Biology Unit 2





All living things are made up of cells.  The structures of different types of cells are related to their functions.  To get into or out of cells, dissolved substances have to cross the cell membranes.



·         Cells are the smallest unit of life.

·         All living things are made of cells.

·         Most human cells, like most other animal cells, have the following parts:

o   nucleus

o   cytoplasm

o   cell membrane

o   mitochondria

o   ribosomes


·         Plant and algal cells also have:

o   cell wall

o   chloroplasts

o   permanent vacuole


Functions of the Parts

·         Nucleus – controls the activities of the cell.

·         Cytoplasm – where most of the chemical reactions take place.

·         Cell membrane - controls the passage of substances in and out of the cell.

·         Mitochondria - where most energy is released in respiration.

·         Ribosomes - where protein synthesis occurs.

·         Cell wall – made of cellulose and strengthens plant cells.

·         Chloroplasts - absorb light energy to make food in plant cells.

·         Permanent vacuole - filled with cell sap in plant cells.



·         Yeast is a single-celled organism.

·         The cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.




·         Bacterium is a single-celled organism.

·         A bacterial cell consists of cytoplasm and a membrane surrounded by a cell wall.

·         The genes are not in a distinct nucleus.

Specialised Cells


Movement into and out of cells


·         To get into or out of cells, dissolved substances have to cross the cell membranes.

·         Solutes = particles in solution eg glucose, sodium ions, chloride ions.

·         Solvent = liquid in which the particles are dissolved eg water.

·         Solute and solvent molecules move around randomly.

·         Solutes can move into and out of cells by diffusion. 



·         Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration.

·         Oxygen required for respiration passes through cell membranes by diffusion.

·         The greater the difference in concentration, the faster the rate of diffusion.





The cells of multicellular organisms may differentiate and become adapted for specific functions.  Tissues are aggregations of similar cells; organs are aggregations of tissues performing specific physiological functions.  Organs are organised into organ systems, which work together to form organisms.


Multicellular organisms

·         Large multicellular organisms develop systems for exchanging materials. 

·         During the development of a multicellular organism, cells differentiate so that they can perform different functions.

·         A tissue is a group of cells with similar structure and


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