CELLS
THE CELL IS THE SMALLEST UNIT OF LIFE.
ALL ORGANISMS ARE COMPOSED OF CELLS.
SOME ARE A SINGLE CELL, OTHERS ARE
MULTICELLULAR. THE CELL THEORY STATES:
1. ALL ORGANISMS RE COMPOSED OF ONE OR
MORE CELLS, WITHIN WHICH THE LIFE
PROCESSES OF METABOLISM AND HEREDITY
OCCUR.
2. CELLS ARE THE SMALLEST LIVING THINGS,
THE BASIC UNIT OF ORGANIZATION OF ALL
ORGANISMS – THE BASIC UNIT OF STRUCTURE
AND FUNCTION.
3. CELLS IN AN ORGANISM ARE ALL
INDIVIDUALS OF THE SAME ORGANIZATIONAL
RANK.
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4. THE ACTION OF AN ORGANISM IS THE SUM
OF MANY ACTIONS OF DIFFERENT
COLLABORATING CELLS.
IS THIS TRUE?
WHY IS IT THE CELL THEORY?
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AN ALTERNATIVE TO THIS IS THE
ORGANISMAL THEORY. THIS STATES THAT
THE WHOLE ORGANISM IS THE BASIC UNIT OF
LIVING ORGANISMS. PLANTS PROVIDE AN
EXCELLENT EXAMPLE BECAUSE THEY HAVE
INCOMPLETE CLEAVAGE BETWEEN DIVIDING
CELLS; CONTINUOUS CYTOPLASMIC
CONNECTIONS BETWEEN CELLS
(PLASMODESMATA) AND ABSENCE OF
CELLULAR MOTILITY DURING DEVELOPMENT.
THE FIXED POSITION AND
INTERCONNECTEDNESS OF ALL CELLS IN A
PLANT CREATE A SYMPLAST – THE ENTIRE
MASS OF A PLANT AS A CONTINUOUS UNIT.
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CELL WALL
PLANT CELLS TYPICALLY HAVE A CELL WALL
CONTAINING THE LIVING PROTOPLAST. THE
LAYER THAT CONTACTS WALLS OF ADJACENT
CELLS IS THE MIDDLE LAMELLA. THIS HAS A
HIGH CONCENTRATION OF PECTIN.
THE PRIMARY CELL WALL – THE WALL
FORMED WHILE THE CELL IS GROWING TO ITS
MATURE SIZE. THIS WALL IS RELATIVELY
THIN; IT IS MADE UP OF CELLULOSE BUT HAS A
HIGH PROPORTION OF HEMICELLULOSES IN A
GEL-LIKE MATRIX ALLOWING MORE
PLASTICITY TO THE GROWING WALL.
FOLLOWING CESSATION OF GROWTH SOME
CELLS DEVELOP A SECONDARY CELL WALL
WHICH IS DEPOSITED ON THE INSIDE SURFACE
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OF THE PRIMARY CELL WALL. IT IS
COMPOSED MAINLY OF CELLULOSE BUT CAN
CONTAIN DEPOSITS OF LIGNIN (A COMPLEX
CARBOHYDRATE, SECOND MOST ABUNDANT
POLYMER IN PLANTS, SERVES TO ADD
RIGIDITY TO THE CELL WALL) OR SUBERIN (A
FATTY SUBSTANCE, FUNCTIONS TO REDUCE
WATER LOSS). THE SECONDARY CELL WALL
OFTEN APPEARS TO BE LAMELLATED.
PRIMARY PIT FIELDS ARE THIN AREAS OF THE
PRIMARY WALL – WHICH CORRESPOND TO
THIN AREAS OF ADJACENT CELLS. THESE
USUALLY HAVE PROTOPLASMIC STRANDS
(PLASMODESMATA) PASSING THROUGH THEM
CONNECTING THE PROTOPLASTS OF
NEIGHBORING CELLS. THIS CONNECTION OF
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LIVING PROTOPLASTS FORM THE SYMPLAST.
THE PRIMARY PIT FIELDS REMAIN EVEN IF
SECONDARY WALLS ARE DEPOSITED – THEN
THEY ARE CALLED PITS OR PIT PAIRS IF
THERE ARE TWO PITS CONNECTING ADJACENT
CELLS.
THE MOVEMENT OF SUBSTANCES FROM CELL
TO CELL BY MEANS OF PLASMODESMATA IS
SYMPLASTIC TRANSPORT AND IS MORE
EFFICIENT APOPLASTIC TRANSPORT WHICH
GOES THROUGH CELL WALLS AND PLASMA
MEMBRANES AND THE MIDDLE LAMELLA. THIS
CAN CARRY ONLY WATER AND INORGANIC
NUTRIENTS AND IS GOOD FOR SHORT
DISTANCES ONLY. THE APOPLAST IS THE NONLIVING SPACE OUTSIDE THE PROTOPLAST.
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PLASMA MEMBRANE
THE PLASMA MEMBRANE (PLASMALEMMA) IS
A SELECTIVELY PERMEABLE MEMBRANE
THAT ENCLOSES THE CONTENTS OF THE CELL
AND CONTROLS THE MOVEMENT OF
MATERIALS INTO AND OUT OF THE CELL.
ORGANELLES
ARE MEMBRANE BOUND COMPARTMENTS
INSIDE THE CELL – EACH WITH ITS SPECIFIC
FUNCTIONS. THOSE THAT CAN BE SEEN WITH A
LIGHT MICROSCOPE INCLUDE:
NUCLEUS
LOCATION OF GENETIC MATERIAL, DNA,
CHROMOSOMES
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VACUOLE
ITS MEMBRANE IS THE TONOPLAST; USUALLY
FILLLED WITH WATER WITH DISSOLVED
SALTS, SUGARS, ENZYMES, ETC. IT MAY
COMPRISE 80-90% OF THE VOLUME OF THE
CELL. SOMETIMES THE CONCENTRATION OF A
PARTICULAR SUBSTANCE IS SUFFICIENT TO
FORM CRYSTALS – CALCIUM OXALATE IS
COMMON.
VACUOLES ARE ALSO SITES WHERE
METABOLIC WASTE PRODUCTS ARE STORED –
THE TONOPLAST DOES NOT ALLOW THEM TO
LEAK BACK INTO THE CYTOPLASM. PLANTS DO
NOT EXCRETE WASTES LIKE ANIMALS;
WASTES STORED IN THE VACUOLE ARE
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NOXIOUS AND BITTER WHICH CAN DETER
ANIMALS FROM EATING THE PLANT.
PLASTIDS
ARE FOOTBALL SHAPED ORGANELLES THAT
INCLUDE CHLOROPLASTS – THE SITE OF
PHOTOSYNTHESIS; AMYLOPLASTS ARE
COLORLESS AND ARE STORAGE SITES FOR
STARCH; CHROMOPLASTS ARE STORAGE SITES
FOR PIGMENTS; LEUCOPLASTS ARE
COLORLESS SITES FOR THE SYNTHESIS OF
STARCH, FATS, LIPIDS, ETC.
ORGANELLES NOT VISIBLE BY THE LIGHT
MICROSCOPE ARE:
ENDOPLASMIC RETICULUM – ER – A TUBULAR
NETWORK THAT FUNCTIONS TO SYNTHESIZE
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LIPIDS AND PROTEINS. THE SMOOTH ER
MAKES LIPIDS,THE ROUGH ER , WITH
RIBOSOMES ON THE MEMBRANE, IS THE SITE
OF PROTEIN SYNTHESIS.
GOLGI APPARATUS (DICTYOSOMES)
ARE STACKS OF FLATTENED SACS WHERE
PRODUCTS OF THE ER ARE PROCESSED
FURTHER. THEY ARE ALSO INVOLVED IN
SECRETION.
MITOCHONDRIA
ARE SITES OF RESPIRATION; RELEASES
CHEMICAL ENERGY STORED IN FOOD AND
TRANSFERS IT TO FORM ATP.
MICROBODIES
ARE VERY SMALL SPHERICAL BODIES.
PEROXISOMES METABOLIZE HYDROGEN
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PEROXIDE AND ARE INVOLVED IN
PHOTORESPIRATION, USUALLY FOUND IN THE
LEAVES (IN ANIMALS PEROXISOMES ARE
COMMON IN LIVER AND KIDNEY CELLS).
GLYOXYSOMES ARE INVOLVED IN
CONVERTING STORRED FATS INTO SUGARS
DURING GERMINATION; THESE ARE FOUND
ONLY IN PLANTS SO PLANTS CAN CONVERT
LIPIDS TO CARBOHYDRATES, ANIMALS
CANNOT.
NON-MEMBRANOUS OREGANELLES
CYTOSOL
IS THE LIQUID MATRIX OF THE CYTOPLASM –
MOSTLY WATER, ENZYMES, ETC.
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RIBOSOMES
ARE THE SITES AT WHICH AMINO ACIDS ARE
LINKED TOGETHER TO FORM PROTEINS. THEY
OCCUR FREELY IN THE CYTOPLASM OR ARE
ATTACHED TO THE ER. THEY ARE COMPOSED
OF PROTEIN AND RNA.
MICROTUBULES & MICROFILAMENTS
ACT AS THE CYTOSKELETON. THE
CYTOSKELETON HOLDS REGIONS OF THE CELL
SURFACE BACK WHILE OTHERS ARE
EXPANDED – OTHERWISE THE CELLS WOULD
JUST BE SPHERES – SO CELL GROWTH AND
EXPANSION ARE DIRECTED TO CERTAIN
AREAS. THEY GUIDE VESICLES TO CERTAIN
SITES/REGIONS AND ARE THE MEANS OF
MOTILITY FOR ORGANELLES WITHIN THE
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CELL. THEY PROVIDE THE FRAMEWORK TO
MOVE CHROMOSOMES DURING CELLULAR
DIVISION.
SPHEROSOMES
ARE LARGE OIL DROPLETS WITHIN THE
CYTOPLASM.
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