoplasm
Schwann cell sheath
Myelin sheath
Synaptic knobs
Cell body
Axon (nerve fibre)
Motor end plate (in muscle)
Much omitted
Impulse direction

16. 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.