Download Unit 5.2 Plant Cells

Unit
5.2
context
Plant cells
The first ever cells to be seen were plant
cells, specifically the cells making up the
bark called cork. Cells are the building
blocks that make up every part of a plant,
regardless of whether the plant is a tiny
and insignificant moss or a towering gum
tree. Like all living things, plants use energy
and gases, grow and reproduce. They draw
water from the soil, release valuable oxygen
gas and produce seeds or spores for
reproduction. Each of these tasks needs a
specific type of cell.
Fig 5.2.1 Cells are the building blocks that make
up all plants.
Structure of a plant cell
Like all living things, plants are made up
of microscopic structures called cells.
However, not all cells are the same. Cells
are specialised to carry out different tasks
in the plant. Despite their variety, all
plant cells share many features.
Cytoplasm: this jelly-like liquid is where energy is released
and new substances are made. The cytoplasm can be
thought of as the chemical factory of the cell.
Cell wall: this must be rigid enough to
support the plant. It contains a tough
fibrous material called cellulose.
Chloroplasts: these contain a
green chemical called chlorophyll.
Chlorophyll traps the light energy
needed for photosynthesis.
Vacuole: In plants, vacuoles are large
and filled with sap. They contain air,
water, wastes and food particles.
Cell nucleus: this controls all chemical
reactions in a cell and how the cell
develops and reproduces. The nucleus
is the ‘control room’ of the cell.
Fig 5.2.2 Plant cells are made
up of parts called organelles.
Each organelle does a specific
job inside the cell.
Cell membrane: this controls
what goes in and out of the
cell. It lines the inner cell wall.
A mitochondrion: mitochondria are energy
capsules that contain glucose (a plant’s food) and
oxygen. Mitochondria are so small they cannot
usually be seen using a light microscope.
Prac 1
p. 153
D ra
g - a n d - d ro p
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Plant cells
Food for plants
Animals get their energy from the food they eat. Plants
don’t eat and instead make their own food in a process
known as photosynthesis. Photosynthesis is a chemical
reaction that uses energy from sunlight to combine
carbon dioxide with water. A sugar called glucose is
produced, as is oxygen gas. The photosynthesis reaction
is best shown as the equation:
carbon dioxide + water + sunlight
glucose + oxygen
A plant obtains its carbon dioxide by drawing it in
from the air through specialised cells, which are located
mainly under its leaves. Water is drawn from the soil
using different specialised cells in the plant’s roots. The
glucose a plant produces is used directly as food or
stored for later use. The oxygen a plant produces is
released back into the air. Specialised cells are needed to
draw in the different materials that photosynthesis
requires. Other specialised cells will transport these
materials to where they are needed, and other cells will
actually carry out the photosynthesis reaction.
Go to
Science Focus 2 Unit 3.2
Fig 5.2.3 The box-like structure of pondweed cells
is obvious when they are viewed under a microscope.
The green dots are the chlorophyll-filled chloroplasts.
150
Diffusion and osmosis
Once a plant has made its food in the form of glucose, it
then needs to use it. This happens within the plant’s
cells. Cells need energy to carry out their required
functions, such as reproduction. They obtain this energy
by ‘burning’ the glucose in a chemical reaction known
as cellular respiration. Respiration requires glucose
and a supply of oxygen gas. The glucose is formed by
photosynthesis and the oxygen is drawn into the leaves.
Carbon dioxide, water and energy are its products. This
reaction is best shown as:
glucose + oxygen
carbon dioxide + water + energy
For this reaction to occur, glucose and oxygen must
flow into the cells (along with some other nutrients and
minerals) and carbon dioxide and water flow out of
them (along with some other wastes). The cell
membrane is therefore not a solid wall but a porous
‘screen’ that allows some chemicals through while
blocking others.
Chemicals move across the cell membrane in two
different ways:
• diffusion (where oxygen, carbon dioxide
and water move from a region of high
concentration to a region of lower
concentration)
• osmosis (where water moves to dilute the
concentrations of larger chemicals and nutrients).
Unit
5.2
Specialised plant cells
There are millions of cells in a plant and different types
of cells perform different jobs. Each cell type has its own
specialised structure and is located in a position on the
plant that maximises its performance.
Photosynthetic cells: These make up a layer near the
top surface of a leaf. They have many chlorophyll-filled
chloroplasts which make the leaf green. Most
photosynthesis happens here.
chloroplasts
nucleus
underside
of leaf
Conducting cells: These are found in the stems and
branches of the plant. They form tubes that transport
water and nutrients to all parts of the plant.
water-conducting
tube
guard
cells
stoma
in open
position
cell magnified
wilted
leaf
sieve
food-conducting
tube
straight
guard
cells
closed
stoma
cell magnified
Root hair cells: Photosynthesis needs water, which is
absorbed through the roots of the plant. Root hairs
increase water absorption by increasing the total
surface area of the root.
root hair
Stomata: small openings, mostly on
the underside of leaves. The plant takes
in carbon dioxide and gives out oxygen
though these openings. Guard cells
open and close to reduce the amount
of water lost through the stomata.
cell in root
nucleus
soil
Prac 2
p. 154
Fig 5.2.4 Plant cells are specialised so that they can perform
their specific job.
Go to
Science Focus 2 Unit 3.1
151
Plant cells
5.2
QUESTIONS
Remembering
7 Photosynthesis is how green plants make their own food.
Identify:
1 State the type of specialised plant cell that:
a the chemicals (reactants) the plant needs to carry out the
reaction
a carries out photosynthesis
b transports water and nutrients
b the chemicals (products) the reaction produces
c absorbs water from the soil
d controls the opening and shutting of the holes, called
stomata.
d from where the reaction gets its energy.
Evaluating
Understanding
8 Propose what would happen to a plant without:
2 Define the term organelle. L
a guard cells
3 Describe the function in the cell of:
b cellulose
a its nucleus
c chlorophyll.
b cell wall
9 Propose a likely reason for each of the following:
c chloroplasts.
a Most plants are green.
4 Explain the purpose of chlorophyll in a plant.
b Plant cells have thicker walls than animal cells.
Applying
c Photosynthetic cells are usually found only on the upper
surface of a leaf.
5 Identify which type of specialised plant cell is:
a ‘hairy’
d There are two different types of conducting cells forming
two different pathways in the plant.
b the ‘gatekeeper’
c a ‘transporter’.
6 From the clues given, identify these substances found in a
plant. Explain the function of each.
10 There are far more types of animal cells than plant cells.
Propose a reason why.
a I’m green.
11 Propose a reason why animals do not need to be able to
carry out photosynthesis like plants do.
b I’m found in the cell wall.
Creating
c I’m found in the vacuole.
12 Construct a 2D or 3D edible model of a plant or animal cell
using biscuits, icing sugar and/or assorted lollies.
5.2
INVESTIGATING
INVESTIGATING
e -xploring
W
The confocal microscope is currently being developed eb Destination
by an Australian company. Investigate this new type of
microscope and how it may be used to observe skin cells without
the removal of skin from the body. To find out more, a list of web
destinations can be found on Science Focus 1 Second Edition
Student Lounge.
152
c the name of the sugar that is used by plants as food