Edexcel GCSE Biology - Cells and Control (2)

Gene

short section of DNA, codes for protein, controls a characteristic, each chromosome contains many genes

Cell Cycle

series of steps cell must take to divide

1^st Stage of Cell Cycle

interphase. cell grows, organelles like ribosomes + mitochondria increase in size + number, protein synthesis occurs, DNA replicates forming X shape, energy stores increase

2^nd Stage of Cell Cycle

mitosis
prophase: chromosomes condense, nuclear membranes break down
metaphase: chromosomes align at cell equator
anaphase: cell fibres pull chromosomes to opposite poles
telophase: spindle fibres break down, 2 nuclear envelopes form around daughter cells, chromosomes disappear

3^rd Stage of Cell Cycle

cytokinesis. 2 genetically identical diploid daughter cells form when cytoplasm and cell membranes divide. same sets of chromosomes in parent and daughter cells

Mitosis in Multicellular Organisms

important in growth, development, replace damage cells. in asexual reproduction, mitosis replicates organism’s own cells

Growth in Animals

division by mitosis and differentiation

Differentiation in Animals

most cells can differentiate at early stage, then lose ability
most specialised cells can reproduce via mitosis
red blood cells lose nucleus, so replaced by adult stem cells, which retain ability to differentiate

Growth in Plants

division - cell cycle
differentiation - many types of cells can differentiate throughout life. only differentiate when reach final position in plant, re-differentiate when moved
in elongation - plants absorb water into vacuoles, controlled by auxins

Cancer

result of small changes in cells causing uncontrolled division - tumour

Monitoring Growth using Percentiles

baby charts using mass, length, head circumference
e.g. 75^th percentile = heavier than 25%
high percentile can indicate health problem

Embryonic Stem Cells

forms when egg + sperm fuse to form zygote
differentiate into any type of cell in body
scientists can culture these cells in lab and direct them to differentiate
replace insulin-producing cells (diabetes)
neural cells (Alzheimer’s)
nerve cells (spinal cord injuries)

Adult Stem Cells

can differentiate into limited range of cell types e.g. bone marrow stem cells make blood cells

Plant Meristems

found in root and shoot tips
differentiate into any type of plant, retain ability across lifespan
can be used to clone parent plant - desirable features / extinction risk

Therapeutic Clonic

embryo produced with same genes as patient
harvested for embryonic stem cells
grown into any required cells e.g. tissues, organs
not rejected - genetically identical to recipient / patient / donor

Benefits of Stem Cell Research

replace damaged / diseased body parts
use unwanted embryos, otherwise be discarded
research differentiation

Problems with Stem Cell Research

don’t fully understand differentiation process, hard to control stem cells to form desired products
stem cell removal destroys embryo
religious + ethical objections to perceived interference with natural reproductive process
contaminated stem cells can transfer infection to recipient
consumes time + money that could go elsewhere

Central Nervous System

brain + spinal cord = CNS. controls consciousness, movements, thoughts, emotions

Cerebrum

2 cerebral hemispheres take up most of skull, sit on left + right-hand sides. control consciousness, memory, intelligence, visual + sensory processing

Cerebellum

large lump-like structure at bottom of brain on rear. controls fine muscle movement and co-ordination

Medulla Oblongata

small elongated structure found in brainstem at base of brain. maintains basic autonomic (automatic) bodily functions e.g. breathing, digestion, swallowing, sneezing.

Applications of CT + PET Scans

examine brain tissue for injury and disease. cutting brain is highly invasive and provides no information RE brain function

Computerised Tomography

X-Rays fired at brain from several different angles, 3D image
examines bleeding within skull
higher radiation exposure not recommended for pregnant women + children

Positron Emission Tomography

radioactive tracer injected pre-scan, which accumulates in areas with higher blood flow, highlighted more brightly, scan is tracer-sensitive
cancerous tumours used more blood than normal tissue

Difficulties in Investigating and Treating Brain Damage + Disease

complex, delicate, easily damaged
drugs cannot always cross membranes surrounding brain
not fully understood which part of the brain does what

Brain Tumours

push against structures + blood vessels, impairing function
buried deep in brain or spinal chord, difficult to remove

Structure of Nervous System

receptor cells convert stimulus to electrical impulse
impulse travels along sensory neurons to CNS
CNS coordinates response, sends electrical impulse to motor neurones
effectors enact response e.g. muscles contracting or glands secreting hormones

Automatic Responses

AKA reflexes occur before you have time to think. prevent individual getting hurt. travel down reflex arc

Reflex Arc

receptors detect stimulus
impulse (action potential) along sensory neuron to CNS
CNS passes impulse to relay neuron to motor neuron to effector

Examples of Reflex Arcs

pupils contracting to avoid damage from light / move hand away from hot surface to prevent damage from heat

Synapses

gaps between 2 neurons. when action potential reaches end of 1^st neuron, neurotransmitter released into + diffuses across synapse, triggers impulse to begin in next neuron
different neurotransmitters → frequency, speed of impulse in next neuron

Myelinated Nerve

surrounded by myelin sheath → action potential travels faster.
myelin produced by schwann cells

Retina

layer of light-sensitive cells at back of eye, send impulse to brain, creating image
rod cells — low light
cone cells — colour vision

Cornea

transparent layer at front of eye, allows light through, curve surfaces refracts + focuses light onto retina

Iris

muscles surrounding pupil
circular muscles contract in bright light
radial muscles relax in low light, more light for better image

Lens

transparent curved surface at front of eye, reflects light onto retina

Ciliary Muscles / Suspensory Ligaments

hold lens in place

Accommodation

near object:
ciliary muscles contract / suspensory ligaments loosen / lens is thicker, more curved / refracts more light

distant object:
ciliary muscles relax / suspensory ligaments tighten / lens is thinner / less curves / less light refracted

Short-Sightedness / Myopia

lens is too curved, distant objects appear blurry
treated with concave spectacle or contact lens (sport) to spread light
laser eye surgery reduces corneal thickness

Long-Sightedness / Hyperopia

lens too flat / cannot refract enough light
treated with convex spectacle or contact lens (sport) to bring rays together
laser eye surgery changer corneal curvature
lens replacement risks retinal damage or cataracts

Cataracts

lens clouding. from birth or over time. treated by lens replacement surgery, swap natural lens for clear plastic one

Colour Blindness

can’t see certain colours. full colour blindness rare, red-green is common
insufficient cone cells in retina, usually genetic