Last minute revision – Human Biological Science test 1
Cell structure and organelles
 Cells are the basic units of life. They contain tiny parts called
organelles that have specific roles for life.
 Cell membrane allows transport in and out of the cell and provides a
boundary from external fluid
 Mitochondria - the place where respiration takes place. High energy
requiring cells will contain lots of mitochondria
 Endoplasmic reticulum is responsible for protein synthesis
 The nucleus contains DNA and controls the cell function.
Structure of the cell membrane as it relates to transport of materials
 Cell membrane made of
mainly lipids and proteins.
Cell membrane is a
phospholipids bilayer (2
layers)
 The lipid molecule contains a
phosphate group =
Phospholipid
 Cell membrane is
differentially permeable –
allows some molecules and
restricts others
 Passive processes – don’t
use energy from respiration,
active processes – requires cells energy.
 3 methods of transfer: Diffusion, carrier mediated and Vesicular
Methods of transporting materials including diffusion, facilitated diffusion, osmosis, active transport,
endocytosis and exocytosis
 Diffusion: Small molecules such as water and gases can passively diffuse through the membrane –
high –low. Oxygen diffuses into the cell along th concentration gradient as the concentration inside
the cell is low as the oxygen is continually being used by respiration. No limitation on rate – the
larger the concentration difference the faster the rate of diffusion
 Osmosis is the diffusion of water –high conc of water (dilute solution) to a low conc of water
(concentrated solution). Water can move through as it is small. Other substances such as salt and
sugar cannot as they are large. As water moves into a cell osmotic pressure increases
 Carrier medicated transport also called Facilitated diffusion. There are special proteins in the
membrane that bind to an ion or molecule and help it move across the membrane- such as glucose
and amino acids. When the transported molecule binds to the carrier protein , the protein changes
shape carrying the molecule to the opposite side of the cell membrane. Once all carrier molecules
in use it cant go any faster
 Active transport –molecules move against the concentration gradient – requires energy. E.g. amino
acids and certain ions that are already more concentrated in the cell can still be absorbed – energy
for active transport comes from cellular respiration.
 Vesicular-Active process – move in and out of the cell in enclosed vesicles –
exocytosis/endocytosis
 Endocytosis – cell surrounds some extracellular material , cell membrane folds and material
enclosed – phagocytosis (cell eating) Pinocytosis (cell drinking)
 Exocytosis – contents of vesiucle pushed out. Membrane around vesicle fuses with cell membrane
and contents are passed to the exterior of cell e.g. milk from breast, saliva
Factors affecting exchange of materials including surface area to volume ratio, concentration gradients.
 All requirements of a cell must pass through cell membrane. Thus relationship between SA and vol
is important. As a cell grows its ability to exchange enough materials to support its increasing
volume diminishes. A large cell could not support itself as it would not have enough cell membrane
surface to absorb all the nutrients required or remove the wastes. Therefore to function – cells need
to be microscopic! Tiny!!
Anabolic and catabolic reactions and organelles involved
 Anabolic – to build uses energy – e.g. Protein synthesis – In the Ribosomes
 Catabolic – to break down – releases energy in the mitochondria
 Most reactions take place in the cytoplasm of the cell
 Respiration is an example of a catabolic reaction that releases energy when molecules of ATP are
broken down
Respiration (aerobic and anaerobic); inputs, outputs and organelles involved
 Ceullar respiration – process by which molecules taken in as food as broken down in the cells using
oxygen to release energy for the cells activities – 7 life processes
 Glucose+ oxygen  carbon dioxide + water + energy
 Most of energy made is used to maintain body temperature. The remaining energy is used to form
a compound called ATP
 Takes place in every cell of the body. The main food material utilized (used) is glucose
 Anaerobic respiration – without oxygen – breaks down glucose into pyruvate then lactic acid via
Glycolysis . This releases enough energy to convert 2 molecules of ADP to ATP
 Anaerobic respiration takes place in the cytoplasm as this is where the enzymes are available for
the reaction. Anaerobic respiration is important during vigorous physical activity e.g. in muscle cells
to supply extra energy. This results in the accumulation of lactic acid in the muscles – pain and
fatigue. Lactic acid is taken to the Liver where it can be re-combined with oxygen to form glucose
and eventually glycogen
 Aerobic respiration uses oxygen – it breaks down 2 pyruvate molecules from glycolysis to carbon
dioxide and water. This happens in the mitochondria
 2 reactions allow this to occur – 1st = Krebs cycle, 2nd electron transport system – produce up to 36
molecules of ATP
Enzyme function including reduction in activation energy, lock and key principle
 Biological catalysts that speed up chemical reactions by
allowing them to occur at normal temperatures. Energy
needed to get a reaction started = activation energy –
enzymes reduced the activation energy needed to begin a
reaction
 Also allow reactions to proceed at a rate that suits body
requirements
 Enzymes are specific and made of proteins. Each
enzymes will combine with one particular substrate. They
have a shape and structure that allow them to fit together –
Lock and Key
 The part of the enzyme that combines with the substrate
is called the active site
Factors that affect enzyme activity including pH, temperature, cofactors, co-enzymes.
 The higher the concentration of enzyme – faster the reaction
 The rate of most enzymes increase as temp increases but only in a limited range
 Above 45oC enzymes become denatured – inactive
 This is because they are proteins and the heat causes their shape to change
 The temp at which as enzyme works best = optimum temperature
 Enzymes are sensitive to pH
 Some enzymes need the help of co-factors such as ions or non-protein molecules
 Co-factors can change the shape of the active site which allow enzyme and substrate to combine