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Cell Structure and Organisation
A cell is the smallest unit that can carry on all the processes of
life
...
Cells are very tiny they could be seen only through a
microscope
...
Planet Cells
2
...
The structures of different types of cells are related to their functions
...
Plant and algal cells also have a cell wall, and often have chloroplasts and a permanent vacuole
...

Dissolved substances pass into and out of cells by diffusion
...
A bacterial cell has a different structure to an animal or plant cell
...

Yeast is a single-celled organism
...
But unlike bacterial cells, yeast cells have a nucleus
...
Their structure will allow them to carry this function out
...
Regular shaped, closely packed
cells form a continuous layer for efficient absorption of sunlight
...

Sperm cell: Fertilises an egg cell - female gamete The head contains genetic information and an enzyme to
help penetrate the egg cell membrane
...
The tail
moves the sperm to the egg
...
Thin outer membrane to let oxygen
diffuse through easily
...
No
nucleus, so the whole cell is full of haemoglobin
...
Pure water has the highest water potential
...
Diffusion is one of the processes that allows this to
happen
...
They move from a region where they
are in high concentration to a region where they are in low concentration
...
This is true in gases
and for particles dissolved in solutions - but diffusion does not occur in solids
...

Examples of diffusion in living organisms
Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion
...

Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:



oxygen moves down a concentration gradient from the air in the alveoli to the blood
carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli

The dissolved substances will only continue to diffuse while there is a concentration gradient
...
A dilute solution
contains a high concentration of water molecules, while a
concentrated solution contains a low concentration of water
molecules
...
They
let some substances pass through them, but not others
...
At this point, the net exchange of water is zero and there is no further change in the liquid
levels
...
This supports the
cell and stops it bursting when it gains water by osmosis
...
Plant cell in a dilute solution (higher water potential than the cell contents) Water enters the cell
by osmosis
...
Plant cell in a concentrated solution (lower water potential than the cell contents) Water leaves
the cell by osmosis
...

Animal cells: Animal cells do not have a cell wall
...


For example, red blood cells:
o
o

gain water, swell and burst in a more dilute solution (this is called haemolysis)
lose water and shrink in a more concentrated solution (they become crenated or wrinkled)

These things do not happen inside the body
...

ACTIVE TRANSPORT
Active transport is the movement of dissolved molecules into or out
of a cell through the cell membrane, from a region of lower
concentration to a region of higher concentration
...

Sometimes dissolved molecules are at a higher concentration inside
the cell than outside, but, because the organism needs these
molecules, they still have to be absorbed
...

Examples of active transport include:
3

o
o

uptake of glucose by epithelial cells in the villi of the small intestine
uptake of ions from soil water by root hair cells in plants

DIFUSSION
DOWN A
CONCENTRATION
GRADIENT
AGAINST A
CONCENTRATION
GRADIENT
ENERGY NEEDED
SUBSTANCE MOVED
NOTES

4

OSMOSIS
Yes

Yes

No

No

No
Dissolved solutes
Gases and dissolved gases
also diffuse

No
Water
Partially permeable
membrane needed

ACTIVE TRANSPORT
No

Yes

Yes
Dissolved solutes
Carrier protein needed



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