As Ocr Biology Definitions - Cells, Exchange And Transport

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Active Transport

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Movement of substances across membranes against their concentration gradient, requiring the use of energy from ATP. Active transport usually involves the use of transport proteins.

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Adhesion

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Force of attraction between molecules of two different substances.

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Affinity

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An attractive force between substances and particles.

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Allele

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A version of a gene.

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Alveoli

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Small air sacs in the lungs.

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Anaphase

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In mitosis, the stage when the newly separated chromatids are pulled towards opposite poles of the nuclear spindle.

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Apoplast Pathway

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The route taken by water between the cells or through the cell walls in a plant.

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Asexual Reproduction

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The production of genetically identical new organisms by a single parent organism.

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Assimilation

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Incorporation. Usually applied to the process of incorporating simple molecules of food produced by digestion into the living cells of an animal for use in metabolism. In plants, refers to the incorporation of carbon from carbon dioxide into organic substances during photosynthesis. The newly formed compounds may be referred to as assimilates.

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ATP

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Adenosine triphosphate - a molecule used as the 'energy currency' in organisms. The molecule is broken down into adenosine diphosphate and phosphate to release energy to drive metabolic processes.

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Atrioventricular Node

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A patch of tissue in the septum of the heart that conducts the electrical stimulus from the atria in the heart through to the Purkyne fibres.

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Atrioventricular Valves

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Valves between the atria and ventricles that prevent back flow of blood.

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Atrium

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One of the upper chambers in the heart.

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Binary Fission

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Method of cell division in bacteria. The DNA replicates and the cell divides into two, each having the same DNA as the parent cell. It does not involve mitosis.

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Bohr Shift/Effect

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The effect of carbon dioxide concentration on the affinity of haemoglobin for oxygen.

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Bronchi

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Airways in the lungs that lead from the trachea to the bronchioles.

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Bronchioles

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Airways in the lungs that lead from the bronchi to the alveoli.

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Buffer

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A chemical system that resists changes in pH by maintaining a constant level of hydrogen ions in solution. Certain chemicals dissolved int he solution are responsible for this.

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Cambium

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Plant tissue in the stem and root that contains dividing cells.

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Carbaminohaemoglobin

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The molecule resulting from the combination of carbon dioxide and haemoglobin.

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Cardiac Cycle

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The sequence of events making up one heartbeat.

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Cardiac Muscle

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The muscle found in the heart. It has its own intrinsic heartbeat (it is myogenic).

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Carrier Protein

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A protein found in membranes, which is capable of carrying a specific molecular ion through the membrane by facilitated diffusion or active transport.

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Cartilage

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A flexible, slightly elastic connective tissue.

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Cartilage Ring

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A flexible ring of cartilage that holds the airways open.

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Casparian Strip

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A strip of waterproof material (suberin) in the cell walls of root endodermis cells. It blocks the apoplast pathway.

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Cell Signalling

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Processes that lead to communication and coordination between cells. Hormones binding to their receptors on the cell surface membrane are an example.

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Centriole

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An organelle from which the spindle fibres develop during cell division in animal cells.

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Centromere

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The region of a chromosome where two sister chromatids are joined together, and where the spindle fibre attaches during cell division.

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Channel Protein

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A protein pore that spans a membrane, through which ions and water-soluble molecules may pass.

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Chloride Shift

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The movement of chloride ions (Cl-) into red blood cells to balance the loss of hydrogen carbonate ions (HCO3-).

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Chlorophyll

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Pigments found in chloroplasts of plant (and some protoctist) cells. Each molecule consists of a hydrocarbon tail and a porphyrin ring head with a magnesium atom. Chlorophyll absorbs red and blue light, trapping the energy, and reflects green light.

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Chloroplast

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An organelle found in pants, which contains chlorophyll and is responsible for photosynthetic activity in the plant.

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Chromatid

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A replicated chromosome appears as two identical strands in early stage of cell division. Each strand is a chromatid.

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Chromatin

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Material staining dark red in the nucleus during interphase of mitosis and meiosis. It consists of nucleic acids and proteins. Chromatin condenses into chromosomes during prophase of cell division.

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Chromosome

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A linear DNA molecule wrapped around histone proteins found in the nucleus. Chromosomes become visible in prophase of cell division.

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Cilia

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Short extensions of eukaryotic cell, typically 2-10 micrometers long and 0.25 micrometers in diameter. They may be used for locomotion or to move fluids or mucus over a surface, for example in the mammalian respiratory tract.

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Ciliated Epithelium

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Epithelial cells that have cilia on their cell surface membrane.

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Clones

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Genetically identical cells or individuals.

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Cohesion

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The attraction between water molecules due to hydrogen bonding.

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Companion Cell

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A cell in the phloem involved in actively loading sucrose into the sieve tube elements. The companion cell is closely associated with a phloem sieve element, to which it is linked by many plasmodesmata.

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Concentration Gradient

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The difference in concentration of a substance between two regions.

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Connective Tissue

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A type of tissue that consists of separate cells held together by a ground substance (matrix).

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Constrict

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To make narrow. For example, vasoconstriction is the narrowing of blood vessels.

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Coronary Arteries

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Arteries that carry blood to the heart muscle.

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Cortex

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Tissue in plant roots and stems between epidermis and vascular tissue.

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Cotransporter Protein

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A protein in a cell membrane that allows movement of one molecule when linked to the movement of another molecule in the same direction by active transport.

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Crenation

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State of animal cells when they have been immersed in a solution of lower water potential and have lost water by osmosis. They become shrivelled.

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Cristae

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The folds found in the inner membrane of a mitochondrion. Stalked particles containing ATP synthase are found on cristae.

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Cytokinesis

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The division of the cell, following nuclear division, to form two new cells.

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Cytoskeleton

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The network of protein fibres and microtubules found within the cell that gives structure to the cell and is responsible for the movement of many materials within it.

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Deoxygenated

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Blood with haemoglobin that carries no or little oxygen.

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Diaphragm

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A sheet of muscular and fibrous tissue separating the chest cavity from the abdominal cavity.

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Diastole

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The period when the heart muscle in the ventricles is relaxing and blood pressure is at its lowest.

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Differentiation

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The development and changes seen in cells as they mature to form specialised cells.

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Diffusion

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The net movement of molecules or ions in a gas or liquid from an area of higher concentration to an area of lower concentration.

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Diffusion Gradient

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The gradient in molecular concentration (the difference in concentrations) that allows diffusion to occur.

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Dilate

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To make wider. For example, vasodilation is when the lumina of blood vessels become wider.

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Diploid

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Cells or organisms that have two copies of each chromosome int heir nuclei.

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Dissociation

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The breakdown of a molecule into two molecules, atoms or ions. For examples, the release of oxygen from oxyhaemoglobin.

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Dissociation Curve (Oxyhaemoglobin)

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The curve on a graph showing the proportion of haemoglobin that is saturated with oxygen at different oxygen tensions.

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Division of Labour

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Any system where different parts perform specialised functions, each contributing to the function of the whole.