Page 19 - Biology_F5

Biology for Advanced Level Secondary Schools

and controls passage of materials into and animal cells, plasma membrane is the
out of the cell. Apart from cell membrane, outermost layer, whereas in plant cells it
other cells such as plant and fungal cells is located beneath the cell wall.

have an additional outer layer called a cell Structure of the cell membrane
wall. It is formed on the outside of the The cell membrane is made up of proteins
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cell membrane to give the cell an extra and lipids (phospholipids). Protein
support, protection, as well as efficiency molecules are embedded in the layers of
and it regulates exchange of materials. the phospholipids. Phospholipids form a

bilayer structure, which is fundamental to
The it nucleus is the largest cellular the selective permeability function of the
structure, located inside the eukaryotic
cell and acts as the control centre of a cell. membrane. Each phospholipid molecule
The material between the cell membrane consists of a hydrophobic (water hating)
and the nuclear envelope is known as tail of two fatty acids and the hydrophilic
cytoplasm. Within the cytoplasm lies (water loving) phosphate head. In the
an intricate arrangement of fine fibres cell membranes, phospholipids arrange
and thousands of miniscule but distinct themselves in a layer of two molecules
structures called cytoplasmic organelles thick (bilayer), with their hydrophobic tails
and cytoplasmic inclusion such as stored pointing inwards, away from the water

nutrients, secretory products, droplets and both inside and outside the cell, while the
pigment granules. Cytoplasmic organelles hydrophilic heads face outwards. There
are small structures that are suspended are two models that can be used to describe
in the cytoplasm of the cell. Examples the structure of the plasma membrane,
of cytoplasmic organelles include: namely Danielli-Davson’s model (1935)
endoplasmic reticulum, ribosomes, and Fluid Mosaic model (1972).
mitochondria, plastids, vacuole, lysosome,
microbodies, cytoskeleton, and Golgi Danielli-Davson’s Model
apparatus. Each type of organelle has a According to Danielli and Davson, the

definite structure and a specific role in the membrane is structurally composed of two
cell. Cytoplasmic inclusions do not have chemical substances made up of proteins
any membrane or specific shapes.
and lipids that form trilaminar layers

Plasma membrane (Figure. 1.6). The outer and inner layers
This is also called plasmalemma or cell are made up of protein molecules which
membrane. It surrounds most of the cell sandwich the phospholipids bilayer. The
organelles, separates the contents of the heads of phospholipids are polar, oriented
cell from the external environment, and towards the protein layers to form the
controls the exchange of materials. In hydrophilic region.

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