E5c: The relation between the structure of different cells and their function
Introduction Brief description of the structure of plant and animal cells in general. Explain that all cells are modifications of
the basic cell design.
Plant cells Palisade cells - palisade mesophyll packed with chloroplasts which are able to move via cytoplasmic
streaming. Therefore get into the best position within a cell for the efficient absorption of light. Elongated
cells, tightly packed with small air spaces, to form a continuous layer which traps most of the incoming light.
Arranged so that perpendicular to epidermis which reduces the number of cell walls through which light has
to pass and so minimises the amount of reflected light.
Plant support achieved by:
Parenchyma cell - when turgid - cell sap in vacuole pushes the cell contents against the cell wall creating
turgor pressure, collenchyma have extra cellulose deposited in the cell wall, sclerenchyma have a
secondary wall impregnated by lignin - dead cells.
Xylem vessels - strengthened by lignin which is waterproof. Lignin is deposited in rings or spiral pattern in
young xylem to enable cell elongation. As soon as the walls are completely lignified the cell dies, and loses its
cell contents. The cell walls between adjacent vessel elements are lost, so that the xylem forms a long
continuous tube through which water can be transported. Pits exist in the walls between xylem elements to
enable lateral movement of water. This is important as water is then able to move via an alternative vessel
should movement become prevented in one vessel. The lignified walls prevent the walls from collapsing
when transporting water under tension.
Sieve tube elements and companion cells - sieve tube elements contain little cell contents, so maximise
the amount of sucrose that can be translocated, but relies upon the companion cells. Companion cells
contain numerous mitochondria as the loading of sucrose into the phloem is an active process.
Guard cells - thickened cell wall on the stoma side of the cell, and rings of thickening mean that the cell
bends when water enters via osmosis. Cell membranes contain potassium pump and ATPase (=proteins in
cell membrane), which are important in creating water potential gradients. Contain chloroplasts, but role is
unclear.
Endodermal cells in root. Suberin in the Casparian strip, results in all water and mineral salts having to move
via the symplastic route - therefore the entry of substances into the xylem is controlled.
Animal cells Red blood cells- contain haemoglobin to carry oxygen. Disc shaped, biconcave- for a larger surface area for
the diffusion of gases in/out. No nucleus - more space for haemoglobin. No mitochondria so respire
anaerobically - therefore do not use oxygen.
White blood cellsProteins in the cell surface membranes of white blood cells enables cell-to-cell recognition. Foreign antigens
are recognised.
Phagocytic neutrophils and macrophages contain numerous lysosomes containing hydrolytic enzymes for the
digestion of foreign bodies taken into the cell. Cytoplasmic streaming in macrophages enables amoeboid
movement in tissues and alveoli.
B-lymphocytes have antibody molecules which are released when foreign antigens are encountered.
Proximal convoluted tubule cell - microvilli to increase the surface area over which substances can be
reabsorbed. Numerous mitochondria for the active uptake of substances. Basal channels to increase the
surface area over which substances can move into the blood.
Muscle fibres - long cells containing myofibrils. Many nuclei. Infolding of the sarcolemma (=cell membrane)
distribute the action potential throughout the muscle fibre. Sarcoplasm containing calcium ions which are
important in starting muscle contraction. Sliding filament theory of muscle contraction - actin and myosin
filaments.
Neurones - sensory and motor. Long axons. Have dendrites which are important in the formation of networks
within the central nervous system. Myelinated sheath (schwann cells) to speed up the speed of transmission
(saltatory conduction). Nissl's granules containing ribosomes which are important in the synthesis of
neurotransmitter. Synaptic knobs have numerous vesicles containing neurotransmitter substance. Numerous
mitochondria in synaptic knob.
Sperm- long tail for swimming. Head containing acrosome, which releases enzymes to penetrate the
membranes surrounding the secondary oocyte. Mid region of sperm contains numerous mitochondria to
provide ATP for the movement of the tail. Contains haploid nucleus, and so diploid number in the zygote is
achieved at fertilisation.
Ciliated epithelial cell - cilia involved in movement, e.g. cells lining the respiratory tract have cilia which can
move mucus to where it can be swallowed.