Plants and animals are multicellular organisms. They are made up of millions of cells working together. Drag the appropriate labels to both plant and animal cells.
The diagrams show a typical plant and a typical animal cell. Animal and plant cells have many parts. The nucleus contains the genetic material and controls the structure and activities of the cell. The nucleus of a cell contains chromosomes, on which many genes are located. These genes control the inherited characteristics of the body, such as eye and hair colour.
Chromosomes are made of a chemical called deoxyribonucleic acid or DNA for short. In humans the normal number of chromosomes in body cells is 46. Click on the graphic to display the number of chromosomes in a human body cell. The exceptions are sperm and egg cells, which only contain 23 chromosomes each, so that when fertilisation takes place, a cell with the normal number of chromosomes is formed. This cell divides many times to form an embryo and then a foetus.
The cell membrane
surrounds the cell and controls the movement of substances in and out of
the cell. The cytoplasm is made up of water and dissolved substances. Many
chemical reactions take place in the cytoplasm. These chemical reactions
are controlled by enzymes. Enzymes are protein molecules that catalyse
biological reactions. For example, the digestion of food is catalysed by
several enzymes. If there were no enzymes, the reactions would only happen
very slowly, if at all.
Plant cells
also have a rigid cell wall, made of cellulose. This strengthens the cell,
and provides support. Please click on the labels to display information
on the structures found in a plant cell. Click on Next when ready to continue.
Plant cells
also have a large permanent vacuole. This contains cell sap, which is water
and dissolved substances. The cell sap is important in maintaining the
shape of the cell. Animal cells may have several small, temporary vacuoles.
These can be where digestion takes place or where excess water is excreted.
Both plant and
animal cells also contain small structures, called organelles. Mitochondria
are the organelles where respiration takes place. Chloroplasts are also
organelles.
Multicellular
plants and animals contain many different types of cells. Each type of
cell is designed to carry out a particular job or function. Click on the
graphic to display information on muscle cells. Muscle cells can contract
and relax. Muscle cells in your gut wall move food along the gut. Cardiac
muscle cells in the heart wall never stop contracting and relaxing throughout
your life. Nerve cells have long fibres to carry messages.
Red blood cells
contain haemoglobin, which can combine with oxygen. Ciliated epithelium
cells have tiny hairs called cilia, which can move substances. They are
found in the mucous membranes lining the windpipe and air passages in the
lungs. Leaf mesophyll cells contain chloroplasts so they can carry out
photosynthesis. Root hair cells have a large surface area to take up water
from the soil.
Cells that are
similar to each other and do the same job are grouped together to form
tissues. Muscle cells make muscle tissue, nerve cells make nerve tissue
and so on. Muscle, blood and nerves are all tissues. Tissues are made up of many cells of the same type, working
together.
Various tissues
together form more complex structures such as organs. The heart, stomach,
and the brain are all organs. The heart is made of muscle tissue, nervous
tissue, and connective tissue. Each organ has its own specific job. Organs
cannot work by themselves, they depend on other organs to supply the things
they need. They are linked together to form organ systems.
A human organism
has a breathing or gas exchange system in order to take in oxygen and get
rid of carbon dioxide. A digestive system breaks down large molecules into
small soluble ones, which can pass into the blood. The digestive system
includes the gullet, stomach, intestines, liver and pancreas.
A circulatory
system transports substances round the body. A nervous system has senses
which respond to stimuli and a brain to co-ordinate and send responses
to different parts of the body. An excretory system gets rid of waste products
like urea, and maintains the balance of water in the body. A muscular-skeletal
system provides support and protection and places for the muscles to attach.
In a healthy
organism, all the systems work together and each responds to changes in
the other, like the parts of a complicated machine. For example, when you run,
your skeleton acts like a set of levers, moving you onwards. But your muscular
system must also contract on the right bones, to make them move. Your nervous
system sends messages to coordinate the muscle movement. The muscles need
a supply of food and oxygen. Food and oxygen are transported to the muscles
and waste carbon dioxide is removed, by your circulatory system.
The three main
mechanisms that living organisms use to move substances in and out of cells
are diffusion, osmosis and active transport.
Diffusion is
the spreading of the particles of a gas, or of a substance in solution,
from a place where they are at a higher concentration to a place where
they are at a lower concentration, along a concentration gradient. The
greater the difference in concentration, the faster the rate of diffusion.
Diffusion takes place because all the particles in gases and liquids are
constantly moving. Small, light particles diffuse more quickly than large
heavy ones. Diffusion is faster at high temperatures, because the particles
have even more kinetic energy.
The oxygen required
for respiration passes through gas exchange surfaces, such as alveoli in
the lungs, by diffusion, which is fairly slow. To make up for this, the
surface area of the lungs is as large as possible, with millions of alveoli.
Diffusion takes place where there is a concentration gradient. Breathing
in air increases the amount of oxygen in the alveoli, so oxygen diffuses
into the blood. Carbon dioxide diffuses out of the blood along a concentration
gradient. Carbon dioxide enters leaves and leaf cells by diffusion. Leaves
are thin and flat, so they provide a large surface area over which diffusion
can take place.
The amount of
liquid inside the dialysis tubing increases because of osmosis. Osmosis
is a special kind of diffusion. Osmosis is the diffusion of water molecules
from a higher concentration of water to a lower concentration of water
through a partially permeable membrane. A partially permeable membrane,
like a cell membrane, is one that allows the passage of water molecules
but not solute molecules, such as glucose. Osmosis takes place because
water molecules can easily pass through the cell membrane. Larger molecules
like glucose are too large to pass through the tiny pores in the cell membrane.
Water moves from the soil into the root hair cells of plant roots by osmosis.
When water moves into plant cells by osmosis it increases the pressure
inside the cell.
Plant cell walls
are sufficiently strong to withstand the pressure from inside the cell.
A plant with plenty of water has firm or turgid cells, which provide support
for the whole plant. The stem is straight and its leaves and petals are
firm. A plant without enough water has soft or flaccid cells. The plant
begins to wilt. Its stem, leaves and petals are soft.
Plants are able
to absorb mineral ions from a very dilute solution in the soil by a process
called active transport. In active transport, substances are moved into
or out of living cells through a partially permeable membrane, against
a concentration gradient. Because this process is against a concentration
gradient, it requires energy. Respiration provides the energy for active
transport. For this reason, root hair cells have a lot of mitochondria.