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 moecules 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 membranesuch 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
 May be waste substances or substances required elsewhere – e.g. milk from breast,
saliva, digestive enzymes from cells liing small intestine all released by exocytosis.
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 however complex foods cush as
carbohydrates, proteins and fats can be broken down.
 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 glicose
and eventually glycogen (livers store of energy0
 After vigerous exercise you continue to breath heavily as you need to ‘pay back” the
Oxygen Debt
 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
ATP and cellular uses of energy
 ATP is formed when ADP is joined with another phosphate group
 Some of the energy is stored in the bond between ADP and the third phosphate group.
 When the third phosphate group is removed – energy is released
 ATP therefore is like the energy store. It is a way of releasing energy from cellular
respiration to the cell processes that need energy
 Uses of energy in a cell – building complex molecules, cell division, growth, movement,
active transport, transmission nerve impulses.
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 enzy,me – 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